Na+-dependent pH regulation by the amitochondriate protozoan parasite Giardia intestinalis

Giardia intestinalis is a pathogenic fermentative parasite, which inhabits the gastrointestinal tract of animals and humans. G. intestinalis trophozoi tes are exposed to acidic fluctuations in vivo and must also cope with acid ic metabolic endproducts. In this study, a combination of independent techn iques (UP NMR spectroscopy, distribution of the weak acid pH marker 5,5-dim ethyl-2,4-oxazolidinedione (DMO) and the fluorescent pH indicator 2 ' ,7 ' -bis (carboxyethyl)-5,6-carboxyfluorescein (BCECF)) were used to show that G. intestinalis trophozoites exposed to an extracellular pH range of 6.0-7. 5 maintain their cytosolic pH (pH(i)) within the range 6.7-7.1. Maintenance of the resting pH(i) was Na+-dependent but unaffected by amiloride (or ana logs thereof). Recovery of pH, from an intracellular acidosis was also Na+- dependent, with the rate of recovery varying with the extracellular Na+ con centration in a saturable manner (K-m = 18 mm; V-max = 10 mM H+ min(-1)). T he recovery of pH. from an acid load was inhibited by amiloride but unaffec ted by a number of its analogs. The postulated involvement of one or more N a+/H+ exchanger(s) in the regulation of pH(i) in G. intestinalis is discuss ed.