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.

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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.

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