Pancreatic α-cells secrete glucagon when plasma glucose concentrations are low. Glucagon secretion declines as glucose concentrations increase, but the mechanism by which secretion is inhibited is unknown. Recent work from our laboratory indicates that in pancreatic β-cells, glucose-induced changes in cell volume may help couple plasma glucose concentrations to insulin secretion (Miley et al. 1997). In the present study we have investigated the effects of glucose on the α-cell volume.
Rats were humanely killed by stunning and cervical dislocation. Islets were isolated from the pancreas by collagenase digestion, and dispersed into single cells using a Ca2+-free medium. Cell volume was measured by video-imaging methods, and α-cells selected on the basis of volume, i.e. < 0.8 pl (Majid et al. 2001). Single α-cells were initially superfused with a control Hepes-buffered solution containing 5 mM glucose. The superfusate was then changed to a solution containing 20 mM glucose (isotonic replacement of mannitol) for 15 min. In this solution, relative cell volume increased to a maximum of 1.08 ± 0.03 (mean ± S.E.M.; n = 8). Cell volume recovered towards control values on return to the 5 mM glucose superfusate.
α-Cell volume was unaffected by superfusion with a solution containing 15 mM 3-O-methyl glucose (3-OMG; a non-metabolisable analogue of glucose) and 5 mM glucose. The volume changes (area under a volume versus time plot) in 20 mM glucose were compared by ANOVA with those with 20 mM 3-OMG or in time-matched controls (5 mM glucose). Cell volume increased in glucose compared with both 3-OMG (n = 4; P < 0.05) and the controls (n = 6; P < 0.01). There was no significant difference between the mean volume changes in 3-OMG and the controls (P > 0.1).
In conclusion, an increase in extracellular glucose concentration causes pancreatic α-cells to swell. These changes in cell volume may affect α-cell electrical activity, and help couple increases in glucose concentration to the inhibition of glucagon secretion.
S.L.D. is supported by a MRC postgraduate studentship.
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