Reduced functions of intracellular Ca2+ in aggregation, secretion and protein phosphorylation of permeabilized...

Aggregation, secretion and 47kDa protein (P47) phosphorylation by various agonists such as thrombin, ADP and ionophore A23187 were markedly reduced in platelets from stroke-prone spontaneously hypertensive rats (SHRSP) compared with those of age-matched Wistar Kyoto rat (WKY) platelets, suggesting defective functions of intracellular Ca2+ in SHRSP platelets (Tomita et al. Hypertension 1989; 14: 304-315). To clarify the mechanism of the platelet hypofunctions, saponin permeabilized platelets were prepared to compare the responses of platelets from both rats in varying concentrations of extracellular Ca2+. The leakage of lactate dehydrogenase from saponin (15 micrograms/ml)-treated platelets was approx. 5% of total activity; the degree of the leakage in both platelets did not differ. In saponin-treated platelets, extracellular Ca2+ alone did not induce either aggregation or secretion in both strains. However, in the presence of 1-oleoyl-2-acetylglycerol (10 micrograms/ml), Ca2+ dose dependently stimulated both aggregation and secretion. Under this condition, Ca2+ sensitivity of aggregation, secretion and P47 phosphorylation in SHRSP platelets were significantly reduced compared with those in WKY platelets. These results strongly suggest that intracellular Ca2+ functions are impaired in SHRSP platelets.