The alpha(1)-subunit of smooth muscle Ca2+ channel preserves multiple openstates induced by depolarization

1. The cloned alpha(1)-subunits of the smooth muscle Ca2+ channel (alpha(1C -b)) from rabbit lung were expressed in Chinese hamster ovary cells. The ef fect of large depolarizations was examined using cell-attached patch clamp techniques. 2. After large, long-duration depolarizations (to +80 mV, 4 s), the cloned smooth muscle Ca2+ channels were still open, and also showed slow channel c losure upon repolarization. The sum of unitary channel currents revealed th at the tail current seen after large conditioning depolarizations had a slo wer deactivation time constant compared to that seen when the cell membrane was depolarized briefly with a test step (to +40 mV), suggesting that larg e depolarizations transform the conformation of the Ca2+ channels to a seco nd open state. 3. The decay time course of the tail current induced by large conditioning depolarizations was prolonged by reducing the negativity of the repolarizat ion step, and vice versa. 4. Using the slow deactivating characteristic, the current-voltage relation ship was directly measured by applying a ramp pulse after a large depolariz ation. Its slope conductance was approximately 26 pS. 5. Since the patch pipettes contained Ca2+ agonists, the transition of the Ca2+ channel conformation to the second, long open state during a large dep olarization was distinct from that caused by Ca2+ agonists, suggesting that the cloned alpha(1)-subunits of smooth muscle Ca2+ channels preserve the c haracteristic features seen in native smooth muscle Ca2+ channels. 6. In addition, when skeletal muscle beta-subunits were coexpressed with th e alpha(1)-subunits, the long channel openings after large, long-duration d epolarizations were frequently suppressed. This phenomenon could be explain ed if the skeletal muscle beta-subunits increased the inactivation rate dur ing the preconditioning depolarization.