IP3 RECEPTOR BLOCKADE FAILS TO PREVENT INTRACELLULAR CA2-1 AND ALPHA-THROMBIN( RELEASE BY ET)

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 intracellula r Ca2+ (ca(i)(2+)) release was examined using fura 2 microspectrofluor ometry in single Chinese hamster ovary cells and myoblasts. Blockade o f the IP3 receptor was achieved by microinjection of heparin or monocl onal antibody (MAb) 18A10 into the IP3 type 1 receptor. Heparin comple tely inhibited Ca-i(2+) release after flash photolysis with caged IP3 and after exposure to PDGF and FGF. In contrast, heparin failed to blo ck Ca-i(2+) release after alpha-thrombin and ET-1. After application o f 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 bu t 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 kina se and certain 7-transmembrane receptors may differ Although both rece ptor types use the IP3-signaling system, the ET-1 and alpha-thrombin r eceptors may have a second, alternative mechanism for activating Ca-i( 2+) release.