Activation of phospholipase C in thrombin-stimulated platelets does not depend on cytoplasmic free calcium concentration.
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.
D016718:Arachidonic Acid; D001095:Arachidonic Acids / Q000097:blood; D001792:Blood Platelets / Q000187:drug effects / Q000201:enzymology*; D002118:Calcium / Q000097:blood*; D003593:Cytoplasm / Q000378:metabolism; D004075:Diglycerides / Q000097:blood; D006801:Humans; D010712:Phosphatidic Acids / Q000097:blood; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols / Q000097:blood; D010740:Phospholipases / Q000097:blood*; D012701:Serotonin / Q000097:blood; D013917:Thrombin / Q000494:pharmacology*; D010738:Type C Phospholipases / Q000097:blood*
D001095:Arachidonic Acids; D004075:Diglycerides; D010712:Phosphatidic Acids; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols; D016718:Arachidonic Acid; D012701:Serotonin; D010740:Phospholipases; D010738:Type C Phospholipases; D013917:Thrombin; D002118:Calcium