Modulation of the gating of unitary cardiac L-type Ca2+ channels by conditioning voltage and divalent ions

Although a considerable number of studies have characterized inactivation and facilitation of macroscopic L-type Ca2+ channel currents, the single channel properties underlying these important regulatory processes have only rarely been examined using Ca2+ ions. We have compared unitary L-type Ca2+ channel currents recorded with a low concentration of Ca2+ ions with those recorded with Ba2+ ions to elucidate the ionic dependence of the mechanisms responsible for the prepulse-dependent modulation of Ca2+ channel gating kinetics. Conditioning prepulses were applied across a wide range of voltages to examine their effects on the subsequent Ca2+ channel activity, recorded at a constant test potential. All recordings were made in the absence of any Ca2+ channel agonists. Moderate-depolarizing prepulses resulted in a decrease in the probability of opening of the Ca2+ channels during subsequent test voltage steps (inactivation), the extent of which was more dramatic with Ca2+ ions than Ba2+ ions. Facilitation, or increase of the average probability of opening with strong predepolarization, was due to long-duration mode 2 openings with Ca2+ ions and Ba2+ ions, despite a decrease in Ca2+ channel availability (inactivation) under these conditions. The degree of both prepulse-induced inactivation and facilitation decreased with increasing Ba2+ ion concentration. The time constants (and their proportions) describing the distributions of Ca2+ channel open times (which reflect mode switching) were also prepulse-, and ion-dependent. These results support the hypothesis that both prior depolarization and the nature and concentration of permeant ions modulate the gating properties of cardiac L-type Ca2+ channels.