Disposal of large urea overloads by the rat kidney: a micropuncture study.

ROCH-RAMEL, FRANCOISE, J. DI~ZI, FRANGOISE CHOM~TY, PIERRETTE MICHOUD, AND G. PETERS. Disposal of large urea overloads by the rat kidney: a micropuncture study. Am. J. Physiol. 2 1 a(6) : 1524-1532. 1970.-In rats overloaded with urea, at plasma concentrations of 60-80 mM, the proximal fractional reabsorption of urea was not impaired, whereas the cortical tissue-to-plasma urea concentration ratio was lowered. This finding argues against active transport of urea from tubular fluid into tubular cells as the cause of high cortical urea concentration. In rats preloaded with urea for several days by the oral route, and subsequently given an intravenous infusion of urea, an amount of urea equal to 25% of the filtered load was added to tubular fluid between the late proximal and early distal tubules. Since in the overloaded animals no urea left the collecting ducts, the source of the added urea could be peritubular blood. Urea appeared to be added in large amounts (27-43s of the filtered load) to tubular fluid between superficial late distal convolutions and pelvic urine in urea-overloaded animals. In collecting duct micropuncture experiments, done on young urea-overloaded rats, no addition of urea could be demonstrated between two sites, 1.5-2 mm apart, within collecting ducts. The addition of urea to collecting duct fluid was, thus, probably due to the addition of a large fraction of filtered urea from deep nephrons.