Partition of portal-drained visceral net flux in beef steers

Abstract

1. Blood from chronic indwelling catheters in the caudal aorta and anterior mesenteric, gastrosplenic and hepatic portal veins was used to measure blood flow and net flux of oxygen, glucose and nitrogenous compounds across hepatic portal-drained viscera (PDV), post-stomach (anterior mesenteric-drained viscera (MDV)) and stomach tissues of two beef steers (390 kg mean live weight).2. Steers were fed in sequence on (I) chopped lucerne (Medicago sariva) hay (twelve meals/d), (2) chopped lucerne hay (two meals/d) and (3) a pelleted concentrate diet containing 780 g ground maize/kg (two meals/d). The lucerne hay and concentrate contained 26.5 and 16.8 g nitrogen/kg respectively.3. Five measurements of net flux (blood flow multiplied by venous-arterial concentration differences (VA)) were obtained hourly on 2 d for each dietary regimen, beginning 0.5 h before feeding at 08.00 hours. Blood flow wdS measured by downstream dilution of p-aminohippurate (PAH).4. Blood flow across MDV averaged 42% of PDV blood flow (665 litres/h). 5. Net use of 0, across MDV accounted for 51 % of net PDV use of 0, (920 mmol/h). This disproportionate use of 0, in relation to blood flow was due to greater VA for 0, across MDV than across stomach tissues. Dietary regimen had no effect on the proportions of PDV blood flow and net 0, consumption attributable to MDV or stomach tissues.6. When lucerne was given, net glucose use across MDV represented 69% of PDV use (35 mmol/h). When concentrate was given, MDV glucose use switched to net absorption (29 mmol/h), reducing net PDV glucose use to 1 mmol/h. 7. When concentrate was given, net MDV absorption of a-amino-N (AAN) increased from 98 to 190 mmol/ h, yet net PDV absorption (101 mmol/h) was not affected. Net stomach AAN flux increased from -7 to -69 mmol/h when concentrate was given, negating the increase in net MDV absorption.8. Net absorption of ammonia-N across MDV represented 28 and 52% of net PDV absorption when lucerne and concentrate were given respectively. Net NH,-N absorption across PDV was lower when lucerne was given than when concentrate was given (295 v. 154 mmol/h), reflecting lower dietary N intake (153 v. 83 g/d). Net MDV absorption of NH,-N was not affected by diet. Net removal of blood urea-N (BUN) across PDV (101 mmol/h) was not affected by diet. Across MDV, BUN removal was lower when concentrate was given than when lucerne was given (32 v. 77 mmol/h). In beef steers, MDV tissues account for substantial portions of net flux of nonprotein-N across PDV and are responsible for essentially all PDV absorption of AAN.

Invited review: Nitrogen in ruminant nutrition: A review of measurement techniques

“…From these observations, Dijkstra et al (1992) deduced a relationship with a saturating stimulatory effect of blood urea and an inhibitory effect of rumen ammonia on urea recycling to the rumen. In agreement with this, Reynolds and Huntington (1988) and Huntington et al (1989) observed a reduced stomach net removal of urea from blood to be associated with higher stomach ammonia absorption.…”

“…The decline in plasma (and milk) urea N concentration due to the increased urine production must be expected to occur with any Na or K salt (Bannink et al, 1999;Eriksson and Rustas, 2014). In line with Reynolds and Huntington (1988), Spek et al (2013b) observed that the fraction of total urea entry rate that returned to the GIT was negatively related to dietary protein content. Reports on this type of measurements allowed Reynolds and Kristensen (2008), and more recently Batista et al (2017), to demonstrate that GER accounts for almost the entire urea entry rate in systemic blood when dietary DM contains 8% or less CP.…”

“…El-Kadi et al (2006) performed regressions on urea net PDV flux in steers fed 4 incremental levels of ME ranging from 0.028 to 0.056 MJ/kg of BW 0.75 per d. They reported a linear increase in urea concentrations in arterial and portal blood, but relative to the average value of 12.8 g urea-N/d, almost no change in PDV urea removal. Also in steers, Reynolds and Huntington (1988) observed that change in diet (chopped alfalfa hay vs. pelleted concentrates) did not affect net urea removal by PDV tissues. However, there was variation in GER within different GIT compartments.…”

“…In some studies, anterior mesenteric vein sampling catheters have been used to quantify the portion of PDV nutrient flux attributable to tissues of the mesenteric-drained viscera (MDV), which depending on catheter placement includes varying portions of the small intestine, mesenteric fat, large intestine, and cecum. Reynolds and Huntington (1988) reported that 52% of net PDV release of ammonia was attributable to the MDV in steers fed a high concentrate (corn and soybean meal) diet, but the MDV accounted for 28% of net PDV release of ammonia when chopped alfalfa hay was fed. Net MDV absorption was not affected by diet.…”

The Vacuolar-TypeH+-ATPase in Ovine Rumen Epithelium is Regulated by Metabolic Signals

“…Resulting from its considerable role in the absorption of nutrients, mainly of short chain fatty acids (SCFAs) and of electrolytes [1–3], the rumen epithelium ranks among the tissues with high metabolic rates [4, 5]. A main proportion of the rumen ATP utilization is related to activity of a Na + /K + -ATPase that has been shown to be expressed at high levels [6–8] in the cell membrane of rumen epithelial cells (REC) [9, 10].…”