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

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Abstract
The effects of external divalent cations on the steady-state open probability (P o), 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 P o versus membrane potential (V m) 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. Our results can be explained if two different mechanisms are involved in the modulation of Na channel gating by extracellular divalent cations in neutral membranes: (1) the screening or binding of Ba2+ at negatively charged groups on the extracellular side of the channel, leading to an increase in the transmembrane electric field at the voltage sensor, as well as an increase in the closing rate and decrease in the opening rate by the same proportion, plus (2) a direct modulatory action on the channel protein which stabilizes the closed state of the channel and accounts for the larger effect of external divalent cations on the opening rate.