Using a technique by which it is possible to study gastro-intestinal absorption

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1 531 J. Physiol. (I956) I34, 53I-537 THE ABSORPTION OF GLUCOSE BY THE INTACT RAT BY P. C. REYNELL AND G. H. SPRAY From the Nuffield Department of Clinical Medicine, University of Oxford (Received 30 May 1956) In the investigation of small-intestinal function it would be valuable to know the maximum rate at which the small intestine of the intact animal was capable of absorbing glucose. This would serve as a measure of intestinal cell function analogous to the glucose Tm as a measure of renal tubular function. Cori (1925a) measured glucose absorption in the intact animal by giving glucose by stomach tube and later killing the animal and measuring the amount of glucose recovered from the gastro-intestinal tract. This method has subsequently been used by many other workers, some of whom have found that the rate of glucose absorption was approximately constant over a wide dosage range and over consecutive periods of time. They have therefore assumed that they are measuring the maximum absorptive capacity of the small intestine. This value has been related to body weight (Cori, 1925 a; Magee & Reid, 1931; Trimble, Carey & Maddock, 1933) or surface area (MacKay & Bergman, 1933) and expressed as an absorption coefficient. However, the validity of the technique has been questioned by other workers (Pierce, Osgood & Polansky, 1928; Fenton, 1945), and it involves the following assumptions: (1) that the rate of absorption is independent of gastric emptying, (2) that there is no gastric absorption of glucose, and (3) that the small intestinal absorptive surface is working at saturation capacity. Using a technique by which it is possible to study gastro-intestinal absorption and motility simultaneously, Reynell & Spray (1956) found that with small glucose loads the rate of absorption was determined mainly by the rate of gastric emptying. Using the same technique, we have studied the effect on gastro-intestinal functions of increasing glucose loads and have attempted to determine the maximum absorptive capacity of the small intestine of the rat. METHODS Adult male rats of the Wistar strain ( g wt.) were used in these experiments. The experimental and analytical methods were those of Reynell & Spray (1956), but the data were analysed somewhat differently. The amount of glucose given to each animal varied from 0*3 to 2 g. The 34-2

2 532 P. C. REYNELL AND G. H. SPRAY dose of glucose was not related to the body weight of the animal and was always given in 4 ml. of solution (7 5-50% w/v solutions). Ten rats were subjected to resection of some two-thirds of the small intestine, and the duodenum was joined by end-to-end anastomosis to the residual lower ileum. Except when investigating the effect of other hypertonic solutions (p. 535), all the animals were killed 1 hr after intubation. Calculations Gastric emptying is the percentage of administered phenol red which has left the stomach during the time since intubation and is calculated as described previously (Reynell & Spray, 1956). Intestinal absorption index for glucose was determined for the third and fourth quarters of the small intestine as described previously (Reynell & Spray, 1956). It is the percentage ofadministered glucose which has been absorbed by the time the intestinal contents have entered the intestinal segment under consideration. Total absorption of glucose is the amount of glucose given less the amount recovered at the time at which the animal is killed. It can be subdivided into the following two fractions. Estimated gastric absorption of glucose is found by subtracting the amount of glucose recovered from the stomach of the test animal from the amount which would have been recovered if no glucose absorption had taken place. It is calculated from the formula Pa Ty Ta, where Pa Py phenol red recovered from the stomach of the test rat, Py = phenol red recovered from the control rat, Ta = glucose recovered from the stomach of the test rat, and Ty = glucose recovered from the control rat. Estimated intestinal absorption of glucose is found by subtracting the amount of glucose recovered from the small intestine of the test animal from the amount of glucose which has left the stomach during the hour since intubation (estimated intestinal glucose load). It is calculated from the formula Py-Pa Ty - Ti, where Ti = amount of glucose recovered from the small intestine of the Py test rat (Pa, Py and Ty as above). (The gastric absorption of glucose is slightly underestimated and the intestinal absorption of glucose correspondingly overestimated, since it is assumed that the ratio of glucose to phenol red leaving the stomach remains constant.) RESULTS Absorption of glucose l'otal absorption ofglucose. The total amount of glucose absorbed is not much increased by increasing the amount of glucose given (total glucose load) over the range studied (Fig. 1). When 0 3-1'0 g glucose was given, the mean absorption was mg in 1 hr. If 1-2 g was given, the mean absorption was mg in 1 hr. Effect of glucose load on gastric emptying. It is well known that gastric emptying is delayed by hypertonic glucose (Quigley & Phelps, 1934; Fenton, 1945), and our own results (Fig. 2) confirm that gastric emptying becomes progressively slower as stronger glucose solutions are introduced into the stomach, so that the intestinal glucose load does not increase in proportion to the total glucose load. Estimated gastric absorption of glucose. The mean estimated gastric absorption of glucose was 70+6 mg when g glucose was given, and mg when 1-2 g was given, so that over the range of glucose loads

3 GLUCOSE ABSORPTION 533 studied there was no significant increase in gastric absorption of glucose when stronger solutions were introduced into the stomach (Fig. 3). 600 _bo 0~~~~~ E 500 c 0~~~~~~~~~~~~~~~~~ O * oo O 0~~~~~~~~~~ ' 0~~~~ Total gluco'se load (mg) Fig. 1. Glucose absorption in I hr after varying doses of glucose. 80 4'6 0 I I I I I I I *.. Fig. 2. ' b* Total glucose load (mg) Percentage C ~~~0 gastric emptying 0 in 1 hr after varying doses of glucose. 0 CPO * * u.,v,140 -o,6o 120 o Wko 10 0 i o- 60-0Oo o *Zg ` Total glucose load (mg) Fig. 3. Glucose absorption from the stomach in1 hr after varying doses of glucose. Estimacted intestinal absorption of glucose. When the estimated intestinal absorption of glucose is plotted against the estimated intestinal load (Fig. 4), the former increas'es throughout the range of intestinal loads studied. At the highest loads the percentage of available glucose which is absorbed falls off slightly, but there is no plateau of 'maximal' intestinal absorption. It is therefore unlikely that the intestinal absorptive capacity has been saturated by the intestinal glucose loads ( mg) achieved in these experiments.

4 534 P. C. REYNELL AND G. H. SPRAY Intestinal absorption indices. Further evidence that the intestinal absorptive capacity is not fully saturated is obtained from the intestinal absorption indices of the third and fourth quarters of the small intestine (Table 1). With total glucose loads of less than 600 mg, practically no glucose is recovered from the lower half of the small intestine which therefore forms a large reserve of unutilized absorptive capacity. Even at total glucose loads of 1-2 g, the high absorption indices show that most of the glucose had been absorbed by the time the intestinal contents had entered the second half of the small intestine, and in only two of these animals was more than 1 mg of glucose recovered from the distal quarter of the small intestine. In no animal had any glucose entered the caecum during the hour following intubation. Fig. 4. r-4j 300.(4;0 200,: E i - 0) Estimated intestinal load(mg) Glucose absorbed from the small intestine in 1 hr with varying intestinal loads. TABLE 1. Intestinal absorption indices for third and fourth quarter of the small intestine with different total glucose loads Mean intestinal absorption index Glucose load No. of,r _ (g) animals third quarter fourth quarter Intestinal resections Further evidence of the great absorptive capacity of the small intestine of the rat for glucose was obtained by studying glucose absorption after massive intestinal resections. In ten rats with a residual small intestine of cm (mean 36 cm, or one-third of initial length) consisting only of duodenum and lower ileum, the mean estimated intestinal absorption of glucose was mg during the hour following intubation with a total glucose load of 400 mg. Since the ratio of absorbing surface to length increases in a linear manner with distance from the ileocaecal valve (Fisher & Parsons, 1950), these animals were probably left with no more than a quarter of their initial absorbing surface, which suggests that the small intestine should be capable of absorbing some 800 mg glucose in 1 hr, a rate exceeding the estimated intestinal load in any of the intact animals.

5 GLUCOSE ABSORPTION 535 Effect of other hypertonic solutions In order to discover whether the action of glucose on gastric emptying was a simple osmotic action, the effect of other hypertonic solutions was studied 1, 2 and 3 hr after intubation (Fig. 5). Sodium chloride, sodium sulphate or sorbitol was dissolved in 10% glucose to make up a solution approximately isosmotic with 50 % glucose, and the same volume of fluid was given in each case. The mean normal figures for comparison are those of Reynell & Spray (1956) using 10% glucose. With 50% glucose and 50% sorbitol there was delayed gastric emptying in every animal. With sodium chloride and sodium sulphate there was some delay at 1 hr, but at 2 and 3 hr after intubation gastric emptying was within normal limits. It thus seems probable that the delay is not explained simply by differences in osmotic pressure &. 80 bo C *s60.- E 40 / e.20.u / " 0 20S,20-8 e Time (hr) Fig. 5. The effect of hypertonic solutions on gastric emptying: O, normal; 0, 50% glucose; 3, 50% sorbitol; Q, 7% NaCl; e, 16% Na2SO4. 1OH2O. DISCUSSION Our experiments show that the basic assumptions implicit in the Cori technique for measuring intestinal absorption are incorrect. (1) Even at high glucose loads, gastric emptying is the most important single determinant of total glucose absorption. (2) Glucose is absorbed from the stomach, and the percentage of administered glucose absorbed by this route will vary with the total glucose load. The absorptive capacity of the stomach is readily saturated and rarely exceeds 100 mg in 1 hr, so that at high glucose loads the proportion of glucose absorbed by the stomach will be small. (3) Under the conditions of our experiments it has not proved possible to saturate the absorptive capacity of the small intestine, and neither total glucose absorption nor even estimated intestinal absorption of glucose can be used as a measure of small intestinal function unless this condition is satisfied. The capacity of the small intestine to absorb glucose could best be estimated by expressing the estimated

6 536 P. C. REYNELL AND G. H. SPRAY intestinal absorption of glucose as a percentage of the estimated intestinal load, although at high intestinal loads this ratio may fall slightly in normal rats. The reason that the total glucose absorption increases so little with increasing total glucose loads is not saturation of absorptive capacity, but delayed gastric emptying which prevents a corresponding increase in intestinal glucose load. Glucose is thus delivered to the small intestine at a rate which remains within its absorptive capacity. Where the Cori technique has been used to show that one substance may interfere with the absorption of another, the effect may be a result of delayed gastric emptying due to the introduction of a more concentrated solution into the stomach (Cori, 1925b, 1926). The small-intestinal absorptive capacity could probably be saturated by substances absorbed less readily than glucose, particularly if their effect on gastric emptying was less striking, and such substances might be better than glucose as tools with which to study the absorptive capacity of the small intestine by our technique. SUMMARY 1. The effect of varying doses of glucose on glucose absorption by the stomach and small intestine has been studied in the intact rat. 2. The stomach absorbs glucose at a rate which is independent of the concentration of glucose given over the range studied. 3. The absorption of glucose by the small intestine increases with increasing intestinal loads over the range studied ( mg). 4. Even at high intestinal loads most of the glucose can still be absorbed by the first half of the small intestine. 5. When two-thirds of the small intestine is removed, the residual small intestine can still absorb glucose at a rate of some 200 mg/hr. 6. It has not proved possible to saturate the absorptive capacity of the small intestine of the rat for glucose. As stronger solutions are introduced into the stomach, gastric emptying becomes slower and glucose is delivered to the small intestine at a rate which is less than its maximum absorptive capacity. 7. The delay in gastric emptying due to hypertonic sugar solutions seems to be more than a simple osmotic effect. We are grateful to Mrs Beryl Godfrey, Miss Shirley Thomas and Miss June Edwards for technical assistance, and to Professor L. J. Witts in whose department this work was done. REFERENCES CoRI, C. F. (1925a). The rate of absorption of hexoses and pentoses from the intestinal tract. J. biol. Chem. 66, CoRI, C. F. (1925b). The rate of absorption of a mixture of glucose and galactose. Proc. Soc. exp. Biol., N.Y., 23, CORI, C. F. (1926). The absorption of glycine and d,l-alanine. Proc. Soc. exp. Biol., N.Y., 24,

7 GLUCOSE ABSORPTION 537 FENTON, P. F. (1945). Response of the gastrointestinal tract to ingested glucose solutions. Amer. J. Physiol. 144, FISHER, R. B. & PARSONS, D. S. (1950). The gradient of mucosal surface area in the small intestine of the rat. J. Anat., Lond., 84, MAcKAY, E. M. & BERGMAN, H. C. (1933). The rate of absorption of glucose from the intestinal tract. J. biol. Chem. 101, MAGEE, H. E. & REID, E. (1931). The absorption of glucose from the alimentary canal. J. Physiol. 73, PIERCE, H. B., OSGOOD, H. S. & POLANSKY, J. B. (1928). The absorption of glucose from the alimentary tract of rats deprived of the vitamin B complex. J. Nutr. 1, QUIGLEY, J. P. & PHELPS, R. R. (1934). The mechanism of gastric motor inhibition from ingested carbohydrates. Amer. J. Physiol. 109, REYNELL, P. C. & SPRAY, G. H. (1956). The simultaneous measurement of absorption and transit in the gastro-intestinal tract of the rat. J. Physiol. 131, TRIMBLE, H. C., CAREY, B. W. Jr. & MADDOCK, S. J. (1933). The rate of absorption of glucose from the gastrointestinal tract of the dog. J. biol. Chem. 100,