Fetal and maternal production and metabolism of gastrin in sheep

In the sheep fetus, plasma levels of gastrin are raised above adult levels from 2 weeks before birth. This observation initiated the present study on the maternal and fetal secretion rate, metabolism and placental transfer of gastrin. The experiments were performed on conscious pregnant ewes with chronically cannulated fetuses and on newborn lambs. Metabolic clearance rate (MCR), production rate (PR) and placental transfer of gastrin were measured by alternate steady-state infusion of gastrin into the mother and fetus. Plasma levels of gastrin were measured by radioimmunoassay. Metabolic clearance rate was similar in the pregnant and non-pregnant ewe (8.4 +/- 1.1 (S.E.M.) and 9.0 +/- 1.4 ml/min per kg) respectively. However, fetal MCR was significantly increased. Term was 145 days. Metabolic clearance rate was 15.5 +/- 1.7 at 110-125 days of gestation, 25.6 +/- 2.9 at 126-135 days, 29.7 +/- 4.9 at 136-145 days and remained raised in the first 2 weeks post partum. Gastrin did not cross the placenta in either direction. Placental destruction of gastrin was not responsible for the increased fetal MCR as umbilical artery and umbilical vein levels were not significantly different during fetal gastrin infusion. Furthermore, MCR remained raised in the newborn lambs. Gastrin PR was significantly increased at all ages. The results showed that the previously reported fetal hypergastrinaemia is from fetal sources and is not a result of immaturity of clearance mechanisms. In fact, fetal MCR was significantly increased. The increased fetal plasma gastrin levels are due to an increased rate of production from the fetus.