1. Intracellular injection, recording and current‐passing methods were used to investigate the role of intracellular Ca in the modulation of electrical behaviour in the ciliate Paramecium caudatum. 2. Injection of EGTA converted graded regenerative responses ascribed to Ca inward current to all‐or‐none action potentials. The EGTA injection also caused a discontinuity in the steady state I—V relations to outward current, but had little effect on hyperpolarizing current—voltage responses. 3. The overshoot of the all‐or‐none spike produced by the EGTA‐injected cell followed an approximate 29 mV increase for a tenfold increase in external Ca concentration and was independent of changes in external K and Na concentrations. 4. The EGTA‐induced all‐or‐none action potential tended to produce plateaus that could last up to 20 sec. During the plateau the membrane slowly repolarized to a critical potential, upon which repolarization occurred precipitously. 5. Injection of 10−6 M‐free Ca2+ as a Ca‐EGTA buffer hyperpolarized the membrane and decreased the potential shifts to inward current pulses. These responses are consistent with an increase in K conductance. 6. During EGTA plateaus reversed beating of the cilia indicated a rise in intracellular Ca, and thus an inability of the EGTA to complex the Ca as rapidly as it entered the cilia. Reversal of the motile apparatus thus appears to be activated at lower concentrations of intracellular Ca than are required to activate the inferred Ca‐dependent K system. 7. In uninjected cells removal of the cilia, which results in a loss of the voltage‐activated Ca channels (Dunlap, 1977), or addition of extracellular Ba both tended to linearize the steady state I—V relations. 8. Injections of Cs and TEA tended to linearize the steady state I—V relations, but did not result in either a conversion to an all‐or‐none spike or a discontinuity in the depolarizing steady‐state I—V relations. 9. It is concluded that in Paramecium a Ca‐activated K conductance short‐circuits the inward current of the regenerative Ca response, preventing all‐or‐none behaviour. The occurrence of plateau spikes following EGTA injection indicates that the Ca conductance inactivates very slowly in face of a maintained depolarization. Such slow Ca‐inactivation is consistent with the slow relaxation of Ca‐dependent ciliary reversal that occurs during maintained depolarization. 10. The possibility is discussed that injection of EGTA may also enhance the Ca conductance.
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