GLYBURIDE AND TOLBUTAMIDE INDUCE DESENSITIZATION OF INSULIN RELEASE IN RAT PANCREATIC-ISLETS BY DIFFERENT MECHANISMS

Insulin secretion was studied in rat pancreatic islets after 24-h exposure to various glyburide or tolbutamide concentrations. Glucose-induced insulin release was significantly (P < 0.05) reduced in islets cultured with 0.1 mum glyburide or 100 mum tolbutamide (2098 +/- 187, 832 +/- 93, and 989 +/- 88 pg/islet.h in control, glyburide-exposed, and tolbutamide-exposed islets, respectively). When glyburide-treated islets were stimulated with glyburide or tolbutamide, insulin release was also impaired compared to that in control islets (P < 0.05). In contrast, tolbutamide-exposed islets showed an impaired response to tolbutamide, but a normal response to glyburide. To investigate the mechanism of the sulfonylurea-induced impairment of insulin secretion, we measured insulin release and Rb+ efflux (a marker of the K+ channel activity) in a perifusion system and islet Ca2+ uptake under static conditions. Insulin release in response to 16.7 mm glucose increased in control islets from 9.4 +/- 1.1 to 131 +/- 19 pg/islet.min (first phase secretion peak). Simultaneously, the fractional Rb-86+ efflux declined from 0.015 +/- 0.002% to 0.006 +/- 0.001% (change in decrement, -63.5%). Glucose-induced insulin release in glyburide- and tolbutamide-treated islets was significantly reduced (first phase peak, 22.1 +/- 5 and 39.7 +/- 8 pg/islet. min, respectively; P < 0.05), and the fractional Rb-86+ efflux decrement was -21 +/- 6% for glyburide (P < 0.005 us. control islets) and -65 +/-4% (not different from control) tor tolbutamide. When glyburide- or tolbutamide-exposed islets were stimulated with the corresponding sulfonylurea, insulin release was impaired compared to that in control islets (P < 0.05), but, again, Rb-86+ efflux was impaired (P < 0.05) only in glyburide-exposed islets. When Ca-45(2+) uptake was studied, the increase in glucose concentration from 2.8 to 16.7 mM increased calcium uptake in control islets from 1.76 +/- 0.58 to 7.27 +/- 1.36 pmol/islet 2 min (n = 4). Preexposure to 0.1 pm glyburide did not change calcium uptake at a glucose concentration of 2.8 mm (1.44 +/- 0.45 pmol/islet-2 min) but significantly reduced calcium uptake stimulated by 16.7 mM glucose (3.21 +/- 0.35 pmol/islet.2 min; n = 4; P < 0.005 compared to control islets). In contrast, preexposure to 100 muM tolbutamide did not change either basal or glucose-stimulated calcium uptake (1.44 +/- 0.45 and 6.90 +/- 0.81 pmol/islet.2 min, respectively; n = 4). These data show that in vitro chronic exposure of pancreatic islets to the sulfonylureas glyburide and tolbutamide impairs their ability to respond to a subsequent glucose or sulfonylurea stimulation. The impaired insulin secretion is accompanied by alterations of ionic fluxes (reduced K+ channel closure and reduced calcium uptake) only in islets preexposed to glyburide, but not to tolbutamide, suggesting a different mechanism for the two sulfonylureas in inducing islet desensitization.