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Submitted on June 29, 2005
Accepted on August 11, 2005
Departments of Medicine and Physiology, University of Toronto, Toronto, Canada, M5S 1A8; Department of Medicine, University of Chicago, Chicago, IL 60637, USA
* To whom correspondence should be addressed. E-mail: yukman.leung{at}utoronto.ca.
We recently reported a transgenic (Mouse Insulin Promoter-Green Fluorescent Protein; MIP-GFP) mouse in which GFP expression is targeted to the pancreatic islet 
-cells to enable convenient identification of -cells as green cells. The GFP-expressing -cells of the MIP-GFP mouse were functionally indistinguishable from -cells of normal mice. Here we characterized the ionic channel properties and exocytosis of MIP-GFP mouse islet - and 
-cells. -cells displayed delayed rectifying K+ and high-voltage-activated (HVA) Ca2+ channels, and exhibited Na+ currents only at hyperpolarized holding potential (VH). -cells were non-green and had both A-type and delayed rectifier K+ channels, both low-voltage-activated (LVA) and HVA Ca2+ channels, and displayed Na+ currents readily at -70 mV VH. -cells had KATP channel density as high as that in -cells, and surprisingly, -cell KATP channels were more sensitive to ATP inhibition (IC50 = 0.16 ± 0.03 mM) than -cell KATP channels (IC50 = 0.86 ± 0.10 mM). While -cells were rather uniform in size (2-4.5 pF), -cells varied vastly in size (2-12 pF). Of note, small -cells (<4.5 pF) showed little exocytosis, whereas medium -cells (5-8 pF) exhibited vigorous exocytosis, but large -cells (>8 pF) had weaker exocytosis. We found no correlation between -cell size and their Ca2+ channel density, suggesting that Ca2+ influx may not be the cause of the heterogeneity in exocytotic responses. The MIP-GFP mouse therefore offers potential to further explore the functional heterogeneity in -cells of different sizes. The MIP-GFP mouse islet is therefore a reliable model to efficiently examine -cell and -cell physiology, and should greatly facilitate examination of their pathophysiology when the MIP-GFP mice are crossed with diabetic models.
-cells •
islet -cells •
ion channels •
exocytosis •
electrophysiology •
capacitance measurements
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