The fate of nitrogen from 15N-labeled nitrate after single intravenous administration of Na15NO3 in sheep

The metabolic fate of nitrogen from 15N-labeled sodium nitrate has been investigated in four healthy Polish Merino ewes. 15N-labeled sodium nitrate was administered intravenously at the dosage of 400 micromol.kg(-1) body weight. Blood plasma and urine concentrations of nitrate, ammonia, and urea and 15N enrichment of ammonia and urea were estimated over a 50-h period following 15N-nitrate administration. Nitrate (NO3-) was slowly eliminated from the blood plasma, and the presence of NO3(-) in the blood plasma above the nitrate "background" was observed for 50 h. 15N enrichment of blood plasma urea already appeared at 15 min and reached the maximum 6 h after 15N-nitrate administration. The urinary excretion of nitrate occured during 50 h after 15N-nitrate injection; the total urine excretion of NO3(-) was 23.63+/-2.39% of the administered dose. The mean urinary recoveries of nitrogen as 15N-urea and 15N-ammonia were 14.76+/-1.32% and 0.096+/-0.015% of the administered 15N-nitrate dose, respectively. It should be pointed out that in total only 38.49% of the administered nitrate-N was excreted in urine (as nitrate, ammonia and urea nitrogen) during 50 h. The results obtained indicate that sheep are able to store nitrate nitrogen in their body. The fate of the remaining approximately 60% of the 15NO3(-) administered dose is unknown. The results obtained do not allow one to conclude what fraction of the unrecovered approximately 60% of the 15NO3(-) dose was utilized by gastrointestinal microorganisms, and (or) metabolized, or stored in sheep tissues.