The effect of intake level of fresh white clover (Trifolium repens) on ammonia and urea kinetics in the ovine liver

Abstract

Ureagenesis typically exceeds the rate of hepatic ammonia-nitrogen (N) extraction and it has been suggested that the shortfall in the N required for ureagenesis is compensated by amino acid (AA)-N. This study tested the hypothesis that the elevated hepatic ammonia extraction required when ruminants are fed fresh white clover (Trifolium repens) would require a concomitant increase in hepatic AA catabolism to supply the additional N required for ureagenesis. Our aim was to quantify the effect of 2 levels of intake (low vs. high) of fresh white clover on ammonia and urea kinetics in the liver. Sheep were prepared with permanent indwelling catheters in the posterior aorta via the femoral artery and in the mesenteric, portal and hepatic veins. The animals were infused for 9.5 h with ¹⁵NH₄Cl via the cranial mesenteric vein to measure the incorporation of 15N in plasma ammonia, urea and AA. For the last 3.75 h of the infusion, the infusate was supplemented with para-aminohippurate in order to measure hepatic blood flow. Blood samples were collected from the posterior aorta and the portal and hepatic veins to quantify the concentration of ammonia, urea and para-aminohippurate and the isotopic enrichment of ammonia, urea and AA in plasma. Although there were significant differences in both DM intake (807 v. 1118 g DM/d; s.e.d. 50.2; P < 0.001) and N intake (31.6 v. 43.9 g N/d; s.e.d. 1.96; P < 0.001) between the low and high groups, there were no significant differences in ammonia uptake or urea production by the liver between these groups. The hepatic ¹⁵N-ammonia transfer, however, was significantly lower in the low intake group compared with the high intake group. This was not the case for the ¹⁵N-urea transfers. The absence of [¹⁵N¹⁵N] urea in plasma confirmed that ammonia contributed only one N atom to urea and, therefore, there was no evidence to suggest that elevated rates of hepatic ammonia detoxification required a disproportionate increase in AA-N catabolism for ureagenesis.