INSULIN-LIKE GROWTH FACTOR-I INFUSION INCREASES IN VIVO SKELETAL MUSCLE AND MAMMARY α-AMINO[1- 14C]ISOBUTYRIC ACID ACCUMULATION IN FOOD RESTRICTED LACTATING RATS

Abstract

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 α-amino[1- 14C]isobutyric acid ([ 14C]-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 ∼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≤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. © 1997 Elsevier Science Inc.