HORMONE ONTOGENY IN THE OVINE FETUS: XXI. THE EFFECT OF INSULIN-LIKE GROWTH FACTOR-I ON PLASMA FETAL GROWTH HORMONE, INSULIN AND GLUCOSE CONCENTRATIONS

We infused intravenously recombinant human Insulin-like Growth Factor-I (IGF-I; 1 microgram/kg/min for 120 minutes after an acute dose of 25 micrograms/kg) into chronically catheterized ovine fetuses (124-132 days gestation) to study its effect on the secretion of fetal ovine Growth Hormone (oGH). In all IGF-I infused fetuses, oGH concentrations fell during the infusion. The maximal change in the concentration of oGH (mean +/- SEM) was -54 +/- 10 ng/ml in contrast to +7 +/- 6 ng/ml in saline controls (p less than 0.005), a decrease of 33 +/- 4% (controls: +6 +/- 5%; p less than 0.005). By 60 minutes after the infusion of IGF-I was completed, the concentration of plasma oGH was comparable to control and pre-infusion values. In IGF-I infused fetuses, the mean concentration of insulin also decreased (p less than 0.02), whereas glucose levels remained unaltered. The results suggest that the lack of inhibitory feedback by the relatively low levels of circulating IGF-I is one factor in the hypersecretion of GH by the fetus.