Insulin-like growth factor-I infusion increases in vivo skeletal muscle and mammary alpha-amino[1-C-14]isobutyric acid accumulation in food restricted lactating rats

Food deprivation in lactating rats causes a significant decrease in circulating levels of IGF-I, which is likely to mediate nutrient partitioning for milk production. To test if circulating IGF-I mediates amino acid uptake and milk production in food-restricted lactating rats, IGF-I was infused using osmotic pumps from d 8 to 14 of lactation into rats which were food restricted during pregnancy and lactation (RPL) or solely during lactation (RL). Amino acid uptake was measured by intravenous infusion of alpha-amino[1-C-14]isobutyric acid ([C-14]-AIB) for 20 min and AIB accumulation was measured in liver, muscle, mammary gland and milk. Milk production was determined by the isotope transfer method using tritiated water. Between d 1 and d 14 postpartum, mean body weight change of control, RL and RPL dams were 26 +/- 21 g, -18 +/- 24 g and -2 +/- 15 g, respectively. Mean milk production of RL and RPL dams were 67% and 57% of control, respectively, and pups of RL and RPL weighed similar to 75% and 67% of control on d 14 postpartum. Serum IGF-I concentrations of RL and RPL dams receiving IGF-I infusion were not significantly different from control levels, while serum IGF-I of untreated RL and RPL dams were 72% and 52% of control (p less than or equal to 0.05), respectively. IGF-I infusion increased AIB uptake by 30% in muscle of RL and RPL dams and 57% in mammary gland of RL dams, but had no effect on mammary uptake of RPL dams. Milk production was decreased by food restriction, but was not influenced by IGF-I infusion. The results of this study showed that tissue AIB accumulation in lactating rats is influenced by the duration of food restriction and that the effect of exogenous IGF-I on AIB uptake is tissue specific. (C) 1997 Elsevier Science Inc.