The activity of mitochondrial enzymes in the muscles of rats subjected to physical training and subchronical intoxication with lead and zinc

The aim of this study was to evaluate the effect of lead and excess zinc on the adaptation of mitochondria from skeletal muscles to physical effort. Rats were intoxicated once a week for 12 weeks by subcutaneous injection of the solution containing 2 mg Zn+2 and/or 3 mg Pb2+ per kg of body weight. During the last 6 weeks, 6 times weekly, rats performed endurance training which involved swimming 15 minutes daily with additional load of 5% of the body weight. The activities of isocitrate (ICD), malate (SDH), succinate (MDH) dehydrogenases, cytochrome oxidase (COX) and protein content (PM) were determined in the mitochondrial fractions obtained from the soleus muscle (ST fibres), and from the superficial (FTb fibres) and deep (FTa fibres) parts of the gastrocnemius muscle. In the control group (C), which was injected with saline, higher activities of ICD and MDH were obtained in FTa and FTb fibres than in the ST fibres. SDH and COX had higher activities in FTa and ST compared to FTb fibres. Zinc treatment (Zn) caused diminution of ICD, SDH and COX activities in ST fibres. Lead intoxication (Pb) resulted in a decrease of MDH activity in all fibre types, and in a decrease of SDH activity in ST fibres. Simultaneous action of zinc and lead produced an increase in ICD activity and diminution of COX activity in FTb fibres. It also resulted in an increase of SDH and decrease of COX activity in ST fibres. These results suggest that the ST fibres are more susceptible to disturbances of adaptation to physical exercise caused by zinc and lead. There are no signs of uniform antagonism between zinc and lead action in the processes under investigation.     

The influence of motor unit composition and stature on fractionated patellar reflex times in untrained men

The purpose of the present study was to investigate the influence of muscle fibre composition and stature on fractionated patellar reflex times in ten healthy untrained men (mean age: 23.3 years, SD 3.1; mass: 65.9 kg, SD 8.5; height: 172.3 cm, SD 5.3). Biopsies were taken from the right vastus lateralis muscle. Using staining for myofibrillar adenosine triphosphatase after pre-incubation at pH 4.3 and 4.6, muscle fibres were classified into slow twitch (ST), fast twitch, oxidative-glycolytic (FTa) and fast twitch, glycolytic (FTb) fibres. Total patellar reflex time (TRT) and its fractionated components--reflex latency (LAT) and reflex motor time (MT)--were obtained from the mean of ten trials in each subject whilst performing Jendrassik's maneuvre. The TRT, LAT and MT were 77.7 ms, SD 16.5, 23.4 ms, SD 1.3 and 54.2 ms, SD 16.3, respectively. The LAT was significantly correlated to the percentage number of ST (r = 0.758, P less than 0.05) and FTa fibres (r = -0.657, P less than 0.05), fast twitch:slow twitch ratio (r = -0.799, P less than 0.01) and to the height of the subjects (r = 0.901, P less than 0.001), whereas TRT and MT were not significantly correlated with either fibre types or the height of the subjects. From these results it can be concluded that the LAT during the patellar reflex is influenced by muscle fibre composition and the length of the sensory and/or motor nerve.     

Glycogen synthesis from lactate in skeletal muscle of the lizardDipsosaurus dorsalis

Summary The capacity of skeletal muscle to synthesize glycogen from lactate was tested in the iliofibularis muscle of the desert iguana,Dipsosaurus dorsalis. Like other reptiles,Dipsosaurus accumulates significant lactic acid concentrations following vigorous exercise. After 5 min of progressively faster treadmill running at 35°C (final speed=2.2 km/h), blood lactate concentration increased over 14 mM, which decreased 11 mM after 2 h of recovery. Blood glucose concentration remained unchanged throughout at 8.6±0.46 mM. The role that muscle gluconeogenesis might play in the removal of post-exercise lactate was evaluated. Animals were run to exhaustion at 1.5 km/h on a treadmill thermostatted at 35°C. Animals (n=43) ran 6.9±0.75 min prior to exhaustion. Animals were sacrificed and iliofibularis muscles of both hindlimbs removed and stimulated at 2 Hz for 5 min, reducing twitch tension to 6% of prestimulus tension. Fatigued muscles were then split into red and white fiber bundles and incubated 2 h or 5 h at 35°C in Ringer solution or in Ringer plus 20 mM lactate. In muscles tested in August, red fiber bundles incubated in lactate demonstrated a rate of glycogen synthesis of approximately 1 mg/(g muscle·h). In muscles tested in December, red fiber bundles synthesized glycogen at a reduced rate that was not statistically different than in fiber bundles incubated in Ringer solution without lactate. Glycogen synthesis from lactate was not evident in white fiber bundles in either August or December. The period of peak gluconeogenic capacity coincides with the field active season ofDipsosaurus. In vivo rates of lactate removal and in vitro rates of glycogen synthesis suggest that muscle gluconeogenesis may potentially account for 20% of the lactate removed during recovery from exhaustive activity.Abbreviations IF iliofibularis - FG fast twitch, glycolytic - FOG fast twitch, oxidative-glycolytic     

Metabolic types of muscle in the sheep: I. Myosin ATPase, glycolytic, and mitochondrial enzyme activities

The metabolic characteristics of 12 skeletal muscles of the sheep were studied. Glycolytic activities (hexokinase, glycogen synthetase I and D, phosphorylase a and b, phosphofructokinase) were measured. Myofibrillar ATPase activity was evaluated. Oxygen consumption, respiratory control and carnitine palmityl transferase, isocitrate dehydrogenase, succinate dehydrogenase and cytochrome oxidase activities were measured in isolated mitochondria. Three metabolic types could be distinguished; (1) essentially oxidative slow twitch muscles, typified by the supraspinatus and infraspinatus, having low ATPase activity, (2) fast twitch red muscles, typified by the longissimus dorsi and the semimembranosus, having a higher ATPase activity and both high oxidative and high glycolytic activity, and (3) essentially glycolytic fast twitch muscles, typified by the tensor fascia lata and the semitendinosus, having the highest ATPase activity.     

Glyconeogenic and glycogenic enzymes in chronically active and normal skeletal muscle

The chronically active (pseudomyotonic) gastrocnemius muscle in the C57B16J dy2J/dy2J mouse contains both elevated lactate and glycogen as well as fibers that have high amounts of glycogen and enhanced glyconeogenic activity. In the present study we analyze the activities of some key glyconeogenic enzymes to assess the causes of elevated muscle glycogen and to determine the pathway for glycogen synthesis from lactate. Glycogen synthase, malate dehydrogenase, phosphoenolpyruvate carboxykinase, and malic enzyme were all elevated in homogenates of the chronically active muscle. Activities of glycogen phosphorylase and fructose 1,6-bisphosphatase were decreased in whole muscle homogenates. Histochemistry demonstrated that the high-glycogen fibers were typically fast-twitch glycolytic fibers that had high glycogen synthase, glycogen phosphorylase, and malic enzyme activities. Malate dehydrogenase activity followed succinate dehydrogenase activity and did not correlate to high-glycogen fibers. Thus the high-glycogen fibers have an elevated enzymatic capacity for glycogen synthesis from lactate, and the pathway may involve use of the pyruvate kinase bypass enzymes.     

Biochemical adaptation in the skeletal muscle of rats depleted of creatine with the substrate analogue beta-guanidinopropionic acid.

Rats were fed on a diet containing 1% beta-guanidinopropionic acid (GPA), a creatine substrate analogue, for 6-10 weeks to deplete their muscle of creatine. This manipulation was previously shown to give a 90% decrease in [phosphocreatine] in skeletal and cardiac muscle and a 50% decrease in [ATP] in skeletal muscle only. Maximal activities of creatine kinase and of representative enzymes of aerobic and anaerobic energy metabolism were measured in the superficial white, medial and deep red portions of the gastrocnemius muscle, in the soleus and plantaris muscle and in the heart. Fast-twitch muscles were smaller in GPA-fed animals than in controls, but the size of the soleus muscle was unchanged. The activities of aerobic enzymes increased by 30-40% in all fast-twitch muscle regions except the superficial gastrocnemius, but were unchanged in the soleus muscle. The activities of creatine kinase and phosphofructokinase decreased by 20-50% in all skeletal-muscle regions except the deep gastrocnemius, and the activity of glycogen phosphorylase generally paralleled these changes. There were no significant changes in the activities of any of the enzymes measured in the heart. The glycogen content of the gastrocnemius-plantaris complex was increased by 185% in GPA-fed rats. The proportion of Type I fibres in the soleus muscle increased from 81% in control rats to 100% in GPA-fed rats, consistent with a previous report of altered isometric twitch characteristics and a decrease in the maximum velocity of shortening in this muscle [Petrofsky & Fitch (1980) Pflugers Arch. 384, 123-129]. We conclude that fast-twitch muscles adapt by a combination of decreasing diffusion distances, increasing aerobic capacity and decreasing glycolytic potential. Slow-twitch muscles decrease glycolytic potential and become slower, thus decreasing energy demand. These results suggest that persistent changes in the [phosphocreatine] and [ATP] are alone sufficient to alter the expression of enzyme proteins and proteins of the contractile apparatus, and that fibre-type-specific thresholds exist for the transformation response.     

Subgrouping of fast twitch fibres in skeletal muscles of man

Summary Subgroups of fast twitch (FT) muscle fibres were identified by histochemical techniques on muscle samples of m. quadriceps femoris from six male and six female subjects, who had been assigned to three groups; untrained, active and well trained (endurance runners). Slow twitch (ST) and FT fibres were initially identified using the histochemical stain for myofibrillar ATPase, preincubated at pH 10.3 and 4.3. Three people, working independently, then identified the subgroups FTa and FTb on the basis of the staining intensity for only one of the following reactions: -glycerophosphate dehydrogenase, -GPD; NADH tetrazolium reductase, NADH-TR; and myofibrillar ATPase preincubated at pH 4.6, ATPase (4.6). FTa fibres were clearly distinguished from the darker staining FTb fibres using the ATPase (4.6) reaction. Differences in the staining within the FT fibres using the -GPD and NADH-TR reactions were more subtle, and differences between subject groups were evident. The percentage of FTa fibres was overestimated for the untrained and underestimated for the well trained subjects using NADH-TR. With the -GPD stain the percentage of FTa fibres was generally underestimated. When the data for all three stains were compared, only 27% of the FT fibres were placed in the same subgroups. These results demonstrate that the subgrouping of FT fibres in man is more reliable using the differences in pH sensitivity for the myofibrillar ATPase reaction compared to histochemical reactions for oxidative or glycolytic enzymes.     

Metabolic characteristics of the myocardium and muscle tissue of the thigh in rats

Myocardium and skeletal muscle of white rats have a number of specific features in metabolism of carbohydrates. The skeletal muscle is characterized by high intensity of glycolytic processes and glycolytic substrate phosphorylation, that is testified to by the activity of the terminal glycolysis stage enzymes (pyruvate kinase, lactate dehydrogenase, its isoenzyme spectrum) and by the content of lactate and pyruvate metabolites. In contrast to skeletal muscles, the activity of NAD-dependent malate dehydrogenase in the myocardium is significant both in cytoplasm and in mitochondria. This activity corresponds to a high level of malate and oxaloacetate metabolites and to the activity of NADP-dependent malate dehydrogenase, playing a connective role between glycolysis, the cycle of tricarboxylic acids and glyconeogenesis. Phosphoenolpyruvate carboxykinase, catalyzing the transformation of cytoplasmatic oxaloacetate into phosphoenolpyruvate is more active in the skeletal muscles where the intensity of the tricarboxylic acids cycle reactions is lower and the activity of glycolysis is higher than that of myocardium.     

A comparative study of glycolysis in red and white muscles of the trout (Salmo gairdneri) and mirror carp (Cyprinus carpio)

The activities of some glycolytic and associated enzymes have been determined in the muscles of trout and carp to investigate the possibility that the discrepancies previously reported between lactate accumulation and anoxic tolerance in these two fish result from underlying differences in glycolytic potential. Steady state concentrations of certain glycolytic intermediates were also determined in freeze-clamped muscles from tankrested fish. The activities of hexokinase, phosphorylase and phosphofructokinase were approximately 2–3 times lower in carp than trout white muscles. Pyruvate kinase and lactate dehydrogenase activities were 5 times lower in carp white muscle. The lower, broader pH optima of lactate dehydrogenase and pyruvate kinase from carp compared to trout muscles is thought to be correlated with the greater anoxic tolerance of the carp. Glycolytic enzyme profiles were markedly different between the red and white muscles of the rainbow trout but broadly similar, with the exception of hexokinase activity, for the corresponding muscles of the carp. The results are discussed in relation to what is known about anaerobiosis in these two species and the comparative physiology of red and white muscles in fish.     

Proteomic analysis of slow- and fast-twitch skeletal muscles

Skeletal muscles are composed of slow- and fast-twitch muscle fibers, which have high potential in aerobic and anaerobic ATP production, respectively. To investigate the molecular basis of the difference in their functions, we examined protein profiles of skeletal muscles using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and two-dimensional gel electrophoresis with pH 4-7 and 6-11 isoelectric focusing gels. A comparison between rat soleus and extensol digitorum longus (EDL) muscles that are predominantly slow- and fast-twitch fibers, respectively, showed that the EDL muscle had higher levels of glycogen phosphorylase, most glycolytic enzymes, glycerol 3-phosphate dehydrogenase, and creatine kinase; while the soleus muscle had higher levels of myoglobin, TCA cycle enzymes, electron transfer flavoprotein, and carbonic anhydrase III. The two muscles also expressed different isoforms of contractile proteins including myosin heavy and light chains. These protein patterns were further compared with those of red and white gastrochnemius as well as red and white quadriceps muscles. It was found that metabolic enzymes showed a concerted regulation dependent on muscle fiber types. On the other hand, expression of contractile proteins seemed to be independent of the metabolic characteristics of muscle fibers. These results suggest that metabolic enzymes and contractile proteins show different expression patterns in skeletal muscles.     

Histochemical characteristics of human expiratory and inspiratory intercostal muscles

The relative occurrence of slow-twitch (ST) and fast-twitch (FTa and FTb) fibers, fiber size, and capillary supply in internal (INT) and external intercostal muscles (EXT), the costal diaphragm (DIA), and vastus lateralis muscle (VAS) was examined post-mortem in eight healthy males. The relative occurrence of ST fibers in INT [64 +/- 3% (SE)] and EXT (62 +/- 3%) was similar but higher than in DIA (49 +/- 3%) and VAS (40 +/- 6%; P less than 0.05). The occurrence of FTa fibers in expiratory INT (35 +/- 3%) was higher than in inspiratory INT and EXT (17 +/- 1%; P less than 0.05) but similar to DIA (28 +/- 6%) and VAS (32 +/- 2%). Accordingly, expiratory INT had fewer FTb fibers (1 +/- 1%) than the others (P less than 0.05). Expiratory INT had a 60% larger fiber area than inspiratory INT and EXT and DIA (P less than 0.05), but the area was similar to that of VAS. The number of capillaries per fiber was higher in expiratory INT (2.3 +/- 0.1) than in inspiratory INT and EXT (1.6 +/- 0.1), DIA (1.9 +/- 0.1), and VAS (1.8 +/- 0.2; P less than 0.05). The results suggest that the occurrence of many large capillary-rich FTa fibers in expiratory INT is bound to function (expiratory vs. inspiratory) rather than to anatomy (INT vs. EXT).     

Histochemical, enzymatic, and contractile properties of skeletal muscle fibers in the lizard Dipsosaurus dorsalis

Lizard skeletal muscle fiber types were investigated in the iliofibularis (IF) muscle of the desert iguana (Dipsosaurus dorsalis). Three fiber types were identified based on histochemical staining for myosin ATPase (mATPase), succinic dehydrogenase (SDH), and alphaglycerophosphate dehydrogenase (alphaGPDH) activity. The pale region of the IF contains exclusively fast-twitch-glycolytic (FG) fibers, which stain dark for mATPase and alphaGPDH, light SDH. The red region of the IF contains fast-twitch-oxidative-glycolytic (FOG) fibers, which stain dark for all three enzymes, and tonic fibers, which stain light for mATPase, dark for SDH, and moderate for alphaGPDH. Enzymatic activities of myofibrillar ATPase, citrate synthase, and alphaGPDH confirm these histochemical interpretations. Lizard FG and FOG fibers possess twitch contraction times and resistance to fatigue comparable to analogous fibers in mammals, but are one-half as oxidative and several times as glycolytic as analogous fibers in rats. Lizard tonic fibers demonstrate the acetylcholine sensitivity common to other vertebrate tonic fibers.     

Geographic distribution of xanthine oxidase, free radical scavengers, creatine kinase, and lactate dehydrogenase enzyme systems in rat heart and skeletal muscle.

The geographic distribution of the following enzyme systems is described in the rat heart (left and right ventricles) and in different skeletal muscles (soleus, plantaris, and red and white gastrocnemius): xanthine oxidase and dehydrogenase, creatine kinase isoenzymes, lactate dehydrogenase isoenzymes, and the free radical scavenger enzymes superoxide dismutase, glutathione reductase, and glutathione peroxidase. No substantial difference in enzyme activities was observed between the left and right ventricles. Skeletal muscles showed a clear distinction between enzyme activities depending on their composition of oxidative fibers and glycolytic fibers.     

Histochemical localization of lactate dehydrogenase isoenzymes in human skeletal muscle fibers.

Lactate dehydrogenase (LDH) activity was histochemically localized in fibers of the vastus lateralis muscle of men and for comparative purpose in the soleus and plantaris muscleo of rats. Human muscle fibers were identified as fast twitch (FT) or slow twitch (ST) from the histochemical stain for myofibrillar adenosine triphosphatase activity. Rat skeletal muscle fibers were classified as fast-twitch-oxidative-glycolytic (FOG), fast-twitch-glycolytic (FG), or slow-twitch-oxidative (SO) on the basis of NADH-diaphorase and myofibrillar adenosine triphosphatase activities. Heart-type (H) LDH was identified by inhibition of the muscle-type (M) isozyme with 4 M urea. Total LDH as estimated histochemically was highest in the human FT and rat FG fibers. This was predominantly the M-LDH isozyme. ST fibers of human and SO fibers of rat skeletal muscle had the least total LDH but the most H-LDH activity. The FOG fibers of rat muscle contained a total LDH activity intermediate to that of the FG and SO fibers and a combination of H- and M-LDH. There were no fibers in the human muscle samples studied that had LDH activities similar to the FOG fibers of rat muscle.     

Fiber-specific responses of muscle glycogen repletion in fasted rats physically active during recovery from high-intensity physical exertion

Mild physical activity performed immediately after a bout of intense exercise in fasting humans results in net glycogen breakdown in their slow oxidative (SO) muscle fibers and glycogen repletion in their fast twitch (FT) fibers. Because several animal species carry a low proportion of SO fibers, it is unclear whether they can also replenish glycogen in their FT fibers under these conditions. Given that most skeletal muscles in rats are poor in SO fibers (<5%), this issue was examined using groups of 24-h fasted Wistar rats (n=10) that swam for 3 min at high intensity with a 10% weight followed by either a 60-min rest (passive recovery, PR) or a 30-min swim with a 0.5% weight (active recovery, AR) preceding a 30-min rest. The 3-min sprint caused 61-79% glycogen fall across the muscles examined, but not in the soleus (SOL). Glycogen repletion during AR without food was similar to PR in the white gastrocnemius (WG), where glycogen increased by 71%, and less than PR in both the red and mixed gastrocnemius (RG, MG). Glycogen fell by 26% during AR in the SOL. Following AR, glycogen increased by 36%, 87%, and 37% in the SOL, RG, and MG, respectively, and this was accompanied by the sustained activation of glycogen synthase and inhibition of glycogen phosphorylase in the RG and MG. These results suggest that mammals with a low proportion of SO fibers can also replenish the glycogen stores of their FT fibers under extreme conditions combining physical activity and fasting.     

Rat skeletal muscle triacylglycerol utilization during exhaustive swimming

The utilization of triacylglycerol in slow oxidative, fast oxidative-glycolytic, and fast glycolytic skeletal muscle fiber types was examined in rats subjected to a prolonged exhaustive swim. Significant reductions of intramuscular triacylglycerol occurred following 2 h and 40 min of swimming in all muscles containing a predominance of slow oxidative and fast oxidative-glycolytic fibers, which possess a high capacity for free fatty acid oxidation. Triacylglycerol content in the soleus decreased by 48%, and reductions of 41, 29, and 27% were measured in the red vastus lateralis, red gastrocnemius, and plantaris muscles, respectively. In the white vastus lateralis and white gastrocnemius muscles (fast glycolytic fibers) triacylglycerol concentrations were unaffected. In all muscles the variability of intramuscular triacylglycerol measurements between animals was 20-50% and the within animal variance (right vs. left hindlimb) was similar. Analytical repeatability was approximately 10% in all muscles and significantly less than the between- and within-animal variances. It was concluded that a real biological variation exists in the triacylglycerol content of all rat skeletal muscles and that intramuscular triacylglycerol is an important energy source during prolonged exercise of moderate intensity.     

Muscle fiber type comparison of PDH kinase activity and isoform expression in fed and fasted rats

Fiber type specificity for expression of all three rat skeletal muscle pyruvate dehydrogenase kinase (PDK) isoforms (PDK1, 2, and 4) was determined in fed and 24-h fasted rats. PDK activity and isoform protein and mRNA contents were determined in white gastrocnemius (WG; fast-twitch glycolytic), red gastrocnemius (RG; fast-twitch oxidative), and soleus (Sol; slow-twitch oxidative) muscles. PDK activity was lower in WG compared with oxidative muscles (RG, Sol) in both fed and fasted rats. PDK activities from fed muscles were 0.12 +/- 0.04, 0.30 +/- 0.01, and 0.36 +/- 0.08 min(-1) in WG, Sol, and RG, respectively, and increased in fasted muscles (0.36 +/- 0.09, 0.68 +/- 0.18, and 0.80 +/- 0.14 min(-1)). This correlated with increased PDK4 protein and to a lesser extent with PDK4 mRNA. PDK2 protein was not different between fiber types in fed or fasted rats, but PDK2 mRNA content was twofold greater in RG from fasted rats compared with fed rats. PDK1 was unaltered by fasting in all muscle types at both the protein and mRNA level, but in both fed and fasted rats had much greater protein and mRNA content in the oxidative vs. glycolytic muscles. In conclusion, PDK activity and PDK1 and 4 protein and mRNA were lower in glycolytic vs. oxidative muscles from fed and fasted rats. Fasting for 24 h induced a two- to threefold increase in PDK activity that was mainly due to increases in PDK4 protein and mRNA. PDK1 and 2 protein and mRNA were generally unaltered by fasting in all fiber types, except for increased PDK2 mRNA in the fast oxidative fibers. Because the PDK isoforms vary greatly in their kinetic properties, their relative proportions in the three fiber types at any given time during fasting could significantly alter the acute regulation of the pyruvate dehydrogenase complex.     

Glyconeogenic and oxidative lactate utilization in skeletal muscle.

In this article we present a synthesis of recent information concerning the fate of lactate in skeletal muscle. This is important since lactate is continuously produced by skeletal muscle at rest and at all levels of exercise. Therefore, the disposal of lactate as an 'intermediary' metabolite is discussed. The two primary fates of lactate in skeletal muscle are (1) oxidation and (2) glycogen synthesis (glyconeogenesis). From recent evidence it seems relatively clear that glycogen formation in muscle is primarily dependent on glucose, although in fast twitch muscles a considerable proportion of lactate can account for muscle glycogen formation, especially immediately after exercise when circulating lactate levels are elevated. Exactly how lactate is converted to glycogen is not known yet, but an extramitochondrial pathway that is divergent from the hepatic gluconeogenic pathway seems likely. Oxidation of lactate is quantitatively the most important means of disposing of lactate, whether in exercising or nonexercising muscle. The lactate gradient between muscle and blood may be an important factor dictating whether lactate is taken up or released by muscle, independent of whether the muscle is active or not. Finally a novel role for epinephrine is considered that may be important for the mitochondrial oxidation of lactate.     

Effect of contractile activity on rat skeletal muscle beta-adrenoceptor properties

The effect of fiber type and endurance exercise training on skeletal muscle beta-adrenoceptor properties were assessed using a direct radioligand binding technique. Six separate muscles, composed of a variety of different fiber types, were examined in treadmill trained and sedentary rats. In trained animals, sarcolemmal preparations from heart and slow twitch soleus muscle exhibited a significantly greater receptor concentration than membranes from white fast twitch glycolytic fibers of the vastus lateralis. No significant changes were observed between trained and sedentary rat muscle beta-adrenoceptor density (beta max, fmole/mg protein) or affinity (Kd, nM) within each muscle type, despite significantly increased myocardial/body weight ratios and skeletal muscle enzyme adaptations associated with the exercise program. These results suggest that muscle beta-adrenoceptor properties may be influenced in part by the motor nerve innervation to that muscle, and are further discussed with respect to a possible relationship between exercise intensity and receptor regulation.     

Correlations between components of the glycolytic pathway

1. The contents of dihydroxyacetone phosphate, fructose diphosphate, pyruvate and lactate and the activities of aldolase and lactate dehydrogenase in the liver, kidney, testis, skeletal muscle, blood cells, sarcoma and hepatoma of rats were measured. 2. Correlations were established between components of the glycolytic pathway as follows: activities of aldolase and lactate dehydrogenase; contents of fructose diphosphate and pyruvate; activity of aldolase and content of fructose diphosphate; activity of lactate dehydrogenase and contents of fructose diphosphate and of pyruvate.