0 199 I International Society for Neurochemistry GTP and Ca2+ Modulate the Inositol 1,4,5-Trisphosphate- Dependent Ca2+ Release in Streptolysin O-Permeabilized

Abstract: The inositol I,4,5-trisphosphate (IP3)-induced Ca2+ release was studied using streptolysin O-permeabilized bovine adrenal chromaffin cells. The IP3-induced Ca2+ release was followed by Cat+ reuptake into intracellular compartments. The 1P3-induced Ca2+ release diminished after sequential ap-plications of the same amount of IP3. Addition of 20 pM GTP fully restored the sensitivity to IP3. Guanosine 5'-0-(3-thio)triphosphate (GTPyS) could not replace GTP but pre-vented the action of GTP. The effects of GTP and GTPyS were reversible. Neither GTP nor GTPyS induced release of Ca2+ in the absence of IP3. The amount of Ca2+ whose release was induced by IP3 depended on the free Ca2+ concentration of the medium. At 0.3 p M free Ca2', a half-maximal Ca2+ release was elicited with-0.1 pM IP3. At 1 pM free Ca2+, no Ca2+ release was observed with 0.1 pM IP3; at this Ca2+ concentration, higher concentrations of IP3 (0.25 pM) were required to evoke CaZ+ release. At 8 pM free Ca2+, even 0.25 pM IP3 failed to induce release of Ca2+ from the store. The IP3-induced Ca2+ release at constant low (0.2 p M) free Ca2+ concentrations correlated directly with the amount of stored Ca2+. Depending on the filling state of the intracellular com-partment, 1 mol of IP3 induced release of between 5 and 3