Using flash photolysis of caged Ca2+ and the membrane capacitance to monitor exocytosis, we have studied the response of single melanotrophs to a step rise in cytosolic Ca2+ concentration ([Ca2+]i). Exocytosis begins with a rapid burst. This burst is followed by a slower phase, which is inhibited at cytosolic pH 6.2, and an ultraslow phase, which is strongly temperature sensitive. The exocytic burst starts with a delay of 6-11 ms and continues at a rate that grows steeply with [Ca2+]i and is half-maximal at [Ca2+]i = 27 μM. At least 3 Ca2+ ions are required to trigger exocytosis. The rate constant at saturating [Ca2+]i suggests that exocytosis of a dense core vesicle takes 40 ms after all Ca2+ ions have bound to their regulatory sites. If docked dense core vesicles cause the exocytic burst, they must decorate the plasma membrane at a mean density of 0.5/μm2.
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