Evidence from binding studies for alpha 2-adrenoceptors directly associated with glomeruli from rat kidney

Membrane trafficking of G protein-coupled receptors

G protein-coupled receptors (GPCRs) modulate diverse physiological and behavioral signaling pathways by virtue of changes in receptor activation and inactivation states. Functional changes in receptor properties include dynamic interactions with regulatory molecules and trafficking to various cellular compartments at various stages of the life cycle of a GPCR. This review focuses on trafficking of GPCRs to the cell surface, stabilization there, and agonist-regulated turnover. GPCR interactions with a variety of newly revealed partners also are reviewed with the intention of provoking further analysis of the relevance of these interactions in GPCR trafficking, signaling, or both. The disease consequences of mislocalization of GPCRs also are described.

alpha(2)-adrenergic receptor-mediated increase in NO production buffers renal medullary vasoconstriction

The present study was designed to investigate the role of nitric oxide (NO) in modulating the adrenergic vasoconstrictor response of the renal medullary circulation. In anesthetized rats, intravenous infusion of norepinephrine (NE) at a subpressor dose of 0.1 microgram. kg(-1). min(-1) did not alter renal cortical (CBF) and medullary (MBF) blood flows measured by laser-Doppler flowmetry nor medullary tissue PO(2) (P(m)O(2)) as measured by a polarographic microelectrode. In the presence of the NO synthase inhibitor nitro-L-arginine methyl ester (L-NAME) in the renal medulla, intravenous infusion of NE significantly reduced MBF by 30% and P(m)O(2) by 37%. With the use of an in vivo microdialysis-oxyhemoglobin NO-trapping technique, we found that intravenous infusion of NE increased interstitial NO concentrations by 43% in the renal medulla. NE-stimulated elevations of tissue NO were completely blocked either by renal medullary interstitial infusion of L-NAME or the alpha(2)-antagonist rauwolscine (30 microgram. kg(-1). min(-1)). Concurrently, intavenous infusion of NE resulted in a significant reduction of MBF in the presence of rauwolscine. The alpha(1)-antagonist prazosin (10 microgram. kg(-1). min(-1) renal medullary interstitial infusion) did not reduce the NE-induced increase in NO production, and NE increased MBF in the presence of prazosin. Microdissection and RT-PCR analyses demonstrated that the vasa recta expressed the mRNA of alpha(2B)-adrenergic receptors and that medullary thick ascending limb and collecting duct expressed the mRNA of both alpha(2A)- and alpha(2B)-adrenergic receptors. These subtypes of alpha(2)-adrenergic receptors may mediate NE-induced NO production in the renal medulla. We conclude that the increase in medullary NO production associated with the activation of alpha(2)-adrenergic receptors counteracts the vasoconstrictor effects of NE in the renal medulla and may play an important role in maintaining a constancy of MBF and medullary oxygenation.

Human IFN gamma receptor cytoplasmic domain: expression and interaction with HuIFN gamma

To investigate the structural basis for human interferon gamma (huIFN gamma) binding to intracellular regions of the human IFN gamma receptor (huIFN gamma R), we have subcloned and expressed the huIFN gamma R free of fusion proteins in the yeast strain Pichia pastoris. HuIFN gamma bound to the cytoplasmic domain of the receptor via the IFN gamma C-terminus. Binding was inhibited by both human and mouse C-terminus peptides. N-terminus peptides failed to inhibit cytoplasmic binding. Thus, while extracellular receptor domain binding is species specific, binding to the cytoplasmic domain of the receptor is species non-specific. In solid-phase binding assays, IFN gamma had a Kd of 3.7 x 10(-8) M for the newly expressed cytoplasmic domain. Peptide competitions showed that IFN gamma bound to a receptor site corresponding to the membrane proximal residues 253-287, which is adjacent to the site of binding of the tyrosine kinase JAK2. The cytoplasmic binding affinity and binding site specificity suggest that the huIFN gamma R cytoplasmic domain can function independent of the extracellular domain to bind huIFN gamma and induce the biological activity previously associated with internalized huIFN gamma.

Renal alpha-adrenergic receptors and genetic hypertension

The hypothesis has been proposed that an increase in the number of renal alpha-adrenergic receptors may contribute to the pathogenesis of genetic hypertension. Herein we review recent findings regarding expression of renal alpha 1 (alpha 1a, alpha 1b)- and alpha 2 (alpha 2a, alpha 2b)-adrenergic subtypes and we provide an updated revision of the above-stated hypothesis. Enhancement in receptor number or in post-receptor components responsible for alpha 1- and alpha 2-adrenergic-mediated sodium reabsorption in proximal tubule may contribute to sodium retention and an elevation in blood pressure. Perhaps such changes contribute to the increase in blood pressure in genetically determined hypertension in humans, although direct tests of this notion have not yet been performed.

Interaction between alpha 2-adrenergic and angiotensin II systems in the control of glomerular hemodynamics as assessed by renal micropuncture in the rat

The hypothesis that renal alpha 2 adrenoceptors influence nephron filtration rate (SNGFR) via interaction with angiotensin II (AII) was tested by renal micropuncture. The physical determinants of SNGFR were assessed in adult male Munich Wistar rats 5-7 d after ipsilateral surgical renal denervation (DNX). DNX was performed to isolate inhibitory central and presynaptic alpha 2 adrenoceptors from end-organ receptors within the kidney. Two experimental protocols were employed: one to test whether prior AII receptor blockade with saralasin would alter the glomerular hemodynamic response to alpha 2 adrenoceptor stimulation with the selective agonist B-HT 933 under euvolemic conditions, and the other to test whether B-HT 933 would alter the response to exogenous AII under conditions of plasma volume expansion. In euvolemic rats, B-HT 933 caused SNGFR to decline as the result of a decrease in glomerular ultrafiltration coefficient (LpA), an effect that was blocked by saralasin. After plasma volume expansion, B-HT 933 showed no primary effect on LpA but heightened the response of arterial blood pressure, glomerular transcapillary pressure gradient, and LpA to AII. The parallel results of these converse experiments suggest a complementary interaction between renal alpha 2-adrenergic and AII systems in the control of LpA.

Cardiovascular pharmacology of adrenergic and dopaminergic receptors: therapeutic significance in congestive heart failure

This review discusses the localization of adrenergic- and dopaminergic-adrenoceptors within the cardiovascular system and describes the cardiovascular and renal changes produced following the activation of these receptors by appropriate agonists. Whereas the role of alpha- and beta-adrenergic agents in the treatment of heart failure is well recognized, recent studies with dopamine (DA)-receptor agonists indicate that they offer a novel approach in the therapy of congestive heart failure. DA-adrenoceptor agonists reduce afterload by causing vasodilation and promote sodium excretion via direct activation of DA1-adrenoceptors located on renal tubules. Fenoldopam is a selective DA1-adrenoceptor agonist found to be effective in heart failure. It reduces afterload by causing peripheral vasodilation and produces natriuresis and diuresis. Dopexamine is a DA1- and beta 2-adrenoceptor agonist, and its efficacy in heart failure is due to its ability to provide mild inotropic support and cause a reduction in afterload. Ibopamine is a prodrug that is converted into its active metabolite, epinine. This compound activates primarily DA1- and DA2-adrenoceptors. It is effective in heart failure, and the mechanism progresses via DA1- and DA2-adrenoceptor-mediated reduction in afterload. Agonists of DA2-adrenoceptors reduce afterload by decreasing the release of norepinephrine and by reducing the levels of renin-angiotensin-aldosterone system. Since both of these systems are active in heart failure, ibopamine offers a rational approach for therapy. The present review addresses the concept of pharmacologic intervention in adrenergic and dopaminergic influence in the cardiovascular and renal systems to produce changes that are desirable for the pharmacotherapy of congestive heart failure.

Renal and myocardial adrenoceptors in steroid contraceptive-induced hypertension in rats

1. Systolic blood pressure (SBP), bodyweight, organ weight, renal beta-adrenoceptor and myocardial beta- and myocardial alpha 1-adrenoceptor characteristics were investigated in female Sprague-Dawley rats after chronic subcutaneous (s.c.) administration of ethynyloestradiol (EE2, 0.2 microgram/day), levonorgestrel (NG, 2.0 micrograms/day) separately and in combination (EE2/NG). 2. EE2 caused a sustained increase in SBP from 6 weeks (maximum at 14 weeks, +22 mmHg compared to control) which was accompanied by increased kidney and ventricle weight after 12 weeks. EE2/NG-treated rats also demonstrated a gradual rise in SBP (maximum at 9 weeks, +18 mmHg compared with control) with renal and ventricular hypertrophy, but were normotensive by week 17 of treatment. In contrast, NG induced only transient SBP increases (maxima at 5 and 10 weeks, +14 mmHg compared with control), unaccompanied by organ hypertrophy. Norethisterone (2 micrograms/day) also produced transient increases (weeks 6-8, +13 mmHg) in SBP. 3. alpha 1- and beta-adrenoceptors were investigated using [3H]-prazosin and (-)-[125I]-iodocyanopindolol (ICYP), respectively. Myocardial alpha 1- and beta-adrenoceptors were unaffected by steroid contraceptive administration for up to 12 weeks. Renal beta-adrenoceptor affinity was markedly reduced in 12 week EE2-treated rats (equilibrium dissociation constant, KD, 53 +/- 7 pmol/L) compared with controls (KD, 31 +/- 4 pmol/L), an effect which was prevented by co-administration of NG (KD, 34 +/- 8 pmol/L). Renal beta-adrenoceptor number was not altered by any treatment. 4. The relatively late onset of organ hypertrophy and beta-adrenoceptor changes appear to result from, rather than cause, EE2-induced hypertension.

Characterization of adrenergic receptors on proximal tubular basolateral membranes

Alpha adrenergic receptors are identified on basolateral plasma membranes derived from proximal tubular epithelial cells. The density of alpha 2 receptors was over two-fold greater than alpha 1 receptors. The basolateral membranes were devoid of beta receptors. These results support previous demonstrations of alpha adrenergic receptors in rat renal cortex and concur with studies which suggest a limited presence of beta receptors on rat proximal tubules.

Atrial natriuretic peptide receptors along the rat and rabbit nephrons: [125I] alpha-rat atrial natriuretic peptide binding in microdissected glomeruli and tubules

Binding of [125I] alpha-rat atrial natriuretic peptide ([125I] alpha-RANP) was measured in glomeruli and pieces of tubule microdissected from rat and rabbit nephrons. High densities of specific ANP binding sites were found only in the glomeruli (10-30 X 10(-18) mol X glom-1), whereas no specific binding could be detected in the proximal tubule, the thin segments of the Henle's loop, the thick ascending limb, the distal tubule and the cortical and outer medullary collecting tubules. Rising the temperature from 4 degrees C to 35 degrees C resulted in biphasic kinetics of binding, suggesting a temperature-dependent inactivation of labelled hormone by glomeruli. At 4 degrees C, specific binding of [125I] alpha-RANP was time and dose-dependent and Scatchard analysis of data indicated an apparent equilibrium dissociation constant of 0.63 nM. Competition experiments revealed the following sequence of stereospecificity for binding to rat glomeruli: RANP 3-28 greater than [125I] alpha-RANP = [125I] alpha-HANP = alpha-RANP = antriopeptin III greater than antriopeptin II, whereas binding was unaffected by pharmacological doses of unrelated peptide hormones, prostaglandins, adrenergic agonists, dopamine, histamine and carbamylcholine. The results indicate that glomerular binding sites might be the physiological ANP receptors.

Characterisation of the alpha-adrenoceptors of the rat renal vascular bed

Postjunctional renal alpha-adrenoceptors were studied (1) in vivo, on the renal vasculature of the anaesthesized rat and compared with those in the femoral vasculature, and (2) in vitro, on the renal vascular bed of isolated perfused rat kidney. In vivo, renal and iliac blood flows were measured with an electromagnetic flow meter. The i.v. injection of (-)-phenylephrine (1-16 micrograms/kg) and B-HT 920 (0.6-600 micrograms/kg) induced an increase in both renal and iliac vascular resistance, inhibited respectively with prazosin (300 micrograms/kg) or yohimbine (300 micrograms/kg). In the kidney, maximum response to B-HT 920 was equivalent to 64% of that to (-)-phenylephrine; on the iliac vasculature, vasoconstrictor responses to both drugs were identical, but only corresponded to 50% of the maximum renal response to (-)-phenylephrine. This indicates the predominance of alpha 1- over alpha 2-adrenoceptors in the renal vascular bed. In vitro, on the isolated perfused rat kidney, vasoconstriction was induced by the preferential alpha 1-adrenoceptor agonists [(-)-phenylephrine, cirazoline and methoxamine] and the preferential alpha 2-adrenoceptor agonists (alpha-methylnoradrenaline, dopamine and clonidine) at concentrations at which they lose their selectivity for the alpha 2-adrenoceptors; all responses were antagonised by prazosin but not by yohimbine. B-HT 920, the selective alpha 2-adrenoceptor agonist, only induced renal vasoconstriction in vitro under concomitant infusion of rabbit plasma.

Adrenoceptors in the kidney: localization and pharmacology

The kidney plays a key role in the regulation of blood pressure. The sympathetic nervous system can influence many aspects of kidney function in relation to blood pressure control, e.g. renal vascular tone, intrarenal renin release and tubular reabsorption of electrolytes and fluid. The intrarenal distribution of adrenoceptors has now been studied on the basis of modern receptor ligand binding techniques combined with microscopic studies. The preferential localization of each adrenoceptor subtype within the kidney is reviewed. Furthermore, an attempt is made to describe the functional correlation of the presence of different adrenoceptor subtypes. Finally, the possible role of renal adrenoceptor abnormalities in the pathogenesis of hypertension is discussed.

Chronic steroid contraceptive treatment decreases renal alpha-adrenoceptor levels in the rat

Female Sprague-Dawley rats were injected s.c. with ethynyloestradiol (EE2, 0.2 microgram/day) and levonorgestrel (NG, 2.0 micrograms/day) separately and in combination (EE2/NG). Binding of [3H]rauwolscine (alpha 2-adrenoceptor specific) and [3H]prazosin (alpha 1-adrenoceptor specific) was examined in crude membrane suspensions prepared from whole rat kidney after 3, 6 and 12 weeks of steroid administration. Receptor affinity was high for both ligands (KD, equilibrium dissociation constant [3H]rauwolscine, congruent to 2.0 nM; [3H]prazosin, congruent to 0.2 nM) and was not altered in rats chronically treated with steroid contraceptives. The Bmax (maximum density of binding sites) for [3H]prazosin binding was not altered, indicating no change in the number of renal alpha 1-adrenoceptors. NG administered alone did not affect the numbers of alpha 1- or alpha 2-adrenoceptors. Catechol metabolites of endogenous oestrogens did not displace the binding of either radioligand, suggesting that these metabolites do not directly interact with renal alpha-adrenoceptors. However, after 12 weeks treatment, the number of [3H]rauwolscine binding sites was reduced in both EE2 (Bmax, 133 +/- 7 fmol/mg protein)- and combined EE2/NG (135 +/- 11 fmol/mg protein)-treated rats, compared to controls (162 +/- 9 fmol/mg protein). Since renal alpha 2-adrenoceptors inhibit renin release, this reduction in alpha 2-adrenoceptor number may contribute to increased renin levels associated with oestrogen-induced hypertension.

Effects of chemical sympathectomy with 6-hydroxydopamine on alpha- and beta-adrenoceptors and muscarinic cholinoceptors in rat kidney

The autonomic receptors in the rat kidney were characterized using the radioligands [3H]prazosin, [3H]clonidine, [3H]dihydroalprenolol (DHA) and [3H]quinuclidinyl benzilate (QNB). The specific binding of [3H]prazosin, [3H]clonidine, [3H]DHA and [3H]QNB to rat kidney membranes was saturable and of high affinity, and showed a pharmacological specificity as well as stereospecificity which characterized renal alpha 1-, alpha 2- and beta-adrenoceptors and muscarinic cholinoceptors, respectively. There was a relatively greater density of alpha-adrenoceptors than beta-adrenoceptors or muscarinic cholinoceptors in the rat kidney. Chemical sympathectomy of rats with 6-hydroxydopamine X HBr (6-OHDA, 50 X 2 mg/kg i.v., 24 h interval) caused a significant increase (21-56%) in the Bmax values for renal [3H]prazosin, [3H]clonidine and [3H]DHA binding at 1 and 2 weeks following the treatment, without a change in the Kd values. 6-OHDA treatment had no significant effect on the Kd and Bmax values for [3H]QNB binding at 1-3 weeks after the treatment. The norepinephrine (NE) concentration was reduced (68-76%) in the 6-OHDA-treated rat kidney. In conclusion, the present study provides biochemical evidence for the possible localization of postsynaptic alpha 1-, alpha 2- and beta-adrenoceptors and muscarinic cholinoceptors in the rat kidney and also for the regulation of these adrenoceptors by the sympathetic nervous system.

Regulation of renal cellular cAMP levels by prostaglandins and alpha 2-adrenoceptors: microdissection studies

Previous studies in our laboratory demonstrated that alpha 2-adrenoceptor activation reversed arachidonic acid induced diuresis in the rat. However, the site of action was not elucidated. Since prostaglandin E2 is the predominant prostaglandin metabolite of arachidonic acid, we studied the effect of renal alpha 2-adrenoceptor stimulation on prostaglandin E2 (PGE2) induced cAMP formation. The study was done in intact single nephron segments and glomeruli. All incubations were done in the presence of 1-methyl-3-isobutylxanthine (phosphodiesterase inhibitor) and propranolol at 37 degrees C for two minutes. PGE2 increased cellular cAMP levels in the thin descending limb of Henle (tDL), cortical collecting tubule (CCT) and glomerulus. Alpha 2-adrenoceptors were activated with varying concentrations of epinephrine (E). In the tDL, alpha 2-adrenoceptor activation with E (5 X 10(-6)M to 5 X 10(-5)M) suppressed (p less than 0.05) PGE2 stimulated cAMP production by 35%. This suppression by E was inhibited by 5 X 10(-6)M yohimbine but not by 5 X 10(-6)M prazosin confirming alpha 2-adrenoceptor mediation of the effects of E. Conversely, in the CCT and glomerulus, E had no effect on PGE2-stimulated increases in cellular cAMP levels. Thus, the capacity of alpha 2-adrenoceptors to inhibit PGE2-stimulated adenylate cyclase is anatomic site-specific. This effect of alpha 2-adrenoceptors on cAMP in the tDL may explain, at least in part, the effect of alpha 2-adrenoceptors on arachidonic acid induced diuresis in the rat.

Light microscopic autoradiography of the distribution of [3H]rauwolscine binding to alpha 2-adrenoceptors in rat kidney

The localization of [3H]rauwolscine binding to microscope slide mounted sections of rat kidney has been examined using the technique of in vitro labelling and autoradiography. Binding to sections equilibrated within 60 min and was reversible following the addition of 10 microM phentolamine. Saturation studies revealed a single population of high affinity (KD 4.27 nM) non-interacting sites (nH 0.97) with a density of 11.1 fmol/section. Stereoselectivity was observed with respect to the isomers of noradrenaline and the relative affinity of a series of alpha-adrenoceptor antagonists suggested binding to alpha 2-adrenoceptors. Autoradiographic studies using 3H-Ultrofilm showed that the binding is largely confined to the renal cortex. More detailed studies using emulsion coated coverslips indicates that the major concentration of binding sites is over the proximal tubules. This study provides evidence that alpha 2-adrenoceptors, known to be coupled in an inhibitory fashion to renal adenylate cyclase, are highly localized to particular structures in the kidney.

[3H]-rauwolscine binding to alpha 2-adrenoceptors in the mammalian kidney: apparent receptor heterogeneity between species

Binding of the alpha 2-adrenoceptor antagonist [3H]-rauwolscine was characterized in membrane preparations from the kidneys of mouse, rat, rabbit, dog, and man. In all species, binding reached equilibrium within 45 min and dissociated at a single exponential rate after addition of phentolamine 10 microM. Saturation studies showed that the affinity of [3H]-rauwolscine was similar in all species (2.33-3.03 nM) except man where it was significantly higher (0.98 nM). Marked differences were seen in the density of binding sites, increasing in the order: man less than dog less than rabbit less than rat less than mouse. In all cases, Hill coefficients were not significantly different from unity. [3H]-rauwolscine binds with low affinity (KD greater than 15 nM) to membranes prepared from guinea-pig kidney. The low affinity binding is not due to the absence of particular ions in the incubation medium or to receptor occupation by endogenous agonist. The binding in all species was found to be stereoselective with respect to the isomers of noradrenaline. However, differences were seen in the characteristics of agonist interactions with the binding site both between isomers and between species. Marked differences in affinity of particular alpha-adrenoceptor antagonists were observed for alpha 2-adrenoceptors labelled by [3H]-rauwolscine. These differences were most evident with the alpha 1-adrenoceptor selective antagonist prazosin which displayed inhibition constants (Ki values) of 33.2, 39.5, 261, 570 and 595 nM in rat, mouse, dog, man and rabbit, respectively. Differences are apparent in the characteristics of alpha 2-adrenoceptors labelled by [3H]-rauwolscine between species and it is suggested that the differences observed for alpha 1-selective antagonists such as prazosin may be related to binding to additional sites in the vicinity of the alpha 2-adrenoceptor.

Specific increase in renal alpha 1-adrenergic receptors following unilateral renal denervation

The effect of unilateral surgical renal denervation on adrenergic receptors in rat kidney was investigated. Denervation was performed by mechanically stripping the nerves from the renal artery and then painting with phenol. This procedure resulted in a decrease in norepinephrine content to less than 10% of the contralateral kidney. The concentrations of alpha 1-, alpha 2-, beta 1- and beta 2-adrenergic receptors were measured in both kidneys of rats 1 week following left renal denervation. Alpha 1-receptor concentrations increased in the denervated left kidneys relative to the intact right kidneys. Alpha 2-, beta 1- and beta 2-receptor concentrations were not different between left and right kidneys. These results indicate that the different classes of adrenergic receptor in the kidney are differentially regulated by agonist stimulation.

Alpha-adrenoceptors: recent development and some comparative aspects

On anatomical and functional bases, alpha-adrenoceptors have been divided into pre- and postsynaptic alpha-adrenoceptors. Recently, alpha-adrenoceptors have been classified as alpha 1 and alpha 2 according to their pharmacological responses, irrespective of their anatomical location. The presynaptic alpha-adrenoceptors, which have been recognized as alpha 2, determine the frequency of the nerve impulses travelling along the axon and also the amount of transmitter released per nerve impulse from the varicose terminal. Postsynaptic alpha-adrenoceptors have been recognized in various tissues including smooth muscle, pancreatic islets, fat cells, platelets and other tissues. Both alpha 1- and alpha 2-adrenoceptors have been located postsynaptically. alpha-Adrenoceptors have been found also in the central nervous system. Generally, they fall into the same categories (alpha 1 and alpha 2) as the peripheral alpha-adrenoceptors. A new class of drugs, the so called calcium blockers, inhibit the postsynaptic response to alpha 2 stimulation but not the alpha 1-mediated response, indicating that the alpha 2-adrenoceptors are dependent on Ca2+ ions for their function. In the most primitive group of vertebrates, the fishes, alpha-adrenoceptors seem to be different in as much as they do not respond to many of the classical drugs employed to distinguish between alpha-adrenoceptors in mammals. In reptiles and amphibians alpha 2-adrenoceptors have been shown to exist. These receptors are involved in the regulation of melanocytes. In the most advanced non-mammalian vertebrates (birds) both peripheral and central alpha-adrenoceptors seem to be qualitatively similar to the mammalian types.