Calcium-dependent sodium current in the marine ciliate Euplotes vannus

Ca and Na inward currents were recorded upon depolarizations in Euplotes after the blockage of K outward currents with intracellular Cs ions. The Na current was analyzed under voltage clamp and had the following properties: it activated to a maximum within 150 msec and partly inactivated during sustained voltage steps. It had a positive equilibrium potential between 25 and 30 mV and could be carried by Na or Li ions but not by K, choline or Tris ions. The current revealed a prominent associated inward tail current which deactivated with a single-exponential time constant of 118 msec. Both the current and its tail were strongly reduced after reduction of the extracellular Na concentration. Externally applied K channel blocker tetraethylammonium chloride did not block the current. Either EGTA injection into the cell or nonlethal deciliation with ethanol eliminated the current and its tail. These results indicate the existence of a Na conductance within the membrane of Euplotes which is activated by the intracellular level of free Ca2+.