Increased cytosolic calcium. A signal for sulfonylurea-stimulated insulin release from beta cells.
The mechanisms by which glyburide and tolbutamide signal insulin secretion were examined using a beta cell line (Hamster insulin-secreting tumor (HIT) cells). Insulin secretion was measured in static incubations, free cytosolic Ca2+ concentration ([Ca2+]i) was monitored in quin 2-loaded cells, and cAMP quantitated by radioimmunoassay. Insulin secretory dose-response curves utilizing static incubations fit a single binding site model and established that glyburide (ED50 = 112 +/- 18 nM) is a more potent secretagogue than tolbutamide (ED50 = 15 +/- 3 microM). Basal HIT cell [Ca2+]i was 76 +/- 7 nM (mean +/- S.E., n = 141) and increased in a dose-dependent manner with both glyburide and tolbutamide with ED50 values of 525 +/- 75 nM and 67 +/- 9 microM, respectively. The less active tolbutamide metabolite, carboxytolbutamide, had no effect on [Ca2+]i or insulin secretion. Chelation of extracellular Ca2+ with 4 mM EGTA completely inhibited the sulfonylurea-induced changes in [Ca2+]i and insulin release and established that the rise in [Ca2+]i came from an extracellular Ca2+ pool. The Ca2+ channel blocker, verapamil, inhibited glyburide- or tolbutamide-stimulated insulin release and the rise in [Ca2+]i at similar concentrations with IC50 values of 3 and 2.5 microM, respectively. At all concentrations tested, the sulfonylureas did not alter HIT cell cAMP content. These findings provide direct experimental evidence that glyburide and tolbutamide allow extracellular Ca2+ to enter the beta cell through verapamil-sensitive, voltage-dependent Ca2+ channels, causing a rise in [Ca2+]i which is the second messenger that stimulates insulin release.
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D005576:Colforsin / Q000494:pharmacology; D006224:Cricetinae; D000242:Cyclic AMP / Q000378:metabolism; D005905:Glyburide / Q000494:pharmacology*; D007328:Insulin / Q000378:metabolism*; D007515:Islets of Langerhans / Q000187:drug effects* / Q000378:metabolism; D014044:Tolbutamide / Q000031:analogs & derivatives / Q000494:pharmacology*; D014700:Verapamil / Q000494:pharmacology
D007328:Insulin; D005576:Colforsin; C006913:carboxytolbutamide; D014044:Tolbutamide; D014700:Verapamil; D000242:Cyclic AMP; D005905:Glyburide; D002118:Calcium