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

EFFECTS OF RILMENIDINE ON PROXIMAL TUBULAR FLUID ABSORPTION IN RATS

The antihypertensive agent rilmenidine has threefold higher affinity for I(1) imidazoline receptors compared with alpha(2)-adrenoceptors and acts on the central nervous system by reducing sympathetic activity and in the kidney by inhibiting Na(+)/H(+) exchange activity. In the present study, we examined: (i) the effects of luminal and peritubular administration of rilmenidine on fluid absorption in superficial proximal tubules; and (ii) the nature of the receptors involved in mediating the action of this drug in the presence of specific antagonists (efaroxan, idazoxan and 2-methoxy-idazoxan). Studies were performed in anaesthetized Sprague-Dawley rats using shrinking split-drop micropuncture. Luminal administration of rilmenidine (10(-5) and 10(-13) mol/L) inhibited proximal tubular fluid absorption. Peritubular rilmenidine at 10(-12) and 10(-13) mol/L also inhibited fluid uptake, whereas rilmenidine at 10(-11) mol/L had a significant stimulatory action. In the presence of the I(2) > I(1)/alpha(2)-adrenoceptor antagonist idazoxan (10(-5) mol/L), luminal rilmenidine (10(-5) mol/L) stimulated fluid absorption. Stimulation of fluid uptake was also observed when rilmenidine (10(-5) mol/L) and the I(1) imidazoline receptor antagonist efaroxan (10(-5) mol/L) were added together in the luminal fluid. Luminal administration of the selective alpha(2)-adrenoceptor antagonist 2-methoxy-idazoxan (10(-5) mol/L) resulted in significant attenuation of the inhibitory action of luminal rilmenidine (10(-5) mol/L). This indicates that both I(1) imidazoline receptors and alpha(2)-adrenoceptors are involved in the luminal actions of rilmenidine. The effects of luminal and peritubular administration of alpha-methylnoradrenaline (an alpha(2)-adrenoceptor agonist) were compared with those of rilmenidine. Luminal alpha-methylnoradrenaline, at higher concentrations (10(-7) and 10(-5) mol/L), inhibited fluid absorption, as was seen with peritubular rilmenidine, but, in contrast with rilmenidine, no stimulatory action was observed. Peritubular alpha-methylnoradrenaline inhibited fluid uptake at higher concentrations (10(-5) and 10(-7) mol/L), whereas rilmenidine at these concentrations had no effect. The differences in the concentration-dependent responses for rilmenidine and alpha-methylnoradrenaline indicate that both imidazoline receptors and alpha(2)-adrenoceptors are involved in the actions of these compounds on proximal fluid uptake.

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.

Expression of multiple alpha-adrenoceptor isoforms in rat CCD

In the rat cortical collecting duct (CCD), epinephrine inhibits vasopressin (AVP)-dependent water permeability and Na+ reabsorption. Although inhibition is reversed by the alpha2-adrenoceptor (AR) antagonist yohimbine, suggesting the epinephrine effect is primarily mediated by an alpha2-AR [C. T. Hawk, L. H. Kudo, A. J. Rouch, and J. A. Schafer. Am. J. Physiol. 265 (Renal Fluid Electrolyte Physiol. 34): F449-F460, 1993], there are also suggestions of an effect at an additional receptor, perhaps an alpha1-AR. For the present experiments, we used RT-PCR of total RNA extracted from 1 to 5 mm of microdissected CCDs from rat kidney to identify the alpha-AR isoforms expressed. Specific primers for the alpha2-ARs amplifying from the 6th transmembrane (TM) to the 3'-untranslated regions, revealed the presence of alpha2A and alpha2B. Western blot analysis also indicated the presence of alpha2B-AR at the protein level. Degenerate alpha1-AR primers that amplify from conserved regions of TM-1 to TM-5, as well as specific primers that amplify either the same region (alpha1B), the carboxy terminus (alpha1A), or within the third cytoplasmic loop (alpha1D), indicated the presence of all three alpha1-ARs. Measurement of transepithelial voltage in isolated perfused renal tubules indicated a small inhibitory effect mediated by alpha1-ARs. Although the functional effects of epinephrine on AVP-dependent transport processes appear to be mediated predominantly by an alpha2-AR, a small contribution to the overall alpha-AR effect may be due to simultaneous activation of an alpha1-AR.

Alpha 2-adrenergic, but not imidazole, agonists activate NaCl cotransport in rabbit tracheal epithelial cells

The adrenergic agonist clonidine activates NaCl cotransport in rabbit tracheocytes. With the use of the high-affinity analogue p-[125I]iodoclonidine, binding of clonidine to cells was determined to fit a two-site model, with one site of high specificity for alpha 2-adrenergic (alpha 2-AR) and the other with a high affinity for I1-imidazol(in)e (I1) receptors. Total density of binding sites for both receptors was similar at 18 fmol/mg protein. Moxonidine displayed a 166-fold greater specificity for I1 receptors compared with cimetidine. Bumetanide-sensitive Na or Cl transport was stimulated by the alpha 2-AR agonists clonidine or guanabenz but not by the I1 agents cimetidine or moxonidine. I1 agonists-stimulated Na transport was detected only in the presence of bumetanide. Prazosin did not block clonidine-stimulated NaCl uptake or efflux, indicating the presence of an alpha 2A-AR subtype. Addition of clonidine either before or after incubation with l-isoproterenol or forskolin did not attenuate the time- and dose-dependent increase in adenosine 3',5'-cyclic monophosphate (cAMP) levels. Thus clonidine stimulates NaCl cotransport in rabbit tracheocytes through an alpha 2A-AR mechanism that does not require cAMP for signal transduction. In addition, I1-imidazol(in)e receptors stimulate Na transport in rabbit tracheocytes through an unidentified pathway.

Renal Effects of α-Adrenoceptor Blockade During Furosemide Diuresis in Conscious Rats

Clearance experiments were performed in conscious rats in order to investigate whether intravenous infusion of the non-selective alpha-adrenoceptor antagonist phentolamine could block compensatory sodium reabsorption during furosemide-induced volume contraction. By measuring inulin clearance, urinary excretion rates of sodium and water, and lithium clearance, the effects on proximal and distal nephron segments were dissociated. The renal effect of intravenous infusion of 0.3 mg/kg/hr phentolamine (n = 6) was compared with time control animals (n = 9). Furosemide was administered as constant intravenous infusion (7.5 mg/kg/hr) with simultaneous phentolamine infusion at four dose levels: 0 (n = 9), 0.3 (n = 6), 1.0 (n = 7) and 3.0 mg/kg/hr (n = 6). Phentolamine infusion reduced norepinephrine-induced increase in blood pressure at all three dose levels (n = 5). Phentolamine infusion induced transient antidiuresis and a prolonged antinatriuretic response. Compared with rats given furosemide only, phentolamine attenuated dose-dependently the diuretic and natriuretic peak response to furosemide. This effect was associated with dose-dependent reductions in mean arterial pressure. The reduced natriuretic response was due to a reduced fractional sodium excretion in the distal nephron segment (at all doses of phentolamine) and a reduction of the glomerular filtration rate (1.0 and 3.0 mg/kg/hr phentolamine). The fractional lithium excretion (FELi) increased to 65 +/- 3% at 0.3 mg/kg/hr phentolamine during the natriuretic peak response of furosemide, while it only increased to 52 +/- 3% during furosemide alone. At steady-state conditions (120-180 min. after start of furosemide infusion) after infusion with furosemide plus 0.3 mg/kg/hr phentolamine the animals were still volume-depleted, but the compensatory tubular Na reabsorption in the proximal tubules was inhibited (FELi = 48 +/- 2% versus 39 +/- 1% in rats given furosemide alone). During furosemide infusion plasma epinephrine increased 700% and plasma norepinephrine increased 50%. These results are compatible with increased systemic sympathetic nervous activity and a contributory role of proximal tubular alpha-adrenoceptors in mediating compensatory sodium reabsorption during acute furosemide-induced volume contraction.

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.

Imidazoline-guanidinium and alpha 2-adrenergic binding sites in basolateral membranes from human kidney

In the present study, we used [3H]idazoxan and [3H]rauwolscine to characterize the imidazoline-guanidinium receptive site (IGRS) and alpha 2-adrenoceptors in the human renal proximal tubule, respectively. In purified basolateral membranes, 11-fold enriched in Na(+)-K+ ATPase. [3H]idazoxan and [3H]rauwolscine binding was twofold higher than in homogenates ([3H]idazoxan: 87 +/- 19 vs. 45 +/- 23.3 fmol/mg protein, P less than 0.05; [3H]rauwolscine: 56.4 +/- 21.4 vs. 25.2 +/- 7.3 fmol/mg protein, P less than 0.01). In competition studies performed at saturating concentration of [3H]idazoxan (15 NM), specific binding was competed for by epinephrine and rauwolscine only by 10-15% but was completely inhibited by imidazoline and guanidinium compounds. Thus, in human renal proximal tubule. [3H]idazoxan mainly binds to an IGRS. The highest density of alpha 2-adrenoceptors in basolateral membranes and of IGRS in partially purified membrane preparations, suggests that these two binding sites have a different subcellular localization. When compared to the rabbit renal IGRS, the human [3H]idazoxan binding site displays different affinities for guanabenz, rilmenidine, clonidine, amiloride and its derivatives that persist after membrane solubilization. In contrast, the human and rabbit renal IGRS share similar regulatory properties such as the sensitivity to K+ and the insensitivity to Na+, divalent cations and 5'-guanylylimidodiphosphate (Gpp(NH)p). In conclusion, we demonstrated that, in the human renal proximal tubule, alpha 2-adrenoceptors are mainly located in basolateral membranes while IGRS appear to be associated with another cell compartment. As indicated by their common interaction with imidazoline and guanidinium derivatives and by similar regulatory properties, human and rabbit IGRS belong to the same family of membrane proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Clonidine inhibits fluid absorption in the rabbit proximal convoluted renal tubule

Previous studies have shown that norepinephrine (NE) and the beta-adrenoceptor agonist, isoproterenol (I), enhance fluid absorption (JV) in isolated, perfused proximal convoluted tubule segments (PCT). Pretreatment of PCT with the beta-adrenoceptor antagonist, propranolol, inhibited the action of NE and produced a significant decline in JV, suggesting modulation of JV by both alpha- and beta-adrenoceptors. The present studies further characterize the alpha-adrenoceptor control of JV in isolated perfused PCT using specific agonists and antagonists. Basal JV declined significantly with the addition of the alpha 2-adrenoceptor agonist, clonidine (10(-4) M), to the bath; however, it was unchanged with the addition of the alpha 1-adrenoceptor agonist, methoxamine (10(-6) or 10(-4) M). With the addition of 10(-6) M isoproterenol JV increased significantly, and returned to control values with the subsequent addition of clonidine (10(-6) or 10(-4) M). Pretreatment of PCT with the alpha 2-adrenoceptor antagonist, yohimbine (10(-5) M), or with pertussis toxin (100 ng/ml) did not interfere with the stimulation of JV by isoproterenol, but abolished the inhibition of isoproterenol-stimulated JV by clonidine. Thus, clonidine inhibits JV in PCT via an alpha 2-adrenoceptor. This effect is mediated by a pertussis toxin inhibitable GTP-binding protein, but not one that is coupled to adenylyl cyclase.

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.

Adrenoceptors in experimental hypertension

Cardiac, aortic and renal alpha- and beta-adrenoceptors were examined in three types of experimental hypertension in rats. In spontaneously hypertensive rats (SHR), cardiac alpha 1-adrenoceptor concentration was increased and cardiac and aortic beta-adrenoceptor concentrations were decreased compared with Wistar-Kyoto rats (WKY). A similar decrease in cardiac beta-adrenoceptor concentration was also shown in two-kidney, one clip (2K, 1C) and deoxycorticosterone acetate(DOCA)-salt hypertensive rats. Renal alpha 1- and alpha 2-adrenoceptor concentrations were increased in SHR, but not in 2K, 1C and DOCA-salt hypertension. In contrast, renal beta-adrenoceptor concentration was increased in 2K, 1C and DOCA-salt hypertension, but unchanged in SHR. The observed increase in cardiac alpha 1- and renal alpha 2-adrenoceptor concentrations may partly contribute to the elevation of blood pressure in SHR.

Inhibition and labeling of the rat renal Na+/H+-exchanger by an antagonist of muscarinic acetylcholine receptors

A covalently binding label for muscarinic acetylcholine receptors, propylbenzilylcholine mustard (PrBCM), irreversibly inhibits the Na+/H+ exchanger in rat renal brush-border membrane vesicles. Substrates of the antiporter, Na+ and Li+, as well as inhibitors, amiloride, 5-(N-ethyl-N-isopropyl)amiloride (EIPA) and propranolol, protect the antiporter from inactivation by PrBCM. With [3H]PrBCM a band with an app. Mr of 65 kDa is predominantly labeled. Amiloride protects this band from labeling with [3H]PrBCM and [14C]-N,N'-dicyclohexylcarbodiimide (DCCD) proving its identity with the renal Na+/H+ exchanger. Our data reveal a specific interaction of PrBCM with the Na+/H+ exchanger and suggest structural relations between antiporter and receptors.

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.