Regulation of hepatic nitrogen metabolism in ruminants

Abstract

As befits its anatomical position between the sites of nutrient absorption and deposition the liver provides the metabolic hub of the body. Although the present review is confined to the regulation of N substrates it must not be forgotten that both lipid and carbohydrate metabolism are important and interactive components of hepatic function. Furthermore, hepatic production of insulin-like-growth-factor (1GF)-1 and partial removal of glucagon and insulin play an important part in regulation of peripheral endocrine balance. These hormonal events are linked to peripheral appearance of two of the major N products of digestion and absorption, NH3 and amino acids (AA), which are both under the control of hepatic metabolism. Thus, in order to provide the required homeostatic and homeorhetic control, complex mechanisms must operate in vivo but, to date, much of our understanding of these involves use of isolated primary hepatocytes and perfused livers derived from nonruminant sources. The commercial species, pigs, cattle and sheep, offer the unique opportunity to establish chronic arterio-venous preparations across the splanchnic tissues and these have provided the basis for the majority of longitudinal studies in vivo. Nonetheless, the data mass is still small and it is currently necessary to assume that acute findings, both in vivo and in vitro, from laboratory animals can be extrapolated to ruminants.Despite an outward homogeneous appearance the structure of the liver is both complex and ordered. For example, hepatocytes have a polar nature between their sinusoidal and canicular surfaces and this important feature of metabolic regulation is lost when isolated hepatocytes are prepared. Furthermore, there is localization of different subpopulations of hepatocytes, with the dominant (>0.9 in the rat) periportal species being specialized for ureagenesis, gluconeogenesis, glutaminolysis and synthesis of plasma albumin (Haussinger et al. 1992a), as well as exhibiting greater activity of the enzymes of AA catabolism (see Haussinger, 1990). In contrast, the minority perivenous cells, which are restricted to a few cell layers around the efferent hepatic vein, can synthesize glutamine and extract aspartate and glutamate from blood .This spatial organization provides a hierarchy for the fates of metabolites and is an important feature for the roles that the liver has to accomplish. The present short review will focus on two of those roles, detoxification of NH3 and the hepatic partition of absorbed AA between catabolism and protein anabolic fates within the liver, as well as the ensuing consequences for the availability of AA sources to the peripheral tissues. UREAGENESIS Ammonia detoxijkationOne vital hepatic function in all mammals is detoxification of NH3, produced from bacterial fermentation or metabolism within the gastrointestinal tract (GIT) and peripheral tissues. Extraction of NH3 by the liver is very efficient with, in sub-overload situations, apparent fractional extractions of 0.75-0.85 (for example, see Reynolds e...

Comparison of fresh and ensiled white and red clover added to ryegrass on energy and protein utilization of lactating cows

“…Bertilsson and Murphy (2003) showed that, whilst clover and clover-grass mixture silages gave higher voluntary DMI than grass silage, the resulting increase in N intake led to reductions in the efficiency of conversion of dietary N to milk N. Noticeable was the low protein utilization for the fresh forage diets compared with the legume-supplemented silage diets, too, as the first resulted in a particularly high N load. Both, plasma and milk urea are indicators of metabolic N excess as it is synthesized in the liver not only from amino acid catabolism in the liver but, in the situation of excess, mainly from ammonia produced in the rumen (Lobley and Milano, 1997). Hence, when no excessive amino acid supply is taking place, urea reflects the extent of CP degradation in the rumen (Broderick and Clayton, 1997).…”

Protein turnover—what does it mean for animal production?

“…Traditionally, data for hepatic removal of AA are expressed as a fraction of the amount absorbed and, indeed, such extractions can be high. For example, absorbed histidine and phenylalanine can be totally removed across the liver in non-growing sheep (Lobley and Milano 1997). In practice, however, the liver is presented not with just absorbed AA but a mixture that also includes re-circulated AA (derived from recently absorbed material that bypassed the liver, plus that released by breakdown of tissue proteins).…”

Amino acid supply and metabolism by the ruminant mammary gland

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Metabolism in the gastrointestinal tract ) and liver (Lobley & Milano, 1997) has been shown to be important in determining the amount and pattern of amino acids (AA) available post-hepatically for mammary gland utilization as well as for muscle, skin and, indeed, the gastrointestinal tract. High rates of metabolism by the non-mammary tissues have been suggested as one reason for the low efficiency (approximately 20 %) of conversion of dietary N into milk protein .

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