Insulin secretion in the fetal and neonatal sheep

The effect on plasma insulin of elevating plasma glucose levels was studied in exteriorized sheep fetuses of 68 to 142 days conceptual age and in 2 lambs, 1 and 7 days old. Fetuses of less than 110 days showed, at most, only a very small and delayed rise in plasma insulin following an injection of glucose (1.67–3.45 g/kg body weight) into the umbilical vein. In 5 out of 6 fetuses of 110 days and older intravenous glucose (0.54–1.38 g/kg) was followed by a marked but delayed rise in plasma insulin reaching a peak 25–100 min after glucose administration. In the lambs intravenous glucose (1 g/kg) gave an immediate response in plasma insulin. Intraduodenal glucose was also seen to give a marked rise in plasma insulin in a 123 day fetus and 2 lambs. Intravenous fructose (0.65 and 0.55 g/kg) to two older fetuses caused no rise in plasma insulin although glucose given subsequently evoked a marked response. The onset of appearance of a marked response to glucose in the fetus was preceded by a proliferation of secondary islets in the pancreas. The changes in plasma insulin in the exteriorized fetuses and in 2 isolated fetuses suggest that the placenta must be relatively impermeable to insulin, fetal insulin must largely be derived from the fetal pancreas and fetal tissues must take up insulin.