Jean-Claude Henquin

Pancreatic β‐cell mass in European subjects with type 2 diabetes

Decreases in both β‐cell function and number can contribute to insulin deficiency in type 2 diabetes. Here, we quantified the β‐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 β‐cell mass was calculated from the volume density of β‐cells (measured by point‐counting methods) and the weight of the pancreas. The pancreatic insulin concentration was measured in some of the subjects. β‐cell mass increased only slightly with body mass index (BMI). After matching for BMI, the β‐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. β‐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 β‐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 β‐cell mass observed within 5 years of onset could cause diabetes in the absence of β‐cell dysfunction.

Evidence that glucose can control insulin release independently from its action on ATP-sensitive K+ channels in mouse B cells.

Glucose stimulation of insulin release involves closure of ATP-sensitive 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 of these channels by diazoxide (100-250 mumol/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 45Ca 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.

Cellular composition of the human diabetic pancreas

SummaryInsulin, 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.

Direct glucocorticoid inhibition of insulin secretion. An in vitro study of dexamethasone effects in mouse islets.

The direct effects of glucocorticoids on pancreatic beta cell function were studied with normal mouse islets. Dexamethasone inhibited insulin secretion from cultured islets in a concentration-dependent manner: maximum of approximately 75% at 250 nM and IC50 at approximately 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 microM 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 Ca2+ in beta cells (only the frequency of the oscillations was modified). Dexamethasone also inhibited insulin secretion induced by arginine, tolbutamide, or high K+. In this case also the inhibition was observed despite a normal rise of cytoplasmic Ca2+. In conclusion, dexamethasone inhibits insulin secretion through a genomic action in beta cells that leads to a decrease in the efficacy of cytoplasmic Ca2+ on the exocytotic process.

Regulation of insulin secretion: a matter of phase control and amplitude modulation

The consensus model of stimulus–secretion coupling in beta cells attributes glucose-induced insulin secretion to a sequence of events involving acceleration of metabolism, closure of ATP-sensitive K+ channels, depolarisation, influx of Ca2+ and a rise in cytosolic free Ca2+ concentration ([Ca2+]c). This triggering pathway is essential, but would not be very efficient if glucose did not also activate a metabolic amplifying pathway that does not raise [Ca2+]c further but augments the action of triggering Ca2+ on exocytosis. This review discusses how both pathways interact to achieve temporal control and amplitude modulation of biphasic insulin secretion. First-phase insulin secretion is triggered by the rise in [Ca2+]c that occurs synchronously in all beta cells of every islet in response to a sudden increase in the glucose concentration. Its time course and duration are shaped by those of the Ca2+ signal, and its amplitude is modulated by the magnitude of the [Ca2+]c rise and, substantially, by amplifying mechanisms. During the second phase, synchronous [Ca2+]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 [Ca2+]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.

Significance of ionic fluxes and changes in membrane potential for stimulus-secretion coupling in pancreatic B-cells

This brief review has tried to shed some light on the mechanisms and significance of the changes in membrane potential and in ionic fluxes occurring in B-cells upon glucose stimulation. There is now strong evidence that, under physiological conditions at least, these electrical events-and the underlying modifications of ionic permeabilities and fluxes — play a causal role in the stimulation of insulin release. It also seems clear that certain accompanying ionic fluxes have no direct stimulatory role, but may be important in maintaining cellular homeostasis. Recent experimental evidence has also shown that the electrical activity in B-cells is not an all-or-none stereotypic response. Not only can its intensity be adjusted to the magnitude of the stimulus, but its characteristics can also be modulated by potentiators Our knowledge of the stimulus-secretion coupling has markedly progressed over the past few years, but elucidation of several important steps remains a challenging goal. There is no doubt that parallel measurements of insulin release, of ionic fluxes and of membrane potential in B-cells will still contribute to that understanding.

Mechanisms by which glucose can control insulin release independently from its action on adenosine triphosphate-sensitive K+ channels in mouse B cells.

Glucose stimulation of insulin release involves closure of ATP-sensitive K+ channels (K(+)-ATP channels), depolarization, and Ca2+ influx in B cells. However, by using diazoxide to open K(+)-ATP channels, and 30 mM K to depolarize the membrane, we could demonstrate that another mechanism exists, by which glucose can control insulin release independently from changes in K(+)-ATP channel activity and in membrane potential (Gembal et al. 1992. J. Clin. Invest. 89:1288-1295). A similar approach was followed here to investigate, with mouse islets, the nature of this newly identified mechanism. The membrane potential-independent increase in insulin release produced by glucose required metabolism of the sugar and was mimicked by other metabolized secretagogues. It also required elevated levels of cytoplasmic Cai2+, but was not due to further changes in Cai2+. It could not be ascribed to acceleration of phosphoinositide metabolism, or to activation of protein kinases A or C. Thus, glucose did not increase inositol phosphate levels and hardly affected cAMP levels. Moreover, increasing inositol phosphates by vasopressin or cAMP by forskolin, and activating protein kinase C by phorbol esters did not mimic the action of glucose on release, and down-regulation of protein kinase C did not prevent these effects. On the other hand, it correlated with an increase in the ATP/ADP ratio in islet cells. We suggest that the membrane potential-independent control of insulin release exerted by glucose involves changes in the energy state of B cells.

Mechanisms of the stimulation of insulin release by saturated fatty acids. A study of palmitate effects in mouse beta-cells

The mechanisms by which fatty acids increase insulin release are not known. In this study, mouse islets were used as a model and palmitate as a reference compound to study in vitro how saturated fatty acids influence pancreatic β-cells. Palmitate (625 μM) was bound to albumin. It did not affect basal insulin release (3 mM glucose) but increased the release induced by 10–15 mM glucose. This effect was dependent on the concentration of free rather than total palmitate. It was reversible and abolished by epinephrine, diazoxide, nimodipine, or omission of extracellular Ca. Bromopalmitate and methyl palmoxirate, two inhibitors of fatty acid oxidation, were ineffective alone, and only bromopalmitate partially inhibited the effects of palmitate on insulin release. The increase in insulin release produced by palmitate could not be ascribed to a blockade of ATP-sensitive K+-channels because the fatty acid only barely decreased 86Rb efflux and did not depolarize β-cells in 3 mM glucose. The small effect on 86Rb efflux might be attributed to a slight rise in the ATP/ADP ratio. No such rise occurred when palmitate was tested in 15 mM glucose, and the fatty acid consistently accelerated 86Rb efflux under these conditions. Measurements of β-cell membrane potential (intracellular microelectrodes) and of free cytoplasmic calcium (Cai 2+) in β-cells (Fura 2 technique) showed that palmitate increases Ca2+ influx; it also caused a very small mobilization of intracellular Ca. The persistence of this stimulation of Ca2+ influx in the presence of diazoxide and high K+ suggests that palmitate might act on Ca2+ channels. The rise in Ca12+ produced by palmitate was accompanied by an increase in insulin release only if the concentration of glucose was sufficiently high. The β-cell response to palmitate thus differs from the responses to glucose and other metabolized nutrients in several respects. Saturated fatty acids appear to potentiate insulin release through an increase in Ca12+ and another, yet unidentified, fuel-dependent mechanism.

Opposite effects of tolbutamide and diazoxide on 86Rb+ fluxes and membrane potential in pancreatic B cells

The effects of tolbutamide and diazoxide on 86Rb+ fluxes, 45Ca2+ uptake, insulin release and B cell membrane potential have been studied in rat or mouse islets. In the presence of 3 mM glucose, tolbutamide rapidly and reversibly decreased Rb+ efflux from perifused islets and depolarised B cells. The effect on Rb+ efflux was paradoxically more marked with 20 than 100 micrograms/ml tolbutamide, at least in the presence of extracellular calcium. Addition of tolbutamide to a medium containing 6 mM glucose and calcium increased Rb+ efflux transiently with 20 micrograms/ml and permanently with 100 micrograms/ml. The drug also inhibited Rb+ influx in islet cells, but had little effect on Rb+ net uptake. Diazoxide rapidly, steadily and reversibly increased Rb+ efflux in a dose-dependent manner (20-100 micrograms/ml). When 20 micrograms/ml tolbutamide and diazoxide were combined in the presence of 3 mM glucose, only a slight decrease in Rb+ efflux was observed. The depolarisation of B cells normally produced by tolbutamide was markedly reduced and the electrical activity completely suppressed by diazoxide. In the presence of 10mM glucose, diazoxide increased Rb+ efflux from the islets and hyperpolarised B cells. Tolbutamide, tetraethylammonium and quinine reversed the increase in Rb+ efflux the inhibition of Ca2+ uptake and the suppression of insulin release produced by diazoxide. Tolbutamide rapidly reversed the hyperpolarisation and restored electrical activity. It is suggested that the stimulation and inhibition of insulin release by tolbutamide and diazoxide are due to their respective ability to decrease and to increase the K permeability of the B cell membrane. This change in K permeability leads either to depolarisation and stimulation of Ca2+ influx or to hyperpolarisation and inhibition of Ca2+ influx.

Hierarchy of the beta-cell signals controlling insulin secretion

The main function of pancreatic β cells is to synthesize and secrete insulin at appropriate rates to limit blood glucose fluctuations within a narrow range. Any alteration in β -cell functioning has a profound impact on glucose homeostasis: excessive secretion of insulin causes hypoglycaemia, and insufficient secretion leads to diabetes. It is therefore not surprising that insulin secretion is subject to very tight control. This control is primarily ensured by glucose itself but also involves an array of metabolic, neural, hormonal and sometimes pharmacological factors (Fig. 1). To integrate all these stimulatory and inhibitory influences, β cells rely on an astonishingly complex stimulus-secretion coupling. This review discusses how the hierarchy between two intracellular pathways, producing triggering and amplifying signals [1], optimizes adequate insulin secretion to changes in blood glucose concentration and enables the β cell to grade the numerous extracellular messages that it receives.