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

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