Liver glycogenolysis during exercise without a significant increase in cAMP.

Liver glycogenolysis may be controlled by glucagon or catecholamine-induced changes in cAMP or by cAMP-independent mechanisms. The purpose of these experiments was to determine whether an increase in liver cAMP occurs during exercise at a time when the rate of liver glycogenolysis is greatly accelerated. Rats were taught to run on a treadmill 10 min/day for 6 wk. They were then run continuously for periods of time ranging from 0 to 120 min at 0.8 mph up a 15% grade. Liver glycogen was depleted by the end of 90 min in fed animals and by 20 min in overnight-fasted animals. Liver cAMP was not significantly increased in fed animals during the first 60 min of exercise. The major increase in liver cAMP occurred after liver glycogen was depleted, at which time the rat must rely entirely on gluconeogenesis for maintenance of blood glucose. This increase in cAMP corresponded to large increases in plasma glucagon and catecholamines. We conclude that liver glycogenolysis in the rat can occur during exercise independently from significant detectable increases in cAMP concentrations.     

Effects of maleate on carbohydrate metabolism in rats.

Intraperitoneal administration of maleate produced an increase in blood alpha-ketoacid, acetoacetate, and free fatty acids. The effect of this treatment on blood glucose levels depended on whether the rats were fed or fasted. In fed rats it was accompanied by slight, transient hyperglycemia connected with depletion of liver glycogen stores. In fasted animals moderate hypoglycemia was observed. The in vivo conversion of various precursors into blood glucose was not inhibited, suggesting that maleate does not affect hepatic gluconeogenesis. Neither was a direct effect on liver glycogenolysis observed. On the other hand, maleate inhibited renal gluconeogenesis from various substrates and stimulated anerobic glycolysis in kidney cortical alices. The data are interpreted in terms of increased utilization and decreased production of glucose by the kidney followed by secondary changes in liver carbohydrate metabolism.     

Muscle and liver glycogen,protein, and triglyceride in the rat

Summary We have previously found that during exercise net muscle glycogen breakdown is impaired in adrenodemedullated rats, as compared with controls. The present study was carried out to elucidate whether, in rats with deficiencies of the sympatho-adrenal system, diminished exercise-induced glycogenolysis in skeletal muscle was accompanied by increased breakdown of triglyceride and/or protein. Thus, the effect of exhausting swimming and of running on concentrations of glycogen, protein, and triglyceride in skeletal muscle and liver were studied in rats with and without deficiencies of the sympatho-adrenal system. In control rats, both swimming and running decreased the concentration of glycogen in fast-twitch red and slow-twitch red muscle whereas concentrations of protein and triglyceride did not decrease. In the liver, swimming depleted glycogen stores but protein and triglyceride concentrations did not decrease. In exercising rats, muscle glycogen breakdown was impaired by adrenodemedullation and restored by infusion of epinephrine. However, impaired glycogen breakdown during exercise was not accompanied by a significant net breakdown of protein or triglyceride. Surgical sympathectomy of the muscles did not influence muscle substrate concentrations. The results indicate that when glycogenolysis in exercising muscle is impeded by adrenodemedullation no compensatory increase in breakdown of triglyceride and protein in muscle or liver takes place. Thus, indirect evidence suggests that, in exercising adrenodemedullated rats, fatty acids from adipose tissue were burnt instead of muscle glycogen.     

Effects of long-term feeding of high-protein or high-fat diets on the response to exercise in the rat

Summary The aim of this work was to find by which mechanisms an increased availability of plasma free fatty acids (FFA) reduced carbohydrate utilization during exercise. Rats were fed high-protein medium-chain triglycerides (MCT), high-protein long-chain triglycerides (LCT), carbohydrate (CHO) or high-protein low-fat (HP) diets for 5 weeks, and liver and muscle glycogen, gluconeogenesis and FFA oxidation were studied in rested and trained runner rats. In the rested state the hepatic glycogen store was decreased by fat and protein feeding, whereas soleus muscle glycogen concentration was only affected by high-protein diets. The percentage decrease in liver and muscle glycogen stores, after running, was similar in fat-fed, high-protein and CHO-fed rats. The fact that plasma glucose did not drastically change during exercise could be explained by a stimulation of hepatic gluconeogenesis: the activity of phosphoenolpyruvate carboxykinase (PEPCK) and liver phosphoenolpyruvate (PEP) concentration increased as well as cyclic adenosine monophosphate (AMPc) while liver fructose 2,6-bisphosphate decreased and plasma FFA rose. In contrast, the stimulation of gluconeogenesis in rested HP-, MCT- and LCT-fed rats appears to be independent of cyclic AMP.     

Mobilization of glucose from the liver during exercise and replenishment afterward.

The liver is anatomically well situated to regulate blood glucose. It is positioned downstream from the pancreas, which releases the key regulatory hormones glucagon and insulin. It is also just downstream from the gut, permitting efficient extraction of ingested glucose and preventing large excursions in systemic glucose after a glucose-rich meal. The position of the liver is not as well situated from the standpoint of experimentation and clinical assessment, as its primary blood supply is impossible to access in conscious human subjects. Over the last 20 years, to study hepatic glucose metabolism during and after exercise, we have utilized a conscious dog model which permits sampling of the blood that perfuses (portal vein, artery) and drains (hepatic vein) the liver. Our work has demonstrated the key role of exercise-induced changes in glucagon and insulin in stimulating hepatic glycogenolysis and gluconeogenesis during exercise. Recently we showed that portal venous infusion of the pharmacological agent 5'-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside leads to a marked increase in hepatic glucose production. Based on this, we propose that the concentration of AMP may be a component of a physiological pathway for stimulating hepatic glucose production during exercise. Insulin-stimulated hepatic glucose uptake is increased following exercise by an undefined mechanism that is independent of liver glycogen content. The fate of glucose taken up by the liver is critically dependent on hepatic glycogen stores, however, as glycogen deposition is greatly facilitated by prior glycogen depletion.     

Hepatic metabolism of meal-fed rats: studies in vivo and in the isolated perfused liver

Hepatic metabolic fluxes (glycolysis, glucose release, glycogenolysis, oxygen consumption, ketogenesis and gluconeogenesis), hepatic glycogen and food ingestion in meal-fed rats were measured and compared to appropriate controls. The following results were obtained: 1) in livers from meal-fed rats a higher fraction of glucosyl units derived from glycogen is used in glycolysis instead of being released in the form of glucose; 2) the rate of glycogen catabolism in livers from meal-fed rats is less than expected when one compares their glycogen levels with those of the appropriate controls; 3) the livers from meal-fed rats become much less ketotic than the livers from rats which were not trained to eat a single meal daily. It was concluded that the liver of meal-fed rats is well adapted to the main characteristics of those animals, e.g., increased lipogenesis from glycolysis products and a reduced need for carbon units from the liver (glucose and ketone bodies) as a consequence of enhanced food intake.     

Lack of influence of glucagon on glucose homeostasis after prolonged exercise in rats

Summary The significance of glucagon for post-exercise glucose homeostasis has been studied in rats fasted overnight. Immediately after exhaustive swimming either rabbit-antiglucagon serum or normal rabbit serum was injected by cardiac puncture. Cardiac blood and samples of liver and muscle tissue were collected before exercise and repeatedly during a 120 min recovery period after exercise. During the post-exercise period plasma glucagon concentrations decreased but remained above pre-exercise values in rats treated with normal serum, while rats treated with antiglucagon serum had excess antibody in plasma throughout. Nevertheless, all other parameters measured showed similar changes in the two groups. Thus after exercise the grossly diminished hepatic glycogen concentrations remained constant, while the decreased blood glucose concentrations were partially restored. Simultaneously concentrations in blood and serum of the main gluconeogenic substrates, lactate, pyruvate, alanine and glycerol declined markedly. During the post-exercise period NEFA concentrations in serum and plasma insulin concentrations remained increased and decreased, respectively, while plasma catecholamines did not differ from basal values. Muscle glycogen concentrations decreased slightly. These findings suggest that in the recovery period after exhaustive exercise the increased glucagon concentrations in plasma do not influence gluconeogenesis.     

Post-exercise glycogen resynthesis in trained high-protein or high-fat-fed rats after glucose feeding

Summary This study examined the effect on glycogen resynthesis during recovery from exercise of feeding glucose orally to physically trained rats which had been fed for 5 weeks on high-protein low fat (HP), high-protein/long-chain triglyceride (LCT) or high carbohydrate (CHO) diets. Muscle glycogen remained low and hepatic gluconeogenesis was stimulated by long-term fat or high-protein diets. The trained rats received, via a stomach tube, 3 ml of a 34% glucose solution immediately after exercise (2 h at 20 m · min^–1), followed by 1ml portions at hourly intervals until the end of the experiments. When fed glucose soleus muscle glycogen overcompensation occurred rapidly in the rats fed all three diets following prolonged exercise. In LCT- and CHO-fed rats, glucose feeding appeared more effective for soleus muscle repletion than in HP-fed rats. The liver demonstrated no appreciable glycogen overcompensation. A complete restoration of liver glycogen occurred within a 2- to 4-h recovery period in the rats fed HP-diet, while the liver glycogen store had been restored by only 67% in CHO-fed rats and 84% in LCT-fed rats within a 6-h recovery period. This coincides with low gluconeogenesis efficiency in these animals.     

Loss of glucagon suppression of feeding after vagotomy in rats

Infusion of pancreatic glucagon through the hepatic-portal vein decreased short-term food intake in sham-vagotomized but not in subdiaphragmatically vagotomized rats. Measurement of hepatic glycogen storage showed that vagotomized rats maintain a lower glycogen level than control animals over the four fasting periods evaluated. To determine whether the absence of a glucagon effect on feeding in vagotomized rats was the result of the reduced amount of substrate for glycogenolysis, vagotomized rats were not fasted and control animals were food deprived for 8h to produce comparable hepatic glycogen levels. Hepatic-portal infusion of glucagon into these differentially fasted animals suppressed feeding in control rats but not in vagotomized rats. It is concluded that the ineffectiveness of glucagon in suppressing feeding in vagotomized rats is not due to reduced concentration of hepatic glycogen. Instead, it is likely that glucagon induces glycogenolysis, but the glucose, or some other correlate of glycogen breakdown, loses its ability to produce satiety subsequent to vagotomy.     

The effect of high-intensity intermittent swimming on post-exercise glycogen supercompensation in rat skeletal muscle

A single bout of prolonged endurance exercise stimulates glucose transport in skeletal muscles, leading to post-exercise muscle glycogen supercompensation if sufficient carbohydrate is provided after the cessation of exercise. Although we recently found that short-term sprint interval exercise also stimulates muscle glucose transport, the effect of this type of exercise on glycogen supercompensation is uncertain. Therefore, we compared the extent of muscle glycogen accumulation in response to carbohydrate feeding following sprint interval exercise with that following endurance exercise. In this study, 16-h-fasted rats underwent a bout of high-intensity intermittent swimming (HIS) as a model of sprint interval exercise or low-intensity prolonged swimming (LIS) as a model of endurance exercise. During HIS, the rats swam for eight 20-s sessions while burdened with a weight equal to 18% of their body weight. The LIS rats swam with no load for 3 h. The exercised rats were then refed for 4, 8, 12, or 16 h. Glycogen levels were almost depleted in the epitrochlearis muscles of HIS- or LIS-exercised rats immediately after the cessation of exercise. A rapid increase in muscle glycogen levels occurred during 4 h of refeeding, and glycogen levels had peaked at the end of 8 h of refeeding in each group of exercised refed rats. The peak glycogen levels during refeeding were not different between HIS- and LIS-exercised refed rats. Furthermore, although a large accumulation of muscle glycogen in response to carbohydrate refeeding is known to be associated with decreased insulin responsiveness of glucose transport, and despite the fact that muscle glycogen supercompensation was observed in the muscles of our exercised rats at the end of 4 h of refeeding, insulin responsiveness was not decreased in the muscles of either HIS- or LIS-exercised refed rats compared with non-exercised fasted control rats at this time point. These results suggest that sprint interval exercise enhances muscle glycogen supercompensation in response to carbohydrate refeeding as well as prolonged endurance exercise does. Furthermore, in this study, both HIS and LIS exercise prevented insulin resistance of glucose transport in glycogen supercompensated muscle during the early phase of carbohydrate refeeding. This probably led to the enhanced muscle glycogen supercompensation after exercise.     

Carbohydrate metabolism and uraemia — Mechanisms for glycogenolysis and gluconeogenesis

Summary Disturbances of carbohydrate metabolism during acute uraemia are characterized by the degradation of liver and muscle glycogen with a simultaneous activation of hepatic gluconeogenesis. After binephrectomy, the substitution of essential amino acids and keto analogues stimulate liver, but not skeletal muscle glycogen synthesis. Serine proves to be an optimal substrate for liver gluconeogenesis and muscle glycogen generation under acute uraemic conditions. Propranolol does not influence glycogenolysis of skeletal muscle in acutely uraemic rats. During starvation, acute uraemia leads to an increase of total carbohydrate content as well as of glycogen and glucose concentrations in heart muscle Alterations in carbohydrate contents are not observed in the kidney after ureter ligation.Enhanced glycogenolysis of skeletal muscle and liver during acute uraemia may be due to activation of phosphorylase kinase caused by the increased serum concentrations of various hormones (glucagon, catecholamines, parathormone) as well as free proteolytic activity, an increase of intracellular Ca²⁺-concentration and finally by alterations in the structure of contractile proteins.This work was supported by the Deutsche Forschungsgemeinschaft (Ho 781/1)     

Effects of somatostatin on glucagon-stimulated glycogenolysis and gluconeogenesis in hepatocytes cultured in vitro

The effects of somatostatin (SS-14) on glycogenolysis and gluconeogenesis in rat hepatocytes cultured in vitro in a serum-free medium were investigated. Somatostatin (122 nmol 1^-1) did not significantly change the basal glucose production with or without pyruvate (10 mmol 1^-1). Glucagon strongly (over 100%) increased the glucose production in hepatocytes incubated in a medium supplemented with 10 mmol 1^-1 pyruvate. This increase in glucose production is the result of increased rates of gluconeogenesis and glycogenolysis. Somatostatin partially inhibited the glucagon stimulated increase in glucose production. Glucagon also significantly increased the glucose production in a glucose-free medium without pyruvate, which resulted from an increase of glycogenolysis. Somatostatin did not inhibit the increase in glucose production in these conditions. After a 4 h ‘fast’, glycogen in hepatocytes fell to a very low level. Glucose production was minimal. After the addition of pyruvate, there was a increase in gluconeogenesis and glucose production. Glucagon stimulated the rate of gluconeogenesis. Somatostatin completely inhibited this glucagon-stimulated increase in gluconeogenesis.     

Effects of somatostatin on glucagon-stimulated glycogenolysis and gluconeogenesis in hepatocytes cultured in vitro.

The effects of somatostatin (SS-14) on glycogenolysis and gluconeogenesis in rat hepatocytes cultured in vitro in a serum-free medium were investigated. Somatostatin (122 nmol l-1) did not significantly change the basal glucose production with or without pyruvate (10 mmol l-1). Glucagon strongly (over 100%) increased the glucose production in hepatocytes incubated in a medium supplemented with 10 mmol l-1 pyruvate. This increase in glucose production is the result of increased rates of gluconeogenesis and glycogenolysis. Somatostatin partially inhibited the glucagon stimulated increase in glucose production. Glucagon also significantly increased the glucose production in a glucose-free medium without pyruvate, which resulted from an increase of glycogenolysis. Somatostatin did not inhibit the increase in glucose production in these conditions. After a 4 h 'fast', glycogen in hepatocytes fell to a very low level. Glucose production was minimal. After the addition of pyruvate, there was a increase in gluconeogenesis and glucose production. Glucagon stimulated the rate of gluconeogenesis. Somatostatin completely inhibited this glucagon-stimulated increase in gluconeogenesis.     

Combined administration of glucose precursors is more efficient than that of glucose itself in recovery from hypoglycemia

The purpose of the present study was to investigate the effect of the combined administration of hepatic gluconeogenic substrates (glycerol + L-lactate + L-alanine + L-glutamine) on glucose recovery during insulin induced hypoglycemia (IIH), in rats. IIH was obtained by an ip injection of regular insulin (1 U/kg). Thus, 150 min after insulin administration the rats received an ip injection of glycerol + L-lactate + L-alanine + L-glutamine (each 100 mg/kg). In these experiments control groups, which received saline, glucose or isolated precursors (100 mg/kg), were employed. Glycemia was measured 30 min later, i.e., 180 min after insulin injection. The results showed that the combined administration of gluconeogenic precursors is more efficient than that of glucose itself to promote glycemia recovery. Since, the blood levels of hepatic glucose precursors were decreased (glycerol, L-lactate and L-alanine) or maintained (L-glutamine) during IIH, the ability of the liver to produce glucose from these gluconeogenic substrates was investigated. The results showed that the maximal capacity of the liver to produce glucose from glycerol (2 mM), L-lactate (2 mM), L-alanine (5 mM) and L-glutamine (5 mM) was increased. To L-alanine and L-glutamine, not only the glucose production was increased (P < 0.05) but also the production of L-lactate, pyruvate and urea. Therefore, the results suggest that the decreased availability of glucose precursors, promoted by insulin administration, limits the participation of hepatic gluconeogenesis to glycemia recovery. However, the administration of gluconeogenic precursors could overcome this limitation and promote better glycemia recovery than glucose itself.     

Role of gluconeogenesis in epinephrine-stimulated hepatic glucose production in humans

To evaluate the contribution of gluconeogenesis to epinephrine-stimulated glucose production, we infused epinephrine (0.06 micrograms X kg-1 X min-1) for 90 min into normal humans during combined hepatic vein catheterization and [U-14C]alanine infusion. Epinephrine infusion produced a rise in blood glucose (50-60%) and plasma insulin (30-40%), whereas glucagon levels increased only at 30 min (19%, P less than 0.05). Net splanchnic glucose output transiently increased by 150% and then returned to base line by 60 min. In contrast, the conversion of labeled alanine and lactate into glucose increased fourfold and remained elevated throughout the epinephrine infusion. Similarly, epinephrine produced a sustained increase in the net splanchnic uptake of cold lactate (four- to fivefold) and alanine (50-80%) although the fractional extraction of both substrates by splanchnic tissues was unchanged. We conclude that a) epinephrine is a potent stimulator of gluconeogenesis in humans, and b) this effect is primarily mediated by mobilization of lactate and alanine from extrasplanchnic tissues. Our data suggest that the initial epinephrine-induced rise in glucose production is largely due to activation of glycogenolysis. Thereafter, the effect of epinephrine on glycogenolysis (but not gluconeogenesis) wanes, and epinephrine-stimulated gluconeogenesis becomes the major factor maintaining hepatic glucose production.     

Non-invasive techniques for assessing carbohydrate flux:

Glycogen forms the smallest yet most labile energy substrate store. Therefore studying carbohydrate flux may be crucial to understanding the regulation of energy balance. Indirect calorimetry has been used to measure carbohydrate oxidation overnight and during exercise in nine fasted subjects. Overnight carbohydrate oxidation (averaging 2.85 ± 0.8 g h^-1) was assumed to be derived primarily from hepatic glycogen since subjects were inactive or asleep, and since glucose oxidized after gluconeogenesis from protein is measured as protein oxidation. Lower-limb muscle glycogen stores were depleted by repeated 30-min periods of cycle ergometry at 45%Vo_2max until exhaustion (8 ± 1 periods). The carbohydrate oxidation rate decreased as exercise progressed. Quadratic curves yielded a close fit to each individual's exercise carbohydrate depletion data (mean multiple correlation r= 0.9996) and provided excellent inter-subject discrimination. Total (muscle plus liver) glycogen stores prior to exercise were estimated by extrapolation of the depletion curves to zero oxidation rate. This produced an estimate (174 ± 61 g) which compared well with predictions (208 ± 43 g) based on reference values for muscle mass and initial glycogen content. The results demonstrate that non-invasive estimates of glycogen status can be obtained from accurate respiratory exchange data.     

The influence of glucagon on hepatic glycogen mobilization in exercising rats

Summary The significance of glucagon for the alterations in carbohydrate and fat metabolism during swimming has been evaluated. Fed, male rats were used. Blood was drawn by cardiac puncture for glucose analysis and either rabbit-antiglucagonserum (A-rats) or normal rabbitserum (N-rats) injected. Twentynine rats were then forced to swim (S-rats) with a tail weight for 60 min, while 16 rats were resting controls (C-rats). Subsequently blood was drawn and samples of liver and muscle tissue collected. In SN-rats glucagon concentrations increased from 152±18 (S.E.) pg/ml (CN-rats) to 332±61 (P<0.05), while liver glycogen decreased (P<0.001) and blood glucose increased (P<0.05). In SA-rats, however, the changes in liver glycogen and blood glucose were halved indicating that increased glucagon secretion enhances hepatic glycogen depletion during prolonged exercise. NEFA rose in SA-rats (P<0.005) as well as in SN-rats (P<0.05). Glycerol concentrations, however, only increased in SA-rats (P<0.05) indicating a shift towards lipid combustion in antibody treated rats. Muscle glycogen and plasma insulin diminished and blood lactate increased uniformly in exercised rats.     

Sympathetic control of metabolic and hormonal responses to exercise in rats

The importance of the sympatho-adrenal system for the pancreatic hormonal response to exercise and, furthermore, the role of glucagon and catecholamines for the hepatic glycogen depletion during exercise were studied. Rats were either surgically adrenomedullectomized and chemically sympathectomized with 6-hydroxydopamine or shamtreated. Two weeks later the rats had either rabbit-antiglucagon serum or normal rabbit serum injected. Subsequently the rats either rested or swam with a tail weight for 75 min. Immediately afterwards cardiac blood was drawn and liver and muscle tissue collected. In control rats in spite of an increase in blood glucose concentrati4ns during exercise plasma insulin concentrations were unchanged, while glucagon concentrations increased. In sympathectomized rats, compared to control rats, glucagon concentrations increased less, and insulin concentrations were higher, although glucose concentrations were lower during exercise. Sympathectomy completely abolished the exercise-induced decrease in liver and muscle glycogen concentrations, whereas neither glycogen depletion nor plasma catecholamine concentrations were influenced by the administration of glucagon antibodies. These findings indicate that the sympatho-adrenal system enhances glucagon secretion as well as muscular and hepatic glycogen depletion but inhibits insulin secretion in exercising rats. The increase in glucagon concentrations, however, does not enhance hepatic glycogen depletion at the work load used.     

Hepatic carbohydrate metabolism in the spontaneously diabetic Bio-Breeding Worcester rat

The effects of diabetes on hepatic carbohydrate metabolism were investigated in spontaneously diabetic Bio-Breeding Worcester (BB/W) rats. The juvenile-onset-type syndrome displayed by these animals is characterized by beta-cell destruction with subsequent ketosis-prone insulinopenia. Livers from diabetic animals demonstrated increased adenosine 3',5'-cyclic monophosphate levels but subnormal total protein and glycogen content. Isolated perfused livers of diabetic BB/W rats demonstrated an increased rate of glucose production from [14C]lactate and an impaired rate of glycogen synthesis. These data were consonant with hepatic enzyme studies demonstrating markedly increased activities of component gluconeogenic (glucose-6-phosphatase, fructose-1,6-diphosphatase, phosphoenolpyruvate carboxykinase) and glycogenolytic (glycogen phosphorylase) enzymes with decreased activities of glycolytic (hexokinase, pyruvate kinase) and glycogenic (glycogen synthase) enzymes. These findings agree with previous studies using alloxan- and streptozotocin-induced diabetic animals and suggest that accelerated hepatic gluconeogenesis and impaired glucose utilization are pathognomonic of all insulin-deficient diabetic syndromes.     

Glucose load diverts hepatic gluconeogenic product from glucose to glycogen in vivo.

Intravenous or oral administration of concentrated glucose solution into fasted rats simultaneously injected with 14C-bicarbonate resulted in an inhibition of [14C]glucose release into the blood and in an accelerated [14C]glycogen formation associated with glycogen synthetase activation and phosphorylase inactivation in the liver. The specific activity of glycogen was much higher than that of blood glucose after the glucose load, indicating that glycogen originated from gluconeogenesis rather than blood glucose. These metabolic changes induced by the glucose load were not mediated by endogenous insulin because they were observed to the same extent in rats treated with anti-insulin serum. However, they were mostly, if not totally, abolished by adrenalectomy, which suppressed gluconeogenesis and glycogenesis. Glucose tolerance was markedly impaired not only by anti-insulin serum, which inhibits peripheral glucose utilization, but also by adrenalectomy, which affects hepatic metabolism. It is concluded that a glucose load diverts the final product of hepatic gluconeogenesis from blood glucose to liver glycogen; these metabolic changes in the liver are an important determinant of glucose tolerance.