Etelvina Andreu

Rapid insulinotropic effect of 17β-estradiol via a plasma membrane receptor

Impaired insulin secretion is a hallmark in both type I and type II diabetic individuals. Whereas type I (insulin‐dependent diabetes mellitus) implies β‐cell destruction, type II (non‐insulin dependent diabetes mellitus), responsible for 75% of diabetic syndromes, involves diminished glucose‐dependent secretion of insulin from pancreatic β‐cells. Although a clear demonstration of a direct effect of 17β‐estradiol on the pancreatic β‐cell is lacking, an in vivo insulinotropic effect has been suggested. In this report we describe the effects of 17β‐estradiol in mouse pancreatic β‐cells. 17β‐Estradiol, at physiological concentrations, closes KATP channels, which are also targets for antidiabetic sulfonylureas, in a rapid and reversible manner. Furthermore, in synergy with glucose, 17β‐estradiol depolarizes the plasma membrane, eliciting electrical activity and intracellular calcium signals, which in turn enhance insulin secretion. These effects occur through a receptor located at the plasma membrane, distinct from the classic cytosolic estrogen receptor. Specific competitive binding and localization of 17β‐estradiol receptors at the plasma membrane was demonstrated using confocal reflective microscopy and immunocytochemistry. Gaining deeper knowledge of the effect induced by 17β‐estradiol may be important in order to better understand the hormonal regulation of insulin secretion and for the treatment of NIDDM.— Nadal, A., Rovira, J. M., Laribi, O., Leon‐Quinto, T., Andreu, E., Ripoll, C., Soria, B. Rapid insulinotropic effect of 17b‐estradiol via a plasma membrane receptor. FASEB J. 12, 1341–1348 (1998)

Junctional communication of pancreatic beta cells contributes to the control of insulin secretion and glucose tolerance.

Proper insulin secretion requires the coordinated functioning of the numerous beta cells that form pancreatic islets. This coordination depends on a network of communication mechanisms whereby beta cells interact with extracellular signals and adjacent cells via connexin channels. To assess whether connexin-dependent communication plays a role in vivo, we have developed transgenic mice in which connexin 32 (Cx32), one of the vertebrate connexins found in the pancreas, is expressed in beta cells. We show that the altered beta-cell coupling that results from this expression causes reduced insulin secretion in response to physiologically relevant concentrations of glucose and abnormal tolerance to the sugar. These alterations were observed in spite of normal numbers of islets, increased insulin content, and preserved secretory response to glucose by individual beta cells. Moreover, glucose-stimulated islets showed improved electrical synchronization of these cells and increased cytosolic levels of Ca(2+). The results show that connexins contribute to the control of beta cells in vivo and that their excess is detrimental for insulin secretion.

Oscillation of gap junction electrical coupling in the mouse pancreatic islets of Langerhans.

1. Pancreatic beta‐cells oscillate synchronously when grouped in islets. Coupling seems essential to maintain this oscillatory behaviour, as isolated cells are unable to oscillate. This allows the islet to be used as a model system for studying the role of coupling in the generation of oscillatory patterns. 2. Pairs of beta‐cells were intracellularly recorded in islets. beta‐Cells oscillated synchronously. Propagated voltage deflections were observed as a function of glucose concentration and of the distance between the recording electrodes. Space constants were smaller in the silent than in the active phases, suggesting a higher intercellular connection in the active phases. 3. Coupling coefficients and estimated coupling conductances were larger in the active than in the silent phases. 4. Coupling coefficients and coupling conductances changed dynamically and in phase with the membrane potential oscillations, pointing to an active modulation of the gap junctions. 5. We hypothesize a role for coupling in the generation of the oscillatory events, providing different levels of permeability dependent on the state of conductance during the oscillatory phases.

Diminished Fraction of Blockable ATP-Sensitive K+ Channels in Islets Transplanted Into Diabetic Mice

The reasons for the poor outcome of islet transplantation in diabetic patients are not well known; a better understanding of the pathophysiology of transplanted islets is needed. To study the mechanism coupling secretagogue stimuli with insulin release in transplanted islets, we determined the effects of glucose, tolbutamide, and carbamylcholine on the β-cell membrane potential and cytosolic calcium concentrations ([Ca2+]i) of islets syngeneically transplanted into normal and streptozocin-induced diabetic mice. In both groups, normoglycemia was maintained after transplantation. Islets transplanted into normal recipients showed similar changes in β-cell membrane potential and [Ca2+]i oscillations to those in control islets. In contrast, when islets were transplanted into diabetic mice, bursts of electrical activity were triggered at lower glucose concentrations (5.6 mmol/l) than in control islets (11 mmol/l), and maximal electrical activity was achieved at lower glucose concentrations (11 mmol/l) than in control islets (22 mmol/l). When membrane potential was plotted as a function of glucose concentration, the dose-response curve was shifted to the left. Compared with control islets, glucose-induced [Ca2+]i oscillations were broader in duration (22.3 ± 0.6 s vs. 118.1 ± 12.6 s; P < 0.01) and higher in amplitude (135 ± 36 nmol/l vs. 352 ± 36 nmol/l; P < 0.01). Glucose supersensitivity was attributed to a resting decrease in the fraction of blockable ATP-sensitive K+ (K+ATP) channels in transplanted islets that maintained normoglycemia with a limited β-cell mass.

Homogeneity in the Electrical Activity Pattern as a Function of Intercellular Coupling in Cell Networks

The aim of this paper is to study changes in the electrical activity of cellular networks when one of the most important electrical parameters, the coupling conductance, varies. We use the pancreatic islet of Langerhans as a cellular network model for the study of oscillatory electrical patterns. The isolated elements of this network, beta cells, are unable to oscillate, while they show a bursting pattern when connected through gap-junctions. Increasing coupling conductance between the elements of the networks leads to the homogeneity of the fast electrical events. We use both experimental data obtained from normal and transgenic animal cells and computational cells and networks to study the implications of coupling strength in the homogeneity of the electrical response.

Balance Between Intercellular Coupling and Input Resistence as a Necessary Requirement for Oscillatory Electrical Activity in Pancreatic beta-Cells

We studied the emergence of oscillatory electrical activity after addition of glucose to insulin secreting cells. In the physiological glucose range (7–20 mM), these cells show a typical square-wave bursting pattern when they are coupled in the islet. Islet of Langerhans consists of some thousands of beta cells, coupled through gap-junctions. When these cells are isolated they also become more excitable in presence of glucose, spiking continuously, but they fail to oscillate. We have hypothesized a role of cell coupling in the generation of oscillatory activity in this system. Now, we examine the phase of continuous activity that appears after a glucose challenge to check our hypothesis. Both experimental data and computer simulations of a small network of β cells, further supports our hypothesis on a role of intercellular coupling in the emergence of oscillatory patterns.

Optimal Range of Input Resistance in the Oscillatory Behavior of the Pancreatic beta-Cell

The pancreatic β-cell produces the hormone insulin in response to the blood glucose levels. This system is damaged in the diabetes. The central region of the s-cell electrical response is oscillatory. In this study, we demonstrate that in this region of the response, the membrane resistance of the cells oscillates in phase with the membrane potential. As the cells input resistance reflects the balance of the ionic conductances, we hypothesize that an optimal range of the input resistance determines the capability of the cells to get into oscillations. Under this scope, we propose an electronic circuit able to reproduce the behavior of the biological system. This circuit would be useful to build an artificial insulin pump able to reproduce the physiological pattern.