Development of structure and function in the mammalian yolk sac: III. The development of amino acid transport by rabbit yolk sac

Abstract

Rabbit yolk sacs obtained from fetuses older than 20 days' gestation actively accumulated l-valine, l-isoleucine, l-phenylalanine, l-alanine, and l-methionine. Glycine, l-proline, betaine, l-lysine, α-methylglucoside, and 6-deoxyglucose were not actively accumulated. The yolk sac is a polarized membrane as it was capable of effecting a net transfer of l-valine across its full thickness from mucosal to serosal compartments against a concentration gradient. l-Valine rapidly equilibrated with intracellular water in tissue obtained from fetuses younger than 20 days' gestation, but accumulation against a concentration gradient did not occur. Inhibition of valine uptake in such tissue by l-methionine and l-isoleucine suggested that a carrier-mediated process was involved. The rate of exit of preloaded l-valine from 17-day yolk sacs was similar to that found in 28- to 30-day yolk sacs. A study of the effect of valine concentration on the initial rate of uptake by the more mature yolk sacs indicated saturation kinetics with an approximate K t of 2.1 m M. No saturation of the transport rate by 17-day yolk sacs was found up to 10 m M concentration of valine.