Research Accomplishment Reports 2007

Assessment and Implications of Carbohydrate Utilization in the Small Intestine of Beef Cattle

D.L. Harmon, K.R. McLeod
Department of Animal and Food Sciences

 

Project Description

Experiments have addressed how gastrointestinal function is regulated and how it can be manipulated nutritionally. Specifically we have looked at how site of digestion affects gastrointestinal peptides, specifically glucagon-like peptide-2 (GLP-2). The role of GLP-2 in ruminants is relatively unknown, despite its importance in the regulation of gastrointestinal growth in non-ruminants. We investigated the mRNA expression pattern of proglucagon (the precursor of GLP-2; GCG) and the GLP-2 receptor (GLP-2R) at different sites of the ruminant digestive tract. Effects of diet on these mRNA expression patterns and measures of gastrointestinal mass were tested using three treatments: water-infused, control; or the addition of 20% of ME intake as starch hydrolysate infused ruminally or abomasally.

Eighteen ruminally and abomasally cannulated Angus steers (250 Kg BW) were randomly assigned to treatment (N = 6). Steers were fed an alfalfa-cube diet at 1.5 x NEM requirement. Steers were infused for 19 to 21 D, with the first 7 d of the period used as an adaptation period to starch hydrolysate infusion in which the amount of starch hydrolysate infused was incrementally increased each day. Steers were harvested and gastrointestinal tissues were weighed and epithelial samples from the rumen (R), omasum (O), abomasum (A), duodenum (D), jejunum (J), ileum (I), and colon (C) were obtained. After extraction of total RNA, real-time PCR was used to determine the relative (to 18S) expression of GCG and GLP-2R mRNA.

Treatment had no effect on empty BW or mass of most gastrointestinal tissues except the small intestine; abomasal starch infusion increased small intestinal weight (% Empty BW) by 18% compared to water or ruminal starch infusion (P < 0.003). Expression of GCG MRNA was over 200-fold greater in intestines (D, J, I, C) than forestomachs (R, O, A). Likewise, GLP-2R expression was over 50-fold greater in the intestines than the forestomachs. Although there was no effect of dietary treatment on expression patterns of GCG and GLP-2R mRNA, these results describe the tissue distribution of mRNA for the GLP-2 precursor and the GLP-2 receptor in ruminants.

Impact

In several species, the peptide hormone glucagon-like peptide-2 (GLP-2) appears to be intimately involved with accurately communicating dietary nutrient status to the cells of the gastrointestinal tract. Nutrients present in the diet stimulate secretion of GLP-2 from specific cells in the gastrointestinal tract, thus establishing the first half of a communication loop.

The second half of this communication loop is achieved by the action of GLP-2 on gastrointestinal cells; GLP-2 stimulates absorption of nutrients by the gastrointestinal tract by several different mechanisms. These mechanisms include increasing total mass of actively absorbing gastrointestinal cells present, increasing intestinal blood flow, and increasing activity and/or amount of the proteins responsible for transporting specific nutrients, namely glucose and amino acids. In this way, the complete loop of communication is established.

The presence of dietary nutrients stimulates GLP-2 secretion which will increase nutrient absorption from the gastrointestinal tract. Consequent removal of dietary nutrients removes the stimulus for GLP-2 secretion and thus the signal for continued gastrointestinal cell proliferation.

While understanding the mechanism of this communication loop is important, the ability to manipulate this system to improve human or animal dietary nutrient absorption is an ultimate goal. In humans, dietary nutrient malabsorption is common in premature infants, the elderly, and several disease states. Improving nutrient absorption may also be advantageous for productive purposes, especially in situations of long-term high total body energy expenditure such as strenuous exercise, rapid growth, or lactation. Because of the demonstrated involvement of GLP-2 in the communication loop between dietary nutrient intake and gastrointestinal growth status, this peptide is a logical point for intervention.

Publications

Atkinson, R.L., C.D. Toone, T.J. Robinson, D.L. Harmon and P.A. Ludden. 2007. Effects of supplemental ruminally degradable protein versus increasing amounts of supplemental ruminally undegradable protein on nitrogen retention, apparent digestibility, and nutrient flux across visceral tissues in lambs fed low-quality forage. J. Anim. Sci. (In Press).

Mouro, G.F., A.F. Branco, D.L. Harmon, L.P. Rigolon, S.M. Coneglian and T.F.M. Ribeiro. 2007. Carbohydrate sources and levels of forage in sheep diets:nitrogen balance digestibility and portal flux of nutrients. R. Bras. Zootec. 36: 489-498.

Merrill, M. L., D. W. Bohnert, D. L. Harmon, A. M. Craig, and F. N. Schrick. 2007. The ability of a yeast-derived cell wall preparation to minimize toxic effects of high ergot-alkaloid tall fescue straw in beef cattle. J. Anim. Sci. 85:2596-2605.

Tricarico, J. M.,M.D. Abney, M.L. Galyean, K.C. Hanson, K.R. McLeod and and D. L. Harmon. 2007. The effects of an Aspergillus oryzae extract containing alpha-amylase activity on performance and carcass characteristics in finishing beef cattle. J. Anim. Sci. 85:802-811.

Guimaraes, K. C., S.M. Rodriguez, J. C. Matthews, K. C. Swanson , D. L. Harmon and A. F. Branco. 2007. Influence of partially hydrolyzed starch and casein administered postruminally on small intestinal brush border sodium-glucose cotransport activity. Braz. Arch. Biol. Tech. 50:963-970.

N.B. Kristensen, A. Storm, B.M.L. Raun, B.A. Rojen and D.L. Harmon. 2007. Metabolism of silage alcohols in lactating dairy cows. J. Dairy Sci. 90:1364-1377.