Properties of inactivation of calcium channel currents in smooth muscle cells of rabbit portal vein

With 10 mM Ba²⁺ as the charge carrier, inactivation of Ca²⁺ channel currents could be subdivided into at least two exponentials in smooth muscle cells dispersed from the rabbit portal vein by use of the wholecell configuration of the patch-clamp technique: fast and slow inactivation. All characteristics of inactivation were independent of the size of the currents. Step changes in the holding potential unveiled an extremely slow recovery and an onset of inactivation of the order of several minutes. Steady-state inactivation critically depended on the duration of the pre-steps. Inactivation curves obtained under steady-state conditions showed as shift by approximately 25 mV towards negative potentials by comparison with curves obtained using 1-s pre-pulses. This shift greatly reduced the window current. Recovery from inactivation studied with double-pulse protocols could be classified into at least two exponentials. The contribution of the slow recovery was accentuated at negative holding potentials. Recovery from inactivation critically depended on the duration of the conditioning voltage step, and was also dependent on the duration of the pre-step: its voltage dependence disappeared when pre-pulses longer than 2 s were applied. Onset of inactivation was composed of at least two exponentials: the fast component was accelerated at less negative pre-step potentials. We propose that several inactivated states are involved in Ca²⁺ channel inactivation. Transitions between these states are voltage dependent and voltage independent.