Urine concentration and dilution in the rat: contribution of papillary structures during high rates of urine flow.

Urine concentration and dilution in the rat: Contribution of papillal'y structures during high rates of urine flow. To examine the contribution of papillary structures to the overall process of urine dilution and concentration at high rates of flow, studies were performed in unilaterally papillectomized kidneys (PX). Compari­ son of kidney weights in sham-operated and PX rats revealed a significant reduction in total weight of the latter. Papillary length removed was 3045 ± 423 p.m. GFR was reduced by 24% and 45% in sham and PX kidneys when compared to their contralateral controls. Under hydropenic conditions, maximal urine concentrat­ ing ability (Uma,.) was comparable in control and sham kidneys but was 59% less in PX kidneys. Fractional sodium excretion during hydropenic conditions was comparable in control and PX kidneys. Nonurea solute [(Na + K) x 2J concentration in the medulla of control and papillectomized kidneys was essentially the same. Free water reabsorption (Tl'J,o) as a function of osmolar clearance (Cos"J was comparable in control, sham, and PX kidneys. At COsm greater than 25%, there was a tencency for Tfi,o to fall in both control and PX kidneys. Free water clearance (CH,0) during hypo­ tonic saline diuresis rose in almost linear fashion as a function of urine flow (Y) without clear-cut differences between control and PX kidneys. There was, however, a tendency for CH,o to be slightly higher at any level of Y in PX than in control kidneys. These experiments suggest that nephrons with long loops reaching into the papilla and the terminal collecting ducts are not essential for the maximal generation and reabsorption of free water.