Calcium dependence of the rate of exocytosis in a synaptic terminal

Rapid calcium-dependent exocytosis underlies neurotransmitter release from nerve terminals. Despite the fundamental importance of this process, neither the relationship between presynaptic intracellular calcium ion concentration ([Ca2+]i) and rate of exocytosis, nor the maximal rate of secretion is known quantitatively. To provide this information, we have used flash photolysis of caged Ca2+ to elevate [Ca2+]i rapidly and uniformly in synaptic terminals, while measuring membrane capacitance as an index of exocytosis and monitoring [Ca2+]i with a Ca(2+)-indicator dye. When [Ca2+]i was abruptly increased to > 10 microM, capacitance rose at a rate that increased steeply with [Ca2+]i. The steepness suggested that at least four calcium ions must bind to activate synaptic vesicle fusion. Half-saturation was at 194 microM, and the maximal rate constant was 2,000-3,000 s-1. A given synaptic vesicle can exocytose with high probability within a few hundred microseconds, if [Ca2+]i rises above 100 microM. These properties provide for the extremely rapid signalling required for neuronal communication.