The membrane potential of Giardia intestinalis

Giardia intestinalis is a primitive microaerophilic protozoan parasite whic h colonises the upper intestine of humans. Despite the evolutionary and med ical significance of this organism, its physiology is very poorly understoo d. In this study we have used a novel flow cytometric technique to make qua ntitative measurements of the electrical potential across the plasma membra ne of G. intestinalis trophozoites. In media lacking both K+ and Na+, G, in testinalis trophozoites maintained a high negative plasma membrane potentia l(Psi (m)) of -134 +/- 3 mV. The Psi (m) was unaffected by the addition of Na+ to the extracellular medium, whereas the addition of K+ resulted in a s ignificant membrane depolarisation, consistent with the G. intestinalis tro phozoite plasma membrane having a significant (electrophoretic) permeabilit y to K+. The membrane was also depolarised by the H+ ionophore m-chlorophen ylhydrazone and by the H+ ATPase inhibitors dicyclohexylcarbodiimide and N- ethylmaleimide. These results are consistent with G. intestinalis trophozoi tes maintaining a high resting Psi (m), originating at least in part from a n electrogenic H+ pump acting in concert with a K+ diffusion pathway. (C) 2 000 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.