β-Adrenoceptor-mediated Relaxation of Urinary Bladder Muscle in β(2)-Adrenoceptor Knockout Mice

Background and Objective: In order to characterize the β-adrenoceptor (AR) subtypes involved in agonist-stimulated relaxation of murine urinary bladder we studied the effects of (-)-isoprenaline and CL 316,243 on tonic contraction and spontaneous contractions in detrusor strips of wild-type (WT) and β(2)-AR knockout (β(2)-AR KO) mice. Materials and Methods: Urinary bladders were isolated from male WT and β(2)-AR KO mice. β-AR subtype expression was determined with quantitative real-time PCR. Intact muscle strips pre-contracted with KCl (40 mM) were exposed to cumulatively increasing concentrations of (-)-isoprenaline or β(3)-AR agonist CL 316,243 in the presence and absence of the subtype-selective β-AR blockers CGP 20712A (β(1)-ARs), ICI 118,551 (β(2)-ARs), and L748,337 (β(3)-ARs). Results: Quantitative real-time PCR confirmed lack of β(2)-AR expression in bladder tissue from β(2)-AR KO mice. In isolated detrusor strips, pre-contraction with KCl increased basal tone and enhanced spontaneous activity significantly more in β(2)-AR KO than in WT. (-)-Isoprenaline relaxed tonic tension and attenuated spontaneous activity with similar potency, but the concentrations required were two orders of magnitude higher in β(2)-AR KO than WT. The concentration-response curves (CRCs) for relaxation were not affected by CGP 20712A (300 nM), but were shifted to the right by ICI 118,551 (50 nM) and L748,337 (10 μM). The -logEC(50) values for (-)-isoprenaline in WT and β(2)-AR KO tissue were 7.98 and 6.00, respectively, suggesting a large receptor reserve of β(2)-AR. (-)-CL 316,243 relaxed detrusor and attenuated spontaneous contractions from WT and β(2)-AR KO mice with a potency corresponding to the drug’s affinity for β(3)-AR. L743,337 shifted the CRCs to the right. Conclusion: Our findings in β(2)-AR KO mice suggest that there is a large receptor reserve for β(2)-AR in WT mice so that this β-AR subtype will mediate relaxation of tone and attenuation of spontaneous activity under physiological conditions. Nevertheless, upon removal of this reserve, β(3)-AR can also mediate murine detrusor relaxation.