Glucose Dependent Insulinotropic Polypeptide (GIP) Infused Intravenously is Insulinotropic in the Fasting State in Type 2 (Non-Insulin Dependent) Diabetes Mellitus

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Abstract
The effects of an intravenous infusion of porcine GIP on β-cell secretion in patients with untreated type 2 diabetes mellitus have been studied. The subjects were studied on two separate days. After a 10 h overnight fast and a further 120 min basal period they were given an intravenous infusion of porcine GIP (2 pmol · kg-1 · min-1) or control solution in random order from 120-140 min. Frequent plasma glucose, insulin, C-peptide and GIP measurements were made throughout and the study was continued until 200 min. Plasma glucose levels were similar throughout both tests. During the GIP infusion there was an early significant rise in insulin concentration from 0.058 ± 0.006 nmol/l to 0.106 ± 0.007 nmol/l (P < 0.01) within 6 min of commencing the GIP infusion and insulin levels reached a peak of 0.131 ± 0.011 nmol/l at 10 min (P < 0.01). Insulin levels remained significantly elevated during the rest of the GIP infusion (P < 0.01-0.001) and returned to basal values 20 min post infusion. No change in basal insulin values was seen during the control infusion. C-peptide levels were similarly raised during the GIP infusion and the increase was significant just 4 min after commencing the GIP infusion (P < 0.05). GIP levels increased from 16 ± 3 pmol/l prior to the infusion to a peak of 286 ± 24 pmol/l 20 min later. At 4 min when a significant β-cell response was observed GIP levels were well within the physiological range. GIP is insulinotropic in the fasting state in patients with untreated type 2 diabetes mellitus and may stimulate the β-cell continuously throughout the day.

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