Further Observations on Asymmetrical Solute Movement across Membranes

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Abstract
The permeability of frog skin under the influence of urea hyperosmolarity has been studied. Flux ratio asymmetry has been demonstrated again for tracer mannitol. The inhibitors DNP [2,4-dinitrophenol], CN", and ouabain have been used to eliminate active Na transport and it was found that urea hyperosmolarity produces asymmetrical mannitol fluxes on frog skins having no short-circuit current. These findings suggest that flux ratio asymmetry is due to solute interaction and is unrelated to Na transport. Studies with a synthetic membrane show clearly that bulk flow of fluid can produce a solvent drag effect and change flux ratios. When bulk flow is blocked and solute gradients allowed their full expression, then solute interaction solute drag is easily demonstrable in a synthetic system.