Permeation and Gating in CaV3.1 (α1G) T-type Calcium Channels Effects of Ca2+, Ba2+, Mg2+, and Na+

We examined the concentration dependence of currents through Ca(V)3.1 T-type calcium channels, varying Ca(2+) and Ba(2+) over a wide concentration range (100 nM to 110 mM) while recording whole-cell currents over a wide voltage range from channels stably expressed in HEK 293 cells. To isolate effects on permeation, instantaneous current-voltage relationships (IIV) were obtained following strong, brief depolarizations to activate channels with minimal inactivation. Reversal potentials were described by P(Ca)/P(Na) = 87 and P(Ca)/P(Ba) = 2, based on Goldman-Hodgkin-Katz theory. However, analysis of chord conductances found that apparent K(d) values were similar for Ca(2+) and Ba(2+), both for block of currents carried by Na(+) (3 muM for Ca(2+) vs. 4 muM for Ba(2+), at -30 mV; weaker at more positive or negative voltages) and for permeation (3.3 mM for Ca(2+) vs. 2.5 mM for Ba(2+); nearly voltage independent). Block by 3-10 muM Ca(2+) was time dependent, described by bimolecular kinetics with binding at approximately 3 x 10(8) M(-1)s(-1) and voltage-dependent exit. Ca(2+)(o), Ba(2+)(o), and Mg(2+)(o) also affected channel gating, primarily by shifting channel activation, consistent with screening a surface charge of 1 e(-) per 98 A(2) from Gouy-Chapman theory. Additionally, inward currents inactivated approximately 35% faster in Ba(2+)(o) (vs. Ca(2+)(o) or Na(+)(o)). The accelerated inactivation in Ba(2+)(o) correlated with the transition from Na(+) to Ba(2+) permeation, suggesting that Ba(2+)(o) speeds inactivation by occupying the pore. We conclude that the selectivity of the "surface charge" among divalent cations differs between calcium channel families, implying that the surface charge is channel specific. Voltage strongly affects the concentration dependence of block, but not of permeation, for Ca(2+) or Ba(2+).