Relationship between hormonal, GTP and Ins(1,4,5)P3-stimulated Ca2+ uptake and release in pancreatic acinar cells.
Electrically permeabilized rat pancreatic acini were used to evaluate the contributions of GTP and Ins(1,4,5) P3 to hormone-stimulated Ca2+ uptake and release from intracellular pools. Treatment of permeabilized acini with Ca2+-mobilizing hormones, GTP or GTP[S] resulted in stimulation of an ATP-dependent, VO4(2-)-sensitive Ca2+ uptake into a non-mitochondrial intracellular pool. GTP and GTP[S] also augmented the hormone-mediated stimulation of Ca2+ uptake. Including oxalate in the uptake medium increased Ca2+ uptake into this pool but did not modify the stimulation of Ca2+ uptake induced by hormones or GTP. Ins(1,4,5)P3 released all the extra Ca2+ accumulated as a result of hormone, GTP or GTP[S] stimulation. Hence, these stimuli activated the Ca2+ pump localized in the membrane of the hormone and Ins(1,4,5)P3-sensitive Ca2+ pool. Including 2,3-diphosphoglyceric acid (PGA) [an inhibitor of Ins(1,4,5)P3 hydrolysis] in the incubation medium blunted the GTP and GTP[S]-stimulated Ca2+ uptake. In the presence of PGA, the hormones inhibited Ca2+ accumulation, and GTP and GTP[S] augmented this effect. Accordingly, PGA stabilized the Ins(1,4,5)P3-evoked Ca2+ release from intracellular pools. Only in the presence of PGA was it possible to demonstrate hormonally-evoked Ca2+ release from permeabilized cells. GTP, and more importantly GTP[S], augmented the hormone-evoked Ca2+ release. Hormones and Ins(1,4,5)P3 in the presence or absence of GTP or GTP[S] released Ca2+ from the same intracellular pool. The extent of Ca2+ release caused by the combination of hormones and GTP or GTP[S] was similar to that evoked by Ins(1,4,5)P3 alone. Taken together, these results suggest that GTP or GTP[S] facilitates stimulation of phospholipase C by hormones. Such stimulation results in stimulation of protein kinase C and increased levels of Ins(1,4,5)P3 and is sufficient to explain the effects of GTP and GTP[S] on Ca2+ uptake and release from pancreatic acinar cells.
Muallem
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Department of Physiology, University of Texas Health Science Center, Dallas 75235.
Beeker
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D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002463:Cell Membrane Permeability; D004560:Electricity; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate / Q000031:analogs & derivatives / Q000494:pharmacology*; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology*; D010070:Oxalates / Q000494:pharmacology; D019815:Oxalic Acid; D010100:Oxygen / Q000494:pharmacology; D010179:Pancreas / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013873:Thionucleotides / Q000494:pharmacology
D010070:Oxalates; D013873:Thionucleotides; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D015544:Inositol 1,4,5-Trisphosphate; D006160:Guanosine Triphosphate; D000255:Adenosine Triphosphate; D019815:Oxalic Acid; D010100:Oxygen; D002118:Calcium