Transfer of urea nitrogen across the rumen wall.

HOUPT, T. RICHARD, AND KATHERINE A. HOUPT. Transfer of urea nitro,gen across the rumen wall. Am. J. Physiol. 2 1 /t(6) : 1296-1303. 1968.-Net movement of urea N across the wall of Pavlov-type rumen pouches was measured in three goats and one sheep as urea concentration difference between pouch and blood was varied. When urease activity of the rumen epitheliurn was depressed by vigorous rinsing or by treatment with antibacterial agents, a large portion of the blood urea that was transferred appeared in the pouch unchanged. Under these conditions transfer of urea N in either direction was linearly related to urea concentration difference and extrapolation of results indicated that transfer would cease at zero concentration difference. When urease activity was undisturbed, nearly all urea transferred was hydrolyzed to NH,; and at normal blood urea levels transfer of urea N was increased manyfold (up to 13 times). It appears that urea per se moves through the rumen wall by diffusion. However, under normal rumen conditions, blood urea is hydrolyzed before all epithelial diffusion barriers are crossed, and the NH3 then diffuses more rapidly than urea would through the remaining epithelial layers. The over-all effect is to enhance transfer of urea N from blood into the rumen.