Effect of bupranolol for BRL37344 and noradrenaline-induced relaxations mediating atypical beta/beta3-adrenoceptor in rat oesophageal muscularis mucosae.

We previously suggested that the existence of atypical beta/beta3-adrenoceptor with pA2-values for bupranolol, a non-selective beta-adrenoceptor antagonist, against BRL37344 and noradrenaline were 5.79 and 5.53 in guinea pig taenia caecum, respectively. We furthermore determined the affinity of bupranolol to subclassify atypical beta/beta3-adrenoceptor in rat oesophageal muscularis mucosae, because it is rich in atypical beta/beta3-adrenoceptor. BRL37344 and noradrenaline produced a concentration-dependent relaxation of rat oesophageal muscularis mucosae. The responses to BRL37344 and noradrenaline were resistant to 3x10(-6) M propranolol, 10(-4) M atenolol, and 10(-4) M butoxamine. However, bupranolol antagonized the responses to BRL37344 and noradrenaline in a concentration-dependent manner. Schild plot analyses of bupranolol against BRL37344 and noradrenaline gave pA2-values of 7.06 and 6.96, respectively. These results suggest that bupranolol can distinguish the difference in affinity between atypical beta/beta3-adrenoceptors in rat oesophageal muscularis mucosae and guinea pig taenia caecum. The difference in behavior of bupranolol confirms the existence of some atypical beta/beta3-adrenoceptors subtypes.