Intravenous energy infusion in lambs: effects on nitrogen retentin, plasma free amino acids and plasma urea nitrogen

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Abstract
1. Acetate, propionate, butyrate and glucose were each administered intravenously (2.09 MJ (0.5 Mcal)/d) to lambs on either high-concentrate (HC) or high-roughage (HR) diets in two experiments with 4 × 4 Latin square design. During a control period all animals received a saline infusion.2. In lambs on the HC diet glucose was the most effective metabolite tested for improving nitrogen retention. Acetate was 51%, butyrate, 75%, propionate, 90% as effective as glucose in promoting N retention. Propionate infusion was most effective in increasing N retention in lambs on the HR diet. Acetate was 60%, butyrate, 75%, and glucose, 80% as effective as propionate. Glucose infusion did not lower plasma urea N (PUN) concentration in lambs receiving the HC diet, but the three volatile fatty acids (VFA) did lower it (P< 0.01). All energy sources significantly reduced the PUN concentrations with the HR diet, but differences between sources were not significant.3. Plasma concentrations of valine, isoleucine, leucine and phenylalanine were depressed to 57, 71, 89 and 90% of their control values during glucose, propionate, butyrate and acetate infusion. Plasma concentrations of lysine and methionine were significantly decreased from control values (P< 0.05) but there was no significant difference between energy sources. Lysine, valine, isoleucine and leucine were significantly (P< 0.01) higher in lambs given the HR diet than in lambs given the HC diet. The concentration of alanine was greater (P< 0.05) during glucose infusion than during acetate and butyrate infusions, indicating net synthesis of alanine and possibly other non-essential amino acids during glucose infusion, since the plasma free amino acid N concentration increased despite decreased concentrations of plasma essential amino acids during glucose infusion.4. The results indicated that glucose and propionate were superior to acetate and butyrate as energy sources for protein formation.5. The poor relationship between PUN and N balance and plasma free essential amino acids suggested that PUN concentration may have been inappropriate to assess the protein status under some dietary conditions.
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