A low affinity Ca²⁺ receptor controls the final steps in peptide secretion from pituitary melanotrophs

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.