The β₂ and β₃-Adrenoceptor-Mediated Relaxation Induced by Isoprenaline and Salbutamol in Guinea Pig Taenia Caecum

Abstract

To understand the receptor subtypes responsible for β-adrenoceptor-mediated relaxation of guinea pig taenia caecum, we investigated the effects of isoprenaline and salbutamol . Isoprenaline and salbutamol caused dose-dependent relaxation of the guinea pig taenia caecum. Propranolol, bupranolol and butoxamine produced shifts of the concentration response curves for isoprenaline and salbutamol. Schild regression analyses carried out for propranolol against isoprenaline and salbutamol gave pA₂ values of 8.43 and 8.88, respectively. Schild regression analyses carried out for butoxamine against isoprenaline and salbutamol gave pA₂ values of 6.46 and 6.68, respectively. Schild regression analyses carried out for bupranolol against isoprenaline and salbutamol gave pA₂ values of 8.60 and 8.69, respectively. However, in the presence of 3 × 10^-4 M atenolol, 10^-4 M butoxamine and 10^-6 M phentolamine to block the β₁-, β₂- and α -adrenoceptor effects, respectively, Schild regression analyses carried out for bupranolol against isoprenaline and salbutamol gave pA₂ values of 5.77 and 5.97, respectively. These results suggest that the relaxant responses to isoprenaline and salbutamol in the guinea pig taenia caecum are mediated by both the β₂- and the β₃-adrenoceptors.