Some properties of inward currents and spontaneous excitation of the cell membrane in smooth muscle

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1.The mechanisms underlying spontaneous excitation in smooth muscle are important, but not resolved. However, there are very few reports describing how ionic currents cause spontaneous excitation in smooth muscle cell membranes. In this study, we used enzymatically isolated smooth muscle cells to investigate the characteristics of inward Ca^<2+> current which is usually thought to be responsible to spike activities during the action potential.2.In guinea-pig detrusor smooth muscle cells, whole-cell patch clamp experiments revealed that when large preconditioning depolarizations (e.g.+80 mV,5 sec) were applied prior to a test step (0 mV,100 msec), test currents were suppressed by only a small amount. Further, subsequent repolarizations to the holding potential (+60 mV) induced very slowly (tau-10 msec) deactivating Ca^<2+> tail currents.3.This phenomenon can be explained by a model in which Ca^<2+> channels are transfered from the normal to a second open state during large depolarizing steps. In the second open state Ca^<2+> channels are not inactivated and deactivate very slowly.4.Using cell-attached patch clamp techniques (50-100 mM Ba^<2+> in the pipette), we recorded single channel currents corresponding to the second open state. After a large depolarization tp transfer Ca^<2+> channels to the long open state, we applied a ramp pulse to measure I-V relationship of unitary Ca^<2+> channel current. The slope conductance was 22-30pS which is similar to that in the normal open state.5.Similar results were obtained from other smooth muscles (e.g.guinea-pig stomach, taenia caeci). The second open state seems to be a common mechanism in smooth muscles, and may play an important role in persistent Ca^<2+> influx and subsequent excitation of the cell membrane in smooth muscle.