Effect of continuous feeding on maternal protein metabolism and fetal growth in the rat

Accelerated maternal catabolism ensures adequate fetal growth in the postabsorptive pregnant mammal during late gestation. However, the relative importance of maternal stores and diet in supplying nutrient requirements to the fetus is unknown. We have measured rates of protein synthesis and breakdown in maternal and fetal tissues during continuous intravenous feeding on days 17 and 20 in the rat, using leucine tracer infusions. Nutrient solutions supplying 180 and 250 kcal.kg-1.day-1 were tested. Changes in fetal size were measured in these rats and in rats maintained on ad libitum chow feeding. In comparison to previous results in the postabsorptive rat, leucine flux from endogenous sources into plasma was significantly suppressed, with only 12-20% of circulating leucine derived from maternal tissues. Muscle protein catabolism (previously described on day 20 of gestation) was abolished with continuous feeding. Fetal growth rate averaged 46%/day on day 17 of gestation in all groups but was reduced to 12%/day on day 20 in the rats intravenously fed 180 kcal.kg-1.day-1 and 22%/day in the rats fed 250 kcal.kg-1.day-1. Fetal growth on day 20 in the rats fed rat chow ad libitum was unaffected. We conclude that by decreasing the maternal contribution to circulating nutrients, continuous intravenous feeding may adversely affect fetal growth in late pregnancy. The findings in the current study underline the physiological importance of maternal-fetal nutrient exchange.