Slow Deactivation and U-Shaped Inactivation Properties in Cloned Cav1.2b Channels in Chinese Hamster Ovary Cells

AbstractWhole-cell patch-clamp techniques were applied to Chinese hamster ovary cells stably expressing cloned smooth muscle Ca2+ channel α1-subunits. In the presence of Ba2+ as a charge carrier, U-shaped inactivation was observed in the presence and absence of Ca2+ agonists. Also, tail currents deactivated slowly when conditioning steps of positive potential were applied. The deactivation time constant was decreased by hyperpolarizing the repolarization step. Application of ATP-γ-S or H-7 had little effect on the conditions necessary to induce slow tail, suggesting involvement of physical processes in the channel protein. In the presence of Bay K 8644, additional application of nifedipine decreased the amplitudes of the test and tail currents induced by a test step preceded by a conditioning step to +80mV, but did not affect the decay time constant of the tail current. From these results and assumptions we have drawn up a kinetic scheme with one closed state, two open states (O1, O2) and two inactivated states linked to the closed state and open state O1, respectively, i.e., open state O2 protected from inactivation. Computer calculation reconstructed slow deactivation and U-shaped inactivation properties. A similar kinetic scheme with Ca2+-agonist-binding states accounted for the results in the presence of Ca2+ agonists