Glucose promotes turnover of Na+ in pancreatic β-cells

AbstractOuabain-induced changes of the free cytoplasmic Na+ concentration ([Na+]i) were monitored in aggregates of cells prepared from β-cell-rich pancreatic mouse: islets and the results were compared with the total islet content of sodium. The steady-state [Na+]i was lower in 20 mM glucose (11 mM) than in 3 mM glucose (14 mM). In the presence of 3 mM glucose the addition of 1 mM ouabain resulted in a rise in [Na+]i with an initial rate of 1.5 mM/min. However, the increase of total sodium corresponded to 2.8 mM/min, suggesting that rapid binding and/or sequestration of Na+ are prominent features for pancreatic β-cells. Elevation of the glucose concentration to 20 mM increased the rate of ouabain-dependent rise of [Na+]i The effect of glucose was mimicked by 1 mM tolbutamide or 100 μM carbachol and was counteracted by 100 nM of the a2-adrenergic agonist clonidine. Glucose also accelerated the lowering of [Na+]i after withdrawal of ouabain. In promoting not only the entry but also the extrusion of Na+ glucose actually enhances the turnover of the ion in pancreatic β-cells

The role of plasma membrane K+ and Ca2+ permeabilities for glucose induction of slow Ca2+ oscillations in pancreatic β-cells

AbstractIn individual pancreatic β-cells the rise of the cytoplasmic Ca2+ concentration ([Ca2+]i), induced by 11 mM glucose, is manifested either as oscillations (0.2–0.5 min−1) or as a sustained elevation. The significance of the plasma membrane permeability of Ca2+ and K+ for the establishment of these slow oscillations was investigated by dual wavelength microfluorometric measurements of [Ca2+]i in individual ob/ob mouse-β-cells loaded with fura-2. Increasing the extracellular Ca2+ to 10 mM or the addition of Ca2+ channel agonist BAY K 8644 (1 μM) or K+ channel blocker tetraethylammonium+ (TEA; 10–20 mM) caused steeper rises and higher peaks of the glucose-induced oscillations. However, when extracellular Ca2+ was lowered to 0.5 mM the oscillations were transformed into a sustained suprabasal level. When the β-cells exhibited glucose-stimulatedsustained elevation of [Ca2+]i in the presence of a physiological Ca2+ concentration (1.3 mM), it was possible to induce slow oscillations by promoting the entry of Ca2+ either by raising the extracellular Ca2+ concentration to 10 mM or adding TEA or BAY K 8644. The results indicate that glucose-induced slow oscillations of [Ca2+]i depend on the closure of ATP-regulated K+ channels and require that the rate of Ca2+ influx exceeds a critical level. Apart from an inherent periodicity in ATP production it is proposed that Ca2+-induced ATP consumption in the submembrane space contributes to the cyclic changes of the membrane potential determining periodic entry of Ca2+

Purinergic Signalling in Pancreatic Islet Endothelial Cells

Both the microvascular endothelium and the endocrine cells in the pancreatic islet can release and react upon ATP. In support for the idea that intermittently released ATP, related to exocytosis of insulin secretory granules, affects the secretory activity of the capillary endothelium, we have demonstrated that isolated endothelial cells respond to activation of P2Y2 receptors with pronounced and extended rises of [Ca2+]i. The presence of such ATP effect is consistent with reports that β-cells regulate the blood flow within islets, where adenosine is a key mediator, and that the endothelial cell produce pro-angiogenic and angiostatic factors. In β-cells down-regulation of P2Y1 receptors results in disappearance of the transients of [Ca2+]i supposed to entrain these cells into a common rhythm. Since the islet endothelial cells respond to activation of P2Y2 receptors with extended elevation of [Ca2+]i, it is likely that the accompanying release of ATP is prolonged. Accordingly, the endothelial cells may have a tonic inhibitory action on the coordination of islet release pulses.</p