The Nature of Water Transport across Frog Skin

A method has been developed for determining simultaneously shortcircuit currents and net water fluxes across frog skin. The basis of the water flux measurement is the determination of changes in weight of a plastic chamber containing the skin and external solution. The accuracy of this method permits net water flows larger than 0.5 mg cm-2hr.-1 to be detected, and the apparatus has been used to investigate the relationship between active Na transport and non-osmotic water flow across the skin. Measurement of Na transport and net water influx across completely short-circuited skins provides no good correlation between the two flows. However, skins exhibiting no net water movement in sulfate Ringer displayed an apparent electroosmotic flow of about 40 water molecules per Na ion when depolarizing current densities of 50 and 100μA cm-2 are used. It is concluded from this and other evidence that the net water influx across frog skin may be partially electroosmotic in character and that there remains another component of water flow unrelated to active Na transport. A theoretical model, based on irreversible thermodynamics, has been developed to explain the non-osmotic water flow across frog skin