Modulation of sodium channel gating by external divalent cations: differential effects on opening and closing rates.

The effects of external divalent cations on the steady-state open probability (Po), opening and closing rates, and conductance of batrachotoxin (BTX)-activated Na channels were studied in planar lipid bilayers. External divalent cations shifted the midpoint of the Po versus membrane potential (Vm) relation (gating curve) to more depolarized potentials. Of the group IIA divalent cations tested, Ca2+ caused the largest depolarizing shift followed by Ba2+, Sr2+ and Mg2+ in that order. In contrast, the order of decreasing efficacy for block of the single channel conductance was Ca2+, Mg2+, Sr2+ and Ba2+. Thus, because it is a relatively good shifter but a poor blocker, Ba2+ was used to study the effects of divalent cations on gating. External Ba2+ decreased the Na channel opening rate and increased the closing rate. At voltages close to the midpoint of the gating curve, the opening rate decreased by a factor of 1/2.8 while the closing rate increased by 2.0-fold. Although Ba2+ would be expected to increase the external surface potential by interacting with negatively charged residues on the channel, this preferential effect on the opening rate indicates that the effects of external Ba2+ cannot be attributed solely to an increase in the potential gradient across the voltage sensing apparatus.(ABSTRACT TRUNCATED AT 250 WORDS)