Progression of Diet-Induced Diabetes in C57BL6J Mice Involves Functional Dissociation of Ca2+ Channels From Secretory Vesicles

  1. Patrik Rorsman1
  1. 1Oxford Centre for Diabetes, Endocrinology and Metabolism, University of Oxford, Churchill Hospital, Oxford, U.K.;
  2. 2Department of Physiology and Biophysics, Institute of Biomedical Sciences, University of São Paulo, São Paulo, Brazil;
  3. 3The Wellcome Trust Centre for Human Genetics, University of Oxford, Oxford, U.K.
  1. Corresponding author: Stephan Collins, stephan.collins{at}drl.ox.ac.uk.

Abstract

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.

RESEARCH 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 ([Ca2+]i) was measured by microfluorimetry. Total internal reflection fluorescence microscope (TIRFM) was used for optical imaging of exocytosis and submembrane depolarization-evoked [Ca2+]i. The functional data were complemented by analyses of histology and gene transcription.

RESULTS After 15 weeks, but not 3 weeks, mice on HFD exhibited hyperglycemia and hypoinsulinemia. Pancreatic islet content and β-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 [Ca2+]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 Ca2+ entry in HFD β-cells. No changes in gene transcription of key exocytotic protein were observed.

CONCLUSIONS HFD results in reduced insulin secretion by causing the functional dissociation of voltage-gated Ca2+ entry from exocytosis. These observations suggest a novel explanation to the well-established link between obesity and diabetes.

Footnotes

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    • Received May 26, 2009.
    • Accepted January 28, 2010.

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  1. Diabetes vol. 59 no. 5 1192-1201
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