Homologous desensitization of ATP-mediated elevations in cytoplasmic calcium and prostacyclin release in human endothelial cells does not involve protein kinase C

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Abstract
Single human umbilical-vein endothelial cells in culture loaded with the Ca2+-sensitive dye fura-2 exhibited characteristic increases in cytosolic Ca2+ concentrations ([Ca2+]i) in response to extracellular ATP. The rapid decline of [Ca2+]i to pre-stimulated levels in the continued presence of ATP, with in most cells no sustained or oscillatory increase in [Ca2+]i, indicated desensization. This was agonist-specific, and contrasted with the [Ca2+]i response to histamine, though each agonist mobilized Ca2+ from the same internal store. In populations of cells, when desensitization was variably induced by a second challenge with ATP after different times, desensitization of the initial peak [Ca2+]i was directly related to desensitization of prostacyclin release. This was not affected by treatment with the protein kinase C inhibitor staurosporine, under conditions where a similar degree of desensitization of peak [Ca2+]i induced by phorbol 12-myristate 13-acetate was blocked. Sequential addition of ATP to cell populations cumulatively desensitized the peak elevation of [Ca2+]i, but did not block the second, sustained, phase of the response. We conclude that desensitization of prostacyclin synthesis by ATP is likely to be due to uncoupling of the P2Y purinoceptor from phosphoinositidase C, but does not involve protein kinase C activation.