Developmental aspects ofL-valine uptake by mouse intestine

¹⁴C-L-Valine uptake by intestinal segments of mice of various ages, ranging between 20-day fetuses and adults, was studied in vitro. 1 mML-Valine was accumulated against a concentration gradient by processes which showed saturation kinetics. There appeared to be a two-fold increase ofL-valine accumulation after the 2nd postnatal day and a three-fold increase in adult mice. Fetal transport of valine only occurred at pH 7.4 but was not Na⁺ dependent. In contrast, valine transport became increasingly Na⁺ dependent and the pH optimum widened, ranging between 5-8. A series of amino acids, including representatives of the imino acid and dibasic groups, failed to inhibit valine uptake while leucine and isoleucine manifested mutual inhibition with valine. It is speculated that in the mouse intestine,L-valine is transported by at least two mechanisms, one functioning in the fetus, not requiring Na⁺, but pH dependent and another which developes postnatally, is Na-dependent and functions over a wide pH optimum.