Glucose suppression of glucagon secretion: metabolic and calcium responses from alpha-cells in intact mouse pancreatic islets.
Glucagon is released from alpha-cells present in intact pancreatic islets at glucose concentrations below 4 mm, whereas higher glucose levels inhibit its secretion. The mechanisms underlying the suppression of alpha-cell secretory activity are poorly understood, but two general types of models have been proposed as follows: direct inhibition by glucose or paracrine inhibition from non-alpha-cells within the islet of Langerhans. To identify alpha-cells for analysis, we utilized transgenic mice expressing fluorescent proteins targeted specifically to these cells. Measurements of glucagon secretion from pure populations of flow-sorted alpha-cells show that contrary to its effect on intact islets, glucose does stimulate glucagon secretion from isolated alpha-cells. This observation argues against a direct inhibition of glucagon secretion by glucose and supports the paracrine inhibition model. Imaging of cellular metabolism by two-photon excitation of NAD(P)H autofluorescence indicates that glucose is metabolized in alpha-cells and that glucokinase is the likely rate-limiting step in this process. Imaging calcium dynamics of alpha-cells in intact islets reveals that inhibiting concentrations of glucose increase the intracellular calcium concentration and the frequency of alpha-cell calcium oscillations. Application of candidate paracrine inhibitors leads to reduced glucagon secretion but did not decrease the alpha-cell calcium activity. Taken together, the data suggest that suppression occurs downstream from alpha-cell calcium signaling, presumably at the level of vesicle trafficking or exocytotic machinery.
Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee 37232, USA.
Sylvain J
SJ
Le Marchand
David W
DW
Piston
D000818:Animals; D001426:Bacterial Proteins; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D005455:Fluorescent Antibody Technique; D005934:Glucagon / Q000378:metabolism*; D050416:Glucagon-Secreting Cells / Q000378:metabolism*; D005941:Glucokinase / Q000378:metabolism; D005947:Glucose / Q000494:pharmacology*; D007004:Hypoglycemic Agents / Q000378:metabolism; D007328:Insulin / Q000378:metabolism; D007515:Islets of Langerhans / Q000166:cytology / Q000378:metabolism*; D008164:Luminescent Proteins; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D009249:NADP / Q000378:metabolism; D013549:Sweetening Agents / Q000494:pharmacology*
D001426:Bacterial Proteins; D007004:Hypoglycemic Agents; D007328:Insulin; D008164:Luminescent Proteins; D013549:Sweetening Agents; C054562:yellow fluorescent protein, Bacteria; D009249:NADP; D005934:Glucagon; D005941:Glucokinase; D005947:Glucose; D002118:Calcium