Involvement of signal transducing GTP‐binding proteins in renal artery α₁‐adrenoceptor mediated smooth muscle contraction

Abstract

rings were assessed in a standard organ bath, the rings pre-tensioned at 2 g. Contractions were evoked by applying the a 1 -adrenoceptor selective agonist phenylephrine (1 nmol/L to 0.3 mmol/L). Isometric contractions of the tissue were registered and stored digitally. Dose-response curves were obtained sequentially with a wash-out of 20 min between each concentration; the maximum contractility of an individual muscle ring was set at 100%. Dose-response curves of inhibitory agents (e.g. WB4101, cholera and pertussis toxins) were determined by comparing the remaining contractility after incubating with the respective drug with a control contraction that was evoked three times (10 m mol/L phenylephrine) and the mean set at 100%. RESULTSPhenylephrine induced dose-dependent and fully reversible isometric contractions with a threshold concentration of 100 nmol/L and an EC 50 of 0.8 m mol/L. The receptor was identified as the a 1A -subtype by the selective antagonist WB4101. Pre-treatment of tissue rings with 5 m g/mL pertussis toxin (120 min, 37 ∞ C) inhibited the control contraction by a mean ( SEM ) of 52.0 (4.6)%, whereas pre-treatment with 1 m g/mL cholera toxin (60 min, 37 ∞ C), leading to a permanent activation of the G Sprotein via blockade of the GTPase activity, decreased the response by 39.0 (8.2)%. CONCLUSIONThese results suggest a coupling of a 1A -adrenoceptors in renal vascular tissue to the heterotrimeric G S -protein and to heterotrimeric G-proteins of the G I -and/or G O -family in the phenylephrine-induced contraction.