Active Cellular Transport of Urea by Frog Renal Tubules

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Abstract
The tubular excretion of urea is characterized by a maximal rate (Tm) which remains constant when plasma urea concentrations are higher than 15 mg percent. 2,4-Dinitrophenol and benemid block completely urea transport in dosages which do not affect glomerular filtration rates. The administration of p-amino-hippurate depresses Tmurea, but not as effectively as does benemid. Although this active transfer process is extremely limited in its distribution among the vertebrates, the tubular excretion of urea shares those general features which characterize the cellular transport of para-amino-hlppuric acid, diodrast, penicillin and phenol red, compounds previously shown to be excreted actively by the renal tubules of all vertebrates.