GTP and Ca2+Modulate the Inositol 1,4,5-Trisphosphate-Dependent Ca2+Release in Streptolysin O-Permeabilized Bovine Adrenal Chromaffin Cells

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Abstract
The inositol 1,4,5‐trisphosphate (IP3)‐induced Ca2+ release was studied using streptolysin O‐permeabilized bovine adrenal chromaffin cells. The IP3‐induced Ca2+ release was followed by Ca2+ reuptake into intracellular compartments. The IP3‐induced Ca2+ release diminished after sequential applications of the same amount of IP3. Addition of 20 μM GTP fully restored the sensitivity to IP3. Guanosine 5′‐O‐(3‐thio)triphosphate (GTPγS) could not replace GTP but prevented the action of GTP. The effects of GTP and GTPγS were reversible. Neither GTP nor GTPγS 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 μM free Ca2+, a half‐maximal Ca2+ release was elicited with ∼0.1 μM IP3. At 1 μM free Ca2+, no Ca2+ release was observed with 0.1 μM IP3; at this Ca2+ concentration, higher concentrations of IP3 (0.25 μM) were required to evoke Ca2+ release. At 8 μM free Ca2+, even 0.25 μM IP3 failed to induce release of Ca2+ from the store. The IP3‐induced Ca2+ release at constant low (0.2 μM) free Ca2+ concentrations correlated directly with the amount of stored Ca2+. Depending on the filling state of the intracellular compartment, 1 mol of IP3 induced release of between 5 and 30 mol of Ca2+.