A Ca-induced Na-current in Paramecium.

Under a voltage clamp, step depolarization and repolarization can induce a sustained inward current and a tail inward current in Paramecium tetraurelia bathed in a solution containing 8 mM-Na+. These currents are best seen in the 'paranoiac' mutant. The I-V plot of the sustained inward current can have a region of negative resistance around -20 mV. This current is absent when Na+ is excluded from the bath solution, and it increases as the Na+ concentration increases from 2 to 8 mM. Injection of Na+ into the cell suppresses this inward current. This current develops very slowly, reaching its maximum seconds after the step depolarization and decays with a time constant of hundreds of milliseconds after the repolarization. This slow current is dependent on Ca2+. It can be suppressed by reduction or deletion of external Ca2+ or by iontophoretic injection of EGTA. 'Pawn' mutants with defective Ca-conductance also lack this current. We conclude that Paramecium has a Ca-induced conductance through which the Na-current flows. Although more prominent in the 'paranoiac' mutant, this Ca-induced Na-current is also seen in the wild type. This conductance may function in generating plateau depolarizations lasting seconds or even minutes and the corresponding prolonged backward swimming away from sources of irritation and stress.