ABSTRACT
Previous work (Phillipson & McAnally, 1942) indicates that absorption from the rumen is not only possible, as was shown by Trautmann (1933), but is rapid enough to prevent passage to the abomasum of volatile acid present in the rumen, although these acids are present in the rumen in a high concentration. This evidence is remarkable, as the epithelium lining the rumen is stratified and squamous qualities not usually associated with membranes through which there is active absorption.
Direct evidence regarding the properties of the rumen as an organ of absorption was necessary, and the experiments described in this paper show that even a large molecule such as sodium ortho-iodo-hippurate can pass through the epithelium of the rumen in significant amounts.
METHOD
Six young lambs up to four months old were used, they were anaesthetized by intravenous injection of nembutal and the abdomen was opened in the mid-line. The abomasum was withdrawn and the thin muscular connexions between it and the rumen and reticulum were severed to allow a ligature to be passed, excluding the epiploic vessels, around the omasal-abomasal orifice, so that the rumen, reticulum and omasum, all of which are lined by stratified squamous epithelium, were isolated from the remainder of the alimentary tract. The abdomen was closed and the animal was taken to the Department of Anatomy and a plate was taken of the abdomen. Sodium ortho-iodo-hippurate was then administered by stomach tube at the rate of approximately 2 g./kg. body weight in watery solution, and its passage into the rumen was observed under the screen. Further exposures were made at suitable intervals after dosing and observations were continued up to 5 hr. Two of the lambs were used as controls, and the same procedure was adopted as with the animals dosed with the opaque salt except that the dose given was water. All animals were kept anaesthetized by further injections of nembutal throughout the course of the experiment.
At the end of 5 hr. the animals were killed, the bladder emptied and the quantity of urine measured. The presence or absence of iodine was demonstrated in the urine by ashing in the presence of excess caustic soda, liberation of iodine with nitrous acid and extraction with chloroform. Finally, the position and efficiency of the ligature at the omasal-abomasal orifice was tested to ensure that no passage of the opaque salt from the rumen to the abomasum had occurred.
RESULTS
Absorption of the opaque salt was demonstrated first by shadow thrown by the bladder and secondly by the presence of iodine in the urine. The details of each experiment were as follows :
Exp. 1
Ram lamb, 10 days old. Weight 3·8 kg. Dose 10 g. iodo-hippurate in 20 c.c. water. No shadow thrown by the bladder 1 hr. after dosing. Distinct shadow thrown hr. after dosing. Positive iodine reaction in urine.
Exp. 2
Ram lamb, 3 months old. Weight 23·7 kg. Dose 40 g. iodo-hippurate in 80 c.c. water. Bladder clearly visible hr. after-dosing; shadow dense hr. after dosing. Bladder contained 189 c.c. urine and gave strong positive reaction for iodine.
Exp. 3
Ram lamb, 3 weeks old. Weight 6·35 kg. Dose 50 c.c. water. No shadow thrown by bladder at 25 min., 2 hr. 40 min. or at 5 hr. after dosing. Bladder contained 67 c.c. urine and gave negative reaction for iodine.
Exp. 4
Ewe lamb, 3 months old. Weight 25·7 kg. Dose 100 c.c. water. No shadow thrown by bladder until hr. after dosing when a faint shadow was visible. Bladder contained 125 c.c. urine and gave negative reaction for iodine.
Exp. 5
Ram lamb, 3 months old. Weight 16·9 kg. Dose 35·3 g. iodo-hippurate in 106 c.c. water. Plain shadow thrown by bladder 5 hr. after dosing. Urine gave strong positive reaction for iodine.
Exp. 6
Ram lamb, 3 months old. Weight 18·7 kg. Dose 37 g. iodo-hippurate in 74 c.c. 1 % sodium acetate. Bladder opaque 5 hr. after dosing. Bladder contained 72 c.c. unne and gave strong positive reaction for iodine.
The greatest absorption, as judged by the opacity of the bladder, occurred in Exp. 2 and the plates of this experiment are shown in Pl. 6 together with one of a control animal.
The plates obtained in Exp. 1 were not as clearly positive as in the other experiments, but the presence of iodine in the urine confirmed that absorption had taken place.
In order to test the stability of sodium ortho-iodo-hippurate during exposure to bacteria of the rumen, the salt was added to liquid rumen ingesta in a concentration comparable to that attained after dosage into the rumen and the material was incubated for 5 hr. at 37° C. At the end of this time no free iodine could be detected in the flask indicating that the benzene ring was not destroyed. This was further confirmed by the fact that iodine excreted in the urine was in organic combination.
Anatomy of the rumen in relation to absorption
The surface area of the reticulum and rumen combined is not large proportionally to the weight of ingesta contained in these organs. The presence of invaginations of the walls forming the muscular pillars, however, makes the absorbing surface larger than is apparent at a cursory glance. The surface area is, in addition, studded with numerous papillae; these are especially large in the anterior region of the rumen and greatly increase the surface area.
The epithelium of the rumen, when studied histologically, differs from that of the skin in the following ways (Pl. 7) :
The layer of cornified cells is thin and rarely exceeds a depth of more than three to four cells. This is due to the continual maceration to which the epithelium is subjected by the semi-liquid contents of the rumen and to the continual friction applied to the epithelium by the movement of the ingesta.
The layer of granula cells, the stratum granu-losum, although distinct in many places is by no means a continuous layer.
No sebaceous glands are present, consequently there is no greasy protection to assist in preventing the passage of water through the epithelium.
The blood vessels of the rumen have been studied in four specimens in which the vessels were injected with gelatine carmine. The vascularity of the rumen in relation to the omasum and abomasum can be judged by reference to Pl. 7. The capillaries come into intimate relation with the deep cells of the malpighian layer and penetrate into the denticular spaces.
The omasum, of which a section through one of the laminae is shown, is similar to the rumen while the number of large vessels distributed through the laminae testifies to vascularity of the organ.
DISCUSSION
These experiments demonstrate conclusively that absorption of a molecule as large as sodium orthoiodo-hippurate can occur through the epithelium of the rumen. The rate of absorption, however, is not rapid, for a considerable quantity of the drug was retained in the rumen at the end of the experimental period. In Exp. 2 an approximate estimation of the quantity of the drug excreted in the urine was made from the quantity of iodine recoverable and found to amount to one-sixth of the dose.
In comparing the epithelium of the rumen to that of the skin it should be pointed out that absorption through the skin is possible and the rapid absolution of boric acid (Kahlenberg, 1924, 1928), and a somewhat slower absorption of salicylic acid (Leslie-Roberts, 1928), has been demonstrated. These compounds, however, were the only ones absorbed of many that were tried. Luciani (1913) states that the removal of the sebum from the skin by ether allows the absorption of certain salts that are not otherwise absorbed.
The skin presents three barriers to prevent absorption. These are: first, the secretion of the sebaceous glands ; secondly, the keratinized external layer of dead cells ; and thirdly, the deep cells of the malpighian layer which, according to Cowdry (1932), offers more resistance than the keratinized layer. Of these barriers the first is absent in the rumen, the second is thin, while only the third is identical with that of the skin. It is not surprising under these circumstances, especially when it is recalled that the epithelium is continually exposed to semi-fluid ingesta, that absorption takes place apparently more readily than it does through the skin.
SUMMARY
The absorption of sodium ortho-iodo-hippurate has been demonstrated from the rumen of lambs. The epithelium of the rumen is described and compared with that of the skin.
ACKNOWLEDGEMENTS
We are greatly indebted to Mr J. F. Fozzard who, by kind permission of Prof. H. A. Hams, performed the radiological part of the work at the School of Anatomy; to Mr A. L. Bacharach and Messrs Glaxo Laboratories who very kindly supplied us with the radio-opaque salt; to Mr J. Freeman who took the microphotographs; and to Mr E. Turner for technical assistance.
REFERENCES
EXPLANATION OF PLATES
Plate 6
Plate 7
Sections taken from the stomach of a sheep the vessals of which were injected with gelatine carmine.