β-Adrenergic, Angiotensin II, and Bradykinin Receptors Enhance Neurotransmission in Human Kidney

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Abstract
Abstract The aim of this study was to investigate angiotensin II (Ang II) receptor–, bradykinin receptor–, and β-adrenergic receptor–mediated modulation of norepinephrine release from human renal sympathetic nerves and to characterize the respective receptor subtypes involved. Human cortical kidney slices were incubated with [ 3 H]norepinephrine, placed in 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 outflow of radioactivity was taken as an index of endogenous norepinephrine release. Ang II and its precursor Ang I (both 0.01 to 1 μmol/L) enhanced stimulation-induced outflow of radioactivity in a concentration-dependent manner, with EC 50 values of 0.03 and 0.05 μmol/L, respectively. The enhancement by Ang I but not that by Ang II was inhibited by the angiotensin-converting enzyme inhibitor captopril (3 μmol/L). The concentration-response curves of Ang I and Ang II were shifted to the right by EXP 3174 (0.01 μmol), the in vitro active form of the Ang II type 1 receptor antagonist losartan, with affinity estimates of 8.72 and 9.30, respectively. A higher concentration of EXP 3174 (0.1 μmol/L) abolished the facilitatory effects of Ang I and Ang II. The Ang II type 2 receptor antagonist PD 123319 (10 μmol/L) did not alter the facilitation by Ang II. In the absence of other drugs, bradykinin (0.01 to 1 μmol/L) failed to modulate stimulation-induced outflow of radioactivity but in the presence of captopril (3 μmol/L) enhanced it in a concentration-dependent manner, with an EC 50 of 0.1 μmol/L. This facilitatory effect of bradykinin was prevented by the bradykinin type 2 receptor antagonist Hoe 140 (0.3 μmol/L). The β 1 /β 2 -adrenergic receptor agonist isoproterenol (0.001 to 0.1 μmol/L) also enhanced stimulation-induced outflow of radioactivity in a concentration-dependent manner, with an EC 50 of 0.008 μmol/L. The facilitatory effect of isoproterenol was abolished by the β 2 -adrenergic receptor antagonist ICI 118551 (0.03 μmoL/L) but unaltered by the β 1 -adrenergic receptor antagonist atenolol (3 μmol/L), captopril (3 μmol/L), or EXP 3174 (0.1 μmol/L). We conclude that activation of prejunctional Ang II type 1, bradykinin type 2, and β 2 -adrenergic 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 β 2 -adrenergic receptor–mediated effect is likely to function independently of a local renin-angiotensin system. However, the facilitatory bradykinin type 2 receptor mechanism can be demonstrated only when bradykinin degradation is prevented by the angiotensin-converting enzyme (kininase II) inhibitor captopril.