Differentiation Between Glucose-Induced Desensitization of Insulin Secretion and beta-Cell Exhaustion in the HIT-T15 Cell Line

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Refractoriness of the pancreatic beta-cell to glucose stimulation plays a role in the secretory defect of NIDDM, but the mechanisms underlying this refractoriness are not clear. The purpose of this study was to determine whether the HIT-T15 pancreatic beta-cell line can be used as an investigative model for refractoriness of glucose-induced insulin secretion and, if so, whether the mechanism for this refractoriness involves alteration in stimulus-secretion coupling (desensitization) or results from exhaustion of insulin stores. In perifusion experiments, acute insulin responses (AIRs) in HIT-T15 cells progressively diminished during consecutive 5-min glucose (11.1 mmol/l) pulses (G) given every 20 min (G1 = 9.2 +/- 1.3, G2 = 4.1 +/- 1.0, G3 = 2.7 +/- 0.7, G4 = 2.5 +/- 1.1 micro U/ml). To determine whether this refractoriness for glucose extended to the potentiating effects of glucose on nonglucose secretagogues, cells were challenged with arginine after desensitization with glucose. In HIT-T15 cells, the response to the arginine pulse (16.7 +/- 5.2 micro U/ml) after three glucose pulses was significantly less (P < 0.01) than the response to a control arginine pulse (29.6 +/- 12.1 micro U/ml) preceded by an infusion of buffer in the absence of glucose pulses. Variable rest periods after desensitization allowed recovery of the AIR in HIT-T15 cells; responses 30, 60, 90, and 120 min after the desensitization procedure increased in a stepwise fashion (3.8 +/- 2.7, 4.5 +/- 2.7, 7.8 +/- 5.2, and 9.7 +/- 5.3 micro U/ml, respectively). To differentiate desensitization from exhaustion of insulin stores, studies were performed in the presence of epinephrine, a potent inhibitor of insulin secretion. In HIT-T15 cells, after three pulses of glucose during the epinephrine infusion, epinephrine was discontinued and the insulin response to a fourth pulse was assessed. The G4 AIR (1.9 +/- 0.6 micro U/ml) was markedly less than a control G4 AIR (5.4 +/- 1.2 micro U/ml) that followed an epinephrine infusion alone with no concurrent glucose pulses. beta-cell refractoriness was also induced in the HIT-T15 cell using 45-min steady-state infusions of glucose. Cells were exposed to a 45-min infusion of either 3.7 or 11.1 mmol/l glucose, rested for 20 min in the absence of glucose, and then challenged with a 5-min, 11.1 mmol/l glucose pulse. In both cases, the AIR to the 5-min pulse (10.2 +/- 5.1 and 2.9 +/- 1.4 micro U/ml after the 3.7 and 11.1 mmol/l infusion, respectively) was lower than the AIR to a control pulse (27.4 +/- 5.9 micro U/ml) not preceded by glucose infusion. These studies demonstrated that the HIT-T15 cell line is an appropriate model for studying mechanisms of beta-cell refractoriness to glucose signaling. The short-term intensive glucose stimulation paradigms used in our studies induced an abnormality in insulin secretion that is consistent with desensitization but not beta-cell exhaustion. Diabetes 47:606-611, 1998

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