Glycogenolysis in the fasting dog

Comp. Biochem. Physiol. Vol. 75B, No. 4, pp. 553-555, 1983 Printed in Great Britain

0305-0491/83$3.00+0.00 © 1983PergamonPress Ltd

GLYCOGENOLYSIS IN THE FASTING DOG JAN J. DE BRUIJNE and PIET DE KOSTER Small Animal Clinic, Faculty of Veterinary Medicine, State University of Utrecht, Utrecht, The Netherlands (Tel: 030-531683) (Received 10 January 1983)

Abstract--l. Glycogen concentrations were determined in liver-biopsy specimenswhich were taken from four dogs during five consecutive days of fasting. 2. It was found that maximal depletion of liver glycogen occurred between the second and the third day. 3. Starvation-induced glycogenolysiswas much slower in the dog than in men and rats. 4. Fuel fluxes are discussed and it is tentatively concluded that in the fasting dog larger amounts of glycerol are available for gluconeogenesisthan in other species.

INTRODUCTION

One of the first metabolic adaptations during fasting is the breakdown of liver glycogen for the maintenance of normoglycemia. The store of glycogen in the liver is limited and in fasting men and rats normoglycemia can only be sustained for 24 hr or less (Hultman and Nilsson, 1971; Goldstein and Curnow, 1979; R6mesar and Alemany, 1980; Shikama et al., 1980). The developing hypoglycemia triggers a number of secondary reactions in men and rats, such as hypoinsulinemia and hyperglucagonemia (Goldstein and Curnow, 1979; Berchtold et al., 1980). As a consequence, gluconeogenesis and lipolysis are stimulated and in some species ketosis may occur (Cahill et al., 1973). In previous studies it was demonstrated that dogs do not develop significant hypoglycemia during 1 week of total starvation (Alszuler et al., 1974; De Bruijne et al., 1981). To our knowledge no data are available on glycogen metabolism in the fasting dog. Therefore, we measured the hepatic glycogen content in dogs that were fasted during 5 days. MATERIALS

AND

()~ + SE). During the first 2 days of starvation the average rate of glycogen disappearance was 22 + 4 mg/g liver per day. In three dogs the hepatic glycogen concentration was minimal on the third day of starvation; in one dog this happened on the second day. The glycogen content of the liver tended to rise slightly during the following days of fasting (Fig. 1). DISCUSSION

This study was undertaken in an attempt to explain why in the dog normoglycemia is maintained during the first week of fasting, in contrast to the situation in man and rat. It was demonstrated that, despite similar hepatic glycogen contents (Table 1), glycogenolysis is much slower in the fasting dog than in fasting man and rat in which liver glycogen is depleted within 24 hr of starvation (Hultman and Nilsson, 1971; R6mesar and Alemany, 1980; Shikama et al., 1980). The rate of glycogenolysis is primarily regulated by insulin and glucagon (Goldstein et al., 1978). In the fasting dog insulin concentrations decline moderately, whereas glucagon concentrations do not change significantly (De Bruijne et al., 1981).

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METHODS

Four female Beagle dogs, 2 years old and weighing 12-14 kg, were fasted without any supplementation. Water was given ad libitum. Liver biopsies were obtained daily by the technique of Menghini (Lettow, 1963) between 8 and 9 a.m.) under local anesthesia. The first samples were taken after an overnight fast. The specimen had at least 10mg fresh weight. They were digested immediatelyin KOH (30~, w/v), followed by precipitation and washing in ethanol (96~o, v/v). Glycogen was hydrolysed in 0.6 M HC1 (Hassid and Abraham, 1957). The glucose concentration in the neutralized solution was measured with glucose analyser (Beckman Instruments, Fullerton, CA).

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70

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60

50 2

S ~o g

RESULTS

The rate at which glycogen disappeared from the liver differed only slightly between the individual dogs (Fig. 1). The average value after an overnight fast was 352_+27/zmol glucose per gram tissue, which equals 5 9 _ 5 mg glycogen per gram liver cBp

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10

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0

t

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i

i

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1 2 3 Z, 5 Period ot storvotion [doysl

Fig. 1. Changes in hepatic glycogen content in fasted dogs.

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JAN J. OE BRUIJNEand PIET DE KOSTER Table 1. Summary of data on hepatic glycogen metabolism in dog, man and rat during the first day of starvation Body weight Liver weight Hepatic glycogen content (rag g tissue) Hepatic glycogenolysisrate (rag g tissue hr) Hepatic glycogenolysis rate [mg (W~/4)]* Glucose turnover rate (g kg hr) Glucose turnover rate [g (W 3/4)hr]* Glycogen contribution to glucose demand on first day ~0

Dog

Man

Rat

13 kg 350 g" 59 -I- 5

70 kg 1500 gb 50 ± 5d

250 g 10 gC 36-80e'r'g

0.9

2.6a

2.9

47

160

84

0.15-0.19h'i'j

0. t 1k

0.54~

0.31 0.40

0.33

0.34

14

40

26

*W = body weight in kg; W 3/4= units of metabolism body sizem. References: ~Ackerknecht (1943), bSchneider (1964), ~Hebel and Stromberg (1976), dHultman and Nilsson ( 1971), ~R6mesarand Alamany (1980), fShikama et al. (1980), gWade (1979), hCowan et al. (1969), iBrady et al. (1977), JSteele et al. (1968), kBerchtold et al. (1980), IFreminet and Leclerc (1980), mKleiber (1975). On the other hand, in man and rat starvation is associated with a significant decrease in circulating insulin (Berchtold et al., 1980; Goodman et al., 1980) and in man, moreover, with a transient rise in glucagon concentration (Berchtold et al., 1980). The different hormonal response upon starvation might explain the lower rate of glycogenolysis in dogs as compared to men and rats. The dog is able to maintain normoglycemia despite the absence of hyperglucagonemia. This phenomenon suggests that during starvation the dog can restrict its peripheral utilization of glucose more efficiently than man and rat. Another possibility is that in the fasting dog endogenous glucose precursors (e.g. glycerol) are used more efficiently for gluconeogenesis. It can be calculated from literature data that the glucose-turnover rate per unit of metabolic body size (Kleiber, 1975) does not differ significantly between the three species (Table 1). Moreover, the glucoseturnover rate drops only 30-35~o in the fasting dog (Steele et al., 1968; Cowan et al., 1969; Brady et al., 1977) whereas in men (Marliss, 1978) and in rats (Freminet and Leclerck, 1980) a drop of more than 50~o has been observed. Hence, the maintenance of normoglycemia during starvation must be caused by a higher rate of gluconeogenesis in the dog as compared to the other species. Freminet and Leclerc (1980) discussed the function of mitochondrial and cytosolic phosphoenolpyruvate carboxykinase (PEPCK; E.C. 4.1.1.32) in the gluconeogenic process in a number of species. A comparison of glucose turnover and the location of PEPCK in the various species led these authors to suggest that the glucose turnover during starvation remains high in species in which the enzyme activity is mainly mitochondrial. About 70~ of PEPCK activity in canine liver is found within the mitochondria (Soling and Kleineke, 1976). In the rat PEPCK is almost exclusively present in the cytosol and in man the enzyme is distributed equally over the two compartments of the liver cell (Hanson, 1974). We reported previously that in the fasting dog protein contributes approx 1 g glucose per kg body

weight per day (De Bruijne, 1979), which equals 1.9 g glucose per unit of metabolic body size. In fasted men and rats the latter figures were, respectively, 1.1 (Marliss, 1978) and 6.0 (Freminet and Leclerc, 1980). It should be noted, however, that in the rat the effect of starvation on gluconeogenesis depends on the amount of adipose tissue that is present in the body (Wade, 1979; Goodman et al., 1980). Recycling of lactate via the Cori cycle is increased in the dog during starvation (R6mesar and Alemany, 1980; Shikama et al., 1980), whereas in the rat a significant decrease has been found upon a transition to the starved state (Freminet and Leclerc, 1980). In man starvation does not induce a significant change in lactate turnover (Marliss, 1978). The glycerol concentration is higher in the dog (0.2 + 0.1 mmol/l) (De Bruijne et al., 1981) than in man (0.09 _+ 0.009 mmol/1) (Streja et al., 1977). It has been demonstrated that the increase of blood glycerol during fasting is higher in the dog than in man (Streja et al., 1977; De Bruijne et al., 1981). Utilization of glycerol is concentration-dependent (Winkler et al., 1969). It can be concluded, therefore, that more glycerol is available for gluconeogenesis in the dog than in man and rat. Acknowledgements--The authors wish to thank Jan Rothuizen D.V.M. for his contribution to this study by taking biopsy specimen.

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