Calcium‐mediated inactivation of the calcium conductance in caesium‐loaded giant neurones of Aplysia californica.

Abstract

The intracellular K in giant neurons of A. californica was replaced with Cs by a method utilizing the ionophore nystatin. Because Cs+ has low permeability through K channels, outward currents during voltage-clamp depolarization were strongly curtailed after the Cs loading procedure and the subsequent wash-out of the ionophore. The Ca current elicited by a test voltage-clamp depolarization (pulse 2) was depressed following the entry of Ca elicited by a prior depolarization (pulse 1). The percentage depression of the test current was a linear function of the pulse 1 current-time integral, and seems related linearly to the amount of Ca carried into the cell during pulse 1. This linear relation was maintained when Ca entry was varied by changes in external Ca concentration, by altered pulse 1 amplitude and altered pulse 1 duration. Depression was substantially reduced by injection of EGTA [ethylene glycol bis (.beta.-aminoethyl ether) tetraacetate], and by substitution of Ba for extracellular Ca. The Ca current was unaffected by prior hyperpolarization of the membrane or by prior depolarizations to about ECa. Depression of the current was not altered by addition of extracellular 50 mM-TEA [tetraethylammonium] or by a strong hyperpolarization between the conditioning and test pulses. The rate relaxation of the inward current during a given depolarization depended on the rate of entry and accumulation of free Ca. Relaxation under a given command potential became slower when Ca was partially replaced with Mg so as to produce a smaller Ca current, or when accumulation of intracellular free Ca was retarded by injected EGTA or by Ba substitution for extracellular Ca. Evidence is considered that accumulation of Ca2+ at the cytoplasmic surface of the membrane leads to inactivation through an action upon the Ca conductance. Reduced driving force and intracellular surface-charge neutralization do not adequately account for the observed depression of the Ca current resulting from intracellular accumulation of Ca2+.