Evidence for a central role for electro-osmosis in fluid transport by corneal endothelium

The mechanism of transepithelial fluid transport remains unclear. The prevailing explanation is that transport of electrolytes across cell membranes results in local concentration gradients and transcellular osmosis. However, when transporting fluid, the corneal endothelium spontaneously generates a locally circulating current of similar to25 muA cm(-2), and we report here that electrical currents (0 to +/- 15 muA cm(-2)) imposed across this layer induce fluid movements linear with the currents. As the imposed currents must be similar to98% paracellular, the direction of induced fluid movements and the rapidity with which they follow current imposition (rise time less than or equal to3 sec) is consistent with electro-osmosis driven by sodium movement across the paracellular pathway. The value of the coupling coefficient between current and fluid movements found here (2.37 +/- 0.11 mum cm(2) hr(-1) muA(-1)) suggests that: 1) the local endothelial current accounts for spontaneous transendothelial fluid transport; 2) the fluid transported becomes isotonically equilibrated. Ca++-free solutions or endothelial damage eliminate the coupling, pointing to the cells and particularly their intercellular junctions as a main site of electro-osmosis, The polycation polylysine, which is expected to affect surface charges, reverses the direction of current-induced fluid movements. Fluid transport is proportional to the electrical resistance of the ambient medium. Taken together, the results suggest that electro-osmosis through the intercellular junctions is the primary process in a sequence of events that results in fluid transport across this preparation.