Pharmacological characterization and anatomical localization of prejunctional beta-adrenoceptors in the rat kidney.

1. The subtype and anatomical localization of beta-adrenoceptors mediating facilitation of stimulus-induced overflow of noradrenaline ('prejunctional beta-adrenoceptors') are not conclusively known to date. The present study was undertaken to characterize these receptors by use of pharmacological methods as well as to define their localization (prejunctional or postjunctional) with radio-ligand binding and autoradiography techniques combined with surgical denervation of the sympathetic innervation to the rat kidney. 2. Exposure of the kidney to (-)-isoprenaline, the nonselective beta-adrenoceptor agonist, resulted in a dose-dependent facilitation of stimulus-induced neurotransmitter overflow. This response was inhibited by propranolol, the beta 1- and beta 2-adrenoceptor antagonist, with a pA2 of 9.20 suggesting that the prejunctional beta-adrenoceptors are not of the beta 3-subtype. 3. The rank order of potency and potency ratios of beta-adrenoceptor agonists at renal prejunctional beta-adrenoceptors (EC50 for agonist/EC50 for (-)-isoprenaline) were: (-)-isoprenaline (1) > procaterol (2) > salbutamol (3) > adrenaline (10) > (+)-isoprenaline (25). However, dobutamine, the beta 1-adrenoceptor agonist, failed to enhance stimulus-induced overflow of noradrenaline. These results are indicative of the presence of beta 2-adrenoceptors as prejunctional beta-adrenoceptors. 4. Facilitation elicited by (-)-isoprenaline and procaterol, the selective beta 2-adrenoceptor agonist, was inhibited by ICI 118,551, the selective beta 2-adrenoceptor antagonist, with pKb values of 9.20 and 9.35, respectively at renal prejunctional beta-adrenoceptors. Similarly, the pKb values of metoprolol, the selective beta 1-adrenoceptor antagonist, at renal prejunctional beta-adrenoceptors were determined to be 6.25 and 6.18 against (-)-isoprenaline and procaterol, respectively. These results suggest the presence of a homogeneous population of beta 2-adrenoceptors as prejunctional beta-adrenoceptors. 5. Radio-ligand binding analysis of renal beta-adrenoceptors revealed the prevalence of the beta 1-subtype as compared to the beta 2-subtype (63% vs 37%). However, surgical denervation of the rat kidney, resulting in more than 90% reduction in renal noradrenaline content, selectively reduced the beta 2-adrenoceptor population by 80%, implying the presence of beta 2-adrenoceptors on renal sympathetic nerve terminals. 6. Autoradiographic analysis demonstrated the presence of beta 1-adrenoceptors on cortical structures such as glomeruli and tubules. beta-Adrenoceptors were found to be present on tubules (minor population), collecting tubules in outer medulla and the adventitia and adventitial-medial border of intraparenchymal branches of the renal artery. Surgical denervation of the rat kidney resulted in the disappearance of Beta2-adrenoceptors associated with the intraparenchymal branches, without affecting the Beta-adrenoceptor populations at other sites. These results support the notion that the Beta2-subtype is present on renal sympathetic nerve terminals and demonstrate that these prejunctional Beta2-adrenoceptors are associated with the renal vasculature and not with renal tubules.7. The results of the present investigation demonstrate that renal prejunctional Beta-adrenoceptors are of the Beta2-subtype in nature. These receptors are present on sympathetic nerve terminals which are associated with the renal vasculature.