Changes in granule mobility and age contribute to changes in insulin secretion after desensitization or rest.

Introduction: Functional impairment of the stimulus secretion coupling in pancreatic beta cells is an essential component of type 2 diabetes. It is known that prolonged stimulation desensitizes the secretion of insulin and thus contributes to beta cell dysfunction. Beta cell rest, in contrast, was shown to enhance the secretory response.

Pharmacological inhibition of thioredoxin reductase increases insulin secretion and diminishes beta cell viability.

Apparently, both a decrease in beta cell function and in beta cell mass contribute to the progressive worsening of type 2 diabetes. So, it is of particular interest to define factors which are relevant for the regulation of insulin secretion and at the same time for the maintenance of beta cell mass. The NADPH-thioredoxin system has a candidate role for such a dual function.

Observer-independent quantification of insulin granule exocytosis and pre-exocytotic mobility by TIRF microscopy.

Total internal reflection fluorescence microscopy of fluorescently labeled secretory granules permits monitoring of exocytosis and the preceding granule behavior in one experiment. While observer-dependent evaluation may be sufficient to quantify exocytosis, most of the other information contained in the video files cannot be accessed this way. The present program performs observer-independent detection of exocytosis and tracking of the entire submembrane population of insulin granules.

Bidirectional insulin granule turnover in the submembrane space during K(+) depolarization-induced secretion.

Like primary mouse islets, MIN6 pseudoislets responded to the depolarization by 40 mm KCl and the resulting increase in the free cytosolic Ca(2+) concentration ([Ca(2+) ](i) ) with a massive increase in insulin secretion, whereas 15 mm KCl had little effect in spite of a clear increase in [Ca(2+) ](i) . Analysis of insulin-enhanced green fluorescent protein (EGFP)-labeled granules in MIN6 cells by total internal reflection fluorescence (TIRF) microscopy showed that 40 mm KCl increased the number of short-term resident granules (<1 second presence in the submembrane space), while the total granule number and the number of long-term resident granules decreased. The rates of granule arrival at and departure from the submembrane space changed in parallel and were two orders of magnitude higher than the release rates, suggesting a back-and-forth movement of the granules as the primary determinant of the submembrane granule number.

Studies of first phase insulin secretion using imposed plasma membrane depolarization.

The first phase of glucose-induced insulin secretion is generally regarded to represent the release of a finite pool of secretion-ready granules, triggered by the depolarization-induced influx of Ca2+ through L-type Ca2+ channels. However, the experimental induction of insulin secretion by imposed plasma membrane depolarization may be more complicated than currently appreciated. A comparison of the effects of high K+ concentrations with those of KATP channel closure, which initiates the electrical activity of the beta cell, suggests that 40 mM K+, which is a popular tool to produce a first phase-like secretion, is of supraphysiological strength, whereas the 20 mV depolarization by 15 mM K+ is nearly inefficient.

Selective enhancement of nutrient-induced insulin secretion by ATP-sensitive K+ channel-blocking imidazolines.

The contribution of ATP-sensitive K(+) channel (K(ATP) channel)-dependent and -independent signaling to the insulinotropic characteristics of imidazolines was explored using perifused mouse islets and beta-cells. Up to a concentration of 100 muM efaroxan had no insulinotropic effect in the presence of a basal glucose concentration, but enhanced the effect of a stimulatory concentration of glucose or nonglucidic nutrients (ketoisocaproate plus glutamine). The secretion by a non-nutrient (40 mM KCl) was not enhanced.