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 microM. 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/micron2.
Department of Human Physiology, School of Medicine, University of California Davis 95616.
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Thomas
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Wong
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Lee
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Almers
D000085:Acetates / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000502:physiology; D001667:Binding, Competitive; D002118:Calcium / Q000378:metabolism*; D002614:Chelating Agents / Q000494:pharmacology; D003600:Cytosol / Q000378:metabolism; D005029:Ethylenediamines / Q000494:pharmacology; D005089:Exocytosis / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D007425:Intracellular Membranes / Q000378:metabolism; D009074:Melanocyte-Stimulating Hormones / Q000378:metabolism*; D009994:Osmolar Concentration; D010455:Peptides / Q000378:metabolism*; D010902:Pituitary Gland / Q000166:cytology / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000378:metabolism / Q000502:physiology*; D013696:Temperature; D013997:Time Factors
D000085:Acetates; D002614:Chelating Agents; D005029:Ethylenediamines; D010455:Peptides; D011956:Receptors, Cell Surface; C057014:DM-nitrophen; D000255:Adenosine Triphosphate; D009074:Melanocyte-Stimulating Hormones; D002118:Calcium