Leucine sources for 10.5-day rat conceptus in vivo

Protein has been shown to be the principal source of leucine for the Day 8.5 to 10.5 rat conceptus in culture. It could be argued that this finding applies only after adaptation to culture conditions and does not apply in vivo. This possibility was investigated using an isotope-dilution technique after i.v. injection of [3H]leucine into Day 10.5 pregnant rats. Specific radioactivity of free leucine in the conceptus was 8 to 10% of that in maternal plasma. Slow exchange of leucine with the maternal circulation and fetal tissue protein turnover were judged to be inadequate as explanations for the observed isotope dilution. Taken together, our results and those from in vitro studies are consistent with a major contribution of leucine coming from the degradation of protein in vivo, probably involving the visceral yolk sac. Our results suggest that mechanisms of amino acid supply to the conceptus identified using whole-embryo culture mirror those in vivo.