Studies on the analysis of endocrinologic and metabolic factors controlling the concentration of plasma insulin-like growth factor-I (IGF-I) in maternal rats

Abstract

The effects of the difference of nutritional intake on the kinetics of IGF-I secretion were investigated by 48 hours refeeding with standard, low-protein, low-fat, or low-sugar diet after 72 hours fasting in nonpergnant rats. The effects of hormonal and nutritional changes on the plasma level of IGF-I were also examined in pregnant rats by administration of hGH (100μg/day) for 3 days from day 12 or day 17 of pregnancy and by fasting for 48 hours from day 18. Plasma levels of GH and IGF-I were measured by radioimmunoassay, and the nitrogen balance was determined by the Plegl-Damas method.When nonpregnant rats were fed diets containing different nutrients after 72 hours of fasting, feeding with a low-protein diet, decreased plasma levels of total protein and albumin, and a decreased nitrogen balance were found to be closely related to a fall in the plasma level of IGF-I. In the nonpregnant rats, it appeared that nutrient intake and protein metabolism were important factors in the control of IGF-I.When the pregnant rats were administered 100μg/day of GH from day 12 to day 14 or day 17 to day 20 of gestation, the plasma levels of IGF-I were not significantly higher than in the control group given physiological saline intramuscularly.The IGF-I level in pregnant rats was 87.6±17.5 ng/ml on day 18. When the rats fasted for 48 hours from day 18, the IGF-I level fell to 60.6±10.7 ng/ml on day 20, a value significantry lower than that in rats allowed free access to food (118.6±24.8 ng/ml). A significant decrease in the plasma levels of total protein, albumin, triglycerides, and total cholesterol was seen after 48 hours of fasting from day 18. The nitrogen balance was 48.6±38.5 mg/day on day 18, and became markedly negative (-279.4±94.1 mg/day) on day 20 after fasting for 48 hours. The plasma level of IGF-I decreased as the nutritional status of the rats worsened, and changes in the IGF-I concentration were in close agreement with changes of the simultaneously measured nitrogen balance.These findings suggest that plasma levels of IGF-I in pregnant rats are influenced by changes of nutrient metabolism, especially protein metabolism.