Key result
Thrombin-stimulated phospholipase C activation in human platelets proceeded at the same rate regardless of cytoplasmic free calcium concentration, indicating a calcium-independent pathway.
Phospholipase C activation in thrombin-stimulated platelets is independent of cytoplasmic free calcium concentration, unlike phospholipase A2.
Does not support altering antiplatelet approaches; leaves open selective targeting of calcium-independent platelet pathways.
Human platelets loaded with the fluorescent Ca2+ indicator quin2 and with different radioactive compounds including [3H]serotonin, [14C]arachidonic acid (AA) and [32P]orthophosphate were stimulated by thrombin under conditions producing secretion. In the absence of external Ca2+ (Ca2+e), cytoplasmic free [Ca2+], [Ca2+]i, increased to 340 nM, against 1685 nM at 1 mM [Ca2+]e. In both cases, diglyceride and phosphatidic acid production proceeded at the same rate, whereas AA release was inhibited at low [Ca2+]i. It is concluded that, at variance with phospholipase A2, phospholipase C activation does not depend on [Ca2+]i. These results give further support to the hypothesis of a Ca2+-independent pathway of cell activation involving phospholipase C and protein kinase C.
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Simon et al. (1984) studied this question. Thrombin stimulation in the absence of external Ca2+ vs. Thrombin stimulation in the presence of 1 mM external Ca2+ was evaluated on Phospholipase C activation (diglyceride and phosphatidic acid production). Thrombin-stimulated phospholipase C activation in human platelets proceeded at the same rate regardless of cytoplasmic free calcium concentration, indicating a calcium-independent pathway.
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