The relationship between cytosolic Ca2+, sn-1,2-diacylglycerol and inositol 1,4,5-trisphosphate elevation in platelet-activating-factor-stimulated rabbit platelets. Influence of protein kinase C on production of signal molecules.

The temporal and dose-response relationships of platelet-activating-factor (PAF)-induced changes in the concentrations of cytosolic Ca2+ ([Ca2+]i), Ins(1,4,5)P3 and 1,2-diacylglycerol (DAG) were examined. In addition, phosphorylation of protein kinase C (PKC) substrate (40-47 kDa protein) was determined. In high-dose PAF-activated platelets, all three signal molecules increased rapidly and transiently, with the peak Ins(1,4,5)P3 concentration preceding maximal elevation of [Ca2+]i by 5 s. In low-dose PAF-activated platelets there were large increases in [Ca2+]i and dense-granule release, without any increase in Ins(1,4,5)P3 and DAG or 40-47 kDa protein phosphorylation. Staurosporine, a non-specific PKC inhibitor, produced enhanced elevations in the concentrations of Ins(1,4,5)P3, DAG and thromboxane B2, and the duration of the Ca2+ signal in platelets stimulated with a high dose, but not a low dose, of PAF. These results suggest there are both phospholipase C-dependent and -independent changes in Ca2+ homoeostasis. Endogenously activated PKC regulates the formation of signal molecules.