Mutants with reduced Ca activation in Paramecium aurelia.

Two heat-sensitive "pawn" mutants of Paramecium aurelia are capable of avoiding reactions when grown at 23 degrees C but not at 35 degrees C. Electrophysiological analyses show that Ca activation is reduces in the mutants even when they are grown at 23 degrees C. The maximal rate of rise and the peak of the evoked action potential (Ca-spike) in the mutants are smaller than those of wild type in a K-solution. After suppression of K conductance by either TEA+ or Ba++, the action potentials of the mutants peak at the same level as that of wild type. However, the maximal rate of rise of the mutants remains only about half that of wild type. Thus, the mutations affect Ca activation but not K activation. Incubation at a high temperature (35 degrees C) further reduces Ca activation to almost zero in the mutants but has little or no effect on wild type. This almost complete loss of Ca activation explains the lack of avoiding reactions when the mutants are grown at high temperatures. A double mutant containing two heat-sensitive mutations shows extremely reduced Ca activation even when grown at 23 degrees C.