MULTIPLE INOSITOL 1,4,5-TRISPHOSPHATE RECEPTOR ISOFORMS ARE PRESENT IN PLATELETS

Platelets are activated by an increase in cytosolic Ca2+, and a portio n of this increase is derived from inositol 1,4,5-trisphosphate (InsP( 3))-mediated Ca2+ release from internal stores via the InsP(3) recepto r. There is some uncertainty concerning the localization of the InsP(3 ) receptor within platelets, and experiments were designed to help res olve this question. [H-3]InsP(3) binding to unphosphorylated and phosp horylated platelet internal membranes revealed both low and high affin ity InsP(3) binding sites, indicating the presence of more than one is oform of InsP(3) receptor within the internal membranes. Phosphorylati on did not significantly affect InsP, binding. In contrast, a single c lass of high affinity sites was observed in plasma membranes indicatin g only one type of InsP, receptor. Western blotting of platelet intern al and plasma membranes with antibodies against the three major InsP(3 ) receptor isoforms revealed that the internal membranes contain both type 1 and type 2 InsP(3) receptors while the plasma membrane contains only InsP, receptor type 2. (C) 1996 Academic Press, Inc.