Abstract
Pulsatile insulin release is the primary means of blood glucose regulation. The loss of pulsatility is thought to be an early marker and possible factor in developing type 2 diabetes. Another early adaptation in islet function to compensate for obesity is increased glucose sensitivity (left shift) associated with increased basal insulin release. We provide evidence that oscillatory disruptions may be linked with overcompensation (glucose hypersensitivity) in islets from diabetes-prone mice. We isolated islets from male 4- to 5-week-old (prediabetic) and 10- to 12-week-old (diabetic) leptin-receptor-deficient (db/db) mice and age-matched heterozygous controls. After an overnight incubation in media with 11 mM glucose, we measured islet intracellular calcium in 5, 8, 11, or 15 mM glucose. Islets from heterozygous 10- to 12-week-old mice were quiescent in 5 mM glucose and displayed oscillations with increasing amplitude and/or duration in 8, 11, and 15 mM glucose, respectively. Islets from diabetic 10- to 12-week-old mice, in contrast, showed robust oscillations in 5 mM glucose that declined with increasing glucose. Similar trends were observed at 4-5-weeks of age. A progressive left shift in maximal insulin release was also observed in islets as db/db mice aged. Reducing glucokinase activity with 1 mM D-mannoheptulose restored oscillations in 11 mM glucose. Finally, overnight low-dose cytokine exposure negatively impacted oscillations preferentially in high glucose in diabetic islets compared with heterozygous controls. Our findings suggest the following: 1) islets from frankly diabetic mice can produce oscillations, 2) elevated sensitivity to glucose prevents diabetic mouse islets from producing oscillations in normal postprandial (11-15 mM glucose) conditions, and 3) hypersensitivity to glucose may magnify stress effects from inflammation or other sources.