Formation of inositol phosphates and Ca²⁺ mobilization in thrombin-activated human platelets

Abstract

In human platelets both membrane phospholipid metabolism and Ca²⁺ mobilization are known to be closely associated with signal transduction. Phosphoinositide metabolism and changes in cytoplasmic Ca²⁺ level were investigated in [³H] inositol-labeled and quin 2-loaded platelets respectively.
In the presence of 1mM CaCl₂ thrombin (0.1-1.0U/ml) caused a transient decrease in [³H] phosphatidylinositol 4, 5-bisphosphate (PIP₂) and the rapid formation of [³H] inositol trisphosphate (IP₃) within 10 seconds. [³H] Inositol bisphosphate was also accumulated in the early phase but [³H] inositol mono-phosphate was accumulated later. The formation of [³H] inositol phosphates was not affected by 1mM EGTA. PIP₂ was resynthesized above the initial level and [³H] phosphatidylinositol 4-phosphate (PIP) increased. [³H] Phosphatidylinositol was found to be a lipid pool for PIP and PIP₂. Cytoplasmic free Ca²⁺ concentration ([Ca²⁺] i) increased rapidly coinciding with the process of IP₃ formation and thereafter declined gradually toward the resting value. Under conditions that IP₃ formation was inhibited a small increase in [Ca²⁺] i was observed. Moreover IP₃ was observed to be a Ca²⁺ releaser from Ca²⁺-accumulating human platelet membranes.
These results indicate that a series of phosphoinositides metabolism may regulate the formation of second messengers to be contributory to human platelet activation.