Pancreatic islets were isolated from 16 nondiabetic organ donors and, after culture for ϳ2 days in 5 mmol/l glucose, were perifused to characterize nutrient-induced insulin secretion in human islets. Stepwise increases from 0 to 30 mmol/l glucose (eight 30-min steps) evoked concentrationdependent insulin secretion with a threshold at 3-4 mmol/l glucose, K m at 6.5 mmol/l glucose, and V max at 15 mmol/l glucose. An increase from 1 to 15 mmol/l glucose induced biphasic insulin secretion with a prominent first phase (peak increase of ϳ18-fold) and a sustained, flat second phase (ϳ10-fold increase), which were both potentiated by forskolin. The central role of ATP-sensitive K ؉ channels in the response to glucose was established by abrogation of insulin secretion by diazoxide and reversible restoration by tolbutamide. Depolarization with tolbutamide or KCl (plus diazoxide) triggered rapid insulin secretion in 1 mmol/l glucose. Subsequent application of 15 mmol/l glucose further increased insulin secretion, showing that the amplifying pathway is operative. In control medium, glutamine alone was ineffective, but its combination with leucine or nonmetabolized 2-amino-bicyclo [2,2,1]-heptane-2-carboxylic acid (BCH) evoked rapid insulin secretion. The effect of BCH was larger in low glucose than in high glucose. In contrast, the insulin secretion response to arginine or a mixture of four amino acids was potentiated by glucose or tolbutamide. Palmitate slightly augmented insulin secretion only at the supraphysiological palmitateto-albumin ratio of 5. Inosine and membrane-permeant analogs of pyruvate, glutamate, or succinate increased insulin secretion in 3 and 10 mmol/l glucose, whereas lactate and pyruvate had no effect. In conclusion, nutrientinduced insulin secretion in normal human islets is larger than often reported. Its characteristics are globally similar to those of insulin secretion by rodent islets, with both triggering and amplifying pathways. The pattern of the biphasic response to glucose is superimposable on that in mouse islets, but the concentration-response curve is shifted to the left, and various nutrients, in particular amino acids, influence insulin secretion within the physiological range of glucose concentrations. Diabetes 55: 3470 -3477, 2006

Rapid and sustained stimulation of ␤-cells with glucose induces biphasic insulin secretion. The two phases appear to reflect a characteristic of stimulus-secretion coupling in each ␤-cell rather than heterogeneity in the time-course of the response between ␤-cells or islets. There is no evidence indicating that biphasic secretion can be attributed to an intrinsically biphasic metabolic signal. In contrast, the biphasic rise in cytoplasmic Ca

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