Expression of urea transporters is affected by dietary nitrogen restriction in goat kidney1

Abstract

Ruminants are known to be able to very effectively recycle urinary urea and reuse it as a source of N for ruminal microbes. It is presumed that urea recycling is accomplished by specialized urea transporters (UT) which are localized in the kidney. This could be especially important in times of increased N requirement, such as during growth or during reduced dietary N intake. The aim of our study was to characterize and to localize UT in the goat (capra hircus) kidney and to investigate its response to reduced dietary N intake in growing goats. Therefore, 12 growing, male goats were fed either a diet containing high (17% CP in complete diet) or low (9% CP in complete diet) N content for 6 wk. After harvesting, blood and kidney samples were taken and analyzed. The mRNA of the different UT isoforms, UT-A1, UT-A2 and UT-B, were detected semiquantitatively in renal tissue by Northern blot analysis. For UT-A2 and UT-B, no statistically significant effect of dietary N restriction on renal mRNA expression could be detected (UT-A2: P = 0.26, UT-B: P = 0.07). However, renal mRNA abundance of UT-A1 significantly increased in the kidney of low-N-fed goats (P = 0.01). Furthermore, protein amounts of UT-B were verified by western blotting; and the localization of UT-A2 and UT-B protein was demonstrated by immunohistochemistry. No significant differences in protein amounts of UT-B could be observed comparing the 2 feeding groups (P = 0.78). The UT-B was localized in renal medulla and papilla, whereas UT-A2 was only found in renal medulla. In addition, comparison of UT-A and UT-BAA sequences of monogastric animals and ruminants showed a high degree of homology, indicating a similar function of the transporters among these species. In summary, we conclude that in ruminants, urea reabsorption in the kidney is most likely increased in response to a low-N diet via an upregulation of UT-A1 mRNA expression. Hypothetically, the reabsorbed urea can then be returned to the rumen via the bloodstream and thus be reused as a source of N for protein synthesis of ruminal microbial community.

Excretions of Urinary Creatinine on Young and Mature Kacang Goat under Different Feeding Levels

“…It showed that the increased creatinine excretion from week 0-2 was relatively constant. It was in accordance with Starke et al (2012) that high protein feed (16%) and low protein feed (11%) resulted in relatively similar creatinine excretion. Chen et al (1995); Pathak et al (2013) reported that the comparison of creatinine daily excretion was relatively constant with the amount of body protein.…”

Modulation of aquaporin 2 expression in the kidney of young goats by changes in nitrogen intake

“…The existence of specific urea transporters in the mammalian kidney of monogastric animals (namely rats) has been described by several authors (Chou and Knepper 1989;Sands et al 1987). Evidence of these specific transporters in (small) ruminants has also been demonstrated for sheep (Artagaveytia et al 2005) and goat kidney (Starke et al 2012).…”

“…Subcellular fractionations and semi-quantification of renal AQP2 and CaR protein Crude membrane fractions from renal cortex and outer medulla were prepared as previously described by Starke et al (2012). The isolation of renal brush-border membranes (BBM) was done by using a divalent-cation precipitation method described by Wilkens et al (2009).…”

“…about 92 % (Muscher et al 2011). In a former study, it could be shown that an N-reduced diet in young goats led to an upregulation of urea transporter UT-A1 mRNA expression in renal medulla (Starke et al 2012), assuming that this is an important mechanism for saving urea in times of N scarcity.…”

“…A reduction in dietary N caused serious changes in urea metabolism and mineral homeostasis in young goats (Muscher et al , 2011Starke et al 2012). Therefore, it is hypothesised that the expression of renal AQPs and the CaR is modulated in young goats fed a reduced-N diet to support renal urea absorption by concentrating tubular fluid.…”