Emerging roles for sodium dependent amino acid transport in mesenchymal cells.

The functional aspects of sodium dependent amino acid transport in mesenchymal cells are the subject of this contribution. In a survey of the cross-talk existing among the various transport mechanisms, particular attention is devoted to the role played by substrates shared by several transport systems, such as L-glutamine. Intracellular levels of glutamine are determined by the activity of System A, the main transducer of ion gradients built on by Na,K-ATPase into neutral amino acid gradients. Changes in the activity of the System are employed to regulate intracellular amino acid pool and, hence, cell volume. System A activity has been found increased in hypertonically shrunken cells and in proliferating cells. Under both these conditions cells have to increase their volume; therefore, System A can be employed as a convenient mechanism to increase cell volume both under hypertonic and isotonic conditions. Although less well characterized, the uptake of anionic amino acids performed by System X(-) AG may be involved in the maintenance of intracellular amino acid pool under conditions of limited availability of neutral amino acids substrates of System A.