Slow-twitch Red Muscle Research Articles

De novo synthesis of adenine nucleotides in different skeletal muscle fiber types.

Management of adenine nucleotide catabolism differs among skeletal muscle fiber types. This study evaluated whether there are corresponding differences in the rates of de novo synthesis of adenine nucleotide among fiber type sections of skeletal muscle using an isolated perfused rat hindquarter preparation. Label incorporation into adenine nucleotides from the [1-14C]glycine precursor was determined and used to calculate synthesis rates based on the intracellular glycine specific radioactivity. Results show that intracellular glycine is closely related to the direct precursor pool. Rates of de novo synthesis were highest in fast-twitch red muscle (57.0 +/- 4.0, 58.2 +/- 4.4 nmol.h-1.g-1; deep red gastrocnemius and vastus lateralis), relatively high in slow-twitch red muscle (47.0 +/- 3.1; soleus), and low in fast-twitch white muscle (26.1 +/- 2.0 and 21.6 +/- 2.3; superficial white gastrocnemius and vastus lateralis). Rates for four mixed muscles were intermediate, ranging between 32.3 and 37.3. Specific de novo synthesis rates exhibited a strong correlation (r = 0.986) with muscle section citrate synthase activity. Turnover rates (de novo synthesis rate/adenine nucleotide pool size) were highest in high oxidative muscle (0.82-1.06%/h), lowest in low oxidative muscle (0.30-0.35%/h), and intermediate in mixed muscle (0.44-0.55%/h). Our results demonstrate that differences in adenine nucleotide management among fiber types extends to the process of de novo adenine nucleotide synthesis.

ATP depletion in slow-twitch red muscle of rat.

Rat slow-twitch muscle, in contrast to fast-twitch muscle, maintains its ATP content near normal during intense stimulation conditions that produce rapid fatigue. An extensive depletion of adenine nucleotide content by the deamination of AMP to IMP + NH3, typical of fast-twitch muscle, does not occur. We evaluated whether this response of slow-twitch muscle could be simply due to failure of synaptic transmission or related to cellular conditions influencing enzyme activity. Stimulation of soleus muscles in situ via the nerve or directly in the presence of curare at 120 tetani/min for 3 min resulted in extensive fatigue but normal ATP contents. Thus the lack of ATP depletion must be related to cellular events distal to neuromuscular transmission. Even nerve and direct muscle stimulation (with curare) during ischemia did not cause a large depletion of ATP or a large elevation of lactate content (12.0 +/- 0.7 mumol/g), even though the decline in tension was essentially complete. However, if the same tension decline during ischemia was prolonged by stimulating for 10 min at 12 tetani/min a large decrease in ATP (2.24 +/- 0.09 mumol/g) and increase in IMP (2.47 +/- 0.16 mumol/g) and lactate (30.4 +/- 2.0 mumol/g) content occurred. Thus adenine nucleotide deamination to IMP can occur in slow-twitch muscle during specific contraction conditions. The cellular events leading to the activation of AMP deaminase require an intense contraction condition and may be related to acidosis caused by a high lactate content.

Glucocorticoid cytosol binding in exercise-induced sparing of muscle atrophy.

Female rats were initially divided into a sedentary or an exercise group that was trained by treadmill running to a final work rate of 31 m/min, 100 min/day, for 13-18 wk. During the last 12 days of training each of these groups were further subdivided into groups that received daily subcutaneous injections of cortisol acetate (CA) (100 mg/kg body wt) or the vehicle (1% carboxymethyl cellulose). Exercise prevented approximately 40% of the gastrocnemius muscle weight loss due to CA treatment. Training did not influence glucocorticoid cytosol-receptor binding concentrations, using [3H]triamcinolone acetonide (TA) as the labeled glucocorticoid in any of the skeletal muscle types investigated. TA-receptor binding capacities were depleted by the multiple injections but were higher in the red fiber types of the CA-treated trained than those in the CA-treated sedentary animals. In a second series of experiments in which receptor depletion and repletion rates were studied using a single injection of cortisol, TA binding capacities 2 h after the cortisol injection were higher in slow-twitch red soleus muscles of trained as compared with sedentary rats (36.4 +/- 2.0 vs. 26.8 +/- 2.5 fmol/mg protein). Similar patterns of TA binding were also observed at 2 h between trained and sedentary animals in the fast-twitch red muscle types, whereas no training related differences were observed in white muscle types. Total and free serum cortisol concentrations also returned to base-line values faster in the trained animals following the single injection protocol.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood flow distribution in tissues of perfused rat hindlimb preparations.

Aerobic muscle metabolism during concentrations requires adequate blood flow and oxygen delivery. Since the perfused rat hindquarter (HQ) has become widely used for muscle stimulation, we examined the blood flow distribution, using 15 microns radiolabeled microspheres, and oxygen consumption of the HQ, using different commonly used perfusion protocols. Perfusion via the abdominal aorta resulted in well-matched (r = 0.90) blood flows between tissues of both hindlimbs that were proportional to total perfusion inflow. Blood flows to the high-oxidative fast-twitch and slow-twitch red muscle sections were three- to fourfold greater than flows to sections of low-oxidative fast-twitch white muscle. However, a large fraction (28%) of the total inflow went to the trunk region, even though all apparent arterial branches to the trunk region were ligated. This trunk mass accounts for at least 40% of the total metabolic responses of the HQ and diverts a large blood flow that is often presumed to supply the hindlimbs. As a result, muscle performance of the distal hindlimb muscle during stimulation can be inordinately poor. Ligation of the iliac artery to the contralateral limb improves blood flow to the remaining hindlimb but does not eliminate trunk blood flow. In contrast, perfusion via the femoral artery restricted 95% of the inflow to the single hindlimb, thereby reducing the tissue mass perfused. Blood flow to the distal limb musculature was high, resulting in an enhanced muscle performance. Thus single hindlimb perfusion provides a preparation where the contracting muscle is a large fraction of the total tissue, and the venous effluent better reflects the metabolic events in the contracting muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of starvation and exercise on concentrations of citrate, hexose phosphates and glycogen in skeletal muscle and heart. Evidence for selective operation of the glucose-fatty acid cycle.

Concentrations of citrate, hexose phosphates and glycogen were measured in skeletal muscle and heart under conditions in which plasma non-esterified fatty acids and ketone bodies were physiologically increased. The aim was to determine under what conditions the glucose-fatty acid cycle might operative in skeletal muscle in vivo. In keeping with the findings of others, starvation increased the concentrations of glycogen, citrate and the fructose 6-phosphate/fructose 1,6-bisphosphate ratio in heart, indicating that the cycle was operative. In contrast, it decreased glycogen and had no effect on the concentration of citrate or the fructose 6-phosphate/fructose 1,6-bisphosphate ratio in the soleus, a slow-twitch red muscle in which the glucose-fatty acid cycle has been demonstrated in vitro. In fed rats, exercise of moderate intensity caused glycogen depletion in the soleus and red portion of gastrocnemius muscle, but not in heart. In starved rats the same exercise had no effect on the already diminished glycogen contents in skeletal muscle, but it decreased cardiac glycogen by 25-30%. After exercise, citrate and the fructose 6-phosphate/fructose 1,6-bisphosphate ratio were increased in the soleus of the starved rat. Significant changes were not observed in fed rats. The data suggest that in the resting state the glucose-fatty acid cycle operates in the heart, but not in the soleus muscle, of a starved rat. In contrast, the metabolite profile in the soleus was consistent with activation of the glucose-fatty acid cycle in the starved rat during the recovery period after exercise. Whether the cycle operates during exercise itself is unclear.

Effect of cold exposure on liver and muscle cAMP content and cAMP phosphodiesterase activity.

Adenosine 3',5'-cyclic monophosphate (cAMP) concentration and 3',5'-cyclic-nucleotide phosphodiesterase (PDE) activity were measured in skeletal muscle, heart, and liver of rats exposed to 1, 3, 5, and 7 days of cold. Cyclic nucleotide concentration increased in fast-twitch red muscle at the same time that PDE activity was decreasing. Nucleotide concentration and enzyme activity of slow-twitch red muscle were not altered by the cold exposure. The PDE activity of fast-twitch white muscle was elevated approximately 50% above control after 1 and 3 days of cold exposure. By the 5th day in the cold, white muscle PDE activity had returned to control levels and remained there through the 7th day of experimentation. cAMP concentration in hearts of cold-exposed rats was significantly (P less than 0.01) elevated above control at all time points measured. Myocardial PDE activity was elevated above control (P less than 0.05) at 1 and 3 days of cold exposure but returned to control levels by the 5th day in the cold. Hepatic cAMP and PDE activity were elevated above control at all time points analyzed. These data suggest that changes in cyclic nucleotide metabolism play a role in attaining homeostasis during acute cold exposure.

Muscle and liver glycogen, protein, and triglyceride in the rat. Effect of exercise and of the sympatho-adrenal system.

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.

Blood flow to different skeletal muscle fiber types during contraction.

Blood flow to fast-twitch red (FTR), fast-twitch white (FTW), and slow-twitch red (STR) muscle fiber sections of the gastrocnemius-plantaris-soleus muscle group was determined using 15 +/- 3-microns microspheres during in situ stimulation in pentobarbital-anesthetized rats. Steady-state blood flows were assessed during the 10th min of contraction using twitch (0.1, 0.5, 1, 3, and 5 Hz) and tetanic (7.5, 15, 30, 60, and 120/min) stimulation conditions. In addition, an earlier blood flow determination was begun at 3 min (twitch series) or at 30 s (tetanic series) of stimulation. Blood flow was highest in the FTR (220-240 ml X min-1 X 100 g-1), intermediate in the STR (140), and lowest in the FTW (70-80) section during tetanic contraction conditions estimated to coincide with the peak aerobic function of each fiber type. These blood flows are fairly proportional to the differences in oxidative capacity among fiber types. Further, their absolute values are similar to those predicted from the relationship between blood flow and oxidative capacity found by others for dog and cat muscles. During low-frequency contraction conditions, initial blood flow to the FTR and STR sections were excessively high and not dependent on contraction frequency. However, blood flows subsequently decreased to values in keeping with the relative energy demands. In contrast, FTW muscle did not exhibit this time-dependent relative hyperemia. Thus, besides the obvious quantitative differences between skeletal muscle fiber types, there are qualitative differences in blood flow response during contractions. Our findings establish that, based on fiber type composition, a heterogeneity in blood flow distribution can occur within a whole muscle during contraction.

Muscle glycogenolysis during exercise: dual control by epinephrine and contractions.

The interaction of epinephrine and contractions on muscle metabolism was studied in the isolated perfused rat hindquarter. Subtetanic contractions (180/min) through 20 min elicited glycogenolysis and increased phosphorylase a activity. In the soleus, a slow-twitch red muscle, these effects were transient, but when epinephrine at a physiological concentration (2.4 X 10(-8) M) was added to the perfusate, glycogenolysis and phosphorylase activity were sustained throughout contractions. At this high frequency of contractions, the effect of epinephrine was much smaller in the fast-twitch red fibers and not significant in the fast-twitch white fibers of the gastrocnemius muscle. However, during less frequent contractions (30/min) epinephrine increased glycogenolysis and phosphorylase a activity in fast-twitch muscle. The data suggest that epinephrine and muscle contractions exert a dual control of muscle glycogenolysis during exercise: contractions principally stimulate glycogenolysis early in exercise, and a direct effect of epinephrine on muscle is needed for continued glycogenolysis. In addition, epinephrine increased oxygen consumption and glucose uptake in both resting and electrically stimulated hindquarters and, under some conditions, it had a positive inotropic effect on contracting muscle.

Metabolic types of muscle in the sheep: II. Lactate dehydrogenase activity and LDH isoenzyme distribution.

Lactate dehydrogenase activity and isoenzyme distribution were measured in fast-twitch red, slow-twitch red and fast-twitch white muscles in the sheep. Total enzyme activity is higher in fast-twitch muscles irrespective of their colour type. Isoenzymes M4 and H4 bear an inverse relationship, M4 being predominant in fast-twitch red and white muscles, and H4 in heart muscle and slow-twitch red muscles.

Uptake of chylomicron triglycerides by contracting skeletal muscle in rats.

The influence of muscle stimulation (3 Hz) on the uptake of exogenously administered chylomicron 14C-labeled triglycerides (14C-TG) in the three different muscle fiber types was evaluated in fed and fasted pentobarbital-anesthetized rats. The fractional uptake of 14C-TG in the nonstimulated muscles was lowest in the fast-twitch white, intermediate in the fast-twitch red, and highest in the slow-twitch red muscle fiber section. Fasting doubled the uptake in both high-oxidative red fibers. These 14C-TG uptakes were directly proportional (r = 0.993) to their respective activities of lipoprotein lipase in these fiber types of fed and fasted animals reported by others. Muscle stimulation caused a significant increase in the fractional 14C-TG uptake in all of the fiber types. Although the TG uptake could account for only a small fraction of the total energy needs of the working muscle, it could contribute to the turnover of endogenous TG, especially in the slow-twitch red fibers. Further, the estimated TG uptake rate is sufficient to replace endogenous TG loss with an overnight rest following exercise. These results suggest that plasma TG could play an important role in lipid metabolism, especially in the high-oxidative slow and fast muscle fiber types.

Androgens regulate mitochondrial cytochrome c oxidase and lysosomal hydrolases in mouse skeletal muscle.

The gastrocnemius, a fast-twitch white muscle, and the soleus, a slow-twitch red muscle, were studied in A/J mice. The specific activities of the lysosomal hydrolases, beta-D-glucuronidase, hexosaminidase, beta-D-galactosidase and arylsulphatase, the inner-mitochondrial-membrane enzyme cytochrome c oxidase, and the outer-mitochondrial-membrane enzyme monoamine oxidase, were greater in the soleus than in the gastrocnemius. The specific activities of the lysosomal hydrolases and cytochrome c oxidase in the gastrocnemius and soleus were substantially higher in male mice than in female mice. Orchiectomy abolished this sex difference. Testosterone increased the activities of the lysosomal hydrolases and cytochrome c oxidase and coincidentally induced muscle hypertrophy and an accretion of protein and RNA, but total DNA remained constant. Monoamine oxidase was unaffected by sex, orchiectomy and testosterone. These findings indicate that endogenous androgens regulate the activity of enzymes associated with lysosomes and the inner mitochondrial membrane, as well as muscle fibre growth in mouse skeletal muscle.

Oxygen consumption in perfused skeletal muscle. Effect of perfusion with aged, fresh and aged-rejuvenated erythrocytes on oxygen consumption, tissue metabolites and inhibition of glucose utilization by acetoacetate.

1. O2 consumption, glucose metabolism and the energy status of skeletal muscle were compared in isolated rat hindquarters perfused with aged (21--35 days), fresh and aged-rejuvenated human erythrocytes. 2. The age of the erythrocytes did not affect O2 consumption, glucose utilization or lactate release either at rest or during exercise. The concentrations of ATP, phosphocreatine and lactate within the muscle were also unaffected by the use of aged erythrocytes. 3. Perfusion with acetoacetate did not inhibit glucose utilization; but, it caused a marked increase in the tissue concentration of citrate in the soleus, a slow-twitch red muscle, and a smaller increase in the gastrocnemius, which contains fast-twitch red and white fibres. Results were similar in hindquarters perfused with aged and aged-rejuvenated erythrocytes. 4. These findings suggest that perfusion with aged human erythrocytes does not cause major alterations in the metabolic performance of the isolated rat hindquarter.

Influence of thyroid status on skeletal muscle LPL activity and TG uptake.

The influence of thyroid status on the clearance of chylomicron 14C-labeled triglycerides (14C-TG), the fractional uptake of 14C-TG, and activity of lipoprotein lipase (LPL) in the different skeletal muscle fiber types was evaluated in pentobarbital-anesthetized rats. The turnover of plasma TG was approximately fourfold greater in the hyperthyroid (HyperT) compared to either euthyroid (EUT) or hypothyroid (HypoT) animals. The uptake of 14C-TG was increased in slow-twitch red and fast-twitch red muscle sections of the HyperT group and normal in the HypoT group. The changes in LPL activity in these two fiber types were inversely related to thyroid influence, with decreases found in the HyperT group and increases found in the HypoT group. Thus, the uptake of 14C-TG in these high-oxidative fibers, relative to the LPL activity, varied directly with thyroid influence. As a result, the normal linear relationship, apparent for the different types of skeletal muscle, between LPL activity measured in vitro and TG uptake determined in situ was not maintained.

Glycogen synthesis from lactate in the three types of skeletal muscle.

Lactate utilization by skeletal muscle was studied in the perfused rat hindlimb. At lactate concentrations above 4 mu, the hindlimb consistently removed lactate from the perfusion medium. At the concentrations examined (up to 26 mu), the rate of lactate uptake was proportional to lactate concentration. When the hindlimb was perfused with medium containing 12 mu lactate as the only substrate, a rapid increase in glycogen occurred in the fast-twitch red and fast-twitch white types of muscle. In contrast, the rate of glycogen accumulation in the slow-twitch red type of muscle was so slow as to be of questionable physiological significance. [W]Lactate was rapidly incorporated into glycogen in the plantaris muscle which is composed predominantly (about 94%) of fast-twitch fibers, while there was little incorporation of [14C]lactate into glycogen in the soleus muscle which consists predominantly of slow-twitch red fibers. Measurement of fructose-1,6-bisphosphatase activity provided an explanation for the very slow rate of lactate conversion to glycogen in slow-twitch red muscle. It was found that in contrast to the fast-twitch red and white types of muscle, the slow-twitch red soleus muscle has an extremely low level of fructose1,6-bisphosphatase activity. There was a 24% inhibition of lactate uptake, and the amount of [14C]lactate incorporated into plantaris muscle glycogen was reduced by 30%, when 8.5 mu glucose and insulin were included in the perfusion medium. However, neither the rate of lactate uptake nor the rate of glycogen accumulation was affected by a high concentration of insulin alone.

Effects of Thyrotoxicosis on Mitochondrial Enzymes of Rat Soleus

Cytochrome oxidase, glycerol-3-phosphate dehydrogenase, and succinate dehydrogenase were measured in mitochondrial fractions obtained from rat soleus muscle of control and 8 week T3 + T4 treated animals. Under these conditions of prolonged treatment, there is a five-fold increase in the specific activities of both cytochrome oxidase and glycerole-3-phosphate dehydrogenase. Significant increases in total cellular mitochondrial content and enzyme activities were observed in T3 + T4 treated animals as compared to controls. These results indicate that thyrotoxicosis can induce selective changes in mitochondrial enzymes in slow twitch red (Type I) muscle fibers.

Regulation of glycogen resynthesis in muscles of rats following exercise.

Following a strenuous bout of exercise, glycogen repletion occurred most rapidly in the fast-twitch red type of muscle, least rapidly in fast-twitch white, and at an intermediate rate in slow-twitch red muscle. There was a linear correlation between glycogen synthase I activity and the rate of glycogen synthesis in the three types of muscle. This finding helps explain the differences between the rates of glycogen resynthesis in the three muscle types, and supports the view that glycogen synthase activity is the most important factor determining the rate of glycogen synthesis when substrate supply is adequate. There was an inverse correlation between muscle glycogen concentration and percent glycogen synthase I. Plasma insulin concentration was low and norepinephrine and glucagon concentrations were elevated in the postexercise period. The finding that rapid glycogen synthesis occurred despite a hormonal milieu conducive to glycogenolysis provides evidence that a low glycogen concentration is a potent stimulus to glycogen synthesis that overrides the effects of low insulin, and high norepinephrine and glucagon levels.

Effect of acetoacetate on glucose metabolism in the soleus and extensor digitorum longus muscles of the rat.

1. The effect of acetoacetate on glucose metabolism was compared in the soleus, a slow-twitch red muscle, and the extensor digitorum longus, a muscle composed of 50% fast-twitch red and 50% white fibres. 2. When incubated for 2h in a medium containing 5 mM-glucose and 0.1 unit of insulin/ml, rates of glucose uptake, lactate release and glucose oxidation in the soleus were 19.6, 18.6 and 1.47 micronmol/h per g respectively. Acetoacetate (1.7 mM) diminished all three rates by 25-50%; however, it increased glucose conversion into glycogen. In addition, it caused increases in tissue glucose, glucose 6-phosphate and fructose 6-phosphate, suggesting inhibition of phosphofructokinase. The concentrations of citrate, an inhibitor of phosphofructokinase, and of malate were also increased. 3. Rates of glucose uptake and lactate release in the extensor digitorum longus were 50-80% of those in the soleus. Acetoacetate caused moderate increases in tissue glucose 6-phosphate and possibly citrate, but it did not decrease glucose uptake or lactate release. 4. The rate of glycolysis in the soleus was approximately five times that previously observed in the perfused rat hindquarter, a muscle preparation in which acetoacetate inhibits glucose oxidation, but does not alter glucose uptake or glycolysis. A similar rate of glycolysis was observed when the soleus was incubated with a glucose-free medium. Under these conditions, tissue malate and the lactate/pyruvate ratio in the medium were decreased, and acetoacetate did not decrease lactate release or increase tissue citrate or glucose 6-phosphate. An intermediate rate of glycolysis, which was not decreased by acetoacetate, was observed when the soleus was incubated with glucose, but not insulin. 5. The data suggest that acetoacetate glucose inhibits uptake and glycolysis in red muscle under conditions that resemble mild to moderate exercise. They also suggest that the accumulation of citrate in these circumstances is linked to the rate of glycolysis, possibly through the generation of cytosolic NADH and malate formation.

Myosin light chains of slow twitch (red) muscle.

ALTHOUGH our results1–3 agree with those of Lowey and Risby4,5 concerning the number of light chain types and their differences in white (fast twitch), cardiac and avian tonic (lat. dorsi ant.) muscle6, there is a discrepancy with respect to rabbit red (slow twitch) muscles. But the difficulty can be resolved with reference to our previous work.