ADENOPHOSTIN-MEDICATED QUANTAL CA2-TRISPHOSPHATE RECEPTOR-TYPE-1( RELEASE IN THE PURIFIED AND RECONSTITUTED INOSITOL 1,4,5)

Kinetics of Ca2+ release by adenophostin, a novel agonist of inositol 1,4,5-trisphosphate (IP3) receptor, in the purified and reconstituted IP3 receptor type 1 (IP(3)R1) was investigated using the fluorescent C a2+ indicator fluo-3. Submaximal concentrations of adenophostin caused quantal Ca2+ release from the purified IP(3)R1 as IP3 did. Adenophost in-induced Ca2+ release by the purified IP(3)R1 exhibited a high posit ive cooperativity (nH = 3.9 +/- 0.2, EC(50) = 11 nM), whereas the IP3- induced Ca2+ release exhibited a moderate one (nH = 1.8 +/- 0.1, EC(50 ) = 100 nM), Inhibition of [H-3]IP3 binding to the purified IP(3)R1 by adenophostin exhibited a positive cooperativity (nH = 1.9, K-i = 10 n M), whereas IP3 did not (nH = 1.1, K-i = 41 nM).