Absorption of amino acids and B vitamins from the rumen
Creators: name: Smith, Frederick Dabell
Publisher: name: University of British Columbia
Year: 2012

description: The purpose of this investigation was to provide direct experimental evidence for or against the suggestion that amino acids and B vitamins are among the nutrients absorbed from the rumen of the ruminant. The investigation was subdivided into two parts: (1) six ruminal fistula experiments and (2) nine blood-sampling experiments. The goat (Capra) was the experimental animal. In the ruminal fistula experiments, solutions of amino acids, B vitamins, and usually propionic acid were added to an empty rumen for 120 or 150 minutes. Propionic acid, a substance known to be absorbed from the rumen, was included in most of the solutions to provide a positive control for ruminal absorption. In addition, in all but the first two experiments, polyethylene glycol was added to the solutions in order to provide a marker substance for differential water movement into or out of the rumen. Controls for the experiments included the taking of a sample from the solution in the rumen at zero time and incubating the sample for the duration of the experiment in a water-bath held at 37°C. The ruminal and control solutions were sampled at periodic intervals during the experiment. The results demonstrated not only marked decreases in the concentrations of propionic acid in the ruminal solutions but also, in most cases, marked decreases in the concentrations of the amino acids and the B vitamins assayed: tryptophan, methionine, tyrosine, glycine, lysine, riboflavin, nicotinic acid, and pantothenic acid. In contrast, the concentrations of these constituents in the control solutions usually demonstrated either no decrease or a small increase. In the first three blood-sampling experiments, the concentrations of amino acids and nicotinic acid in the plasma draining the rumen were compared with the concentrations in the peripheral plasma. The results demonstrated that seldom were the concentrations of an amino acid or nicotinic acid higher in the plasma draining the rumen than in the peripheral plasma. The results did suggest, however, that there had been slight increases in the plasma concentrations of the amino acids with time. Accordingly, the next six blood-sampling experiments were designed to determine if time-related increases in the blood concentration of alpha amino nitrogen occurred after the addition of amino acids to the rumen. Blood samples were removed periodically from anesthetized goats, both before and after the addition of a solution of amino acids to an empty rumen. The results demonstrated time-related increases in the blood concentrations of alpha amino nitrogen. These results and those obtained for the ruminal fistula experiments are interpreted as supporting the suggestion that amino acids and B vitamins are absorbed from the rumen. In the General Discussion, a preliminary attempt is made to answer three questions: 1) What is the manner of the passage of amino acids and B vitamins across the ruminal epithelium? 2) Does this passage of amino acids and B vitamins occur under normal feeding conditions? 3) How does this passage of amino acids and B vitamins across the ruminal epithelium fit into the scheme pictured for the metabolism and final fate of these compounds in the ruminal contents? As a part of the attempt to answer the first question, literature reviews are presented on the manner of absorption of amino acids and B vitamins in the small intestine, kidney, placenta, and other tissues. Then, after a literature review on the manner of absorption of substances other than amino acids and B vitamins from the rumen, the data of the present investigation are examined for evidence as to the manner of absorption of amino acids and B vitamins from the rumen. The examination led to the following statement: the movement of amino acids across the ruminal wall of the goat is determined by a summation of the effects of chemical, electrical, and possibly metabolic potentials. Unfortunately, other than to suggest that the process of simple diffusion is involved, the nature of the data for the B vitamins was not such as to yield much information on their manner of absorption. In the attempt to answer the second question, the fulfillment or non-fulfillment of the following conditions is considered: (1) the demonstration of the presence of amino acids and B vitamins in the rumen, (2) the demonstration of the relation of the concentrations of individual amino acids and B vitamins in the ruminal liquor to their concentrations in the blood plasma, and (3) the demonstration of the absorption of individual amino acids and B vitamins from an ingesta-filled rumen. Although fulfillment of the third condition was not obtained, fulfillment of the first two conditions was obtained and led to the statement that a strong possibility exists that the absorption of amino acids and B vitamins occurs from the rumen under normal feeding conditions. In the attempt to answer the third question, a number of factors are discussed in turn: (1) the environmental conditions of the rumen, (2) the influence of the diet on the composition of the population of microorganisms in the rumen, (3) the physical and chemical nature of the dietary constituents, (4) the relative stability of amino acids and B vitamins in the ruminal ingesta, and (5) the physiological state of the animal. Evidence is presented to demonstrate that each of these factors probably exerts an important influence on the content of amino acids and B vitamins in the rumen. It is suggested that, when the interaction of the many factors is such as to produce higher concentrations of amino acids and B vitamins in the rumen, the proportions of amino acids and B vitamins that pass from the rumen by absorption through its wall will be an important pathway for these nutrients. To conclude the General Discussion, the possible nutritional implications of the absorption of amino acids and B vitamins from the rumen are considered. One suggestion made is that a general absorption of nutrients from the rumen would permit the simultaneous arrival at the liver, or elsewhere in the body, of volatile fatty acids, amino acids, B vitamins, and inorganic ions. This should promote more efficient utilization of each of the nutrients. Another suggestion made is that ruminal absorption of amino acids and B vitamins places the host ruminant in a better competitive position relative to its contained microbial population in the competition for nutrients. Finally, a suggestion is made that the action of the ruminal wall in absorbing amino acids helps to explain the results that have been obtained for certain nitrogen retention experiments in ruminants.