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* Département des sciences animales, Université Laval, Ste-Foy, Qc, Canada, G1K 7P4
Department of Agriculture, The University of Reading, Reading, UK, RG6 6AT
Rowett Research Institute, Bucksburn, Aberdeen, UK, AB21 9SB
Faculté de médecine vétérinaire, Université de Montréal, St-Hyacinthe, Qc, Canada, J2S 7C6
|| Dairy and Swine Research and Development Centre, Agriculture and Agri-Food Canada, Lennoxville, Qc, Canada, J1M 1Z3
Corresponding author:
H. Lapierre; e-mail:
lapierreh{at}agr.gc.ca.
The effect of the supply of metabolizable protein on splanchnic fluxes of nutrients and hormones was measured in six catheterized late-lactation Holstein cows in a crossover design. Two isonitrogenous diets (16.3% CP), but differing in rumen protein degradability and estimated metabolizable protein (MP) supply (1654 g/d, Lo-MP; 1930 g/d, Hi-MP) were fed, each over a 35-d experimental period. On d 34 or 35, net fluxes of nutrients and hormones across the portal-drained viscera, the liver, and total splanchnic tissues were determined. Portal absorption of total, essential, nonessential, and branched-chain amino acids (AA) increased with the Hi-MP diet. Approximately 76% of the additional metabolizable protein supply was recovered as extra AA-N absorption in the portal vein. Liver removal of AA was not different between diets, and this resulted in a greater net release across the splanchnic tissues for the Hi-MP diet. This extra AA supply provided substrates for the observed increased milk protein yield for the Hi-MP diet. Fractional efficiencies of conversion of absorbed individual essential AA into milk protein ranged from 0.42 to 0.68. The corresponding efficiencies for utilization of postsplanchnic AA supply were 0.42 to 1.80. Provision of methionine, phenylalanine, and histidine beyond the liver were similar to outputs in milk protein but the other essential AA were supplied to peripheral tissues in excess of milk output, indicative of oxidative mechanisms in nonhepatic tissues. Net fluxes of glucose, NH3-N, and urea were not affected by the diets. Neither arterial concentrations of insulin, somatotropin, or IGF-1, nor net transfers across the portal-drained viscera or liver of insulin, were affected by the diets. Although portal release of glucagon was not different between the diets, a smaller proportion was removed by the liver on the Hi-MP diet. Metabolism of AA across the splanchnic tissue bed is a major determinant of the quantity and the profile of AA delivered to peripheral tissues.
Key Words: amino acids • splanchnic • metabolism • cow
Abbreviation key: BCAA = branched-chain AA, EAA = essential AA, Hi-MP = high supply of metabolizable protein, Lo-MP = low supply of metabolizable protein, MP = metabolizable protein, NEAA = nonessential AA, pAH = para-aminohippurate, PDV = portal-drained viscera, TAA = total AA, TSP = total splanchnic tissues, VA = venous-arterial
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