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The present study analyzes the changes in plasma oxidative state with age and their influence on the contractions induced by noradrenaline (NA) in endothelium-denuded segments from the tail artery of 6- (young), 24- (old) and 30- (very old) month-old Sprague Dawley rats. The sensitivity (-log EC50) to NA increased with age, this increase being higher in old than in very old animals. Moreover, the maximum response (Emax) to NA did not change in old rats, whereas decreased in very old animals. We also found a progressive increase in the plasma oxidative state with age, measured as malondialdehyde (MDA) levels, that was accompanied by a decrease in the plasma antioxidative state, measured as glutathione peroxidase activity. In addition, MDA (0.5, 1 and 10 microM) potentiated the NA responses in 6-, 24- and 30-month-old rats, respectively, without affecting Emax. In young animals, catalase (1000 U/ml) or dimethylsulfoxide (7 mM), scavengers of hydrogen peroxide or hydroxyl radicals, respectively, did not modify either the contractions induced by NA in control situation or the potentiation of these responses caused by MDA. However, the superoxide anion scavenger, superoxide dismutase (SOD, 150 U/ml), completely reversed the increase in sensitivity to NA caused by MDA, without affecting NA responses in control situation. These results suggest that the increase in NA sensitivity with age could be due, at least in part, to the enhancement of plasma oxidative state during aging. In addition, in this alteration of the responses to NA caused by MDA, the generation of superoxide anions appears to be involved. This study supports the hypothesis that the enhancement of plasma oxidative state could play an important role in the increase of vascular resistance with age.

1. The effect of L-arginine (L-Arg), the nitric oxide synthase (NOS) substrate, on the responses to prostaglandin F2alpha (PGF2alpha, 10 microM) and K+ (120 mM) in rat middle cerebral artery (MCA) segments was analysed. 2. PGF2alpha induced a stable contraction of 0.35+/-0.06 mN mm(-1); the subsequent addition of bradykinin (BK, 1 microM) produced a relaxation of 42+/-9% of the PGF2alpha-induced tone. K+ induced a response consisting of a rapid basal tone increase (1.42+/-0.16 mN mm(-1)) followed by a decrease to a stable phase (1.24+/-0.15 mN mm(-1)). 3. L-Arg (0.1 mM), but not D-Arg, decreased the basal tone and reduced the contraction to PGF2alpha in segments with and without endothelium. The contractile response to K+ was also reduced and not maintained in the presence of L-Arg. 4. The inhibitory effect of L-Arg on the PGF2alpha- and K+-induced contractions was completely reversed by the NOS inhibitor, NG-monomethyl-L-arginine (L-NMMA, 0.1 mM). 5. Pre-incubation of segments with dexamethasone (1 microM), to inhibit inducible NOS (iNOS), or with the antibiotic polymyxin B (10 microg ml(-1)) reduced the L-Arg inhibition, whereas it was increased by lipopolysaccharide (LPS, 100 ng ml(-1)), an inductor of iNOS. L-NMMA antagonized the effects of dexamethasone and LPS. 6. The present results suggest that L-Arg inhibition of the PGF2alpha- and K+-induced contractions in rat MCA is the result of NO synthesis by iNOS stimulation.

Objective: To investigate the endothelial modulation of the rat thoracic aorta sodium-potassium ATPase activity and its possible alteration by ageing and hypertension., Methods: Male spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) aged 5 weeks, 3, 6, 12 and 18 months were anaesthetized. Their aortae were dissected and divided into cylindrical segments and vasomotor responses obtained in segments with or without endothelium were recorded on a polygraph., Results: Endothelium removal increased ouabain responses in young (aged 3 and 6 months) WKY rat aortic segments, but reduced those obtained in old (aged 18 months) WKY rat segments and in segments from SHR aged 3, 6 and 12 months. An enhancement of contractions in response to ouabain with age was observed in both strains, but it occurred at earlier ages in vessels from SHR. At ages 3 and 6 months, responses to ouabain were higher in intact vessels from SHR than they were in those from WKY, but were lower in endothelium-denuded vessels. In aortic segments from 6-month-old WKY rats and SHR, ouabain reduced or abolished potassium-induced relaxations in segments with or without endothelium, respectively. In vessels from SHR and WKY rats aged 18 months, ouabain abolished potassium-induced relaxations both in intact and in endothelium-denuded vessels., Conclusions: Endothelium of WKY rat aorta might release a factor in response to ouabain that modulates its contraction negatively. Endothelium of SHR and old WKY rat aorta releases a contracting factor that modulates ouabain contractions positively. Thus, the inhibitory effect of endothelium on contractions induced by ouabain might be lost as a consequence both of hypertension and of age, being replaced by an endothelium-dependent contracting factor that facilitates ouabain responses.

1. AI and AII induced contractions in cat femoral arteries, which were inhibited by saralasin. 2. The response to AI was reduced by captopril and endothelium removal and by chymostatin in endothelium-denuded segments. 3. AII contractions were increased by indomethacin, L-NAME and endothelium removal. 4. AII and AI facilitated the adrenergic neurotransmission. This facilitation was inhibited by saralasin and/or captopril. 5. These data suggest: (1) AI is converted into AII in the endothelial and adventitial layer; (2) the contractions caused by AI and AII are mediated by AII receptors and are modulated by endothelial release of NO and PGI2; and (3) the existence of presynaptic AII receptors mediating the facilitation of neurotransmission caused by AI and AII.

1. Isolated aortic segments from transgenic rats for the mouse renin gene Ren-2 were more sensitive than those from control Sprague-Dawley ones to the vasoconstrictions induced by angiotensin II and to the potentiation of norepinephrine contractions by this peptide. 2. In transgenic, but not in control aorta, pretreatment with angiotensinogen potentiated norepinephrine-induced vasoconstrictions, this effect being abolished by captopril. 3. These results suggest that in the aorta of transgenic rats there is a higher functional tissue renin-angiotensin system that potentiates the vascular reactivity to norepinephrine.

The objective of this study was to analyze the effect of chronic treatment of rabbits for 4, 8 and 12 weeks with the anabolic steroid, nandrolone, on the contractile responses induced by different agents in segments of thoracic aorta and mesenteric and femoral arteries. In the three types of arteries, the contractions elicited by noradrenaline, 5-hydroxytryptamine and angiotensin II were increased by endothelium removal. The treatment reduced the contractions elicited by the three agents (mainly those caused by 5-hydroxytryptamine) in aorta, and only those caused by 5-hydroxytryptamine in mesenteric arteries. Ca(2+)-free medium containing 0.1 mM ethylene glycol bis (beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) reduced the responses elicited by 1 microM noradrenaline, 10 microM 5-hydroxytryptamine and 0.1 microM angiotensin II in aorta segments from control rabbits. Addition of CaCl2 to this medium restored the initial responses elicited by the three agents in normal medium, both in arteries from control and treated rabbits. In aorta, the contractions elicited by phorbol 12,13-dibutyrate (PDB), an activator of protein kinase C, were reduced by the treatment. Staurosporine, an inhibitor of protein kinase C, reduced the responses evoked by PDB. Likewise, the contractions caused by noradrenaline, 5-hydroxytryptamine and especially by angiotensin II were also reduced by staurosporine. These results suggest that the thoracic aorta is the most affected by the treatment, and that the reduction of contractile responses appears to be due to changes in protein kinase C activity and/or in a mechanism situated beyond protein kinase C activation.

The influence of endothelium on the neurogenic component of ouabain-induced contractions in isolated perfused guinea pig carotid arteries was analyzed. Ouabain (0.1 mumol/L to 0.1 mmol/L) evoked concentration-dependent increases of perfusion pressure. Phentolamine (0.3 to 10 mumol/L) and prazosin (30 nmol/L to 10 mumol/L) (nonselective antagonist of alpha-adrenergic receptors and selective antagonist of alpha 1-adrenergic receptors, respectively) induced a concentration-dependent relaxation in segments precontracted with ouabain (0.1 mmol/L). When the arteries were preincubated with those blockers (both at 3 mumol/L) or the animals were pretreated with reserpine, the contractions to the glycoside were diminished, indicating that they are partially mediated by norepinephrine release from adrenergic nerve endings. De-endothelialization abolished the effect of adrenergic blockade on ouabain-induced contractions. On the other hand, de-endothelialization did not modify significantly the effect of the adrenergic blockade on norepinephrine-induced contractions. The nitric oxide blocker oxyhemoglobin, at concentrations (10 mumol/L) that abolished endothelium-dependent relaxations induced by 3 mumol/L acetylcholine; or the cyclooxygenase blocker indomethacin (10 mumol/L) did not modify the relaxation caused by phentolamine. In bioassay experiments, 30 mumol/L phentolamine induced a relaxation on the ouabain-elicited contraction only when the glycoside was added through a donor segment with endothelium. Ouabain-induced tritiated norepinephrine release was significantly reduced by the removal of endothelium but not by 1 mumol/L oxyhemoglobin or 1 mumol/L indomethacin. These results suggest that the endothelium modulates the neurogenic component involved in contractions evoked by the glycoside by a diffusible factor (or factors) whose nature is unknown; however, the factor is neither nitric oxide nor a cyclooxygenase-related compound.

The aim of the present study was to analyse the ability of phorbol 12,13-dibutyrate (PDB) to activate protein kinase C (PKC), measured by its capacity to translocate the enzyme from the cytosol to the membrane fraction, as well as to induce vasconstrictive responses in segments from branches of bovine cerebral arteries. PDB (0.1 microM) produced a marked translocation of PKC activity from the cytosolic to the membranous fraction. This drug induced concentration-dependent contractions which were slow in onset. The contraction elicited by PDB was reduced by the PKC inhibitor, staurosporine (1 and 10 nM), but unaltered by both Ca(2+)-free medium containing 3 mM EGTA and the Ca(2+)-channel antagonist, nifedipine (1 microM). Preincubation of segments with PDB (10 and 30 nM) reduced the vasoconstriction elicited by 5-hydroxytryptamine (5-HT) in a concentration- and preincubation time-dependent manner. These data indicate that bovine cerebral arteries possess cytosolic and membranous PKC activities, that the vasoconstrictive responses induced by PDB were independent of extracellular Ca2+, that cytosolic C-kinase is translocated to the membrane and probably down-regulated by PDB, and that this enzyme is not involved in 5-HT responses, but is down-regulated by PDB.

Endothelial interference with ouabain effects on vascular smooth muscle was analyzed in isolated human placental arteries and veins, carotid arteries from reserpinized guinea pigs (to eliminate the adrenergic actions of the glycoside), and aorta from Wistar-Kyoto (WKY) rats that were 5 weeks, 3, 6, 12, and 18 months old. After endothelium removal, ouabain-evoked contractions were increased in human placental vessels and guinea pig carotid arteries. These effects were not mimicked by the blockade of nitric oxide, prostaglandin, or leukotriene synthesis. Bioassay experiments suggest the release of an endothelial diffusable factor(s). 86Rb+ uptake, a method to measure sodium-pump activity, was significantly reduced by the removal of endothelium. In aortas from WKY rats, ouabain caused contractions that were increased with the age of animals until 12 months. Endothelium removal increased ouabain effects in vessels from animals of 3 and 6 months but not in older ages. These results suggest (a) that there is an inhibitory endothelial modulation of ouabain-induced vasoconstriction by the release of a diffusible factor(s); (b) that this substance is not related to nitric oxide, prostaglandins, or leukotrienes; (c) that its mechanism of action could be stimulating the activity of vascular sodium pump and/or antagonizing its inhibition by ouabain; and (d) that the endothelial modulation is lost in the elderly.

The vascular effects of endothelin-1 (ET-1) were compared with those elicited by phorbol 12,13-dibutyrate (PDB), an activator of the protein kinase C (PKC), to analyze the involvement of this enzyme on ET-1 responses. PDB and ET-1 caused slow-developing contractions (sustained and transient, respectively), which were reduced by the PKC inhibitor, staurosporine (1 and 10 nM). Only the contractile effects evoked by ET-1 were reduced in Ca-free medium and by the Ca channel antagonist, nifedipine (1 microM), and increased by the Ca channel agonist, BAY K 8644 (10 nM). PDB (10 and 30 nM) preincubation reduced the vasoconstriction elicited by 5-hydroxytryptamine (5-HT; 0.01, 0.1 and 1 microM) in a way dependent on phorbol concentration and preincubation time, whereas ET-1 (1 nM) increased the contractile response to 5-HT (0.1 microM). Furthermore, PDB (0.1 microM) also reduced the responses elicited by ET-1 (30 microM) and vice versa. ET-1 (0.1 microM) induced transient translocation of PKC activity from the cytosol to the membrane, which was less than that produced by PDB (0.1 microM). Electrical stimulation induced [3H]noradrenaline (NA) release, which was increased by PDB (10 and 100 nM) and not affected by ET-1 (10 nM). These results indicate: (1) the responses induced by PDB and ET-1 were independent and dependent on extracellular Ca, respectively; (2) PKC is involved in NA release and 5-HT responses, but mainly in desensitization of these responses, and (3) PKC is activated by ET-1 and is implicated in vascular actions of ET-1, but other mechanisms, such as the activation of ET-1 receptors and opening of dihydropyridine-sensitive Ca channels also appear to be involved.

The influence of endothelium on the direct contractile effects of ouabain in vascular smooth muscle was analyzed in isolated perfused guinea pig carotid arteries. After blocking the neurogenic component of the glycoside contraction with alpha-adrenergic receptor blocking drugs or treating the animals with reserpine, ouabain-induced contractions were markedly reduced in vessels with intact endothelium. However, removal of the vascular endothelium from reserpinized carotid arteries resulted in ouabain-induced contractions similar to those observed in control arteries. These effects were not mimicked by the inhibitor of nitric oxide NG-monomethyl L-arginine or by the cyclooxygenase blocker indomethacin. Bioassay experiments suggested that these endothelial effects are mediated by diffusible factors. Uptake of 86Rb to measure sodium pump activity was significantly reduced by removal of the endothelium. These results suggest the existence of an inhibitory modulation by the endothelium of contractions induced by ouabain, likely mediated by a diffusible factor (or factors) released from these cells. The nature of this substance is unknown, but it is neither related to prostaglandins nor a nitric oxide-related compound. Its mechanism of action could be the stimulation of vascular sodium pump activity, the antagonism of the pump's inhibition by ouabain, or both.

1. AI and AII induced concentration-dependent contractions in cat femoral artery segments, the potency of AII being greater than that of AI. 2. The antagonist of AII receptors, saralasin (1 and 10 nM), inhibited the AI and AII responses. 3. Indomethacin (10 microM) and endothelium removal increased the responses to AII, whereas those induced by AI were barely affected. 4. The response induced by AI was reduced by captopril (200 microM). 5. These data suggest: (1) the contractions induced by AII are mediated by AII receptors and endothelial products derived from cyclooxygenase, (2) the response to AI is, in part, produced by its conversion into AII.

The aim of the present study was to determine the influence of the sodium pump on the responses elicited by 5-hydroxytryptamine (5-HT) in segments of human placental veins. 5-HT (10(-9)-3 x 10(-7) M) elicited potent concentration-dependent vasoconstrictor responses, but a fall in tone was observed at higher concentrations. In the presence of 10(-7) M ouabain, this fall in tension was abolished. A single concentration of 5-HT (10(-6) M) produced a biphasic response, consisting in a fast early contraction followed by a slow relaxation. This relaxant phase was concentration dependently inhibited by ouabain (10(-7) and 10(-6) M), and abolished by preincubating the vessels in a K(+)-free solution and reducing bath temperature to 28 degrees C, methods usually employed to inhibit the sodium pump. After adding 7.5 mM K+ or returning the temperature to 37 degrees C, marked relaxation was observed. On the other hand, the relaxant phase with the amine remained unchanged by pretreatment with phenidone, oxyhemoglobin, indomethacin (all at 10(-5) M) and endothelium removal. 5-HT (10(-7) and 10(-6) M) elicited increases in ouabain-sensitive 86Rb+ uptake. These results suggest that: (1) 5-HT activates Na+,K(+)-ATPase, likely by an indirect mechanism that involves an increase of intracellular sodium concentration; and (2) the relaxant phase of 5-HT-evoked vasoactive responses is not mediated by the release of nitric oxide or prostacyclin from the endothelium.

Ouabain (1 x 10(-7)-3 x 10(-5) M) elicited concentration-dependent vasoconstriction in human placental arteries and veins. These responses, but not those produced by 10(-6) M 5-hydroxytryptamine, were increased after the removal of vascular endothelium. In placental arteries, the respective blockade of cyclooxygenase or lipoxygenase with indomethacin of 5,8,11,15-eicosatetraynoic acid as well as the inactivation of nitric oxide with phenidone or oxyhemoglobin and the inhibition of nitric oxide synthesis with NG-monomethyl L-arginine (all at 10(-5) M) did not mimic the effects of endothelial denudation on ouabain-evoked contractions. Bioassay experiments suggested that the above-mentioned endothelial effects are mediated by diffusible factors. 86Rb+ uptake, a method to measure sodium pump activity, was significantly reduced by the removal of endothelium. These results suggest the existence of an inhibitory modulation by the endothelium of the contractions induced by ouabain, likely mediated by a diffusible factor(s) released from these cells. The nature of this substance is unknown but is not related to prostaglandins or leukotrienes, and neither is it a nitric oxide-related compound. Its mechanism of action could be stimulating the activity of vascular sodium pump and/or antagonizing its inhibition by ouabain.

1. Endothelin (ET-1) induced concentration-dependent contractions, which were slowly developed in segments of bovine cerebral arteries. Furthermore, this agent produced tachyphylaxis. 2. The contractions evoked by ET-1 were markedly reduced in Ca-free medium containing 1 mM EGTA and by the Ca channel antagonist, nifedipine (1 microM), but increased by the Ca channel agonist, BAY K 8644 (10 nM). 3. The contractions caused by ET-1 were significantly reduced by the protein kinase C (PKC) inhibitor, staurosporine (1 and 10 nM). 4. These results indicate that ET-1 induced potent vasoconstrictive responses, probably mediated by PKC activation, which were mainly dependent on extracellular Ca; this Ca enters the smooth muscle cells via dihydropyridine sensitive Ca channels.

Phorbol 12,13-dibutyrate (PDB), an activator of protein kinase C (PKC), induced slow-developing sustained contractions in segments of cat middle cerebral arteries. PDB-induced responses were not affected by phentolamine (1 microM) and endothelium removal, and were reduced by 1-(5-isoquinoline sulfonyl)-2-methylpiperazine (25 microM) and staurosporine (10 nM), PKC inhibitors. Forskolin (25 microM) produced a rapid and marked vasodilation in segments contracted with PDB. The 4 alpha-phorbol 12,13-didecanoate, an inactive compound, induced slight vasodilation. Preincubation with nifedipine diminished the responses elicited by PDB at all concentrations used. Ca-free medium containing 3 mM ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA), but not 1 mM, markedly reduced the phorbol-induced responses at concentrations up to 10 nM. Nifedipine (0.1 microM) and forskolin (25 microM) produced a rapid and marked relaxation of PDB (10 nM)-evoked contractions in segments incubated in a Ca-free solution (1 mM EGTA), but PBD responses in 3 mM EGTA were not affected by nifedipine. PDB (10 and 100 nM) practically did not modify K-induced contractions, but reduced vasoconstrictions elicited by different norepinephrine concentrations; this effect was phorbol concentration and preincubation time-dependent. These results indicate that: 1) PDB induced PKC activation and contraction mainly produced by Ca entry (essentially at low PDB concentrations) through dihydropyridine-sensitive Ca channels; 2) the activated PKC has elevated sensitivity for Ca; 3) PKC may be involved in the alpha adrenoceptors desensitization, but did not play an important role in the norepinephrine-induced contraction in these arteries.

1. Vascular endothelium releases different substances (endothelium-derived contractile factors, EDCFs), which mediate vasoconstrictor responses induced by several agents. 2. Clear differences have been reported in endothelium-dependent contractions, which suggest at least three distinct EDCFs, named EDCF1, EDCF2 and EDCF3, respectively. 3. EDCF1 is a cyclooxygenase metabolite(s) of arachidonic acid. EDCF2 is a polypeptide released from cultured endothelial cells. It has been isolated and identified as a 21-amino acid peptide called endothelin, which is described as the most potent vasoconstrictor agent known to date. EDCF3 is an unidentified contractile factor(s), which is neither EDCF1 nor EDCF2. 4. The physiological role of these endothelial contractile factors is not yet clear. However, they have been implicated in the local mechanisms involved in blood flow regulation, as well as in some pathological conditions, such as hypertension or cerebral vasospasm.

1. 5-Hydroxytryptamine (5-HT) induced dose-dependent contractions in the isolated cat femoral artery, which were reduced by LSD, methysergide, phentolamine and reserpine pretreatment (only at low doses). 2. Pentobarbital (PB) and Mn/+ relaxed the arteries previously contracted with 5-HT. These drugs reduced the contraction evoked by this amine as it was Ca2+-suppression. 3. High concentrations of 5-HT and K+ induced tritium release from vessels prelabelled with 3H-noradrenaline. Ca2+-deprivation and PB unmodified the release caused by 5-HT, but that elicited by K+ was abolished. 4. These data indicate that 5-HT-induced contraction is essentially due to direct interaction of this agent with 5-HT-receptors, and that PB interferes with Ca2+ entry to the cell.

1. Tyramine and noradrenaline (NA) caused dose-dependent contractions in middle cerebral and femoral arteries of cat, which were decreased by phentolamine. 2. Gangliectomy increased the contraction evoked by NA in brain arteries. 3. Reserpine pretreatment and/or gangliectomy reduced the contraction caused by tyramine, the maximal responses being unmodified in the cerebral vessels. 4. Tyramine induced Ca2+-dependent tritium release from brain and femoral arteries, which was reduced by reserpine pretreatment and/or gangliectomy. 5. These data suggest that tyramine has a direct component, apart from an indirect one, in brain arteries. The mechanisms by which NA induces slight contraction in them, including the role of Ca2+, are postulated.

This study analyzes the changes induced by subarachnoid hemorrhage (SAH) on the contractile responses and the noradrenaline release evoked in cat cerebral arteries by histamine. The dose-dependent vasoconstriction induced by histamine on the cerebral arteries of normal cats was significantly reduced by diphenhydramine and phentolamine. When SAH was produced 3 and 7 days before the experiment, the histamine-induced vasoconstriction also decreased. Thereafter, a tendency to normalization in the contractile vascular responses was observed such that in 15 days after the hemorrhage it was not significantly different from that found in controls animals. The decrease in the contractile responses to histamine provoked by SAH was similar to that seen after pretreatment with intracisternal injections of 6-hydroxydopamine. The amount of radioactivity released by histamine following preincubation with 3H-noradrenaline from the cerebral arteries of cats exposed to SAH 3, 7, and 15 days before the experiment was significantly reduced when compared with controls. Moreover, the basal level of tritium release and the radioactivity retained at the end of the experiment were also decreased after SAH. These decreases were less marked 15 days after SAH. Intracisternal injections of 6-hydroxydopamine 3, 7, and 15 days prior to the assay, and the removal of both superior cervical ganglia 15 days before the experiment, also markedly reduced these three parameters. These results indicate that histamine releases noradrenaline from cat cerebral arteries, and SAH produces a transient denervation of the perivascular adrenergic nerve endings. The inhibition of the histamine-induced vasoconstriction observed after SAH might be explained by the impairment of the indirect adrenergic mechanism involved in the overall contractile response elicited by this amine in cerebral arteries. According to the present findings, histamine does not seem to play a significant role in the production of the cerebral vasospasm occurring after SAH.

Thiopental and pentobarbital induced dose-dependent vasodilations in human cerebral arteries previously contracted with noradrenaline (NA), serotonin (5-HT) and KCl. Preincubation with both barbiturates decreased the contractions evoked by the three agents. Pentobarbital and thiopental reduced the Ca2+-induced contractile effects in K+-depolarized arteries and 5-HT-Ca2+ and NA-Ca2+ contractions dose-dependently. The tritium release evoked by K+ from these vessels prelabelled with [3H]NA was significantly reduced by both barbiturates at 10(-3) M and by Ca2+ removal. These results indicate that pentobarbital and thiopental essentially produce a similar interference with Ca2+ influx inhibiting the contractile responses induced by the three vasoactive agents and the exocytotic NA release evoked by K+.

1. Segments of cat posterior communicating artery and femoral artery submitted to field electrical stimulation showed frequency-dependent contractile responses which were decreased by tetrodotoxin, bretylium, and reserpine pretreatment. Phentolamine could only reduce the response of femoral artery segments. Superior cervical gangliectomy also decreased the contractions of cerebral cylinders. 2. The tritium released by electrical stimulation from both kinds of vessels preloaded with [3H]noradrenaline appeared reduced in the presence of tetrodotoxin, bretylium, and after reserpine pretreatment. It was also decreased in cerebral segments after superior cervical gangliectomy. 3. These results suggest that the electrical field stimulates the sympathetic nerve endings of these vessels.

The vasoconstrictor responses induced by isoproterenol in cylindrical segments of posterior communicating and femoral arteries of the cat were analyzed. For this purpose, dose-response curves for isoproterenol before and after addition of propranolol (5 X 10(-9) to 10(-6) M) or phentolamine (10(-6) M) to the bath were determined. Propranolol did not significantly change the contraction induced by isoproterenol in cerebral and femoral arteries. Phentolamine reduced the contractile responses of cerebral arteries evoked by all concentrations of isoproterenol. However phentolamine shifted the dose-response curve to isoproterenol in femoral arteries to the right, the pA2 value for this antagonist being 6.74. These results indicate that the alfa adrenoceptors are involved in the contraction caused by isoproterenol in femoral arteries, whereas in brain arteries other receptors or mechanisms are also likely involved.

The vasoconstrictor effect elicited by field electrical stimulation of the posterior communicating cerebral artery of the goat was analyzed before and after treatment with pharmacological agents to find out if the adrenergic system was involved in this response. For this purpose, trains of 300 square wave pulses (1-32 Hz, 0.5 msec.) at supramaximal voltage were applies to these arteries producing a frequency-dependent increase in tension. The vasoconstrictor response was significantly reduced by tetrodotoxin (3 x 10(-6) M), phentolamine (10(-6) M) and bretylium (5 x 10(-4) M), but it was not modified by cocaine (10(-6) M). The contraction produced by electrical stimulation of arterial segments from goats pretreated with reserpine (0.02 mg/kg/day for three days) and from goats on which a bilateral superior cervical gangliectomy had been performed 12 days previously, was significantly reduced as compared with controls. These results show that a large part of the vasoconstrictor response of the goat cerebral vessels to field electrical stimulation is mediated by an adrenergic mechanism.

The effects of field electrical stimulation on the contractile response of the isolated middle cerebral artery of the goat were evaluated before and after the use of experimental procedures designed to test the adrenergic component involved. Supramaximal stimuli produced frequency-dependent increases in tension. This response was significantly reduced by phentolamine (10(-6) M), tetrodotoxin (3 X 10(-6) M), and bretylium (5 X 10(-5) M), but not by cocaine (10(-6) M). Arterial segments from goats pretreated with reserpine and from goats in which both superior cervical sympathetic ganglia had been removed 12 days prior to the experiment also showed a significant decrease in the contraction elicited by electrical stimulation. The norepinephrine concentration of the arteries of the circle of Willis from control goats was 2.10 microgram per gram of tissue. Reserpine or gangliectomy reduced the catecholamine content to undetectable levels. It is likely that a major part of the contractile response of cerebral arteries to electrical stimulation is due to release of endogenous norepinephrine which in turn activates the alpha-adrenergic receptors.

5-Hydroxytryptamine (5-HT) induced dose-dependent increases in tension on the isolated posterior communicating artery (PCA) of the cat were significantly antagonized by lysergic acid diethylamide (LSD, 6 X 10(-9). In the presence of the phentolamine (10(-6) M) the contraction induced by the two lowest doses of 5-HT was significantly reduced. Pretreatment of the animals with reserpine (3 mg kg-1, i.p., total dose) did not modify the dose-response curve to 5-HT except for the lowest dose. Removal of both superior cervical sympathetic ganglia 15 days before the experiment brought about a significant increase in the vasoconstriction induced by 5-HT at all the doses compared with the control. Cocaine (10(-6) M) induced a significant shift to the left of the dose-response curve to 5-HT but the maximum response was the same as in the control. The augmented response to 5-HT after denervation was partially antagonized by LSD (6 X 10(-9) M) but not by phentolamine (10(-6) M). These results show that the vasoconstriction elicited by 5-HT in the PCA of the cat is mainly due to direct stimulation of tryptaminergic receptors. The participation of an indirect adrenergic component in the contractile effects of 5-HT seems to be negligible.