Ca2+ Current-Deficient Pawn Mutants are Promoted to Queens During Chronic Depolarization of Paramecium tetraurelia

Chronic KCl-induced depolarization of Paramecium tetraurelia enhances Ca²⁺-dependent backward swimming behavior over a period of 8–24 hr. Here, we investigated the electrophysiological mechanisms underlying this adaptive phenomenon using voltage-clamp techniques. Cells that had been adapted to 20 mm KCl showed several significant changes in the properties of the Ca²⁺ current that mediates ciliary reversal in Paramecium (I _Ca ), including a positive shift in voltage sensitivity and a significant slowing of inactivation. In seeking an explanation for these changes, we examined the effects of chronic depolarization on mutants that do not normally express a Ca²⁺ current or swim backward. Surprisingly, pawn B mutant cells slowly regained the ability to reverse their cilia during KCl exposure with a time course that mirrored behavioral adaptation of the wild type. This behavior was accompanied by expression of a novel Ca²⁺ current (I _QUEEN ) whose voltage sensitivity was shifted positive with respect to the wild-type Ca²⁺ current and that was slow to inactivate. Coincidental expression of I _QUEEN in the wild type during adaptation would readily explain the observed changes in I _Ca kinetics. We also examined the effects of chronic depolarization on Dancer, a mutant suggested previously to have an I _Ca inactivation defect. The mutant phenotype could be suppressed or exaggerated greatly by manipulating extracellular KCl concentration, suggesting that Dancer lesion instead causes inappropriate regulation of I _QUEEN . Received: 23 April 1999/Revised: 29 June 1999