Osmoregulation in the Parasitic Protozoan Tritrichomonas foetus

Abstract

ABSTRACT Tritrichomonas foetus was shown to undergo a regulatory volume increase (RVI) when it was subjected to hyperosmotic challenge, but there was no regulatory volume decrease after hypoosmotic challenge, as determined by using both light-scattering methods and measurement of intracellular water space to monitor cell volume. An investigation of T. foetus intracellular amino acids revealed a pool size (65 mM) that was similar to that of Trichomonas vaginalis but was considerably smaller than those of Giardia intestinalis and Crithidia luciliae . Changes in amino acid concentrations in response to hyperosmotic challenge were found to account for only 18% of the T. foetus RVI. The T. foetus intracellular sodium and potassium concentrations were determined to be 35 and 119 mM, respectively. The intracellular K + concentration was found to increase considerably during exposure to hyperosmotic stress, and, assuming that there was a monovalent accompanying anion, this increase was estimated to account for 87% of the RVI. By using light scattering it was determined that the T. foetus RVI was enhanced by elevated external K + concentrations and was inhibited when K + and/or Cl − was absent from the medium. The results suggested that the well-documented Na + -K + -2Cl − cotransport system was responsible for the K + influx activated during the RVI. However, inhibitors of Na + -K + -2Cl − cotransport in other systems, such as quinine, ouabain, furosemide, and bumetanide, had no effect on the RVI or K + influx in T. foetus .