Pharmacological Characterization of Inositol 1,4,5-tris Phosphate Receptors in Human Platelet Membranes

The phosphatidylinositol (PI) hydrolysis signaling system has been shown to be altered in platelets of depressed and schizophrenic subjects. Inositol (1,4,5) trisphosphate (Ins(1,4,5)P 3 ), an integral component of the PI signaling system, mobilizes Ca 2+ by activating Ins(1,4,5)P 3 receptors. To eventually investigate the role of Ins(1,4,5)P 3 receptors in depression and other mental disorders, we characterized [ 3 H]Ins(1,4,5)P 3 binding sites in crude platelet membranes prepared from small amounts of blood obtained from healthy human control subjects. We found a single, saturable binding site for [ 3 H]Ins(1,4,5)P 3 to crude platelet membranes, which is time dependent and modulated by pH, inositol phosphates, and heparin. Since cyclic adenosine monophosphate (cAMP) and Ca 2+ have been shown to be important modulators in Ins(1,4,5)P 3 receptors, in the present study we also determined the effects of various concentrations of CaCI 2 and forskolin on Ins(1,4,5)P 3 binding to platelet membranes. CaCI 2 modulated [ 3 H]Ins(1,4,5)P 3 binding sites in a biphasic manner: at lower concentrations it inhibited [ 3 H]Ins(1,4,5)P 3 binding, whereas at higher concentrations, it stimulated [ 3 H]Ins(1,4,5)P 3 binding. On the other hand, forskolin inhibited [ 3 H]Ins(1,4,5)P 3 binding. Our results thus suggest that the pharmacological characteristics of [ 3 H]Ins(1,4,5)P 3 binding to crude platelet membranes are similar to that of Ins(1,4,5)P 3 receptors; and that both Ca 2+ and cAMP modulate [ 3 H]Ins(1,4,5)P 3 binding in crude platelet membranes.