The Mechanism of Absorption from the Rumen as Exemplified by the Behaviour of Acetic, Propionic and Butyric Acids

Abstract

The following conclusions were reached by placing a solution of sodium acetate, propionate and buty-rate in the rumen of sheep and studying the rate at which these substances are absorbed: (1) When the rumen contents are at pH 7.5, fatty acid anion only is absorbed, i.e. no free fatty acid is absorbed, and the anion is accompanied by a roughly equivalent amount of sodium. The anion is absorbed by passage through water-filled pores, the diameter of which is large compared with retric acid. These pores probably lie in the intercellular cement of the rumen epithelium. (2) When the rumen contents are at pH 5.8, free acid is lost from the rumen in large amounts in addition to the loss of fatty acid anion. Part of this free acid is absorbed by passage through water-filled pores, but the greater part of it passes through the lipoid membranes of the epithelial cells. (3) In consequence of the difference in the two mechanisms, at alkaline pH the substances are lost in the order acetate > propionate > butyrate, whereas at acid pH the order is butyrate > propionate > acetate. (4) All the substances studied (fatty acid anions, sodium and water) move from regions of higher to regions of lower concentration. (5) The total fatty acid absorbed from the rumen of a sheep on pasture grass may readily be twice that absorbed from the rumen by the same sheep on hay. (6) The permeability of the rumen epithelium is such that the pH of the ingesta tends to move towards neutrality, independently of the neutralizing action of the saliva.

The Absorption of Volatile Fatty Acids from the Rumen

“…This latter finding is in conflict with the conclusion of Danielli et al (1945). The direct measurement of the absorption of acetic acid from an 'isolated' rumen, carried out at the same time as the indirect measurement employed by these workers, has shown that their procedure may give rise to misleading results.…”

“…This allowed observations to be made on the essentially intact rumen in its physiological state. The procedure first used by Barcroft, McAnally & Phillipson (1944) and adopted by Danielli et al (1945) was, however, repeated in some experiments. In these the rumen and its associated organs were isolated from the rest of the alimentary tract by tying off the abomasum just below the omasum, taking care to exclude the epiploic vessels.…”

“…when phosphate was present. Danielli et al (1945) have pointed out that the change from acid to neutral reaction must, in part, be brought about by the passage of free acid through the rumen wall. In the older view the change would have been ascribed entirely to the advent of alkaline saliva into the rumen.…”

“…Danielli and his colleagues (Danielli et al 1945), however, concluded that the relative rates of absorption were reversed when the medium was slightly alkaline, and experimental data were presented which, it was claimed, proved that the relative velocities of absorption from a mixture of fatty acids at pH 7-5.fell in the series acetic > propionic > butyric. These latter experimental data were unconvincing.…”

“…In a previous paper the relative rates of absorption of acetic acid and propionic acid through the rumen wall were discussed (Gray, 1947a). While the manuscript was being prepared, Danielli, Hitchcock, Marshall & Phillipson (1945) described a series of experiments from which it was evident that the lower homologues of the fatty acids are absorbed through the rumen wall at different rates when the rumen contents are acid, and that the relative velocities of absorption fall in the series butyric > propionic > acetic. The observations made in this laboratory (Gray, 1947 a) independently and by different methods were in agreement with these conclusions.…”