|Baldwin, Ransom - Randy|
Submitted to: Journal of Nutrition
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/1/2006
Publication Date: 5/15/2006
Citation: El-Kadi, S.W., Baldwin, R.L., Sunny, N.E., Owens, S.L., Bequette, B.J. 2006. Metabolism of amino acids and glucose by the sheep gatrointestinal tract in response to post-ruminal protein supplies. Journal of Nutrition. 136(5):1261-1269. Interpretive Summary: The greatest users of nutrients in the productive ruminants on a weight basis are the visceral organs resulting in limited availability of nutrients for productive function. Because lysine is most often a limiting AA for growth and milk production in ruminants fed corn or corn-silage based diets, the basis for GIT metabolism of lysine and the BCAA will need to be understood more completely, and furthermore, represented in feeding models for ruminants as well as for other species. For this reason, essential AA and glucose metabolism by the visceral organs was assessed in sheep when the protein supply to the small intestines was increased from marginal to adequate for maintenance and growth requirements. The efficiency of absorption of the essential AA remained fixed across the various levels of protein supply, and in particular the BCAA and lysine were net metabolized in increasing amounts. By better understanding efficiency of absorption, balancing nutrient delivery with tissue requirements will offer a tool to the producer to improve the efficiency and effectiveness of feeding regimes.
Technical Abstract: The gastrointestinal tract (GIT) utilizes a considerable portion of the intestinal supply of essential and nonessential amino acids (AA) to fulfill biosynthetic and energetic requirements. This study sought to establish whether net metabolism of essential AA by the GIT of ruminants represents a fixed amount or a fixed proportion of intestinal AA supply and whether metabolism of individual AA differs when protein supply is altered. Wether sheep (n = 4, 33 ± 2.0 kg) were surgically fitted with vascular catheters for measurement of net flux (absorption) of AA by the mesenteric (MDV) and portal-drained (PDV) viscera, and a catheter was inserted into the duodenum for casein infusions. Sheep were fed a fixed amount of a diet adequate in energy (10.0 MJ ME/ kg DM) but low in protein content (9.5% CP, as fed). Sheep were assigned to four levels of casein infusion (0, 35, 70, and 105 g/d in water), each infused for 5.5 days, according to a 4 x 4 Latin square design. Estimates of net and kinetic fluxes of AA, leucine and glucose by the GIT were made by infusion (8 h) of [methyl-2H3]leucine into the duodenum and [1-13C]leucine plus [6-2H2]glucose into a jugular vein. For most AA, the relationship of net AA absorption and casein-AA infusion rate was linear (P < 0.05, R2= 0.6 to 0.8 for MDV and R2 = 0.55 to 0.95 for PDV). Apparent absorptive efficiencies (regression slope) by the PDV were for lysine, leucine, valine and isoleucine (48-75%) significantly (P < 0.05) less than 100%, thus the GIT net metabolized a fixed proportion, but increasing amounts, of the additional intestinal supply of these AA. By contrast, absorptive efficiencies of all essential AA were greater than 100% for the MDV. Plasma glucose entry rate increased (linear, P < 0.05) with casein infusion rate and, despite greater net metabolism of AA by the GIT, glucose use by the GIT remained constant. Transfer of urea (recycling) to the PDV increased (P < 0.05) with casein infusion in a linear (P < 0.05) relationship with arterial urea concentration. In ruminants, the GIT net metabolizes a constant proportion of the intestinal supply of certain essential AA, even across a range of intestinal protein supplies.