The augmenting effect on insulin secretion by oral versus intravenous glucose is exaggerated by high-fat diet in mice

To study whether the incretin effect is involved in adaptively increased insulin secretion in insulin resistance, glucose was infused at a variable rate to match glucose levels after oral glucose (25 tug) in normal anesthetized C57BL/6j female mice or in mice rendered insulin resistant by 8 weeks of high-fat feeding. Insulin response was markedly higher after oral than i.v. glucose in both groups, and this augmentation was even higher in high-fat fed than normal mice. In normal mice, the area under the curve (AUC(insulin)) was augumented from 4.0 +/- 0.8 to 8.0 +/- 1.8 nmol/1 x 60min by the oral glucose, i.e. by a factor of 2 (P=0.023), whereas in the high-fat fed mice, AUC(insulin) was augmented from 0.70 +/- 0.4 to 12.4 +/- 2.5 nmol/1X60 min, i.e. by a factor of 17 (P < 0.001). To examine whether the incretin hormone glucagon-like peptide-1 (GLP-1) is responsible for this difference, the effect of i.v. GLP-1 was compared in normal and high-fat fed mice. The sensitivity to i.v. GLP-1 in stimulating insulin secretion was increased in the high-fat diet fed mice: the lowest effective dose of GLP-1 was 650 pmol/kg in normal mice and 13 pmol/kg in the high-fat diet fed mice. We conclude that 1) the incretin effect contributes by similar to 50% to insulin secretion by the oral glucose in normal mice, 2) this effect is markedly exaggerated in insulin-resistant mice fed a high-fat diet, and 3) this augmented incretin contribution in the high-fat fed mice may partially be explained by GLP-1.