Sulphonylurea effects on insulin secretion in islets desensitized to glucose

Prolonged, continuous exposure of the islets of Langerhans to high glucose concentrations results in desensitization of the beta cell to glucose stimulation. This study tested the ability of a sulphonylurea to stimulate insulin secretion in this setting. Normal isolated rat islets were cultured for 18-20 h in RPMI-1640 with 300 mg/dl glucose to induce desensitization or with 100 mg/dl as a control. Islets were then placed into a perifusion system and perifused with 60 mg/dl glucose followed by a stimulus. After preincubation at 300 mg/dl, a significant 50% suppression of glucose-induced insulin secretion compared with secretion in the control group preincubated at 100 mg/dl glucose was observed (p less than 0.025-0.001). This confirmed the occurrence of desensitization to glucose in this in vitro model. In contrast, stimulation of insulin secretion by glyburide (500 ng/ml) was unaffected compared with control. We also tested whether glyburide corrects the defective response to glucose stimulation in glucose-desensitized islets. Control islets (preincubated at 100 mg/dl) were stimulated with 300 mg/dl glucose or with this glucose concentration plus glyburide. Peak incremental insulin responses were similar (0.81 +/- 0.07 and 0.77 +/- 0.12 microU/ml.islet). After preincubation at 300 mg/dl, perifusion with 300 mg/dl glucose alone or with glyburide was associated with smaller, but similar, peak insulin responses (0.53 +/- 0.13 and 0.62 +/- 0.06 microU/ml.islet). In conclusion, islets in which the insulin-secretory response is compromised by desensitization to glucose are nevertheless completely responsive to the direct stimulatory effects of a sulphonylurea. However, the sulphonylurea does not correct the defect in glucose-induced insulin secretion.