] i has not been raised by the first pathway; i.e., as long as glucose has not reached its threshold concentration. The alteration of this hierarchy by long-acting sulfonylureas or genetic inactivation of K ATP channels may lead to inappropriate insulin secretion at low glucose. The amplifying pathway serves to optimize the secretory response not only to glucose but also to nonglucose stimuli. It is impaired in ␤-cells of animal models of type 2 diabetes, and indirect evidence suggests that it is altered in ␤-cells of type 2 diabetic patients. Besides the available drugs that act on K ATP channels and increase the triggering signal, novel drugs that correct a deficient amplifying pathway would be useful to restore adequate insulin secretion in type 2 diabetic patients.

Decreases in both b-cell function and number can contribute to insulin deficiency in type 2 diabetes. Here, we quantified the b-cell mass in pancreas obtained at autopsy of 57 type 2 diabetic (T2D) and 52 non-diabetic subjects of European origin. Sections from the body and tail were immunostained for insulin. The b-cell mass was calculated from the volume density of b-cells (measured by point-counting methods) and the weight of the pancreas. The pancreatic insulin concentration was measured in some of the subjects. b-cell mass increased only slightly with body mass index (BMI). After matching for BMI, the b-cell mass was 41% (BMI < 25) and 38% (BMI 26-40) lower in T2D compared with non-diabetic subjects, and neither gender nor type of treatment influenced these differences. b-cell mass did not correlate with age at diagnosis but decreased with duration of clinical diabetes (24 and 54% lower than controls in subjects with <5 and >15 years of overt diabetes respectively). Pancreatic insulin concentration was 30% lower in patients. In conclusion, the average b-cell mass is about 39% lower in T2D subjects compared with matched controls. Its decrease with duration of the disease could be a consequence of diabetes that, with further impairment of insulin secretion, contributes to the progressive deterioration of glucose homeostasis. We do not believe that the small difference in b-cell mass observed within 5 years of onset could cause diabetes in the absence of b-cell dysfunction.

No abstract

] c oscillations in all beta cells of an individual islet induce pulsatile insulin secretion, but these features of the signal and response are dampened in groups of intrinsically asynchronous islets. Glucose has hardly any influence on the amplitude of [Ca 2+ ] c oscillations and mainly controls the time course of triggering signal. Amplitude modulation of insulin secretion pulses largely depends on the amplifying pathway. There are more similarities than differences between the two phases of glucose-induced insulin secretion. Both are subject to the same dual, hierarchical control over time and amplitude by triggering and amplifying pathways, suggesting that the second phase is a sequence of iterations of the first phase.

Glucose stimulation of insulin release involves closure of ATPsensitive K+ channels, depolarization, and Ca2+ influx in B cells. Mouse islets were used to investigate whether glucose can still regulate insulin release when it cannot control ATP-sensitive K+ channels. Opening ofthese channels by diazoxide (100-250 Mmol/liter) blocked the effects of glucose on B cell membrane potential (intracellular microelectrodes), free cytosolic Ca2+ (fura-2 method), and insulin release, but it did not prevent those of high K (30 mmol/liter). K-induced insulin release in the presence of diazoxide was, however, dose dependently increased by glucose, which was already effective at concentrations (2-6 mmol/liter) that are subthreshold under normal conditions (low K and no diazoxide). This effect was not accompanied by detectable changes in B cell membrane potential.Measurements of 'Ca fluxes and cytosolic Ca2" indicated that glucose slightly increased Ca2+ influx during the first minutes of depolarization by K, but not in the steady state when its effect on insulin release was the largest. In conclusion, there exists a mechanism by which glucose can control insulin release independently from changes in K+-ATP channel activity, in membrane potential, and in cytosolic Ca2+. This mechanism may serve to amplify the secretory response to the triggering signal (closure of K+-ATP channels -depolarization -Ca2" influx) induced by glucose. (J. Clin. Invest. 1992. 89:1288-1295

The direct effects of glucocorticoids on pancreatic ␤ cell function were studied with normal mouse islets. Dexamethasone inhibited insulin secretion from cultured islets in a concentration-dependent manner: maximum of ‫ف‬ 75% at 250 nM and IC 50 at ‫ف‬ 20 nM dexamethasone. This inhibition was of slow onset (0, 20, and 40% after 1, 2, and 3 h) and only slowly reversible. It was prevented by a blocker of nuclear glucocorticoid receptors, by pertussis toxin, by a phorbol ester, and by dibutyryl cAMP, but was unaffected by an increase in the fuel content of the culture medium. Dexamethasone treatment did not affect islet cAMP levels but slightly reduced inositol phosphate formation. After 18 h of culture with or without 1 M dexamethasone, the islets were perifused and stimulated by a rise in the glucose concentration from 3 to 15 mM. Both phases of insulin secretion were similarly decreased in dexamethasone-treated islets as compared with control islets. This inhibition could not be ascribed to a lowering of insulin stores (higher in dexamethasone-treated islets), to an alteration of glucose metabolism (glucose oxidation and NAD(P)H changes were unaffected), or to a lesser rise of cytoplasmic Ca 2 ϩ in ␤ cells (only the frequency of the oscillations was modified). Dexamethasone also inhibited insulin secretion induced by arginine, tolbutamide, or high K ϩ . In this case also the inhibition was observed despite a normal rise of cytoplasmic Ca 2 ϩ . In conclusion, dexamethasone inhibits insulin secretion through a genomic action in ␤ cells that leads to a decrease in the efficacy of cytoplasmic Ca 2 ϩ on the exocytotic process. ( J. Clin. Invest. 1997. 99:414-423.)

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

Insulin, glucagon, somatostatin and pancreatic polypeptide cells were stained by immunoperoxidase techniques and quantitated morphometrically in sections of pancreases obtained from eight control subjects, four Type 1 (insulin-dependent) and eight Type 2 (non-insulin-dependent) diabetic patients. The whole pancreas was studied to take into consideration the heterogeneous distribution of the different cell types. From the volume density of each cell type, and the weight of each lobe of the pancreas, the total mass of endocrine tissue was calculated. It averaged 1395 mg in control subjects, 413 mg in Type 1 and 1449 mg in Type 2 diabetic patients. The loss of endocrine tissue observed in the Type 1 patients was almost restricted to the lobe poor in pancreatic polypeptide cells. In these patients, B cells were practically absent (at the most seven per section), but the 'atrophic islets' still contained numerous A, D, or pancreatic polypeptide cells. The mass of A, D and pancreatic polypeptide cells and the ratio of D to A cells were not different from those measured in the control subjects. This shows that the disappearance of B cells in Type 1 diabetes has no preferential effect on any other endocrine cell of the pancreas. In Type 2 diabetes, the mass of A cells was increased, whereas that of B, D and pancreatic polypeptide cells was not changed. This hyperplasia of A cells leads to a decrease in the ratio of B to A and of D to A cells. These alterations may enlighten certain aspects of the physiopathology of Type 2 diabetes.