Adaptation to hyperglycemia enhances insulin secretion in glucokinase mutant mice

The present study was undertaken to test the hypothesis that exposure to high glucose concentrations enhances insulin secretion in pancreatic islets from glucokinase-deficient mice. Insulin secretion and intracellular calcium ([Ca2+]i) were measured as the glucose concentration was increased from 2 to 26 mmol/l in islets from heterozygous glucokinase (GK)-deficient mice (GK+/-) and their wild-type littermates (GK+/+). Results obtained in islets incubated in 11.6 or 30 mmol/l glucose for 48-96 h were compared. GK+/- islets that had been incubated in 30 mmol/l glucose showed improved although not normal insulin secretory and [Ca2+]i responses to the standard glucose challenge as well as an enhanced ability to sense small amplitude glucose oscillations. These effects were associated with increased glucokinase activity and protein. In contrast, exposure of GK+/+ islets to 30 mmol/l glucose increased their basal insulin secretion but reduced their incremental secretory responses to glucose and their ability to detect small amplitude glucose oscillations. Thus exposure of GK+/- islets to 30 mmol/l glucose for 48-96 h enhanced their ability to sense and respond to a glucose stimulus, whereas similar exposure of GK+/+ islets induced evidence of beta-cell dysfunction. These findings provide a mechanistic framework for understanding why glucokinase diabetes results in mild hyperglycemia that tends not to increase over time. In addition, the absence of one allele of the glucokinase gene appears to protect against glucose-induced beta-cell dysfunction (glucose toxicity).