A mutation that alters properties of the calcium channel in Paramecium tetraurelia.

Abstract

The membrane properties of a new mutant of P. tetraurelia, were compared under voltage clamp with those of the wild type. The Ca2+ current was isolated and examined using CsCl-filled electrodes and tetraethylammonium in the bath solution to block K+ channels. The amplitude of the Ca2+ transient was not altered by the mutation. The Ca2+ current in the mutant inactivated more slowly and less extensively: hence a larger sustained Ca2+ current remained in the mutant. A change in the time course of the deactivation of the Ba2+ current was observed in the mutant. This mutational change is not likely to be the consequence of the Ca2+-channel inactivation because it is seen in the Ba2+ solution where there is little inactivation of the current. Other measured properties of the Ca2+ channel, the voltage-dependent K+ current and the resting properties of the membrane were normal in the mutant. The Ca2+-activated K+ current and the Ca2+-activated Na+ current were larger in the mutant than in the wild type, consistent with a greater elevation of free intracellular Ca2+ during depolarization in the mutant. It is likely that the mutation causes an alteration in the Ca2+-channel structure or in its immediate environment and thereby affects the inactivation and deactivation processes of the Ca2+ channel. As would be expected from the greater Ca2+ current, the mutant tends to generate all-or-none Ca action potentials as opposed to the graded action potentials in the wild type.