Possible contribution of long open state to noninactivating Ca2+ current in detrusor cells

The whole cell patch-clamp technique was used to measure Ca2+ current in isolated smooth muscle cells from guinea pig urinary bladder. Noniactivating Ca2+ channel current was modeled incorporating the long open state of the Ca2+ channel. When inactivation was examined over a wide voltage range, a completely U-shaped curve was obtained. Lack of inactivation at +80 mV could be attributed to the long open state induced by large depolarization as well as to minimal Ca2+ influx and Ca(2+)-dependent inactivation. Activation parameters were obtained by comparing the amplitudes of conditioned (by +80 mV, 5 s) and unconditioned test potentials. With the use of the activation curve and the U-shaped inactivation curve, a noninactivating current that peaks around +20 mV was obtained. This current is composed of a so-called "window" current and a persistent current brought about by the long open state. Differences in the voltage dependence of the development of the long open state in various smooth muscles, as well as differences in the equilibrium constant between open and inactivated states, could underlie the different patterns of contractile behavior that characterize smooth muscles.