Calcium‐mediated inactivation of calcium current in Paramecium

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Abstract
The Ca current response to depolarization was investigated in P. caudatum under voltage clamp. Inactivation of the current was measured with the double pulse method; a fixed test pulse of an amplitude sufficient to evoke maximal inward current was preceded by a conditioning pulse of variable amplitude (0-120 mM). The amplitude of the current recorded during the test pulse was related to the potential of the conditioning pulse. Reduction of test pulse current was taken as an index of Ca current inactivation. The current recorded during a test pulse showed a progressive decrease to a minimum as the potential of the conditioning pulse approached +10 to +30 mV. Further increase in conditioning pulse amplitude was accompanied by a progressive restoration of the test pulse current. Conditioning pulse near the Ca equilibrium potential had only a slight inactivating effect on the test pulse current. Injection of a mixture of Cs and TEA [tetraethylammonium] which blocked late outward current had essentially no effect on the inward current or its inactivation. Elevation of external Ca from 0.5-5 mM was accompanied by increased inactivation of the test pulse current. The enhanced inactivation of the test pulse current was approximately proportional to the increase in current recorded during the conditioning pulse. Following injection of the Ca chelating agent, EGTA [ethylene glycol bis(.beta.-aminoethyl ether) N,N,N''N''-tetraacetic acid], the inactivation of the test pulse current was diminished; the transient inward current relaxed slightly more slowly, and the transient was followed by a steady net inward current. The time course of recovery from inactivation in the double pulse experiment approximated a single exponential having a time constant of 80-110 ms. Injection of EGTA shortened the time constant by as much as 50%. Interference was the entry of Ca or enhanced removal of intracellular free Ca2+ interferes with the process of Ca current inactivation, while enhanced entry of Ca promotes the process of inactivation. While the mechanism of inactivation is unknown, arguments are presented that the accumulation of intracellular Ca influences the Ca channel conductance.