Abnormalities of carbohydrate metabolism in spontaneously hypertensive rats

Summary The present study was performed to investigate as to whether peripheral insulin resistance exists in spontaneously hypertensive rats (SHR). After a 12 h fasting period, SHR had significantly higher serum glucose and higher plasma glucagon values in comparison to normotensive control rats (WKY). There was a tendency for higher serum insulin concentrations as well, but this difference did not reach significance. After oral glucose loading or glucose/insulin administration, serum glucose and insulin levels were also higher in SHR compared to WKY rats. Muscle glycogen and glucose concentrations were identical in fasted SHR and WKY rats. With an oral glucose load or glucose/insulin treatment there was a significant increase in muscle glycogen, whereas glucose values declined in skeletal muscle. Both total (a+b-form) phosphorylase activity as well as the active a-form of the enzyme were similar in skeletal muscle of SHR and WKY rats. Glucose/insulin administration or oral glucose loading induced a considerable reduction of both a+b-form and a-form activities. The decrease in muscle phosphorylase activities was almost identical in both groups of animals. There was also a comparable activity of muscle glycogen synthetase activity in all groups of rats. Despite subtile changes of glucose, glucagon and to a lesser degree insulin levels which would be suggestive of insulin resistance, the data obtained from skeletal muscle argue against peripheral insulin resistance in spontaneously hypertensive rats.     

Activity of glycogen synthase and glycogen phosphorylase in normal and cirrhotic rat liver during glycogen synthesis from glucose or fructose

Cirrhotic patients often demonstrate glucose intolerance, one of the possible causes being a decreased glycogen-synthesizing capacity of the liver. At the same time, information about the rates of glycogen synthesis in the cirrhotic liver is scanty and contradictory. We studied the dynamics of glycogen accumulation and the activity of glycogen synthase (GS) and glycogen phosphorylase (GP) in the course of 120 min after per os administration of glucose or fructose to fasted rats with CCl₄-cirrhosis or fasted normal rats. Blood serum and liver pieces were sampled for examinations. In the normal rat liver administration of glucose/fructose initiated a fast accumulation of glycogen, while in the cirrhotic liver glycogen was accumulated with a 20 min delay and at a lower rate. In the normal liver GS activity rose sharply and GPa activity dropped in the beginning of glycogen synthesis, but 60 min later a high synthesis rate was sustained at the background of a high GS and GPa activity. Contrariwise, in the cirrhotic liver glycogen was accumulated at the background of a decreased GS activity and a low GPa activity. Refeeding with fructose resulted in a faster increase in the GS activity in both the normal and the cirrhotic liver than refeeding with glucose. To conclude, the rate of glycogen synthesis in the cirrhotic liver is lower than in the normal one, the difference being probably associated with a low GS activity.     

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.     

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.     

耗竭心肌细胞内糖原对缺血后再灌注心肌的保护作用及其机制

本工作在离体大鼠等容收缩心脏模型上观察耗竭心脏细胞内糖原对心肌缺血再灌注损伤的影响。心脏富氧灌流30 min后,随机分为三组:Ⅰ、富氧组:富氧灌流75min。Ⅱ、对照组:常规K-H液富氧灌流15min,旷置30min,再灌流30min。Ⅲ、耗竭糖原组:先用充N_2(95%N_2:5%CO_2)的K-H液灌流15min,余同组Ⅱ。结果表明。耗竭心肌细胞内糖原可提高再灌注后心脏血液动力学的恢复;冠脉流出液中LDH活性及心肌组织MDA含量降低,线粒体及胞浆液中GSH-Px有较高的活性;心肌组织Na~+,Ca~(2+)超负荷减轻。说明耗竭心肌细胞内糖原可通过减少细胞内H~+的生成抑制Na~+/H~+交换,从而明显减轻心肌缺血再灌注损伤。     

Effects of age on glycogen synthase and phosphorylase activities in rat liver

Six exopeptidases present in human diploid fibroblasts were identified by separation on polyacrylamide gel electrophoresis and their activity profiles against 17 dipeptides, two tripeptides and L-leucine-p-nitroanilide determined. No differences in relative activity or in the electrophoretic patterns of any of the six exopeptidases were detected with ageing. Aminoacylarylamidase activity assayed spectrophotometrically showed significantly increased activity in the middle age-group cells as opposed to the enzyme isolated from young and old cells. Heat-inactivation studies using the same substrate suggested the possibility of an increased proportion of heat-labile enzyme in the old cells but interpretation of the data was difficult because of the complex nature of the inactivation curves obtained. Overall, the results tended to refute the hypothesis that age-related changes in the free amino acid pool of human diploid fibroblasts were associated with significant alterations in the activities of cellular exopeptidases.     

吡咯烷二硫代氨基甲酸酯对2型糖尿病大鼠肝糖原合成的影响

目的:探讨吡咯烷二硫代氨基甲酸酯(PDTC)对糖尿病大鼠肝糖原合成的影响及其机制。方法:雄性Wistar大鼠,随机分为2组:正常饮食组和高脂饮食组。喂养8周后,高脂饮食组大鼠腹腔注射单剂量链脲佐菌素(STZ)27 mg/kg复制2型糖尿病大鼠模型,2型糖尿病大鼠造模成功后随机分为3组:糖尿病模型组、PDTC治疗组和胰岛素治疗组。PDTC治疗组大鼠每天腹腔注射PDTC(50 mg/kg)1次;其它各组每天同一时间注射相同体积的生理盐水,胰岛素治疗组大鼠在处死前1 h腹腔注射胰岛素(1 U/kg)1次。治疗1周后尾静脉采血测定各组大鼠血糖水平,然后断头处死大鼠,测定肝组织中肝糖原的含量,采用Western blotting分析大鼠肝脏中蛋白激酶B(PKB/Akt)和糖原合成酶激酶-3β(GSK-3β)磷酸化水平的变化。结果:糖尿病模型组与正常饮食组大鼠相比血糖显著升高(P0.01);肝糖原含量明显减少(P0.01);肝脏中Akt及GSK-3β磷酸化水平明显降低(P0.01)。与糖尿病模型组大鼠相比,PDTC治疗组与胰岛素治疗组大鼠肝糖原合成均显著增加(P0.01);血糖均明显降低(P0.01);肝脏中Akt和GSK-3β磷酸化水平均明显增加(P0.01)。结论:PDTC可通过调控Akt/GSK-3β活性,增加肝糖原合成,降低血糖。     

Increased glucagon secretion during hyperthermia in a sauna

Summary Plasma glucagon, adrenaline, noradrenaline, insulin and glucose concentrations were measured in 7 healthy young males during hyperthermia in a sauna bath: plasma glucagon levels increased from baseline values of 127.0±12.9 (SEM) pg · ml⁻¹ to a maximum of 173.6±16.1 (SEM) pg · ml⁻¹ at the 20th min of exposure. No change in plasma insulin and a slight increase in plasma glucose concentration were seen. Since a concomitant moderate increase in plasma catecholamine levels was also present, the adrenergic stimulus is believed to trigger glucagon release during hyperthermia. Diminished visceral blood flow, known to occur in sauna baths, may cause a decrease in the degradation of plasma glucagon and thus contribute to the elevated plasma glucagon levels.     

The effect of exercise training on glycogen,glycogen synthase and phosphorylase in muscle and liver

Summary It is thought that exercise training in both man and the rat results in a protective effect against the depletion of carbohydrate stores during exercise (glycogen-sparing). However there has been no comprehensive study of the effects of training on glycogen anabolic and catabolic enzymes with liver or muscle. The aim of this study was to examine whether changes in these enzymes occur and whether these changes may provide an explanation for the glycogen-sparing which results from exercise training.Male rats were trained by a treadmill running program at three different workloads. In addition, there were three control groups: free eating (SF), food restricted (SR), and one SF with a single bout of exercise prior to sacrifice.Exercise training was associated with a 60–150% increase in glycogen synthase and phosphorylase and a 50–70% increase in glycogen content in soleus, an intermediate muscle, but not in extensor digitorum longus (EDL), a white muscle nor in liver. The increase in glycogen synthase and phosphorylase in intermediate muscle was proportional to the degree of training and there was a significant correlation between glycogen content, glycogen synthase, and phosphorylase activity in intermediate muscle. Cytochrome c oxidase activity, an indicator of respiratory capacity, increased 50% in gastrocnemius of trained rats and was significantly correlated with glycogen synthase and phosphorylase in soleus.These results indicate a significant effect of exercise training on glycogen anabolic and catabolic enzymes in intermediate muscle, with no significant effects in white muscle or liver. The changes do not provide an explanation for glycogen-sparing, but are consistent with improved capacity of intermediate muscle for rapid glycogen mobilisation and repletion.     

Effect of hyperglycaemia on muscle glycogen mobilization during muscle contractions in the rat

Summary In the rat, muscle glycogen is mobilized during the first stage of exercise, despite normoglycaemia. The aim of the present study was to examine if this process could be prevented or reduced by hyperglycaemia. Three experiments were carried out: in the first, rats were forced to run on a treadmill; in the second the gastrocnemius muscle group was made to contract by stimulation of the sciatic nerve and in the third adrenaline was administered subcutaneously. Each group was divided into two subgroups: control and enriched with glucose (hyperglycaemic). It was shown that hyperglycaemia has no effect on running-induced glycogen mobilization in hind-limb muscles of different fibre composition but prevented it totally in diaphragm muscle. Hyperglycaemia also did not affect the glycogen mobilization induced by stimulation of the sciatic nerve. However, it delayed and reduced markedly the glycogenolytic effect of adrenaline. It is concluded that increased glycogenolysis in muscles at the beginning of exercise may be a consequence of a delay in the activation of glucose transporting mechanisms in muscle cells.     

On the mechanism of glucagon stimulation of hepatic gluconeogenesis

Summary The addition of L-alanine as substrate to a perfused rat liver preparation produced a five-fold increase in the rate of glucose production. This enhancement of the gluconeogenic flux seems to be a consequence of a rise in the steady-state levels of pyruvate and oxaloacetate subsequent to the rise in alanine concentration.Glucagon (2×10^–9 M) increased the gluconeogenic flux from alanine (10 mM) by 50 percent, even though the concentration of the substrate in the perfusion fluid was at saturation. This effect was accompanied by a rise in the intracellular concentrations of alanine. However, the steady-state concentrations of pyruvate and oxaloacetate were decreased, probably as a consequence of a more reduced state of the nicotinamide-nucleotide system. In vivo, the intraperitoneal administration of glucagon to starved rats was accompanied by a decrease in the hepatic alanine and pyruvate concentrations despite the striking effects raising the plasma glucose levels. These observations seem to indicate that the effect of the hormone increasing the hepatic glucose output must be mediated through some other mechanism(s) independent of the intracellular variations in the hepatic amino acids levels.     

Effect of glucose on plasma glucagon and free fatty acids during prolonged exercise

Summary The effects of glucose ingestion on the changes in blood glucose, FFA, insulin and glucagon levels induced by a prolonged exercise at about 50% of maximal oxygen uptake were investigated. Healthy volunteers were submitted to the following procedures: 1. a control test at rest consisting of the ingestion of 100 g glucose, 2. an exercise test without, or 3. with ingestion of 100 g of glucose. Exercise without glucose induced a progressive decrease in blood glucose and plasma insulin; plasma glucagon rose significantly from the 60th min onward (+45 pg/ml), the maximal increase being recorded during the 4th h of exercise (+135 pg/ml); plasma FFA rose significantly from the 60th min onward and reached their maximal values during the 4th h of exercise (2177±144 ΜEq/l, m±SE). Exercise with glucose ingestion blunted almost completely the normal insulin response to glucose. Under these conditions, exercise did not increase plasma glucagon before the 210th min; similarly, the exercise-induced increase in plasma FFA was markedly delayed and reduced by about 60%. It is suggested that glucose availability reduces exercise-induced glucagon secretion and, possibly consequently, FFA mobilization. Chercheur Qualifié of the Fonds National Belge de la Recherche Scientifique     

Meal related effects of IV glucose,glucagon and insulin on blood glucose level: Role in glucoregulation exhibited by self-injecting rats

In previous works, it was demonstrated that rats self-injected with (IV) glucose, insulin and/or glucagon and that the self-administration pattern was related to the meal. It was suggested that the different preprandial or postprandial blood glucose level (B.G.L) determinated the meal and were dependent on it. Previously self-injected amounts of glucose, glucagon and insulin were here injected by an automatic device and their effect on B.G.L. were compared in the post and preprandial conditions. Results indicated that the effects of insulin, glucagon and glucose on B.G.L., which were different during the postprandial hyperglycemia compared to the preprandial normoglycemia, accounted for the different self-injection patterns. Furthermore successive injection of glucose and insulin or glucagon and insulin suggest that rats, by pressing alternatively two levers performed a regulatory correction of variations of their B.G.L. It is concluded that rats may learn to perform a behavioral regulation of their B.G.L. by IV self-injections of glucose and glucoregulatory hormones.     

Phosphagens and glycogen content in skeletal muscle after treadmill training in young and old rats

Summary The concentration of creatine phosphate (CrP), ATP, ADP, AMP and glycogen were measured in extensor digitorum longus (EDL) and in quadriceps muscles of 3, 6, 24, and 27 months old male Wistar rats groups. Young (3 months) and old (24 months) rats were trained for 12 weeks, 3 days a week, with running exercise session. Each training session was of 2 h. In sedendary goups and for both muscles, CrP, ATP, ADP, and glycogen contents decrease with aging (between 6 and 27 months). In spit of an AMP increase, total adenosine nucleotides (TAN) decrease significantly between 6–27 months (P<0.01) from 6.11 to 5.11 (EDL) and from 5.59 to 4.65 (quadriceps) mol·g^–1 wet weight muscle. After 12 weeks of physical training, the mean values of CrP, TAN, and glycogen were improved in both young and old rat groups. Moreover, the ATP/ADP ratios and the energy charge of the adenylate system were unrelated to age, but training decreases significantly the mean value of energy charge in both young and old groups. These results suggest that, as far as energy-rich phosphagen metabolism is concerned young and old muscles show the same pattern response to training.     

Muscle glycogen repletion and pre-exercise glycogen content: effect of carbohydrate loading in rats previously fed a high fat diet

We have recently reported that rates of muscle glycogen repletion during the early period of recovery were increased by carbohydrate (CHO) loading in rats previously fed a high fat diet. However, the reason for this remained unanswered. The purpose of this study was to examine whether an increase of glycogen utilization due to an elevated pre-exercise glycogen store would enhance rates of glycogen repletion in muscle. Despite an equal degree of glycogen depletion, the rates of glycogen repletion of soleus, red and white gastrocnemius muscles by postexercise administration of glucose (3.0 g · kg^–1 body mass) and citrate (0.5 g · kg^–1 body mass) were faster in the CHO loaded (3 days) rats than in the nonloaded rats, as a result of elevated pre-exercise glycogen content and consequently the greater glycogen utilization. The higher rate of muscle glycogen repletion may in part be explained by increased postexercise glycogen synthase activity.     

Adiponectin 基因转染对肌细胞糖原合成和葡萄糖氧化的影响

目的：研究adiponectin对C2C12肌细胞糖原合成和葡萄糖氧化的影响。 方法： 用阳离子脂质体介导转染和随后G418筛选建立稳定转染小鼠adiponectin cDNA真核表达质粒（pcDNA3.0-mad）及空载pcDNA3.0的C2C12细胞株并鉴定。C2C12肌细胞糖代谢实验分对照组、空载体组和pcDNA3.0-mad(mad)组共3组进行，每组又分 0、0.5、5、100 nmol/L胰岛素刺激4个亚组。通过液闪测定细胞合成的糖原中［14C］的放射活性和氧化产生的［14CO2］，分别检测肌细胞的糖原合成和葡萄糖氧化情况。 结果： Western blotting和免疫组化检测证实mad组细胞表达adiponectin蛋白。Mad组葡萄糖氧化量随胰岛素浓度增加的速率较其它两组快，对照、空载体和mad组线性回归系数分别为23.34、23.23和26.06。Mad组C2C12肌细胞基础状态下和胰岛素刺激下的葡萄糖氧化和糖原合成与其它两组无显著差异（P>0.05）。 结论： 转染adiponectin基因对C2C12肌细胞葡萄糖氧化和糖原合成无显著影响。     

Decreased feeding in rats following hepatic-portal infusion of glucagon

Male rats received a single hepatic-portal injection of glucagon following mild food deprivation. Cumulative food intake measured after 0.5–20 hr was decreased by the hormone. The absence of a concomitant decrease in water intake suggested a specific effect of glucagon on feeding. This specificity was further demonstrated by the use of an hepatic-portal infusion of glucagon as the unconditioned stimulus for the formation of a conditioned taste aversion which failed to produce avoidance of a novel taste. In contrast, pairing the taste with an intraperitoneal injection of lithium chloride did produce a learned taste aversion. Thus, the decreased feeding following infusion of a low concentration of pancreatic glucagon through a chronic hepatic-portal cannula cannot be attributed to visceral malaise. The relatively specific effect of this hormone on short-term feeding probably results from the activation of hepatic glycogenolysis, with the long-term effect on feeding possibly due to gluconeogenesis.     

Fuel substrate turnover and oxidation and glycogen sparing with carbohydrate ingestion in non-carbohydrate-loaded cyclists

This study examined the effects of ingesting 500 ml/h of either a 10% carbohydrate (CHO) drink (CI) or placebo (PI) on splanchnic glucose appearance rate (endogenous + exogenous) (R _a), plasma glucose oxidation and muscle glycogen utilisation in 17, non-carbohydrate-loaded, male, endurance-trained cyclists who rode for 180 min at 70% of maximum oxygen uptake. Mean muscle glycogen content at the start of exercise was 130 ± 6 mmol/kg ww; (mean ± SEM). Total CHO oxidation was similar in CI and PI subjects and declined during the trial. R _a increased significantly during the trial (P < 0.05) in both groups. Plasma glucose oxidation also increased significantly during the trial, reaching a plateau in the PI subjects, but was significantly (P < 0.05) higher in CI than PI subjects at the end of exercise [(98 ± 14 vs. 72 ± 10 μmol/min/kg fat-free mass) (FFM) (1.34 ± 0.19 vs. 0.93 ± 0.13 g/min)]. However, mean endogenous R _a was significantly (P < 0.05) lower in the CI than PI subjects throughout exercise (35 ± 7 vs. 54 ± 6 μmol/min/kg FFM), as was the oxidation of endogenous plasma glucose, which remained almost constant in CI subjects, and reached values at the end of exercise of 42 ± 13 and 72 ± 10 μmol/min/kg FFM in the CI and PI groups respectively. Of the 150 g CHO ingested during the trial, 50% was oxidised. Muscle glycogen disappearance was identical during the first 2 h of exercise in both groups and continued at the same rate in PI subjects, however no net muscle glycogen disappearance occurred during the final hour in CI subjects. We conclude that ingestion of 500 ml/h of a 10% CHO solution during prolonged exercise in non carbohydrate loaded subjects has a marked liver glycogen-sparing effect or causes a reduction in gluconeogenesis, or both, maintains plasma glucose concentration and has a muscle glycogen-sparing effect. Received: 25 August 1995/Received after revision: 25 March 1996/Accepted: 29 April 1996     

The influence of free fatty acids on glycogen recovery in rat heart after exercise

Summary Glycogen supercompensation is the term used to denote the abnormally high levels of glycogen found in the heart shortly after an exercise-induced reduction of the substrate. Using rats, we tested whether this condition was linked to the use of plasma free fatty acids (FFA), which normally rise with exercise. Before a 1-h swim, animals received an injection of either saline (S) or nicotinic acid (NA). The nicotinic acid treatment dramatically suppressed the rise in plasma FFA observed in the S-group. Exercise caused a significant but similar reduction (35–38%) of the myocardial glycogen content in both groups. After 1 h of recovery in the S-group, myocardial glycogen reached a value of 30.3±1.7 Μmol·g⁻¹ or 113% of that measured before the exercise began. In contrast, the value for hearts from the NA-group with reduced FFA levels was 24.0±1.9 Μmol·g⁻¹ or only 91% of that measured before exercise. After 2 h the values were 33.8±1.4 and 29.0±1.9 Μmol·g⁻¹ respectively. These data indicate that glycogen repletion in rat heart after exercise is related to the amount of FFA present in the plasma. We suggest that carbohydrate metabolism is diverted towards synthesis and storage as a result of the glycolytic inhibition exerted by the increased use of fat as an energy source as previously observed in hearts from fasted or diabetic animals. This work was supported by a grant from the Utah Heart Association and the Deseret Gym Corporation     

Effects of glucagon on hepatic glycogen and smooth endoplasmic reticulum

The action of glucagon on hepatic glycogen and smooth endoplasmic reticulum (SER) was studied in samples of liver taken sequentially from anesthetized rats. The physiological state of each animal was assessed by continuously monitoring aortic blood pressure and blood lactate/pyruvate ratios. High hepatic glycogen levels were established by using 10–12 hr fasted control-fed rats infused continuously with glucose. In rats receiving glucose only, hepatic glycogen levels remained above 5.0% during the 4-hr period of glucose administration. Centrilobular hepatocytes displayed an abundance of glycogen which often appeared dispersed with elements of SER between the glycogen particles. Periportal cells had dense clumps of glycogen with few vesicles of SER restricted to the periphery of the glycogen masses. The addition of glucagon to the glucose infusate caused a marked stimulation of glycogenolysis. In these rats, the hepatic glycogen level (x?±SE) was 6.71±.15% 1 hr after glucose and declined after initiation of glucagon infusion as follows: 5.86±.29% (15 min), 4.89±.26% (1 hr), 2.16±.40% (2 hr), and 1.66±.29% (3 hr). The fine structure of hepatocytes showed a dramatic response to the administration of glucagon. The glycogen regions of the cells were noticeably decreased in size and number of glycogen granules 3 hr after initiation of glucagon infusion, and SER was abundant in both periportal and centrilobular hepatocytes. The interpretation offered is that glucagon induces the formation of new SER membranes which participate in glycogen breakdown and/or glucose release from hepatocytes.