Facilitation of sodium currents in frog neuroglia by nerve impulses: dependence on external calcium

Facilitation of voltage-gated sodium currents in glial membranes by nerve impulses has been studied by using both the whole cell and loose-patch clamp techniques in the isolated intact optic nerve of the frog. During facilitation there is a shift in the voltage dependence of glial Na+ channels such that a given depolarization produces a larger inward Na+ current. Decreasing external calcium from 4 times normal to 0.2 times normal produced a similar shift in the current-voltage relation. Increasing the external calcium concentration to 4-5 times normal blocks facilitation. In reduced calcium, 0.1-0.2 times normal, the peak of facilitation was unaffected, but its decay was slowed. The addition of 1 mM nickel and 2 mM cobalt or 2 mM cadmium, to prevent depletion of extracellular calcium that might result from voltage-dependent entry of calcium into the axons, did not block the facilitation. The results suggest that, even though facilitation is blocked by high extracellular calcium, a decrease in extracellular calcium produced by axon impulses is not the cause of the facilitation.