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 osmolality resulted in a transient increase in cell volume which was followed by an efficient 'regulatory volume decrease' (RVD). H. inflata contains high 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-activated pathways. The efflux of amino acids and K⁺, together with a charge-balancing counter-anion, accounted almost fully for the observed RVD. The pharmacological properties of the swelling-activated pathways differ from those of volume-sensitive transporters and channels described previously in other cell types.