Water in normal muscle and muscle with a tumor

The total water content, the amount of non-freezable water, and the Na⁺ and K⁺ contents in the gastrocnemius muscle of albino mice with and without a solid tumor were determined. The spin-lattice relaxation time (T₁) for the water protons in the two kinds of muscle were measured at six resonance frequencies ranging from 4.5 to 60 MHz over the temperature range +37 to −65°C. Quantitatively calculated T₁ values are given. The difference in T₁ for the two types of muscle at temperatures above −5°C is attributed to the difference in the distribution ratio of water between hydration and free states, and bears no direct relation to the concentration of Na⁺.     

Purine and pyrimidine metabolism in human muscle and cultured muscle cells

Using radiochemical methods, we determined the activities of various enzymes of purine and pyrimidine metabolism in homogenates of human skeletal muscle and of cultured human muscle cells. Results show a large discrepancy between the enzyme activities in muscle and cultured cells. With regard to purine metabolism, adenylate (AMP) deaminase activity was only 1-3% in cultured cells compared to that in muscle, whereas the activity of adenosine deaminase, purine-nucleoside phosphorylase, adenosine kinase, adenine phosphoribosyltransferase and hypoxanthine phosphoribosyltransferase was 7-15-fold higher in the cultured cells. The enzymes of pyrimidine metabolism, orotate phosphoribosyltransferase, orotidine 5'-monophosphate decarboxylase and uridine kinase showed activity of 100-200-fold higher in cultured cells than in adult muscle. The differences in enzyme activity are probably related to the low differentiation stage and the absence of contractile activity in the cultured muscle cells. Care must be taken when using these cells as a model for studying purine and pyrimidine metabolism of adult myofibers.     

Shoulder muscle load and muscle fatigue among industrial sewing-machine operators

Physiological responses to physical work were assessed for 29 female industrial sewing-machine operators during an 8-h working day under ordinary working conditions. During sewing-machine work, the average (left and right) static load in the trapezius muscle was 9% of the maximal electromyogram (EMG) amplitude (% EMG_max), while the average mean load was 15% EMG_max, and the average peak load was 23% EMG_max. The static load level was unrelated to the muscle strength of the sewing-machine operators, which for the group as a whole was within the normal range. The load levels remained unchanged during the working day, while changes in the EMG mean power frequency and zero crossing frequency rate occurred, both indicating the development of muscle fatigue in left and right trapezius muscle during the working day. In line with this, the rating of perceived exertion in the shoulder and neck region increased during. the working day. Dividing the group of sewing-machine operators into two groups, those with the highest frequency and those with the lowest frequency of shoulder/neck troubles showed that the former group had significantly lower muscle strength, despite the fact that no differences in the surface EMG during sewing were found between the two groups. It was concluded that industrial sewing-machine work involves a pattern of shoulder muscle activity which induces fatiguing processes in the shoulder and neck regions. Furthermore, since the static shoulder muscle load was independent of muscle strength, factors other than working posture may be of significance for the static shoulder muscle load.     

Effect of testosterone on muscle mass and muscle protein synthesis

We have studied the effect of a pharmacological dose of testosterone enanthate (3 mg.kg-1.wk-1 for 12 wk) on muscle mass and total-body potassium and on whole-body and muscle protein synthesis in normal male subjects. Muscle mass estimated by creatinine excretion increased in all nine subjects (20% mean increase, P less than 0.02); total body potassium mass estimated by 40K counting increased in all subjects (12% mean increase, P less than 0.0001). In four subjects, a primed continuous infusion protocol with L-[1-13C]leucine was used to determine whole-body leucine flux and oxidation. Whole-body protein synthesis was estimated from nonoxidative flux. Muscle protein synthesis rate was determined by measuring [13C]leucine incorporation into muscle samples obtained by needle biopsy. Testosterone increased muscle protein synthesis in all subjects (27% mean increase, P less than 0.05). Leucine oxidation decreased slightly (17% mean decrease, P less than 0.01), but whole-body protein synthesis did not change significantly. Muscle morphometry showed no significant increase in muscle fiber diameter. These studies suggest that testosterone increases muscle mass by increasing muscle protein synthesis.     

Voluntary muscle activation varies with age and muscle group.

The consistency and the number of attempts required to achieve maximal voluntary muscle activation have not been documented and compared between young and old adults. Furthermore, few studies have contrasted activation between functional pairs of muscle groups, and no study has tested upper limb muscles. The purpose of this study was to measure and compare voluntary muscle activation of the elbow flexors and extensors in young and old men over two separate test sessions. With the method of twitch interpolation to measure activation, six young (24 +/- 1 yr) and six old (83 +/- 4 yr) men performed five maximal voluntary contractions (MVC) during each session for each muscle group. Elbow flexion and extension MVC was less (43 and 47%, respectively) in the old men, yet the best maximal voluntary muscle activation was similar between age groups. However, when all 10 attempts at MVC were compared, the mean activation scores were slightly less (approximately 5%) in the elbow extensors but were approximately 11% less (P < 0.001) in the elbow flexors of old men, compared with young men. During the second session, there was a significant improvement of 13% (P < 0.005) in mean elbow flexor activation in the old men. There were no session differences for either muscle group for the young men. The results indicate that, for aged men, elbow flexor maximal activation is achieved less frequently compared with elbow extensors, and thus mean activation for elbow flexors is less than for elbow extensors. However, if sufficient attempts are provided, the best effort for the old men is not different from that of the young men for either muscle group.     

Myogenesis and muscle regeneration

Skeletal muscle has received much attention with regard to developmental origin, control of cell differentiation and regeneration. In this article, early landmarks in skeletal muscle research are reviewed and recent findings on myogenesis are addressed with particular focus on novel regulatory molecules including miRNAs, as well as on the topographical heterogeneity of skeletal muscle origin. The latter has developed into a central theme of keen interest in the past years, particularly since overlaps in genetic and embryological background between head muscle subsets and heart muscle have been described. As embryonic myogenesis and regenerating myofibers employ common molecules, the heterogeneity in embryonic sources from which skeletal muscle groups in the vertebrate body take origin is closely reflected by differences in the susceptibility to particular muscle dystrophies as well as their regeneration potential. In the regeneration chapter of this review the progress that has been made in the field of muscle stem cell biology, with special focus on the satellite cells, is outlined. Satellite cells are considered the most promising source of muscle stem cells possessing a high regenerative potential. We shall discuss recent insights into the heterogeneous nature of these satellite cells not just in terms of their expression profile but also their regeneration potential. Latest findings about the motility of the satellite cell shall also be discussed. Furthermore, we shall outline the impact of an improved understanding of muscle stem cells within their environment, and of satellite cells in particular, on efficient stem cell replacement therapies for muscular dystrophies, putting embryological findings and stem cell approaches into context.     

Relationship between respiratory nerve and muscle activity and muscle force output.

To demonstrate the most satisfactory way of using electrical activities of respiratory nerves and muscles, activities of phrenic nerve and external intercostal muscle (ICM) and the airway pressure changes generated by respiratory muscle contraction were recorded in anesthetized cats during complete airway occlusion. Electrical activities were rectified, integrated and processed in terms of peak and average inspiratory rates per 0.1 s and of total activity per breath. Peak rate of phrenic nerve activity exhibited a high linear correlation (r = 0.974) with peak inspiratory pressure. Average phrenic rate showed a similar high correlation (r = 0.973). Peak rate of external ICM was linearly related to peak pressure but the correlation was less good (r = 0.915). Total phrenic activity per breath was too dependent upon inspiratory duration to be a satisfactory correlate (r = 0.674). In this experiment occlusion pressure was an index of muscle force generation and respiratory control system output. It is concluded that peak or average rates of phrenic activity provide an electrical index of output changes. On theoretical grounds, peak rate is probably better.     

Microinjection of nonmuscle and smooth muscle caldesmon into fibroblasts and muscle cells

Caldesmon is present in a high molecular mass form in smooth muscle and predominantly in a low molecular mass form in nonmuscle cells. Their biochemical properties are very similar. To examine whether these two forms of caldesmon behave differently in cultured cells, we microinjected fluorescently labeled smooth muscle and nonmuscle caldesmons into fibroblasts. Simultaneous injection of both caldesmons into the same cells has revealed that both high and low relative molecular mass caldesmons are quickly (within 10 min) and stably (over 3 d) incorporated into the same structures of microfilaments including stress fibers and membrane ruffles, suggesting that nonmuscle cells do not distinguish nonmuscle caldesmon from smooth muscle caldesmon. The effect of calmodulin on the incorporation of caldesmon has been examined by coinjection of caldesmon with calmodulin. We have found that calmodulin retards the incorporation of caldesmon into stress fibers for a short period (10 min) but not for a longer incubation (30 min). The behavior of caldesmon in developing muscle cells was also examined because we previously observed that caldesmon disappears during myogenesis (Yamashiro, S., R. Ishikawa, and F. Matsumura. 1988. Protoplasma Suppl. 2: 9-21). We have found that, in contrast to its stable incorporation into stress fibers of fibroblasts, caldesmon is unable to be incorporated into thin filament structure (I-band) of differentiated muscle.     

Growth hormone and muscle function responses to skeletal muscle ischemia.

We examined the effects of ischemia (ISC) alone and with low-intensity exercise (ISC+EX) on growth hormone (GH) and muscle function responses. Nine men (22 +/- 0.7 yr) completed 3 study days: an ISC day (thigh cuff inflated five times, 5 min on, 3 min off), an ISC+EX day [knee extension at 20% maximal voluntary contraction (MVC) with ISC], and a control day. MVCs and submaximal contraction tasks (15 and 30% MVC) were performed before and following the perturbations. Surface electromyogram signals were collected from thigh muscles and analyzed for median frequency and root mean square alterations. Blood samples were collected every 10 min (190 min total) and analyzed for GH concentrations. Peak GH concentrations and GH area under the curve were highest (P < 0.01) on the ISC+EX day (7.5 microg/l and 432 microg.l(-1).min(-1), respectively) compared with the ISC (0.9 microg/l and 76.4 microg.l(-1).min(-1)), and CON (1.1 microg/l and 83.8 microg.l(-1).min(-1)) days. A greater GH pulse amplitude, mass/pulse, and production rate were also observed on the ISC+EX day (P < 0.05). Following the intervention, force production decreased on the ISC and ISC+EX days by 16.1 and 55.8%, respectively, and did not return to baseline values within 5 min of recovery. During the submaximal contractions, median frequency shifted to lower frequencies for most of the muscles examined, and root mean square electromyogram was consistently elevated for ISC+EX day. In conclusion, ISC coupled with resistance exercise acutely increases GH levels and reduces MVC, whereas ISC alone decreases force capacity, without alterations in GH levels.     

Apoptosis-inducing factor regulates skeletal muscle progenitor cell number and muscle phenotype

Apoptosis Inducing Factor (AIF) is a highly conserved, ubiquitous flavoprotein localized in the mitochondrial intermembrane space. In vivo, AIF provides protection against neuronal and cardiomyocyte apoptosis induced by oxidative stress. Conversely in vitro, AIF has been demonstrated to have a pro-apoptotic role upon induction of the mitochondrial death pathway, once AIF translocates to the nucleus where it facilitates chromatin condensation and large scale DNA fragmentation. Given that the aif hypomorphic harlequin (Hq) mutant mouse model displays severe sarcopenia, we examined skeletal muscle from the aif hypomorphic mice in more detail. Adult AIF-deficient skeletal myofibers display oxidative stress and a severe form of atrophy, associated with a loss of myonuclei and a fast to slow fiber type switch, both in "slow" muscles such as soleus, as well as in "fast" muscles such as extensor digitorum longus, most likely resulting from an increase of MEF2 activity. This fiber type switch was conserved in regenerated soleus and EDL muscles of Hq mice subjected to cardiotoxin injection. In addition, muscle regeneration in soleus and EDL muscles of Hq mice was severely delayed. Freshly cultured myofibers, soleus and EDL muscle sections from Hq mice displayed a decreased satellite cell pool, which could be rescued by pretreating aif hypomorphic mice with the manganese-salen free radical scavenger EUK-8. Satellite cell activation seems to be abnormally long in Hq primary culture compared to controls. However, AIF deficiency did not affect myoblast cell proliferation and differentiation. Thus, AIF protects skeletal muscles against oxidative stress-induced damage probably by protecting satellite cells against oxidative stress and maintaining skeletal muscle stem cell number and activation.     

Properties of calcium channels in cardiac muscle and vascular smooth muscle

The voltage-dependent slow channels in the myocardial cell membrane are the major pathway by which Ca²⁺ ions enter the cell during excitation for initiation and regulation of the force of contraction of cardiac muscle. The slow channels have some special properties, including functional dependence on metabolic energy, selective blockade by acidosis, and regulation by the intracellular cyclic nucleotide levels. Because of these special properties of the slow channels, Ca²⁺ influx into the myocardial cell can be controlled by extrinsic factors (such as autonomic nerve stimulation or circulating hormones) and by intrinsic factors (such as cellular pH or ATP level). The slow Ca²⁺ channels of the heart are regulated by cAMP in a stimulatory fashion. Elevation of cAMP produces a very rapid increase in number of slow channels available for voltage activation during excitation. The probability of a slow channel opening and the mean open time of the channel are increased. Therefore, any agent that increases the cAMP level of the myocardial cell will tend to potentiate I_si, Ca²⁺ influx, and contraction. The myocardial slow Ca²⁺ channels are also regulated by cGMP, in a manner that is opposite to that of CAMP. The effect of cGMP is presumably mediated by means of phosphorylation of a protein, as for example, a regulatory protein (inhibitory-type) associated with the slow channel. Preliminary data suggest that calmodulin also may play a role in regulation of the myocardial slow Ca²⁺ channels, possibly mediated by the Ca²⁺-calmodulin-protein kinase and phosphorylation of some regulatory-type of protein. Thus, it appears that the slow Ca²⁺ channel is a complex structure, including perhaps several associated regulatory proteins, which can be regulated by a number of extrinsic and intrinsic factors.VSM cells contain two types of Ca²⁺ channels: slow (L-type) Ca²⁺ channels and fast (T-type) Ca²⁺ channels. Although regulation of voltage-dependent Ca²⁺ slow channels of VSM cells have not been fully clarified yet, we have made some progress towards answering this question. Slow (L-type, high-threshold) Ca²⁺ channels may be modified by phosphorylation of the channel protein or an associated regulatory protein. In contrast to cardiac muscle where cAMP and cGMP have antagonistic effects on Ca²⁺ slow channel activity, in VSM, cAMP and cGMP have similar effects, namely inhibition of the Ca²⁺ slow channels. Thus, any agent that elevates cAMP or cGMP will inhibit Ca²⁺ influx, and thereby act to produce vasodilation. The Ca²⁺ slow channels require ATP for activity, with a K_0.5 of about 0.3 mM. C-kinase may stimulate the Ca²⁺ slow channels by phosphorylation. G-protein may have a direct action on the Ca²⁺ channels, and may mediate the effects of activation of some receptors. These mechanisms of Ca²⁺ channel regulation may be invoked during exposure to agonists or drugs, which change second messenger levels, thereby controlling vascular tone.