Inactivation of Ca conductance dependent on entry of Ca ions in molluscan neurons.

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Abstract
Inactivation of the Ca channel of Aplysia [californica] neurons was studied in the absence of K current in cells that were Cs-loaded with the aid of the ionophore nystatin. Inactivation was substantially decreased by methods that limited Ca entry. Depolarizations commensurate with the equilibrium potential of Ca resulted in minimal inactivation. Replacement of extracellular Ca by Ba also decreased inactivation. Inactivation of the Ca channel is probably a function of the extent of Ca entry rather than membrane potential, differing fundamentally from the purely voltage-dependent mechanism for Na inactivation.