The Production-of Fatty Acids in the Alimentary Tract of the Dog

The production of a mixture of acetic, propionic, and butyric acids is a characteristic feature of alimentary fermentation in the ruminant and in the horse, pig, rabbit, and rat. The proportions of these acids present in the digesta of the large intestine, and of the stomach in the case of the ruminant, is similar in these species, with the exception of the rat, in which acids higher than acetic form a bigger proportion of the whole. In order to complete the series, the investigation was extended to include the dog, to see (a) whether alimentary fermentation was appreciable in a carnivorous animal, and (b) whether the products of fermentation were the same.

Four dogs were used. They were fed on a diet consisting of one-third meat and two-thirds bread for at least a week before the experiment. Dogs 1 and 2 were fed within 2 hr. of the experiment, dog 3 received its last meal the morning before the experiment, while dog 4 received its last meal the evening before the experiment.

Dog 1 (18 · 7 kg.) was aged, dog 2 (16 · 4 kg.) and dog 3 (10 · 2 kg.) were probably 1–4 years old, and dog 4 (8·0 kg.) was approximately 6–8 months old.

The animals were anaesthetized with Nembutal by intravenous injection. The abdomen was opened and blood was withdrawn from vessels draining the stomach, small intestine, and the large intestine and in some cases the portal vein. Blood was also taken from the carotid artery and the jugular vein. After death, the whole alimentary tract was removed and divided into stomach, anterior two-thirds of small intestine, posterior third of small intestine, and large intestine. The contents of each portion were weighed and sampled in the manner previously described (Elsden, Hitchcock, Marshall & Phillipson, 1946).

The procedures adopted for the determination of volatile acids in the digesta were identical with those previously described. In all cases, however, distillates were redistilled according to the method of Friedemann in the presence of acid mercuric sulphate in order to remove any formic or lactic acids present.

Volatile acid in the blood was distilled by the method described by McClendon (1944) with the exception that three 30 ml. fractions were collected and titrated. This was found to be necessary after the distillation of known quantities of acetic acid. Proteins of blood were precipitated by the addition of 3 c.c. of a freshly prepared solution of 25 % metaphosphoric acid to 5 c.c. of blood already luked in water in a 50 ml. volumetric flask. The volume was made up to 50 ml. and 5 or 10 c.c. of the filtrate were used for distillation. This method was found by Elsden (personal communication) to be more reliable than that recommended by McClendon.

The values for volatile acids in blood show clearly that more is present in the blood leaving the large intestine than in any of the other venous and arterial samples collected. This is clear proof that absorption of volatile acid occurs from the large gut. In dog 2, the evidence is incomplete as technical errors spoiled the samples of peripheral blood that were taken. This dog defaecated immediately before anaes-thetization and its colon was practically empty during the experiment. This is reflected by the fact that the figure found for blood draining the colon is no greater than that found for the stomach.

Analysis of the digesta in the various parts of the alimentary canal support this finding, for appreciable quantities of volatile acid are present only in the digesta of the large gut, as shown by Table 2.

The values obtained after the second Friedemann distillation, that is the figures for volatile acid after the removal of formic and lactic acids, are expressed in Table 3 as a percentage of the dry matter of the digesta. The concentrations found in the large intestine are appreciable and are comparable to those found in the large intestine of herbivores.

The figures are surprising in that the values for volatile acid found in the blood draining the large gut are highest in dog 4, although the total amount of volatile acid present in the digesta of this organ is lower in this animal, and the concentration less than that of dog 1 and dog 3.

The bulk of the volatile acidity found in the digesta of the large intestine is apparently due to fatty acid other than formic acid, as the volatile acid obtained after the second Friedemann distillation was in no case less than 80% of the total volatile acidity of the first distillation. This was not so with the remainder of the digesta, as the reduction here was in most cases in the region of 50%.

Chromatographic analysis, according to the method of Elsden (1946), of the acids obtained from the digesta of the large intestine gave the following results:

The identity of the individual acids was not confirmed by analysis other than the speed at which they passed through the column of silica gel, but their behaviour in this respect was the same as that of the acids named in Table 4.

In order to make the results comparable to those previously obtained in other animals, the total volatile acidity after the first distillation is given in Table 5 as g./kg. The figures show that the production of volatile acids is insignificant compared to production in herbivorous animals and is less than that in omnivorous animals such as the pig and the rat on this basis ; but when it is recalled that the concentration of volatile acid in the colon varied between 3·3 and 5·3 g. acetic acid per 100 g. dry matter (Table 3), figures that fall in the middle of the range found in the caeca of herbivorous animals, it appears probable that the principle circumstance which limits the part played by fermentation in the digestion of the dog is the size of the large intestine : otherwise there is nothing to distinguish the dog from the ruminant in the intensity of fermentation when this is judged by the concentration of volatile acid found in the digesta.

The concentration of volatile acid found in the blood draining the large gut was equal to that found in blood draining the rumen in one case and in the remainder was higher than that in peripheral blood. Absorption therefore does occur and the acids are not all excreted in the faeces.

Thanks are due to Miss J. F. New for her assistance in some of the analytical procedures used in this work.