K+- and Na+-gradient-dependent transport of l-phenylalanine by mouse intestinal brush border membrane vesicles

In the presence of an Na⁺- or a K⁺-gradient ($\text{outside > inside}$), l-phenylalanine uptake exhibited an overshoot phenomenon indicating active transport. The amplitudes of the overshoots were increased by increasing either Na⁺ or K⁺ concentrations in the incubation media, indicating that binding alone cannot account for the K⁺ effect. The K⁺-induced overshoot is not due to the presence of a membrane potential alone, as a gradient of choline chloride failed to produce it. Li⁺ could also substitute for Na⁺ though less potent than Na⁺ in inducing an overshoot. Uptake of l-leucine also showed Na⁺- and K⁺-effects and l-leucine and l-alanine could inhibit the Na⁺- and K⁺-overshoots obtained with phenylalanine. These results lead us to postulate the presence of a carrier for neutral amino acids dependent on monovalent cation with higher affinity for Na⁺ in mouse intestine. The Na⁺- and K⁺-driven active transport of l-phenylalanine were shown to be dependent on the presence of a membrane potential, as short-circuiting the membrane with FCCP reduced the amplitude of the overshoots seen with both ions. However, substitution of Cl^? by more lipophilic anions (NO₃^?, SCN^?) produced an inhibition of uptake. A preliminary analysis of the interrelations between Na⁺ and K⁺ for l-phenylalanine uptake showed complex interactions which can be best explained by mutual competition for a common carrier at both sides of the membrane. These results suggest the presence of a new transport system or a variant of an ASC-type system for l-phenylalanine (and neutral amino acids) in the mouse intestine. However, our studies do not rule out the possible involvement of more than one system for neutral amino acid uptake.