Developmental aspects of amino acid transport. Suggestion of switching-on and switching-off mechanisms.

L-Lysine-14 C uptake was studied in vitro in intestinal segments of rats, in various ages, ranging between 15-day fetuses and adults. L-Lysine in 0.065 mM concentrations was accumulated against a chemical concentration gradient by processes which obeyed saturation kinetics. There appeared to be two peaks of lysine uptake values, one in the fetus, diminishing by the 17th fetal day and another on the 2nd postnatal day. Fetal transport of lysine was not inhibited by anaerobic conditions and was not Na+ dependent, in contrast with increasing O2 and Na+ dependency postnatally. A series of amino acids including representatives of the neutral, imino acid and dibasic groups, failed to inhibit lysine uptake, with the exception of L-arginine, which was also antagonized by L-lysine. These findings suggest that in the rat intestine L-lysine is transported by at least two mechanisms, one fetal, not requiring O2 and Na+, and another developing postnatally with a peak during the 2nd day, which is Na-dependent and requires aerobic conditions.