Functional identification of beta3-adrenoceptors in the guinea-pig ileum using the non-selective beta-adrenoceptor antagonist (+/-)-bupranolol.

Abstract

1. To clarify whether there is a species difference or a tissue difference in beta3-adrenoceptors, the beta3-adrenoceptors mediating relaxations to catecholamines ((-)-isoprenaline, (-)-noradrenaline and (-)-adrenaline), a selective beta3-adrenoceptor agonist BRL37344 and a non-conventional partial beta3-adrenoceptor agonist (+/-)-CGP12177A (a potent beta1- and beta2-adrenoceptor antagonist with a partial beta3-adrenoceptor agonist property) were investigated in the guinea-pig ileum. 2. Catecholamines and beta3-adrenoceptor agonists induced concentration-dependent relaxations of pre-contracted strips of the guinea-pig ileum. The rank order for their relaxing potency was (-)-isoprenaline (pD2: 7.60) > BRL37344 (7.05) > (-)-noradrenaline (6.38) > (+/-)-CGP12177A (6.25) > (-)-adrenaline (6.07). 3. In the presence of the non-selective beta1- and beta2-adrenoceptor antagonist (+/-)-propranolol (1 microM), only small rightward shifts of the concentration-response curves (CRCs) to these agonists were observed and the rank order of potency of agonists was BRL37344 (pD2: 7.00) > (+/-)-CGP12177A (6.17) > (-)-isoprenaline (6.01) > (-)-noradrenaline (5.69) > (-)-adrenaline (5.41). 4. In the presence of (+/-)-propranolol (1 microM), the additional presence of (+/-)-bupranolol (3-30 microM), a non-selective beta1-, beta2- and beta3-adrenoceptor antagonist, caused a concentration-dependent rightward shift of the CRCs to catecholamines and beta3-adrenoceptor agonists. Schild plot analyses of (+/-)-bupranolol against these agonists gave pA2 values of 6.02 ((-)-isoprenaline), 6.03 ((-)-noradrenaline), 6.01 ((-)-adrenaline), 6.56 (BRL37344) and 5.74 ((+/-)-CGP12177A), respectively. All Schild plot slopes were not significantly different from unity. The pA2 values of (+/-)-bupranolol obtained for the guinea-pig beta3-adrenoceptors were about one log unit less than the values obtained for the rat beta3-adrenoceptors and about two log units less than the values obtained for dog beta3-adrenoceptors. 5. These results confirm that functional beta3-adrenoceptors are present in the guinea-pig ileum and that the relaxations of these agonists are mainly mediated via beta3-adrenoceptors in this tissue. The differential antagonistic potency of (+/-)-bupranolol may suggest that there is a species difference between the three species (guinea-pig, dog and rat) in their beta3-adrenoceptors.