Inhibitory mechanism of BRL37344 on muscarinic receptor-mediated contractions of the rat urinary bladder smooth muscle

We examined the inhibitory mechanism of BRL37344, a beta-adrenoceptor agonist that is considered to be specific to beta(3)-subtype, on muscarinic receptor-mediated contraction of the rat urinary bladder smooth muscle. BRL37344 produced apparently biphasic concentration-relaxation curves in the urinary bladder smooth muscle contracted with carbachol (0.6 muM). The first and second phases had estimated pD(2) (-logEC(50)) values of 7.80 +/- 0.34 and 4.62 +/- 0.18, respectively (n=6). The first component of the BRL37344 concentration-response curve was not affected by propranolol (1 p,M), whereas it was inhibited by higher concentrations of the drug (10 p M or 30 muM). The second component was completely resistant to propranolol. On the other hand, BRL37344 produced monophasic concentration-relaxation of 30 mM KCl-precontracted urinary bladder smooth muscle with a pD2 value of 8.34 +/- 0.18 (n=6). Pretreatment of the urinary bladder smooth muscles with BRL37344 (30, 100 and 300RM) significantly (P<0.05) shifted the concentration-response curves for carbachol-induced contractions. In radioligand binding experiments, BRL37344 concentration-dependently displaced the specific binding of [H-3]N-methyl scopolamine to muscarinic receptors on rat urinary bladder smooth muscle membranes. Additionally, BRL37344 inhibited [H-3]N-methyl scopolamine binding to cloned human muscarinic receptors (M-1-M-5) expressed in Chinese hamster ovary cells. These results suggest that BRL37344 attenuates muscarinic receptor-mediated contractions through prevention of the agonists binding to their receptors, in addition to stimulation of beta(3)-adrenoceptors, in rat urinary bladder.