alpha 2-Adrenergic receptors are associated with renal proximal tubules

Wakefulness-inducing area in the brainstem excites warm-sensitive and inhibits cold-sensitive neurons in the medial preoptic area in anesthetized rats

Sleep-wakefulness and body temperature are known to influence each other. The body temperature rises during wakefulness and falls during sleep. The midbrain reticular formation is one of the areas in the brainstem that induces wakefulness, while the preoptico-anterior hypothalamic area is the main thermoregulatory center in the brain. In order to understand the neural mechanism for simultaneous regulation of these functions we hypothesized that the wakefulness area in the brainstem is likely to have an opposite influence on warm- and cold-sensitive neurons in the preoptico-anterior hypothalamic area. Hence, first, the wakefulness-inducing area was identified in the brainstem by stimulating the site with high-frequency rectangular wave electrical pulses (100 Hz, 100 microA, 200 microsec for 5-8 sec) in freely behaving chronically prepared experimental rats. Then, single neuronal activity from the medial preoptico-anterior hypothalamic area was recorded and their thermosensitivity was established. Thereafter, the influence of such a confirmed wakefulness-inducing area in the brainstem on the responsiveness of the single neuronal activity of predetermined warm- and cold-sensitive neurons as well as on temperature-insensitive neurons was studied by overlapping stimulus (1 Hz, 500 microA, 200 microsec) bound responses. It was observed that the warm-sensitive neurons were excited and the cold-sensitive neurons were inhibited by stimulation of the wakefulness-inducing area in the brainstem. Most of the temperature-insensitive neurons remained unaffected. The results confirm our hypothesis and help in understanding the mechanism of simultaneous modulation of body temperature in association with changes in wakefulness at the single neuronal level.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Alpha 2-adrenoceptors in brain and kidney during development of hypertension in Dahl-Iwai salt-sensitive rats

BACKGROUND

Both renal and extrarenal factors have been considered to contribute to the development of hypertension in Dahl salt-sensitive rats, but contents of both factors have not been established precisely.

AIM

To clarify the role of those factors in the sympathetic nervous system, we examined the regulation of alpha2-adrenoceptors in the lower brainstem and the renal tubular basolateral membranes simultaneously during the development of salt-induced hypertension in Dahl-Iwai salt-sensitive rats.

METHODS

Dahl-Iwai salt-sensitive or resistant rats were fed a high (8.0% NaCl)- or low (0.3%)- salt diet from 4 to 6 or 10 weeks of age. At 4, 6 and 10 weeks of age, the plasma membranes of the lower brainstem and the renal tubular basolateral membranes were obtained simultaneously and alpha 2-adrenoceptors were quantified by a radioligand binding assay using 3H-rauwolscine.

RESULTS

In the salt-sensitive rats, systolic blood pressure was significantly higher in those fed a high-salt diet than in those fed a low-salt diet. In the salt-resistant rats, both the high- and the low-salt groups showed similar blood pressure levels. At 6 weeks of age, alpha 2-receptor densities of the salt-sensitive rats fed a high-salt diet were lower in the lower brainstem and higher in the renal basolateral membranes than those fed a low-salt diet. In contrast, in the salt-resistant rats, both the high- and the low-salt groups had similar densities. At 10 weeks of age, the difference between the high- and the low-salt groups in the salt-sensitive rats disappeared in both the brainstem and the renal basolateral membranes.

CONCLUSIONS

Alpha 2-adrenoceptor regulation in the brainstem and the renal basolateral membranes differs between Dahl-Iwai salt-sensitive and salt-resistant rats. The modulation of alpha 2-adrenoceptors by a high salt intake may be essential particularly in the early phase of the development of salt-induced hypertension.

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.

Autoradiographic localization and computerized quantification of alpha 1- and alpha 2-adrenoceptors in spontaneously hypertensive rat kidney: [3H]bunazosin and [3H]yohimbine binding studies

The autoradiographic localization of alpha 1- and alpha 2-adrenoceptors was identified in 22-week-old Wistar-Kyoto rat kidney, using alpha 1-selective ([3H]bunazosin) and alpha 2-selective ([3H]yohimbine) antagonists. [3H]Bunazosin binding was distributed predominantly over the cortex, less over the outer medulla and was absent from the inner medulla. [3H]Yohimbine binding was distributed predominantly over the medulla, less over the renal cortex and was absent from the inner medulla. In addition, the distribution of renal alpha-adrenoceptors was investigated in 3-, 7- and 22-week-old spontaneously hypertensive rats (SHRs) using a computerized image analysis system. Renal alpha-adrenoceptors were both found to be increased in SHRs at all ages tested compared with their respective controls and were increased in both the cortex and the outer medulla. The increase in renal alpha-adrenoceptors was already present in 3-week-old SHRs whose systolic blood pressures did not differ significantly from those of the controls. The results strongly suggest that these abnormalities of renal alpha-adrenoceptors may play a critical role in the development of hypertension in SHRs.

Characterization of alpha 2-adrenoceptors in the rat: proximal tubule, renal membrane and whole kidney studies

In the present study, alpha 2-adrenoceptors have been characterized in rat renal proximal tubules which were isolated by a Percoll gradient technique. Competitive binding curves with [3H]rauwolscine (0.5 nM) and alpha 2-adrenoceptor agonists and antagonists were consistent with an alpha 2B-adrenoceptor subtype. However, the rank order of potency (Ki) for clonidine and UK 14,304 was reversed from that reported for other tissues (clonidine, 48 nM greater than UK 14,304, 330 nM). This rank order was confirmed in a crude renal membrane preparation consisting of whole kidney as well as separated medullary and cortical segments. An intrarenal infusion of clonidine at 11 nmol/kg per min resulted in a greater diuresis and natriuresis than an equimolar dose of UK 14,304 suggesting that clonidine also had a greater affinity in the collecting tubules. Further displacement studies in proximal tubules with [3H]rauwolscine and calcium channel blockers demonstrated that verapamil was the most potent (Ki, 2.3 microM), followed by diltiazem (48% displacement at 100 microM) and then nifedipine (no displacement at 100 microM). These studies indicated that alpha 2-adrenoceptor in the rat proximal tubule may be of the alpha 2B-adrenoceptor subtype. Further studies will be required to determine whether the reverse rank order of potency of clonidine and UK 14,304 is consistent with an alpha 2-adrenoceptor subtype which is different from that found in other tissue.

High NaCl diets increase alpha 2-adrenoceptors in renal cortex and medulla of NaCl-sensitive spontaneously hypertensive rats

Diets high in NaCl simultaneously elevate renal alpha 2-adrenoceptor binding and exacerbate hypertension in young NaCl-sensitive spontaneously hypertensive rats (SHR-S). The present study tests the hypothesis that in SHR-S on a high NaCl diet, an upregulation of renal alpha 2-adrenoceptors is present in densely innervated areas of the kidney, and this precedes the increase in blood pressure. Seven week old SHR-S fed on a high (8%) compared to basal (1%) NaCl diet for 2 weeks displayed significantly exacerbated hypertension and elevated renal alpha 2-adrenoceptor binding in both cortex and medulla. In contrast one week on the high NaCl diet did not alter renal alpha 2-adrenoceptor number or blood pressure in SHR-S. Autoradiographic experiments demonstrated that the NaCl-induced upregulation of alpha 2-adrenoceptors occurs in all areas of the renal cortex and medulla. None of these differences were observed in NaCl-resistant, Wistar-Kyoto rats (WKY). Further, the high NaCl diet did not alter renal alpha 1-adrenoceptor binding in SHR-S or WKY. Together with previous findings, these data suggest that the NaCl-induced upregulation of renal alpha 2-adrenoceptors is not specific to densely innervated regions of the kidney.

Alpha 2-adrenoceptor regulation of parathyroid hormone function in the isolated perfused kidney

We investigated physiological interactions between alpha 2-adrenoceptors and parathyroid hormone (PTH) in the isolated buffer-perfused kidney. PTH infusion (10nM) caused a rapid and significant rise in cAMP excretion which diminished despite continued hormone infusion. PTH also caused a delayed phosphaturia which peaked 20-30 minutes following the start of PTH infusion. alpha 2-Adrenoceptor stimulation with epinephrine (28nM) diminished PTH-stimulated cAMP accumulation by 68-71% (p less than 0.05) but had no effect on PTH-induced phosphaturia. None of the experimental interventions affected renal hemodynamics. In additional studies, we used lithium (1mM) as a marker of proximal tubular sodium transport. PTH caused a rapid rise in lithium excretion which was temporally distinct (maximum response in 0-10 minutes) from the phosphaturia. alpha 2-Adrenoceptor stimulation completely blocked the inhibitory effect of PTH on lithium reabsorption. These results suggest that alpha 2-adrenoceptors regulate the stimulatory effect of PTH at proximal tubular adenylate cyclase. However, alpha 2-adrenoceptors play no role in phosphate transport in this segment. alpha 2-Adrenoceptor stimulation reverses PTH-induced lithium excretion, suggesting physiological antagonism in the proximal tubule, most likely involving Na/H exchange.

Effects of extracellular fluid volume changes on renal response to low-dose dopamine infusion in normal women

Dopamine (DA) i.v. infused in a low dose (0.1 microgram/kg/min) in healthy women during sustained hypotonic polyuria, produced different renal functional effects as variations in extracellular fluid volume occurred. (1) In hydro-saline retention (n = 23), induced by desoxycorticosterone acetate treatment, DA produced typical vasodilator and hydro-natriuretic effects (Goldberg, 1972). (2) In hydro-saline depletion (n = 19), induced by diuretic treatment and low dietary sodium intake, DA lost its vasodilator and natriuretic efficacy, manifesting, on the other hand, renal sodium conserving effects mainly dependent on the increase in distal sodium reabsorption and a trend towards afferent arteriolar vasoconstriction. (3) Treatment with prazosin (n = 9) or propranolol (n = 9) in hydro-saline depletion, was efficacious in partly restoring the typical vasodilator and natriuretic effects of DA. Thus, in hydro-saline depletion, DA produced sympathomimetic effects which were sufficiently intense to outweigh those due to activation of specific DA renal receptors.

Alpha 2 adrenergic agonists stimulate Na+-H+ antiport activity in the rabbit renal proximal tubule

The role of adrenergic agents in augmenting proximal tubular salt and water flux, was studied in a preparation of freshly isolated rabbit renal proximal tubular cells in suspension. Norepinephrine (NE, 10(-5) M) increased sodium influx (JNa) 60 +/- 5% above control value. The alpha adrenergic antagonist, phentolamine (10(-5) M), inhibited the NE-induced enhanced JNa by 90 +/- 2%, while the beta adrenergic antagonist, propranolol, had a minimal inhibitory effect (10 +/- 2%). The alpha adrenergic subtype was further defined. Yohimbine (10(-5) M), an alpha2 adrenergic antagonist but not prazosin (10(-5) M), an alpha1 adrenergic antagonist completely blocked the NE induced increase in JNa. Clonidine, a partial alpha2 adrenergic agonist, increased JNa by 58 +/- 2% comparable to that observed with NE (10(-5) M). Yohimbine, but not prazosin, inhibited the clonidine-induced increase in JNa, confirming that alpha2 adrenergic receptors were involved. Additional alpha2 adrenergic agents, notably p-amino clonidine and alpha-methyl-norepinephrine, imparted a similar increase in JNa. The clonidine-induced increase in JNa could be completely blocked by the amiloride analogue, ethylisopropyl amiloride (EIPA, 10(-5) M). The transport pathway blocked by EIPA was partially inhibited by Li and cis H+, but stimulated by trans H+, consistent with Na+-H+ antiport. Radioligand binding studies using [3H]prazosin (alpha1 adrenergic antagonist) and [3H]rauwolscine (alpha2 adrenergic antagonist) were performed to complement the flux studies. Binding of [3H]prazosin to the cells was negligible. In contrast, [3H]rauwolscine showed saturable binding to a single class of sites, with Bmax 1678 +/- 143 binding sites/cell and KD 5.4 +/- 1.4 nM. In summary, in the isolated rabbit renal proximal tubular cell preparation, alpha2 adrenergic receptors are the predominant expression of alpha adreno-receptors, and in the absence of organic Na+-cotransported solutes, alpha2 adrenergic agonists enhance 22Na influx into the cell by stimulating the brush border membrane Na+-H+ exchange pathway.

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.

The ontogeny of renal alpha 1- and alpha 2-adrenoceptors in the Dahl rat model of experimental hypertension

[3H]prazosin (PRAZ) and [3H]rauwolscine (RAUW) were used to examine the ontogeny of renal alpha 1- and alpha 2-adrenoceptors in the inbred Dahl hypertension-sensitive (S/JR) and -resistant (R/JR) rat. PRAZ and RAUW each bound to a single population of non-interacting sites. The binding of each ligand was saturable and reversible. The greatest proliferation of each receptor subtype occurred between 5 and 25 days of age. During this period, a 4 to 5-fold increase in the density of each was observed. Adult levels of each were reached by 50 days of age. The alpha 2-adrenoceptor was the predominant subtype present in renal tissue. However, its ratio to the alpha 1 subtype was influenced by strain and age: the ratio was greatest in the S/JR strain and decreased with age in both strains. The profile of alpha 1-adrenoceptor development was similar in S/JR and R/JR rats. In contrast, the density of alpha 2-adrenoceptors was similar in S/JR and R/JR rats at 5 and 15 days of age but significantly greater in the S/JR rat between 25 and 150 days of age. The elevated density of alpha 2-adrenoceptors could not be explained by strain-related differences in blood pressure or alterations in the affinity of the receptor. The results suggest that a relationship may exist between elevated renal alpha 2-adrenoceptor density and the genetic predisposition to hypertension in the S/JR rat. However, because this relationship is not apparent during the neonatal period of development, the possibility that the elevated density of sites may be secondary to some other event should also be considered.

Characterization of alpha 1- and alpha 2-adrenoceptors directly associated with basolateral membranes from rat kidney proximal tubules

We have used 2-(beta-(3-125iodo-4-hydroxyphenyl)-ethylaminoethyl)-tetr alo ne ([125I]HEAT or BE2254), an alpha 1-selective antagonist, and [3H]yohimbine, an alpha 2-selective antagonist, to demonstrate and characterize binding sites in basolateral membranes from rat kidney cortex. Parathyroid hormone (PTH) stimulated the adenylate cyclase activity of the basolateral membranes, whereas thyrocalcitonin, arginine vasopressin (AVP) and isoproterenol did not. Therefore, the basolateral membranes were probably derived from the proximal tubules. The specific binding of [125I]HEAT and [3H]yohimbine to basolateral membranes was rapid, reversible, saturable and of high affinity. The maximum densities of alpha 1- and alpha 2-receptors were 364 and 1130 fmoles/mg protein, indicating that the ratio of alpha 1- to alpha 2-adrenoceptors was about 1:3. The specific binding of [125I]HEAT and [3H]yohimbine to the basolateral membranes was displaced by various adrenergic agents in a manner that suggests that the labeled sites probably represent alpha 1- and alpha 2-adrenoceptors respectively. These results suggest that the binding sites of [125I]HEAT and [3H]yohimbine, which appear to be alpha 1- and alpha 2-adrenoceptors, exist in the basolateral membranes of the proximal tubules.

Alpha 2 adrenergic receptors in hyperplastic human prostate: identification and characterization using [3H] rauwolscine

[3H]Rauwolscine ([3H]Ra), a selective ligand for the alpha 2 adrenergic receptor, was used to identify and characterize alpha 2 adrenergic receptors in prostate glands of men with benign prostatic hyperplasia. Specific binding of [3H]Ra to prostatic tissue homogenates was rapid and readily reversible by addition of excess unlabelled phentolamine. Scatchard analysis of saturation experiments demonstrates a single, saturable class of high affinity binding sites (Bmax = 0.31 +/- 0.04 fmol./microgram. DNA, Kd = 0.9 +/- 0.11 nM.). The relative potency of alpha adrenergic drugs (clonidine, alpha-methylnorepinephrine and prazosin) in competing for [3H]Ra binding sites was consistent with the order predicted for an alpha 2 subtype. The role of alpha 2 adrenergic receptors in normal prostatic function and in men with bladder outlet obstruction secondary to BPH requires further investigation.

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.

Effect of dietary sodium on platelet alpha 2-adrenergic receptors in essential hypertension

To study the aggregation, adhesion, and specific binding of an alpha 2-antagonist, [3H]rauwolscine, to the platelet membrane fractions, platelets were obtained from 30 patients with essential hypertension and nine normotensive subjects fed a high sodium diet (NaCl, 16-18 g/day) for 7 days and thereafter a low sodium diet (NaCl, 1-3 g/day) for 7 days. The patients with essential hypertension were classified as either salt responders (all those who had greater than 7% decrease in mean arterial pressure from the high to low sodium period) or salt nonresponders (all others). In salt responders, the number of alpha 2-adrenergic receptors on platelet membrane fraction was increased from 523.4 +/- 55.4 fmol/mg of protein in the high sodium period to 669.4 +/- 84.0 fmol/mg of protein in the low sodium period (p less than 0.01), whereas it did not change in salt nonresponders. In contrast, the epinephrine-induced platelet aggregation through alpha 2-adrenergic receptors was decreased in nonresponders, from 47.3 +/- 7.4% in the high sodium period to 24.5 +/- 9.3% in the low sodium period (p less than 0.05), while it did not change in responders. No significant change in the number of alpha 2-adrenergic receptors or epinephrine-induced platelet aggregation was observed in the normotensive subjects.

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.

Effect of prazosin and yohimbine on systolic blood pressure and on renal norepinephrine content in DOCA-salt rats

We studied the effect of alpha-1 and alpha-2 blockers (prazosin and yohimbine) on systolic blood pressure (SBP) and on renal norepinephrine (NE) content in Sprague-Dawley normotensive and DOCA-salt rats. The administration of desoxycorticosterone acetate (DOCA) to these rats for 6 weeks increased their SBP from 137 to 183 mmHg (p less than .001). This increase was prevented by simultaneous administration of prazosin (p less than .001), yohimbine (p less than .01), or prazosin + yohimbine (p less than .001). DOCA rats on saline and on yohimbine had lower renal NE content (p less than .05 and p less than .001, respectively) than normotensive rats. Renal NE content of DOCA rats on yohimbine decreased with respect to those treated with prazosin (p less than .001) or prazosin + yohimbine (p less than .05). Besides, renal NE content of DOCA rats on prazosin increased when compared to control DOCA rats (p less than .05). However, these drugs showed no effect on SBP and on renal NE content in normotensive rats. These findings further confirm that the alpha adrenoceptor blockade can prevent the hypertension of DOCA-salt rats in such a way that their blood pressure stabilizes at similar levels to those observed in normotensive treated animals.

Functional characterization of the alpha adrenergic receptor modulating the hydroosmotic effect of vasopressin on the rabbit cortical collecting tubule

To characterize the type of alpha adrenergic receptor, the effects of specific alpha adrenergic agonists and antagonists on antidiuretic hormone [( Arg8]-vasopressin [AVP])-induced water absorption were evaluated in cortical collecting tubules isolated from the rabbit kidney and perfused in vitro. In the presence of AVP (100 microU/ml), net fluid volume absorption (Jv, nanoliters per minute per millimeter) was 1.39 +/- 0.09 and osmotic water permeability coefficient (Pf, X 10(-4) centimeters per second) was 150.2 +/- 15.0. The addition of 10(-6) M phenylephrine (PE), an alpha adrenergic agonist, resulted in a significant decrease in Jv and Pf to 0.72 +/- 0.11 (P less than 0.005) and 69.9 +/- 10.9 (P less than 0.005). The addition of 10(-4) M prazosin (PZ), an alpha adrenergic antagonist, did not cause any significant change in Jv and Pf, which were 0.71 +/- 0.09 (P = NS vs. AVP + PE) and 67.8 +/- 9.5 (P = NS vs. AVP + PE), respectively. In a separate group of tubules, in the presence of AVP (100 microU/ml) and PE (10(-6) M), Jv and Pf were 0.78 +/- 0.17 and 76.1 +/- 18.0, respectively. The addition of 10(-6) M yohimbine (Y), an alpha 2 adrenergic antagonist, resulted in a significant increase in Jv to 1.46 +/- 0.14 (P less than 0.01) and Pf to 157.5 +/- 22.3 (P less than 0.005). Y (10(-4) M) or PZ (10(-4) M) alone did not significantly affect Jv and Pf in the presence of AVP )100 microU/ml). The effect of the natural endogenous catecholamine norepinephrine (NE) on Jv and Pf in the presence of AVP and propranolol (PR) was next examined. Jv and Pf were 1.53 +/- 0.07 and 176.3 +/- 5.2, respectively, in the presence of AVP (100 microU/ml) and PR (10(-4) M). The addition of NE (10(-8) M) resulted in a significant decrease in Jv to 1.19 +/- 0.11 (P less than 0.05) and Pf to 127.0 +/- 11.3 (P less than 0.02). Increasing the concentration of NE to 10(-6) M resulted in a further decrease in Jv and Pf to 0.70 +/- 0.10 (P less than 0.01 vs. NE 10(-8) M) and 68.5 +/- 10.6 (P less than 0.01 vs. NE 10(-8) M), respectively. The inhibitory effect of NE on AVP-induced water absorption was blocked by Y, but not by PZ. The effect of the alpha 2 adrenergic agonist clonidine (CD) on Jv and Pf was also examined. In the presence of AVP (10 microU/ml) Jv and Pf were 1.65 +/- 0.04 and 175.1 +/- 13.1, respectively. The addition of CD (10(-6) M) resulted in a significant decrease in Jv to 1.08 +/- 0.12 (P < 0.01) and Pf to 108.1 +/- 15.4 (P < 0.01). Increasing the concentration of CD to 10(-4) M resulted in a further significant decrease in Jv and Pf to 0.57 +/- 0.13 (P < 0.02 vs. CD 10(-6) M) and 54.7 +/- 13.8 (P < 0.01 vs. CD 10(-6) M), respectively. Similar results were obtained in the presence of AVP (100 microU/ml). The inhibitory effect of CD on AVP-induced water absorption was blocked by Y. CD did not significantly affect Jv and Pf in the presence of 8-bromo adenosine 3',5'-cyclic monophosphate. These studies indicate that alpha adrenergic agonists directly inhibit AVP-mediated water absorption at the level of renal tubule, an effect that can be blocked by specific alpha2 adrenergic antagonists, but not by specific alpha1 adrenergic antagonists. Alpha2 adrenergic stimulation directly inhibits AVP-mediate water absorption at the level of the tubule, an effect that can be blocked by a specific alpha2 adrenergic antagonist. This effect appears to be exerted at the level of activation of adenylate cyclase, since it is absent in the present of cyclic AMP.

Autoradiographic localization of prazosin binding sites in rat kidney

[3H]Prazosin is a selective ligand for alpha 1-adrenoceptors. Using an in vitro autoradiographic technique specific binding of [3H]prazosin has been demonstrated, mainly to the renal cortex of the rat. These results provide further evidence that, in the rat, there is a high density of alpha 1-adrenoceptors within the renal cortex.

Evidence from binding studies for beta 1-adrenoceptors associated with glomeruli isolated from rat kidney

The beta adrenoceptor antagonist radioligand [3H] dihydroalprenolol (DHA) has been used to characterise beta adrenoceptors in membranes prepared from rat renal glomeruli. Association of the ligand was rapid and had reached equilibrium within 10 mins at 37 degrees C. Dissociation occurred in two distinct phases, a rapidly dissociating phase (low affinity site) and a slowly dissociating phase (high affinity site). The KD value for the high affinity site calculated from the kinetic experiments was 0.8 nM. Saturation analysis of binding gave comparable values for KD (1.77 nM) and demonstrated that membranes from glomeruli had four times the density of binding sites measured in renal cortex. In all saturation studies Hill coefficients were not significantly different from unity. Binding was stereoselective with respect to the (-) isomers of isoprenaline and propranolol and the potency of the selective displacing agents betaxolol (beta 1 adrenoceptors) and ICI 118,551 (beta 2 adrenoceptors) indicated that the receptors are of the beta 1 subtype.

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

Binding of the alpha 2-adrenoceptor radioligands [3H]clonidine and [3H]rauwolscine but not the alpha 1-adrenoceptor radioligand [3H]prazosin was enhanced in membranes prepared from rat isolated renal glomeruli. [3H]Rauwolscine binding to glomeruli was stereoselective with respect to the (-)-isomer of noradrenaline and the order of potency of a series of antagonists for displacement of binding indicated that the alpha 2-adrenoceptors in this preparation differ somewhat from those in some other species and tissues. Chemical sympathectomy produced no significant change in the number of sites labelled by [3H]rauwolscine indicating that few of the alpha 2-adrenoceptors in glomerular membranes are associated with sympathetic nerve terminals.

Renal alpha-adrenergic receptor abnormality in the spontaneously hypertensive rat

Activation of renal alpha-adrenergic receptors induces vasoconstriction, proximal tubular reabsorption of sodium, and inhibition of renal release. Excesses of these effects are present in varying degrees in animal models of, and in patients with, "essential" hypertension. Since essential hypertension is genetically determined, we sought abnormalities of renal alpha-adrenergic receptors in the Okamoto-Aoki strain of spontaneously hypertensive rats (sr-SHR) and their stroke-prone variant (sp-SHR). Total alpha-adrenergic receptor concentrations were determined by Scatchard analysis of [3H]dihydroergocryptine binding to a renal membrane fraction and were found to be increased (p less than 0.02) in male sr-SHR at 4, 16, and 32 weeks of age and in female sr-SHR at 16 weeks of age as compared to age- and sex-matched Wistar-Kyoto controls. They were also increased in 9-week-old sp-SHR renal membranes (p less than 0.005). Further studies revealed that this increase in renal alpha-adrenergic receptors was due entirely to an increase in alpha 2-receptors as measured by [3H]yohimbine binding rather than to an increase in alpha 1-receptors as quantitated by [3H]prazosin binding. No difference in binding affinities of the various radioligands could be demonstrated between any of the hypertensive and normotensive groups of rats. Plasma norepinephrine levels were elevated (p less than 0.01) in the 4-, 9- and 16-week-old SHR, but not in the 32-week-old hypertensive rats. Thus, high renal alpha 2-adrenergic receptor number is coupled with a significant increase in plasma norepinephrine concentrations during the development of hypertension in SHR. By mediating an enhanced receptor-coupled response, such as increased proximal tubular sodium reabsorption, this abnormality of renal alpha-adrenergic receptors may contribute to some or all of the pathophysiologic derangements leading to hypertension in SHR.

Catecholamines in discrete kidney regions. Changes in salt-sensitive Dahl hypertensive rats

Steady state levels of catecholamines (dopamine, norepinephrine, and epinephrine) were measured by the use of radioenzymatic techniques in discrete areas of the kidney (outer and inner cortex, outer and inner medulla) dissected by a "punch" technique from frozen kidney sections of salt-sensitive (DS) and salt-resistant (DR) Dahl rats fed a low or high salt diet. All three catecholamines were present in all areas of the kidney examined. There were gradients of concentrations of each catecholamine in different kidney areas. Renal medullary areas contained proportionally more dopamine than cortical areas. The proportion of epinephrine with respect to the total catecholamine content was relatively high in the inner medulla. Genetic factors and the amount of dietary salt influenced the catecholamine content in specific kidney areas, and these changes were different according to the area considered. DS rats when fed a high salt diet presented increased systolic blood pressure but no increased levels of dopamine in the inner medulla nor of norepinephrine in the outer medulla and outer cortex. Results suggest that either the uptake, release, storage, synthesis, or catabolism of kidney catecholamines is altered in Dahl salt-sensitive (DS) hypertensive rats and suggest specific roles for each catecholamine in discrete areas of the kidney.

Localisation of [3H]clonidine binding to membranes from guinea pig renal tubules

The selective radioligand [3H]clonidine has been used to localise alhpa 2 adrenoceptors in guinea pig kidney. Chemical sympathectomy with 6-hydroxydopamine produced no significant change in the number of sites labeled by [3H]clonidine indicating that the majority of binding sites were not located on sympathetic nerve terminals. Binding was enhanced in membranes prepared from renal tubules and considerably reduced in preparations from glomeruli. Subcellular fractions of renal cortex revealed that binding was to plasma membranes and that the greatest binding capacity was present in the fraction rich in basal lateral membranes. It is concluded that the major concentration of renal alpha 2 adrenoceptors are present on renal tubules and that they may be localised to a particular pole of the renal tubule cell.