Role of beta-adrenoceptor subtypes in mediating relaxation of the pig bladder trigonal muscle in vitro.

Aims To investigate the role of β-adrenoceptor subtypes in mediating relaxation of the pig bladder trigonal muscle in vitro. Materials and Methods Longitudinal strips of trigonal muscle were isolated, and the mucosa and serosa removed. Tissues were precontracted with KCl, and β-adrenoceptor agonists (isoprenaline or salbutamol) were added cumulatively, and concentration–relaxation curves (CRCs) were obtained. CRCs to agonists were obtained in the absence and presence of antagonists and antagonist affinity values were calculated. Results Isoprenaline (non-selective β-agonist) relaxed with high potency (pEC50 = 7.2). Propranolol antagonized CRCs to isoprenaline with a high affinity (apparent pKB = 8.8), but the Schild plot had a slope significantly (P < 0.01) less than unity (0.61), suggesting that responses were mediated by more than one β-adrenoceptor subtype. CGP20712A (β1-antagonist) antagonized responses to isoprenaline with a low affinity (apparent pKB = 5.13), indicating β1-adrenoceptors did not participate in the response. The affinity of ICI118551 (β2-antagonist) for antagonism of responses to isoprenaline was also relatively low (apparent pKB = 6.9), and the Schild slopes were significantly (P < 0.01) less than unity (0.58). SR59230A (β3-antagonist) antagonized CRCs to isoprenaline with a relatively low affinity (apparent pKB = 7.5), and with a Schild slope significantly (P < 0.01) less than unity (0.86), indicating that responses may be mediated by more than one β-adrenoceptor subtype. In contrast to that observed with isoprenaline, the β2-adrenoceptor selective agonist salbutamol induced relaxation with a relatively low potency (pEC50 = 6.6) and with maximum responses of 80% of that to isoprenaline. ICI118551 competitively antagonized (Schild plot of unity) responses to salbutamol with a high affinity (pA2 = 8.3). Conclusions These data suggest that β-adrenoceptor mediated responses to isoprenaline of the bladder trigone are mediated via both the β2- and β3-adrenoceptor subtypes whilst responses to salbutamol appear to be mediated only via the β2-adrenoceptor. Neurourol. Urodynam. 22:338–342, 2003. © 2003 Wiley-Liss, Inc.