The passive permeability of the skin of anuran amphibia: a comparison of frogs (Rana pipiens) and toads (Bufo marinus).

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Abstract
Efflux of Na+ across dorsal skin, in vitro (bathed on both sides with Ringer solution), of frogs and toads was similar, but it was greater in ventral skin from the latter. The efflux of Na+ declined, in both species, when the external surface of ventral, but not dorsal, skin was exposed to hyposmotic solutions with a low Na+ concentration. This change in Na+ permeability was influenced by the low osmotic concentration and the low Na concentration. Efflux of Cl- was similar in both the dorsal and ventral skin preparations (Ringer on both sides) from frogs and toads. Cl- efflux declined in all skin preparations exposed on the external surface to dilute Ringer solution. Solute replacement with sucrose, choline or Na2SO4 showed that the decreased Cl- efflux was principally due to the low Cl- concentration, though Na+ may also contribute. This observation suggests the presence of Cl-/Cl- exchange diffusion mechanism. Cutaneous urea permeability was less in toads than frogs and the dorsal and ventral skin was similar with respect to this solute in each species. The presence of external hyposmotic solutions reduced the efflux, but not the influx, of urea across frog, but not toad, skin and it appeared that solvent drag may contribute to this change. Diffusion permeability to water was greater in frogs than toads and in the latter the ventral skin was more permeable than the dorsal. The passive permeability of amphibian skin shows considerable interspecific and regional differences to various molecules which do not necessarily parallel each other. The control of passive cutaneous permeability appears to involve a variety of specific mechanisms which evolved during their adaptation to different environments.