Regulation of amino acid and glucose transport activity expressed in isolated membranes from untransformed and SV 40‐transformed mouse fibroblasts

Membrane vesicles isolated from untransformed Balb/c and Swiss mouse fibroblasts and their SV 40‐transformed derivatives were shown to catalyze carrier‐mediated, intravesicular uptake of α‐aminoisobutyric acid and D‐glucose. Concentrative uptake of α‐aminoisobutyric acid required the presence of a Na + ‐gradient (external >internal) and could occur independently of endogenous (Na + + K + )ATPase activity. A K + diffusion gradient (internal > external) in the presence of valinomycin, or the addition of the Na + salt of a highly permeant anion, conditions expected to create an interior‐negative membrane potential, stimulated Na + ‐gradient‐dependent uptake, suggesting this process is electrogenic. D‐Glucose uptake was nonconcentrative and did not require ion gradients or metabolic conversion. Na + ‐gradient‐dependent transport of α‐aminoisobutyric acid was reduced both in initial rate and extent of uptake in vesicles from confluent untransformed cells and increased in those from SV 40‐transformed cells, compared with activities observed in vesicles from proliferating untransformed cells. No changes in D‐glucose carrier activity were observed when assayed at low glucose concentrations.