IP3 receptor blockade fails to prevent intracellular Ca2+ release by ET-1 and α-thrombin

The effect of inositol 1,4,5-trisphosphate (IP3) receptor blockade on platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), endothelin-1 (ET-1), or alpha-thrombin receptor-mediated intracellular Ca2+ (ca(i)(2+)) release was examined using fura 2 microspectrofluorometry in single Chinese hamster ovary cells and myoblasts. Blockade of the IP3 receptor was achieved by microinjection of heparin or monoclonal antibody (MAb) 18A10 into the IP3 type 1 receptor. Heparin completely inhibited Ca-i(2+) release after flash photolysis with caged IP3 and after exposure to PDGF and FGF. In contrast, heparin failed to block Ca-i(2+) release after alpha-thrombin and ET-1. After application of ligand, IP3 levels were five- to sevenfold higher for alpha-thrombin than for ET-1 or PDGF. IP3 levels after PDGF and ET-1 were comparable. Similar to heparin, MAb 18A10 blocked Ca-i(2+) release after PDGF but failed to block Ca-i(2+) release after ET-1 or alpha-thrombin. These data suggest that the mechanisms of Ca-i(2+) release by tyrosine kinase and certain 7-transmembrane receptors may differ Although both receptor types use the IP3-signaling system, the ET-1 and alpha-thrombin receptors may have a second, alternative mechanism for activating Ca-i(2+) release.