Gastric Inhibitory Polypeptide and Insulin: Response to Intraduodenal and Intravenous Glucose Infusions in Fetal and Neonatal Pigs*

Abstract

The responses of gastric inhibitory polypeptide (GIP) and insulin to intraduodenal (OGTT) and iv (IVGI) glucose infusions were measured in 14 late fetal and 9 neonatal pigs. Basal plasma glucose (P < 0.01) and GIP (P < 0.001) concentrations were lower in the fetal than in the neonatal pigs, while basal plasma insulin was not significantly different in the two groups. In the fetal pigs, almost identical plasma glucose curves were obtained during the OGTT and the IVGI, but plasma insulin did not change during either test. In these pigs, plasma GIP increased 3-fold (P < 0.01) during the OGTT, whereas no significant changes in plasma GIP were observed during the IVGI. In the neonatal pigs, plasma glucose increased more during the OGTT than during the IVGI, but plasma insulin exhibited similar increments during both tests. Thus, the insulinogenic index of the IVGI was almost 3 times higher than that of the OGTT (P < 0.05). In contrast to the normal increment in plasma GIP observed in fetal pigs, plasma GIP decreased significantly during both tests in the neonatal pigs. It is concluded that β- cells of fetal pigs are unresponsive to acute elevations of plasma glucose with or without a concomitant increase in plasma GIP. Conversely, β-cells of neonatal pigs exhibit a significant response to hyperglycemia, but the enteroinsular axis is defective, since more insulin is released after iv than during intraduodenal glucose administration. The defective enteroinsular axis in neonatal pigs might be due to the observed fall in plasma GIP during the OGTT.