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Engling, R. and Föhr, K. J. and Kemmer, T. P. and Gratzl, Manfred (1991): Effect of GTP and Ca2+ on inositol 1,4,5-trisphosphate induced Ca2+ release from permeabilized rat exocrine pancreatic acinar cells. In: Cell Calcium, Vol. 12: pp. 1-9
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Abstract

The effects of Ca2+ and GTP on the release of Ca2+ from the inositol 1,4,5-trisphosphate (IP3) sensitive Ca2+ compartment were investigated with digitonin permeabilized rat pancreatic acinar cells. The amount of Ca2+ released due to IP3 directly correlated with the amount of stored Ca2+ and was found to be inversely proportional to the medium free Ca2+ concentration. Ca2+ release induced by 0.18 μM IP3 was half maximally inhibited at 0.5 μM free Ca2+, i.e. at concentrations observed in the cytosol of pancreatic acinar cells. GTP did not cause Ca2+ release on its own, but a single addition of GTP (20 μM) abolished the apparent desensitization of the Ca2+ release which was observed during repeated IP3 applications. This effect of GTP was reversible. GTPγS could not replace GTP. Desensitization still occurred when GTPγS was added prior to GTP. The reported data indicate that GTP, stored Ca2+ and cytosolic free Ca2+ modulate the IP3 induced Ca2+ release. EGTA, Ethylene-glycol-bis (2-aminoethylether)-N,N,N′,N′- tetra acetic acid; GTPγS, Guanosine 5′-O-[3-thio]triphosphate; GDPβS, Guanosine 5′-O-[2-thio]diphosphate; IP3, Inositol 1,4,5-trisphosphate; IP2, Inositol 1,4-bisphosphate; IP4, Inositol 1,3,4,5-tetrakisphosphate; MOPS, Morpholinopropane sulfonic acid; HEPES, 4-(2-hydroxyethyl)-1-piperazine ethanesulfonic acid; pHMB, Parahydroxymercuribenzoate