Amino acid transport by membrane vesicles of virally transformed and nontransformed cells: Effects of sodium gradient and cell density

Abstract

Mixed membrane vesicle populations composed of plasma membrane and endoplasmic reticulum were prepared from Balb/c 3T3 and simian virus 40‐transformed Balb/c 3T3 mouse fibroblasts. The initial rates of uptake of L‐leucine and α‐aminoisobutyric acid by these vesicles were stimulated by a NaCl gradient (external > internal). Cation specificity for stimulation of L‐leucine uptake was Na⁺ > Li⁺ > K⁺. NaSCN was as effective as NaCl. Stimulation of uptake of both amino acids by a NaCl gradient was twice as great in vesicles from transformed as compared to non‐transformed cells. The NaCl gradient produced transient accumulation of both L‐leucine and α‐aminoisobutyric acid to twice the equilibrium level in vesicles from transformed cells. No such “overshoot” was observed in vesicles from nontransformed cells. In vesicles from the contact‐inhibitable Balb/c 3T3 cells, transport of α‐aminoisobutyric acid, but not L‐leucine, exhibited a density‐dependent decrease in Na⁺ gradient induced stimulation, from 248% for sub‐confluent to 109% with confluent cells. No density‐related changes in uptake were noted with vesicles from the transformed cells. These studies suggest that variations in amino acid uptake associated with viral transformation may be related, at least in part, to alterations in Na⁺ permeability of the surface membrane.