Toxic effects after exposure to mercury are well documented in human. Little is, however, known about how Hg2+ affect host defense in general and neutrophil functions in particular. We show here that exposure of human neutrophils to HgCl2 dose-dependently impairs chemoattractant-stimulated motility. Long-term exposure (5-10 min) to Hg2+ yields a rapid influx of extracellular Ca2+ followed by leakage of cytosolic fluorophores, as assessed using fura-2 and ratio imaging microscopy. The inhibition on motility was partly reversible, since pre-treated neutrophils placed in an Hg2+-ftee environment displayed higher migration rates. The Hg2+-induced fluxes were prevented by addition of small-sized polyethylene glycols (PEG 200-400), which also dose-dependently inhibited neutrophil transmigration. Localized, minute micropipette additions of Hg2+ or PEG caused retraction of the leading edge and redirection of cell migration. Since Hg2+ increases and PEGs decrease membrane permeability in a partially competitive manner, we suggest that the known aquaporin-inhibitor Hg2+ alters membrane permeability by affecting the bidirectional flux through the leukocyte aquaporin-9 (AQP9) while small-sized PEGs yield decreased membrane permeability by becoming trapped in the promiscuous channel. The local additions of Hg2+ or PEG probably force other cell regions to take over from those with blocked AQPs. Hence, the cells turn direction of motility away from the micromanipulator needle. (C) 2004 Elsevier Inc. All rights reserved.