Dependence of cytoplasmic calcium transients on the membrane potential in isolated nerve endings of the guinea pig

Abstract

The relation of changes in internal, free Ca 2+, measured with arsenazo III, to the membrane potential, measured with the cyanine dye di-SC 2(5) or 86Rb + distribution ratio, was studied in isolated guinea pig cortical nerve endings. Depolarization of the plasma membrane with veratridine or gramicidin as well as addition of ionophore A23187 led to an increase in cytosolic Ca 2+. Only the response to veratridine was inhibited by tetrodotoxin. The dependence of the depolarization-induced increase in intraterminal, free Ca 2+ on the membrane potential between about −50 to 0 mV was sigmoidal. A maximal increase in cytosolic Ca 2+ was reached when the membrane potential was depolarized from the resting level, about −64 mV, to about −40 mV. These results show that in isolated nerve endings the activation of voltage-sensitive Ca 2+ channels concomitantly leads to an increase in cytosolic, free Ca 2+. Comparison of the results of the present study with the previous electrophysiological observations indicate that Ca 2+ channels in synaptosomes, presynaptic nerve terminals of the squid giant synapse and cardiac cells have essentially similar voltage dependency.