Functional characterization of beta-adrenoceptors mediating relaxation in sheep gallbladder.

The purpose of this study was to characterize beta-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 x 10(-7) M) contracted smooth muscles of the sheep gall bladder. Isoprenaline-induced relaxation was significantly antagonized by propranolol with -logKB 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 beta 1-adrenoceptor antagonist, also significantly antagonized isoprenaline-induced relaxation with -logKB values of 5.82 +/- 0.11 and 6.09 +/- 0.09 in the fundic and ductal strips respectively. However, ICI 118551, a selective beta 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 beta 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 x 10(-7) M). This would confirm that the response was mediated via beta 3-adrenoceptors. It was concluded that beta 1- and beta 3-adrenoceptors coexist in the sheep gallbladder and mediate smooth muscle relaxation.