The glucose-induced gastrointestinal stimulation of insulin secretion in man: relation to age and to gastrin release.

An oral 50 g glucose tolerance test and a simple intravenous glucose infusion test were performed in 20 young (20 - 32 years), 20 middle-aged (42 -55 years), and 20 old (65 - 81 years) normal subjects. Blood glucose, serum insulin, and serum gastrin concentrations were measured during all tests. The intravenous glucose infusion duplicated the oral blood glucose curve in young, middle-aged, and half of the old subjects. In the remaining old subjects the intravenous blood glucose curve was below the oral blood glucose curve. Glucose and insulin concentrations were of similar magnitude in all groups, but maximum concentrations were reached later in the old subjects. Glucose per os induced a rapid rise in serum gastrin concentrations of the order of 10 pmol/1 and a subsequent decrease of about 20 pmol/1 in all groups. The intravenous glucose infusion induced no significant changes in serum gastrin concentrations. The non-glycaemic (i.e. enteral) stimulation of insulin secretion was expressed as the difference between the integrated incremental areas of the oral and intravenous insulin curves in subjects with identical glucose curves during the two tests. This stimulation was of similar magnitude in all age groups, and it was not correlated to variations in gastrin concentrations. However, in the young subjects the enteral stimulation was greater during the first 30 minutes and smaller during the two last hours when compared to the old subjects. The results suggest that: 1. A simple intravenous glucose infusion test can be used to copy the oral blood glucose curve. 2. The size of the enteral stimulation of insulin secretion during the whole test is independetn of age. 3. The action of the glucose-induced enteral stimulation is delayed with age, and closely linked to the dynamics of the glycaemic stimulus. 4. The glucose-induced gastrin release is probably too small to affect insulin secretion significantly.