The intracellular acid-extractable (acid-soluble) amino acid pool in mammalian cells: 4 An hypothesis to explain the effects of preloading on the exchange of amino acids across the cell membrane.

Intracellular acid-extractable pools preloaded with amino acids before the cells were placed in lower concentrations of labelling amino acid showed a massive infiltration of the label at resuspension. In some the overshoot was rapid, in others it was slower developing, but in all cases the final level returned to the control value. Some amino acids can produce an overshoot in preloaded pools of others, whereas in certain combinations, e.g. alanine preloaded cells challenged with leucine, no discernible change in uptake kinetics was seen. Discharge of the preloaded pool is a necessary prerequisite for overshoot since cells placed in challenges of equal concentration to the preloading conditions do not exhibit this phenomenon. There is no energy requirement for the rapid phase of overshoot to occur since it follows similar kinetics at 2 degrees and 37 degrees C. The results are discussed in terms of exchange reactions occurring within the acid-extractable pool which is composed of amino acids in a complexed form, and a hypothesis is offered to explain the phenomenon. This hypothesis is consistent with the cyclical perfusion model of Wheatley and Inglis (1980), and does not contravene Fick's law ans the second law of thermodynamics.