Role of Mitochondria in the Generation of Spontaneous Activity in Detrusor Smooth Muscles of the Guinea Pig Bladder.

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Abstract
Purpose: The rhythmic electrical activity of gastrointestinal smooth muscles is associated with mitochondrial Ca2+ handling. We examined the role of mitochondria in the generation of spontaneous activity in detrusor smooth muscles. Materials and Methods: Changes in the membrane potential and intracellular Ca2+ concentration ([Ca2+]i) were measured in detrusor smooth muscles of the guinea pig using conventional microelectrode techniques and Fura-PE3 (Calbiochem, San Diego, California) fluorescence, respectively. Results: Detrusor smooth muscle cells showed spontaneous action potentials and associated transient increases in [Ca2+]i (Ca transients). The mitochondrial protonophore CCCP (carbonyl cyanide m-chlorophenyl hydrazone) (10 μM) depolarized the membrane, increased [Ca2+]i and caused activation followed by suppression of action potentials and Ca transients. High K solution potassium concentration ([K+]o = 30 mM) depolarized the membrane and increased [Ca2+]i to levels similar to those produced by 10 μM CCCP but this depolarization did not suppress action potentials. Nifedipine (10 μM) decreased the amplitude of CCCP induced increases in [Ca2+]i by about 50%. CCCP induced increases in [Ca2+]i were further reduced by about 70% in Ca2+-free solution and by about 30% in the presence of 10 μM SKF96365, a blocker for store operated Ca entry. In the presence of 10 μM nifedipine and 10 μM cyclopiazonic acid, CCCP induced [Ca2+]i responses were suppressed to about 25% of control values. Under these conditions repetitive applications of 10 μM acetylcholine chloride successively decreased [Ca2+]i responses and finally failed to increase [Ca2+]i. Subsequent CCCP failed to elevate [Ca2+]i. Conclusions: These results suggest that mitochondria have an important role in Ca2+ buffering in bladder smooth muscles. Mitochondrial Ca2+ is presumably supplied by Ca2+ transport from internal stores and also by capacitative calcium entry through nonselective cation channels. Mitochondrial Ca2+ handling may also be critical for the generation of spontaneous activity in detrusor smooth muscle.