Nutrient Control of Insulin Secretion in Isolated Normal Human Islets

Abstract

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

Lysosomal acid lipase and lipophagy are constitutive negative regulators of glucose-stimulated insulin secretion from pancreatic beta cells

“…2e). This is consistent with the mechanism of action of forskolin (raising cAMP), which is not thought to contribute to the amplification pathway employed by glucose [3,4]. Collectively these results suggest that LAL inhibition has a greater impact on the secretory response to glucose than non-nutrient secretagogues, and acts in the amplification pathway.…”

“…To investigate the amplification pathway [3,4], MIN6 cells were stimulated in the combined presence of diazoxide (to circumvent metabolic closure of K + ATP channels) and KCl (to provide an independent depolarisation stimulus). Under these conditions, raising glucose from 2 to 20 mmol/l doubled insulin secretion, indicative of the amplification pathway (Fig.…”

“…Glucose-stimulated insulin secretion (GSIS) comprises two separate but interconnected metabolic pathways in the beta cell: the initiation pathway, which is K + ATP channel dependent and results in increased calcium influx [1,2], and the amplification pathway, which potentiates insulin secretion independently of further changes in cytosolic calcium [3,4]. The mechanisms underlying amplification are much less well understood than those of initiation [5][6][7][8] but the turnover of endogenous lipid stores, and consequent generation of a lipid signalling molecule, is one possibility [5,6,9,10].…”

Oscillatory control of insulin secretion

“…The dose-response relationship for glucose-induced insulin secretion is sigmoidal with a species-dependent stimulatory threshold at 3-7 mM and a steep increase, reaching maximum at 15-30 mM of the sugar (Henquin et al, 2006;Salehi et al, 2006). Such glucose dependence is consistent with the fact that high K m GLUT2 is the dominating glucose transporter in rodent β-cells Johnson et al, 1990).…”

Voltage-Gated Ion Channels in Human Pancreatic β-Cells: Electrophysiological Characterization and Role in Insulin Secretion

“…Diabetes 57:1618-1628, 2008 V oltage-gated plasmalemmal ion channels play a fundamental role in stimulus secretion coupling in ␤-cells, and Ca 2ϩ influx through voltage-gated Ca 2ϩ channels triggers exocytosis of insulin-containing secretory granules (1). Voltage-gated Ca 2ϩ channels are activated by coordinated fluctuations of the cell membrane potential (electrical activity), which are initiated by the glucose-induced closure of ATP-sensitive K ϩ channels (K ATP channels) (2,3) and dependent on voltagegated Na ϩ and K ϩ channels (4). Due to the limited availability of human islets, few electrophysiological studies of voltage-gated ion channels in human ␤-cells have been published, and the identity of the ␤-cells was not unequivocally established (5)(6)(7)(8), although 45% of normal human islets cells are non-␤-cells (9).…”