Micropuncture study of urea transport in rat renal medulla

Abstract

Urea concentration may be higher in rat renal papilla than in urine under circumstances in which the urine urea is decreased. To investigate this further, anesthetized young rats were given urea-C14 and inulin-methoxy-H3, and C14 and H3 activities were determined in vena cava plasma and in fluid obtained by micropuncture from loops of Henle, vasa recta, and collecting ducts in the papilla, and from distal convolutions on the surface of the kidney. Mean urea concentration was greater in vasa recta (VR) than in adjacent collecting ducts (CD) in protein-depleted rats (mean VR/CD = 1.14; P<0.02), and also in rats on a normal diet during osmotic diuresis (mean VR/CD =1.08; P<0.10). Urea concentration in loops of Henle did not differ significantly from that in collecting ducts in protein-depleted diuretic rats. Comparison or urea and inulin fluid-to-plasma ratios in distal convolutions and collecting ducts suggest movement of urea out of collecting ducts into the renal medullary interstitium. These results suggest active transport of urea out of collecting ducts.