Effect of GTP and Ca2+ on inositol 1,4,5-trisphosphate induced Ca2+ release from permeabilized rat exocrine pancreatic acinar cells

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