Urea recycling from the renal pelvis in sheep: a study with [14C]urea

Abstract

To test the hypothesis that urea can be recycled from the renal pelvis, [14C]urea diluted in native urine (1 microCi/ml) was perfused (0.5 ml/min) into one of the pelvises of sheep fed either normal (NP) or low (LP)-protein diets. Blood samples were obtained from the ipsilateral renal vein and from the carotid artery throughout the perfusions. 14C activity determinations in urine and plasma demonstrated a flux of [14C]urea from the pelvis to renal vein blood (40,000 in NP and 130,000 disintegrations/min in LP sheep, P less than 0.01). The corresponding flux of native urea was only 1.5 times higher in NP than in LP sheep (6.8 +/- 1.1 vs. 4.7 +/- 2.9 mumol/min, not significant) despite their 8 times higher urinary concentration of urea. The fraction of filtered urea that was reabsorbed in the pelvis was larger in LP sheep (7.5 +/- 3.7 vs. 1.9 +/- 0.7% in NP sheep, P less than 0.05). A fraction of urea is thus actually recycled from the renal pelvis in sheep, and this pelvic retention is enhanced in LP animals. The importance of this phenomenon in the nitrogen economy is discussed.

Microbial protein production determined by urinary allantoin and renal urea sparing in normal and low protein fed Corriedale sheep

“…As in previous results [8,22], the kidneys of sheep adapted to restricted protein intake by reducing the amounts of urea eliminated in the urine. This decrease was larger than what we expected from the simple fall of the urea blood produced by the reduction of the N intake.…”

“…This process is nutritionally advantageous for ruminants because ruminal bacteria are able to use urea nitrogen to synthesize proteins that will be absorbed in the small intestine. When protein intake is restricted, renal elimination of urea is reduced by (a) a decreased filtered load of urea, consecutive to a reduction in renal plasma flow and glomerular filtration rate, and (b) an enhanced capacity for urea reabsorption from tubules and the renal pelvis [8]. This adaptive process prevents an excessive reduction of the blood urea pool.…”

The Renal Response of Sheep Fed a High Protein Diet to Treatment with Vasopressin Analogue

“…The adaptation ability of sheep kidneys to a reduced delivery of protein by food includes the morphological (Koãi‰ová et al 1997), biochemical (Faixová et al 1998) and functional response (Leng et al 1985) in order to increase the renal urea reabsorption for its subsequent recycling through the digestive tract. The main change in renal function responsible for a high urea retention in sheep appears to be a significant reduction of glomerular filtration rate (GFR, Leng et al 1985;Cirio and Boivin 1990). That is why the regulation of GFR in ruminants became an interesting research subject.…”

Urea transporter mRNA in papilla and pelvic epithelium of kidney in normal and low protein fed sheep

“…Indeed, LP diets reverse the normal medullary urea gradient, shifting its highest concentration to the outer medulla and creating a steep urea concentration gradient from the outer medulla to the fi nal urine fi lling the fornices of the upper pelvic space (reviewed in SchmidtNielsen, 1990). Since the extremely thin pelvic epithelium at this level allows the exchange of urea between pelvic urine and tissue (Cirio and Boivin, 1990;SchmidtNielsen, 1990), a decrease in urea permeability due to a lesser expression of the UTs could be relevant for urea conservation, reducing its loss from the outer medulla.…”

“…The advantage of this adaptation should be relevant in ruminants in which endogenous urea serves as nitrogen source for protein synthesis in their forestomachs. In low protein fed sheep, urea conservation is achieved by a reduction in fi ltered load of urea and an enhanced urea reabsorption from renal tubules and pelvis (Cirio and Boivin, 1990). Characterization of UT-A mRNAs in urea-transporting epithelia of the sheep kidney is lacking.…”