Micropuncture study on urea movements in the kidney cortical tubules of low protein fed sheep.

Micropuncture studies of late proximal, early and late distal cortical tubules were carried out on kidneys of normal (NP) and low (LP) protein fed sheep in order to investigate the participation of these segments in the urea sparing induced by protein restriction in the diet. A low protein diet induced significant reductions in the fractional (-54%) and total (-84%) urea excretion, revealing an enhanced capacity for urea conservation. Micropuncture data did not show any difference in the proximal tubule functions between both groups of sheep. In distal cortical tubules the fractional delivery of urea (early distal, 0.61 +/- 0.06 for NP and 0.77 +/- 0.06 for LP sheep, not significant (NS); late distal, 0.45 +/- 0.07 for NP and 0.71 +/- 0.09 for LP sheep, P < 0.05) showed a relatively larger amount of urea present in the late distal tubule of protein restricted sheep. The tubular fluid-to-plasma inulin ratio in the late distal tubule was found to be lower in LP sheep (4.33 +/- 0.23 versus 8.58 +/- 0.9 in NP sheep, P < 0.01). The tubular flow rate, reduced in the early distal tubules of LP sheep (10.87 +/- 0.99 versus 18.92 +/- 2.58 nL.min-1 in NP sheep, P < 0.05), was not different in the late distal tubules from values in normally fed animals (6.65 +/- 0.90 versus 7.73 +/- 0.94 nL.min-1 in NP sheep, NS). These findings suggest a decreased distal water reabsorption coincident with the relatively larger amounts of intraluminal urea in LP sheep. This relatively larger urea delivery to the initial collecting duct could increase the subsequent urea reabsorption in protein restricted sheep.