Glucose and pharmacological modulators of ATP-sensitive K+ channels control [Ca2+]c by different mechanisms in isolated mouse alpha-cells

Electrophysiology of islet cells

Stimulus-Secretion Coupling (SSC) of pancreatic islet cells comprises electrical activity. Changes of the membrane potential (V(m)) are regulated by metabolism-dependent alterations in ion channel activity. This coupling is best explored in beta-cells. The effect of glucose is directly linked to mitochondrial metabolism as the ATP/ADP ratio determines the open probability of ATP-sensitive K(+) channels (K(ATP) channels). Nucleotide sensitivity and concentration in the direct vicinity of the channels are controlled by several factors including phospholipids, fatty acids, and kinases, e.g., creatine and adenylate kinase. Closure of K(ATP) channels leads to depolarization of beta-cells via a yet unknown depolarizing current. Ca(2+) influx during action potentials (APs) results in an increase of the cytosolic Ca(2+) concentration ([Ca(2+)](c)) that triggers exocytosis. APs are elicited by the opening of voltage-dependent Na(+) and/or Ca(2+) channels and repolarized by voltage- and/or Ca(2+)-dependent K(+) channels. At a constant stimulatory glucose concentration APs are clustered in bursts that are interrupted by hyperpolarized interburst phases. Bursting electrical activity induces parallel fluctuations in [Ca(2+)](c) and insulin secretion. Bursts are terminated by I(Kslow) consisting of currents through Ca(2+)-dependent K(+) channels and K(ATP) channels. This review focuses on structure, characteristics, physiological function, and regulation of ion channels in beta-cells. Information about pharmacological drugs acting on K(ATP) channels, K(ATP) channelopathies, and influence of oxidative stress on K(ATP) channel function is provided. One focus is the outstanding significance of L-type Ca(2+) channels for insulin secretion. The role of less well characterized beta-cell channels including voltage-dependent Na(+) channels, volume sensitive anion channels (VSACs), transient receptor potential (TRP)-related channels, and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is discussed. A model of beta-cell oscillations provides insight in the interplay of the different channels to induce and maintain electrical activity. Regulation of beta-cell electrical activity by hormones and the autonomous nervous system is discussed. alpha- and delta-cells are also equipped with K(ATP) channels, voltage-dependent Na(+), K(+), and Ca(2+) channels. Yet the SSC of these cells is less clear and is not necessarily dependent on K(ATP) channel closure. Different ion channels of alpha- and delta-cells are introduced and SSC in alpha-cells is described in special respect of paracrine effects of insulin and GABA secreted from beta-cells.

Pancreatic islet cells: a model for calcium-dependent peptide release

In mammals the concentration of blood glucose is kept close to 5 mmol∕l. Different cell types in the islet of Langerhans participate in the control of glucose homeostasis. β-cells, the most frequent type in pancreatic islets, are responsible for the synthesis, storage, and release of insulin. Insulin, released with increases in blood glucose promotes glucose uptake into the cells. In response to glucose changes, pancreatic α-, β-, and δ-cells regulate their electrical activity and Ca(2+) signals to release glucagon, insulin, and somatostatin, respectively. While all these signaling steps are stimulated in hypoglycemic conditions in α-cells, the activation of these events require higher glucose concentrations in β and also in δ-cells. The stimulus-secretion coupling process and intracellular Ca(2+) ([Ca(2+)](i)) dynamics that allow β-cells to secrete is well-accepted. Conversely, the mechanisms that regulate α- and δ-cell secretion are still under study. Here, we will consider the glucose-induced signaling mechanisms in each cell type and the mathematical models that explain Ca(2+) dynamics.

Glucose suppression of glucagon secretion: metabolic and calcium responses from alpha-cells in intact mouse pancreatic islets

Glucagon is released from alpha-cells present in intact pancreatic islets at glucose concentrations below 4 mm, whereas higher glucose levels inhibit its secretion. The mechanisms underlying the suppression of alpha-cell secretory activity are poorly understood, but two general types of models have been proposed as follows: direct inhibition by glucose or paracrine inhibition from non-alpha-cells within the islet of Langerhans. To identify alpha-cells for analysis, we utilized transgenic mice expressing fluorescent proteins targeted specifically to these cells. Measurements of glucagon secretion from pure populations of flow-sorted alpha-cells show that contrary to its effect on intact islets, glucose does stimulate glucagon secretion from isolated alpha-cells. This observation argues against a direct inhibition of glucagon secretion by glucose and supports the paracrine inhibition model. Imaging of cellular metabolism by two-photon excitation of NAD(P)H autofluorescence indicates that glucose is metabolized in alpha-cells and that glucokinase is the likely rate-limiting step in this process. Imaging calcium dynamics of alpha-cells in intact islets reveals that inhibiting concentrations of glucose increase the intracellular calcium concentration and the frequency of alpha-cell calcium oscillations. Application of candidate paracrine inhibitors leads to reduced glucagon secretion but did not decrease the alpha-cell calcium activity. Taken together, the data suggest that suppression occurs downstream from alpha-cell calcium signaling, presumably at the level of vesicle trafficking or exocytotic machinery.

Characterization of Erg K+ channels in alpha- and beta-cells of mouse and human islets

Voltage-gated eag-related gene (Erg) K(+) channels regulate the electrical activity of many cell types. Data regarding Erg channel expression and function in electrically excitable glucagon and insulin producing cells of the pancreas is limited. In the present study Erg1 mRNA and protein were shown to be highly expressed in human and mouse islets and in alpha-TC6 and Min6 cells alpha- and beta-cell lines, respectively. Whole cell patch clamp recordings demonstrated the functional expression of Erg1 in alpha- and beta-cells, with rBeKm1, an Erg1 antagonist, blocking inward tail currents elicited by a double pulse protocol. Additionally, a small interference RNA approach targeting the kcnh2 gene (Erg1) induced a significant decrease of Erg1 inward tail current in Min6 cells. To investigate further the role of Erg channels in mouse and human islets, ratiometric Fura-2 AM Ca(2+)-imaging experiments were performed on isolated alpha- and beta-cells. Blocking Erg channels with rBeKm1 induced a transient cytoplasmic Ca(2+) increase in both alpha- and beta-cells. This resulted in an increased glucose-dependent insulin secretion, but conversely impaired glucagon secretion under low glucose conditions. Together, these data present Erg1 channels as new mediators of alpha- and beta-cell repolarization. However, antagonism of Erg1 has divergent effects in these cells; to augment glucose-dependent insulin secretion and inhibit low glucose stimulated glucagon secretion.

Inhibitory effects of leptin on pancreatic alpha-cell function

OBJECTIVE

Leptin released from adipocytes plays a key role in the control of food intake, energy balance, and glucose homeostasis. In addition to its central action, leptin directly affects pancreatic beta-cells, inhibiting insulin secretion, and, thus, modulating glucose homeostasis. However, despite the importance of glucagon secretion in glucose homeostasis, the role of leptin in alpha-cell function has not been studied in detail. In the present study, we have investigated this functional interaction.

RESEARCH DESIGN AND METHODS

The presence of leptin receptors (ObR) was demonstrated by RT-PCR analysis, Western blot, and immunocytochemistry. Electrical activity was analyzed by patch-clamp and Ca(2+) signals by confocal microscopy. Exocytosis and glucagon secretion were assessed using fluorescence methods and radioimmunoassay, respectively.

RESULTS

The expression of several ObR isoforms (a-e) was detected in glucagon-secreting alphaTC1-9 cells. ObRb, the main isoform involved in leptin signaling, was identified at the protein level in alphaTC1-9 cells as well as in mouse and human alpha-cells. The application of leptin (6.25 nmol/l) hyperpolarized the alpha-cell membrane potential, suppressing the electrical activity induced by 0.5 mmol/l glucose. Additionally, leptin inhibited Ca(2+) signaling in alphaTC1-9 cells and in mouse and human alpha-cells within intact islets. A similar result occurred with 0.625 nmol/l leptin. These effects were accompanied by a decrease in glucagon secretion from mouse islets and were counteracted by the phosphatidylinositol 3-kinase inhibitor, wortmannin, suggesting the involvement of this pathway in leptin action.

CONCLUSIONS

These results demonstrate that leptin inhibits alpha-cell function, and, thus, these cells are involved in the adipoinsular communication.