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 ¹⁵N-labeled potassium nitrate (K¹⁵NO₃) and with ¹⁵N-labeled sodium nitrite (Na¹⁵NO₂). 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 K¹⁵NO₃ 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 ¹⁵N in body fluids and tissues. Two sheep not adapted to dietary nitrate were dosed intravenously with K¹⁵NO₃ or Na¹⁵NO₂ 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 ¹⁵N dose in urea after 46 h and total urinary recovery of ¹⁵N 141 h after dosing were greater in sheep fed the Nitrate diet relative to CON (P < 0.05). Recoveries of ¹⁵N 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 ¹⁵N 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.

饮食中的硝酸盐对于其代谢物的药理作用及其抑制肠道甲烷生成的能力越来越引起人们的关注。进行了三个实验序列，以研究绵羊胃肠道（GIT）中硝酸盐和亚硝酸盐的吸收，代谢和排泄，并确定静脉注射^15剂后体液，组织和粪便中硝酸盐和亚硝酸盐的命运。N标记的硝酸钾（K ¹⁵ NO ₃）和¹⁵ N标记的亚硝酸钠（Na ¹⁵ NO ₂）。在实验1中，将十二只雌性美利奴绵羊分配给两种饮食疗法之一，并在两周内适应了实验饮食。给六只绵羊饲喂小麦谷壳混合物的对照日粮（600克小麦糠谷壳加200 g小麦籽粒，CON），向六只绵羊饲喂CON日粮，其中包含18 g硝酸盐/ kg DM（硝酸盐）。适应饮食后，所有绵羊均接受单次静脉内剂量的K ¹⁵ NO ₃，并置于代谢笼中，以每天收集6天的总粪便和尿液。实验2研究了¹⁵ N静脉注射剂量在体液和组织中的运动。给两只不适应硝酸盐饮食的绵羊静脉注射K ¹⁵ NO ₃或Na ¹⁵给药60分钟后收集NO ₂以及体液和组织样品。最后，进行了实验3，以识别和量化体内硝酸盐和亚硝酸盐转移的主要部位，重点是麻醉羊的吸收，分配和分泌到GIT中。将单剂量的硝酸钠（NaNO ₃）或亚硝酸钠（NaNO ₂）引入瘤胃或厌恶或小肠中，并测定其他池（包括血浆，尿液和唾液）中硝酸盐和亚硝酸盐的浓度变化。实验1的结果显示，尿素在46小时后的尿量恢复为¹⁵ N，总尿量恢复为¹⁵饲喂硝酸盐饮食的绵羊在服药后141 h的N值要比CON高（P <0.05）。组织中¹⁵ N的回收表明硝酸盐和亚硝酸盐主要积累在绵羊的皮肤和肌肉中（实验2）。最后，实验3表明硝酸盐和亚硝酸盐被迅速从瘤胃，皱胃和小肠吸收到血液中。亚硝酸盐在血浆中被氧化，生成的硝酸盐被浓缩并通过唾液再循环。尿素中¹⁵ N的出现证实了血浆硝酸盐通过唾液或经瘤胃流通过消化道而被肠道菌群还原为氨。