Ca2+ efflux from platelets

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Abstract
Large amounts of Ca2+ (almost 20 nmol/108 cells) are released from platelets by exocytosis. This secretory-granule-associated Ca2+ does not contribute to the cytosolic free Ca2+ ([Ca2+]i), which is controlled by the much smaller agonist-sensitive Ca2+ pool, unless high (1 μM), but not low (0.04 μM) concentrations of ionomycin are present. Low concentrations of ionomycin release Ca2+ almost exclusively from the agonist-sensitive stores. In aspirinated platelets incubated in the presence of 0.5 mM EGTA the extensive depletion of the agonist-sensitive stores is obtained by the combined action of low ionomycin and the endomembrane Ca2+-ATPase inhibitor thapsigargin (which individually promote only a partial depletion). The subsequent decay of [Ca2+]i is increased by phorbol-myristate acetate, confirming that Ca2+ efflux from platelets is potentiated by the activation of protein kinase C [Pollock, W. K, Sage, S. O. & Rink, T. J. (1987) FEBS Lett. 210, 132–140]. A novel type of control of Ca2+ efflux appears to be exerted by the filling state of the stores. Treatment with low ionomycin or thapsigargin determines the release of a fraction of the stores-associated Ca2+; the subsequent decay of [Ca2+]i is slow. The decay rate of [Ca2+]i accelerates after extensive depletion of the stores following the addition of thapsigargin or ionomycin. If the depletion of the stores is induced by thrombin, added alone or in combination with thapsigargin, the increases of [Ca2+]i are the same and the subsequent decay rates are largely superimposable; however a large fraction of [Ca2+]i is reaccumulated into the stores in the absence, but not in the presence of thapsigargin, indicating that Ca2+ efflux is activated when the stores are empty. Ca2+ efflux can proceed against a concentration gradient. In 45Ca-loaded platelets, the thrombin-promoted 45Ca efflux is potentiated by thapsigargin. The protein-kinase-C-dependent and store-depletion-dependent stimulations of 45Ca efflux are additive. These observations indicate that, in addition to being activated by protein kinase C, Ca2+ efflux from platelets is activated by the depletion of the stores. The two activations appear to be additive.

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