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

Abstract

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)P3), an integral component of the PI signaling system, mobilizesCa2+by activating Ins(1,4,5)P3receptors. To eventually investigate the role of Ins(1,4,5)P3receptors in depression and other mental disorders, we characterized [H3]Ins(1,4,5)P3binding 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 [H3]Ins(1,4,5)P3to crude platelet membranes, which is time dependent and modulated by pH, inositol phosphates, and heparin. Since cyclic adenosine monophosphate (cAMP) andCa2+have been shown to be important modulators in Ins(1,4,5)P3receptors, in the present study we also determined the effects of various concentrations ofCaCI2and forskolin on Ins(1,4,5)P3binding to platelet membranes.CaCI2modulated [3H]Ins(1,4,5)P3binding sites in a biphasic manner: at lower concentrations it inhibited [3H]Ins(1,4,5)P3binding, whereas at higher concentrations, it stimulated [3H]Ins(1,4,5)P3binding. On the other hand, forskolin inhibited [3H]Ins(1,4,5)P3binding. Our results thus suggest that the pharmacological characteristics of [3H]Ins(1,4,5)P3binding to crude platelet membranes are similar to that of Ins(1,4,5)P3receptors; and that bothCa2+and cAMP modulate [3H]Ins(1,4,5)P3binding in crude platelet membranes.