Further evidence that (+/-)-carteolol-induced relaxation is mediated by beta2-adrenoceptors but not by beta3-adrenoceptors in the guinea pig taenia caecum.

The properties of the beta1- and beta2-adrenoceptor partial agonist (+/-)-carteolol were investigated against the beta2- and beta3-adrenoceptors of the taenia caecum of the guinea pig. (--)-Isoprenaline and (+/-)-carteolol induced concentration-dependent relaxation in this tissue. The non-selective beta1- and beta2-adrenoceptor antagonist (+/-)-propranolol (10-100 nM), the selective beta2-adrenoceptor antagonist ICI 118,551 (10-100 nM) and the non-selective beta1-, beta2- and beta3-adrenoceptor antagonist (+/-)-bupranolol (10-100nM), caused a concentration-dependent rightward shift of the concentration-response curves for (--)-isoprenaline and (+/-)-carteolol. Schild regression plot analyses carried out for (+/-)-propranolol against (--)-isoprenaline and (+/-)-carteolol gave pA2 values of 8.35 and 8.24, respectively. Schild plot analyses of ICI 118,551 against (--)-isoprenaline and (+/-)-carteolol gave pA2 values of 8.47 and 8.41, respectively. Schild plot analyses of (+/-)-bupranolol against (--)-isoprenaline and (+/-)-carteolol gave pA2 values of 8.47 and 8.53, respectively. Slopes of the Schild plots were not significantly different from unity. These results suggest that the relaxant effects of (+/-)-carteolol in the guinea pig taenia caecum are mediated by beta2-adrenoceptors but not by beta3-adrenoceptors.