Serum IGF-I concentrations in rats decrease significantly in late pregnancy. To determine if the reduction in serum IGF-I concentrations is attributable to circulating GH or maternal nutritional status, we investigated the effect of treatment with recombinant human GH (rhGH: 100 microgram/rat per day) on IGF-I concentrations during late pregnancy, and evaluated the relationship between maternal nitrogen balance and IGF-I concentrations. Serum IGF-I concentrations and maternal nitrogen balance ((nitrogen intake)-(nitrogen content in faeces and urine)-(nitrogen content in fetus and placenta)) were measured by RIA and the Dumas method. In non-pregnant rats treated with rhGH for 3 days, serum IGF-I concentrations (835.4+/-59.5 ng/ml; P<0.01) were significantly greater than in those animals treated with saline (319.6+/- 95.6 ng/ml). In the pregnant rats, however, there was no significant difference in serum IGF-I between those treated with rhGH (151. 1+/-43.0 ng/ml) and those treated with saline (142.0+/- 39.9 ng/ml) from day 17 to 19 of pregnancy. Maternal nitrogen balance in the pregnant rats increased significantly from day 4 to day 10 of pregnancy (169.5+/-57.4 and 196.1+/- 33.4 mg/day, respectively; P<0. 05) compared with non-pregnant controls (31.9+/-19.9 mg/day) and decreased markedly from day 12 of pregnancy (79.8+/-60.1 mg/day; P<0. 05) onwards, to 14.9+/-47.8 mg/day on day 20 of pregnancy (P<0.01), significantly different from the value on day 10 of pregnancy. The mean difference in maternal nitrogen balance between pregnant and non-pregnant rats was positively correlated (r=0.87, P<0.01) with the mean difference in maternal IGF-I concentrations, using linear regression analysis. These results support the conclusion that the circulating concentration of IGF-I in pregnant rats is associated with the change in nitrogen balance, but not with circulating GH.

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