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

School of Biochemistry and Molecular Biology, University of New South Wales, Sydney, NSW 2052, Australia¹
Division of Biochemistry and Molecular Biology, Australian National University, Canberra, ACT 0200, Australia²

Author for correspondence: Giancarlo A. Biagini. Tel: +61 2 9385 2043. Fax: +61 2 9385 1483. e-mail: G.Biagini{at}unsw.edu.au

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.

Keywords: anaerobic protozoa, osmoregulation, volume regulation, membrane transport, alanine

Abbreviations: [1-¹⁴C]AIB, 2-amino[1-¹⁴C]isobutyrate; DIDS, 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid disodium salt; NEM, N-ethylmaleimide; NPPB, 5-nitro-2-(3-phenylpropylamino)benzoic acid; RVD, regulatory volume decrease

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