Calcium‐mediated inactivation of calcium current in Paramecium

Abstract

SUMMARY1. The Ca current seen in response to depolarization was investigated in Paramecium 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 mV).2. 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 pulses near the calcium equilibrium potential had only a slight inactivating effect on the test pulse current.3. Injection of a mixture of Cs and TEA which blocked late outward current had essentially no effect on the inward current or its inactivation.4. Elevation of external Ca from 0 5 to 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.5. Following injection of the Ca chelating agent, EGTA, the inactivation of the test pulse current was diminished; in addition, the transient inward current relaxed slightly more slowly, and the transient was followed by a steady net inward current.6. The time course of recovery from inactivation in the double pulse experiment approximated a single exponential having a time constant of 80-110 msec. Injection of EGTA shortened the time constant by as much as 50 %.7. It is concluded that interference with 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.

Ca2+-Binding Protein-1 Facilitates and Forms a Postsynaptic Complex with Cav1.2 (L-Type) Ca2+Channels

“…5C). In addition, for Ca v 1.2 alone, a plot of I res /I pk versus test voltage was U-shaped, with maximal inactivation at ϩ5 mV, which was near the peak of the I-V curve, as would be expected of a process depending on incoming Ca 2ϩ (Brehm et al, 1980) (Fig. 5D).…”

Calcium current inactivation in identified neurones of Helix aspersa.

“…It is inherently difficult to separate such a mechanism experimentally from the effects of IK(ca) activation since procedures altering Ca entry would affect either process. Experiments in intact cells have not so far enabled elimination of all outward current, at least with large depolarizations (Brehm & Eckert, 1978;Tillotson, 1979;Brehm, Eckert & Tillotson, 1980). Intracellular perfusion experiments, which give better suppression of outward currents, could alter Ca inactivation if this does depend on [Ca]i and if the perfusion affects intracellular Ca buffering.…”

Communications

“…It is proposed that the inward K rectifier (iK1) is 'activated' when, under (Tillotson, 1979;Brehm, Eckert & Tillotson, 1980). A similar situation is found in frog heart for the slow inward current (Irn).…”