Efflux of alanine by Giardia intestinalis

Giardia intestinalis trophozoites synthesise and then secrete large amounts of alanine into the external medium during growth. This efflux of alanine was studied by preloading cells with L-[2,3-3H]alanine, and determining efflux of radiolabel from intact trophozoites. The efflux of alanine was also determined by measurement of alanine concentration in trophozoites and external medium using high pressure liquid chromatography amino acid analysis. Over the temperature range 4 degrees C to 37 degrees C there was a slow efflux of alanine, but this efflux was greatly stimulated by a number of amino acids structurally similar to alanine, notably glycine, L-serine, L-threonine, L-asparagine and L-glutamine. In contrast, 2-aminoisobutyrate, D-amino acids, and other naturally occurring amino acids had no effect. Those amino acids which stimulated the efflux of intracellular alanine are the same amino acids which inhibited uptake of extracellular alanine. This concordance suggests that an alanine antiport functions for both the influx and efflux of alanine, and acts to maintain a balance between intracellular and extracellular alanine concentrations.