The effect of repeating a 60 min glucose infusion at a 40 to 70 min interval was investigated after an overnight fast in 14 healthy, non-obese subjects with normal glucose tolerance and normal insulin response to glucose administration. When a hyperglycaemic plateau of around 300 mg/100 ml was induced by the first glucose infusion, the insulin response to a second challenge was enhanced over the range of stimulations used. Both the early and late phase insulin responses were amplified, the enhancement being more marked with higher stimulatory levels of glucose. The blood glucose-insulin dose-response curve became steeper after pretreatment with glucose, the stimulatory threshold level not being altered. These findings suggest that the synergism between the glucose pretreatment, and the insulin releasing effect of glucose, is of multiplicative type, resulting in increase of the maximum effect of the glucose.
The dose-dependency of this potentiation was investigated by keeping the second glucose challenge at a constant level and altering the dose of the first infusion. It was necessary to reach hyperglycaemias around and above 300 mg/100 ml during the first infusion in order to obtain enhancement of the insulin response to the second stimulus. The dose-response curve of the potentiating effect of glucose is thus displayed towards the right when compared with that describing the insulin releasing effect of glucose, which has its threshold around 100 mg/100 ml.
It is suggested that glucose exerts a dual effect on the pancreatic islets: an immediate one which initiates the release of insulin, and a time-bound one which modulates the first action.