Active transport of urea across the skin of the euryhaline toad, Bufo viridis.

Urea is accumulated in considerable amounts (greater than 100 mM) in the blood of the euryhaline toad Bufo viridis, under conditions of adaptation to high salinities. Salt adaptation increases active transport of urea (inward direction) in the skin, which was measured in vitro. The active transport of urea is insensitive to ADH, and was inhibited nearly 50% by 0.5 mM phloretin. This transport system is different from the facilitated diffusion of urea which has been studied extensively in the toad urinary bladder, and may offer a simple model system for the study of active urea transport.