Summary: The effect of medium hypo-osmolality on cell volume and intracellular amino acid composition was studied in the protozoan parasite . When was exposed to hypotonic medium, it initially swelled by 40% of its original volume and then decreased its volume, thus demonstrating a regulatory volume decrease process (RVD). These processes were accompanied by a rapid release of intracellular neutral amino acids, especially alanine, but not by amino acids with net charges such as glutamate and ornithine. The net alanine efflux was Na and Cl independent, and sensitive to medium osmolality. Alanine efflux was sigmoidal with respect to medium osmolality, with an approximately linear relationship over a range of 250 mOsm kg. Alanine efflux was also sensitive to temperature, and an Arrhenius plot gave a of 3.6 and an activation energy of 25 kcal mol (105 kJ mol), suggesting that a carrier-type transport protein, or uniport, was involved in the net alanine efflux under hypotonic conditions. This volume-activated (VA) alanine efflux was not inhibited by ionophores or chloride channel blockers. Of the potential inhibitors tested, only -hydroxymercuribenzoate inhibited net alanine efflux. This thiol reagent also inhibited giardial RVD, suggesting that alanine efflux plays a significant role in this process. The VA (alanine) uniport was able to transport 2-aminoisobutyric acid (AIB), a structural analogue of alanine which is frequently used for the characterization of eukaryotic alanine transport, but which is not transported by under isotonic conditions. On the basis of AIB uptake under hypotonic conditions and lack of transactivation of AIB efflux from AIB-loaded cells by external 10 mM alanine or glycine under isotonic conditions, it is evident that the VA (alanine) uniport is different from the previously reported (alanine) antiport.


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