AUTHOR=Propping Stefan , Lorenz Kristina , Michel Martin C. , Wirth Manfred P. , Ravens Ursula 

TITLE=β-Adrenoceptor-mediated Relaxation of Urinary Bladder Muscle in β2-Adrenoceptor Knockout Mice

JOURNAL=Frontiers in Pharmacology

VOLUME=7

YEAR=2016

URL=https://www.frontiersin.org/journals/pharmacology/articles/10.3389/fphar.2016.00118

DOI=10.3389/fphar.2016.00118

ISSN=1663-9812

ABSTRACT=<p><bold>Background and Objective:</bold> 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<sub>2</sub>-AR knockout (β<sub>2</sub>-AR KO) mice.</p><p><bold>Materials and Methods:</bold> Urinary bladders were isolated from male WT and β<sub>2</sub>-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<sub>3</sub>-AR agonist CL 316,243 in the presence and absence of the subtype-selective β-AR blockers CGP 20712A (β<sub>1</sub>-ARs), ICI 118,551 (β<sub>2</sub>-ARs), and L748,337 (β<sub>3</sub>-ARs).</p><p><bold>Results:</bold> Quantitative real-time PCR confirmed lack of β<sub>2</sub>-AR expression in bladder tissue from β<sub>2</sub>-AR KO mice. In isolated detrusor strips, pre-contraction with KCl increased basal tone and enhanced spontaneous activity significantly more in β<sub>2</sub>-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<sub>2</sub>-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<sub>50</sub> values for (-)-isoprenaline in WT and β<sub>2</sub>-AR KO tissue were 7.98 and 6.00, respectively, suggesting a large receptor reserve of β<sub>2</sub>-AR. (-)-CL 316,243 relaxed detrusor and attenuated spontaneous contractions from WT and β<sub>2</sub>-AR KO mice with a potency corresponding to the drug’s affinity for β<sub>3</sub>-AR. L743,337 shifted the CRCs to the right.</p><p><bold>Conclusion:</bold> Our findings in β<sub>2</sub>-AR KO mice suggest that there is a large receptor reserve for β<sub>2</sub>-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<sub>3</sub>-AR can also mediate murine detrusor relaxation.</p>