Metabolism of nitrate by cattle

Abstract

A cow receiving a direct addition of 70-100 g of potassium nitrate a day into the rumen through a fistula showed no methemoglobin in the blood for several days. This delay in methemoglobin formation could not be attributed to failure of the micro-organisms of the rumen to form nitrite, for they immediately showed vigorous nitrate reduction. Potassium [15N]nitrate was added to the rumen to trace the formation, absorption and elimination of nitrate and the compounds formed from it. Nitrate, nitrite and ammonium ions were absorbed in considerable quantities directly from the rumen into the blood. The highest concentrations of 15n in the ammonium ion in the blood appeared 3-4 hr. after potassium [15N]nitrate was added to the rumen, whereas the higest 15N concentration in the nitrate plus nitrite fraction appeared in the blood after 5-6 hr., and the high concentration persisted. The curves for excretion of the 15N-labeled total, nitrate, urea and ammonium nitrogen fractions in the urine lagged behind their corresponding curves for appearance and disappearance of these components in the blood by about an hour. The formation of methemoglobin in the blood followed the time course of nitrite formation in the rumen rather closely, suggesting that nitrite was passed rapidly and directly from the rumen to the blood. There was no direct evidence for nitrite formation from nitrate in the blood. Nitrite was bound tightly to the heme of methemoglobin, for it could be recovered in heme that had been subjected to treatment with acetone containing 1.2 ml. of 12N-hydro-chloric acid/100 ml.