Effects of membrane depolarization on intracellular calcium in single nerve terminals

The [Ca²⁺]_i of individual neurosecretory nerve terminals loaded with the fluorescent probe fura-2 was monitored during depolarizing stimuli and in the presence of substances known to induce or block neurohormone release. Induction of membrane depolarization with elevated [K⁺] or veratridine led to a rapid increase in [Ca²⁺]_i that was sensitive to block by substances which block voltage-sensitive L-type Ca²⁺ channels such as the dihydropyridine nicardipine and by D-888. Relaxin, cholecystokinin and enkephalin which have been reported to regulate vasopressin and oxytocin secretion at the nerve endings were without effect on basal [Ca²⁺]_i or K⁺-stimulated increases in [Ca²⁺]_i.