sentences:  Since the transport is electrogenic (alters the membrane potential), depolarization of the membrane can reverse the exchanger's direction if the cell is depolarized enough, as may occur in excitotoxicity.In addition, as with other transport proteins, the amount and direction of transport depends on transmembrane substrate gradients.This fact can be protective because increases in intracellular Ca2+ concentration that occur in excitotoxicity may activate the exchanger in the forward direction even in the presence of a lowered extracellular Na+ concentration.