Beta-adrenoceptors in the rat kidney. Immunohistochemical study

Immunofluorescent imaging of beta 1- and beta 2-adrenergic receptors in rat kidney

BACKGROUND

beta-Adrenergic receptors (beta-ARs) are known to participate in the regulation of glomerular filtration, NaCl reabsorption, acid-base balance, and renin secretion; however, the precise histologic localization of beta-AR at putative signaling sites involved in these processes remains an open issue.

METHODS

We used a set of subtype-specific rabbit antibodies to visualize beta(1)- and beta(2)-AR in rat kidney by immunohistochemistry and specified cells and segments of the nephron thought to be regulated by catecholamines. In addition, the relative proportion of beta-AR subtypes in cortical and medullary portions of rat kidney was determined by Western blotting and by competing [(125)I]-cyanopindolol binding with the beta(1)- or beta(2)-selective antagonists bisoprolol or ICI 118,551, respectively.

RESULTS

Immunoreactivity for beta(1)-AR was found in mesangial cells, juxtaglomerular granular cells, the macula densa epithelium, proximal and distal tubular segments, and acid-secreting type A intercalated cells of the cortical and medullary collecting ducts. Immunoreactivity for beta(2)-AR was predominantly localized in the apical and subapical compartment of proximal and, to a lesser extent, distal tubular epithelia (suggesting interactions with luminal fluid catecholamines). Both subtypes were dense in the membranes of smooth muscle cells from renal arteries. Concordant data were obtained by radioligand binding and immunoblotting of membranes prepared from cortical and medullary portions of the kidney.

CONCLUSION

Our data provide an immunohistochemical basis for the cellular targets of beta-adrenergic regulation of renal function. Moreover, they could help to devise therapeutic strategies directed at renal beta-ARs.

beta-Adrenergic agonists stimulate Mg(2+) uptake in mouse distal convoluted tubule cells

beta-Adrenergic agonists influence electrolyte reabsorption in the proximal tubule, loop of Henle, and distal tubule. Although isoproterenol enhances magnesium absorption in the thick ascending limb, it is unclear what effect, if any, beta-adrenergic agonists have on tubular magnesium handling. The effects of isoproterenol were studied in immortalized mouse distal convoluted tubule (MDCT) cells by measuring cellular cAMP formation with radioimmunoassays and Mg(2+) uptake with fluorescence techniques. Intracellular free Mg(2+) concentration ([Mg(2+)](i)) was measured in single MDCT cells by using microfluorescence with mag-fura-2. To assess Mg(2+) uptake, MDCT cells were first Mg(2+) depleted to 0.22 +/- 0.01 mM by culturing in Mg(2+)-free media for 16 h and then placed in 1.5 mM MgCl(2), and the changes in [Mg(2+)](i) were determined. [Mg(2+)](i) returned to basal levels, 0.53 +/- 0.02 mM, with a mean refill rate, d([Mg(2+)](i))/dt, of 168 +/- 11 nM/s. Isoproterenol stimulated Mg(2+) entry in a concentration-dependent manner, with a maximal response of 252 +/- 11 nM/s, at a concentration of 10(-7) M, that represented a 50 +/- 7% increase in uptake rate above control values. This was associated with a sixfold increase in intracellular cAMP generation. Isoproterenol-stimulated Mg(2+) uptake was completely inhibited with RpcAMPS, a protein kinase A inhibitor, and U-73122, a phospholipase C inhibitor, and partially blocked by RO 31-822, a protein kinase C inhibitor. Accordingly, isoproterenol-mediated Mg(2+) entry rates involve multiple intracellular signaling pathways. Aldosterone potentiated isoproterenol-stimulated Mg(2+) uptake (326 +/- 31 nM/s), whereas elevation of extracellular Ca(2+) inhibited isoproterenol-mediated cAMP accumulation and Mg(2+) uptake, 117 +/- 37 nM/s. These studies demonstrate that isoproterenol stimulates Mg(2+) uptake in a cell line of mouse distal convoluted tubules that is modulated by hormonal and extracellular influences.

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.

Evidence for the presence of functional beta-adrenoceptor along the proximal tubule of the rat kidney

Evidence for the presence of beta adrenoceptors on proximal tubules from the rat kidney has been obtained using enriched tubule suspensions prepared by Percoll centrifugation. Intact tubules demonstrated simultaneous enrichment of parathyroid hormone and isoproterenol sensitive cAMP production with no enrichment of antidiuretic hormone sensitive cAMP production. Both norepinephrine and epinephrine were less potent than isoproterenol and the stimulatory effect of catecholamines could be blocked with propranolol but not phentolamine. The stimulatory effect of norepinephrine on cellular phenylalanine uptake is blunted by co-addition of isoproterenol suggesting that the beta receptor may modulatory catecholamine stimulated transport.

Beta-adrenoceptors in kidney tubules of spontaneously hypertensive and normotensive rats

Beta-adrenoceptor binding characteristics were determined in different fractions of rat kidney tubules using a [125Iodo]-(-)-cyanopindolol (ICYP) binding assay. The highest amount of binding sites was found in a fraction containing predominantly distal tubular fragments. In a separate series of experiments the ICYP binding characteristics were compared in whole tubular fractions from spontaneously hypertensive (SHR) and normotensive Wistar Kyoto rats (WKY) of different ages. The maximum number of binding sites was significantly higher both in young (3 weeks) and adult (14 weeks) SHR when compared to age-matched WKY. These studies showed the presence of beta-adrenoceptor binding sites in rat kidney tubules and support the potential importance of tubular beta-adrenoceptors in the development of spontaneous hypertension and in the mechanism of antihypertensive action of beta-blockers.

Autoradiographic localization of beta-adrenoceptor subtypes in guinea-pig kidney

The distribution of beta-adrenoceptor subtypes in slide-mounted sections of guinea-pig kidney has been examined by the technique of in vitro labelling combined with autoradiography. Binding of (-)-[125I]-cyanopindolol (Cyp) to kidney sections equilibrated and dissociated slowly, was saturable and stereoselective with respect to the isomers of propranolol and pindolol. These characteristics were appropriate for binding to beta-adrenoceptors. Delineation of beta-adrenoceptor subtypes was achieved by use of betaxolol (beta 1-adrenoceptors) and ICI 118,551 (beta 2-adrenoceptors) and computer assisted curve fitting techniques. Both beta 1- and beta 2-adrenoceptors were present in the proportions 1:2. 3H-Ultrofilm images of (-)-[125I]-Cyp binding to guinea-pig kidney sections showed localized patches of binding in the cortex and concentrated binding in the outer stripe of the medulla. Cortical receptors were of the beta 1 subtype and those associated with the outer stripe of the medulla were of the beta 2-adrenoceptor subtype. beta 1-Adrenoceptors were concentrated over glomeruli and beta 2-adrenoceptors over the straight portion of the proximal tubule.