Identification of fiber types in rat skeletal muscle based on the sensitivity of myofibrillar actomyosin ATPase to copper

Experiments are reported demonstrating that differential rates of inactivation of the histochemical staining for myofibrillar actomyosin ATPase in rat skeletal muscle fibers exist following inclusion of low concentrations of Cu2+ in the preincubation medium. This response of rat muscle occurs at near neutral (7.40), acid (4.60), and alkaline (10.30) pH. The response to Cu2+ appears to result from a binding of Cu2+ onto the myofibrillar complex, probably on myosin itself, as it can be reversed by soaking of the pretreated muscle sections in sodium cyanide or the Cu2+ chelator diethyldithiocarbamate. The pattern of modification of the staining pattern following pretreatment with Cu2+ is the mirror image of that produced by pretreatment with acid. The results demonstrate that the inclusion of Cu2+ in the preincubation media for the myofibrillar actomyosin ATPase can be a useful tool to differentiate fiber types. They also support the earlier conclusion that three distinct types of type II fibers can be identified in rat skeletal muscle based on the histochemical staining for myofibrillar actomyosin ATPase.

Fiber number and size in overloaded chicken anterior latissimus dorsi muscle

The relative contribution of increases in fiber area and number was evaluated in the chicken anterior latissimus dorsi (ALD) muscle in which enlargement was induced by hanging a weight on one wing. ALD muscles from wings to which weights had been attached for periods ranging from 6 to 65 days weighed an average of 105% (range 22-225%) more than control muscles. Total muscle fiber number, determined by direct counts after nitric acid digestion and fiber dissection, and the frequency of branched fibers were unchanged by muscular enlargement. Fiber cross-sectional area was greater (P less than 0.01) in the enlarged muscles. A close relationship existed (r = 0.78) between actual muscle weight and weight calculated as the product of fiber volume, total fiber number, and muscle density for the control and enlarged muscles. Histochemical staining revealed a conversion of type IIa to type I fibers in the stretched muscles. These results support the concept that skeletal muscle enlargement in response to chronic overload is produced by hypertrophy of preexisting fibers and not be a formation of new fibers.

Fluid shifts and muscle function in humans during acute simulated weightlessness

Head-down tilt is considered an effective experimental model to simulate weightlessness. To determine the acute effects of simulated weightlessness on transcapillary fluid balance, tissue fluid shifts, muscle function, and triceps surae reflex time, eight supine subjects were tilted 5 degrees head down for 8 h. A cephalic fluid shift from the legs was indicated by facial edema, nasal congestion, increased urine flow, decreased creatinine excretion, reduced calf girth, and decreased lower leg volume. As measured by wick catheters inserted under local anesthesia, interstitial fluid pressure in the tibialis anterior muscle (4.6 +/- 0.6 mmHg) and subcutaneous tissue (0.6 +/- 0.5 mmHg) of the lower leg fell significantly to -2.8 +/- 0.5 and -3.8 +/- 0.4 mmHg, respectively. Other transcapillary pressures (capillary and interstitial fluid colloid osmotic pressures) were relatively unchanged. Needle-biopsy specimens, obtained just before and after tilt, indicated that total water content of soleus muscle was unchanged during 8 h of head-down tilt. After head-down tilt, isometric strength and isokinetic strength of the plantar flexors were unchanged. Triceps surae reflex time associated with plantar flexion movement slowed slightly after the tilt maneuver. Collectively these results demonstrated a dehydration effect of head-down tilt on muscle and subcutaneous tissues of the lower leg that may affect muscle function.

Interaction of ribulose bisphosphate carboxylase/oxygenase with 2-carboxyhexitol 1,6-bisphosphates

2-C-Carboxy-D-glucitol 1,6-bisphosphate (CGBP) and 2-C-carboxy-D-mannitol 1,6-bisphosphate (CMBP) have been synthesized, isolated, and the structures of these compounds and the derived lactones elucidated by NMR spectroscopy and periodate oxidation. Both carboxyhexitol bisphosphates, which are homologs of the transition state analog 2-C-carboxy-D-arabinitol 1,5-bisphosphate, exhibit competitive inhibiton of ribulose bisphosphate carboxylase/oxygenase (EC 4.1.1.9) isolated from spinach (Spinacia oleracea), with respect to ribulose 1,5-bisphosphate. CMBP was a more potent inhibitor (100-fold) displaying an inhibition constant (Ki at pH 8.0 and 30 degrees C) of 1-2 microM with enzymes from spinach, barley (Hordeum vulgare), and Chromatium vinosum. In contrast the Rhodospirillum rubrum enzyme was inhibited about 40-fold more weakly (Ki = 53 microM at pH 8.0 and 30 degrees C). Both CGBP and CMBP potentiated activation of RuBP carboxylase from spinach and R. rubrum.

Differentiation of fiber types in skeletal muscle from the sequential inactivation of myofibrillar actomyosin ATPase during acid preincubation

A method is described for identifying fiber types of skeletal muscle from several mammalian species on the basis of the sequential inactivation of myofibrillar actomyosin ATPase during acid preincubation. When this method is used in combination with the standard alkaline preincubation at least 5 types of fibers can be identified. Of these, 2 are type I fibers with those of the slow twitch soleus muscle being different from those that exist in mixed muscles. The 3 subtypes of type II fibers exist independent of their metabolic properties. The need for careful standardization of histochemical methods for the visualization of myofibrillar actomyosin ATPase and the implication of the existence of different fiber types in apparently homogeneous muscle for the preparation of antibodies used for immunocytochemical methods of fiber identification are discussed.

Acute fasting and fiber number in rat soleus muscle

The influence of fasting on fiber number in the soleus muscle (SM) of weanling male and female rats was investigated. For female rats, comparisons were made among groups of animals fed normally, rats fasted and then fed until prefast body weight was regained, and animals that grew to maturity. For male rats, comparisons were made only between control and fasted groups. Prior to the experimental treatments the SM was surgically removed from one leg. There was a 40% loss in body weight after fasting. Although major weight losses occurred in most muscles and organs, there was no change in the SM. Over the same period SM weight increased 31% in normal animals. Total fiber number (direct counts after nitric acid digestion) was unaltered by the treatments. Although wide variation existed between animals, total fiber number between legs for the same animal was closely correlated (r = 0.98). SM weight for male rats calculated from fiber length, cross-sectional area, and total fiber number could account for from 91 to 99% of the total muscle weight. There was no change in fiber number from weaning to maturity. It is concluded that fiber number is unchanged by fasting or during normal maturation.

Effect of training on beta-adrenergic receptor number in rat heart

Female Sprague-Dawley rats were subjected to endurance-training programs, and the effect of training on myocardial beta-adrenergic receptor number, receptor-binding characteristics, and adenylate cyclase (AC) activities associated with the receptor were examined. Training produced a 45% (P less than 0.01) increase in the succinate dehydrogenase activity of the plantaris muscle. Specific (-)-[3H]dihydroalprenolol (DHA)-binding data were subjected to Scatchard plot analysis to quantify beta-adrenergic receptor number and DHA-binding characteristics of myocardial membranes. The DHA concentrations at which 50% of the total binding sites were occupied were similar for membranes from sedentary (1.95 +/- 0.51) and trained (1.59 +/- 0.34 nM) groups. Total DHA-binding sites of membranes from control (91.6 +/- 13.3) and trained (83.1 +/- 7.6 fmol/mg) groups were also similar. Basal and maximally stimulated AC activities were also unchanged by endurance training. Fluoride-stimulated AC activities of crude homogenate and 10,000 g fractions decreased 47 and 49%, respectively, with training. No differences were observed in a 40,000 g fraction. The specific activities of a ouabain-sensitive Na+-K+-ATPase (a sarcolemmal membrane marker) of crude homogenate, 10,000 g, and 40,000 g membrane fractions were similar. These data indicate that training produces no detectable difference in the potential for adrenergic responses at the receptor level.

Peripheral factors as limitations to exercise capacity

The paper focuses on two aspects of the peripheral limitations to exercise: (a) intracellular and extracellular substrates used for energy transduction and (b) the metabolic potential of muscle. The major substrates used to support contractile activity have been discussed in reference to high-intensity, short-term exercise and moderately-intense, long-term exercise. It has been suggested that substrate availability is a prime candidate as a limiting factor to prolonged exercise, whereas, it is clearly not a limiting factor in short-term maximal efforts. Consideration of the metabolic potential of muscle as a possible limiting factor to exercise has been divided into anaerobic metabolism, aerobic metabolism and regulatory factors. It appears that the enzymatic capacity of the energy transducing pathways is not a limiting factor to any type of exercise.

Relationship of strength and endurance with skeletal muscle structure and metabolic potential

Human skeletal muscle consists of fast and slow twitch motor units. The FT units can be further subdivided into three types on the basis of staining procedures that identify the proteins in the contractile elements. Presently it does not appear that a conversion of motor units occurs with training. Major metabolic differences exist between the different motor units. With endurance training there is an increase in the enzymes for end-terminal oxidation. This reduces the differences between fiber types. This reduction is the differences between fiber types is most dramatic when viewed from histochemical stains. Training does increase the concentration of glycogen stored in the muscle. Muscular strength is related to the total cross-sectional area of the muscle. This can be increased with heavy resistance exercise. This enlargement is due to hypertrophy of the pre-existing fibers and not to a proliferation of fibers as the result of splitting of the fibers.

Significance of skeletal muscle oxidative enzyme enhancement with endurance training

A theoretical model is proposed to explain how the increase in mitochondrial protein concentration, and therefore of the oxidative enzymes, that occurs with endurance training could operate to alter the choice of substrate during submaximal exercise in a manner such that the oxidation of fatty acids increases, glycogen depletion and lactate production are reduced, and work capacity is enhanced. The model is based on the control of enzyme activities both by enzyme and substrate concentrations. The effect of altering enzyme concentration on reaction velocities is presented on the basis of standard Henri-Michaelis-Menten kinetics. It is shown that the reaction velocity at a given substrate concentration is a function of total enzyme concentration. With an increase in total enzyme concentration there is a parallel increase in reaction velocity at the same substrate level. This would have its greatest impact at substrate levels below the Km of the enzyme. It would have an effect of enhancing fatty acid flux through the oxidative pathways while inhibiting the Embden-Meyerhof pathway. The model, as proposed, is consistent with known alterations in metabolism as they occur in man during submaximal exercise following endurance training.

Exercise-induced glycogenolysis in sympathectomized rats

The role of the adrenergic system in regulating glycogenolysis during exercise was studied in rats. Alterations in the adrenergic system were produced by injections of 6-hydroxydopamine (6-OHD), surgical removal of the adrenal medulla (ADMX), or the combination of ADMX and 6-OHD injection. Exercise was treadmill running at 22 m/min for 60 min. Colonic temperature averaged 2.8 degrees C higher in the exercised than control rats. Exercise reduced the glycogen of the liver and skeletal muscles of all groups. The glycogen concentrations of the soleus and red portion of the gastrocnemius muscles of the ADMX and ADMX-6-OHD groups were about 3.8 and 2.5 times higher after exercise than those of the normal-exercised rats. Glycogen depletion of the white portion of the gastrocnemius muscle was similar for all exercised groups. 6-OHD treatment depleted the catecholamines of the myocardium. These results demonstrate that glycogen depletion during exercise occurs in rats devoid of adrenergic control. However, differences between types of skeletal muscle suggest that factors other than the adrenergic system may be involved in controlling glycogen metabolism during exercise.

Response of ventilatory muscles of the rat to endurance training

The effect of endurance training on the oxidative and glycolytic potentials of the diaphragm and intercostal muscles of rats has been studied. Training consisted of treadmill running (28 m/min, 60 min/day, 5 days/wk) for periods ranging from 8-26 weeks. Exercise of similar duration and intensity produced a glycogen depletion in the diaphragm and intercostal muscles of nontrained rats. Oxidative potential was estimated from the activity of the mitochondrial marker enzyme succinate dehydrogenase (SDH). The activities of phosphorylase (PHOS), hexokinase (HK), and lactate dehydrogenase (LDH) were determined as well as the distribution of the LDH isozymes. SDH activity averaged 44 (42-51) and 17 (10-22)% (P less than 0.0l) greater in the plantaris and diaphragm muscles, respectively, after 8-12 weeks of endurance running as compared to the sedentary animals. There was no change in the SDH activity of the intercostal muscles or in the activities of the glycolytic enzymes. There was also no change in the distribution of the isozymes of LDH. Extending the duration of the training program to 26 weeks did not produce any additional alteration in the magnitude of the adaptation observed after the initial training period. Comparative studies of different types of muscles demonstrated that the diaphragm, although having a fiber composition somewhat similar to that of a fast-twitch skeletal muscle, has a metabolic profile that is intermediate between pure slow twitch skeletal muscle and cardiac muscle.

Muscular enlargement and number of fibers in skeletal muscles of rats

The effect of muscular enlargement produced by surgical ablation of a synergist and the combination of synergist ablation and exercise on the number of fibers in the soleus (S), plantaris (P), and extensor digitorum longus (EDL) muscles of the rat was studied. The number of fibers per muscle was determined by direct counts of individual fibers dissected from HNO3-treated muscles. Ablation of a synergist produced average enlargements of about 25, 45, and 29% for the S, P, and EDL muscles, respectively. Exercise and synergist ablation produced increases in wet weight to about 44 and 88% for the S and P muscles, respectively, whereas no further increases were observed in the EDL muscles. Intra-animal comparisons revealed that no differences existed for total fiber number or the incidence of fibers with bifurcations between the enlarged and contralateral control muscles. The difference in dry weight of fibers from the enlarged as compared with control muscles was closely correlated to differences in total muscle wet weight. These data demonstrate that hypertrophy rather than hyperplasia was responsible for increases from 10 to over 100% in the weight of skeletal muscles.

Lipolysis and cAMP accumulation in adipocytes in response to physical training

Adenylate cyclase activity is lower in membrane preparations of fat cell homogenates from exercise-trained compared with sedentary rats (J. Appl. Physiol.: Respirat. Environ, Exercise Physiol. 42: 884-888, 1977). In the present investigations lipolysis and cyclic adenosine 3':5'-monophosphate (cAMP) accumulation were measured in isolated parametrial fat cells prepared from sedentary and trained rats. The purpose of these investigations was to determine whether the normal catecholamine-induced increases in cAMP accumulation is affected in isolated adipocytes from endurance-trained rats. The increases in cAMP accumulation in response to isoproterenol (0.01-10 microM) was reduced in fat cells isolated from trained rats. However, glycerol release in response to the same hormonal challenge was greater in these adipocytes. cAMP phosphodiesterase activity measured at 0.125 and 1.025 microM cAMP was greater in the particulate fraction of fat cell homogenates obtained from trained rats as compared with their sedentary counterparts. Hormone-sensitive lipase activity was reduced in crude fat pad homogenate preparations from trained rats if the animals were killed at rest. However, if the animals were run to exhaustion immediately prior to being killed, there were no differences in the hormone-sensitive lipase activity between preparations from trained and nontrained rats. These data indicate that, although cAMP accumulation by isolated fat cells in response to isoproterenol is markedly lower in trained rats, lipolysis and hormone-sensitive lipase activation is not reduced.

Lactic acidosis as a result of iron deficiency

Iron-deficient rats have an impaired work performance, even when their anemia is corrected by exchange transfusion. Muscle activity is associated with a higher blood lactate concentration than is observed in iron-replete animals. The accumulation of lactate is a result of excessive production as lactate clearance from the blood was shown to be unaffected. By adjusting the work load to a lower level, it was possible to divide iron-deficient animals into two groups, one capable of continued treadmill running and another in which animals stopped before 20 min. In the former, blood lactate concentration reached a plateau at moderate levels, whereas it continued to increase in the latter until the animal stopped running. Levels of alpha-glycerophosphate oxidase in skeletal muscle mitochondria were found to be much lower in the second group (P < 0.001). Lactate infusion into normal animals was shown to interfere with work performance, and maintenance of a normal pH in iron-deficient and iron-replete animals did not prevent the impairment in work associated with high blood lactate concentrations. Additional evidence was obtained that energy substrate (blood glucose and free fatty acids, muscle glycogen) was adequate in irondeficient animals. Oxygen tension in their vena caval blood was higher than in controls. Furthermore, the in situ behavior of electrically stimulated gastroenemius and soleus muscles appeared similar to that of control animals. Because the stimulation of the single muscle in the iron-deficient animal did not result in appreciable elevation of blood lactate and did not show impaired contractility further supported the hypothesis that the elevation of blood lactate caused the decreased work performance. It is concluded that iron deficiency by a depletion in the iron-containing mitochondrial enzyme, alpha-glycerophosphate oxidase, impairs glycolysis, resulting in excess lactate formation, which at high levels leads to cessation of physical activity.

Phosphorylase a in human skeletal muscle during exercise and electrical stimulation

Experiments were conducted to examine the conversions of phosphorylase b to phosphorylase a in human skeletal muscle during bicycle exercise or isometric contractions. Muscle biopsies were obtained from the vastus lateralis with the needle technique at rest and either during or immediately after activity and frozen in liquid nitrogen within 2--4 s. Total phosphorylase and phosphorylase a activities were differentiated by measurement in the presence and absence of AMP, respectively. At rest 8.5% of the total phosphorylase activity existed in the a form. Little or no change in the percent of phosphorylase in the a form occurred during voluntary dynamic or static muscular activity that produced muscle lactate concentrations in excess of 18 mmol.kg-1 wet muscle. Electrical stimulation of the vastus lateralis muscle also failed to produce an increase in the percentage of phosphorylase a. These data suggest that during exercise the conversion of phosphorylase to the a form is of minor importance. An increased activity of phosphorylase b due to changes in muscle concentrations of ATP, AMP, and inorganic phosphate may regulate glycogenolysis during voluntary exercise in man.