Adaptations of hepatic amino acid uptake and net utilisation contributes to nitrogen economy or waste in lambs fed nitrogen- or energy-deficient diets

Abstract

We investigated the effect of relative changes in dietary nitrogen (N) and energy supply and the subsequent variations in net portal appearance (NPA) of nitrogenous and energy nutrients on the net amino acid (AA) uptake by the liver and net N supply to the peripheral tissues. Six lambs were catheterised across the splanchnic tissues and received, in a replicated Latin square, one of three dietary treatments. The diets were formulated to either match the requirements of N and energy (C), or supply only 0.8 of the N requirement (LN) or 0.8 of the energy requirement (LE). Net fluxes of AA and urea-N were measured across the portal-drained viscera, and estimation of arterial hepatic flow allowed the estimation of hepatic fluxes. Catheters were implanted into the portal and hepatic veins as well as in the abdominal aorta for the measurement of AA fluxes. Animals fed the LN diet showed more efficient N retention (0.59 of digested N) than did the C and LE diet (0.50 and 0.33, respectively;P< 0.001). The NPA of total AA-N for the LN diet was only 0.60 of the value measured for the control (C) diet (P< 0.01). Despite this, the total estimated AA-N net splanchnic fluxes were not significantly different across the three diets (3.3, 1.9 and 2.6 g total AA-N/day for C, LN and LE, respectively,P= 0.52). Thus, different metabolic regulations must have taken place across the liver between the three experimental diets. A combination of decreased net uptake of total AA-N by the liver of animals in the LN diet (0.61 of the C diet;P= 0.002) and reduced urinary urea-N production (0.52 of the C diet;P= 0.001) spared AA from catabolism in the LN diet relative to the other two diets. For the LE diet, the urinary urea-N output was 1.3 times the value of the C diet (P= 0.01). This may relate to an increased catabolism of AA by the muscle and/or, to a lesser extent, to an increased utilisation of AA for gluconeogenesis in the liver. These effects may explain the reduced whole body protein retention observed with the LE diet.

Relationship between level of forage intake, blood flow and oxygen consumption by splanchnic tissues of sheep fed a tropical grass forage

“…In turn, the nutrients metabolized by liver originate not only from PDV but also from peripheral tissues (i.e. fatty acids from adipose and amino acids from muscle tissue), at increasing rates when the level of ME intake is below maintenance (Emery et al., ; Kraft et al., ). Moreover, the rates of increase in blood flow and oxygen consumption by PDV and total ST with increasing OM intake obtained in the present study were higher than those reported by Vernet et al.…”

Net hepatic release of glucose from precursor supply in ruminants: a meta-analysis

“…Majdoub et al, 2003). Whether it is the high level of precursor supply or the imbalanced nutrient supply (as in Kraft et al, 2011 with N deficient diets) which is responsible for this plateau would remain to be confirmed. This reduced efficiency of hepatic conversion of precursors into glucose suggests a supply of precursors in excess of glucose demand.…”

“…Our results also showed that the average level of α-amino-N hepatic uptake (LSMeans) can be modified by the dietary N concentration, availability in propionate and hepatic glucose release. Direct experimental evidence was reported by Kraft et al (2011) who showed that a N deficient diet significantly limited the net hepatic uptake of α-amino-N, as a sparing mechanism, without modifying net glucose release (Loncke et al, 2009a).…”

Nutritional regulation of the anabolic fate of amino acids within the liver in mammals: concepts arising fromin vivostudies

“…As an example, in a ruminant study carried out by our group, a variation of N supply from 31 % to 20 % of energy intake did not affect total export or synthesis of albumin. In this case, hepatic uptake of EAA decreased at low N intakes but albumin synthesis did not, suggesting that export protein synthesis was already at its maximum, and that export protein synthesis was prioritised relative to hepatic catabolism via oxidative pathways ( 30 , 46 ) .…”

“…As for TAA, the hepatic uptake of four EAA (His, Met, Thr, Trp) is highly responsive to EAA influx to the liver ( 2 , 45 ) , whereas BCAA and Lys hepatic extraction rates are very small whatever their influx ( 25 , 46 ) . Only NEAA involved in inter-organ N and C cycling (Asn, Asp, Ala, Gln and Glu) present a hepatic uptake less responsive to influx ( 2 , 45 ) .…”