Multiple inositol 1,4,5-trisphosphate receptor isoforms are present in platelets.

Platelets are activated by an increase in cytosolic Ca(2+), and a portion of this increase is derived from inositol 1,4,5-trisphosphate (InsP3)-mediated Ca(2+) release from internal stores via the InsP3 receptor. There is some uncertainty concerning the localization of the InsP3 receptor within platelets, and experiments were designed to help resolve this question. [3H]InsP3 binding to unphosphorylated and phosphorylated platelet internal membranes revealed both low and high affinity InsP3 binding sites, indicating the presence of more than one isoform of InsP3 receptor within the internal membranes. Phosphorylation did not significantly affect InsP3 binding. In contrast, a single class of high affinity sites was observed in plasma membranes indicating only one type of InsP3 receptor. Western blotting of platelet internal and plasma membranes with antibodies against the three major InsP3 receptor isoforms revealed that the internal membranes contain both type 1 and type 2 InsP3 receptors while the plasma membrane contains only InsP3 receptor type 2.