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. Nonin activating Ca2+ channel current was modeled incorporating the long ope n state of the Ca2+ channel. When inactivation was examined over a wid e voltage range, a completely U-shaped curve was obtained. Lack of ina ctivation at +80 mV could be attributed to the long open state induced by large depolarization as well as to minimal Ca2+ influx and Ca2+-de pendent 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 ina ctivation curve, a noninactivating current that peaks around +20 mV wa s obtained. This current is composed of a so-called ''window'' current and a persistent current brought about by the long open state. Differ ences in the voltage dependence of the development of the long open st ate in various smooth muscles, as well as differences in the equilibri um constant between open and inactivated states, could underlie the di fferent patterns of contractile behavior that characterize smooth musc les.