Progression of Diet-Induced Diabetes in G57BL6J Mice Involves Functional Dissociation of Ca~(2+) Channels From Secretory Vesicles

摘要

Objective-The aim of the study was to elucidate the cellular mechanism underlying the suppression of glucose-induced insulin secretion in mice fed a high-fat diet (HFD) for 15 weeks.rnResearch design and methods-C57BL6J mice were fed a HFD or a normal diet (ND) for 3 or 15 weeks. Plasma insulin and glucose levels in vivo were assessed by intraperitoneal glucose tolerance test. Insulin secretion in vitro was studied using static incubations and a perfused pancreas preparation. Membrane currents, electrical activity, and exocytosis were examined by patch-clamp technique measurements. Intracellular calcium concentration ([Ca~(2+)]_i) was measured by microfluorim-etry. Total internal reflection fluorescence microscope (TIRFM) was used for optical imaging of exocytosis and submembrane depolarization-evoked [Ca~(2+)]_i. The functional data were complemented by analyses of histology and gene transcription.rnResults-After 15 weeks, but not 3 weeks, mice on HFD exhibited hyperglycemia and hypoinsulinemia. Pancreatic islet content and (3-cell area increased 2- and 1.5-fold, respectively. These changes correlated with a 20-50% reduction of glucose-induced insulin secretion (normalized to insulin content). The latter effect was not associated with impaired electrical activity or [Ca~(2+)]_i signaling. Single-cell capacitance and TIRFM measurements of exocytosis revealed a selective suppression (>70%) of exocytosis elicited by short (50 ms) depolarization, whereas the responses to longer depolarizations were (500 ms) less affected. The loss of rapid exocytosis correlated with dispersion of Ca~(2+) entry in HFD β-cells. No changes in gene transcription of key exocytotic protein were observed.rnConclusions-HFD results in reduced insulin secretion by causing the functional dissociation of voltage-gated Ca~(2+) entry from exocytosis. These observations suggest a novel explanation to the well-established link between obesity and diabetes.