Glucagon, insulin and glucose homeostasis in the fetal lamb

The actions of insulin and glucagon in the fetal lamb and regulation of their secretion from the fetal pancreas have been examined to assess the possible roles of these hormones in regulating glucose homeostasis in the lamb during fetal life. Much evidence indicates that insulin stimulated glucose utilization in the fetal lamb and that glucagon can promote mobilization of fetal liver glycogen. Glucose stimulates and adrenaline inhibits insulin secretion by fetal pancrease pieces in vitro from 50 days gestation onwards, but alanine and glycine have little effect on insulin release. Alanine and glycine stimulate glucagon secretion by fetal pancreas pieces in vitro from 50 days gestation. The effects are potentiated by caffeine. Adrenaline has a small stimulatory effect but glucagon release is not altered by glucose. In vivo adrenaline infusion increases fetal plasma glucagon concentrations but glycine infusion does not. Glycine infusion into post-natal lambs increases plasma glucagon. Fasting pregnant ewes for two days decreases plasma insulin but does not alter plasma glucagon in either ewe or fetus. The observations suggest insulin secretion in the fetal lamb is an important determinant of glucose uptake and utilization by the fetus during at least the last third of pregnancy. The quantitative importance of glucagon in regulating fetal hepatic glucose metabolism remains uncertain.