Effects of isoproterenol on spontaneous excitations in detrusor smooth muscle cells of the guinea pig

Purpose: Because beta -adrenoceptor agonists would be a useful tool for the pharmacological treatment of unstable bladder, we investigated the cellula r mechanisms underlying beta -adrenoceptor mediated inhibition on spontaneo us excitation in detrusor smooth muscle. Materials and Methods: Detrusor smooth muscle bundles were isolated from gu inea pig bladders. Changes in membrane potential were recorded using an int racellular recording technique. In preparations loaded with the calcium ind icator fura-PES changes in the concentration of intracellular calcium ions were measured simultaneously with membrane potential. Effects of isoprotere nol on spontaneous changes in the membrane potential and intracellular Ca2 were examined. Results: Detrusor smooth muscle cells exhibited spontaneous action potentia ls that were associated with transient increases in intracellular Ca2+ (cal cium transients). Isoproterenol, which hyperpolarized the membrane, prevent ed action potentials and calcium transients. This induced inhibition of cal cium transients was not affected by cyclopiazonic acid. Isoproterenol induc ed hyperpolarization was inhibited by inhibitors of protein kinase A, N-[2- ((p-bromocinnamyl)amino)ethyl] -5-isoquinolinesulfonamide, hydrochloride an d Rp-adenosine-3',5' -cyclic phosphorothioate. Hyperpolarization was blocke d by a solution containing 30 mM. potassium but not by a range of potassium channel blockers. Ouabain and a solution of 0.5 mM. potassium also inhibit ed hyperpolarization. Conclusions: Our results suggest that isoproterenol prevented spontaneous a ction potential discharges and associated calcium transients through the ac tivation of protein kinase A. The isoproterenol induced inhibition of intra cellular Ca2+ largely depends on the prevention of spontaneous action poten tials since the contribution of the intracellular calcium store was small. Isoproterenol hyperpolarizes the membrane, probably by stimulating sodium p ump activity.