Effects of insulin excess and deficiency on glucose-dependent insulinotropic polypeptide (GIP) was examined in rats following insulinoma transplantation or streptozotocin (STZ) administration. Over 14 days, food intake was increased (p<0.001) in both groups of rats, with decreased body weight (p<0.01) in STZ rats. Non-fasting plasma glucose levels were decreased (p<0.01) and plasma insulin levels increased (p<0.001) in insulinoma-bearing rats, whereas STZ treatment elevated glucose (p<0.001) and decreased insulin (p<0.01). Circulating GIP concentrations were elevated (p<0.01) in both animal models. At 14 days, oral glucose resulted in a decreased glycaemic excursion (p<0.05) with concomitant elevations in insulin release (p<0.001) in insulinoma-bearing rats, whereas STZ-treated rats displayed similar glucose-lowering effects but reduced insulin levels (p<0.01). GIP concentrations were augmented in STZ rats (p<0.05) following oral glucose. Plasma glucose and insulin concentrations were not affected by oral fat, but fat-induced GIP secretion was particularly (p<0.05) increased in insulinoma-bearing rats. Exogenous GIP enhanced (p<0.05) glucose-lowering in all groups of rats accompanied by insulin releasing (p<0.001) effects in insulinoma-bearing and control rats. Both rat models exhibited increased (p<0.001) intestinal weight but decreased intestinal GIP concentrations. These data suggest that circulating insulin has direct and indirect effects on the synthesis and secretion of GIP.
Irwin, N., Francis, J. M. E., & Flatt, P. (2011). Insulin modulates glucose-dependent insulinotropic polypeptide (GIP) secretion from enteroendocrine K cells in rats. Biological Chemistry, 392(10), 909-918. https://doi.org/10.1515/BC.2011.176