Anomalous transport of sucrose and urea in toad skin.

Abstract

1. Increased permeability to sucrose and anomalous transport of this substance in the inward direction are elicited in toad skin when the outside bathing solution is made hyperosmotic by addition of 200 millimoles of urea per liter. Influx and efflux of sucrose are measured on paired halves of skins from the belly of the toad, using 14C-sucrose as a tracer. 2. When the hydrostatic pressure of the inside solution is raised 25 cm H2O over that of the outside solution, the flux asymmetry for sucrose is substantially reduced. 3. Similar experiments with 14C-urea as the ‘driven species’ and inactive urea as the ‘driving species’ gave similar results: Increased permeability and anomalous inward transport. The effect of a hydrostatic pressure difference on the flux ratio was, however, less apparent than in the case of sucrose. 4. At low urea concentrations there seems to be a slow inward active transport of urea between identical solutions. The phenomenon is absent in frog skin. 5. The anomalous transports of sucrose and urea are discussed in the light of a model involving ‘anomalous solvent drag’.