Control Mechanisms of the Oscillations of Insulin Secretion In Vitro and In Vivo

Abstract

The mechanisms driving the pulsatility of insulin secretion in vivo and in vitro are still unclear. Because glucose metabolism and changes in cytosolic free Ca(2+) ([Ca(2+)](c)) in beta-cells play a key role in the control of insulin secretion, and because oscillations of these two factors have been observed in single isolated islets and beta-cells, pulsatile insulin secretion could theoretically result from [Ca(2+)](c) or metabolism oscillations. We could not detect metabolic oscillations independent from [Ca(2+)](c) changes in beta-cells, and imposed metabolic oscillations were poorly effective in inducing oscillations of secretion when [Ca(2+)](c) was kept stable, which suggests that metabolic oscillations are not the direct regulator of the oscillations of secretion. By contrast, tight temporal and quantitative correlations between the changes in [Ca(2+)](c) and insulin release strongly suggest that [Ca(2+)](c) oscillations are the direct drivers of insulin secretion oscillations. Metabolism may play a dual role, inducing [Ca(2+)](c) oscillations (via changes in ATP-sensitive K(+) channel activity and membrane potential) and amplifying the secretory response by increasing the efficiency of Ca(2+) on exocytosis. The mechanisms underlying the oscillations of insulin secretion by the isolated pancreas and those observed in vivo remain elusive. It is not known how the functioning of distinct islets is synchronized, and the possible role of intrapancreatic ganglia in this synchronization requires confirmation. That pulsatile insulin secretion is beneficial in vivo, by preventing insulin resistance, is suggested by the greater hypoglycemic effect of exogenous insulin when it is infused in a pulsatile rather than continuous manner. The observation that type 2 diabetic patients have impaired pulsatile insulin secretion has prompted the suggestion that such dysregulation contributes to the disease and justifies the efforts toward understanding of the mechanism underlying the pulsatility of insulin secretion both in vitro and in vivo.

Disorganization of cytoplasmic Ca 2+ oscillations and pulsatile insulin secretion in islets from ob / ob mice

“…With a few exceptions [43], the characteristics of their responses were comparable to those obtained by others in single beta cells from lean mice [12]. On the other hand, only a few [Ca 2+ ] i measurements have been carried out in clusters of cells [38] or intact islets from ob/ob mice [26,31].…”

G Protein-independent Activation of an Inward Na+Current by Muscarinic Receptors in Mouse Pancreatic β-Cells

“…A depolarizing effect of ACh persisted after inhibition of the Na ϩ /K ϩ ATPase by omission of extracellular K ϩ , and reactivation of the Na ϩ /K ϩ pump after its blockade (by K ϩ removal or ouabain) induced a transient repolarization that was not suppressed by ACh. 2 Moreover, ACh slightly increased initial 86 Rb ϩ uptake (14), which is opposite to the effect observed after blockade of the pump with ouabain (48,49).…”

“…This stimulation involves two complementary pathways. Glucose generates a triggering signal, a rise in cytosolic Ca 2ϩ concentration ([Ca 2ϩ ] c ), 1 through the following sequence of events: the acceleration of cell metabolism increases the ATP/ADP ratio, which closes ATP-sensitive K ϩ (K ϩ -ATP) channels in the plasma membrane; the resulting decrease in K ϩ conductance leads to membrane depolarization, opening of voltage-dependent Ca 2ϩ channels, and Ca 2ϩ influx (1)(2)(3)(4)(5). Glucose also produces amplifying signals that increase the efficacy of Ca 2ϩ on exocytosis (6,7).…”

Loss of Connexin36 Channels Alters β-Cell Coupling, Islet Synchronization of Glucose-Induced Ca2+ and Insulin Oscillations, and Basal Insulin Release

“…First, although [Ca 2ϩ ] i was found to oscillate in individual ␤-cells from Cx36 Ϫ/Ϫ mice, the islets of these animals no longer showed the synchronous oscillations of [Ca 2ϩ ] i that normally occur during glucose stimulation (17)(18)(19)(20)(21)(22). These oscillations are caused by a depolarizationmediated, periodic influx of Ca 2ϩ into ␤-cells (22,23), as shown by their temporal relation with membrane potential changes (17,21) and their abolition by the omission of extracellular Ca 2ϩ , blockers of L-type Ca 2ϩ channels, or membrane repolarization (17,22). Synchronization of glucose-induced oscillations of membrane potential (4), and hence of [Ca 2ϩ ] i oscillations (17,23), has been attributed to the electrical coupling of ␤-cells connected by gap junctions.…”

“…These oscillations are caused by a depolarizationmediated, periodic influx of Ca 2ϩ into ␤-cells (22,23), as shown by their temporal relation with membrane potential changes (17,21) and their abolition by the omission of extracellular Ca 2ϩ , blockers of L-type Ca 2ϩ channels, or membrane repolarization (17,22). Synchronization of glucose-induced oscillations of membrane potential (4), and hence of [Ca 2ϩ ] i oscillations (17,23), has been attributed to the electrical coupling of ␤-cells connected by gap junctions. Several studies measuring [Ca 2ϩ ] i in whole islets or clusters of islet cells have indirectly supported this interpretation (5,17,24,25) but could not conclusively prove it, mainly because specific inhibitors of connexin channels are not yet available (26).…”