Electrophysiological evidence suggests a defective Ca2+ control mechanism in a new Paramecium mutant

A new mutant of Paramecium tetraurelia, k-shyA, was characterized behaviorally and electrophysiologically. The mutant cell exhibited prolonged backward swimming episodes in response to depolarizing conditions. Electrophysiological comparison of k-shyA with wild type cells under voltage clamp revealed that the properties of three Ca2+-regulated currents were altered in the mutant. (i) The voltage-dependent Ca2+ current recovered from Ca2+-dependent inactivation two- to 10-fold more slowly than wild type. Ca2+ current amplitudes were also reduced in the mutant, but could be restored by EGTA injection. (ii) The decay of the Ca2+-dependent K+ tail current was slower in the mutant. (iii) The decay of the Ca2+-dependent Na+ tail current was also slower in the mutant. All other membrane properties studied, including the resting membrane potential and resistance and the voltage-sensitive K+ currents, were normal in k-shyA. Considered together, these observations are consistent with a defect in the ability of k-shyA to reduce the free intracellular Ca2+ concentration following stimulation. The possible targets of the genetic lesion and alternative explanations are discussed. The k-shy mutants may provide a useful tool for molecular and physiological analyses of the regulation of Ca2+ metabolism in Paramecium.