1985 Volume 16 Issue 2 Pages 183-186
The enhancement of cytoplasmic Ca2+ level upon stimulation is known to trigger the rapid response of secretory cells, such as platelets. The role of extracellular Ca2+ on activation of phospholipases A2 and C activities was studied.
In the presence of 1mM EGTA, the level of platelet aggregation by thrombin was reduced to 20% of that in the presence of 1mM CaCl2. The EGTA-suppressed platelet restored aggregability by addition of extra Ca2+ concentration. Thrombin (0.3U/ml)-induced serotonin release assessed at 1min was independent of the concentrations of EGTA or CaCl2. The increased phosphatidylinositol (PI) turnover was observed as inferred by a decreased level of PI and increased 1, 2-diacyl glycerol and phosphatidic acid in [3H] arachidonic acid (AA)-labeled cells stimulated with thrombin (0.1 and 1U/ml). The increment of radioactivity in arachidonate and its metabolites was accounted for by the loss of [3H] phosphatidylcholine (PC) independent of the extracellular Ca2+ The degradation products of phospholipase A2, lysoPI and lysoPC were detected in stimulated cells with or without Ca2+ in the medium. Extracellular Ca2+ levels do not affect the degradation or resynthesis of phosphoinositides, PI, PI-4-monophosphate (DPI), and PI-4, 5-bisphosphate (TPI). These results indicate that the activation processes of phospholipases A2 and C in thrombinstimulated platelets are independent of extracellular Ca2+ level.
