Presynaptic inhibition of sympathetic neurotransmission by adenosine in the rat kidney

Endogenous adenosine contributes to renal sympathetic neurotransmission via postjunctional A1 receptor-mediated coincident signaling

Adenosine A(1) receptor antagonists have diuretic/natriuretic activity and may be useful for treating sodium-retaining diseases, many of which are associated with increased renal sympathetic tone. Therefore, it is important to determine whether A(1) receptor antagonists alter renal sympathetic neurotransmission. In isolated, perfused rat kidneys, renal vasoconstriction induced by renal sympathetic nerve simulation was attenuated by 1) 1,3-dipropyl-8-p-sulfophenylxanthine (xanthine analog that is a nonselective adenosine receptor antagonist, but is cell membrane impermeable and thus does not block intracellular phosphodiesterases), 2) xanthine amine congener (xanthine analog that is a selective A(1) receptor antagonist), 3) 1,3-dipropyl-8-cyclopentylxanthine (xanthine analog that is a highly selective A(1) receptor antagonist), and 4) FK453 (nonxanthine analog that is a highly selective A(1) receptor antagonist). In contrast, FR113452 (enantiomer of FK453 that does not block A(1) receptors), MRS-1754 (selective A(2B) receptor antagonist), and VUF-5574 (selective A(3) receptor antagonist) did not alter responses to renal sympathetic nerve stimulation, and ZM-241385 (selective A(2A) receptor antagonist) enhanced responses. Antagonism of A(1) receptors did not alter renal spillover of norepinephrine. 2-Chloro-N(6)-cyclopentyladenosine (highly selective A(1) receptor agonist) increased renal vasoconstriction induced by exogenous norepinephrine, an effect that was blocked by 1,3-dipropyl-8-cyclopentylxanthine, U73122 (phospholipase C inhibitor), GF109203X (protein kinase C inhibitor), PP1 (c-src inhibitor), wortmannin (phosphatidylinositol 3-kinase inhibitor), and OSU-03012 (3-phosphoinositide-dependent protein kinase-1 inhibitor). These results indicate that adenosine formed during renal sympathetic nerve stimulation enhances the postjunctional effects of released norepinephrine via coincident signaling and contributes to renal sympathetic neurotransmission. Likely, the coincident signaling pathway is: phospholipase C → protein kinase C → c-src → phosphatidylinositol 3-kinase → 3-phosphoinositide-dependent protein kinase-1.

Purinergic receptors in the splanchnic circulation

There is considerable evidence that purines are vasoactive molecules involved in the regulation of blood flow. Adenosine is a well known vasodilator that also acts as a modulator of the response to other vasoactive substances. Adenosine exerts its effects by interacting with adenosine receptors. These are metabotropic G-protein coupled receptors and include four subtypes, A(1), A(2A), A(2B) and A(3). Adenosine triphosphate (ATP) is a co-transmitter in vascular neuroeffector junctions and is known to activate two distinct types of P2 receptors, P2X (ionotropic) and P2Y (metabotropic). ATP can exert either vasoconstrictive or vasorelaxant effects, depending on the P2 receptor subtype involved. Splanchnic vascular beds are of particular interest, as they receive a large fraction of the cardiac output. This review focus on purinergic receptors role in the splanchnic vasomotor control. Here, we give an overview on the distribution and diversity of effects of purinergic receptors in splanchnic vessels. Pre- and post-junctional receptormediated responses are summarized. Attention is also given to the interactions between purinergic receptors and other receptors in the splanchnic circulation.

ATP modulates the release of noradrenaline through two different prejunctional receptors on the adrenergic nerves of rat prostate

1. The effects of adenosine and ATP receptor agonists on the release of endogenous noradrenaline from electrically stimulated (2 Hz, 0.1 msec) rat prostate were examined in order to clarify the pharmacological properties of prejunctional receptors for adenosine and ATP on the adrenergic nerve varicosities in the prostate. Noradrenaline was quantified by HPLC coupled with electrochemical detection techniques. 2. Both adenosine and ATP receptor agonists (1 micromol/L) inhibited noradrenaline release and the relative order of inhibitory effect was N(6)-cyclopentyl-adenosine (CPA) > 5'-N-ethylcarboxamidoadenosine > 2-chloroadenosine > adenosine > 2-methylthio-ATP (2mSATP) > AMP > ATP. 3. The adenosine receptor agonist CPA (1 nmol/L-1 micromol/L) and the ATP receptor agonist 2mSATP (100 nmol/L-100 micromol/L) inhibited the stimulation-induced release of noradrenaline in a concentration-dependent manner. The concentrations of CPA and 2mSATP that produced 50% inhibition of noradrenaline release were 9.6 nmol/L and 1.4 micromol/L, respectively. 4. 1,3-Dipropyl-8-cyclopentylxanthine, an adenosine A(1) receptor antagonist, significantly reduced the inhibitory effects of not only CPA, but also 2mSATP. 5. Suramin, an ATP receptor antagonist, significantly reduced the inhibitory effects of 2mSATP, but not those of CPA. 6. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, another ATP receptor antagonist, had no effect on the inhibitory action of either agonist. 7. These results suggest that, in the sympathetic nerve terminals of rat prostate, adenosine and ATP induce inhibition of noradrenaline release via the activation of adenosine A(1) and/or xanthine-sensitive ATP receptors, which play an inhibitory regulatory role in adrenergic neurotransmission in the prostate.

The role of intraspinal adenosine A1 receptors in sympathetic regulation

Using a splanchnic nerve-spinal cord preparation in vitro, we have previously demonstrated that tonic sympathetic activity is generated from the thoracic spinal cord. Here, we sought to determine if adenosine receptors play a role in modulating this spinally generated sympathetic activity. Various adenosine analogs were applied. N6-Cyclopentyladenosine (CPA, adenosine A1 receptor agonist) and 5'-N-ethylcarboxamidoadenosine (NECA, adenosine A1/A2 receptor agonist) reduced, while N6-[2-(4-aminophenyl)ethyl]adenosine (APNEA, non-selective adenosine A3 receptor agonist) did not alter sympathetic activity. The inhibitory effect of CPA or NECA on sympathetic activity was reversed by 8-cyclopentyltheophylline (CPT, adenosine A1 receptor antagonist) or abolished by CPT pretreatment. In the presence of 3,7-dimethyl-1-propargylxanthine (DMPX, adenosine A2 receptor antagonist), sympathetic activity was still reduced by CPA or NECA. Sympathetic activities were not changed by applications of the more selective adenosine A2 or A3 receptor agonists or antagonists, including 4-[2-[[6-amino-9-(N-ethyl-beta-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid (CGS21680), 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM241385), 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Chloro-IB-MECA), and 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS1191). These findings exclude a possible involvement of A2 or A3 receptors in sympathetic regulation at the spinal levels. Interestingly, CPT alone did not affect sympathetic activity, suggesting that adenosine A1 receptors are endogenously quiescent under our experimental conditions. We conclude that intraspinal adenosine A1 receptors may down-regulate sympathetic outflow and serve as a part of the scheme for neuroprotection.

Purinergic modulation of vascular sympathetic neurotransmission

It is generally agreed that the release of norepinephrine (NE) is inhibited by activation of prejunctional purinoceptor. We examined the pharmacological properties of purinoceptors on vascular sympathetic nerve terminals and the source of endogenous adenyl purines. Electrically (1 Hz) evoked NE-release was inhibited by not only P1-agonists but also P2-agonists. Although the inhibition induced by P2-agonists was blocked by P1-antagonists, P2-agonists-induced inhibition was not due to the breakdown to adenosine. Therefore, there may be a new class of purinoceptor that is activated by both P1- and P2-agonists and antagonized by P1-antagonists. Electrical stimulation at 8 Hz but not at 1 Hz evoked the release of adenyl purines such as ATP, ADP, AMP and adenosine, in addition to NE; and the purines-release was blocked by an alpha1-antagonist. Methoxamine, an alpha1-agonist, also evoked the release of purines. Electrically (1 Hz)-evoked NE-release was inhibited by methoxamine, and this inhibition was blocked by not only an alpha1-antagonist but also a P1-antagonist. Therefore, the activation of alpha1-adrenoceptor appeared to release purines, which in turn inhibited NE-release via prejunctional purinoceptors. From these results, it is suggested that the unique purinoceptor and the endogenous purines released from alpha1-adrenoceptor-sensitive sources participate in the antidromic transsynaptic modulation of vascular sympathetic neurotransmission.

Characterization of prejunctional purinoceptors inhibiting noradrenaline release in rat mesenteric arteries

The effects of purinoceptor agonists on noradrenaline NA release by electrical stimulation in rat mesenteric arteries were examined to clarify the pharmacological properties of prejunctional purinoceptors on adrenergic nerves. Adenosine and the other P1-receptor agonists, 5'-(N-ethylcarboxamido) adenosine and 2-chloroadenosine, significantly inhibited the release of NA. Also beta,gamma-methylene ATP and 2-methylthio ATP, P2-receptor agonists, significantly inhibited NA releases. The inhibitory effect of adenosine was significantly reduced by adenosine deaminase, but those of beta,gamma-methylene ATP and 2-methylthio ATP were not affected. This suggests that the inhibitory effects of P2-receptor agonists are not due to conversion into adenosine. 1,3-Dipropyl-8-cyclopentylxanthine (DPCPX), a P1 (A1)-receptor antagonist, significantly reduced the inhibitory effects of not only the P1- but also P2-receptor agonists. Therefore, DPCPX appears to act on both prejunctional P1- and P2-receptor as an antagonist. Pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), a P2-receptor antagonist, significantly reduced the inhibitory effects of the P2-receptor agonists, but not those of the P1-receptor agonists. From these findings in the rat mesenteric artery, the P1-receptor agonist-induced inhibition of NA-release appears to be mediated via a well-known prejunctional P1-receptor of the A1-subtype, but the P2-receptor agonist-induced inhibition appears to be mediated via an unidentified purinoceptor that is blocked not only by P2-receptor antagonists but also by P1-receptor antagonists.

Exogenous and endogenous adenosine inhibits fetal calf serum-induced growth of rat cardiac fibroblasts: role of A2B receptors

BACKGROUND

Because proliferation of cardiac fibroblasts participates in cardiac hypertrophy/remodeling associated with hypertension and myocardial infarction, it is important to elucidate factors regulating cardiac fibroblast proliferation. Adenosine, a nucleoside abundantly produced by cardiac cells, is antimitogenic vis-à-vis vascular smooth muscle cells; however, the effect of adenosine on cardiac fibroblast proliferation is unknown. The objective of this study was to characterize the effects of exogenous and endogenous (cardiac fibroblast-derived) adenosine on cardiac fibroblast proliferation.

METHODS AND RESULTS

Growth-arrested cardiac fibroblasts were stimulated with 2.5% FCS in the presence and absence of adenosine, 2-chloroadenosine (stable adenosine analogue), or modulators of adenosine levels, including (1) erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA; adenosine deaminase inhibitor); (2) dipyridamole (adenosine transport blocker); and (3) iodotubericidin (adenosine kinase inhibitor). All of these agents inhibited, in a concentration-dependent manner, FCS-induced cardiac fibroblast proliferation as assessed by DNA synthesis ([3H]thymidine incorporation) and cell counting. EHNA, dipyridamole, and iodotubericidin increased extracellular levels of adenosine by 2.3- to 5.6-fold when added separately to cardiac fibroblasts, and EHNA+iodotubericidin or EHNA+iodotubericidin+dipyridamole increased extracellular adenosine levels by >690-fold. Both KF17837 (selective A2 antagonist) and DPSPX (nonselective A2 antagonist) but not DPCPX (selective A1 antagonist) blocked the antimitogenic effects of 2-chloroadenosine, EHNA, and dipyridamole on DNA synthesis, suggesting the involvement of A2A and/or A2B but excluding the participation of A1 receptors. The lack of effect of CGS21680 (selective A2A agonist) excluded involvement of A2A receptors and suggested a major role for A2B receptors. This conclusion was confirmed by the rank order potencies of four adenosine analogues.

CONCLUSIONS

Cardiac fibroblasts synthesize adenosine, and exogenous and cardiac fibroblast-derived adenosine inhibits cardiac fibroblast proliferation via activation of A2B receptors. Cardiac fibroblast-derived adenosine may regulate cardiac hypertrophy and/or remodeling by modulating cardiac fibroblast proliferation.

Cyclic AMP-adenosine pathway inhibits vascular smooth muscle cell growth

In this study we determined whether cAMP is metabolized to adenosine in vascular smooth muscle cells and whether cAMP-derived adenosine modulates vascular smooth muscle cell growth. Confluent smooth muscle cells were exposed to cAMP (0.01 to 30 mumol/L) in the presence and absence of 3-isobutyl-1-methylxanthine (IBMX, 1 mmol/L; an inhibitor of both extracellular and intracellular phosphodiesterase), alpha, beta-methyleneadenosine 5'-diphosphate (AMP-CP, 100 mumol/L; an ecto-5'-nucleotidase inhibitor), and 1,3-dipropyl-8-p-sulfophenyl-xanthine (DPSPX, 100 mumol/L; a xanthine that can inhibit extracellular phosphodiesterase) for 0 to 60 minutes. Medium was then sampled and assayed for AMP, adenosine, and inosine. cAMP increased the amount of AMP, adenosine, and inosine in the medium in a time- and concentration-dependent manner. The conversion of cAMP to adenosine and inosine was inhibited by blockade of phosphodiesterase with IBMX, of ecto-phosphodiesterase with DPSPX, and of ecto-5'-nucleotidase with AMP-CP. To evaluate the physiological relevance of cAMP-derived adenosine in vascular smooth muscle cell proliferation, we studied the inhibitory effects of cAMP (10(-4) mol/L) and 8-bromo-cAMP (10(-4) mol/L) on fetal calf serum-induced DNA synthesis ([3H]thymidine incorporation) in the presence and absence of erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA, an inhibitor of adenosine deaminase), dipyridamole (a blocker of adenosine transport), KF17837 (a selective A2 adenosine receptor antagonist), and DPSPX (a nonselective adenosine receptor antagonist). cAMP inhibited DNA synthesis, and both EHNA and dipyridamole enhanced this effect. Both KF17837 and DPSPX significantly reduced the inhibitory effects of cAMP on DNA synthesis; however, they did not reduce the inhibitory effects of 8-bromo-cAMP on DNA synthesis. These results indicate that vascular smooth muscle cells metabolize cAMP to adenosine via the sequential action of ecto-phosphodiesterase and ecto-5'-nucleotidase and provide the first evidence that cAMP-derived adenosine can inhibit vascular smooth muscle cell growth. Hence, this cAMP-adenosine pathway may importantly contribute to the regulation of vascular biology.

Adenosine inhibits growth of rat aortic smooth muscle cells. Possible role of A2b receptor

Abnormal growth of vascular smooth muscle cells (SMC) is frequently associated with hypertension and atherosclerosis, and homeostasis within a normal vessel is maintained by the balanced generation of both vasoconstrictors and vasodilators. Moreover, several endogenous vasoconstricting factors induce SMC growth, whereas several vasodilators inhibit SMC growth. Inasmuch as adenosine is a potent vasodilator, it is possible that it too could inhibit SMC growth. Hence, the effects of adenosine (10(-8) to 10(-3) mol/L), 2-chloroadenosine (a stable analogue of adenosine; 10(-8) to 10(-3) mol/L), and 8-bromo-cAMP (10(-8) to 10(-3) mol/L) on fetal calf serum (FCS; 2.5%)-induced growth of rat aortic SMC were evaluated. Growth was analyzed by assaying DNA synthesis (thymidine incorporation in SMC pulsed for 4 hours with 1 microCi/mL [3H]thymidine) and cell proliferation (change in cell number). Growth-arrested SMC were treated with 2.5% FCS in the presence and absence of adenosine, 2-chloroadenosine, or 8-bromo-cAMP for 24 hours for DNA synthesis or 4 days for cell proliferation. All three substances inhibited DNA synthesis and cell proliferation in a concentration-dependent manner. Compared with adenosine, 2-chloroadenosine was more potent in inhibiting growth. The inhibitory effects of 2-chloroadenosine were reversed by KF17837 (a specific A2 receptor antagonist) but not by DPCPX (a specific A1 receptor antagonist). Furthermore, the inhibitory effects of 2-chloroadenosine were not mimicked by CGS21680 (an A2a receptor agonist), and the effects of N6-cyclopentyladenosine (CPA; an A1 receptor agonist) were not markedly more potent than those of 2-chloroadenosine, suggesting that the inhibitory effects of adenosine are possibly mediated via A2b receptors. These studies provide evidence that adenosine inhibits SMC growth and suggest that a decrease in local levels of adenosine may initiate SMC growth and contribute to the vascular remodeling process observed in hypertension and atherosclerosis.

Smooth muscle cell-derived adenosine inhibits cell growth

Several endogenous factors generated within the vessel wall have been implicated in contributing to the vascular remodeling process associated with hypertension and atherosclerosis. Furthermore, substances generated by smooth muscle cells (SMCs) are known to regulate SMC proliferation in an autocrine fashion. Adenosine is a vasodilator synthesized by SMCs, and exogenous adenosine inhibits SMC proliferation. However, whether adenosine produced endogenously has antimitogenic effects is not known. Hence, we evaluated the effects of SMC-derived adenosine on 2.5% fetal calf serum-induced proliferation of rat aortic SMCs. SMC proliferation was assayed by measurement of DNA synthesis ([3H]thymidine incorporation) and cell counting. To determine the effects of endogenous adenosine on SMC proliferation, we stimulated growth-arrested SMCs with 2.5% fetal calf serum in the presence and absence of modulators of adenosine levels, including (1) erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride (EHNA; inhibits adenosine deaminase), (2) dipyridamole (blocks adenosine transport and inhibits phosphodiesterase), (3) dipyridamole plus EHNA, and (4) adenosine with or without EHNA. [3H]Thymidine incorporation and cell number were measured after 24 and 96 hours, respectively. EHNA and dipyridamole inhibited both FCS-induced DNA synthesis and cell proliferation in a concentration-dependent manner. Furthermore, extracellular (in medium) adenosine levels were significantly increased when cultured cells were treated with EHNA, and the inhibitory effects of dipyridamole as well as exogenous adenosine were enhanced in the presence of EHNA. Additionally, the inhibitory effects of dipyridamole and EHNA on DNA synthesis were significantly reduced in the presence of KF17837, an A2 adenosine receptor antagonist. These results indicate that SMC-derived adenosine can inhibit SMC proliferation. Hence, it is possible that a defect in localized adenosine synthesis within the vessel wall could contribute to vascular thickening and neointima formation.

Effects of adenosine on adrenergically induced renal vasoconstriction in dogs

The role of exogenous and endogenous adenosine in the neural control of renal blood flow was studied in anesthetized dogs. The plasma norepinephrine (NE) concentration was measured by high-performance liquid chromatography and the renal NE secretion rate was calculated. Renal nerve stimulation (1-3 Hz) reduced renal blood flow and increased NE secretion rate. The intrarenal arterial injection of NE (0.3-1.0 micrograms) also reduced renal blood flow. Infusion of adenosine (10-100 micrograms/min) into the renal artery attenuated the increase in NE secretion rate induced by renal nerve stimulation, but the nerve stimulation-induced decrease in renal blood flow was unaffected. On the other hand, adenosine potentiated the NE-induced renal blood flow response. Similar results were obtained with an adenosine potentiator, dipyridamole (1-10 micrograms/min). An adenosine receptor blocker, theophylline (0.3-1.0 mg/min), potentiated the NE secretion rate response induced by nerve stimulation, without any change in the renal blood flow response. The NE-induced renal blood flow response was attenuated by theophylline. These results suggest that adenosine inhibits neural NE release and enhances vasoconstriction in the dog kidney during sympathetic stimulation under in vivo conditions. These post- and presynaptic mechanisms may thus be activated by endogenous adenosine.

Modulation of noradrenaline release in slices of rat kidney cortex through alpha 1- and alpha 2-adrenoceptors

Slices of rat kidney cortex were incubated in [3H]noradrenaline, then placed in a flow cell and subjected to electrical field stimulation. At a stimulation frequency of 5 Hz, both the alpha 2-adrenoceptor antagonist idazoxan (0.1 microM) and the alpha 1-adrenoceptor antagonist prazosin (0.1 microM) significantly enhanced the stimulation-induced (S-I) outflow of radioactivity from the slice. However, neither clonidine (0.1 microM) nor methoxamine (10 microM), alpha 2- and alpha 1-agonists respectively, affected the S-I outflow of radioactivity at this stimulation frequency. At a lower stimulation frequency (1 Hz), the S-I outflow of radioactivity was not affected by idazoxan or prazosin, but was inhibited by both clonidine and methoxamine. The effect of clonidine was prevented by idazoxan (0.1 microM), but not by prazosin (0.1 microM). The effect of methoxamine was abolished by prazosin (0.1 microM), but not by idazoxan (0.1 microM). The inhibitory effect of methoxamine was not prevented by the prostaglandin synthesis inhibitor indomethacin (10 microM) or the adenosine receptor antagonist 8-phenyltheophylline (1 microM) and thus was not mediated by either prostaglandins or adenosine. The results suggest that both prejunctional alpha 1- and alpha 2-adrenoceptors are directly involved in modulation of noradrenaline release from the renal sympathetic nerves of the rat.

Evaluation of the effects of SKF 82526 and LY 171555 on presynaptic (DA2) and postsynaptic (DA1) dopamine receptors in rat kidney

Effects of dopamine receptor agonists, SKF 82526 and LY 171555 on presynaptic (DA2) and postsynaptic (DA1) dopamine receptors were studied in the isolated perfused rat kidney. SKF 82526 failed to alter [3H]NA release elicited during periarterial nerve stimulation in the kidney whereas LY 171555 caused a concentration-dependent inhibition of the stimulus-induced release of [3H]NA. This inhibitory action of LY 171555 could be antagonized by sulpiride but not by SCH 23390. SKF 82526 caused concentration-dependent renal vasodilation which could be antagonized by SCH 23390. LY 171555 did not produce significant changes in renal perfusion pressure over a wide range of concentrations. These results show that SKF 82526 and LY 171555 are selective agonists at DA1 and DA2 receptors respectively. These agents represent an important class of compounds that would be useful for pharmacological characterization of peripheral dopamine receptors.

Minireview. Presynaptic receptors and alterations in norepinephrine release in spontaneously hypertensive rats

The ability of blood vessels to constrict to a given stimulus is significantly increased in spontaneously hypertensive rats (SHR). Such an increase in the vasoconstrictor responsiveness contributes to the elevated peripheral vascular resistance noted in SHR. The present review discusses evidence in support of the concept that an increased release of norepinephrine during sympathetic nerve stimulation may contribute to the increase in vasoconstrictor responsiveness and, subsequently, to an increase in vascular resistance in the SHR. Several studies suggest that the exocytotic release of norepinephrine from sympathetic nerves may be altered by endogenously occurring neurohumoral substances which produce their effects by interacting with presynaptic receptors located on postganglionic sympathetic nerves. Therefore, it is postulated that alterations in presynaptic regulation of norepinephrine release, resulting from changes in the functioning of one or more of these presynaptic receptors, may lead to a greater release of norepinephrine in the SHR. This review summarizes the results of studies evaluating presynaptic receptor mechanisms and norepinephrine release in the SHR. These studies suggest that norepinephrine release during sympathetic nerve stimulation is greater in the SHR and that alterations in some of the presynaptic receptor mechanisms may be responsible for this phenomenon.

Effect of epinephrine on norepinephrine release from rat kidney during sympathetic nerve stimulation

Experiments were performed to study presynaptic beta-adrenoceptor facilitation of sympathetic neurotransmitter release in the isolated perfused rat kidney and evaluate the effect of epinephrine on norepinephrine release during sympathetic nerve stimulation. The right kidney was isolated and perfused with Krebs-Ringer solution. Norepinephrine storage sites were labelled with [3H]norepinephrine. Increasing concentrations of isoproterenol and salbutamol when perfused through the kidney, caused an enhancement of the stimulus-induced release of [3H]norepinephrine at 0.5 and 2 Hz, with the maximum facilitatory effect being observed at 0.5 Hz. The effect of salbutamol on [3H]norepinephrine release was concentration-dependent and more pronounced than that of isoproterenol. While propranolol (10(-9)-10(-5)M) by itself did not cause any significant changes in the stimulus-induced release of [3H]norepinephrine, it antagonized the facilitatory action of salbutamol on [3H]norepinephrine release during periarterial nerve stimulation. When epinephrine (10(-10)-10(-7)M), was perfused through the kidney in the presence of cocaine, it caused a concentration-dependent inhibition of the stimulus-induced release of [3H]norepinephrine release elicited during periarterial nerve stimulation. However, when epinephrine was perfused in the presence of cocaine, phentolamine and corticosterone it caused a slight but significant increase in the stimulus-induced release of [3H]norepinephrine; the highest concentration (10(-7)M) still caused a decrease in the [3H]norepinephrine release. These results, while providing evidence for the existence of presynaptic facilitatory beta-adrenoceptors on renal sympathetic nerves, fail to support the hypothesis that these receptors have a physiological role in the regulation of sympathetic neurotransmitter release.

Increased norepinephrine release during sympathetic nerve stimulation and its inhibition by adenosine in the isolated perfused kidney of spontaneously hypertensive rats

The present study was performed to measure norepinephrine release during sympathetic nerve stimulation and determine the inhibitory action of adenosine on stimulus-induced release of norepinephrine in the isolated perfused kidney of WKY and SHR. Norepinephrine release during periarterial nerve stimulation was measured as total 3H-overflow since greater than 75% of total 3H-overflow was 3H-norepinephrine in both the WKY and SHR. A significantly greater increase in 3H-norepinephrine overflow was observed during periarterial nerve stimulation in SHR in comparison with WKY. Adenosine (0.3, 1.0, 3.0 and 10.0 micrograms/ml) produced dose-dependent inhibition of 3H-norepinephrine overflow elicited by periarterial nerve stimulation. However, the effect of adenosine on transmitter release was more pronounced in the SHR in that the threshold dose required to cause inhibition of stimulus-induced release of 3H-norepinephrine was smaller in the SHR. These results demonstrate that while norepinephrine release during sympathetic nerve stimulation is greater in the SHR, this finding can not be explained on the basis of a decrease in the presynaptic inhibitory action of adenosine. Therefore, the mechanism responsible for the increased release of norepinephrine in the SHR remains to be determined.