Amino acid transport and cell volume regulation in Ehrlich ascites tumour cells

Abstract

Cellular and extracellular concentrations of amino acids were measured in Ehrlich ascites cells by amino acid analysis and by distribution of radioactive amino acids between cells and medium. Dilution of the medium results in a reduction in the cellular concentration of non-essential amino acids and taurine and an equivalent increase in the extracellular content of these amino acids. The membrane potential and the electrochemical gradient of sodium were measured. The decrease in the cellular to extracellular gradient of taurine and glycine is not a consequence of a decrease in the sodium gradient but is caused by the changes in osmolarity and cell volume. The unidirectional influx and efflux of glycine and taurine were measured as the initial influx of [14C]glycine and [35S]taurine, using a rapid filter technique. For both amino acids the rate constants for influx are decreased and the rate constants for efflux increased following a reduction in osmolarity. The taurine and glycine fluxes were analysed as a simple pump and leak system. Reduction in osmolarity increases the leak permeability to both taurine and glycine. The results for glycine are also discussed in relation to a Na+-glycine co-transport model, where reduction in osmolarity increases the leak permeability to glycine. The cellular permeability to taurine and glycine increases as a function of increasing cell volume. The taurine permeability increases 7-fold and the glycine permeability 1.5-fold with an increase in cell volume of 30%. The absolute increase in permeability is equal for both taurine and glycine.