title: Evidence for multiple metabolic pools of phosphatidylinositol in stimulated platelets.
abstract: Stimulation of platelets with ionophore A23187 or thrombin indicates the existence of three distinct metabolic fractions of phosphatidylinositol. Two of those pools of phosphatidylinositol are degraded by phosphatidylinositol-specific phospholipase C and the third one by a phospholipase A2 activity. Low concentrations of ionophore A23187 (100 nM) or thrombin (0.25 units/ml) induce the degradation by phospholipase C of a minor fraction of phosphatidylinositol which is involved in the phosphatidylinositol cycle. In addition, thrombin, but not ionophore A23187, leads to the degradation by phospholipase C of a larger fraction of phosphatidylinositol and the subsequent accumulation of phosphatidic acid. A third fraction of phosphatidylinositol, sensitive to thrombin (0.5-2 units/ml) or ionophore A23187 (0.5-2 microM), can be degraded by phospholipase A2 to lysophosphatidylinositol with the concomitant liberation of arachidonic acid. Degradation of phosphatidylinositol by the phospholipase C pathway precedes that of the phospholipase A2 pathway. The results also suggest that the phosphatidylinositol cycle is sensitive to a small rise in cytosolic Ca2+ concentration. A further mobilization of cytosolic Ca2+ interrupts the phosphatidylinositol cycle by inhibiting conversion of phosphatidic acid to phosphatidylinositol and also activates phospholipases of the A2 type.
mesh_terms: D000818:Animals; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D010716:Phosphatidylinositols / Q000097:blood*; D013917:Thrombin / Q000494:pharmacology; D013997:Time Factors
chemical_list: D010716:Phosphatidylinositols; D000001:Calcimycin; D013917:Thrombin; D002118:Calcium