Volume-regulatory amino acid release from the protozoan parasite Crithidia luciliae

The unicellular protozoan parasite, Crithidia luciliae, responded to osmotic swelling by undergoing a regulatory volume decrease. This process was accompanied by the efflux of amino acids (predominantly alanine, proline and glycine). The relative loss of the electroneutral amino acids proline, valine, alanine and glycine was greater than that for the anionic amino acid, glutamate; there was negligible loss of the cationic amino acids, lysine, arginine and ornithine. The characteristics of amino acid release were investigated using a radiolabeled form of the nonmetabolized alanine analogue alpha-aminoisobutyrate. alpha-Aminoisobutyrate efflux was activated within a few seconds of a reduction of the osmolality, and inactivated rapidly (again within a few seconds) on restoration of isotonicity. The initial rate of efflux of alpha-aminoisobutyrate from cells in hypotonic medium was unaffected by the extracellular amino acid concentration. Hypotonically activated alpha-aminoisobutyrate efflux (as well as the associated regulatory volume decrease) was inhibited by the sulfhydryl reagent N-ethylmaleimide but was not inhibited by a range of anion transport blockers. As in the efflux experiments, unidirectional influx rates for alpha-aminoisobutyrate increased markedly following reduction of the osmolality, consistent with the swelling-activated amino acid release mechanism allowing the flux of solutes in both directions. Hypotonically activated alpha-aminoisobutyrate influx showed no tendency to saturate up to an extracellular concentration of 50 mM. The functional characteristics of the amino acid release mechanism are those of a channel, with a preference for electroneutral and anionic amino acids over cationic amino acids. However, the pharmacology of the system differs from that of the anion-selective channels that are thought to mediate the volume-regulatory efflux of organic osmolytes from vertebrate cells.