ELEVATING MATERNAL INSULIN-LIKE GROWTH FACTOR-I IN MICE AND RATS ALTERS THE PATTERN OF FETAL GROWTH BY REMOVING MATERNAL CONSTRAINT

Fetal growth is normally constrained by maternal factors. This constraint is demonstrated by the usual inverse linear relationship between litter size and mean fetal weight. Cross-breeding experiments between mice of lines selected for high or low plasma insulin-like growth factor (IGF-I) levels suggested that elevations in maternal IGF-I abolish (P<0.01) this constraining effect and reverse the usual positive relationship between fetal and placental size in late gestation. This was confirmed by treating mice and rats throughout pregnancy with IGF-I. In normal mice and in low IGF-I line mice treatment with IGF-I (10-mu-g 8-hourly s.c. from day 1 to 19 of pregnancy) abolished maternal constraint whereas 0.9% (w/v) NaCl treatment did not. In Wistar rats osmotic pumps were implanted to deliver IGF-I (1-mu-g/g body weight per day), bovine GH (bGH; 0.6-mu-g/g body weight per day) or saline from day 1 to 19 of pregnancy. IGF-I therapy but not bGH or saline abolished (P<0.01) maternal constraint and altered (P<0.01) the relationship between placental and fetal weight. When high or low IGF-I line mice embryos were transplanted into a normal line of mice, the expected negative relationship (P<0.05) between mean fetal weight and litter size was maintained. However the embryos of the high line were heavier (P<0.05) than those from the low line irrespective of fetal number, suggesting a direct role for IGF-I in the regulation of fetal growth. Thus both endogenous and exogenous elevations in maternal IGF-I indirectly promote fetal growth either by altering nutrient delivery to the placenta or by affecting placental function.