Uphill transport of water by electroosmosis

Summary Lepismatidae are able to gain water from subsaturated atmosphere above a relative humidity of 45%, surmounting a water potential difference of at least 1.1×10⁸ Pa (1,100 bar). This extraordinary task is performed by the monolayered epithelium of the posterior rectum. The particle coat of the folded apical membrane of this epithelium suggests the presence of the electrogenic, lumen-directed cation transport, which is commonly found in insects. Assuming this kind of transport and considering the anatomy of the organ, a working hypothesis for this hyposmotic water transport has been developed: The electrogenic cation transport maintains the circulation of the transported ion species across the apical membrane; the voltagedriven inward current transfers water by electroosmosis against its chemical potential from the extracellular space into the cytoplasm. Voltage and current measurements and synchronous measurements of water flow across the epithelium of the posterior rectum ofLepisma saccharina strongly corroborate this hypothesis. The transepithelial voltage is up to 200 mV (lumen positive); the short-circuit current averages 200 A per cm² of the epithelium. Both depend acutely on oxidative metabolism as does spontaneous water uptake. Exogenous transepithelial current (I) induces, independently of anoxia, a proportional change in volume flow (J_v). The induced flow has the direction of the cation flow. Its mean coupling ratio (J_v/I) is 1.5×10^–9m³/A·sec corresponding to 7 to 8 H₂O per positive unit charge. Critical evaluation of experimental data reveals that water uptake by electroosmosis may quantitatively account forin vivo performance without requiring any unusual assumption.