Urea transport in a distributed loop model of the urine-concentrating mechanism

Continuously distributed loops of Henle were used in a central core model of the rat kidney's urine-concentrating mechanism to investigate the importance of overlapping loops for three different modes of urea transport in the long loops of Henle: 1) urea-impermeable loops, 2) urea-permeable loops (as indicated by perfused tubule experiments), and 3) loops with urea-permeable descending limbs and active urea transport out of thin ascending limbs. Mode 1 produces high papillary tip osmolality in accordance with tissue slice experiments, but the relative contribution of urea to the osmolality of the central core and the long descending limbs is below experimental measurements. Mode 2 generates no significant osmolality increase in the inner medulla, in agreement with other model studies. Mode 3 produces high papillary tip osmolality with a substantial contribution of urea to the osmolality of the core and the descending limbs, which is more in accordance with experiments. The results suggest that 1) overlapping loops may produce a cascade effect that contributes to the inner medullary concentrating mechanism and that 2) new experiments are needed to more certainly ascertain the urea transport characteristics of the thin ascending limbs.