Effect of Intravenous Amino Acid Infusion on Leucine Oxidation Across the Mammary Gland of the Lactating Goat

Abstract

Changes in the kinetics of leucine in the mammary gland were examined in four lactating goats (25, 38, 45, and 135 DIM) that were given an i.v. infusion of a mixture of 18 AA, not including leucine, to alter the availability of leucine to the gland relative to other AA. Arteriovenous monitoring of [1-13C]leucine kinetics across one-half of the mammary gland was conducted on the last day (d 6 or 7) of the saline (control) and the AA infusion periods. Although blood flow to the mammary gland and the arterial concentration of most AA other than leucine were increased by the AA infusion, milk and protein yields did not change. For goats in early lactation (n = 3), arterial leucine concentrations fell considerably during AA infusion; however, the arteriovenous difference of leucine was maintained, resulting in uncommonly low leucine concentrations in venous plasma (8 microM). Whole body leucine flux (protein synthesis plus oxidation) was unaffected by AA infusion, but, because whole body leucine oxidation was reduced, whole body utilization of leucine for protein synthesis increased. The AA infusion reduced mammary oxidation of leucine to approximately one-third of control values. These results suggest that leucine oxidation can be reduced considerably without affecting milk protein output; thus, leucine oxidation may not be an irrevocable consequence of mammary metabolism. If catabolism of other AA either by the gland or in the whole body can be reduced, then the efficiency of milk yield can be improved.

Effects of ruminally protected methionine and/or phenylalanine on performance of high producing Holstein cows fed rations with very high levels of canola meal

“…It has been suggested that it is not blood AA concentrations per se that limits milk protein production, but rather the metabolic machinery which determines maximum velocity of milk protein production (Cant et al, 2001) or a gastrointestinal event (Bequette et al, 1996), both of which are under hormonal control.…”

Determining the optimal ratio of canola meal and high protein dried distillers grain protein in diets of high producing Holstein dairy cows

“…Even though both lysine and leucine are taken up in excess of requirements, mainly to oxidize and synthesize other AA, Lapierre et al (2009) suggested that excess uptake of lysine across the mammary gland was required to maintain milk protein production while leucine oxidation decreased if leucine supply was limited, thereby indicating that excess leucine is not required to sustain milk protein yields. Bequette et al (1996) reported that increasing the supply of leucine to the mammary gland did not enhance milk protein output, but did increase its oxidation in the mammary gland. Lapierre et al (2002) also showed that only 0.16 of the increased supply of leucine available for absorption ended up in milk protein.…”

Responses of cows to abomasal infusion of lysine and methionine at two levels of dietary protein

“…In the current study, uptake was calculated from free plasma AA concentrations alone and the peptide-bound AA that might contribute to the mammary tissue metabolism was not considered. Up to 16% of Lys secretion in milk was contributed from plasma peptides in lactating goats at different stages of lactation (Bequette et al, 1996;Mabjeesh et al, 2000). Interestingly, this fraction was not affected by the availability of free Lys in plasma.…”

“…A third point that should be borne in mind that these balance measurements across the mammary gland were done under the assumption that AA are taken up by the gland and contribute to a biosynthetic pathway (anabolism). The oxidative pathway was activated in the mammary gland of goats when surplus EAA (e.g., Leu, Lys), relative to requirements, were supplied intravenously (Bequette et al, 1996;Mabjeesh et al, 2000). Up to 31% of the Lys taken up by the gland during LM infusion was oxidized (Mabjeesh et al, 2000).…”