Fig. 4. Effect of membrane potential on the amino acid transport activity elicited by b^o,+AT in reconstituted systems. b^o,+AT-expressing MDCK cells were used for reconstitution in liposomes. Initial transport rates of 0.5 µM l-[³H]arginine (l-arg_o) into liposomes in the exchange mode l-arg_o/l-leu_i or l-arg_o/l-arg_i were measured. Liposomes containing no amino acids or 2 mM leucine (l-leu_i) or arginine (l-arg_i) in the internal medium were used. Transport in exchange with the internal amino acid was calculated by subtracting transport in the absence of amino acid in the internal medium from transport in its presence. Valinomycin was added when indicated to liposomes containing 120 mM KP_i in the internal medium to impose a membrane potential (negative inside). Data (mean ± SEM) correspond to a representative experiment run in triplicate.