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.