Is Na+-dependent exchange diffusion a true exchange?

Trans-stimulation of glycine uptake by cellular glycine in Ehrlich cells is a Na+-dependent phenomenon. In contrast trans-stimulated methionine or leucine uptake is Na+-independent. Trans-stimulated uptake of glycine does not show any characteristics of an ex change process but rather appears to be due to changes in membrane potential which occur as a result of a net Na+-dependent loss of cellular amino acids. Trans-stimulated influx of glycine occurs during the time of net loss of cellular glycine and is absent when the cellular amino acid level is at steady or when the cell is depolarized. Exchange of leucine or methionine occurs when the amino acid level is at steady state and it is not directly affected by depolarizing agents such as gramicidin.