Trapping of urea by red cells in the kidney

Abstract

Outflow patterns of T-1824, urea, thiourea, and creatinine have been determined in the dog kidney in vivo and in situ as a function of the arterial hematocrit by means of the multiple indicator-dilution technique. The mean transit times of T-1824, urea, and creatinine decrease and converge as the hematocrit decreases. The outflow patterns of urea are anomalous: a) there is precession of urea over creatinine and thiourea; b) the transit times of urea are shorter than those of creatinine and thiourea; c) the recoveries of urea are greater than those of creatinine at normal and high hematocrit values. Thiourea has a longer equilibration time with red cells than urea. Prior incubation of thiourea with red cells results in precession of thiourea over urea. These results are considered evidence of transient trapping of urea in red cells during their passage through the glomerular capillaries. The similarity of the urea outflow curves to curves of substances known to participate in a membrane carrier transport system by the tubule cells from the antiluminal side suggest that urea may participate in a similar system.