Dihydropyridine-sensitive skeletal muscle Ca channels in polarized planar bilayers. 1. Kinetics and voltage dependence of gating.

Rabbit skeletal muscle transverse tubule (T) membranes were fused with planar bilayers. Ca channel activity was studied with a "cellular" approach, using solutions that were closer to physiological than in previous studies, including asymmetric extracellular divalent ions as current carriers. The bilayer was kept polarized at -80 mV and depolarizing pulses were applied under voltage clamp. Upon depolarization the channels opened in a steeply voltage-dependent manner, and closed rapidly at the end of the pulses. The activity was characterized at the single-channel level and on macroscopic ensemble averages of test-minus-control records, using as controls the null sweeps. The open channel events had one predominant current corresponding to a conductance of 9 pS (100 mM Ba2+). The open time histogram was fitted with two exponentials, with time constants of 5.8 and 30 ms (23 degrees C). Both types of events were virtually absent at -80 mV. The average open probability (fractional open time) increased sigmoidally from 0 to a saturation level of 0.08, following a Boltzmann function centered at -25 mV and with a steepness factor of 7 mV. Ensemble averages of test-minus-control currents showed a sigmoidal activation followed by inactivation during the pulse and deactivation (closing) after the pulse. The ON time course was well fitted with "m3h" kinetics, with tau m = 120 ms and tau h = 1.2 s. Deactivation was exponential with tau = 8 ms. This study demonstrates a technique for obtaining Ca channel events in lipid bilayers that are strictly voltage dependent and exhibit most of the features of the macroscopic ICa. The technique provides a useful approach for further characterization of channel properties, as exemplified in the accompanying paper, that describes the consequences on channel properties of phosphorylation by cAMP dependent protein kinase.