DIFFERENTIATION BETWEEN GLUCOSE-INDUCED DESENSITIZATION OF INSULIN-SECRETION AND BETA-CELL EXHAUSTION IN THE HIT-T15 CELL-LINE

Refractoriness of the pancreatic beta-cell to glucose stimulation play s a role in the secretory defect of NIDDM, but the mechanisms underlyi ng this refractoriness are not clear. The purpose of this study was to determine whether the HIT-T15 pancreatic beta-cell line can be used a s an investigative model for refractoriness of glucose-induced insulin secretion and, if so, whether the mechanism for this refractoriness i nvolves 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 diminis hed during consecutive 5-min glucose (11.1 mmol/l) pulses (G) given ev ery 20 min (G(1) = 9.2 +/- 1.3, G(2) = 4.1 +/- 1.0, G(3) = 2.7 +/- 0.7 , G(4) = 2.5 +/- 1.1 mu U/ml). To determine whether this refractorines s for glucose extended to the potentiating effects of glucose on nongl ucose secretagogues, cells were challenged with arginine after desensi tization with glucose. In HIT-T15 cells, the response to the arginine pulse (16.7 +/- 5.2 mu U/ml) after three glucose pulses was significan tly less (P < 0.01) than the response to a control arginine pulse (29. 6 +/- 12.1 mu U/ml) preceded by an infusion of buffer in the absence o f glucose pulses. Variable rest periods after desensitization allowed recovery of the AIR in HIT-T15 cells; responses 30, 60, 90, and 120 mi n 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 mu U/ml, respe ctively). To differentiate desensitization from exhaustion of insulin stores, studies were performed in the presence of epinephrine, a poten t inhibitor of insulin secretion. In HIT-T15 cells, after three pulses of glucose during the epinephrine infusion, epinephrine was discontin ued and the insulin response to a fourth pulse was assessed. The G(4) AIR (1.9 +/- 0.6 mu U/ml) was markedly less than a control G(4) AIR (5 .4 +/- 1.2 mu U/ml) that followed an epinephrine infusion alone with n o concurrent glucose pulses. beta-cell refractoriness was also induced in the HIT-T15 cell using 45-min steady-state infusions of glucose. C ells were exposed to a 45-min infusion of either 3.7 or 11.1 mmol/l gl ucose, rested for 20 min in the absence of glucose, and then challenge d with a 5-min, 11.1 mmol/l glucose pulse. In both cases, the AIR to t he 5-min pulse (10.2 +/- 5.1 and 2.9 +/- 1.4 mu U/ml after the 3.7 and 11.1 mmol/l infusion, respectively) was lower than the AIR to a contr ol pulse (27.4 +/- 5.9 mu U/ml) not preceded by glucose infusion. Thes e studies demonstrated that the HIT-T15 cell line is an appropriate mo del for studying mechanisms of beta-cell refractoriness to glucose sig naling. The shortterm intensive glucose stimulation paradigms used in our studies induced an abnormality in insulin secretion that is consis tent with desensitization but not beta-cell exhaustion.