Partitioning of nutrient net fluxes across the portal-drained viscera in sheep fed twice daily: effect of dietary protein degradability

Abstract

Extrusion is used to decrease leguminous seed protein degradability in the rumen in order to shift part of the dietary protein digestion towards the small intestine. The effect of such displacement of digestion site on the partitioning of nutrient net fluxes across the gastrointestinal tract was studied using four sheep fitted with catheters and blood-flow probes, allowing measurements across the rumen, the mesenteric-drained viscera (MDV) and the portal-drained viscera (PDV). Two diets containing 34 % of pea seeds were tested in a crossover design. They differed only according to pea treatment: raw pea (RP) or extruded pea (EP) diet. Rumen undegradable protein (RUP) accounted for 23 and 40 % of dietary crude protein for RP and EP diets, respectively. Across the rumen wall, ammonia net flux was lower with EP diet, whereas urea net flux was not different. Across the MDV, free amino acid (FAA) net flux was greater with EP diet, whereas peptide amino acid net flux was not different, accounting for 7 % of the non-protein amino acid net release. From RP to EP diet, PDV net flux of ammonia decreased by 23 %, whereas FAA net release increased by 21 %. The difference in dietary RUP did not affect the PDV net flux of SCFA, 3-hydroxybutyrate, lactate and glucose. In conclusion, the partial shift in pea protein digestion from the rumen to the small intestine did not affect the portal net balance of N, but decreased N loss from the rumen, and increased amino acid intestinal absorption and portal delivery.Amino acids: Peptides: Short-chain fatty acids: Portal-drained viscera Proteins consumed by ruminants are largely degraded in the rumen, up to ammonia production, in order to provide N required for microbial growth. Consequently, the outflow from the rumen of synthesized microbial protein ensures the largest contribution to amino acids (AA) entering the small intestine. Nevertheless some dietary proteins, more resistant, can escape ruminal degradation and complement the microbial AA source quantitatively and qualitatively to meet maintenance and production needs (1) . Conversely, dietary protein readily degraded in the rumen often leads to N wastage through excessive ammonia absorption from the rumen. Processing has thus been developed in order to decrease rumen degradability of feedstuff, and increase the AA delivery to the intestine. However, an increased flow of protein-N to the duodenum does not necessarily imply an increased availability of AA for anabolic purposes due to the intense metabolic activity of the splanchnic bed. More specifically, the digestive tract is one of the most important AA consumers within the body and participates to about 30 % of the whole-body protein synthesis (2) . This metabolism greatly impacts on the quantitative and qualitative net release of AA in the portal vein (3) . Yet, even if the digestive tract is metabolically very active and is withdrawing important quantities of AA, increasing dietary protein supply usually results in a greater portal absorption of AA, as far as the energy supp...

Modulation of urea transport across sheep rumen epithelium in vitro by SCFA and CO2

“…In these situations, the backflow of urea into the gut approaches zero, despite an increase in the level of urea in plasma (35). Conversely, the highest rates of urea influx into the rumen are observed postprandially, when fermentational processes lead to a drop in ruminal pH (63,64). However, in this study, the lowering of luminal pH alone merely had minor effects on urea flux (Table 3).…”

“…These studies were later challenged (38, 89), and, for a number of years, an uneasy consensus supporting ruminal urea transport via lipid diffusion reemerged. However, this hypothesis could not explain the outcome of feeding studies (13,34,36,41,59,60), leading to the suggestion that the recycling of urea into the rumen is not determined by plasma urea levels but involves changes in the permeability of the rumen epithelium in response to dietary stimuli (35).Current models of ruminal urea transport suggest that, whereas the concentration gradient serves as a driving force for the entry of urea from the blood into the rumen, the rate of influx is regulated in a far more complex manner (1,35,41,44,67,90), involving both long-term adaptation and a system of short-term regulation corresponding to a postprandial increase in the extraction rate of arterial urea by the rumen (44,50,64,65). Thus, when feed is supplemented with a rapidly fermentable energy source, the daily flux of urea across the rumen wall increases up to twofold (41, 58 -60, 86) and, in the 3-h interval after a meal, urea flux into the rumen can rise even further (64).…”

“…Current models of ruminal urea transport suggest that, whereas the concentration gradient serves as a driving force for the entry of urea from the blood into the rumen, the rate of influx is regulated in a far more complex manner (1,35,41,44,67,90), involving both long-term adaptation and a system of short-term regulation corresponding to a postprandial increase in the extraction rate of arterial urea by the rumen (44,50,64,65). Thus, when feed is supplemented with a rapidly fermentable energy source, the daily flux of urea across the rumen wall increases up to twofold (41, 58 -60, 86) and, in the 3-h interval after a meal, urea flux into the rumen can rise even further (64).…”

“…Thus, when feed is supplemented with a rapidly fermentable energy source, the daily flux of urea across the rumen wall increases up to twofold (41, 58 -60, 86) and, in the 3-h interval after a meal, urea flux into the rumen can rise even further (64). This mechanism allows the animal to capitalize on available carbohydrates by an endogenous supply of nitrogen (33).…”

Rumen Microbiology: From Evolution to Revolution

“…Urea transporters in the rumen wall are differentially expressed depending on dietary N concentration (Marini and Amburgh 2003 ). Amount of N intake (Marini et al 2004 ), total dry matter intake (Sarraseca et al 1998 ), type and frequency of feeding dietary N that is degraded in the rumen (Wickersham et al 2008 ;Rémond et al 2009 ;Kiran and Mutsvangwa 2010 ), ruminal fermentable carbohydrate intake, and organic matter digestibility (Kennedy and Milligan 1980 ) are the major dietary factors regulating the proportion of hepatic urea-N output returning to the gut. This mechanism is proposed to have constant supply of nitrogen to preserve rumen microbial population when the nitrogen supply in feed is limited.…”