The Regulatory Mechanisms of Inositol Trisphosphate Generation and Ca++ Release from the Storage Sites of Human Umbilical Vein Endothelial Cells with References to the Prostacyclin Generation

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The role of Ca++ mobilization from storage sites on the prostacyclin (PGI2) generation was investigated utilizing saponized human umbilical vein endothelial cells (HUVEC). Bradykinin and thrombin increased PGI2 generation, Inositol 1, 4, 5-trisphosphate (IP3) formation and cytosolic free Ca++ concentration ([Ca++]i). And these increases induced by bradykinin or thrombin were not observed in Ca++ free solution, while their basal levels were maintained (Fig. 4, 6, 7). IP3 increased 45Ca release from the storage sites in saponized HUVEC, and it was 35.7% of Ca ionophore releasable 45Ca. Its increase was not influenced by the pretreatment of antimycin and oligomycin (Fig. 8). Through these results it was concluded that; 1) The extracellular Ca++ was not required for the maintenance of basal PGI2 generation. 2) The enhancement of PGI2 generation induced by bradykinin or thrombin was brought about from the increase of [Ca++]i, which was induced by the enhanced Ca++ influx from the extracellular space and by the Ca++ release from the storage sites triggered by IP3 generation. 3) The first direct evidence was shown that IP3 enhanced Ca++ release from the non-mitochondrial storage sites in HUVEC, and the amount was 35.7% of Ca ionophore releasable Ca++.
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