Hepatic response to increased exogenous supply of plasma amino acids by infusion into the mesenteric vein of Holstein- Friesian cows in late gestation

Abstract

The hepatic responses of late gestation, dry dairy cows to acute (6 h) infusions of an amino acid (AA) mixture (Synthamin; 0.0, 1.1, 2.2, 4.4, 8.8 and 17.6mmoVmin) into the mesenteric vein were determined. Neither blood flow nor 0 2 consumption across the portal-drained viscera (PDV) and liver was significantly altered by infusion. Similarly, there were no effects on net absorption, or hepatic removal, of acetate, propionate, butyrate or NH3. Glucose PDV appearance was unchanged but hepatic glucose production increased (P = 0.032) by 0.2 mmoVmin per mmoVmin of AA infused.Additional extraction of alanine, glycine (both infused) and glutamine (not infused) by the liver was sufficient to account for most of the extra C required for glucose synthesis. The N that would be liberated from these glucogenic AA would also account for a large proportion of the increase in urea-N produced in response to the AA infusion. This supports the concept of a correlation between gluconeogenesis and ureagenesis. Furthermore, the amide-N liberated from the extracted glutamine would contribute up to 0.17 of hepatic NH, flux and assist in balancing N inputs into the carbamoyl phosphate and arginosuccinate entry points of the ornithine cycle. Rates of fractional extraction of the various AA by the liver were best fitted by linear equations, indicating that even at the highest rates of administration (approximately twice maximal physiological absorption) the transport systems were not saturated. Hepatic fractional extractions of infused essential AA were highest for methionine (083) and phenylalanine (0.87) with the lowest proportion removed observed for valine (0.25), leucine (0.30), lysine (0.31) and isoleucine (0.49). For the non-essential AA, the highest apparent fractional extractions were for glycine (0.73), arginine (0.79) and tyrosine (0.63) followed by alanine (0.54), proline (0.47) and serine (0.37). Hepatic removal of AA-N exceeded the increase in urea-N formation such that, at the highest rate of infusion, approximately 10mmoVmin of the extracted AA was apparently available for hepatic anabolism, more than is required to account for assumed increases in liver mass and export protein synthesis. Similarly, the amount of AA available for peripheral tissue protein gain, when assessed against phenylalanine supply as the limitation, would be the equivalent of a maximum of 0.5 g protein retainedmin (6 mmol AA-N/min). This would provide suNicient AA for replenishment of peripheral (muscle) protein stores plus support of the placenta and fetus.Amino acids: Urea: Liver: Cattle Only a small proportion of the total amino acids (AA) absorbed from the small intestine into the hepatic portal vein reaches the peripheral circulation in free form because

Effect of intake on whole body plasma amino acid kinetics in sheep

“…Furthermore, between 30 and 80% of AA disappearing from the lumen do not appear in the portal vein [15,[27][28][29] and are used as energy substrates to support protein turnover within the tissues of the digestive tract. Although, the essential AA do not appear to be used preferentially by the GIT [15,27] this is not the case for the liver where net removals may range from 10-100% [30,31]. Hepatic metabolism therefore will alter both the amount and pattern of AA that enters the systemic circulation and is available to [32].…”

“…Hepatic metabolism therefore will alter both the amount and pattern of AA that enters the systemic circulation and is available to [32]. For example, the ovine liver removes small amounts of absorbed BCAA [30,31] but a large proportion of histidine and phenylalanine are removed to support the synthesis of export proteins [33]. For the peripheral tissues (especially muscle), the BCAA have a major role in the regulation of protein synthesis [34,35] and hepatic bypass may act as an important signal of increased nutrient (protein) supply.…”

Protein turnover—what does it mean for animal production?

“…Re-circulation accounts for the largest proportion of hepatic supply, ranging from 72 to 98% in sheep, pigs and cattle (Reynolds et al 1994;Hanigan et al 1998;Le Floc'h et al 1999;Lobley et al 2001a). Thus, of the total AA inflow to the liver, only 1-20% is extracted per pass (e.g., Reynolds et al 1994;Wray-Cahen et al 1997;Lobley et al 2001a). Furthermore, over a wide range of total inflows, the fractional extraction for most individual AA appears to be constant (Wray-Cahen et al 1997;Hanigan et al 1998;Lobley et al 2001a).…”

“…Thus, of the total AA inflow to the liver, only 1-20% is extracted per pass (e.g., Reynolds et al 1994;Wray-Cahen et al 1997;Lobley et al 2001a). Furthermore, over a wide range of total inflows, the fractional extraction for most individual AA appears to be constant (Wray-Cahen et al 1997;Hanigan et al 1998;Lobley et al 2001a).…”

The effect of jugular or abomasal infusion of amino acids on milk yield in lactating cows fed a protein deficient diet

“…This regulation was demonstrated by the increased hepatic removal of histidine, methionine, phenylalanine representing 50 to 87% of their increased portal absorption during the mesenteric infusion of a mixture of amino acids in late gestation dry dairy cows [48]. For these amino acids, their postsplanchnic supply matches closely their milk output in lactating cows [30] and their ratio of milk output to mammary uptake is close to unity [36].…”