Selective loss of glucose-induced amplification of insulin secretion in mouse pancreatic islets pretreated with sulfonylurea in the absence of fuels

The beta cell metabolism of glucose, and some other fuels, initiates insulin secretion by closure of ATP-sensitive K⁺ channels and amplifies the secretory response via unknown metabolic intermediates. The aim of this study was to further characterise the mechanism responsible for the metabolic amplification of insulin secretion.

Pancreatic islets were isolated from albino mice by collagenase digestion. Insulin secretion in perifused islets was determined by ELISA. Bioluminometry was used to determine the ATP and ADP content of the incubated islets.

After perifusing islets for 60 min with 2.7 μmol/l glipizide (closing all ATP-sensitive K⁺ channels) in the absence of any fuel, perifusion with a test medium containing 2.7 μmol/l glipizide plus 30 mmol/l glucose did not enhance insulin secretion. However, test media supplemented with 2.7 μmol/l glipizide plus either 10 mmol/l α-ketoisocaproate or 10 mmol/l 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid amplified the glipizide-induced insulin secretion. In pancreatic islets preincubated for 60 min with 2.7 μmol/l glipizide in the absence of any fuel, 40 min incubations in the presence of 2.7 μmol/l glipizide plus 30 mmol/l glucose or plus 10 mmol/l α-ketoisocaproate produced an increase in the ATP content, no change in the ADP content and a rather small increase in the ATP:ADP ratio. The corresponding effects of glucose and α-ketoisocaproate were similar.

These results suggest that metabolic amplification of fuel-induced insulin secretion is not mediated by changes in the beta cell content of ATP and ADP, but might be due to export of citrate cycle intermediates to the beta cell cytosol.