BETA-ADRENERGIC, ANGIOTENSIN-II, AND BRADYKININ RECEPTORS ENHANCE NEUROTRANSMISSION IN HUMAN KIDNEY

The aim of this study was to investigate angiotensin II (Ang II) recep tor-, bradykinin receptor-, and beta-adrenergic receptor-mediated modu lation of norepinephrine release from human renal sympathetic nerves a nd to characterize the respective receptor subtypes involved. Human co rtical kidney slices were incubated with [H-3]norepinephrine, placed i n superfusion chambers between two platinum electrodes, and superfused with Krebs-Henseleit solution. The sympathetic nerves were stimulated electrically at 2.5 Hz for 1 minute, and the stimulation-induced outf low of radioactivity was taken as an index of endogenous norepinephrin e release. Ang II and its precursor Ang I (both 0.01 to 1 mu mol/L) en hanced stimulation-induced outflow of radioactivity in a concentration -dependent manner, with EC(50) values of 0.03 and 0.05 mu mol/L, respe ctively. The enhancement by Ang I but not that by Ang II was inhibited by the angiotensin-converting enzyme inhibitor captopril (3 mu mol/L) . The concentration-response curves of Ang I and Ang II were shifted t o the right by EXP 3174 (0.01 mu mol), the in vitro active form of the Ang II type 1 receptor antagonist losartan, with affinity estimates o f 8.72 and 9.30, respectively. A higher concentration of EXP 3174 (0.1 mu mol/L) abolished the facilitatory effects of Ang I and Ang II. The Ang II type 2 receptor antagonist PD 123319 (10 mu mol/L) did not alt er the facilitation by Ang II. In the absence of other drugs, bradykin in (0.01 to 1 mu mol/L) failed to modulate stimulation-induced outflow of radioactivity but in the presence of captopril (3 mu mol/L) enhanc ed it in a concentration-dependent manner, with an EC(50) of 0.1 mu mo l/L. This facilitatory effect of bradykinin was prevented by the brady kinin type 2 receptor antagonist Hoe 140 (0.3 mu mol/L). The beta(1)/b eta(2)-adrenergic receptor agonist isoproterenol (0.001 to 0.1 mu mol/ L) also enhanced stimulation-induced outflow of radioactivity in a con centration-dependent manner, with an EC(50) of 0.008 mu mol/L. The fac ilitatory effect of isoproterenol was abolished by the beta(2)-adrener gic receptor antagonist ICI 118551 (0.03 mu mol/L) but unaltered by th e beta(1)-adrenergic receptor antagonist atenolol (3 mu mol/L), captop ril (3 mu mol/L), or EXP 3174 (0.1 mu mol/L). We conclude that activat ion of prejunctional Ang II type 1, bradykinin type 2, and beta(2)-adr energic receptors facilitates renal norepinephrine release in humans. Ang II can be formed locally within the human renal cortex from Ang I by angiotensin-converting enzyme to activate prejunctional Ang II type 1 receptors. The beta(2)-adrenergic receptor-mediated effect is likel y to function independently of a local renin-angiotensin system. Howev er, the facilitatory bradykinin type 2 receptor mechanism can be demon strated only when bradykinin degradation is prevented by the angiotens in-converting enzyme (kininase II) inhibitor captopril.