The smooth muscle of the guinea pig bladder demonstrates in vitro spontaneous electrical activity in the form of action potentials which are associated with contraction. The action potential frequency is highly voltage-sensitive. The relative contributions of Na, Ca and K to the action potential elicited by depolarizing current have been studied using intracellular microelectrodes. In solutions in which NaCl is replaced by sucrose, the membrane hyperpolarizes and the rate of rise and after-hyperpolarization of the elicited action potential is increased. The amplitude is unaffected. In Ca-deficient solutions, the membrane depolarizes, the rate of rise and amplitude of the action potential is reduced, and the after-hyperpolarization is decreased. Nifedipine reduces amplitude and rate of rise but does not affect after-hyperpolarization. In the presence of the K-channel antagonist TEA, the duration of the action potential is prolonged, but the amplitude and rate of rise are unaffected. After-hyperpolarization is not reduced. It is concluded that the action potential of guinea pig bladder muscle, like many other smooth muscles studied, is Ca-based. Repolarization depends on changes in K conductance. The after-hyperpolarization is voltage-sensitive.
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