Role of K+ and amino acids in osmoregulation by the free-living microaerophilic protozoon Hexamita inflata

The primitive free-living protozoon Hexamita inflata was found to maintain a cell volume of approximately 260 fl under standard culture conditions, On increasing the extracellular osmolality the volume decreased and the cells remained shrunken for >30 min, By contrast, a decrease in the external osm olality resulted in a transient increase in cell volume which was followed by an efficient 'regulatory volume decrease' (RVD). H. inflata contains hig h concentrations of amino acids, with alanine constituting over 70% of the total amino acid pool, Exposure to hypo-osmotic medium resulted in the loss from the cell of both amino acids and K+, via one or more swelling-activat ed pathways. The efflux of amino acids and K+, together with a charge-balan cing counter-anion, accounted almost fully for the observed RVD, The pharma cological properties of the swelling-activated pathways differ from those o f volume-sensitive transporters and channels described previously in other cell types.