Functional characterization of β‐adrenoceptors mediating relaxation in sheep gallbladder

— The purpose of this study was to characterize β‐adrenoceptor subtype(s) mediating relaxation in smooth muscle strips of the sheep gallbladder. Experiments were performed on isolated smooth muscle strips suspended in tissue baths containing Krebs' solution. Isoprenaline (10 −8 M‐10 −5 M) and salbutamol (10 −7 M‐10 −4 M) produced concentration‐dependent relaxation of carbachol (10 −7 M‐3 × 10 −7 M) contracted smooth muscles of the sheep gall bladder. Isoprenaline‐induced relaxation was significantly antagonized by propranolol with ‐log K B values of 7.81 ± 0.11 ( n = 7) and 7.73 ± 0.12 ( n = 6) in the fundic and ductal strips respectively. Atenolol (10 −5 M), a selective β 1 ‐ adrenoceptor antagonist, also significantly antagonized isoprenaline‐induced relaxation with ‐log K B values of 5.82 ± 0.11 and 6.09 ± 0.09 in the fundic and ductal strips respectively. However, ICI 118551, a selective β 2 ‐adrenoceptor antagonist, at concentrations up to 10 −6 M had little or no effect on isoprenaline‐induced relaxation in either of these preparations. BRL 37344A, a selective β 3 ‐adrenoceptor agonist produced concentration‐dependent relaxation of carbachol‐precontracted fundic and ductal strips. BRL 37344 was approximately 9‐fold more potent in the ductal than fundic strips. In both preparations, BRL 37344‐induced relaxation was not significantly ( p > 0.05) antagonized by propranolol (3 × 10 −7 M). This would confirm that the response was mediated via β 3 ‐adrenoceptors. It was concluded that β 1 ‐ and β 3 ‐adrenoceptors coexist in the sheep gallbladder and mediate smooth muscle relaxation.