Active urea transport through isolated skins of frog and toad

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Abstract
Urea influxes (Ji) and effluxes (Je) were studied across the isolated skins of Rana esculenta, Bufo bufo, and B. viridis. Two symmetrical pieces of the same skin, bathed in Ringer + 2 mM urea, were used for the two fluxes. In R. esculenta the urea fluxes are passive when the animals are kept in running water but become active after dehydration in air or preadaptation in saline solutions. The ratio Ji/Je can vary between 3 and 27 and the Ji between 2 and 22 nmol . h-1 . cm-2 according to preadaptation. Only the active fluxes obey saturation kinetics. Urea absorption is always independent of sodium transport. In toads, active urea transport occurs even when hydrated. It is markedly stimulated by saline preadaptation. A correlation between the degree of active urea transport across the skin and the capacity of the species to endure dehydrating conditions would appear to exist. The physiological significance of this transport mechanism is discussed.