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It has been suggested that at least the majority of mammalian presynaptic alpha2-autoreceptors belong to the genetic alpha2A/D-adrenoceptor subtype. The aim of the present study was to re-examine the alpha2-autoreceptors in tissues in which previous assignments conflicted with this alpha2A/D rule: in the rat vena cava and rat heart atria, where the autoreceptors were classified as alpha2B or similar to, but not identical with, alpha2D, and in the human kidney, where they were classified as alpha2C. Also re-examined were the autoreceptors in the guinea-pig urethra, where they were suggested to be alpha2A, in agreement with the rule, but in contrast to indications that the alpha2A/D-adrenoceptor of the guinea pig possesses alpha2D pharmacological properties. Tissue pieces were preincubated with 3H-noradrenaline and then superfused and stimulated electrically under autoinhibition-free or almost autoinhibition-free conditions. The Kd values of up to 14 antagonists (including the partial agonist oxymetazoline) against the release-inhibiting effect of the alpha2 agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) were determined. UK 14,304 reduced the evoked overflow of tritium with an EC50 between 6.3 and 13.2 nM. All antagonists (except prazosin in one case) shifted the concentration-inhibition curve of UK 14,304 to the right. Comparison of the Kd values thus obtained with Kd values at known alpha2 subtypes indicated that the autoreceptors in the rat vena cava, rat atria and the guinea-pig urethra were alpha2D and those in the human kidney alpha2A. For example, the pKd values of the antagonists in the rat vena cava, in rat atria and in the guinea-pig urethra were closely correlated with pKd values at the prototypic alpha2D radioligand binding sites in the bovine pineal gland (r = 0.96, P < 0.001; r = 0.92, P < 0.01; and r = 0.95; P < 0.001) and with the pKd values at the alpha2D-autoreceptors of guinea-pig atria (r = 0.99, P < 0.001; r = 0.95, P < 0.001; and r = 0.98, P < 0.001). The pKd values at the autoreceptors in rat vena cava, rat atria and guinea-pig urethra were not significantly or more loosely correlated with pKd values at alpha2A, alpha2B and alpha2C binding sites and alpha2A-autoreceptors. On the other hand, the pKd values of the antagonists in the human kidney were closely correlated with pKd values at the prototypic alpha2A radioligand binding sites in HT29 cells (r = 0.95; P < 0.001) and with pKd values at the alpha2A-autoreceptors of the pig brain cortex (r = 0.97; P < 0.001), but were not significantly or more loosely correlated with pKd values at alpha2B, alpha2C and alpha2D binding sites and alpha2D-autoreceptors. In contrast to previous suggestions, the autoreceptors in rat vena cava and atria are alpha2D, those in the human kidney alpha2A, and those in the guinea-pig urethra equally alpha2D. All, therefore, conform to the rule that alpha2-autoreceptors belong at least predominantly to the genetic alpha2A/D subtype of the alpha2-adrenoceptor. The apparent paradox of an alpha2A-autoreceptor in the urethra of the guinea pig, a species in which the genetic alpha2A/D-adrenoceptor otherwise has alpha2D pharmacological properties, is removed.

Osteoporosis is a frequent and severe complication of transplantation that substantially reduces the recipient's quality of life. However, strategies to prevent or to treat osteoporosis have as yet not been established. Determination of bone mass and density is the basis for interventions following transplantation. Therefore, bone mineral density should be measured in all patients awaiting transplantation to assess their individual risk of fracture. Prophylaxis should be considered in patients at high risk in view of the rapid initial bone loss after transplantation. Treatment of acute rejection with corticosteroids induces more bone loss than maintenance therapy with corticosteroids. Once manifest osteoporosis has occurred, treatment with a triple therapy consisting of calcium, vitamin D and either calcitonin or bisphosphonates is possible without negative effects on graft function. Therapy should be initiated according to the general guidelines for osteoporosis therapy, but several aspects specific for this patient group have to be considered. The present recommendations for the management of osteoporosis after transplantation need to be proven by further intervention studies.

The aim of our study was to characterize functionally prejunctional neuropeptide Y (NPY) receptors in human and rabbit renal cortex, as well as in human right atrium. Segments of human atrial appendages and of human and rabbit renal cortex were preincubated with [3H]noradrenaline, superfused with Krebs-Henseleit solution and stimulated electrically in superfusion chambers. The stimulation-induced outflow of radioactivity was taken as an index of endogenous noradrenaline release. The effects of subtype-selective NPY analogs on the stimulation-induced noradrenaline release were studied. NPY, its endogenous analog, peptide YY, and its C-terminal fragment, NPY13-36, but not its analog, [Leu31,Pro34]NPY, concentration dependently (1-100 nM) inhibited [3H]noradrenaline release in all tissues studied. NPY-induced inhibition of [3H]noradrenaline release in human and rabbit kidney was abolished by pretreatment with pertussis toxin. We conclude that prejunctional inhibition of noradrenaline release in human heart and human and rabbit kidney occurs through NPY receptors of the Y2 subtype, which appear to couple to a pertussis toxin-sensitive G protein.

The purpose of this prospective, angiographically controlled study was to determine the diagnostic value of color Doppler sonography with two differing approaches of scanning in patients with clinical clues for renal artery stenosis (RAS). In 135 hypertensive patients peak systolic velocity in the main renal arteries and the resistive index (RI) of the intrarenal arteries were measured. The RI side-to-side-difference (delta RI) greater than 0.05 or peak systolic velocity greater than 2 m/s were used to discriminate normal from stenotic renal arteries, defined as angiographically confirmed RAS > 50%. In 88 of 135 patients 107 RAS were demonstrated by selective arteriography. Five renal artery occlusions occurred, which were all diagnosed by color Doppler sonography. Seventy-six RAS were detected by increased peak systolic velocity, and an additional 19 stenoses were identified by delta RI. The combined analysis of peak systolic velocity and delta RI resulted in a sensitivity of 89% and specificity of 92%. The positive predictive value was 92% and the negative predictive value was 88%. Our data suggest that only the combination of intra- and extrarenal scanning with color Doppler sonography represents an effective screening method for significant renal artery stenosis in hypertensive patients.

The aim of the present study was to investigate whether the pharmacokinetic profile of cyclosporin A under Sandimmun Optoral compared to Sandimmun has an effect on blood pressure in patients with stable renal allograft function. Twenty-eight patients were randomized into two groups. Two pharmacokinetic profiles (I, II; time interval 7 days) were obtained in each group. One group (n = 18) was switched from Sandimmun to an equal dose (1:1) of Sandimmun Optoral after the first kinetic and the other (n = 10) received Sandimmun for both kinetics. Eight blood samples were taken during each kinetic and cyclosporine values were measured. Ambulatory blood pressure measurement was performed during each kinetic. There were no differences in the pharmacokinetic parameters between kinetics I and II in the control group but significant differences in the switched group with increased Cmax (+64%), AUC (+44%), C0 (+17%) and decreased Tmax (-39%) in kinetic II compared to kinetic I. Correlation of C0 with AUC was stronger for Sandimmun Optoral (r = 0.792) than for Sandimmun (r = 0.718). Correlation of C0 with Cmax was similar for Sandimmun Optoral (r = 0.53) and Sandimmun (r = 0.56). Mean 12 h blood pressure of kinetic II tended to be lower than that of kinetic I in the control group, but the opposite was true for the switched group. When blood pressure levels around Tmax +/- 1 h were analyzed only, RR (syst/diast) in mmHg was 135 +/- 3.7/89 +/- 2.2 (kinetic I) and 130 +/- 2.8/85 +/- 2.8 (kinetic II) for control as compared to 134 +/- 2.4/85 +/- 2.9 and 137 +/- 3.1/86 +/- 2.8 for the switched group, respectively. Sandimmun Optoral shows an improved pharmacokinetic profile. However, higher blood concentrations of cyclosporine under Sandimmun Optoral tend to increase slightly systemic blood pressure as compared to that under Sandimmun.

Pharmacological and molecular cloning techniques have identified six human subtypes of alpha-adrenoceptors which are designated alpha 1A, alpha 1D, alpha 2A, and alpha 2C. At the protein level human kidney expresses predominantly alpha 2A-adrenoceptors while other alpha 2-adrenoceptor subtypes or alpha 1-adrenoceptors have not been detected consistently in radioligand binding studies. However, the presynaptic receptors, which inhibit noradrenaline release in the human kidney, appear to belong to the alpha 2C-subtype. Intrarenal infusion of the nonselective alpha-adrenoceptor antagonist, phentolamine, and of the selective alpha 2-adrenoceptor antagonist, yohimbine, but not of the selective alpha 1-adrenoceptor antagonist, doxazosin, increase renal blood flow and renin release in hypertensive patients undergoing diagnostic renal angiography. Thus, alpha 2- but not alpha 1-adrenoceptors appear to mediate a tonic renal vasoconstriction and inhibition of renin release. Effects of systemically given alpha 1-adrenoceptor agonists and antagonists are difficult to interpret on a mechanistic level since direct effects in the kidney and indirect effects due to baroreflex activation and peripheral presynaptic and central sympatholytic actions may at least partially offset each other. Moreover, some of these drugs may additionally act independent of alpha-adrenoceptors, for example, via imidazoline recognition sits. The net result in a given subject may depend on the endogenous sympatho-adrenal tone. Thus, for each target population of interest, effects have to be described empirically for each drug.

Objectives: The aim of the present study was to evaluate the role of vascular alpha 1- and alpha 2-adrenoceptors in kidneys of spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY)., Methods: SHR and WKY kidneys (12-14 weeks) were isolated and perfused with Krebs-Henseleit solution. Concentration-response curves for the alpha 1-adrenoceptor agonist methoxamine and the alpha 2-adrenoceptor agonist UK 14304 were constructed alone and in the presence of the alpha 1-adrenoceptor antagonist prazosin, the alpha 2-adrenoceptor antagonist idazoxan and exogenous angiotensin II., Results: Methoxamine induced a maximal pressor response of 247 +/- 9 mmHg with an EC50 of 1.3 +/- 0.1 microM in SHR and of 193 +/- 4 mmHg with an EC50 of 1.1 +/- 0.1 microM in WKY. The concentration-response curve for methoxamine was shifted to the right by prazosin with a pA2 value of 9.29 (SHR) and 9.26 (WKY) and by idazoxan with a pA2 value of 6.45 (SHR) and 6.33 (WKY). UK 14304 induced a maximal pressor response of 41 +/- 12 mmHg in SHR and of 37 +/- 8 mmHg in WKY. Angiotensin II (0.1 nM) did not significantly alter pressor responses to methoxamine but caused a marked shift to the left of the concentration-response curve for UK 14304. UK 14304 then induced a maximal pressor response of 92 +/- 13 mmHg with an EC50 of 0.07 +/- 0.01 microM in SHR and of 78 +/- 14 mmHg with an EC50 of 0.14 +/- 0.01 microM in WKY. In the presence of angiotensin II (0.1 nM) the concentration-response curve for UK 14304 was shifted to the right by prazosin with a pKB of 6.36 (SHR) and 6.33 (WKY) and by idazoxan with a pKB of 7.68 (SHR) and 7.65 (WKY)., Conclusions: The results demonstrate a predominant role of vasoconstrictory alpha 1-adrenoceptors over alpha 2-adrenoceptors in SHR and WKY isolated kidneys. Low physiological concentrations of angiotensin II unmask functional vascular alpha 2-adrenoceptors which are slightly more sensitive in SHR than in WKY kidneys.

1. The aim of the present study was to characterize the presynaptic alpha 2-autoreceptors in human right atrium in terms of the alpha 2A-D system. Segments of atrial appendages were preincubated with [3H]-noradrenaline and then superfused in the presence of cocaine and stimulated electrically. pEC30% values of eight alpha-adrenoceptor antagonists with discriminatory power were determined. pEC30% is the negative logarithm of the antagonist concentration that increased the stimulation-induced overflow of tritium by 30%. For four antagonists, the dissociation constant KD was determined, in addition to pEC30%, against the overflow-inhibiting effect of 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) under autoinhibition-free conditions. 2. pEC30% and KD values yielded identical rank orders of antagonist affinity (rauwolscine > WB 4101 > phentolamine > prazosin) suggesting that both released noradrenaline and the exogenous agonist UK 14,304 activated the same receptor to inhibit release. 3. The eight antagonist pEC30% values obtained in right atrium correlated significantly with their pEC30% values, reported in the literature, at the presynaptic alpha 2C-autoreceptors in human kidney (r = 0.817; slope of the regression line 1.03). No significant correlation was obtained between pEC30% values at atrial autoreceptors and pKD values at previously characterized alpha 2A-autoreceptors in rabbit and alpha 2D-autoreceptors in rat, mouse and guinea-pig tissues. 4. Comparison of antagonist pEC30% values with their pKD values at native alpha 2 binding sites in cells or tissues that express a single subtype only, and with pKD values at alpha 2 binding sites in membranes of COS cells transfected with human alpha 2 subtype genes confirms the alpha 2C character of the atrial autoreceptors: significant correlations were obtained exclusively with the alpha 2C binding sites. 5. Ratios of KD values were computed for alpha 2-autoreceptors in human right atrium and for binding sites in COS cells transfected with human alpha 2 subtype genes. The autoreceptor ratios corresponded well with the respective ratios for the alpha 2C binding sites (maximal three fold deviation) but were, in part, markedly different from ratios calculated for alpha 2A and alpha 2B binding sites (up to 166 fold deviation). This outcome supports the alpha 2C designation of the autoreceptors. 6. In conclusion, the presynaptic alpha 2-autoreceptors in human right atrium are alpha 2C. In this they agree with the previously characterized alpha 2-autoreceptors in human kidney. The alpha 2C classification possibly separates, in general, human alpha 2-autoreceptors from those in lagomorph (rabbit) and rodent (rat, mouse, guinea pig) species that have been proposed to be predominantly alpha 2A or alpha 2D.

The aim of this study was to investigate angiotensin II (Ang II) receptor-, bradykinin receptor-, and beta-adrenergic receptor-mediated modulation of norepinephrine release from human renal sympathetic nerves and to characterize the respective receptor subtypes involved. Human cortical kidney slices were incubated with [3H]norepinephrine, placed in superfusion chambers between two platinum electrodes, and superfused with Krebs-Henseleit solution. The sympathetic nerves were stimulated electrically at 2.5 Hz for 1 minute, and the stimulation-induced outflow of radioactivity was taken as an index of endogenous norepinephrine release. Ang II and its precursor Ang I (both 0.01 to 1 mumol/L) enhanced stimulation-induced outflow of radioactivity in a concentration-dependent manner, with EC50 values of 0.03 and 0.05 mumol/L, respectively. The enhancement by Ang I but not that by Ang II was inhibited by the angiotensin-converting enzyme inhibitor captopril (3 mumol/L). The concentration-response curves of Ang I and Ang II were shifted to the right by EXP 3174 (0.01 mumol), the in vitro active form of the Ang II type 1 receptor antagonist losartan, with affinity estimates of 8.72 and 9.30, respectively. A higher concentration of EXP 3174 (0.1 mumol/L) abolished the facilitatory effects of Ang I and Ang II. The Ang II type 2 receptor antagonist PD 123319 (10 mumol/L) did not alter the facilitation by Ang II. In the absence of other drugs, bradykinin (0.01 to 1 mumol/L) failed to modulate stimulation-induced outflow of radioactivity but in the presence of captopril (3 mumol/L) enhanced it in a concentration-dependent manner, with an EC50 of 0.1 mumol/L.(ABSTRACT TRUNCATED AT 250 WORDS)

The aim of the present study was to investigate dopamine receptor- and alpha-adrenergic receptor-mediated modulation of norepinephrine release in human atria. Right atrial appendages were incubated with 3H-norepinephrine, placed in superfusion chambers, and field-stimulated by platinum electrodes at a frequency of 5 Hz. The stimulation-induced (S-I) outflow of radioactivity was taken as an index of norepinephrine release. The dopamine D2-receptor agonist quinpirole (0.03-3 microM) concentration dependently inhibited the S-I outflow of radioactivity with an EC50 of 0.03 microM. The concentration-response curve of quinpirole was potently shifted to the right by the D2-receptor antagonists domperidone (0.003 microM, pKB approximately 9.2) and S(-)-sulpiride (0.1 microM, pKB approximately 8.6). The D1-receptor antagonist SCH 23390 (1 microM) slightly (pKB approximately 6.9) shifted the concentration-response curve of quinpirole, whereas the alpha 2-adrenergic antagonist rauwolscine (0.01 microM) and the alpha 1-adrenergic antagonist prazosin (1 microM) had no effect. The D1-receptor agonist did not affect fenoldopam (0.03 and 0.3 microM), but fenoldopam (3 microM) enhanced the S-I outflow of radioactivity. The facilitatory effect of fenoldopam (3 microM) was unaltered by SCH 23390 (0.1 microM) but prevented by rauwolscine (0.01 microM). The alpha 2-adrenergic agonist UK 14304 (0.01-1 microM) (EC50: 0.06 microM), but not the alpha 1-adrenergic agonist methoxamine (0.3-30 microM), inhibited S-I outflow of radioactivity. The concentration-response curve of UK 14304 was shifted to the right by rauwolscine (0.01 microM, pKB approximately 8.6).(ABSTRACT TRUNCATED AT 250 WORDS)

1. Strips of human saphenous veins and of human renal arteries and veins were superfused with Krebs-Henseleit solution at 37 degrees C. Constrictor responses were elicited by exogenous noradrenaline and the P2x-purinoceptor-selective agonist, beta, gamma-methylene-L-ATP. 2. In human saphenous veins, beta, gamma-methylene-L-ATP (0.3-30 microM; EC50 2.2 microM) induced marked constrictor responses. The maximal response to beta, gamma-methylene-L-ATP was similar to the maximal response to noradrenaline. The P2-purinoceptor antagonist suramin (30 microM) shifted the concentration-response curve of beta, gamma-methylene-L-ATP to the right (apparent pKB value 4.8); suramin (100 microM) markedly inhibited the responses to beta, gamma-methylene-L-ATP. The preferential P2x-purinoceptor antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 3 microM) slightly reduced the response to beta, gamma-methylene-L-ATP. At a ten times higher concentration (30 microM), PPADS almost abolished the responses to beta, gamma-methylene-L-ATP. PPADS (30 microM), in contrast, caused no significant change in the concentration-response curve of noradrenaline. 3. In extrarenal and intrarenal arteries, EC50 values and maximal responses to noradrenaline were similar when compared with responses to noradrenaline in saphenous veins. Noradrenaline also constricted extrarenal veins. However, in contrast to the results obtained on saphenous veins, beta, gamma-methylene-L-ATP caused almost no constrictor responses in extrarenal veins and arteries and only moderate responses in intrarenal arteries. 4. The results demonstrate marked differences in responsiveness of human blood vessels to the selective P2x-purinoceptor agonist, beta, gamma-methylene-L-ATP, suggesting tissue differences in the occurrence or operation of P2x-purinoceptors in human vascular tissues. Moreover, the results indicate that PPADS blocks P2x-purinoceptors in human isolated blood vessels as previously demonstrated in animal blood vessels.

The present study investigates sympathetic cotransmission and its alpha-adrenoceptor-mediated modulation in kidneys of spontaneously hypertensive rats (SHR, 12 to 14 weeks) and age-matched normotensive Wistar-Kyoto rats (WKY). In the presence of cocaine and corticosterone, renal nerve stimulation at 1 Hz (30 seconds) induced a greater outflow of norepinephrine in SHR (4.2 +/- 0.2 pmol/g kidney) than in WKY (3.0 +/- 0.2 pmol/g kidney). The alpha 2-adrenoceptor antagonist rauwolscine (0.01 to 1 mumol/L) increased the stimulation-induced norepinephrine outflow to a greater extent in SHR than in WKY. In contrast, the alpha 1-adrenoceptor antagonist prazosin (0.03 to 3 mumol/L) increased the stimulation-induced norepinephrine outflow to a greater extent in WKY than in SHR. This difference was not observed in the presence of the P1-purinoceptor antagonist 8-(p-sulfophenyl)theophylline (100 mumol/L). Stimulation at 4 Hz (30 seconds) induced an outflow of ATP (SHR, 12.7 +/- 3.3 pmol/g kidney; WKY, 16.7 +/- 2.1 pmol/g kidney; perfusion solution without cocaine and corticosterone). Prazosin (0.03 mumol/L) markedly reduced pressor responses to stimulation and inhibited the induced ATP outflow by 60% to 70%. When prazosin (0.03 mumol/L) was present, rauwolscine (0.1 mumol/L) increased the induced outflow of norepinephrine and ATP and markedly enhanced prazosin-resistant pressor responses. These pressor responses were abolished by the P2-purinoceptor antagonist suramin (300 mumol/L). The results demonstrate an increased alpha 2-adrenoceptor-mediated automodulation of norepinephrine release in SHR kidneys caused by increased intrasynaptic norepinephrine levels.(ABSTRACT TRUNCATED AT 250 WORDS)

The aim of the present study was to investigate alpha-adrenoceptor modulation of neurotransmission in kidneys of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) and to classify the prejunctional alpha 2-adrenoceptor subtype involved. Kidneys of SHR (12-14 weeks) and age-matched WKY were isolated and the sympathetic nerves were stimulated electrically. The renal nerve stimulation-induced outflow of endogenous noradrenaline was measured by high-pressure liquid chromatography with electrochemical detection (HPLC-ECD). In the presence of uptake blockade by cocaine and corticosterone, the renal nerve stimulation-induced outflow of endogenous noradrenaline was significantly greater in SHR than in WKY at 1 Hz (688 +/- 42 pg/g vs. 525 +/- 46 pg/g) and 4 Hz (5265 +/- 495 pg/g vs. 3544 +/- 245 pg/g). At a stimulation frequency of 1 Hz the alpha 2-adrenoceptor agonist UK 14304 (0.01-1 microM) potently inhibited the renal nerve stimulation-induced outflow of noradrenaline with a pEC50 of 7.49 (7.32-7.68) and a maximum of 91% in SHR and with a pEC50 of 7.46 (7.30-7.74) and a maximum of 92% in WKY. alpha-Adrenoceptor antagonist affinity estimates (pKB values) against UK 14304 at the prejunctional alpha 2-autoreceptors were determined. The rank order of affinities (phentolamine > rauwolscine > 2-(2,6-dimethoxyphenoxyethyl) aminomethyl-1,4-benzodioxane HCl (WB 4101) > prazosin) was identical in SHR and WKY, and comparison with data from radioligand binding, molecular cloning and functional studies in other tissues and cell lines indicates that prejunctional alpha 2-autoreceptors of SHR and WKY kidneys are of the alpha 2D subtype.

The aim of the present study was to investigate beta-adrenergic receptor and angiotensin II (Ang II) receptor modulation of norepinephrine release in human atria. Slices of human atrial appendages were incubated with [3H]norepinephrine, superfused with Krebs-Henseleit solution, and electrically stimulated in superfusion chambers. Pretreatment of the tissue with 6-hydroxydopamine (1.2 mmol/L) before the [3H]norepinephrine incubation to destroy sympathetic nerves reduced the uptake of radioactivity and abolished the stimulation-induced (S-I) outflow of radioactivity. Furthermore, S-I outflow of radioactivity was prevented by the addition of tetrodotoxin (1 mumol/L) to and omission of extracellular Ca2+ from the superfusion solution. Separation of [3H]norepinephrine from its metabolites revealed that the S-I outflow of radioactivity was mainly composed of intact [3H]norepinephrine. Thus, the S-I outflow of radioactivity was taken as an index of norepinephrine release. Isoproterenol (0.001 to 0.1 mumol/L) dose-dependently enhanced the S-I outflow of radioactivity. The concentration-response curve of isoproterenol was shifted to the right by the selective beta 2-adrenergic receptor antagonist ICI 118551 (0.01 and 0.1 mumol/L) but not by the beta 1-adrenergic receptor-selective antagonist atenolol (0.3 and 30 mumol/L). Ang II (0.001 to 1.0 mumol/L) also dose-dependently enhanced S-I outflow of radioactivity. The facilitatory effect of Ang II was blocked by either the peptide Ang II receptor antagonist saralasin (1.0 mumol/L) or EXP 3174 (0.1 mumol/L), the in vitro active form of the nonpeptide Ang II receptor antagonist losartan. The cell-permeable cAMP analogue 8-bromo-cAMP (30 to 300 mumol/L) dose-dependently enhanced S-I outflow of radioactivity.(ABSTRACT TRUNCATED AT 250 WORDS)

The aim of the present study was to investigate whether or not activation of imidazoline receptors modulates noradrenaline release in the rat isolated kidney. Kidneys were pre-exposed to 3H-noradrenaline and the renal nerves were stimulated with 6 pulses at 100 Hz. The stimulation induced (S-I) outflow of radioactivity was taken as an index of endogenous noradrenaline release. The imidazoline derivatives clonidine (1-1000 nmol/l) and moxonidine (10-1000 nmol/l) inhibited S-I outflow of radioactivity with an EC50 of 6.8 nmol/l and 62.5 nmol/l and a maximum of 88% and 97%, respectively. The concentration response curves for clonidine and moxonidine were shifted to the right by the selective alpha 2-adrenoceptor antagonist rauwolscine (0.1 mumol/l) in a parallel manner with identical pKB's of 8.52 and 8.46, respectively. Furthermore, the alpha-adrenoceptor agonist (-)-alpha-methylnoradrenaline (0.1-30 nmol/l), which has no affinity for imidazoline binding sites, inhibited S-I outflow of radioactivity with and EC50 of 2.4 nmol/l and a maximum of 94%. Rauwolscine (0.1 mumol/l) again shifted the concentration response curve for this alpha-adrenoceptor agonist to the right with a pKB of 8.40. Moreover, the selective alpha 2-adrenoceptor antagonist 2-[2-(2-methoxy-1,4- benzo-dioxanyl)]imidazoline HCl (RX821002, 0.01 mumol/l) shifted the concentration response curves for clonidine and moxonidine to the right with pKB's of 9.46 and 9.18, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

1. Strips of human saphenous veins were superfused with Krebs-Henseleit solution at either 25 degrees C or 37 degrees C. Constrictor responses to electrical stimulation (10 Hz, 40 s) but not to exogenous noradrenaline (0.1, 1 microM) were abolished by guanethidine (10 microM) and tetrodotoxin (1 microM). Hence, responses to electrical stimulation are due to action potential-induced release of sympathetic neurotransmitters. 2. Constrictor responses to electrical stimulation and noradrenaline were reduced by the alpha 1-adrenoceptor antagonist, prazosin (0.3 microM) as well as by the alpha 2-adrenoceptor antagonist, rauwolscine (1 microM). The combination of prazosin and rauwolscine abolished constrictor responses to noradrenaline at 25 degrees C and 37 degrees C. However, constrictor responses to electrical stimulation were partly resistant to alpha-adrenoceptor blockade by prazosin and rauwolscine (at 25 degrees C about 30%). Residual constrictor responses to electrical stimulation were also observed in the presence of the combination of prazosin (3 microM) and rauwolscine (10 microM) as well as in the presence of phenoxybenzamine (10 microM). 3. Veins, incubated with [3H]-noradrenaline, released tritium upon electrical stimulation (10 Hz, 40 s). Moreover, electrical stimulation also induced an overflow of ATP amounting to 4.8 +/- 1.5 pmol g-1 at 25 degrees C and 2.0 +/- 0.5 pmol g-1 at 37 degrees C. Both tritium and ATP overflow were abolished by tetrodotoxin (0.5 microM). The combination of prazosin (0.3 microM) and rauwolscine (1 microM) increased tritium overflow at either 25 degrees C or 37 degrees C by about 120%, but reduced ATP overflow by about 70%. Hence, a significant percentage of the electrically evoked ATP overflow seems to be released from non-neuronal cells upon activation of alpha-adrenoceptors by endogenous noradrenaline. The remaining ATP overflow, which was resistant to alpha-adrenoceptor blockade, may reflect neuronally released ATP.4. ATP (300 MicroM) and alpha,Beta-methylene-ATP (1, 10 MicroM), both induced constrictor responses. The P2-purinoceptor antagonist, suramin (300 MicroM) markedly inhibited constrictor responses to ATP and alpha, beta-methylene-ATP, but not those to electrical stimulation and to noradrenaline. Moreover, suramin(300 MicroM) failed to diminish the alpha-adrenoceptor blockade-resistant constrictor response to 10 Hz.5. In conclusion, constrictor responses to sympathetic nerve stimulation in human saphenous veins are mainly but not exclusively mediated by neuronally released noradrenaline. There is a concomitant release of ATP and noradrenaline. P2-purinoceptors which mediate vasoconstriction are present; however,a role of neuronally released ATP in constrictor responses to electrical stimulation could not be established. Therefore, the nature of the sympathetic transmitter responsible for alpha-adrenoceptor blockade-resistant constrictor responses remains unknown.

The effects of the alpha 1-adrenoceptor agonist methoxamine and the alpha 2-adrenoceptor agonist bromoxidine (UK 14034) on the stimulation induced (S-I) outflow of radioactivity at 100 Hz/6 pulses from rat isolated kidney preincubated with 3H-noradrenaline were investigated. Methoxamine (0.3-30 mumol/l) inhibited S-I outflow of radioactivity to a maximum of 83% with a pEC50 of 5.85 (5.71-5.94). UK 14304 (0.0003-0.3 mumol/l) inhibited S-I outflow of radioactivity to a maximum of 99% with a pEC50 of 8.35 (8.26-8.47). alpha-Adrenoceptor antagonist affinities (pKD) against methoxamine and UK 14304 at prejunctional alpha-adrenoceptors were determined. The concentration response curve of methoxamine was shifted to the right by the alpha 1/alpha 2B-adrenoceptor antagonist prazosin (0.1 mumol/l) with a pKD of 7.41 and that of UK 14304 by prazosin (0.3 mumol/l) with a pKD of 6.24. The alpha 2-adrenoceptor antagonist rauwolscine (0.1 mumol/l) shifted the concentration response curve of UK 14304 potently to the right with a pKD of 8.34. The concentration response curve of methoxamine was shifted also to the right by rauwolscine (0.1 mumol/l) and the alpha 2-adrenoceptor antagonist idazoxan (0.1 mumol/l), however, both antagonists suppressed the maximum response of methoxamine to 46% and 56%, respectively. A ten times lower concentration of rauwolscine (0.01 mumol/l) did not shift the concentration response curve of methoxamine but the inhibitory effect of methoxamine still reached only a maximum of 59%.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine receptor modulation of noradrenaline release from renal sympathetic nerves was investigated. Human kidney slices were incubated with 3H-noradrenaline, placed into superfusion chambers between two platinum electrodes and field-stimulated at 5 Hz. The slices accumulated radioactivity. Pretreatment of the kidney slices with 6-hydroxy-dopamine (1.2 mM) prior to the 3H-noradrenaline incubation reduced the accumulation of radioactivity. The stimulation induced (S-I) outflow of radioactivity was mainly composed of intact 3H-noradrenaline. The sodium channel blocker tetrodotoxin (1 microM), 6-hydroxy-dopamine pretreatment and omission of calcium from the superfusion solution abolished S-I outflow of radioactivity. The DA1-receptor agonist fenoldopam (SKF 82526; 0.01 and 0.1 microM) did not alter but fenoldopam (1 microM) increased S-I outflow of radioactivity. However, in the presence of either the non-selective alpha-adrenoceptor antagonist phentolamine (1 microM) or the selective alpha 2-adrenoceptor antagonist idazoxan (1 microM) fenoldopam (1 microM) had no effect. The DA2-receptor agonist quinpirole (LY 171555; 1 microM) inhibited S-I outflow of radioactivity, an effect blocked by the selective DA2-receptor antagonists S(-)-sulpiride (10 microM) and domperidone (0.3 microM) but unaltered either by the DA1-receptor antagonist SCH 23390 (1 microM) or by phentolamine (1 microM). The alpha 2-adrenoceptor agonist UK 14304 (0.1 microM) inhibited S-I outflow of radioactivity, and this effect was blocked by phentolamine (1 microM) and idazoxan (1 microM) but unaltered by S(-)-sulpiride (10 microM). Phentolamine and idazoxan, in contrast to S(-)-sulpiride, domperidone and SCH 23390, enhanced S-I outflow of radioactivity by themselves.(ABSTRACT TRUNCATED AT 250 WORDS)

1 Rat kidneys were perfused with Krebs-Henseleit solution and incubated with [3H]-noradrenaline. The renal nerves were electrically stimulated at either 1 Hz for 30 s or 100 Hz for 0.06 s. The stimulation induced (S-I) outflow of radioactivity was taken as an index of endogenous noradrenaline release. 2 At a frequency of 1 Hz for 30 s the alpha-adrenoceptor antagonists BRL 44408 (0.01, 0.1 microM) and imiloxan (0.1, 1.0 microM) enhanced S-I outflow of radioactivity. However, at a frequency of 100 Hz for 0.06 s the alpha-adrenoceptor antagonists, idazoxan (0.1, 1.0 microM), imiloxan (0.1, 1.0 microM), BRL 44408 (0.1, 1.0 microM), BRL 41992 (0.1, 1.0 microM) and prazosin (0.01 microM) failed to enhance S-I outflow of radioactivity. 3 Thus, the rat isolated kidney stimulated at 100 Hz for 0.06 s, avoids autoinhibition by endogenous noradrenaline and alpha-adrenoceptor antagonist affinities (pKB) at the prejunctional alpha-autoreceptor were estimated without disturbance by the endogenous activator. 4 The alpha 2-adrenoceptor agonist, clonidine, inhibited the S-I outflow of radioactivity with a maximum of 90% and an EC50 of 7.2 nM. 5 All alpha-adrenoceptor antagonists used caused parallel shifts of the concentration-response curve for clonidine to the right. The rank order of potencies was: rauwolscine (alpha 2A/B) > idazoxan (alpha 2A/B) > phentolamine (alpha 2A/B) > WB 4101 (alpha 2A) > BRL 44408 (alpha 2A) > BRL 41992 (alpha 2B) > prazosin (alpha 2B) = imiloxan (alpha 2B).(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of the dopamine D2-receptor agonist carmoxirole on noradrenaline release was investigated in human and rat cortical kidney slices. After preincubation with 3H-noradrenaline, the slices were electrically stimulated at 5 Hz in superfusion chambers, and the stimulation-induced (S-I) outflow of radioactivity was taken as the index of noradrenaline release. In human but not in rat cortical kidney slices, carmoxirole (0.03 microM) inhibited the S-I outflow of radioactivity. Carmoxirole (0.3 microM) also failed to inhibit the S-I outflow of radioactivity from human kidney slices. When alpha-adrenoceptors were blocked by the non-selective alpha-adrenoceptor antagonist phentolamine (1 microM), carmoxirole (0.03 microM, 0.3 microM) inhibited S-I outflow to a similar extent. The inhibitory effect of carmoxirole (0.03 microM) was prevented by the D2-receptor antagonist (-)-sulpiride (10 microM) but not by the D1-receptor antagonist SCH 23390 (1 microM) in human kidney slices. Phentolamine (1 microM) by itself induced a five-fold greater enhancement of the S-I outflow of radioactivity in rat than in human cortical kidney slices. The data suggest that activation of prejunctional D2-receptors by carmoxirole inhibits noradrenaline release from human renal sympathetic nerves. Carmoxirole in higher concentrations (0.3 microM) blocks inhibitory prejunctional alpha-autoreceptors, which seems to mask the inhibitory D2-receptor mediated effect. The different effects of phentolamine and carmoxirole in human and rat kidney may indicate a difference of the prejunctional alpha-autoreceptor mechanism in the two species.

1. The effects of selective alpha 1- and alpha 2-adrenoceptor agonists and antagonists on the stimulation-induced (S-I) outflow of radioactivity at 2 Hz were investigated in superfused rabbit renal arteries incubated with [3H]-noradrenaline. 2. The alpha 1-adrenoceptor agonist methoxamine (10 microM) inhibited S-I outflow of radioactivity and this effect was abolished by the alpha 1-adrenoceptor antagonist prazosin (0.1 microM) but not by the alpha 2-adrenoceptor antagonist rauwolscine (1 microM). Neither the prostaglandin synthesis inhibitor indomethacin (10 microM) nor the adenosine receptor antagonist 8-phenyl-theophylline (1 microM) prevented the inhibitory effect of methoxamine. 3. The alpha 2-adrenoceptor agonists clonidine (0.1 microM) and UK 14304 (0.1 microM) both inhibited S-I outflow of radioactivity. The inhibitory effect of clonidine was blocked by rauwolscine but not by prazosin. The inhibitory effect of UK 14304 was markedly reduced by rauwolscine. 4. Prazosin (0.1 microM) alone did not enhance the S-I outflow of radioactivity at 2 Hz and slightly enhanced S-I outflow at 4 Hz. Rauwolscine (1 microM) alone markedly enhanced S-I outflow of radioactivity at 2 and 4 Hz. 5. Pretreatment of the arteries with pertussis toxin (1 microgram ml-1) did not significantly alter the inhibitory effects of methoxamine or UK 14304 or the potentiation by rauwolscine. However, pretreatment of the arteries with a higher concentration of pertussis toxin (5 micrograms ml-1) prevented the inhibitory effect of methoxamine but still did not affect the responses to UK 14304 and rauwolscine. 6. Pretreatment of the arteries with N-ethylmaleimide (NEM, 10 microM) for 30 min did not alter the inhibitory effect of methoxamine but markedly attenuated the inhibitory effect of UK 14304 and the facilitatory effect of rauwolscine. 7. The results suggest that both alpha 1- and alpha 2-adrenoceptors take part in the modulation of noradrenaline release from sympathetic nerves in rabbit renal arteries. Alpha 1-adrenoceptor mediated inhibition may be coupled to G-proteins which are pertussis toxin sensitive and alpha 2-adrenoceptor mediated inhibition to G-proteins which are NEM-sensitive.

The effects of the classical dopamine DA2-receptor agonist quinpirole (LY 171555) and the recently characterized DA2-receptor agonist, carmoxirole (EMD 45609), on neurotransmission in rat isolated kidney were investigated. After preincubation with 3H-noradrenaline, the renal nerves were electrically stimulated. The stimulation induced (S-I) outflow of radioactivity was taken as an index of noradrenaline release. Quinpirole (0.3 mumol/l) inhibited S-I outflow of radioactivity and pressor-responses to renal nerve stimulation (RNS) at 1 Hz. Both effects of quinpirole were blocked by the DA2-receptor antagonist S(-)-sulpiride (10 mumol/l). The alpha 1, alpha 2-adrenoceptor antagonist phentolamine (1 mumol/l) did not block the inhibitory effect of quinpirole. Carmoxirole (0.003 and 0.03 mumol/l) did not alter and carmoxirole (0.3 mumol/l) even enhanced S-I outflow of radioactivity, however, pressor responses to RNS were markedly reduced by carmoxirole (0.003-0.3 mumol/l). Pressor responses to RNS were also markedly reduced by the alpha 1-adrenoceptor antagonist prazosin (0.1 mumol/l). Carmoxirole (0.3 mumol/l), prazosin (0.1 mumol/l) and phentolamine (1 mumol/l) totally abolished pressor responses to exogenous noradrenaline (0.05 mumol/l). In contrast, quinpirole (0.3 mumol/l) did not alter pressor responses to exogenous noradrenaline (0.05 mumol/l). Furthermore, carmoxirole (0.003-0.3 mumol/l) markedly reduced pressor responses induced by the alpha 1-adrenoceptor agonist methoxamine (1 mumol/l) but even the highest concentration of carmoxirole (0.3 mumol/l) had no effect on pressor responses induced by bolus injections of either neuropeptide Y (1.5 ng) or angiotensin II (1 ng). Phentolamine (1 mumol/l) by itself markedly enhanced S-I outflow of radioactivity and pressor responses to RNS were virtually unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

The aim of the present study was to investigate beta-adrenoceptor modulation of noradrenaline release from sympathetic nerves in superfused cortical kidney slices of 4-week-old spontaneously hypertensive rats (SHR) and age-matched controls (WKY). After preincubation with 3H-noradrenaline the kidney slices were electrically stimulated in superfusion chambers. The stimulation induced (S-I) outflow of radioactivity was mainly composed of unmetabolized 3H-noradrenaline in both strains and thus taken as an index of noradrenaline release. There was a frequency-dependent (1.25-20 Hz) increase in the S-I outflow of radioactivity. At all stimulation frequencies tested S-I outflow of radioactivity was similar or even slightly lower in SHR than in WKY kidney slices in either the absence or presence of cocaine (10 mumol/l). The non-selective beta-adrenoceptor agonists isoprenaline (0.1 mumol/l) and adrenaline (0.01 and 0.1 mumol/l) enhanced S-I outflow of radioactivity. The facilitatory effects of isoprenaline (0.1 mumol/l) and adrenaline (0.1 mumol/l) were blocked by the selective beta 2-adrenoceptor antagonist ICI 118551 (0.1 mumol/l) but not by the selective beta 1-adrenoceptor antagonist atenolol (0.3 mumol/l). The cell-permeable cAMP analogue 8-bromo-cAMP (300 mumol/l) enhanced S-I outflow of radioactivity to a similar extent in both SHR and WKY kidney slices. A combination of 8-bromo-cAMP (300 mumol/l) and adrenaline (0.1 mumol/l) did not enhance S-I outflow of radioactivity to a greater extent than 8-bromo cAMP (300 mumol/l) alone in both strains.(ABSTRACT TRUNCATED AT 250 WORDS)

The aim of the present study was to investigate alpha 2-adrenoceptor modulation of noradrenaline release in superfused strips of human and rabbit renal arteries. The arteries were field-stimulated after incubation with [3H]noradrenaline. The stimulation-induced outflow of radioactivity was taken as an index of noradrenaline release. At a high stimulation frequency (4 Hz), the alpha 2-adrenoceptor agonist clonidine (0.1 mumol/l) failed to inhibit stimulation-induced outflow of radioactivity in human and rabbit renal arteries whereas the alpha 2-adrenoceptor agonist UK 14304 (0.1 mumol/l) did inhibit stimulation-induced outflow. The inhibitory effect of UK 14304 in human renal arteries was blocked by the alpha 2-adrenoceptor blocking drug rauwolscine (1 mumol/l). At a lower stimulation frequency (2 Hz), both clonidine and UK 14304 inhibited stimulation-induced outflow of radioactivity from rabbit renal arteries; both effects were blocked by rauwolscine. Rauwolscine by itself enhanced stimulation-induced outflow of radioactivity in both preparations. The results suggest that activation of prejunctional alpha 2-adrenoceptors in human and rabbit renal arteries inhibits noradrenaline release. Neuronally released noradrenaline exerts inhibitory feed-back modulation of its own release through activation of prejunctional alpha 2-adrenoceptors. At a higher stimulation frequency most of the prejunctional alpha 2-adrenoceptors are already occupied by endogenous noradrenaline and clonidine fails to inhibit noradrenaline release since it seems to act as a partial agonist at these prejunctional alpha 2-adrenoceptors.

Rat kidney slices were incubated with [3H]-noradrenaline and placed into a superfusion chamber between two platinum electrodes. The kidney slices accumulated and stored radioactivity. In kidney slices taken from rats whose sympathetic nerve terminals were destroyed by pretreatment with 6-hydroxydopamine accumulation of radioactivity was abolished. The alpha 2-adrenoceptor antagonist idazoxan (0.1-1 microM) enhanced but tetrodotoxin (TTX, 1 microM) or omission of calcium from the superfusion solution abolished the stimulation induced (S-I) outflow of radioactivity. Angiotensin (A) I (3-300 nM) and AII (1-100 nM) enhanced S-I outflow of radioactivity. The effect of AI was markedly attenuated by the angiotensin converting enzyme inhibitor captopril (3 microM) and that of AII was blocked by the AII receptor antagonist saralasin (1 microM). These results suggest that the kidney slice preparation is a valid technique to study modulation of renal noradrenaline release. Endogenous noradrenaline released from sympathetic nerves in rat kidney slices activates prejunctional alpha 2-adrenoceptors to inhibit its own release. AII, which can also be formed locally from AI in these kidney slices, activates prejunctional AII receptors to facilitate renal noradrenaline release.

Renal neurotransmission and its modulation by prostaglandin (PG) E2 (0.06 microM) and indomethacin (10 microM) was investigated in isolated kidneys of adult spontaneously hypertensive (SHR) and normotensive control (WKY) rats. After preincubation with [3H]-noradrenaline the renal nerves were stimulated. The stimulation induced (S-I) outflow of radioactivity was taken as an index of noradrenaline release. The S-I outflow of radioactivity from SHR and WKY kidneys was similar but S-I pressor responses were enhanced in kidneys of SHR. PGE2 inhibited and indomethacin enhanced the S-I outflow of radioactivity in kidneys of WKY and SHR to a similar extent. This inhibitory effect of PGE2 is due to activation of inhibitory prejunctional PGE2 receptors. Neuronally released noradrenaline stimulates a local formation of PGE2 which then transjunctionally inhibits noradrenaline release. Renal noradrenaline release in the adult SHR and its modulation by PGs is not altered. An enhanced postjunctional responsiveness of the renal vasculature may contribute to the maintenance of hypertension in the adult SHR.

Renal sympathetic nerve varicosities possess a variety of receptors which when activated by appropriate agonists can modulate noradrenaline release at the local level of the kidney. Thus, activation of prejunctional alpha 1- and alpha 2-adrenoceptors, prostaglandin (PG), dopamine, adenosine and serotonin receptors inhibits, whereas activation of prejunctional beta 2-adrenoceptors and angiotensin (A) II receptors enhances renal noradrenaline release. Moreover, neuronally released noradrenaline itself activates prejunctional inhibitory alpha 1- and alpha 2-adrenoceptors forming a "negative feedback loop" of its own release (autoinhibition). PGE2 and adenosine locally formed in the kidney by renal nerve stimulation inhibits noradrenaline release through activation of their specific prejunctional receptor system (transjunctional inhibition).

[3H]noradrenaline was infused intravenously into pentobarbitone-anesthetized rabbits to reach a steady-state plasma (3H]noradrenaline level, from which the noradrenaline plasma clearance was calculated. The plasma level of endogenous noradrenaline was determined simultaneously, and the rate of noradrenaline release was then derived. The effects of a series of selective alpha 1- and alpha 2-adrenoceptor blocking drugs on the noradrenaline release rate and noradrenaline clearance were investigated. Yohimbine (1 mg/kg i.v.), rauwolscine (1 mg/kg i.v.), corynanthine (1 mg/kg i.v.), prazosin (0.3 and 1 mg/kg i.v.), phenoxybenzamine (4 mg/kg i.v.), and sodium nitroprusside (10 micrograms/kg/min i.v.) decreased the plasma noradrenaline clearance. The selective alpha 2-adrenoceptor blocking drugs yohimbine and rauwolscine, as well as phenoxybenzamine, increased the noradrenaline release rate more than equihypotensive doses of the directly acting vasodilators hydralazine and sodium nitroprusside. The selective alpha 1-adrenoceptor blocking drugs prazosin and corynanthine increased the noradrenaline release rate less than equihypotensive doses of the vasodilators. These results suggest that, in vivo, blockade of alpha 2-adrenoceptors results in increased noradrenaline release, probably due to blockade of inhibitory presynaptic alpha 2-adrenoceptors at sympathetic nerve endings. Blockade of alpha 1-adrenoceptors, on the other hand, appears to depress baroreceptor-mediated increases in noradrenaline release in response to a fall in blood pressure.

The aim of this study was to investigate alpha-adrenoceptor modulation of norepinephrine (NE) release from sympathetic nerves in rat isolated perfused kidney. After preincubation with [3H]NE, the renal nerves were stimulated. The stimulation-induced (S-I) outflow of radioactivity was used as an index of NE release. Clonidine (0.1 mumol/L) decreased the S-I outflow of radioactivity. This effect was abolished by the alpha 1-adrenoceptor antagonist idazoxan (0.1 mumol/L) but not by the alpha 2-adrenoceptor antagonist prazosin (0.1 mumol/L). Methoxamine (10 mumol/L) also had an inhibitory effect; this was abolished by prazosin (0.1 mumol/L), but not by idazoxan. Individually, these alpha-blocking drugs and the alpha 1-adrenoceptor antagonist corynanthine (0.3 mumol/L) enhanced S-I outflow of radioactivity. In the presence of indomethacin (10 mumol/L), the inhibitory effect of methoxamine was abolished but clonidine still inhibited S-I outflow of radioactivity. The facilitatory effect of prazosin was also unaltered by indomethacin. These results suggest the existence of inhibitory prejunctional alpha 1- and alpha 2-adrenoceptors in the kidney. The inhibitory effect of methoxamine seems to be mediated through prostaglandin inhibition of NE release. However, the evidence for inhibitory prejunctional alpha 1-adrenoceptors rests solely on the facilitatory effects of prazosin and corynanthine.

1. Rat kidneys were perfused with Krebs-Henseleit solution and the perfusion pressure was monitored. After incubation with [3H]-noradrenaline the renal nerves were stimulated. The stimulation-induced (S-I) outflow of radioactivity was taken as an index of noradrenaline release. The effect of prostaglandins on perfusion pressure, pressor responses to renal nerve stimulation (RNS) and S-I outflow of radioactivity was assessed. 2. Prostaglandin E2 (PGE2, 0.06 and 0.6 microM), PGF2 alpha (0.6 microM), PGI2 (0.6 and 3 microM) and iloprost (0.6 microM) increased perfusion pressure and enhanced pressor responses to RNS. These facilitatory effects of the prostaglandins were not a result of an enhanced transmitter release. In contrast, PGE2 dose-dependently inhibited, whereas the other prostaglandins failed to modulate S-I outflow of radioactivity. PGE2 (0.6 microM) also enhanced pressor responses to exogenous noradrenaline. 3. Arachidonic acid (1 microM) increased perfusion pressure and enhanced pressor responses to RNS. These effects were abolished in the presence of indomethacin (10 microM) suggesting that local production of prostaglandins from exogenous arachidonic acid was responsible for this facilitation. However, arachidonic acid (1 microM) did not modulate S-I outflow of radioactivity. Arachidonic acid (10 microM), despite causing a marked increase in perfusion pressure, failed to alter pressor responses to RNS and only slightly inhibited S-I outflow of radioactivity. 4. The thromboxane A2 (TxA2) receptor agonist U-46619 (0.1 microM) increased vascular tone and enhanced pressor responses to RNS. These effects were blocked by the newly developed selective TxA2 receptor antagonist, daltroban (BM 13505; 3 microM), suggesting that these facilitatory effects of U-46619 were due to activation of TxA2 receptors. However, U-46619 failed to alter the S-I outflow of radioactivity from rat isolated kidney. 5. The alpha 1-adrenoceptor agonist methoxamine (1 microM) also increased perfusion pressure and enhanced pressor responses to RNS without affecting the S-I outflow of radioactivity in the presence of the prostaglandin synthesis inhibitor indomethacin (10 microM). 6. The results suggest that PGE2 modulates noradrenaline release through an inhibitory prejunctional receptor mechanism. There is no evidence for prejunctional PGF2 alpha, PGI2 or TxA2 receptors in the rat isolated kidney. All prostaglandins increased vascular tone in the rat isolated kidney and this alone may provide a condition for enhanced pressor responses to RNS since methoxamine also enhanced pressor responses to RNS without affecting S-I outflow of radioactivity. It is probable that postjunctionally active PGF2 and PGI2 is formed locally from exogenous arachidonic acid, but not enough prejunctionally active PGE2 is synthesized to modulate renal transmitter release.