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Effect of Supply of Metabolizable Protein on Whole Body and Splanchnic Leucine Metabolism in Lactating Dairy Cows

H. Lapierre^*, J. P. Blouin^,1, J. F. Bernier, C. K. Reynolds, P. Dubreuil and G. E. Lobley^||

^* Dairy and Swine Research and Development Centre, Agriculture and Agri-Food Canada, Lennoxville, Qc, Canada J1M 1Z3
Département des sciences animales, Université Laval, Ste-Foy, Qc, Canada G1K 7P4
Department of Agriculture, The University of Reading, Reading, UK RG6 6AT
Faculté de médecine vétérinaire, Université de Montréal, St-Hyacinthe, Qc, Canada J2S 7C6
^|| Rowett Research Institute, Bucksburn, Aberdeen, UK AB21 9SB

Corresponding author:
H. Lapierre; e-mail:
lapierre{at}agr.gc.ca.

The effect of the supply of metabolizable protein (MP) on protein metabolism across the splanchnic tissues was determined in six catheterized lactating Holstein cows. In a crossover design, two isonitrogenous (16.3% CP) diets balanced to provide a low (Lo-MP) or high (Hi-MP) supply of MP were fed over 35-d periods. After 24 d of feeding, N balance was determined over a 6-d period. On d 33, [¹³C] sodium bicarbonate was infused into one jugular vein for 6 h, and hourly breath samples were collected. On d 34 or 35, L[1-¹³C] leucine was infused into one jugular vein, and between 2 to 6 h of infusion, breath and blood samples were taken hourly from the portal and hepatic veins and an artery. Isotopic enrichments of plasma leucine, 4-methyl-2-oxopentanoate, and expired CO₂ were determined for calculation of leucine kinetics. Net leucine absorption was greater, either on a direct basis (leucine transfer only) or corrected for portal-drained viscera metabolism to 4-methyl-2-oxopentanoate and CO₂ for the Hi-MP diet. There were no effects of diet on hepatic net flux of leucine across the liver, and, thus, more leucine was available to peripheral tissues with the Hi-MP diet. Combined with an increment in portal absorption of most of essential AA, this led to increased milk protein output, although it only represented 16% of the additional available leucine. Whole body leucine oxidation was also greater for the Hi-MP diet, as was leucine used for protein synthesis. Despite these changes, MP supply did not affect irreversible loss rate of leucine by portal-drained viscera and the liver; these averaged 35 and 20% of whole body irreversible loss rate, respectively. These ratios confirm the high metabolic activity of splanchnic tissues in lactating dairy cows, which are even greater than previously reported in growing ruminants.

Key Words: protein • splanchnic • metabolism • leucine

Abbreviation key: BCAA = branched-chain AA, Hi-MP = high supply of metabolizable protein, IE = isotopic enrichment, ILR = irreversible loss rate, Lo-MP = low supply of metabolizable protein, MP = metabolizable protein, MOP = 4-methyl 2-oxopentanoate, pAH = para-aminohippurate, PDV = portal-drained viscera

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