Giardia intestinalis: volume recovery in response to cell swelling

The trophozoite from of the protozoan parasite Giardia intestinalis is subjected to a changing osmotic environment in the small intestine of the host, and consequently effective osmoregulation and control of cell volume are essential to its survival. As a first step toward investigating the mechanism by which hypoosmotically-activated transport is controlled in this organism, we used a light scattering technique to monitor continuously changes in cell volume after osmotic challenge. There was a hyperbolic relationship between A550 and giardial protein concentration, resulting in linear double reciprocal plots and allowing the calculation of relative Giardia cell volumes from A550 values. The initial rate of cell swelling was directly proportional to the hypoosmotic gradient when the hypoosmotic difference was greater than 50 mOsm kg-1. However, a hypoosmotic challenge of < 30 mOsm kg-1 had little effect on cell swelling, suggesting that giardial cell rigidity can resist small changes in medium osmolarity. The use of light scattering as a measure of giardial cell volume changes was validated using a rapidly penetrating solute, ethylene glycol, to induce isoosmotic cell swelling. We have previously shown that trophozoites swelled initially when subjected to a hypoosmotic challenge and that a subsequent regulatory volume decrease was accompanied by rapid alanine efflux and activation of the uptake of an alanine analog, 2-aminoisobutyrate. The ethylene glycol-induced isoosmotic cell swelling was also followed by a regulatory volume decrease, accompanied by a similar rapid release of intracellular alanine and activation of 2-aminoisobutyric acid uptake. This suggests that an increase in cell volume is the primary stimulus for the rapid alanine efflux after hypoosmotic challenge.