Impacts of protein and amino acid supply on milk production, intermediary metabolism, and mRNA expression in liver of early lactation dairy cows.

摘要

The central hypothesis explored in this dissertation is that changes in abundance of ornithine transcarbamoylase (OTC) and aminoadipate semialdehyde synthase (AASS) mRNA in liver may be responsible for inefficiency of amino aciduse and these changes are regulated by altered protein or AA supply. The objective of this work was to examine molecular responses in liver and mammary tissue with increased supply of metabolizable protein and a modified profile of AA.;Post-ruminal infusion of protein and AA mixtures have increased milk and milk protein production but these responses have been variable due, in part, to differences in tissue response to altered N supply. Early-lactation dairy cows fitted with rumen cannulae were used to determine the effects of post-ruminal infusion of milk protein isolate on expression of OTC, ASS-1, and AASS. Data indicate post-ruminal protein infusion increases N supply resulting in increased ureagenesis and up-regulation of non-essential AA catabolism and Lys oxidation.;In a separate infusion study, post-ruminal delivery of 63 g/d of lysine did not alter milk production, milk composition, or liver mRNA expression of OTC, AASS, or ASS-1 despite a 99% increase in plasma lysine and 51% increases in plasma &agr;-aminoadipic acid. Though there was no difference in expression of AASS mRNA in the liver and mammary individually, post-ruminal infusion of Lys resulted in an 86% increase in the ratio of AASS mRNA in liver compared with mammary mRNA. These changes suggest hepatic Lys metabolism is responsive to Lys supply more so than mammary tissue, suggesting a hierarchy in Lys utilization by individual tissues.;To examine alternative strategies to change post-ruminal protein and AA supply, addition of dietary divalent metals and rumen-protected AA were evaluated in long-term feeding experiments. Effect of addition of dietary metals, Zn, Mn, and Fe, in altering microbial ruminal degradation of protein, was evaluated by feeding low protein diets with and without addition of metals, or a high protein diet with and without addition of metals to sixty early lactation dairy cows. Metal inclusion reduced rumen degradation of crude protein by 5%, while increasing estimated intestinal digestibility of RUP by 1.5%, and dry matter intake decreased by 12% resulting in a 9% increase in efficiency of feed use for milk synthesis. Reducing dietary protein by 1.5% decreased milk protein yield by 8%, blood urea N concentrations by 9%, milk urea N concentrations by 11%, while also reducing abundance of OTC mRNA by 32% and AASS mRNA by 33.5%. These data suggest a role of metals in altering ruminal protein degradation while a 1.5% reduction in dietary protein decreased indices of N utilization supporting increased efficiency of N utilization and corresponding decreased ureagenesis.;Response of milk production and abundance of OTC and AASS were also examined when feeding rumen-protected AA to early lactation dairy cows. Thirty-six multiparous early-lactation Holstein cows were fed diets containing 16.5 % CP, 17.5% CP, or a 16.5% CP diet containing rumen-protected Lys and Met for an 84 day period. Inclusion of rumen-protected Lys and methionine reduced milk urea N by 21.5%, while milk and milk component yield were unchanged. Abundance of OTC and AASS mRNA were not altered by inclusion or rumen-protected AA or reduced dietary protein. Increased milk urea N without additional responses to dietary N suggests that the urea cycle contains enough reserve capacity to respond to the moderate increases in N supply.;Together, these data indicate a role of N and AA supply in regulating ureagenesis and Lys catabolism in bovine. Ability of increased circulating N to regulate ureagenesis without alteration in mRNA associated with milk protein synthesis suggests that differential metabolism of AA by liver alters profile of AA available for by mammary tissue for milk protein synthesis. (Abstract shortened by UMI.).