Post-mortem physiological and morphological changes of rumen mucosal tissue

Glucose and amino acid transport and metabolism in flat duodenal sheets of dairy cattle at three stages of lactation

The apparent duodenal transport and metabolism of amino acids and glucose in early-, mid-, late- and non-lactating dairy cows was investigated. Km values for glucose were not affected by stage of lactation. The capacity (Iscmax) of duodenal sheets to transport glucose was greater in lactating than in non-lactating cows. Lactating cows had a greater transport capability for amino acids than non-lactating cows. Duodenal sheets of early-lactation cows metabolized a greater percentage of absorbed glucose carbon to carbon dioxide than cows in mid-, late- and non-lactating cows.

In vitro production of beta-hydroxybutyrate from 1,3-butanediol by bovine liver, rumen mucosa, and kidney

A model ketosis can be produced in dairy cows by restricting feed intake plus feeding 1,3-butanediol. Increases in D-beta-hydroxybutyrate in blood result from metabolism of the 1,3-butanediol, but the site of metabolism has not been established. In vitro production of D-beta-hydroxybutyrate from butyrate and from isomers of 1,3-butanediol was measured in bovine liver, rumen papillae, and kidney cortex, all obtained at slaughter from nonlactating, nonpregnant Holstein cows. Production of D-beta-hydroxybutyrate from butyrate by the tissues was greatest for liver and rumen and much less for kidney. Only liver, however, produced appreciable amounts of D-beta-hydroxybutyrate from R-, S-, or RS-1,3-butanediol, and rates were maximal at substrate concentrations of 5 mM. Production of D-beta-hydroxybutyrate from R-1,3-butanediol by liver was greater than from S-1,3-butanediol. In vitro rates of production of D-beta-hydroxybutyrate were consistent with liver being the primary tissue involved in the metabolism of 1,3-butanediol, and capacity of the liver probably is sufficient to account for metabolism of 1,3-butanediol when 1 kg is fed daily to dairy cows.

Occurrence, absorption and metabolism of short chain fatty acids in the digestive tract of mammals

Short chain fatty acids (SCFA) also named volatile fatty acids, mainly acetate, propionate and butyrate, are the major end-products of the microbial digestion of carbohydrates in the alimentary canal. The highest concentrations are observed in the forestomach of the ruminants and in the large intestine (caecum and colon) of all the mammals. Butyrate and caproate released by action of gastric lipase on bovine milk triacylglycerols ingested by preruminants or infants are of nutritional importance too. Both squamous stratified mucosa of rumen and columnar simple epithelium of intestine absorb readily SCFA. The mechanisms of SCFA absorption are incompletely known. Passive diffusion of the unionized form across the cell membrane is currently admitted. In the lumen, the necessary protonation of SCFA anions could come first from the hydration of CO2. The ubiquitous cell membrane process of Na+-H+ exchange can also supply luminal protons. Evidence for an acid microclimate (pH = 5.8-6.8) suitable for SCFA-protonation on the surface of the intestinal lining has been provided recently. This microclimate would be generated by an epithelial secretion of H+ ions and would be protected by the mucus coating from the variable pH of luminal contents. Part of the absorbed SCFA does not reach plasma because it is metabolized in the gastrointestinal wall. Acetate incorporation in mucosal higher lipids is well-known. However, the preponderant metabolic pathway for all the SCFA is catabolism to CO2 except in the rumen wall where about 80% of butyrate is converted to ketone bodies which afterwards flow into bloodstream. Thus, SCFA are an important energy source for the gut mucosa itself.

Volatile fatty acid metabolism by rumen mucosa from cattle fed hay or grain

Effects of an all-grain versus an all-hay diet on metabolic activity of rumen mucosa of cattle were investigated. After diets had been fed for 3 to 4 mo, rumen papillae were collected at slaughter from the dorsal rumen sac and incubated with one of various volatile fatty acids. Rates of substrate utilization were in the order: n-butyrate greater than n-valerate approximately propionate greater than iso-butyrate approximately iso-valerate. Over-all, papillae from hay-fed steers utilized greater amounts of volatile fatty acids. Dietary treatment did not significantly affect extent of conversion of volatile fatty acids to lactate and to ketone bodies. Lactate was the major metabolite from propionate and n-valerate. Ketone body formation accounted for more than 90% of n-butyrate uptake by papillae. Ketone formation from n-valerate was restricted to beta-hydroxybutyrate while that from iso-valerate was essentially acetoacetate plus acetone. Metabolic systems in rumen mucosa of physiologically mature ruminants seem to adapt little to varying individual volatile fatty acids available for absorption in vivo.

Activity of selected gluconeogenic and lipogenic enzymes in bovine rumen mucosa, liver and adipose tissue

The activities of phosphoenolpyruvate carboxykinase, ;malic enzyme', citrate-cleavage enzyme and glucose 6-phosphate dehydrogenase were assayed in homogenates of rumen mucosa, liver and adipose tissue of cattle. Rumen mucosa cytoplasm contained activities of ;malic enzyme' approximately sevenfold those of phosphoenolpyruvate carboxykinase, suggesting that the conversion of propionate into lactate by rumen mucosa involves ;malic enzyme'. Neither starvation for 8 days nor feeding with a concentrate diet for at least 3 months before slaughter produced enzyme patterns in the tissues different from those in cattle given only hay, except that the all-concentrate diet caused increased activities of glucose 6-phosphate dehydrogenase and ;malic enzyme' in adipose tissues. Rumen mucosa, liver and adipose tissue contained phosphoenolpyruvate carboxykinase activity. ;Malic enzyme' was absent in liver. Citrate-cleavage enzyme activity was present in liver and adipose tissue but was quite low in rumen mucosa. Liver contained much less glucose 6-phosphate dehydrogenase activity than rumen mucosa or adipose tissue.