Protein Turnover in Skeletal Muscle

Glucocorticoid sites in skeletal muscle: adrenalectomy, maturation, fiber type, and sex

The glucocorticoid receptor population in skeletal muscle of the rat was examined. Data are included that address the following: tissue preparation, receptor stabilization, method of assay and analysis, cross-reactivity of a large variety of steroids, time to equilibrium, and the effect of adrenalectomy on the number of sites as well as the apparent binding affinity. In addition, we have observed the following: 1) an exponential decline in the concentration of sites from 27 to 160 days after birth; 2) a significantly higher concentration of sites in muscle from male animals as compared with female animals; and 3) a significantly higher concentration of sites in the slow-fiber soleus muscle as compared with the fast-fiber extensor digitorum longus muscle.

Effects of stretch and denervation on protease activities of normal and dystrophic chicken muscle

Changes of protease activities that follow passive stretch, denervation, and denervation plus stretch were followed in the patagialis muscle of normal and dystrophic chicks between 6 and 7 weeks of age. The baseline activities of cathepsin C, cathepsin D, and leucine aminopeptidase in dystrophic muscle were 2 to 3.5 times higher than in normal muscle. Passive stretch and denervation induced increases in protease activities by 40 to 120% in normal muscle, whereas the same treatments did not significantly affect the activities of the enzymes in dystrophic muscle. We conclude that the level of protease activity in dystrophic chicken muscle at 6 weeks of age had already attained a maximum limit and could not be increased even by denervation. In spite of protease activities, which were not different from control dystrophic muscle, denervated dystrophic muscles lost muscle weight rapidly whether they were stretched or not. They weighed 60% less than the innervated control muscle after 7 days. Inherently high protease activities in dystrophic muscle do not vary at this age regardless of whether or not the muscle is gaining or losing weight.

Intracellular protein degradation: basis of a self-regulating mechanism for the proteolysis of endogenous proteins

The intracellular basal proteolysis system, as distinct from the lysosomal system, is important in sustaining a high flux of proteins required for maintenance, growth and adaptability of cells. Its activity automatically fluctuates with changes in protein synthetic activity, but with a considerably slower response time, since the two processes are only indirectly or passively linked. Since as much as one-third of intracellular proteolysis in mammalian cells is directed as nascent proteins, the consequences are more fully discussed in relation to cell growth state. During rapid growth, cells have to accumulate more than double their original protein mass in order to achieve a 100% increase between divisions. The effects of reducing protein synthesis by inducing quiescence, serum step-down or cycloheximide treatment on intracellular proteolysis are considered, and the possibility that this leads to enhanced degradation of existing proteins has been explored. No substantial evidence was found to support this latter notion. The basal proteolysis system is seen as a constitutive, pervasive and broad-spectrumed collection of hydrolytic enzymes. It destroys proteins randomly, having no means of distinguishing young from old, aberrant from normal. The rate of demise of protein substrates depends on two factors, the ease of access of the hydrolytic enzymes to their peptide bonds, and the length of time that any species of protein remains at risk to this hydrolytic potential. While the former has long been recognized, the importance of the second factor in relation to the ability of proteins to become integrated in the living fabric of the cell is only beginning to be appreciated. The discussion also suggests elaborate regulatory mechanisms akin to those for protein synthesis would be unnecessary for protein degradation, especially if it can now be substantiated that substrate availability determines the turnover rates of proteins by a pervasive and relatively unlimited proteolytic system (Grisolía, 1964).

Occurrence of reduced alpha 2-macroglobulin and lowered protease inhibiting capacity in plasma of amyotrophic lateral sclerosis patients

Lowered levels of plasma alpha 2-macroglobulin were found in 13 patients with amyotrophic lateral sclerosis using anti-alpha 2-macroglobulin-embedded agar plates. Levels of this major protease inhibitor in ALS patients were contrasted with those in disease controls, consisting of patients with a variety of neurologic disorders, as well as with those in normal individuals. The patients' levels, in milligrams per deciliter, were 191 +/- 39 (SD). The disease controls had levels of 269 +/- 33. Local normal values compared with the reported normal values in the literature (260 +/- 70). A correlation was not made for either the form of motor neuron disease, disease severity, or age. As a measure of functional protease inhibition, trypsin inhibitory capacity of sera was estimated using a radioactive fibrinolytic assay. An increase in the half-maximal inhibitory concentration of sera was found in the amyotrophic lateral sclerosis group. These findings likely relate to the known increases in proteolytic activity in denervated muscle and collagenolytic activity in skin of patients with amyotrophic lateral sclerosis. The determination of whether altered protease inhibitory potential in such patients is primary or secondary may help in understanding the pathogenesis of this enigmatic disorder and provide suggestions for future approaches to intervention.

The effect of acute and chronic glucocorticoid excess on leucine kinetics and protein turnover in vivo

The present studies were undertaken to assess the effects of excess cortisol on amino acid exchange in the conscious dog. Three groups of 18-hr fasted dogs with catheters chronically implanted in the femoral artery were studied: Group I (n = 6) received saline; Groups II and III (n = 5, each) received ACTH intravenously (1 U/min) for 7 hr; in addition, Group III received ACTH, 500 U/day intramuscularly for 4 days. Leucine rates of appearance (Ra) and clearance were measured using a constant infusion of L-4,5-[3H]leucine. ACTH treatment resulted in a 9-fold increase in plasma cortisol in Groups II and III (from 2 +/- 1 to 18 +/- 1 and 17 +/- 2 micrograms/dl, in II and III, respectively P less than 0.001), with no effect on either plasma insulin or glucagon. Plasma leucine (mmole/liter) increased from 118 +/- 6 (I) to 153 +/- 6 (II, P less than 0.005) to 275 +/- 35 (III, P less than 0.001). Leucine Ra (micromoles/kilogram/minute) did not change in II, but rose by 39% (P less than 0.005) in III. Clearance (milliliters/kilogram/minute) dropped from 25 +/- 2 (I) to 18 +/- 2 (II, P less than 0.005), to 15 +/- 2 (III, P less than 0.001). It is concluded that acute elevations of cortisol increased plasma leucine only by inhibiting its rate of disposal, whereas chronic elevations had a dual effect; they inhibited leucine disposal and increased its entry into the plasma compartment, suggesting an inhibition of protein synthesis and stimulation of protein breakdown.

Ca2+-activated protease in denervated rat skeletal muscle measured by an immunoassay

Right hind leg muscles of adult male rats were denervated by transecting the sciatic nerve, while muscles of the left leg served as a control. On days 4, 9, and 18 after denervation, the rats were killed and the extensor digitorum longus and soleus muscles removed. Extracts were prepared which contained the Ca2+-activated protease. The molecular weight of the protease in the extracts, measured immunologically on Western blots of SDS-polyacrylamide gels, was identical to that of purified standard protease and was unaffected by denervation. The amount of protease in each extract was measured by a solid phase immunoassay using monospecific IgG labeled with 125I, and the results were expressed as units of protease activity per milligram noncollagen protein. There were increases of 40 to 80% in the mean values obtained for both denervated muscles compared with controls. Because the protease is probably localized on the cell membrane, these modest increases are likely to be a consequence of a generalized over-development of muscle membranes which follows denervation.

A fluorescent microscopy study of biopsied muscles from infantile neuromuscular disorders

The Acridine Orange (AO) stain for muscle biopsies is particularly useful to identify regenerating and ongoing hypertrophic muscle fibers under fluorescent microscopy. This method was applied to muscle biopsies from 65 patients who suffered from various childhood neuromuscular disorders. While normal fibers showed dull green cytoplasm with small green-yellow nuclei, striking fluorescent fibers were observed in eight cases of congenital muscular dystrophy (CMD) and 12 cases of Duchenne muscular dystrophy (DMD); these fibers were characterized as follows: (1) small fibers with big oval or spherical nuclei which fluoresced strongly with a bright orange color; (2) fibers of various sizes and different degrees of orange fluorescence; and (3) opaque fibers with bright yellow cytoplasm. The small diameter fibers in Werdnig-Hoffmann (WH) disease, nemaline myopathy, and congenital fiber type disproportion failed to show apparent AO-RNA fluorescence. Although all the atrophic fibers in Kugelberg-Welander (KW) disease showed a vague orange fluorescent color, this was obviously different from that of regenerating fibers seen in CMD and DMD. In addition to these findings, the hypertrophic fibers in a case of unclassified myopathy also showed moderate orange fluorescence around the entire periphery of the cytoplasm.

Age-related growth of the spleen in normal and glucocorticoid treated rats

1. Age-related changes in the growth, nucleic acid content and protein turnover of the spleen have been studied in normal male rats. 2. A rapid and marked atrophy of the spleen, was found 24 hr after exposure to cortisone or dexamethasone; increased rates of protein breakdown being primarily responsible. 3. Nonetheless, the total amount of protein synthesized in the spleen (measured in vivo) was significantly decreased (30-50%) 24 hr after exposure to these steroids. 4. This compared with only a 15% decrease in whole body protein synthesis, indicating a more pronounced hormonal effect on the spleen than on most other body tissues.

The catabolic effect of glucocorticoids on different types of skeletal muscle fibres and its dependence upon muscle activity and interaction with anabolic steroids

Myofibrillar protease activity was found to be elevated, and content of RNA and DNA reduced in m. quadriceps femoris, especially in the fast-twitch white fibers, after dexamethasone treatment. Moderate physical activity prevents, to some extent, the effect of synthetic glucocorticoids. Exhaustive exercise augments the myofibrillar protease activity in both types of muscle fibers. The fast-twitch white type of muscle fibers in exhausted rats are found to have an elevated anticatabolic activity after the treatment of rats with anabolic steroid. It appears that there exists a close correlation between myofibrillar protease activity and muscle weight in the fast-twitch white fibers in dexamethasone treated and exhausted rats.

Dynamic aspects of structural proteins in vertebrate skeletal muscle

In this review, our current knowledge on the structural proteins of vertebrate skeletal muscle is briefly outlined. Structural proteins include the contractile proteins (actin and myosin), the major regulatory proteins (troponin and tropomyosin), the minor regulatory proteins (M-protein, C-protein, F-protein, I-protein, and actinins), and the scaffold proteins (connectin, desmin, and Z-protein). In addition, the relative turnover rates of the muscle proteins (M-protein greater than or equal to troponin greater than soluble protein as a whole greater than tropomyosin not equal to alpha-actinin greater than myosin greater than 10S-actinin greater than actin) are discussed. The changes in the turnover of muscle proteins are compared in denervated and dystrophic muscles. The properties of the various proteases in muscle, including alkaline protease, calcium-activated neutral protease (CANP), and acidic protease (cathepsins), and the structural alterations of myofibrils by these proteases are also described. Finally, the role of proteases and their inhibitors in diseased muscle is summarized, with focus on CANP and its inhibitors, leupeptin and E-64.

The contractile apparatus of striated muscle in the course of atrophy and regeneration. I. Myosin and actin filaments in the denervated rat soleus

The ultrastructure of the contractile apparatus of the rat soleus muscle during the course of denervation atrophy was investigated. It was found that the ratio of thin the thick filaments increased in myofibrils of atrophying muscle fibers. Elevation of the ratio was observed as early as the second day after denervation, and became more pronounced with the progress of atrophy. Parallel measurements of the amounts of actin and myosin in the myofibrils and in the muscle protein extracts revealed a lower proportion of myosin heavy chains to actin in the fractions from denervated muscles, compared with the control values. Both the electron-microscopic observations and the biochemical evaluation of the actin content of the muscle, suggests that the elevated ratio of thin of thick filaments seen in the course of the muscle atrophy appears as the results of an earlier and more intensive disappearance of thick filaments. Thin filaments disappeared more slowly, in parallel to the decrease in muscle weight. On the basis of the results presented a mechanism of progress of "simple atrophy" of muscle in suggested.

Effects of dexamethasone on fibre subtypes in rat muscle

The extent to which dexamethasone treatment produced atrophy of fast-twitch (EDL) and slow-twitch (SOL) muscles in rat was investigated. The mean weight of steroid-treated EDL muscles was decreased as compared to normal, whereas SOL muscles from normal and dexamethasone-treated animals showed no significant difference. Muscle fibre diameters also showed comparatively minor changes in SOL, which consists of Type 1 (slow oxidative) and Type 2A (fast oxidative/glycolytic) fibres. Rat EDL contains, in addition to Type 1 and Type 2A fibres, two sub-populations of fast glycolytic fibres (Types 2B and 2B'). These fibre types showed the most severe degree of atrophy both after dexamethasone treatment and after denervation. The mean ratio of the weights of denervated to innervated EDL muscles was lower in steroid-treated rats than in normal animals suggesting that the atrophy produced by steroid treatment in conjunction with denervation was more than simply additive. Analysis of the proportions of histochemical fibre types in SOL and EDL showed that dexamethasone treatment produced no major alterations in the fibre type constitution of these muscles. However, further histochemical studies showed that there was relatively severe impairment of myophosphorylase activity in Type 2B' (fast glycolytic) fibres as compared to other fibre types; conversely Type 1 fibres frequently contained increased myophosphorylase. Levels of beta-hydroxybutyrate dehydrogenase were low in both normal and steroid-treated EDL but high in SOL which also showed higher general oxidative activity. It is suggested that the particular susceptibility of fast glycolytic fibres to atrophy as a result of steroid treatment may be linked to: 1 the relatively severe reduction of myophosphorylase activity in these fibres and 2 their comparative inability to utilize alternative energy sources, especially substrates derived from free fatty acids.

A possible role for glucocorticoids in denervation atrophy

Skeletal muscle atrophies after denervation. Glucocorticoids also cause skeletal muscle to atrophy. The data presented in this report demonstrate that after denervation there is a significant increase in the number of glucocorticoid receptors in the cytosol of skeletal muscle of rats. The results are consistent with the hypothesis that a skeletal muscle atrophies after denervation because it becomes hypersensitive to glucocorticoids.

The role of proteases in experimental glucocorticoid myopathy

Mature, male, New Zealand white rabbits were treated with the synthetic glucocorticoid betamethasone (0.3 mg/kg body weight/day) for 2 weeks. The glucocorticoid treatment caused a 30% decrease in muscle weight of the type 2 psoas muscle, but had no apparent effect on the type 1 soleus muscle. Cathepsin D activity was elevated twofold in the psoas muscle of treated rabbits, a finding suggesting an active role for lysosomes in mediating muscle breakdown in glucocorticoid-induced myopathy of the rabbit. There was no detectable alkaline serine protease activity in the muscles from either treated or control rabbits. Alkaline protease is localized in mast cells in some species, particularly the rat. Toluidine blue staining for mast cells was absent in rabbit muscles, a finding indicating that this species does not contain these cells. This protease, previously implicated in glucocorticoid myopathy, apparently plays no role in rabbit myopathy. There was no detectable elevation of the Ca-activated protease in muscles from glucocorticoid-treated animals. This finding suggests that if this protease plays a role in muscle degradation, its activity is controlled in vivo by special conditions (such as elevated CA levels, inhibitors, and compartmentalizations).

Clinical usefulness of urinary 3-methylhistidine excretion in indicating muscle protein breakdown

Urinary excretion of the post-translationally modified amino-acid 3-methylhistidine, derived from the contractile proteins actin and myosin, was measured in patients with conditions associated with nitrogen loss. The ratio of 3-methylhistidine:creatinine excretion, a measure of the fractional catabolic rate of myofibrillar protein was increased in severe injury, thyrotoxicosis, neoplastic disease, prednisolone administration, and sometimes Duchenne muscular dystrophy. In myxoedema, osteomalacia, and hypothermia the ratio was decreased; and starvation, elective operations, and rheumatoid arthritis had little effect. Provided that the diet is meat free, measurement of urinary 3-methylhistidine may provide useful information on the cause of protein loss.

The influence of testosterone on the alkaline proteolytic activity in rat skeletal muscle

The effects of testectomy and subsequent administration of testosterone propionate on the activity of the alkaline proteinases in rat skeletal muscle were investigated. Castration of the mature rat was followed by a short-term delay in protein accretion in skeletal muscle tissue as measured by the protein/DNA ratio and was paralleled by a 2-3-fold increase in specific activity of the alkaline proteinase(s). This increase of proteolytic activity was equally significant when expressed relative to microgram DNA. Although the gain in body weight was significantly lower in the castrated rats, nevertheless the protein/DNA ratio in muscle after 6 weeks approximated the values of sham-operated control rats without normalization of the proteolytic activity. Treatment of the castrated rats with testosterone propionate resulted in restoring normal levels of previously elevated levels of alkaline proteolytic activity in muscle tissue. The normalization of enzyme activity as well as protein accretion in muscle was dose-dependent. Treatment of the rats with a low dose (0.1 mg/day) of testosterone propionate failed to restore the proteolytic activity, but led to a small increase of th protein/DNA ratio as well as to a progressive increase in body weight. These data indicate a regulatory role of testosterone in the adaptive behaviour of the alkaline proteolytic system in rat skeletal muscle.

Effects of denervation on the composition and synthesis of sarcolemmal proteins

In vitro synthesis of proteins and changes in polypeptide composition of sarcolemma were studied in innervated and denervated extensor digitorum longus muscle of the rat. A technique of evacuating myoplasm from muscle slices was used as a preliminary step in the preparation of three membrane fractions, M, H and S, containing sarcolemma. On the basis of findings from the previous study and the present investigation, it was concluded that the M fraction was most enriched with extrajunctional sarcolemma. In vitro incorporation of [3H]leucine into membrane proteins of the M fraction showed an apparent linear increase in the rate of protein synthesis from 1-10 days after denervation. The relative increase at 10 days was 137% greater than that of innervated controls. Fractions H and S showed a smaller relative increase. Polypeptide composition of M, H and S fractions based on SDS gel electrophoresis of innervated and denervated muscle, showed qualitative and quantitative changes. The most striking difference was a nominal 29 000 component in M that constituted a disproportionately large peak. Following 10 days of denervation the M fraction underwent significant compositional changes in its electrophoretic profile, the most dramatic of which was a large reduction in the proportion of the 19 000 component. The denervation-induced compositional change is discussed in light of known alterations in the chloride conductance of the muscle plasmalemma.

Effects of glucocorticoids on muscle protein turnover in perfused rat hemicorpus

Normal and adrenalectomized rats treated with cortisone lost 2% of their initial body weight per day, whereas controls gained weight at a rate of 2%/day. Five days of treatment resulted in a 25% reduction in the weights of a number of mixed fiber type muscles, but did not affect the weights of heart or soleus, a muscle consisting of slow-twitch red fibers. Reductions in muscle weights were accompanied by a loss of protein and RNA. Perfused hemicorpus preparations from rats receiving 5 days of treatment released several amino acids in greater amounts than the controls. Protein synthesis in perfused gastrocnemius was reduced 50-60% after 3 or 5 days of steroid treatment. This reduction was due to a loss of RNA and to an inhibition of translation resulting from an impairment in peptide-chain initiation. In contrast, RNA content and initiation were not altered in heart and soleus. Protein degradation in perfused hemicorpus and cathepsin D activity in gastrocnemius were unaffected by cortisone treatment, suggesting that loss of muscle weight and protein content as well as increased amino acid release resulted from the reduction in protein synthesis.

Effects of a mild weight-lifting program on the progress of glucocorticoid-induced atrophy in rat hindlimb muscles

The effect of a mild weight-lifting program on the progress of glucocorticoid-induced atrophy was investigated. Groups of rats were either injected daily for 6 weeks with triamcinolone acetonide, 1 mg/kg (group S), subjected to a weight-lifting program 4 times per week (group T), or subjected to the injection and weight-lifting programs concurrently (group ST). The training program, besides decreasing the normal body weight gain of control (group C) rats, had no effects on gastrocnemius and soleus muscles. In group ST, the extent of gastrocnemius atrophy was less severe than in group S, and the mean areas of all fiber types were greater in similar proportion. Gastrocnemius muscles were also tetanically stronger (g, g/g body weight) in group ST than group S. The soleus muscles of groups ST and S were similar in all indices of size and strength, except for a significantly decreased fast-twitsch-oxidative-glycolytic (FOG) mean fiber area in the ST seleus muscles. The extent of fast-twitsch muscle atrophy resulting from chronic glucocorticoid treatment can be lessened by mild weight-lifting exercise. Differences in fiber area responses between soleus and gastrocnemius may reflect recruitment and/or metabolic differences of similar fiber types in the two muscles.

Local effect of burn injury on glucose and amino acid metabolism by skeletal muscle

Previous studies from our laboratory demonstrated that there is a difference in glucose metabolism by skeletal muscles from the burned versus unburned regions of the body. To further investigate the effect of proximity to the burn wound on muscle metabolism, in vitro glucose uptake as well as lactic and amino acid releases by soleus muscles, were studied 3 days following a 3-second scald burn on one hind limb of the rat. No differences in glucose uptake or lactic and amino acid releases were observed between soleus from the unburned limb of burned rats and that of controls. In comparison to these two groups, soleus from the burned limb took up 125% more glucose (p less than 0.001), and released 80% more lactic acid (p less than 0.01), 229% more alanine (p less than 0.001), 84% more glutamic acid (p less than 0.01), and over 36% more glutamine (p less than 0.05). The relatively enhanced release of lactic acid by soleus muscle from the burned limb was reduced but not eliminated by the omission of glucose from the medium. Simultaneously, the omission of glucose had no effect on the release of alanine, glutamic acid, and glutamine by the burned limb soleus. The data indicate that a mild thermal injury stimulates glucose utilization and enhances amino acid release by skeletal muscle from the burned region. Since such an effect is absent in muscle from the contralateral unburned region of the same animal, the changes are not likely to be mediated by systemic alterations in the metabolic and endocrine environment. The persistence of an enhanced amino acid release in the presence of varying glycolytic rates suggests that the burn-induced local alterations in amino acid metabolism by skeleton muscle are independent of coincident changes in glucose utilization.

Experimental corticosteroid myopathy: effect on myofibrillar ATPase activity and protein degradation

Corticosteroid myopathy was studied in young, mature New Zealand white rabbits given daily injections of betamethasone (0.3 mg/kg body weight/day) for two weeks. Control rabbits were pair-fed and received saline injections. Bethamethasone treatment caused significant wasting of type 2 gluteus medius and psoas muscles but did not cause any atrophy of type 1 soleus and gluteus minimus muscles. The Mg2+- and Ca2+-activated myofibrillar ATPase activities of the corticosteroid-treated rabbits did not differ from controls despite a 30% reduction in muscle wet weight and pronounced reduction in cross-sectional area of fibers. SDS-polyacrylamide gel electrophoresis profiles of myofibrillar proteins did not differ quantitatively or qualitatively between experimental and control rabbits. Studies of net muscle protein degradation (using 3H-leucine) in betamethasone-treated and control rabbits indicate that both type 1 and type 2 muscle fiber proteins are degraded several times faster in the corticosteroid-treated group. This suggests that a compensatory mechanism exists for those type 1 and mixed fiber type muscles which have increased degradation but do not undergo wasting.

The effect of trenbolone acetate with time on the various responses of protein synthesis of the rat

1. The anabolic agent trenbolone acetate (3-oxo-17beta-hydroxy-4,9,11-oestratriene acetate; TBA) given subcutaneously to female rats increased their growth rate compared with that of the placebo-treated controls. 2. The increased growth rate of the TBA-treated rats was not a consequence of an increased water retention. The TBA-treated rats had a significantly higher (P less than 0.01) total carcass nitrogen content but the total carcass fat content decreased by a non-significant (P greater than 0.05) 8.3%. 3. There was evidence of a time-lag in the response of the fractional synthetic rate of certain individual tissues to TBA. The fractional synthetic rates of the uterus and skeletal muscle mixed tissue proteins were significantly reduced in the TBA-treated rats. 4. The measured reduction in fractional synthetic rates was concluded to reflect true changes in the synthetic rate rather than a result of an alteration in the specific activity of the tyrosine pool used for protein synthesis.

The uptake of 3H(G)L leucine into single muscle fibers in Charcot-Marine-Tooth disease

In previous studies, the incorporation of 3H(G)L-leucine into muscles of patients with Charcot-Marie-Tooth (CMT) disease was shown to be increased in comparison with that observed in motor neuron disease (MND). To determine the cause of the increased uptake in CMT, studies of single fiber leucine incorporations have been undertaken. The results of this study indicate that the increased incorporation is into those muscle fibers which are undergoing regeneration following reinnervation. These results do not support the thesis that there is an associated myopathic process in CMT.

The turnover of lactate dehydrogenase in skeletal muscle of the mouse. Influence of fibre type and exercise regimen

1. The influence of fibre-type and an exercise regimen on the turnover characteristics of proteins in mouse skeletal muscle has been studied. 2. Exercise induced a weight increase in most individual muscles and a decrease in lactate dehydrogenase activity which was more marked in white muscle than red. The coincident decrease in M-subunit activity was more noticeable in fast-twitch muscles than in slow-twitch types. 3. Exercise caused a considerable and consistent increase in the rate of degradation of total soluble proteins in individual muscles, with turnover being higher in muscles with a high red fibre content. 4. The response of lactate dehydrogenase to exercise varied considerably between individual muscles as indicated by the comparative rate constants for synthesis and degradation. In general, degradation was greater in muscle with a high slow-twitch or red fibre content. 5. Overall, a considerable redistribution of protein resources was observed during this physiological perturbation (exercise) with the main emphasis away from the pool of total soluble protein towards the myofibrillar constituents. Highly individualistic responses in relation to rates of synthesis and degradation were observed for the specific protein, lactate dehydrogenase, in the separate muscle of this animal.

Regulation of cardiac protein balance by hydrocortisone: interaction with insulin

In fetal mouse hearts in organ culture the rate of protein synthesis was substantially reduced and the rate of protein degradation slightly increased by hydrocortisone in the absence of insulin, but in the presence of insulin the steroid caused a small increase in protein synthesis and a significant reduction in protein degradation. Hydrocortisone promoted the net uptake (or reduced the net release) of branched-chain amino acids independent of insulin and independent of simultaneous changes in protein balance. The specific activities of the lysosomal enzymes cathepsin D and glucosaminidase were reduced by hydrocortisone in all media, whereas the specific activity of creatine kinase increased when the medium contained insulin but decreased in the absence of insulin. It is concluded that hydrocortisone regulates cardiac protein balance via alterations both in synthesis and in degradation. Some of the hormone's myocardial effects are influenced by insulin so that hydrocortisone is anabolic in its presence but catabolic in its absence.

Leupeptin, a protease inhibitor, decreases protein degradation in normal and diseased muscles

The protease inhibitor leupeptin decreases protein degradation in rat skeletal and cardiac muscle incubated in vitro, while protein synthesis remains unaltered. Leupeptin also lowers protein breakdown in denervated rat muscles and affected muscles from mice with hereditary muscular dystrophy. Leupeptin may thus be useful in retarding tissue atrophy. Since homogenates of leupeptin-treated muscles had decreased cathepsin B activity, this lysosomal protease may play a role in protein turnover in normal and diseased muscles.

The effects of denervation on protein turnover of the soleus and extensor digitorum longus muscles of adult mice

1. Changes in protein turnover of the soleus and EDL muscles of adult mice have been studied 1, 7 and 80 days after denervation. 2. Increased rates of protein degradation 7 and 80 days post-denervation correlated with the atrophy and loss of protein from these muscles. 3. Rates of protein synthesis in the EDL decreased 24 hr after nerve section. However, these synthetic rates increased again to become higher in the 7 day denervated muscles compared with their controls. These latter anabolic changes are inconsistent with the concept of a denervated muscle being inactive. 4. These findings have been compared with a similar study on muscles of growing rats. Any passive stretching of the denervated muscles by continued bone growth appears unlikely to be a crucial factor explaining the increased rates of protein synthesis 7 days after denervation.

Morphometric and autoradiographic studies on the growth of red and white axial muscle fibres in the shark Etmopterus spinax

The axial muscle growth of the shark Etmopterus spinax has been studied by means of morphometry, slide histology and autoradiography from radioactive thymidine and amino acids. Comparisons have been made between the red and white muscles fibres. While growing from 10 to 23 cm in length, the white muscle transverse sectional (ts) area increases 5.4 times, while the red area increases 3.7 times. The mean ts area of individual white fibres increases by 2.7, and the ts area of red fibres increases by 1.6. The number of white fibres does not increase significantly, while the red fibres increase slightly in number. Red fibres incorporate amino acids faster than the white fibres, and have a higher turnover rate. The myonuclei of the red fibres (satellite cell nuclei) incorporate thymidine more often than do those of white fibres, but the frequency of the satellite cells is similar in the two fibre types. Mitoses are extremely rare in the myonuclei. The nucleocytoplasmic ratio is higher in the red than in the white fibres.

Fractional catabolic rates of myosin and actin estimated by urinary excretion of Ntau-methylhistidine: the effect of dietary protein level on catabolic rates under conditions of restricted food intake

1. Critical studies on the distribution of Ntau-methylhistidine (3-methylhistidine; Me-His) among organs and tissues in adult rats are reported. Adult rats contained 46-5+/-3-6 mg Me-His/kg body-weight. Almost 90% of the Me-His in the body was recovered from skeletal muscle. These results support the hypothesis that fractional catabolic rates of myosin and actin in skeletal muscle can be estimated by measuring urinary excretion of Me-His. 2. Dietary protein level did not affect the total amount of Me-His in the body. However, urinary excretion of Me-His increased as dietary protein intake was increased. 3. From these results it was concluded that fractional catabolic rates of myosin and actin increase as dietary protein intake increases.