Short-term vasopressin analogue treatment does not influence the nitrogen balance in sheep

The influence of vasopressin (dDAVP) on the nitrogen balance in 12 sheep, allotted to 2 groups of 6, fed a daily ration containing 16.01 g of nitrogen, was examined. The experimental animals received 12.5 μg dDAVP injected subcutaneously twice daily for 10 days before N balance was determined. Water intake and urine flow rate decreased due to dDAVP. The urine N concentration was significantly higher in the dDAVP group than in controls. Urinary and faecal N excretion and retained N (g/d and % of N intake) did not differ between groups. It is concluded that short-term administration of the vasopressin analogue does not affect the nitrogen balance in sheep.


INTRODUCTION
Our recent results with sheep injected with a vasopressin analogue (dDAVP) resulted in a 26% increase in the plasma urea, while renal urea reabsorption was concomitantly elevated by 63% (Boldižárová et al., 1999).This difference has been explained by extensive urea recycling into the digestive tract.The microbial population in the forestomachs enhances utilization of nitrogen from recycled endogenous urea.Thus, the kidneys of ruminants contribute to a N saving mechanism for protein synthesis (Faixová et al., 1998).These data confirm the important role of the ruminant kidneys in nitrogen metabolism.
The aim of this experiment was to investigate the effects of vasopressininduced formation of highly concentrated urine on the nitrogen balance in sheep.

MATERIAL AND METHODS
The experiments were carried out on 12 wethers of 33 to 38 kg BW housed individually with free access to water and minerals, allotted to 2 groups of 6.The control group was injected subcutaneously with 125 μl of glycerol while the experimental one was given 12.5 μg of a long-acting synthetic vasopressin analogue (1-desamino-8-D-arginine vasopressin, dDAVP, Adiuretin-SD, Ferring-Léčiva a.s., Prague) twice daily in 125 μl of glycerol.Animals were adapted to the experimental ration (Table 1) for 3 weeks before the 10 day-dDAVP treatment followed by nitrogen balance measurements.Total faeces and urine were collected two times daily for 5 days.The urine samples were collected using a device described by Kowalczyk et al. (1996).Representative samples were taken at 8.00 a.m. and 8.00 p.m. for analysis.Jugular blood was sampled twice daily during each balance period into heparinized test tubes and stored in deep freeze until analysis.Diets, faeces, urine and blood were analysed for N according to AOAC (1980) methods.
Statistical significance was assessed by Student's t-test.The results are given as arithmetic means and standard error of means (SEM).

RESULTS AND DISCUSSION
No significant differences in blood urea concentrations were found between control and experimental animals (6.5±0.3 vs 7.1±0.4,mmol .L -1 ), respectively.Shortterm (10-day) administration of the vasopressin analogue resulted in significant reduction of water intake and urine flow rate and a significantly higher nitrogen concentration in the urine of experimental group animals (Table 2).No differences were found in nitrogen intake, faecal and urine nitrogen excretion between the control and experimental groups.The higher concentration of nitrogen in urine of animals treated with dDAVP was related to a lower urine flow rate, which was described earlier (Boldižárová et al., 1999).1.9 ± 0.5 1.9 ± 0.3 NS Total N excretion, % N intake 94 ± 3 97 ± 3 NS On the other hand, long-term treatment (5 months) with the same vasopressin analogue in growing lambs resulted in the relative enlargement of the kidney medulla and reduction in the size of the cortex.The surface area of the renal pelvis as a site of urea reabsorption was enlarged too.It was suggested that such vasopressin-induced changes in the kidneys of growing lambs could support more efficient renal recycling and subsequent reabsorption of urea into the blood stream (Bizup and Leng, 1997).Urea is known to be reabsorbed across the terminal inner medullary collecting duct by the vasopressin-regulated, facilitated urea transporter, UT-A1 (Artegaveytia et al., 2005).Vasopressin-induced urea transport is facilitated by a process acting via the adenylate cyclase-dependent pathway and activation of specialized urea transporters (Tsukaguchi et al., 1998).Ruminants have the ability to accumulate N by passive transfer of plasma urea by N recycling to the rumen and/or large intestine, where it can be used for synthesis of microbial protein (Boda et al., 1974).It has been documented that ruminants fed a low nitrogen diet are able to recycle a larger portion of endogenous urea into the digestive tract than animals on a high nitrogen intake (Havassy et al., 1974).Nonetheless, the vasopressin induced short-term formation of highly concentrated urine showed to be an insufficiently strong stimulator to increase the retention of nitrogen in our experimental sheep.
The results of this study show that 10-day long treatment of sheep with a vasopressin analogue had no effect on their nitrogen balance.

Table 1 .
Composition of daily ration for sheep