THE FATE OF SUGAR IN THE ANIMAL

Transcription

1 THE FATE OF SUGAR IN THE ANIMAL BODY. III. THE RATE OF GLYCOGEN FORMATION IN THE LIVER OF NORMAL AND INSULINIZED RATS DURING THE ABSORP- TION OF GLUCOSE, FRUCTOSE, AND GALACTOSE. BY CARL F. CORI. (From the State Institute for the Study of Malignant Disease, Bu$alo.) freceived for publication, August 26, 1926.) The experiments of the preceding paper give a clear picture of the fate of ingested glucose. In the normal animal during 4 hours of sugar absorption for 100 parts of the absorbed sugar, 38 parts are oxidized, while 36 parts are deposited as glycogen in the muscles and 16 parts in the liver. In the insulinized animals for 100 parts of absorbed sugar, 50 parts are oxidized, 38 parts are deposited in the muscles, and only 4 parts in the liver. It seemed of interest to supplement these findings by a detailed study of the glycogen formation in the liver at shorter and longer absorption periods and to include other sugars. In this way curves for the rate of glycogen formation in the liver could be construtited and the percentage of the absorbed sugar that was retained as liver glycogen could be calculated. EXPERIMENTAL. The experiments were made in the fall of 1925 and in the winter of Male rats were used that weighed between 120 and 150 gm. The animals were fasted for 48 hours in order to reduce the glycogen content of the liver. They were then fed a known amount of sugar solution by stomach tube or fed with sugar and injected with insulin (15 units per 100 gm. of body weight). Enough sugar was introduced to let the absorption proceed for 6 hours. Groups of 4 to 6 rats were killed 1, 2, 3, 4, and 5 hours after the sugar feeding. The whole liver was removed as quickly as possible, frozen with compressed Cot, weighed in the frozen state, minced, and introduced into boiling 60 per cent 577

2 578 Rate of Glycogen Formation in Liver KOH. Glycogen was then determined by Pflueger s method. The amount of sugar absorbed was determined in the manner described in the first paper (1). The glycogen values obtained during the absorption of sugar were corrected for the preformed glycogen. Table I shows that the preformed glycogen, determined on suitable control rats, was, as an average, per cent. This figure was subtracted in each case. The variation in the preformed glycogen of &0.090 per cent introduces no serious error, since the amount of glycogen deposited in the course of the sugar absorption is large in comparison. TABLE I. Glycogen Content of the Liver of Male Rats, Fasted Previously for 48 Hours. Liver in per cent of body weight Average Glycogen content per 100 gm. of liver The glycogen values of Tables II to IV are represented graphically in Fig. 1. The curve illustrating the rate of glycogen formation from glucose is S-shaped. The rate increases gradually up to the 2nd hour, reaches its maximum between the 2nd and 3rd hour and diminishes again in the following hour. Between the 4th and 5th hour no glycogen is deposited, in spite of the fact that the absorption of sugar from the intestine proceeds at an undiminished rate. A definite glycogen maximum of the liver has been reached. The curve for fructose follows a straight line between the 2nd and 4th hour of absorption. In the following hour the amount of glycogen deposited in the liver is very small. The curve for galactose is flat, indicating that this sugar is very slowly converted into liver glycogen. A comparison of the curves 0.397

3 C. E. Cori 579 shows that glucose and fructose are on a par as glycogen farmers. This is very remarkablc, since the former sugar is absorbed twice as fast as the latter. Galactose, on the other hand, which is absorbed at about the same rate as glucose, plays an unimportant rolc as a source of liver glycogen. This is not due to a low permcability of the liver cells for this sugar, since in former experiments (2) all three sugars were found t,o permeate with equal rapidity into the liver cells. TABLE IZate of Glvcogen Forma.tion in Liver of Rut &wing Absorption of Glucose Jrom Ihe Intestine. A, AJter Giving Glucose Alone. 13, AIter Giving Glucose plus Insulin (15 Units Per 100 Gm. of Body Weight). Each Figure Is an Average of Five to Sin: Experiments Yrn. 1.1s: 3.1% 3.17E 3.17( 3.17:.1. Glucose alone. I B. Glucose plus insulin. -?c 2 a 8 - z mg. 16s t 17: ym zko.2; 2.66&0.4! : 3.65ztO.5 < - II. (vn ! ( : ! OO.lS! mg. gm zko tO f zt hrs * The absorption coefficient is the amount of sugar absorbed per 100 gm., of body weight per hour. The curves for glucose plus insulin and fructose plus insulin revcal how profoundly the glycogen formation in the liver is inhibited by an excess of the pancreatic hormone. The inhibiting influence is stronger in the case of fructose than in the case of glucose. In the first 3 hours of fructose absorption no liver glycogen was formed, while after 4 and 5 hours a small amount was deposited. During the absorption of glucose, glycogen was formed in the 2nd hour and this could not be prevented, even if

4 580 Rate of Glycogen Formation in Liver the largest doses of insulin were injected. It should be noted that the insulinized animals absorbed nearly the same amount of sugar as the normal animals and that hypoglycemic symptoms were absent. The significance of the lessened deposition of glycogen in the liver of insulinized animals has been discussed in the preceding paper and need, therefore, not be repeated here. When only 1 unit instead of 15 units was injected, the rate of glycogen formation from glucose was only slightly inhibited, as TABLE Rate of Glycogen Formation in Liver of Rat during Absorption of Fructose from the Intestine. A, After Giving Fructose Alone. B, Ajter Giving Fructose plus Insulin (15 Units per 100 Gm. of Body Weight). Each Figure is an Average of Four to Five Experiments A. Fructose alone. B. Fructose plus insulin. gm. gm zko k-o III. ~-- gm.?n!?. sm f f0.2 * The absorption coefficient is the amount of sugar absorbed per 100 gm. of body weight per hour. T hrs is shown in Table V. In previous experiments (3) on the liver of sugar-fed and insulinized rabbits, a glycogen deposition of about the same rate as in control animals was observed. The insulin dose in these experiments was apparently too small to show the strong inhibiting effect. Table IV shows that the percentage of the absorbed galactose that is excreted in the urine increases with increasing length of absorption, in spite of the fact that the rate of absorption remains constant. Thus in 1 hour 27 per cent appears in the urine, in 2 hours 41 per cent, in 3 hours 51 per cent, and in 4 hours 60 per

5 C. F. Cori cent. The excretion of galactose in the urine shows many striking features and will be dealt with in the sixth paper of this series. The previous observations on the mechanism of absorption of sugars from the intestine are confirmed by the present investigation. When the data of Tables II to IV are used for a graphic TABLE Rate of Glycogen Formation in Liver and Rate of Excretion of Sugar in Urine during Absorption of Galactose from the Intestine. Each Figure Is an Average o.f Four to Five Experiments. IV. Percentage Percentage Percentage Liver in Glycogen of sugar per cent Absorption,$$~$l formed per absorbed regf::%at zs$$ig of body coefficient.* that is lo0 gm, of liver, that is depos- is deposited period. weight. excreted. lted as liver as liver glyglycogen. cogen. ~ gm. sm. hrs f ko * The absorption coefficient is the amount of sugar absorbed per 100 gm. of body weight per hour. TABLE V. Rate of Glycogen Formation from Glucose After Giving 1 Unit of Insulin. (In Experiments in Tables II and Ill 15 Units Were Injected). Each Figure Is an Average of Four Experiments. c7m. mg. gm. hrs zko illustration of the rate of absorption, it will be found that the absorption of glucose, fructose, and galactose follows a straight line. The absorption coefficients for these three sugars were the same as previously reported, as is shown by the following summary. It will also be noted that insulin has no influence on the rate of absorption of sugars from the intestine.

6 582 Rate of Glycogen Formation in Liver J.8 I.6 I.4 /.2 I FIG. 1. The rate of glycogen formation in the liver of normal and insulinized rats. Average Absorption Coqficient. Glucose. Fructose. Galactose Former experiments Cl) Present (normal animals) (insulinized animals).

7 C. F. Cori 583 DISCUSSION. In previous work on the glycogen formation in the liver quantitative data on the part,icipation of this organ in the carbohydrate metabolism of the whole body have been lacking. The result was that the rale of the liver has generally been overestimated. In Tables II to V the percentage of the total amount of sugar absorbed that is retained in the liver as glycogen, has been calculated. The retention of glucose reaches its maximum in 4 hours and corresponds then to 17 per cent of the amount absorbed. Even if a liberal estimate for the glucose oxidation in the liver is made, one finds that in 4 hours the liver can dispose of only onefifth to one-fourth of the total amount of sugar metabolized in the body. Since the glycogen formation ceases after 4 hours of glucose absorption, the participation of the liver in the disposal of sugar is negligible in later periods. On the other hand, one will be prevented from committing an injustice, if one considers that the liver of rats fasted for 48 hours constitutes only 3 per cent of the body weight. If one compares the percentage retention from glucose and from fructose, one has to take into account that the former sugar is absorbed twice as fast as the latter, while the rate of glycogen formation from both sugars is the same. The retention of fructose also reaches its maximum in 4 hours and amounts to 39 per cent of the absorbed sugar. The difference between the two sugars is still greater after 2 hours, since 34 per cent from fructose and only 7 per cent from glucose are retained as liver glycogen. Hence, the liver is of greater importance for the removal of the former sugar than for the latter. This may be the experimental explanation, why the tolerance for fructose rather than for glucose is used as a test for liver function. The influence of insulin on liver glycogen has been discussed by the author in 1925 (4). Since that time several new papers have appeared, which seem to confuse the issue. A discussion of the pertinent data would, therefore, not seem out of place. Grevenstuk and Laqueur (5) made experiments on fasting rabbits using the abdominal window method of the author. They found in confirmation of the author s results that large doses of insulin have either no influence or lead to a decrease in the liver glycogen. Frank, Hartmann, and Nothmann (6) maintained that small

8 584 Rate of Glycogen Formation in Liver doses of insulin (0.1 unit per kilo) increase the liver glycogen of fasting rabbits, but this could not be substantiated in a later communication of Grevenstuk and Laqueur (7). The latter authors imply that, since insulin does not cause glycogenesis in the liver of fasting animals, it could not have this effect under any conditions. This is in opposition to a number of observations. Glycogen deposition in the liver under the influence of insulin has been demonstrated for fasting and phlorhizinized animals by Cori (4) and for sugar-fed and phlorhizinized animals by Nash (8). The same effect was observed by Cori (4) and by Hedon (9) on the liver of fasting and depancreatized animals and by Banting and coworkers (10) on depancreatized animals that were fed with carbohydrates. Another error of Grevenstuk and Laqueur has to be pointed out. They mention Cori s experiments on the liver of fasting and insulinized mice (16 injected mice showed, as an average, 39 per cent less liver glycogen than 16 control mice) and confront them with the experiments of Bissinger and Lesser (II), who found an increase in the glycogen content of the whole mouse following an injection of glucose plus insulin. The fallacy of such a comparison is fully explained by the data of the preceding paper. Even if no glycogen is deposited in the liver, an abundant glycogen deposition may occur in the muscles. SUMMARY. 1. Curves illustrating the rate of glycogen formation in the liver of the rat during the absorption of glucose, fructose, and galactose have been presented. 2. Glucose and fructose are on a par as glycogen formers, even though the former sugar is absorbed twice as fast as the latter. Galactose plays an unimportant role as a source of liver glycogen. 3. After 4 hours of glucose or fructose absorption the glycogen formation in the liver ceases or is markedly diminished, indicating that a glycogen maximum has been reached. 4. Large doses of insulin almost completely suppress the glycogen formation in the liver from glucose and from fructose. 5. The role of the liver in the carbohydrate metabolism of the whole body could be measured quantitatively by calculating the percentage of the total amount of sugar absorbed that is retained as liver glycogen. The maximum retention occurred in 4 hours

9 C. F. Cori and amounted to 17 per cent in the case of glucose and to 39 per cent in the case of fructose. BIBLIOGRAPHY. 1. Cori, C. F., J. Biol. Chem., 1925, lxvi, Cori, C. F., and Goltz, H. L., Proc. Sot. Ezp. Biol. and Med., 1925, xxiii, Cori, C. F., Cori, G. T., and Pucher, G. W., J. Pharmacol. and Exp. Therap., 1923, xxi, Cori, C. F., J. Pharmacol. and Exp. Therap., 1925, xxv, Grevenstuk, 8., and Laqueur, E., Biochem. Z., 1925, clxiii, Frank, E., Hartmann, E., and Nothmann, M., Klin. Woch., 1925, iv, Grevenstuk, A., and Laqueur, E., Biochem. Z., 1926, clxxiii, Nash, T. P., Jr., J. Biol. Chem., 1925, Ixvi, HCdon, E., Compt. rend. Sot. biol., 1925, xciii, Banting, F. J., Best, C. H., Collip, J. B., Macleod, J. J. R., and Noble, E. C., Tr. Roy. Sot. Canada, 1922, xvi, Sect. V, Bissinger, E., and Lesser, E. J., Biochem. Z., 1926, clxviii, 398.

10 THE FATE OF SUGAR IN THE ANIMAL BODY: III. THE RATE OF GLYCOGEN FORMATION IN THE LIVER OF NORMAL AND INSULINIZED RATS DURING THE ABSORPTION OF GLUCOSE, FRUCTOSE, AND GALACTOSE Carl F. Cori J. Biol. Chem. 1926, 70: Access the most updated version of this article at Alerts: When this article is cited When a correction for this article is posted Click here to choose from all of JBC's alerts This article cites 0 references, 0 of which can be accessed free at ml#ref-list-1