Demonstration of a Specific [3H]INS(1,4,5)P3 Binding Site in Rat Heart Sarcoplasmic Reticulum

Previous studies have suggested that inositol (1,4,5)-trisphosphate (InsP3) might induce release of Ca2+ by sarcoplasmic reticulum (SR) vesicles isolated from heart muscle. In this study binding sites specific for InsP3 in isolated SR vesicles from heart muscle were demonstrated. Scatchard analysis of a binding isotherm indicated two binding sites, viz a high affinity site with a Kd = 20 nM and Bmax = 4.2 pmol/mg protein and a low affinity site with a Kd = 424 nM and Bmax = 31 pmol/mg protein. Specificity of binding was demonstrated by using structural analogues. In addition to the finding of binding sites for InsP3, this membrane preparation also demonstrated a possible binding site for InsP4.