Density regulation of amino acid transport in cultured, androgen-responsive tumor cells.

In androgen-responsive cells, activity of a sodium-dependent transport system for neutral amino acids is shown to be density-dependent, whereas activity of a sodium-independent system is not. Transport of the non-metabolizable amino acids, alpha-aminoisobutyric acid )alpha-aib) was largely sodium-dependent, wherease transport of cycloleucine (cyleu) was by both sodium-dependent and sodium-independent systems. The km for cyleu transport changed with density because only the sodium-dependent portion of this transport was density-dependent. By contrast, the Vmax for alpha-aib transport changed with density, indicating that either amount of functional membrane carrier of the activity of the sodium pump was affected.