Cellular responses to Ca2+ from extracellular and intracellular sources are different as shown by simultaneous measurements of cytosolic Ca2+ and secretion from bovine chromaffin cells.
Bovine adrenal medullary cells, cultured on quartz plates, were superfused with buffer to which pulses of stimulant were added. Cytosolic Ca2+ was measured by the fura-2 fluorescence method and the simultaneously released catecholamine was measured electrochemically. When stimulant concentrations were adjusted to given equivalent elevations of cytosolic Ca2+, secretion depended entirely on whether Ca2+ came from internal stores or from the extracellular medium. Calcium from internal stores did not support secretion under these conditions. This nonequivalence of the two sources of cytosolic Ca2+ points to important differences in the physiological roles of the two sources of calcium. Dimethylphenylpiperazinium (a cholinergic agonist) and elevated K+ increased cytosolic Ca2+ and caused secretion only in the presence of external Ca2+. Bradykinin, muscarine, and ATP elevated cytosolic Ca2+ in the presence and absence of extracellular Ca2+ but caused secretion only in the presence of extracellular Ca2+. UTP, which in the absence of extracellular Ca2+ elevated cytosolic Ca2+ as effectively as ATP, did not cause detectable secretion under any circumstance. Because of the high Ca2+-buffering capacity of the cytosol, we expected that Ca2+ gradients, perhaps quite steep, would be produced by a pulse of Ca2+ entering the cytosol. Fura-2 fluorescence measures only the average free cytosolic Ca2+. Our data show that Ca2+ entering across the plasma membrane was much more effective at triggering exocytosis than was Ca2+ released from internal stores, suggesting that the two sources of Ca2+ are effectively compartmentalized, probably by concentration gradients in the cytosol.
Kim
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KT
Program in Molecular and Cellular Biology, University of Massachusetts, Amherst 01003.
Westhead
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D000313:Adrenal Medulla / Q000187:drug effects / Q000378:metabolism*; D000818:Animals; D001572:Benzofurans; D001920:Bradykinin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002395:Catecholamines / Q000378:metabolism; D002417:Cattle; D003600:Cytosol / Q000378:metabolism; D005089:Exocytosis / Q000187:drug effects; D005456:Fluorescent Dyes; D016257:Fura-2; D007700:Kinetics; D009116:Muscarine / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D013050:Spectrometry, Fluorescence; D014544:Uridine Triphosphate / Q000494:pharmacology
D001572:Benzofurans; D002395:Catecholamines; D005456:Fluorescent Dyes; D009116:Muscarine; D011188:Potassium; D001920:Bradykinin; D002118:Calcium; D016257:Fura-2; D014544:Uridine Triphosphate