Westenbrink, 1936 a). (Donhofler, 1935) or ingested (Mackay & Clark, 1941), and by making

Transcription

1 276 J. Physiol. (I95') II3, THE ABSORPTION OF GLUCOSE AND XYLOSE BY ADULT AND NEWBORN RATS BY PHILIP E. H. JONES From the Medical Research Council Department of Experimental Medicine, University of Cambridge (Received 23 June 195) The rate of absorption of glucose, xylose and other sugars by adult rats was investigated by Cori (1925) and some of his conclusions may be summarized as follows: (a) The rate did not depend on the concentration of the solution or the volume given. (b) The rate was constant from the beginning until the end of absorption. (c) Each sugar had its characteristic rate. If the rate for glucose was given the value of 1, that for xylose was 15. Other workers have confirmed that the pentose is absorbed more slowly than the hexose (Wilbrandt & Laszt, 1933; Verzar, 1935; Westenbrink & Middlebeck, 1936; Westenbrink, 1936a; Davidson & Garry, 1939). Verzar suggested that the glucose was preferentially absorbed by an active process which involved phosphorylation (Verzar & McDougall, 1936) but- absorption of both glucose and xylose is hindered by monoiodoacetate (Wilbrandt & Laszt, 1933; Westenbrink, 1936 a). Cori's results have not been well supported by later work. High absorption coefficients have been obtained by increasing the volume of solution administered (Donhofler, 1935) or ingested (Mackay & Clark, 1941), and by making rats diabetic (Pauls & Drury, 1942). It has also been found that the rate varied during the absorption period (Pierce, Osgood & Polansky, 1929; Burget, Moore & Lloyd, 1932; Feyder & Pierce, 1935) and was not the same in all parts of the intestine (Westenbrink, 1936 b). The rate at which the stomach empties must affect the rate of intestinal absorption, and the relationship has been investigated by Macleod, Magee & Purves (193), Pierce (1935), Fenton & Pierce (194, 1944), Pierce, Haege & Fenton (1942) and by Fenton (1945). It has been established that there is a close connexion between gastric evacuation and intestinal absorption in the rat, and a similar relationship probably exists in man. If glucose solutions are

2 ABSORPTION OF SUGARS BY RATS 277 given to some subjects who have had a gastro-enterostomy the rise in blood sugar is more rapid in onset than in people who have normal pyloric function, and in one case reported by Evensen (1942) the obliteration of the gastroenterostomy restored the glucose tolerance curve to the normal pattern. Confirmation was obtained by comparing the different rises of blood-sugar levels in normal people who were given the sugar solution by mouth and by duodenal intubation. It is probably the motility of the stomach which is chiefly responsible for its rate of evacuation, and this motility is inhibited in dogs by the presence of glucose in the duodenum (Quigley & Hallaran, 1932; Quigley & Phelps, 1934). A similar mechanism may operate in rats but there is no direct evidence either on this point or on the effect of xylose solutions on the motility of the stomach. All previous experiments have been carried out on adult animals. The present investigations were undertaken to compare the rate of absorption of glucose and xylose by adult and newborn rats. METHODS Hooded rats from the colony maintained by the Department were used for the experiments. The strain was originally derived from the Lister Institute and has inbred for many years. The adults were given the stock diet and were allowed to feed until a few hours before the experiments. Their stomachs were usually found to contain some hair and other indigestible residues, and sometimes a little food. Newborn rats were not allowed to suckle and were removed from their mothers as soon as possible after birth. During the experimental period the adult rats were kept at laboratory temperatures in cages with wire-meshed bottoms. No food or water was allowed. The temperature regulation of newborn rats is poorly developed and they were therefore kept in an incubator at 37 C. The rats were given 2-66 M solutions of glucose (48%) or xylose (4%) by stomach tube. The adults were intubated with a No. 3 soft rubber catheter attached to a syringe. For the newborn animals polyvinyl chloride insulating sleeving, 2 mm. in external diameter, was drawn out after careful heating over a small Bunsen flame. This gave a thin, pliable tube which was mounted on a needle attached to a glass -5 ml. tuberculin syringe. The technique of intubation required gentleness of touch but with practice failures became very rare. After the doses had been given the adults were returned to their cage and the newborns were placed in the incubator. The newborns lay on ifiter-paper, which showed up any loss of alimentary contents by regurgitation or diarrhoea. If this occurred, the experiment was abandoned. A period of j hr. or 1 hr. was allowed for absorption. The adults were then killed by coal gas, but the newborns, who are relatively insensitive to this poison, were usually killed by decapitation. The stomach and intestine were dissected out rapidly without loss of their contents, after forceps had been placed on the oesophagus, the pylorus and the ileo-caecal junction. In the infant rat it was also necessary to ligate the cardiac and pyloric ends of the stomach. The standard dose adopted for the experiments was -1 ml. solution/1 g. body weight. The desired amount was measured in the syringe and delivered through the tube. When smalldoses were given, none of the solution entered the large intestine of the adults, even by the end of 1 hr. Progress through the infants' gut was difficult to follow exactly, because at birth the small and large intestines are poorly differentiated. When larger doses were given, certain modifications of technique were necessary. There was a tendency for the adult and infant animals to regurgitate stomach contents after death. In the adults this was avoided by carrying out the dissections on an inclined board with the head higher than the abdomen, and clamping the oesophagus in the thorax at the earliest possible moment.

3 278 P. E. H. JONES 278 P.H O E These methods were not effective in the infants and they were therefore killed by tying a ligature tightly round the neck. It was found that if a large enough volume of solution was given, sufficient sugar to exceed the absorptive capacity of the intestine before the animal was killed passed through the pylorus at once. In some experiments therefore some of the fluid which had just been delivered was withdrawn from the stomach after a pause of about 15 sec. This prevented the stomach becoming so distended by the volume administered, and by the gastric secretions, that regurgitation became unavoidable. When the adults were given large doses, the period of absorption had to be limited to 3 min. to avoid diarrhoea. When the withdrawal technique was used, the final contents of the syringe were analysed so that the amount of sugar actually given could be determined. Glucose and xylose were estimated by the method of Nelson (1944). The colour was read in a 'Spekker' photoelectric absorptiometer using an Ilford 64 spectrum green filter, which transmitted maximally at a wavelength of 52 m,u. Satisfactory agreement with Beer's Law was obtained over the range used. RESULTS Table 1 shows the average coefficients of absorption obtained from the experiments in which adults and newborn rats were given the standard dose. The 1-hr. rates for the adults are in close agreement with those of Cori (1925) and Feyder & Pierce (1935). The results showed that (i) the rate of absorption of glucose was the same whether the absorption period was i hr. or 1 hr.; (ii) glucose was absorbed more rapidly than xylose by the adult rats; (iii) the rate of absorption of xylose for i hr. was nearly double the rate for 1 hr. Very little absorption, therefore, can have occurred in the second i hr.; (iv) the infants absorbed xylose twice as fast as the adults; (v) the glucose : xylose absorption ratios for the adults were about 2-5: 1 for i hr. and 4:1 for 1 hr. The corresponding ratios for the infants were 1:1 for i hr., and 1-5:1 for 1 hr. Table 1 also shows the amount of the alimentary residues. In all the dissections it was noticed that the stomach contained more fluid than had been introduced. This was confirmed by measurement. It was also observed that when glucose had been given, the intestine was apparently empty, but that it was much more filled after xylose. This was related to the amounts of sugar found by analysis in the intestine. The residue of xylose was always large; in the newborn rats at the end of I hr. it was, however, the same as the residue of glucose. The effect of increastng the volume of sugar solution For reasons already given, the duration of the experiments in which large volumes of solution were administered was limited -to i hr. The results are given in Table 2. The total amount of sugar passing the pylorus (intestinal residue plus sugar absorbed) increased with the size of the dose in both adult and infant rats, even

4 ABSORPTION OF SUGARS BY RATS 279. ts*o 44., [ C $ > o VC,@S. S. ~~~~~~~~~~~~~~c 4) _, - o t Go cq 4' CO,. 4 o -O C.._ B 4,._ 4 6: Cs 44 o 44 Z S._ 4) '4 ) - r-3t -O. Cd 1 C oc. o..c O C~~~~~~~. *5 9c~9 999w r *E P.cO _ f Q a s m. ". O& o $ o '.8.3~~~~~~' Cq8 " 88nn v CO C CO'oo P < 'Xo % 4) 4 2 V dso4 c4 ~o~ N) v) Cs 4)~ fz Cs O.,F Co ;1 - CO CO CO t-.s's O O '' 4, 41) t- t-. 4Q, -4. P.. -4) Ca E, o c P4 & C) ~C to44.. ".. E4, C) * '4o EH,, o CO +q M Cl t- 4oC (Z4OCOOO co' 1 k kto9c t.d Cg 6 6 ;, C's C> 4 M xo *I co xc 44 M oi o P c M n o to >Cq QQt OO gt-s t - "CO X 1 rel F c'l2s s-p co COCO QC)P3-oCOPCO COCO e CPnCOtt ~~~~~~~~~~~~ c. o c C) c (5 5 ( k x k 55 o> s>..) C C CO) 4) ()4 4) 4) 'L, : t- L=o o ko &M lo u-lqocoo-4 o C to^e9 9 noo o4c ce : E-ị E4 O)C z

5 28 P. E. H. JONES when the withdrawal technique was used. The effect of thus increasing the quantity of sugar presented to the intestinal mucosa was to increase enormously. the rates of absorption. There is no sign that the maximum rates possible had been attained, and it was unfortunate that, because of their tendency to cause diarrhoea, doses greater than 3 mg./1 g. body weight (a total of about 8 ml.) could not be given to the adults. If this difficulty could have been avoided, the adult rates might have reached much higher levels. Glucose was always absorbed much more rapidly than xylose by the adults. The infants absorbed both sugars equally well until the dose exceeded x2 ml./1 g. body weight, but then the rate for xylose fell off slightly. DISCUSSION The records in the literature of absorption coefficients for sugars given to the adult rat mostly lie between 16 and 23 mg./1 g. body weight/hr. (Cori, 1925; Feyder & Pierce, 1935; Westenbrink, 1936a; Fenton & Pierce, 194; Fenton, 1945). They were nearly all obtained by giving doses about 3 ml. in volume, and the present results show that they did not truly indicate the absorptive capacity of the intestine. This appears to depend on the volume of fluid reaching the intestine and on the rate at which it is delivered. The greater these are, the larger will be the area of absorbing surface in contact with the sugar solution. Absorption may also be affected by alterations in the concentration of sugar reaching the mucosa; in the present experiments this concentration was probably much greater than normal because dilution in the stomach was eliminated. There has been some controversy about the effect of concentration, but the evidence has nearly always been confused by a lack of knowledge of the actual concentration at the absorbing surface. The evidence of Table 1 is in favour of the presence in the rat of a mechanism which maintains a more or less constant passage of glucose out of the stomach, and therefore a constant rate of absorption. The simplest device would undoubtedly be one similar to that possessed by the dog (Quigley & Hallaran, 1932; Quigley & Phelps, 1934) for inhibiting the motility of the stomach, and therefore its evacuation. Presumably the stomach contents are being constantly diluted, and as dilution progresses, the inhibitory effect of glucose solutions in the duodenum declines and the stomach empties more rapidly. The figures in Table 1 also suggest that the absorption of xylose almost stops during the second i hr. both in adults and newborns, although the amounts left in the intestine are still comparatively large, and are greater at the end of 1 hr. The fact that the residues increase suggests that the presence of xylose in the intestine does not have the effect of preventing the release of more of the sugar from the stomach. This is difficult to explain, but it is perhaps worth noting that McCance & Madders (193) found that the absorption of the pentose

6 ABSORPTION OF SUGARS BY RATS281 sugar in man ceased after hr. while there was still sugar in the alimentary tract. It is difficult to believe that the small differences in the rates of absorption of glucose by adult and infant rats over the range of dosage at which they can be compared have any fundamental importance. It does seem, however, that the process which results in the more favourable treatment of glucose is not developed in the infant. Glucose and xylose are absorbed equally well when the dosages are moderate and the rate is measured over i hr. During that j hr. the newborns absorbed the xylose at a higher rate than the adults, but no new theory is needed to explain this apparent anomaly. Sphincteric control is imperfectly developed in the immature animal: milk given to the human infant starts to pass out of the stomach almost immediately (Waller, 1949; Ford, 1949) and a similar observation was made by dissecting newborn rats, such as those used for the experiments here described, directly after they had been given milk by stomach tube. It has been shown that absorption depends chiefly on the amount presented to the intestine, and the high rate for xylose may merely represent a high rate of flow through the almost functionless pylorus as the sugar is being administered. SUMMARY 1. Adult and newborn rats absorbed glucose at approximately the same rates/1 g. of body weight when the sugar solutions were administered by stomach tube. 2. The rate of absorption of glucose did not decrease with time. 3. The xylose was apparently not absorbed after the first - hr. by either adults or newborns. For that time the rate of absorption/1 g. of body weight was greater for the infants than for the adults. 4. By increasing the volumes of xylose and glucose administered, the sugar absorbed in the first hr. was increased in adults and newborns and very high rates of absorption were obtained. 5. Most ofthese results can be explained by supposing that (a) strong solutions of glucose (but not perhaps of xylose) in the duodenum decrease the motility of the stomach, (b) sphincteric control is much more easily overcome at birth than in adult life. Evidence can be produced in favour of both suppositions. I am grateful to Dr R. F. A. Dean for his help in the preparation of this paper. REFERENCES Burget, G. E., Moore, P. H. & Lloyd, R. W. (1932). Amer. J. Phy8iol. 11, 565. Cori, C. F. (1925). J. biol. Chem. 66, 691. Davidson, I. N. & Garry, R. C. (1939). J. Phy8iol. 96, 172. Donhoffer, S. (1935). Pfliug. Arch. ges. Phy8iol. 235, 569. Evensen,. K. (1942). Acta. med. 8cand., Supp. 126.

7 282 P. E. H. JONES Fenton, P. F. (1945). Amer. J. Phy8iol. 144, 69. Fenton, P. F. & Pierce, H. B. (194). J. biol. Chem. 133, xxxi. Fenton, P. F. & Pierce, H. B. (1944). Fed. Proc. 3, 57. Feyder, S. & Pierce, H. B. (1935). J. Nutrit. 9, 435. Ford, F. J. (1949). Lancet, 1, 987. McCance, R. A. & Madders, K. (193). Biochem. J. 24, 75. Mackay, E. M. & Clark, W. G. (1941). Amer. J. Phy8iol. 135, 187. Macleod, J. J. R., Magee, H. E. & Purves, C. B. (193). J. Phy8iol. 7, 44. Nelson, N. (1944). J. biol. Chem. 153, 375. Pauls, F. & Drury, D. R. (1942). Amer. J. Physiol. 137, 242. Pierce, H. B. (1935). J. Nutrit. 1, 689. Pierce, H. B., Haege, L. F. & Fenton, P. F. (1942). Amer. J. Physiol. 135, 526. Pierce, H. B., Osgood, H. S. & Polansky, J. B. (1929). Proc. Soc. exp. Biol., N.Y., 26, 347. Quigley, J. P. & Hailaran, W. R. (1932). Amer. J. Phy8iol. 1, 12. Quigley, J. P. & Phelps, K. R. (1934). Amer. J. Physiol. 19, 133. Verza,r, F. (1935). Biochem. Z. 276, 17. VerzAr, F. & McDougall, E. J. (1936). Absorption from the intestine, 1st ed. London: Longmans Green. Waller, H. (1949). Lancet, 1, 17. Westenbrink, H. G. K. (1936a). Arch. ngerl. Physiol. 21, 433. Westenbrink, H. G. K. (1936b). Arch. negerl. Physiol. 21, 18. Westenbrink, H. G. K. & Middlebeck, A. (1936). Arch. nrerl. Physiol. 21, 283. Wilbrandt, W. & Laszt, L. (1933). Biochem. Z. 259, 398.