The beta subunit increases Ca2+ currents and gating charge movements of human cardiac L-type Ca2+ channels

The properties of the gating currents (nonlinear charge movements) of human cardiac L-type Ca2+ channels and their relationship to the activation of the Ca2+ channel (ionic) currents were studied using a mammalian expression system. Cloned human cardiac alpha(1) + rabbit alpha(2) subunits or human cardiac alpha(1) + rabbit alpha(2) + human beta(3) subunits were transiently expressed in HEK293 cells. The maximum Ca2+ current density increased from -3.9 +/- 0.9 pA/pF for the alpha(1) + alpha(2) subunits to -11.6 +/- 2.2 pA/pF for alpha(1) + alpha(2) + beta(3) subunits. Calcium channel gating currents were recorded after the addition of 5 mM Co2+, using a -P/5 protocol, The maximum nonlinear charge movement (Q(max)) increased from 2.5 +/- 0.3 nC/mu F for alpha(1) + alpha(2) subunit to 12.1 +/- 0.3 nC/mu F for alpha(1) + alpha(2) + beta(3) subunit expression. The Q(ON) was equal to the Q(OFF) for both subunit combinations, The Q(ON)-V-m data were fit by a sum of two Boltzmann expressions and ranged over more negative potentials, as compared with the voltage dependence for activation of the Ca2+ conductance. We conclude that 1) the beta subunit increases the number of functional alpha(1) subunits expressed in the plasma membrane of these cells and 2) the voltage-dependent activation of the human cardiac L-type calcium channel involves the movements of at least two nonidentical and functionally distinct gating structures.