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Amphetamine induces xerogenic effects, but its mechanism of action and xerogenic potency are unknown. In the current in vivo study on the rat parotid gland, the effects of amphetamine on reflex-evoked and acetylcholine-evoked salivation were examined in the absence and presence of adrenergic and dopaminergic antagonists. Under anaesthesia, amphetamine increased the secretion of salivary fluid and the amount of protein therein in response to acetylcholine. Phentolamine abolished the increase in salivary flow and had no effect on the salivary protein concentration, whereas propranolol only reduced the salivary protein concentration. Reflex activation of the secretion evoked a well-maintained level of secretion that was reduced by amphetamine [50% inhibitory dose (ID50) 1.9 ± 0.1 mg kg−1 intravenously); the salivary protein concentration was increased in the presence of amphetamine. Phentolamine and haloperidol reduced the amphetamine-inhibitory effect on the reflex-evoked fluid response, whereas propranolol had no effect on the fluid response. The xerogenic effect of amphetamine is mainly exerted by central mechanisms involving α-adrenoceptors, while, indirectly, amphetamine causes secretion of protein by inducing the release of noradrenaline from glandular nerve terminals. [ABSTRACT FROM AUTHOR]

Background and purpose:The internal anal sphincter has been shown to contract in response to α1-adrenoceptor stimulation and therefore α1-adrenoceptor agonists may be useful in treating faecal incontinence. This study characterizes the α1-adrenoceptor subtype responsible for mediating contraction of the internal anal sphincter of the pig.Experimental approach:The potency of agonists and the affinities of several receptor subtype selective antagonists were determined on smooth muscle strips for the pig internal anal sphincter. Cumulative concentration–response curves were performed using phenylephrine and noradrenaline.Key results:The potency of the α1A-adrenoceptor selective agonist A61603 (pEC50=7.79±0.04) was 158-fold greater than that for noradrenaline (pEC50=5.59±0.02). Phenylephrine (pEC50=5.99±0.05) was 2.5-fold more potent than noradrenaline. The α1D-adrenoceptor selective antagonist BMY7378 caused rightward shifts of the concentration–response curves to phenylephrine and noradrenaline, yielding low affinity estimates of 6.59±0.15 and 6.33±0.13, respectively. Relatively high affinity estimates were obtained for the α1A-adrenoceptor selective antagonists, RS100329 (9.01±0.14 and 9.06±0.22 with phenylephrine and noradrenaline, respectively) and 5-methylurapidil (8.51±0.10 and 8.31±0.10, respectively). Prazosin antagonized responses of the sphincter to phenylephrine and noradrenaline, yielding mean affinity estimates of 8.58±0.10 and 8.15±0.08, respectively. The Schild slope for prazosin with phenylephrine was equal to unity (1.01±0.24), however the Schild slope using noradrenaline was significantly less than unity (0.50±0.11, P<0.05).Conclusion and implications:The results suggest that contraction of circular smooth muscle from the pig internal anal sphincter is mediated via a population of adrenoceptors with the pharmacological characteristics of the α1A/L-adrenoceptor, most probably the α1L-adrenoceptor form of this receptor.British Journal of Pharmacology (2008) 155, 110–117; doi:10.1038/bjp.2008.217; published online 2 June 2008 [ABSTRACT FROM AUTHOR]

Abstract: We investigated the pharmacological profile of the vasoconstrictive response to clonidine in the isolated rabbit ear vein, and compared the characteristics of clonidine with those of noradrenaline and moxonidine. The maximal vasoconstrictive responses to clonidine and moxonidine in the rabbit ear vein were 35.94±11.18% and 88.78±11.54% of the maximum response to noradrenaline, respectively. Prazosin 0.1 μM inhibited the vasoconstriction induced by lower concentrations of noradrenaline, and the concentration-dependent response curve for noradrenaline was significantly shifted to the right by 1 μM prazosin. Yohimbine (0.1 and 0.5 μM) only decreased the vasoconstrictive response to lower concentrations of noradrenaline, but did not affect the response to higher concentrations. Vasoconstrictive responses to lower but not higher concentrations of clonidine and moxonidine were inhibited by 0.1 μM yohimbine. In contrast, the same concentration of yohimbine significantly potentiated the maximal response to a high concentration of clonidine by 24.06%. In isolated rabbit ear vein pretreated with 0.1 μM yohimbine, prazosin competitively inhibited the concentration–response curve for clonidine with a pA2 value of 8.05±0.06. We conclude that clonidine acts mainly on α2-adrenoceptors to produce vasoconstriction in the rabbit ear vein; however, in the preparation pretreated with yohimbine, the clonidine-induced vasoconstriction is mediated via α1-adrenoceptors and its maximal vasoconstriction is significantly potentiated. [Copyright &y& Elsevier]

Abstract: The vasculature of the equine digit fulfils an important role in thermoregulation. In other species, it has been found that cooling may enhance the response of cutaneous vessels to 5-hydroxytryptamine (5-HT) and α2-adrenoceptor agonists. Translocation of α2-adrenoceptors to the smooth muscle cell membrane, mediated by Rho kinase, is thought to be involved in the cooling-enhanced response in mouse tail arteries. However, little is known about the effect of cooling on 5-HT receptor function. The present investigation compared the response of 5-bromo-6-(2-imidazolin-2-ylamino) quinoxaline (UK14304:1 nM to 30 μM), methoxamine (0.1 nM to 30 μM; in the presence of yohimbine 0.1 μM), 5-carboxamidotryptamine (5-CT; 0.1 nM to 10 μM) and α-methyl 5-HT (0.1 nM to 10 μM) in the isolated equine digital vein at 30 °C and 22 °C. The effect of the Rho kinase inhibitor, fasudil (1 μM), and the recovery of the response after the irreversible blockade of surface receptors with phenoxybenzamine (10 μM) or 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ;10 μM), was established. Moderate cooling significantly increased the maximum response to α-methyl 5-HT, 5-CT and UK14304 and shifted their response curves to the left. Cooling also augmented the phenoxybenzamine- and EEDQ-resistant response to UK14304 and 5-CT, respectively. Fasudil had no effect on the contractile response at 30 °C, but completely abrogated the effect of cooling on the response to 5-CT and UK14304. The response to methoxamine was not significantly affected by cooling. These results suggest that Rho kinase plays an important role in the cooling-enhanced response mediated by 5-HT1B/D receptors and α2-adrenoceptors. The exact mechanism by which Rho/Rho kinase enhances the functional responses mediated by these receptors in these vessels has yet to be determined. [Copyright &y& Elsevier]

Abstract: Our previous study demonstrated that successive electroacupuncture (EA) at ST36 acupoint reduces IgE production in BALB/c mice immunized with 2,4-dinitrophenylated keyhole limpet protein (DNP-KLH) by suppression of the Th2 cell lineage development. Here, we report that pretreatment of phentolamine (α-adrenoceptor antagonist, 10mg/kg, i.p.) completely blocks the EA-induced suppression of antigen-specific and total IgE levels in serum and IL-4 production in anti-CD3 mAb-activated splenocytes in DNP-KLH immunized mice. The results suggest that α-adrenoceptor play an important role in mediating the suppressive effects of EA on IgE production and Th2 cell response in DNP-KLH immunized mice. [Copyright &y& Elsevier]

The article discusses a study on the mechanisms by which norepinephrine regulates protein levels of bumetanide-sensitive cotransporter type 1 in an immobilized thick ascending limb epithelial cell line. It concludes that bumetanide-sensitive cotransporter type 1 regulation proceeds via the β-adrenoceptor receptor-cyclic adenosine monophosphate (cAMP)-protein kinase A pathway and α-adrenoceptor activation negatively regulates bumetanide-sensitive cotransporter type 1 protein levels.

1 In isolated smooth muscle tissues taken from rats, rabbits and guinea-pigs, all at 37.5 °C, the equilibrium dissociation constant ( K β) of the competitive, reversible α-adrenoceptor antagonist phentolamine varied between 4 and 28 nm. 2 The concentration of the antagonist required to inhibit contractions to direct- or indirect-acting α-adrenenoceptor agonists by 50% (IC50) also varied between 5 and 30 nm. 3 From one tissue to another, the IC50/ K β ratio of the blocker varied from 1 to 2.5, the values being close to those predicted by classical receptor theory based on the law of mass action. 4 At 27.5 °C, using phenylephrine as the spasmogen in rat aorta, the IC50/ K β ratio for phentolamine was 3.1. 5 A significantly higher IC50 compared with K β for phentolamine indicates that the procedures for estimating affinity constants for a competitive antagonist are not equivalent. [ABSTRACT FROM AUTHOR]

Objective: This study tested whether α -adrenoceptor-mediated coronary vasoconstriction is augmented in the metabolic syndrome and is accompanied by the alteration of specific α1 - and α2 -coronary adrenoceptors. Methods: Studies were conducted in control and chronically high-fat-fed (6 weeks of 60% calories from fat) dogs with metabolic syndrome. Alterations in coronary α1B -, α 1D -, and α2A -adrenoceptor mRNA and protein expression were examined by real-time PCR and Western analyses, respectively. Coronary blood flow and its response to intracoronary infusion of either the α1 -adrenoceptor agonist methoxamine (0.1–3 mg) or the α2 -adrenoceptor agonist BHT-933 (0.1–3 mg) were measured in anesthetized dogs. Results: Basal plasma epinephrine and norepinephrine levels were higher in the high-fat-fed dogs compared to controls. Real-time PCR revealed no alterations of coronary artery or arteriole α 1B -, α1D -, and α 2A -adrenoceptor mRNA expression. However, Western blot analysis showed a significant decrease in α2A -adrenoceptor protein density with no change in α1B - or α1D -adrenoceptors. Methoxamine and BHT-933 produced dose-dependent decreases in coronary blood flow, but the decrease in coronary flow to methoxamine was significantly greater (∼20%) in dogs with the metabolic syndrome. No differences in the coronary flow response to BHT-933 were noted. Conclusions: These results indicate that the metabolic syndrome is associated with sensitization of α1 - and α2 -adrenoceptor signaling that could significantly limit control of coronary blood flow when the sympathetic nervous system is activated. [ABSTRACT FROM AUTHOR]

1 Among the potential effects of exposure to high levels of radio-frequency energy (RFE) (which includes microwaves), an increase in body temperature is the primary consequence. Release of autacoids and activity of the autonomic nervous system may influence (or be directly responsible for) some of the physiological changes that occur in conjunction with this hyperthermia. 2 The main focus of this review is the interaction of autacoids and the autonomic nervous system with cardiovascular changes during heating. Differences between environmental and RFE-induced heating (such as rate of temperature change and degree of skin vs. core heating) may be important when considering these effects. 3 Antihistamines exhibited no beneficial effect on circulatory collapse during RFE-induced heating. The serotonergic blocker methysergide decreased survival time in rats during terminal RFE exposure, despite no effects on heart rate (HR) or blood pressure. Although blockade of platelet-activating factor resulted in lower HR before RFE exposure, there was a lack of effect on the subsequent increase in HR during heating. Nitric oxide did not contribute to the hypotension that occurs due to rapid heating by RFE exposure. There have been either no or very limited studies of effects of prostaglandins, bradykinin, or angiotensin on RFE-induced heating responses. 4 β-Adrenoceptor antagonism with propranolol resulted in significantly decreased survival times and lower final colonic temperatures during RFE exposure. A lack of effects of nadolol on survival time and temperature, coupled with its poor ability to traverse the blood–brain barrier, suggests that central β-adrenergic stimulation rather than peripheral stimulation may alter thermoregulation. 5 Effects of the autonomic nervous system (as studied by adrenoceptor blockade) on potassium changes during heating have not been fully investigated. Such changes could be important in animals’ responses to RFE and other modalities of heating, and should be studied in future. [ABSTRACT FROM AUTHOR]

A literature search of PubMed documented publications and abstracts from proceedings of scientific meetings was made to review the available data on benign prostatic hyperplasia/lower urinary tract symptoms (BPH/LUTS) and erectile dysfunction (ED) with a special focus on the role of α-adrenoceptors as critical mediators of pathophysiology. The reader is introduced to clinical results on the therapeutic potential of α-blockers alone and in combination with phosphodiesterase type 5 (PDE-5) inhibitors in the treatment of ED associated with LUTS/BPH. Epidemiological studies clearly show that an association exists between ED and LUTS/BPH. The severity of LUTS is correlated with the risk for ED. A significant number of LUTS/BPH patients are nonresponsive to the common ED treatment with PDE-5 inhibitors. As smooth muscle contractility is regulated by adrenoceptors in the corpus cavernosum, prostate and detrusor, the α-adrenoceptor system may be considered a common pathophysiological mediator in the development of ED and LUTS/BPH. Blockade of α-adrenoceptors for the treatment of BPH/LUTS may have the potential of improving sexual function. Conversely, PDE-5 inhibitors may exhibit positive effects in LUTS patients. Pilot studies on combination regimens of α-adrenoceptor antagonists and PDE-5 inhibitors have yielded encouraging results in LUTS patients with persistent ED. On the basis of pharmacological and clinical evidence, it is established that the α-adrenoceptor system plays an important role in the pathophysiology of ED and LUTS secondary to BPH. Larger trials on the combination of α-adrenoceptor antagonists with PDE-5 inhibitors are necessary to develop an integrated treatment approach for BPH/LUTS patients with comorbid ED. [ABSTRACT FROM AUTHOR]

Aims/hypothesis: Brown adipocytes provide a potentially important model system for understanding AMP-activated protein kinase (AMPK) regulation, where adrenergic stimulation leads to mitochondrial uncoupling through uncoupling protein-1 (UCP1) activity. AMPK is a sensor of energy homeostasis and has been implicated in glucose and lipid metabolism in several insulin-sensitive tissues. The aim of this study was to characterise the potential role of AMPK in adrenergically mediated glucose uptake and to find out whether UCP1 is involved in the adrenergic activation of AMPK. Methods: We used primary brown adipocytes differentiated in culture and measured AMPK phosphorylation and glucose uptake following adrenergic activation. Results: Treatment of adipocytes with noradrenaline (norepinephrine) caused phosphorylation of AMPK via β-adrenoceptors and not α1- or α2-adrenoceptors. This effect was not β3-adrenoceptor specific, since responses remained intact in adipocytes from β3-adrenoceptor knock-out mice. These effects were also mimicked by forskolin and cAMP analogues. Treatment of cells with adenine 8-β-Darabinofuranoside, an AMPK inhibitor, partially blocked β-adrenoceptor-mediated increases in glucose uptake. Brown adipocytes are characterised by the production of UCP1, which can uncouple the mitochondria. Using adipocytes from Ucp1+/+ and Ucp1-/- mice, we showed that noradrenaline-mediated phosphorylation of AMPK does not require the presence or activity of UCP1. Conclusions/interpretation: These results suggest a pathway where increases in cAMP mediated by β-adrenoceptors leads to activation of AMPK in brown adipocytes, which contributes in part to β-adrenoceptor-mediated increases in glucose uptake, an effect independent of the presence or function of UCP1. [ABSTRACT FROM AUTHOR]

1 Experiments were carried out to characterize the possible adrenergic properties of the 5-HT1A antagonists WAY 100635 and MM-77 using the mouse isolated vasa deferentia preparation. 2 When vasa deferentia were preincubated for 10 min in the presence of MM-77 (10−8–10−6 m) or WAY100635 (10−8–7 × 10−7 m), a concentration-dependent inhibition of the contractile response to submaximal electrical field stimulation (10 Hz, 50 V, 50 ms) was observed with pIC50 values of 7.05 ± 0.01 and 6.85 ± 0.1 respectively. 3 MM-77 (10−8–10−6 m) antagonized the contractile responses of the vasa deferentia to phenylephrine (PE) (10−6–10−3 m) in a concentration-dependent manner. Schild plots of these data were linear and yielded a mean ρ A2 value of 6.81 ± 0.084. The mean slope was 1.42 ± 0.22. 4 WAY100635 (10−8–10−6 m) antagonized the contractile responses of the vasa deferentia to PE (10−6–10−3 m) in a concentration-dependent manner. Schild plots of these data were linear and yielded a mean ρ A2 value of 7.05 ± 0.08. The mean slope was 0.97 ± 0.1. 5 The results suggest that while WAY100635 acts as a competitive antagonist at α1-adrenoceptors, MM-77 displays non-competitive antagonist characteristics at this receptor subtype. 6 These results may have important implications for the use of these compounds as 5-HT1A receptor antagonists in in vivo studies. [ABSTRACT FROM AUTHOR]

We have recently shown that the β-adrenoceptor ligands CGP 12177, bupranolol, and SR 59230A (aryloxypropanolamines), but not BRL 37344 and CL 316243 (phenylethanolamines), exhibit significant affinity for α1-adrenoceptors and that CGP 12177 displays partial agonist properties at α-adrenoceptors in rat pulmonary artery. In this study, bupranolol and SR 59230A were further evaluated for their potential α-adrenoceptor mediated effects (i.e., agonist and/or antagonist properties) in rat intralobar pulmonary artery and compared with BRL 37344 and CL 316243. Bupranolol induced a relaxation in phenylephrine-precontracted arteries, but had no effect in prostaglandin $${\text{F}}_{{2{\text{ $ \alpha $ }}}} {\left( {{\text{PGF}}_{{2{\text{ $ \alpha $ }}}} } \right)}$$-precontracted ones. SR 59230A also elicited a relaxation in phenylephrine-precontracted arteries. In $${\text{PGF}}_{{2{\text{ $ \alpha $ }}}} $$-precontracted arteries, SR 59230A induced a contractile response that was insensitive to the irreversible α-adrenoceptor antagonist phenoxybenzamine. BRL 37344 at high concentrations, but not CL 316243, produced slight relaxation in both phenylephrine- and $${\text{PGF}}_{{2{\text{ $ \alpha $ }}}} $$-precontracted arteries. The contractile response to phenylephrine was antagonized by bupranolol and SR 59230A in a competitive manner (pA2: 6.38 and 7.08 respectively). The concentration–response curve to phenylephrine was also shifted to the right by BRL 37344 (mean pKb: 4.45), but not by CL 316243 (100 μM). This study indicates that the aryloxypropanolamine derivatives bupranolol and SR 59230A exhibit competitive antagonist, but no agonist properties on α1-adrenoceptors, SR 59230A also inducing α-adrenoceptor-independent contraction. Among the phenylethanolamines, BRL 37344 but not CL 316243, also exerts an antagonist effect on α1-adrenoceptors, with a much lower potency than the aryloxypropanolamines studied. [ABSTRACT FROM AUTHOR]

The functional role of the sodium–calcium exchanger in mouse ventricular myocardium was evaluated with a newly developed specific inhibitor, SEA0400. Contractile force and action potential configuration were measured in isolated ventricular tissue preparations, and cell shortening and Ca2+ transients were measured in indo-1-loaded isolated ventricular cardiomyocytes. SEA0400 increased the contractile force, cell shortening and Ca2+ transient amplitude, and shortened the late plateau phase of the action potential. α-adrenergic stimulation by phenylephrine produced a sustained decrease in contractile force, cell shortening and Ca2+ transient amplitude, which were all inhibited by SEA0400. Increasing the contraction frequency resulted in a decrease in contractile force in the absence of drugs (negative staircase phenomenon). This frequency-dependent decrease was attenuated by SEA0400 and enhanced by phenylephrine. Phenylephrine increased the Ca2+ sensitivity of contractile proteins in isolated ventricular cardiomyocytes, while SEA0400 had no effect. These results provide the first pharmacological evidence in the mouse ventricular myocardium that inward current generated by Ca2+ extrusion through the sodium–calcium exchanger during the Ca2+ transient contributes to the action potential late plateau, that α-adrenoceptor-mediated negative inotropy is produced by enhanced Ca2+ extrusion through the sodium–calcium exchanger, and that the negative staircase phenomenon can be explained by increased Ca2+ extrusion through the sodium–calcium exchanger at higher contraction frequencies. [ABSTRACT FROM AUTHOR]

1 β-Adrenoceptor (AR) ligands have been the mainstay of cardiovascular therapy for decades, withβ-AR antagonist being used for hypertension, angina and myocardial infarction and adrenaline in use for cardiopulmonary resuscitation for nearly 100 years.2 Ischaemia of the heart through coronary artery occlusion causes cell injury and death through necrosis and apoptosis. Reperfusion of the ischaemic myocardium results in cardiac dysfunction and infarction.3 Stimulation ofα- andβ-ARs in the ischaemic heart have variable and inconsistent effects depending on when the agonist is applied. This review describes the different effects of stimulation of the three establishedβ-AR subtypes (β1-,β2- andβ3-ARs) either before ischaemia (preconditioning) or during ischaemia and reperfusion of the heart (postconditioning).4 Brief periods of ischaemia preceding a major ischaemic episode can have a protective effect against post-ischaemia–reperfusion damage, known as ischaemic preconditioning. This review considers the role of endogenous catecholamines released during preconditioning and the nature of the adrenoceptor subtypes that mediate these effects. The clinical significance of this to the use ofβ-AR antagonists is considered.5 The transduction pathways and effects on apoptosis of the cardioprotective and deleterious effects of AR activation are considered.6 This commentary reviews the literature and attempts to bring together a unified synopsis of the effects of adrenoceptor stimulation in myocardial ischaemia and the potential clinical relevance. [ABSTRACT FROM AUTHOR]

Changes in autonomic receptor systems (α- and β-adrenoceptors and muscarinic receptors) were compared in the aging and failing human heart. In both settings responsiveness of β-adrenoceptors and all other receptor systems that evoke their effects via cyclic AMP accumulation was diminished. Muscarinic receptor function, on the other hand, was decreased in the aging, but unchanged in the failing heart; in contrast, G protein-coupled receptor kinase activity was increased in the failing, but unchanged in the aging heart. α-Adrenoceptor function was unchanged or slightly decreased in the failing heart. However, nothing is known on α-adrenoceptor changes in the aging heart. These results indicate that in the failing human heart all autonomic receptor systems appear to be altered in the direction to attenuate β-adrenoceptor responses to sympathetic (over)stimulation while in the aging human heart autonomic receptor systems appear to be altered in a direction that protects the heart against too pronounced reduction in β-adrenoceptor responsiveness. [Copyright &y& Elsevier]

1. Three differentα1-adrenoceptor subtypes, designatedα1A,α1B andα1D, have been cloned and identified pharmacologically in cardiomyocytes.In vitrostudies have suggested thatα1-adrenoceptors play an important role in facilitating cardiac hypertrophy. However, it remains controversial as to which subtype ofα1-adrenoceptors is involved in this response. In the present study, we investigated the different role of eachα1-adrenoceptor subtype in mediating cardiomyocyte protein synthesis, which is a most important characteristic of cardiac hypertrophy in cultured neonatal rat cardiomyocytes.2. Cardiomyocyte hypertrophy was monitored by the following characteristic phenotypic changes: (i) an increase in protein synthesis; (ii) an increase in total protein content; and (iii) an increase in cardiomyocyte size.3. The role of eachα1-adrenoceptor subtype in mediating cardiomyocyte protein synthesis was investigated by the effect of specificα1-adrenoceptor subtype-selective antagonists on noradrenaline-induced[3H]-leucine incorporation. In addition, pKB values forα1-adrenoceptor subtype-selective antagonists were calculated and compared with the corresponding pKi values to further identify their effects.4. Activation ofα1-adrenoceptors by phenylephrine or noradrenaline in the presence of propranolol significantly increased[3H]-leucine incorporation, protein content and cell size.5. Pre-incubating cardiomyocytes with 5-methyl-urapidil, RS 17053 or WB 4101 significantly inhibited noradrenaline-induced[3H]-leucine incorporation. However, there was no effect when cardiomyocytes were pre-incubated with BMY 7378. The correlation coefficients between pKB values forα1-adrenoceptor subtype-selective antagonists and pKi values obtained from clonedα1A-,α1B- orα1D-adrenoceptors were 0.92 (P < 0.01), 0.66 (P > 0.05) and 0.24 (P > 0.05), respectively.6. Our results suggest that theα1-adrenoceptor is dominantly responsible for adrenergic hypertrophy of cultured cardiomyocytes in neonatal rats. The efficiency in mediating cardiomyocyte protein synthesis isα1A > α1B ≫ α1D. [ABSTRACT FROM AUTHOR]

We used genistein (Gen) and tyrphostin 23 (Tyr-23) to evaluate the importance of tyrosine phosphorylation in norepinephrine (NE)-induced changes in intracellular free calcium concentration ([Ca2+]i) in rat afferent arterioles. [Ca2+]i was measured in microdissected arterioles using ratiometric photometry of fura 2 fluorescence. The control [Ca2+]i response to NE (1 µM) consisted of a rapid initial peak followed by a plateau phase sustained above baseline. Pretreatment with the tyrosine kinase inhibitor Tyr-23 (50 µM, 10 min) caused a slow 40% increase in baseline [Ca2+]i. Tyr-23 attenuated peak and plateau responses to NE, both by ∼70%. In the absence of extracellular Ca2+ (0 Ca), Tyr-23 reduced the immediate [Ca2+]i response to NE by ∼60%, indicative of mobilization of internal stores, and abolished the plateau phase. In other arterioles, the [Ca2+]i response to depolarization induced by KCl (50 mM) was not attenuated by Tyr-23, indicating no direct effect on L-type Ca+ channels activated by depolarization. The Ca2+ channel blocker nifedipine (1 µM) inhibited the NE response by ∼50%; the effects of nifedipine and Tyr-23 were not additive. Nifedipine had no inhibitory effect after Tyr-23 pretreatment, indicating Tyr-23 inhibition of Ca2+ entry. Another tyrosine kinase inhibitor, Gen (5 and 50 µM), did not affect baseline [Ca2+]i. High-dose Gen inhibited the peak and plateau response to NE by 87 and 75%, respectively; low-dose Gen attenuated both responses by ∼20%. In 0 Ca, Gen (50 µM) abolished the immediate [Ca2+]i mobilization response. Combined nifedipine and Gen (50 µM) inhibited the rapid NE response by ∼90% in the presence of extracellular Ca2+. Gen (50 µM) also inhibited by 60% the [Ca2+]i response to 50 mM KCl, indicating a direct interaction with voltage-sensitive, L-type Ca2+ entry channels. These results indicate that tyrosine phosphorylation is an important link in the chain of events leading to α-adrenoceptor-induced Ca2+ recruitment (both entry and release) in afferent arteriolar smooth muscle cells. Furthermore, different blockers of tyrosine kinase appear to have different modes of action in renal microvessels. [ABSTRACT FROM AUTHOR]

A suboptimal fetal environment has been linked to increased risk of cardiovascular disease in adulthood. We investigated whether intrauterine stress (IUS) alters the development of adrenergic reactivity in different types of rat arteries. Intrauterine stress was induced by ligation of the uterine arteries at day 13 of pregnancy in Wistar rats. First-order mesenteric, renal, femoral and saphenous arteries of the 21-day-old male offspring were studied in a myograph. IUS in the rat changes arterial adrenergic reactivity in a regionally selective manner. Adrenoceptor-mediated responses are altered in the renal artery. Maximal contractile responses to phenylephrine were increased, while sensitivity to the α1-adrenoceptor agonist was decreased. Intrauterine stress significantly reduced contractile responses to norepinephrine and enhanced relaxing responses to isoproterenol in the renal artery. Adrenergic responses were not modified in mesenteric, femoral and saphenous arteries. In the kidneys the densities of [3H]prazosin binding sites, periarterial adrenergic nerves and of the glomeruli were not altered after intrauterine stress at day 13 of gestation. The observed regionally selective alterations in arterial reactivity might link a suboptimal fetal environment to the development of cardiovascular disease in the adult. [Copyright &y& Elsevier]

Several α2-adrenoceptor agonists containing an imidazoline moiety inhibit insulin secretion when applied to beta cells. In the present study we investigated such imidazolines in regard to membrane potential effects in MIN 6 cells. We confirmed the inhibition of insulin release as reported in previous studies and showed an additional hyperpolarizing activity of the imidazolines using bisoxonol for membrane potential measurements. Pertussis toxin pre-incubation of MIN 6 cells converted the inhibitory imidazoline activity with regard to insulin release into a stimulatory effect. In addition, the marked hyperpolarization caused by the α-adrenoceptor agonists containing an imidazoline moiety was converted into a depolarizing effect after pertussis toxin pre-incubation. Adrenaline, an α-adrenoceptor agonist lacking an imidazoline moiety, also inhibited insulin release and hyperpolarized MIN 6 cells. Pertussis toxin pre-incubation led only to a loss of the inhibitory adrenaline effect and of the hyperpolarization but not to the stimulatory effects observed with the imidazolines. Thus a stimulatory effect of α-adrenoceptor agonists containing an imidazoline moiety was demasked by PTX in this study. It remains to be clarified to which extent a blockade of KATP channels is responsible for this effect. [ABSTRACT FROM AUTHOR]

The concentration and functionality of the α-adrenoceptor (α-AR) subtypes in the genital tract of cyclic heifers were investigated. In each tissue sample, a single class of α1-ARs was observed, whereas two distinct classes of α2-ARs were discriminated: low-affinity (LA) and high-affinity (HA) α2-ARs. Statistical analysis showed the presence of significantly ( p < 0.05) higher concentrations of all α-AR subtypes in the follicle than in the corpus luteum. No significant differences were found in the ovary or myometrium between the luteal and follicular phases. In the ovary, the density of α1-ARs was significantly ( p < 0.05) higher than that of α2-ARs. By contrast, there were significantly ( p < 0.05) more α2-ARs than α1-ARs in the myometrium. As far as α2-ARs are concerned, LA α2-ARs were significantly ( p < 0.05) higher than HA α2-ARs in all tested tissues. Competition studies suggested that the rank order of potency of antagonists for α1-ARs was prazosin > phentolamine > yohimbine, whereas for α2-ARs the order of potency was yohimbine ≥ phentolamine > prazosin. Functional assays performed on myometrium showed that noradrenaline, phenylephrine and clonidine elicited concentration-dependent contractions only in dioestrus and pro-oestrus preparations and that clonidine was more effective than phenylephrine as a contractile agent. It appeared that there were no significant modifications in α-AR affinity or concentration during the different stages of bovine oestrous cycle, whereas the uterine spontaneous activity and the responsiveness to α-adrenergic stimulation was strongly influenced by hormonal levels. The modifications of uterine contractility observed during the oestrous cycle may be related to modifications induced in the transductional mechanisms of α-ARs. [ABSTRACT FROM AUTHOR]

The effects of nipradilol, a drug used in the treatment of glaucoma, on the contractions induced by noradrenaline and phenylephrine in isolated dog central retinal, external and internal ophthalmic arteries and pig ciliary arteries were investigated. In dog ocular arteries treated with oxyhemoglobin (1.6 × 10[sup –5] mol/l) to adsorb nitric oxide, noradrenaline (2 × 10[sup –8] to 10[sup –5] mol/l) produced a concentration-related contraction which was markedly inhibited by prazosin but not by yohimbine. Nipradilol (10[sup –9] to 10[sup –7] mol/l) slightly but significantly inhibited the noradrenaline-induced contraction in a concentration-related manner, but the inhibitory potency and efficacy were much less than those of prazosin. However, nipradilol inhibited the phenylephrine-induced contraction with a similar PA[sub 2] value of prazosin. In pig ciliary arteries treated with oxyhemoglobin, noradrenaline-induced contraction was slightly inhibited by prazosin but markedly inhibited by yohimbine. Nipradilol, similarly to timolol, did not inhibit, but rather tended to potentiate, the contraction elicited by noradrenaline. The contraction induced by phenylephrine was significantly inhibited by prazosin and nipradilol. It is concluded that nipradilol acts as an α[sub 1] -adrenoceptor antagonist (but not as an α[sub 2] -adrenoceptor antagonist) in the ocular arteries, which may partially explain its ocular-pressure-lowering mechanism. Taken together with the results of our previous studies, the potencies of the nipradilol-induced vascular actions in ocular arteries are found to be in the following order: β-adrenoceptor inhibition > α[sub 1] -adrenoceptor inhibition ≒ direct vasodilation via a release of nitric oxide.Copyright © 2002 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Abstract Infusion of a β-adrenoceptor antagonist into the basolateral nucleus of the amygdala (BLA) blocks memory enhancement induced by systemic or intra-BLA administration of a glucocorticoid receptor (GR) agonist. As there is evidence that glucocorticoids interact with the noradrenergic signalling pathway in activating adenosine 3′,5′-cyclic monophosphate (cAMP), the present experiments examined whether glucocorticoids influence the β-adrenoceptor–cAMP system in the BLA in modulating memory consolidation. Male, Sprague–Dawley rats received bilateral infusions of atenolol (a β-adrenoceptor antagonist), prazosin (an α1-adrenoceptor antagonist) or Rp-cAMPS (a protein kinase A inhibitor) into the BLA 10 min before inhibitory avoidance training and immediate post-training intra-BLA infusions of the GR agonist, RU 28362. Atenolol and Rp-cAMPS, but not prazosin, blocked 48-h retention enhancement induced by RU 28362. A second series of experiments investigated whether a GR antagonist alters the effect of noradrenergic activation in the BLA on memory consolidation. Bilateral intra-BLA infusions of the GR antagonist, RU 38486, administered 10 min before inhibitory avoidance training completely blocked retention enhancement induced by α1-adrenoceptor activation and attenuated the dose–response effects of post-training intra-BLA infusions of clenbuterol (a β-adrenoceptor agonist). However, the GR antagonist did not alter retention enhancement induced by post-training intra-BLA infusions of 8-Br-cAMP (a synthetic cAMP analogue). These findings suggest that glucocorticoids influence the efficacy of noradrenergic stimulation in the BLA on memory consolidation via an interaction with the β-adrenoceptor–cAMP cascade, at a locus between the membrane-bound β-adrenoceptor and the intracellular cAMP formation site. [ABSTRACT FROM AUTHOR]

Although thoracic and abdominal rat aorta are often used as a classical pharmacological preparation for the assessment of vascular drug effects, little is known on regional differences among these two parts of the aorta with regard to their reaction to Gq/11-coupled receptor activation. Thus, we determined, in rings from thoracic and abdominal aorta from 12-week-old male Wistar rats, the effects of noradrenaline (NA; 10–8–10–4 M), endothelin-1 (ET-1; 10–10–10–6 M) and the thromboxane A2 mimetic U 46619 (10–8–10–5 M) on inositolphoshate (IP) formation (assessed as accumulation of total [3H]IPs in [3H]myo-inositol prelabelled rings). NA, ET-1 and U 46619 concentration-dependently increased IP formation; maximum increases were, however, significantly more pronounced in thoracic than in abdominal aorta. Similarly, NA, ET-1 and U 46619 evoked significantly larger maximum contractions in thoracic than in abdominal aorta. NA-induced [3H]IP formation could be inhibited with BMY 7378 (10–9–10–4 M) and with 5-methyl-urapidil (5-MU; 10–9–10–5 M) both exhibiting biphasic concentration-inhibition curves. The pKi-values for BMY 7378 at the high affinity site were in thoracic aorta 8.93±0.28 (n=5), and in abdominal aorta 8.76±0.35 (n=4) and at the low affinity site 6.45±0.2 (thoracic aorta) and 6.55±0.27 (abdominal aorta). pKi-Values for 5-MU in thoracic aorta at the high affinity site were 8.25±0.34 (n=4), and at the low affinity site 6.61±0.39 . In abdominal aorta reliable pKi-values could not be calculated for 5-MU due to a low signal-to-noise ratio. On the other hand, in both preparations the ETA-receptor antagonist BQ-123 (10–9–10–5 M) and the TP-receptor antagonist SQ 29548 (10–9–10–5 M) inhibited ET-1- and U 46619-induced IP formation, respectively, with monophasic concentration-inhibition curves: pKi-values for BQ-123 were: 8.16±0.24 (thoracic aorta) and 8.10±0.35 (abdominal aorta) and for SQ 29548: 8.2±0.3 (thoracic aorta) and 8.5±0.3 (abdominal aorta). The amount of immunodetectable Gq/11-protein was similar in both tissues. We conclude that responses to NA, ET-1 and U 46619 (IP formation and contractile force) are larger in thoracic than in abdominal aorta. ET-1 effects on IP formation are mediated by ETA-receptors and U 46619 effects by TP-receptors. NA effects are mediated by α1D- and α1A-adrenoceptors; α1B-adrenoceptors seem to play a minor role. [ABSTRACT FROM AUTHOR]

1. The contractile function of diabetic hearts is impaired. In addition, the responsiveness of diabetic cardiac muscle to sympathetic stimulation is altered. Previous studies have revealed a depressed response to β-adrenoceptor stimulation; however, the response to α-adrenoceptor activation remains controversial. Because α- and β-adrenoceptor agonists increase cardiac contractility, largely through increased mobilization of intracellular Ca2+, the aim of the present study was to investigate the effects of α- and β-adrenoceptor stimulation on intracellular Ca2+ handling in cardiac myocytes from streptozotocin-induced diabetic rats. 2. Intracellular Ca2+ was measured using fura-2. Under basal conditions (27°C, 2.5 mmol/L extracellular [Ca2+], 0.3 Hz stimulation), there was no significant difference in resting or peak Ca2+ levels between control and diabetic cardiomyocytes. However, the time course of the intracellular Ca2+ transient was significantly prolonged in cells from diabetic hearts. 3. The β-adrenoceptor agonist orciprenaline (at 10-7 and 10-6 mol/L) increased the amplitude of the Ca2+ transient in both groups; however, the extent of potentiation was less in diabetic compared with control cardiomyocytes. Orciprenaline decreased the duration of the transient to the same extent in both groups. 4. The α-adrenoceptor agonist phenylephrine (at 10-7 and 10-6 mol/L) had no effect on the Ca2+ transient in control myocytes but caused a significant concentration-dependent increase in its amplitude in diabetic cardiomyocytes. Phenylephrine had no effect on the time course of the transient in either group. 5. These results demonstrate differential effects of insulin- dependent diabetes on the responsiveness of cardiomyocytes to α- and β-adrenoceptor stimulation. The... [ABSTRACT FROM AUTHOR]

Aims We investigated the effect of angiotensin converting enzyme inhibitors (ACEIs) on postsynaptic adrenoceptor sensitivity and compared the effect of the lipophilic ACEI, quinapril, and that of hydrophilic agent, enalapril in human vessels. Methods α-adrenoceptor sensitivity was evaluated using the dorsal hand vein compliance technique. The dose–response curves of vasoconstriction to phenylephrine and prostaglandin F2α were obtained in healthy male volunteers. Results The ACEIs shifted the dose–response curve of phenylephrine to the right and raised the median effective dose (ED50; 189.3 (57.6 ng min-1 ) of phenylephrine. Following quinapril administration, ED50 increased to 481.1 (101.8 ng min-1 compared with 266.8 (55.8 ng min-1 after enalapril (95% CI for differences; 31.1–397.5 ng min-1 ). Quinapril administration had no effect on the dose–response curve of PGF2α. Conclusions ACE inhibition attenuates α-adrenoceptor sensitivity in human vessels. The effect of quinapril, a lipophilic ACEI, was greater than that of enalapril, a hydrophilic ACEI. Lipophilic ACEIs may be more potent in vasodilating effect than hydrophilic ACEIs. Angiotensin II concentration in tissue rather than that in plasma may contribute to the α-adrenoceptor sensitivity of the vessels. [ABSTRACT FROM AUTHOR]

Skeletal muscle can release or take up lactate depending on the lactate concentration gradient across the cell membrane. In the perfused rat hindlimb without arterial lactate, both noradrenaline (NA) infusion and increased flow promote lactate release and oxygen consumption (VO2). However, it is unclear whether NA or increased flow rate have similar effects on lactate uptake. The present study compares these effects in the rat hindlimb perfused at a basal flow rate of 0.33 mL min-1 g-1 and 25 °C in the presence of added arterial lactate. When 10 mmol L-1 L-(+)-lactate was added to the arterial perfusate, lactate was taken up (16 ± 1.0 μmol g-1 h-1, n = 13) by the hindlimb with a 35% higher VO2 than that without added lactate. Doubling perfusion flow rate enhanced lactate uptake and VO2 by 120% and 40%, respectively. Glucose uptake was also increased (by 253%) with increased flow. Infusion of NA increased perfusion pressure, VO2 and glucose uptake similarly to those induced by increased flow rate. However, lactate uptake was inhibited by NA. This inhibition was not altered by the β-adrenergic antagonist propranolol. Vasopressin also showed similar effects to NA to decrease lactate uptake associated with increased VO2 and vasoconstriction. These data indicate that in the presence of a high arterial lactate concentration, NA has opposite effects from increased flow rate on skeletal muscle lactate uptake although both have similar effects on lactate release in the absence of arterial lactate. Inhibition of lactate uptake may relate to the vasoconstrictive action of NA. [ABSTRACT FROM AUTHOR]

Guinea pig vas deferens was used as an animal model for alpha-1 adrenoceptor (α-receptor) mediated contractions in human hyperplastic prostatic tissue. The selective α-receptor agonist, phenylephrine (PE), induced fully reversible, dose-dependent contractions antagonized by increasing concentrations of the α-receptor blockers prazosin (1-100 nM) and YM 617 (0.1-10 nM). Removal of extracellular Ca reduced PE-evoked contractions in a time-dependent manner. Nifedipine (1-1000 nM), a blocker of voltage-dependent L-type Ca channels (VDCC), inhibited the PE-induced response by up to 65%. Removal of extracellular Ca abolished the α-agonist reactivity in a time-dependent fashion. To elucidate the participation of intracellular Ca stores in α-receptor contractions, the tissue was pretreated with ryanodine (10 μM) or thapsigargin (0.1 μM), established inhibitors of Ca release from intracellular pools. Both substances reduced the PE contractions by up to 80%. Nifedipine suppressed the remaining contractions completely. This provides evidence that Ca influx through VDCC and Ca release from intracellular stores contribute to α-receptor contractions in the guinea pig vas deferens and may be important in obstructive benign prostatic hyperplasia. [ABSTRACT FROM AUTHOR]

Benzyltetrahydropalmatine hydrochloride (BTHP) exhibited antiarrhythmic action in animal models. I. Electrophysiological effects of BTHP were investigated in various heart preparations. I. BTHP showed a competitive α-adrenoceptor-blocking effect with pA2 value of 5.8 and 5.86 in rat anococcygeus muscle and rabbit aortic strips respectively. Radioligand binding assays showed that BTHP had affinity for both α and α-adrenoceptor. The results from these experiments show that BTHP is a new K channel blocker of Chinese medicinal origin with α-adrenoceptor-blocking action. [ABSTRACT FROM AUTHOR]

The membrane conductance and reversal potential were estimated for neurones in toad dorsal root ganglia with intracellular recording technique during depolarization or hyperpolarization induced by noradrenaline (NA). The effects of blocking agents for potassium or calcium ion current on NA-induced membrane potential responses were observed as well. The NA depolarization was accompanied by a 32.6% decrease of membrane conductance. In a few neurones, the membrane conductance showed an initial increase and a following decrease. The NA hyperpolarization was associated with an increase of membrane conductance by 16.2%. The mean reversal potential of NA depolarization was -88.5 ± 0.9mV. The NA hyperpolarization responses were nullified at -89 to -92 mV of membrane potential. TEA superfusion enhanced NA depolarization amplitude by 73.7 ± 11.9% and depressed NA hyperpolarization amplitude by 40.5%. CsCl (intracelluar injection) increased phenylephrine depolarization by 34.5 %. MnCl superfusion decreased the amplitudes of NA depolarization by 50.5 ± 9.9%, and of NA hyperpolarizalion by 89.5±4.9% respectively. The results suggest that depolarization or hyperpolarization induced by NA might be mediated by the alteration of K or Ca channels. [ABSTRACT FROM AUTHOR]

In mouse peritoneal macrophages, α-adrenoceptor stimulation evokes a Ca-dependent K current [ I(Adr)] [Hara et al. (1991) Pflügers Arch 419:371-379]. The roles of D- myo-inositol 1,4,5-trisphosphate (Ins P) and a GTP-binding protein (G protein) in I(Adr) were investigated with tight-seal whole-cell recordings and fura-2 fluorescence measurements. Intracellular injection of lns P (5-50 μM) evoked transient outward currents [ I(Ins P)] with or without damped oscillations in membrane currents at -40 mV. Dialysis with 0.2 mM guanosine 5′-[3-thio]triphosphate (GTP[ γS], a poorly hydrolysable GTP analogue) at -40 mV activated oscillatory outward currents or a slowly developing steady current on which such oscillations were superimposed after a delay of 10-90 s. I(Ins P) and the GTP[ γS]-induced current { I(GTP[ γS])} were accompanied by an increase in conductance. Reversal potentials of both responses closely depended on the extracellular K concentration. Fura-2 measurements revealed that I(Ins P) and I(GTP[ γS]) result from a rise in intracellular free Ca concentration ([Ca]). Removal of extracellular Ca did not abolish I(Ins P) and I(GTP[ γS]). Both were blocked by bath-applied charybdotoxin. Intracellular D- myo-inositol 1,3,4,5-tetrakisphosphate (Ins P, 50 μM) did not evoke any responses, whereas D- myo-inositol 2,4,5-trisphosphate [Ins P(2,4,5), 20 μM] elicited an outward current at -40 mV. I(Ins P) was completely blocked by prior dialysis with the Ins P receptor antagonist heparin (5 mg/ml). Inclusion of guanosine 5′-[2-thio] diphosphate (GDP[ βS], 2 mM) or heparin (5 mg/ml) together with GTP[ γS] in the patch pipette solution completely blocked I(GTP[ γS]). These results indicate that intracellular injection of Ins P or GTP[ γS] mimic I(Adr). Furthermore, intracellular dialysis with heparin (3 mg/ ml) or GDP[ βS] (2 mM) greatly accelerated a run-down of I(Adr). On the other hand, I(Adr) was markedly prolonged in a cell dialysed with GTP[ γS] (0.2 mM). Therefore, it is concluded that I(Adr) results from stimulation of α-adrenoceptor and Ins P formation via a G protein. [ABSTRACT FROM AUTHOR]

Responses to adrenaline in mouse peritoneal macrophages were investigated with perforated and cell-attached patch-clamp recording, and with a combination of the perforated-patch recording and fura-2 fluorescence measurements. Extracellularly applied adrenaline induced a transient outward current (4-10s in duration, 100-500 pA in amplitude) at −40 mV associated with a marked increase in conductance. The adrenaline-induced current [ I (Adr)] reversed polarity near −80 mV. The reversal potential depended distinctly on the external K concentration but not on external Cl concentration. Removal of external Ca did not affect I(Adr) within 2-4 min but subsequent responses to adrenaline were progressively depressed. In contrast, treatment with an intracellular Ca chelator, the acetoxymethyl ester of 1,2-bis-(2-aminophenoxy)ethane- N,N,N′,N′-tetraacetic acid completely abolished I(Adr). Furthermore, I(Adr) was blocked by bath-applied quinidine and charybdotoxin, but not by tetraethylammonium or apamin. Extracellular application of an α-adrenoceptor agonist phenylephrine and of noradrenaline mimicked I(Adr). On the other hand, I(Adr) was antagonized by a non-selective α-adrenoceptor antagonist phentolamine (0.2 μM) and an α-adrenoceptor antagonist prazosin (0.2 μM), but was not affected by an α-adrenoceptor antagonist yohimbine (1 μM) or a β-adrenoceptor antagonist propranolol (1 μM). Cell-attached single-channel recordings with the pipette solution containing 145 mM KCl revealed the activation of single-channel currents with a conductance of 40 pS during application of adrenaline outside the patch. Parallel measurements of membrane current and fura-2 fluorescence in the same cell demonstrated a correlation between the rise in [Ca] and an increase in K conductance. Therefore, it is concluded that adrenaline activates a Ca-dependent K conductance by release of Ca from internal stores through an activation of an α-adrenoceptor. [ABSTRACT FROM AUTHOR]

To clarify the effects of hypoxia on adrenergic transmission, we examined the contractile responses of isolated rabbit aortic strips to electrical stimulation, the concentration-response relationships for noradrenaline and KCl, and the electrical stimulation-evoked overflows of total [H] and [H]noradrenaline from strips preloaded with [H]noradrenaline in media equilibrated with gas mixtures containing various concentrations of 0. Contractile responses to electrical stimulation were completely inhibited by tetrodotoxin and α-adrenoceptor antagonists such as phentolamine and phenoxybenzamine, but were not affected by indomethacin. When the concentration of O in the gas mixture was decreased from 95% to 20%, the contractile responses to electrical stimulation remained unchanged, but as the concentration of OZ was further decreased, the responses were inhibited concentration-dependently. At 0% O, the response was inhibited by about 80% when compared with control values obtained at 95% O, and the electrical stimulation-evoked overflows of total [H] and [H]noradrenaline into the superfusates were decreased by about 55%. At 0% 02, the concentration-response curve for exogenous noradrenaline was shifted to the right about 50-fold and the maximum response was decreased by 25%. The maximum contractile responses of aortic strips from animals pretreated with reserpine or 6-hydroxydopamine to high KCl were decreased slightly (about 15%). These results suggest that inhibition of adrenergic transmission under hypoxic conditions is mainly the result of a decrease in the stimulus-evoked release of noradrenaline and of a decrease in the affinity of α-adrenoceptor for noradrenaline and/ or inhibition of signal transduction mechanisms, although hypoxia also causes a slight decrease in the contractility of vascular smooth muscle. [ABSTRACT FROM AUTHOR]

1. The α-adrenoceptor agonist clonidine (0.03 and 0.1 μol/l) significantly inhibited stimulation-induced overflow of radioactivity from mouse isolated atria pre-incubated with [H]-noradrenaline. This effect of clonidine was blocked by idazoxan (0.3 gmol/l) but not prazosin (0.3 μol/l), indicating that an α-adrenoceptor was involved. 2. In some experiments mice were injected with pertussis toxin (1.5 μg/mouse) 4 days before their atria were removed and subsequently incubated with [H]-noradrenaline. Alternatively, isolated atria from untreated mice were suspended in Krebs-Henseleit solution, incubated for 16 h with pertussis toxin (1.0 and 4.0 μg/ml) or vehicle and subsequently incubated with [H]-noradrenaline. The effectiveness of pertussis toxin pretreatment was assessed indirectly using carbachol. Carbachol caused a dose dependent fall in both the rate and force of contraction of isolated, spontaneously beating atria from mice pretreated with vehicle in vivo or in vitro. This effect of carbachol was not seen in atria from mice pretreated with pertussis toxin in vivo or in vitro, suggesting that active toxin penetrated the myocardium. 3. Pertussis toxin pretreatment, either in vivo or in vitro did not alter the inhibitory effect of clonidine (0.03 and 0.1 gmol/l), or the facilitatory effect of the α-adrenoceptor antagonist phentolamine (1.0 μmol/l), on the stimulation-induced overflow of radioactivity. These results suggest that α-adrenoceptor modulation of noradrenaline release from sympathetic nerve terminals is not dependent on an inhibitory guanine-nucleotide-binding protein. [ABSTRACT FROM AUTHOR]

The effect of N-ethylmaleimide (NEM), which has been shown to abolish rather selectively inhibition of adenylate cyclase, on the α-adrenoceptor modulation of noradrenaline release was studied. Slices of the rabbit hippocampus were loaded with H-noradrenaline, superfused continuously and stimulated twice electrically. NEM (30 μmol/l) applied for 30 min enhanced both basal and stimulation-evoked tritium overflow significantly. Occupation of the receptor by the α-adrenoceptor agonist clonidine prior to and during NEM treatment did not protect the α-adrenoceptor-mediated autoinhibitory feedback system from being affected by NEM. Preincubation of the hippocampal slices with NEM was without any influence on H-noradrenaline uptake. The inhibitory effect of clonidine on H-noradrenaline release was attenuated in a non-competitive manner. In addition, the facilitatory effect of the α-adrenoceptor antagonist yohimbine on the stimulusevoked tritium overflow was reduced. The facilitation of the evoked noradrenaline release by yohimbine or yohimbine and NEM converged with increasing concentrations of yohimbine, suggesting that yohimbine and NEM were acting at the same signal-transduction system. These results are compatible with the idea that NEM, by alkylating the N-unit of a presynaptically located adenylate cyclase, prevents the α-adrenoceptor-mediated modulation of noradrenaline release. [ABSTRACT FROM AUTHOR]

The influence of clonidine on the stimulation-evoked overflow of tritium was studied in brain slices preincubated withH-noradrenaline. The slices were prepared from parietal cortex (Cx), nucleus anterior hypothalami (nah) and nucleus tractus solitarii (nts). After preincubation, the tissues were superfused at 23°C or 37°C with a medium containing the noradrenaline uptake inhibitor desipramine. Electrical field stimulation was applied using stimulation frequencies of 0.3-10 Hz. [ABSTRACT FROM AUTHOR]

The effect of islet-activating protein (IAP) on α-adrenoceptor mediated modulation of noradrenaline release in the rabbit hippocampus was studied. Slices of the hippocampus were incubated for 6 h with IAP, subsequently loaded withH-noradrenaline and superfused continuously. IAP-pretreatment significantly enhanced the electrically evoked transmitter release and diminished the facilitatory effect of the α-adrenoceptor antagonist yohimbine. In addition, the inhibitory effect of the α-adrenoceptor agonist clonidine was reduced. These results provide circumstantial evidence that an inhibitory guanine-nucleotide-binding protein, most probably N of a presynaptically located adenylate cyclase, is involved in the α-autoreceptor mediated modulation of noradrenaline release. [ABSTRACT FROM AUTHOR]

Vasoconstrictor responses mediated by the α-adrenoceptor agonist TL99, were particularly sensitive to blockade by the calcium antagonist drug diltiazem in isolated perfused tail arteries of spontaneously hypertensive rats (SHR). In contrast, the vasoconstrictor responses induced by the α-adrenoceptor agonist methoxamine were significantly more resistant to antagonism by diltiazem. At higher concentrations (>300 nmol/l) diltiazem became an effective antagonist of all α-adrenoceptor mediated responses. In normotensive Wistar Kyoto (WKY) or Sprague-Dawley (SD) rats diltiazem was significantly less potent againts vasoconstrictor responses to TL99 than in SHR. The blockade of α-adrenoceptor mediated vasoconstriction by diltiazem was not significantly different when normotensive rats and SHR were compared. The vasoconstrictor responses evoked by 5HT in the perfused tail arteries were particularly resistant to blockade by diltiazem in SHR arteries. The responses to endogenously released noradrenaline, evoked by electrical field stimulation, were significantly antagonised by diltiazem (30 nmol/l-3 μmol/l) in SHR-tail arteries, while they were not modified in WKY-tail arteries. At the concentrations of diltiazem which blocked end organ responses to field stimulation, there was no modification of total tritium overflow SHR-tail arteries after labelling the tissue withH-noradrenaline, indicating that diltiazem does not inhibit transmitter release at these concentrations. The tail artery preparation of SHR contains a population of postsynaptic α-adrenoceptors which mediate contraction in this blood vessel and the calcium entry blocker diltiazem is a potent antagonist of vasoconstrictor responses mediated by vascular α-adrenoceptors in hypertensive rats. These findings may be relevant to the antihypertensive action of diltiazem. [ABSTRACT FROM AUTHOR]

In adrenal medullary cells, carbachol evokes the secretion of catecholamines with simultaneous uptake ofCa. Highly selective agonists for α-adrenoceptors, clonidine, naphazoline, guanfacine, tramazoline and oxymetazoline inhibited carbachol evoked secretion of catecholamines in a dose-dependent manner. These α-agonists also inhibited the uptake ofCa evoked by carbachol with similar dose-response curve to inhibition of catecholamine secretion. On the contrary, highly selective agonists for α-adrenoceptors, phenylephrine and norfenefrine did not inhibit the secretion of catecholamines and cellular uptake ofCa. The inhibition by α-agonists was not restored either by the increase in carbachol, or Ca concentrations, suggesting that the mode of inhibition was distinct from competition at cholinergic receptors or Ca-channels. It is likely that α-agonists inhibited the secretion of catecholamines via the inhibition of Ca uptake which was probably caused through the activation of α-adrenoceptors. [ABSTRACT FROM AUTHOR]

The effects of cyclocytidine and norepinephrine on the release of renin and esterase from mouse submandibular gland were compared in in vivo and in vitro experiments. Cyclocytidine (150 mg/kg i.p.) produced the depletion of renin and esterase in in vivo experiments as did the α-adrenoceptor agonist norepinephrine (1 mg/kg i.v.). Cyclocytidine was more effective in depleting renin and esterase than norepinephrine. The α-adrenoceptor antagonist phenoxybenzamine (10 mg/kg i.p.), but not the β-adrenoceptor antagonist propranolol (10 mg/kg i.p.), attenuated the depletion of renin and esterase by both cyclocytidine and norepinephrine. These results suggest that the mechanism of action of cyclocytidine on the release of renin and esterase in mouse submandibular gland could be similar to that of norepinephrine, and the secretory process for both cyclocytidine and norepinephrine could be initiated by activation of α-adrenoceptors. In in vitro experiments using dispersed cells, cyclocytidine (10 M-10 M) had no effect on renin and esterase release, but dose-dependent release of both was clearly observed in response to norepinephrine (10 M-10 M). Immunocytochemical studies of renin in both in vivo and in vitro preparations showed similar enzymatic activity as measured by radioimmunoassay. The results of in vitro experiments did not correspond to those of in vivo experiments, which may suggest that the site of action of cyclocytidine in mouse submandibular gland may be different than that of norepinephrine. Pretreatment with bretylium (20 mg/kg, i.p.) prior to i.p. injection of cyclocytidine (150 mg/kg) completely blocked the esterase depletion induced by cyclocytidine. From these results, it seems possible to conclude that the in vivo effect of cyclocytidine on mouse submandibular gland is an indirect one and is mediated via reflex activation of sympathetic nerves. [ABSTRACT FROM AUTHOR]

The effects of noradrenaline (NA), adrenaline (Adr) and isoprenaline (Iso) on rat isolated uterus were studied, throughout the natural oestrous cycle. The β-inhibitory effects of the catecholamines were measured as a percentage inhibition of a standard acetylcholine (ACh)-induced contraction. Iso produced approximately 80% maximum inhibition of the standard ACh-contraction in all 4 stages of the oestrous cycle. Adr and NA produced 80% maximum inhibition in oestrus only, and 50-60% maximum inhibition in proestrus, metoestrus and dioestrus. The differences in the degree of inhibition produced by the catecholamines were overcome (i.e. the maximum inhibition produced by Adr and NA was increased to become equal to that produced by Iso) when experiments were repeated in the presence of the uptake inhibitors desmethylimipramine (DMI) and normetanephrine (NMN), but not in the presence of an α-adrenoceptor antagonist (azapetine 10 M). Variations in the uptake ofH-Iso andH-NA were observed in the different stages of the oestrous cycle. Small α-adrenoceptor mediated motor responses to NA and Adr were observed only in the presence of a β-antagonist (propranolol 10 M), in uteri from rats in oestrus, metoestrus and dioestrus, but not proestrus. It is concluded that in the rat isolated uterus, β-inhibitory receptors predominate throughout the natural oestrous cycle, although the existence of α-excitatory receptors has been shown in 3 of the 4 stages. [ABSTRACT FROM AUTHOR]

Several α-adrenoceptor agonists given intracerebroventricularly or subcutaneously to rats were assessed for their effects on gastric secretion under condition of pylorus ligation. Intracerebroventricular injection of α-adrenoceptor agonists reduced gastric secretion, in the following order (relative potency, clonidine = 1): oxymetazoline (100) > clonidine (1) > methoxamine (0.024) > phenylephrine (0.003). The antisecretory effects of oxymetazoline and clonidine given intracerebroventricularly were antagonized with yohimbine administered by the same route. Subcutaneous injection of α-adrenoceptor agonists also reduced gastric secretion, in the following order (relative potency, clonidine = 1): clonidine (1) > oxymetazoline (0.3) ≫ phenylephrine (0.001) ∼ methoxamine (0.0006). Oxymetazoline, when given intracerebroventricularly, was most effective in decreasing the volume and titratable acidity of gastric secretion. Pretreatment with 6-hydroxydopamine intracerebroventricularly (250 μg/rat × 2; 72 and 120 h before) reduced the antisecretory effect of clonidine given intracerebroventricularly. Thus, gastric secretion appears to be regulated in an inhibitory manner by α-, but not by α- in the central and peripheral nervous systems, in pylorus-ligated rats. [ABSTRACT FROM AUTHOR]

Brain cortex slices from rabbits were preincubated with [H]noradrenaline and then superfused and stimulated electrically at 3Hz. In the presence of cocaine 30 μM, unlabelled noradrenaline, α-methylnoradrenaline, clonidine, oxymetazoline, xylazine and guanabenz decreased, whereas yohimbine, corynanthine, phentolamine, tolazoline and azapetine increased the stimulation-evoked overflow of tritium. Phenylephrine and prazosin had no effect on the evoked overflow except at concentrations that greatly accelerated the basal outflow of tritium. The results indicate that the noradrenergic axons of rabbit brain cortex are endowed with presynaptic α-adrenoceptors which are exclusively of the α-type. Addition of various concentrations of cocaine, addition of pargyline, or stimulation at different current strengths was used to obtain either a high or a low stimulation-evoked overflow of tritium. Independently of the method used, a low evoked overflow coincided with a large percentage inhibition produced by 0.1μM clonidine, whereas a high evoked overflow coincided with a smaller percentage inhibition produced by clonidine. The results indicate that drugs which block the re-uptake of noradrenaline diminish the presynaptic inhibitory effect of α-adrenergic agonists by increasing the biophase concentration of released noradrenaline. [ABSTRACT FROM AUTHOR]

The influence of thyroid hormones on the concentration and properties of α-adrenoceptors in a crude membrane fraction obtained from the rat cerebral cortex was investigated using the [H]-WB 4101 binding assay. Animals were made hypothyroid by feeding 6-propyl-2-thiouracil for 8 weeks. Hyperthyroidism was induced by triiodothyronine injections (50 μg/100 g body weight) for 9 days. It is concluded that thyroid hormones regulate the number of α-adrenoceptors in membranes of the rat cerebral cortex, leaving their affinities unchanged. [ABSTRACT FROM AUTHOR]

Tritiated prazosin was used to characterize high affinity binding sites with characteristics similar to α adrenoceptors in rat brain membranes. These sites were compared with α adrenoceptors labeled with tritiated clonidine. The prazosin sites had an association constant of 2 nM and bound the ligand optimal around pH 7.0. The density of the sites was 300 fmoles per mg of protein; the half time of dissociation of prazosin was 7 min at 30° C. The order or potencies of agonists, determined from binding-inhibition experiments with labeled prazosin, was: naphazoline > clonidine > adrenaline > noradrenaline > phenylephrine > α-methylnoradrenaline > dophamine. The order of potencies of antagonists was: prazosin > phenoxybenzamine > phentolamine > clozapine > yohimbine. Sodium ions and divalent cations as well as guanyl nucleotides have little or no effect on the binding of the labeled antagonist. This is in contrast to the binding of the labeled agonist clonidine (Glossmann and Presek, 1979a, 1979b). Labeled prazosin may be a useful tool to characterize α adrenoceptors. [ABSTRACT FROM AUTHOR]

Electrophysiological and mechanical effects of phenylephrine were observed in the rabbit papillary muscle which was depolarized and rendered inexcitable by an elevation of potassium concentration to 27 mM in the presence of a β-adrenoceptor blocking drug, bufetolol (10 M). In conclusion, the restoration by phenylephrine of Ca-dependent action potential may be ascribed to an enhancement of the inward Ca current, mediated by α-adrenoceptors and this mechanism may play an important role in the α-adrenoceptor-mediated positive inotropic effect. [ABSTRACT FROM AUTHOR]

The hypotensive action of a new compound, B-HT 933 (2-amino-6-ethyl-4,5,7,8-tetrahydro-6H-oxazolo-[5,4-d]-azepin) was characterized to be of the 'clonidine-type' by the use of 6 pharmacological test procedures. (1) Intravenous injection of the substance elicited a typical cardiovascular reaction pattern of initial hypertension, followed by hypotension with bradycardia and decreased cardiac output. (2) A direct α-adrenoceptor stimulation at peripheral vascular sites was demonstrated by a hypertensive action in spinal rats. (3) A central nervous site of action was shown as 30 μg/kg injected intracisternally (i.ci.) lowered blood pressure and heart rate in cats, but the same dose i.v. was practically ineffective. (4) The decrease in sympathetic activity was revealed by a decrease in preganglionic splanchnic nerve discharges in cats. (5) The drug facilitated the vagally-mediated baroreceptor reflex as shown by the increase in reflex bradycardia following i.v. angiotensin injection in dogs. (6) The involvement of central nervous α-adrenoceptors was suggested because the α-adrenoceptor blocking drug piperoxan antagonized the facilitation of the baroreceptor reflex induced by B-HT 933. Furthermore, the drug facilitated the flexor reflex in rats with spinal section at midthoracic level. These results were unexpected as the chemical structure of B-HT 933 differs considerably from hitherto known substances of the 'clonidine-type'. [ABSTRACT FROM AUTHOR]

Single oral administration of modafinil (16-64 mg/kg) in monkeys induced an increase in nocturnal activity and in behavioural arousal without stereotyped behaviour. Modafinil-induced increase in nocturnal activity was prevented by the centrally acting α adrenoceptor antagonist prazosin. After repeated oral administration of modafinil, b.i.d. for 5 days, nocturnal activity was still increased, but was lower than after a single administration; the gradual decrease appeared on the second night. No rebound of sleep and no residual effect on night 5 (when modafinil was administered for 4 days and placebo on day 5) were seen after the withdrawal of the drug. Therefore, modafinil appeared to produce strong behavioural stimulation and awakening in a non-human primate species, after single, as well as after repeated administration, without other obvious side-effects (no sign of anxiety, no behavioural trouble). These awakening properties involved a modulation (stimulation) of a central α adrenergic system. [ABSTRACT FROM AUTHOR]

Objective: The cardiac sarcolemmal Na+/Ca2+-exchanger (NCX) plays an important role in the maintenance of the myocardial Ca2+ homeostasis which is altered in cardiac hypertrophy and failure. The aim of the present study was to investigate whether α-adrenergic stimulation known to induce cardiac hypertrophy might be involved in the regulation of the sarcolemmal NCX. Methods: Adult rat ventricular cardiocytes (ARC) were isolated from male Sprague–Dawley rats. Phenylephrine, an α-adrenergic agonist, was used as hypertrophic agent. NCX expression was measured by competitive RT-PCR and Western blot analysis. Results: α-Adrenergic stimulation of ARC with 10 μM phenylephrine for 24 h resulted in a significant increase of the NCX mRNA (2.5-fold) and the NCX protein level (1.8-fold). The changes on the expression level were blocked by the α1-adrenoceptor antagonist prazosin. Conclusions: The data demonstrate that the NCX expression level is up-regulated by the activation of the α-adrenergic signal transduction pathway. The increased NCX mRNA level induced by α-adrenergic stimulation appeared to be translated into an increased NCX protein level. [ABSTRACT FROM PUBLISHER]