Nitrate and nitrite absorption, recycling and retention in tissues of sheep

Dietary nitrate is of increasing interest both for the pharmacological effects of its metabolites as well as its capacity to inhibit methanogenesis in the gut. A sequence of three experiments was conducted to investigate the absorption, metabolism and excretion of nitrate and nitrite through the gastrointestinal tract (GIT) of sheep, and to determine the fate of nitrate and nitrite in body fluids, tissues and faeces after intravenous dosing with N-15-labeled potassium nitrate ((KNO3)-N-15) and with N-15-labeled sodium nitrite ((NaNO2)-N-15). In Experiment 1, twelve female Merino sheep were assigned to one of two dietary treatments and adapted to the experimental diet over two weeks. Six sheep were fed a control diet of wheaten chaff mixture (600 g wheaten chaff plus 200 g wheat grain, CON) and six sheep were fed the CON diet with the inclusion of 18 g nitrate/kg DM (Nitrate). After acclimation to the diets, all sheep received a single intravenous dose of (KNO3)-N-15 and were placed in metabolic cages for daily collection of total faeces and urine over 6 days. Experiment 2 studied movement of an intravenous dose of N-15 in body fluids and tissues. Two sheep not adapted to dietary nitrate were dosed intravenously with (KNO3)-N-15 or (NaNO2)-N-15 and body fluids and tissue samples were collected 60 min after dosing. Finally, Experiment 3 was conducted to identify and quantify the major sites of nitrate and nitrite transfer within the body, focusing on absorption, partitioning and secretion into the GIT of anaesthetised sheep. A single dose of sodium nitrate (NaNO3) or sodium nitrite (NaNO2) was introduced into the rumen or abomasum or small intestine, and changes in nitrate and nitrite concentrations in other pools, including plasma, urine and saliva, were determined. Results from Experiment 1 showed that urinary recovery of N-15 dose in urea after 46 h and total urinary recovery of N-15 141 h after dosing were greater in sheep fed the Nitrate diet relative to CON (P < 0.05). Recoveries of N-15 in tissues indicated that nitrate and nitrite principally accumulated in the skin and muscle of sheep (Experiment 2). Finally, Experiment 3 indicated that nitrate and nitrite were rapidly absorbed from the rumen, abomasum and small intestine into the bloodstream. Nitrite was oxidized in plasma and the resultant nitrate was concentrated and recycled via saliva. Appearance of N-15 in urinary urea confirmed the passage of plasma nitrate to the digestive tract, via saliva or transruminal flow to be reduced by gut biota to ammonia.

Automated summary

Dietary nitrate is increasingly studied for its pharmacological effects and its impact on gut microbiota, with experiments showing that sheep fed a nitrate-rich diet had higher urinary recovery of N-15 dose and accumulation of nitrate and nitrite in skin and muscle. Nitrate and nitrite were found to be rapidly absorbed from the rumen, abomasum and small intestine into the bloodstream, with nitrite oxidized in plasma and resulting nitrate being recycled via saliva.