omega-Conotoxin inhibits glucose- and arachidonic acid-induced rises in intracellular [Ca2+] in rat pancreatic islet beta-cells.
Earlier studies suggest that the accumulation of non-esterified arachidonic acid (AA) in islets following stimulation with glucose participates in the glucose-induced secretion of insulin. A possible role for AA might include the facilitation of Ca2+ influx into islet beta-cells. Recently, we demonstrated that AA induces Ca2+ influx into purified rat pancreatic islet beta-cells, prepared by fluorescence-activated cell sorting (FACS). This effect was abolished in the presence of the Ca(2+)-chelator EGTA, but was only partially reduced by the dihydropyridine (DHP) L-type Ca(2+)-channel blocker, nifedipine. This raised the possibility that DHP-insensitive Ca2+ entry mechanisms may exist in pancreatic beta-cells, in addition to the known DHP-sensitive L-type Ca2+ channels. Here we report that omega-conotoxin (CTX), which blocks omega-type Ca(2+)-channels, inhibits AA-induced Ca2+ influx by a magnitude similar to that of nifedipine and that the combination of omega-CTX and nifedipine results in a nearly additive decrement in AA-induced increases in beta-cell cytosolic [Ca2+]. We further demonstrate that bovine serum albumin, which complexes free AA and prevents AA-induced increases in cytosolic [Ca2+], also inhibits the glucose-induced increase in beta-cell [Ca2+]. These results suggest that rat pancreatic FACS-purified islet beta-cells express omega-type (DHP-insensitive) Ca(2+)-channels, in addition to DHP-sensitive Ca(2+)-channels. They further suggest that the glucose-induced accumulation of non-esterified AA in the membranes of beta-cells serves to amplify glucose-mediated Ca2+ influx into the beta-cells.
Ramanadham
S
S
Washington University School of Medicine, Department of Medicine, St. Louis, Missouri.
Turk
J
J
D000818:Animals; D016718:Arachidonic Acid / Q000037:antagonists & inhibitors*; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D002469:Cell Separation; D002478:Cells, Cultured; D005434:Flow Cytometry; D005947:Glucose / Q000037:antagonists & inhibitors*; D007515:Islets of Langerhans / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D009543:Nifedipine / Q000494:pharmacology; D010455:Peptides / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D020866:omega-Conotoxin GVIA
D002121:Calcium Channel Blockers; D010455:Peptides; D016718:Arachidonic Acid; D020866:omega-Conotoxin GVIA; D009543:Nifedipine; D005947:Glucose; D002118:Calcium