The stimulation of protein degradation in muscle by Ca2+ is mediated by prostaglandin E2 and does not require the calcium-activated protease

The dose-response effects of flurbiprofen, indomethacin, ibuprofen, and naproxen on primary skeletal muscle cells

BACKGROUND

Non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen, flurbiprofen, naproxen sodium, and indomethacin are commonly employed for their pain-relieving and inflammation-reducing qualities. NSAIDs work by blocking COX-1 and/or COX-2, enzymes which play roles in inflammation, fever, and pain. The main difference among NSAIDs lies in their affinity to these enzymes, which in turn, influences prostaglandin secretion, and skeletal muscle growth and regeneration. The current study investigated the effects of NSAIDs on human skeletal muscle cells, focusing on myoblast proliferation, differentiation, and muscle protein synthesis signaling.

METHODS

Using human primary muscle cells, we examined the dose-response impact of flurbiprofen (25-200 µM), indomethacin (25-200 µM), ibuprofen (25-200 µM), and naproxen sodium (25-200 µM), on myoblast viability, myotube area, fusion, and prostaglandin production.

RESULTS

We found that supraphysiological concentrations of indomethacin inhibited myoblast proliferation (-74 ± 2% with 200 µM; -53 ± 3% with 100 µM; both p < 0.05) compared to control cells and impaired protein synthesis signaling pathways in myotubes, but only attenuated myotube fusion at the highest concentrations (-18 ± 2% with 200 µM, p < 0.05) compared to control myotubes. On the other hand, ibuprofen had no such effects. Naproxen sodium only increased cell proliferation at low concentrations (+36 ± 2% with 25 µM, p < 0.05), and flurbiprofen exhibited divergent impacts depending on the concentration whereby low concentrations improved cell proliferation (+17 ± 1% with 25 µM, p < 0.05) but high concentrations inhibited cell proliferation (-32 ± 1% with 200 µM, p < 0.05).

CONCLUSION

Our findings suggest that indomethacin, at high concentrations, may detrimentally affect myoblast proliferation and differentiation via an AKT-dependent mechanism, and thus provide new understanding of NSAIDs' effects on skeletal muscle cell development.

Inhibition of prostaglandin-degrading enzyme 15-PGDH rejuvenates aged muscle mass and strength

Treatments are lacking for sarcopenia, a debilitating age-related skeletal muscle wasting syndrome. We identifed increased amounts of 15-hydroxyprostaglandin dehydrogenase (15-PGDH), the prostaglandin E₂ (PGE₂)-degrading enzyme, as a hallmark of aged tissues, including skeletal muscle. The consequent reduction in PGE₂ signaling contributed to muscle atrophy in aged mice and results from 15-PGDH-expressing myofibers and interstitial cells, such as macrophages, within muscle. Overexpression of 15-PGDH in young muscles induced atrophy. Inhibition of 15-PGDH, by targeted genetic depletion or a small-molecule inhibitor, increased aged muscle mass, strength, and exercise performance. These benefits arise from a physiological increase in PGE₂ concentrations, which augmented mitochondrial function and autophagy and decreased transforming growth factor-β signaling and activity of ubiquitin-proteasome pathways. Thus, PGE₂ signaling ameliorates muscle atrophy and rejuvenates muscle function, and 15-PGDH may be a suitable therapeutic target for countering sarcopenia.

Low-dose aspirin and COX inhibition in human skeletal muscle

Skeletal muscle health has been shown to benefit from regular consumption of cyclooxygenase (COX)-inhibiting drugs. Aspirin, especially at low doses, is one of the most commonly consumed COX inhibitors, yet investigations of low-dose aspirin effects on skeletal muscle are nonexistent. The goal of this study was to examine the efficacy of low-dose aspirin on skeletal muscle COX production of the inflammatory regulator prostaglandin (PG)E₂ at rest and after exercise. Skeletal muscle biopsies (vastus lateralis) were taken from eight individuals [4 men, 4 women; 25 ± 1 yr; 81.4 ± 3.4 kg; maximal oxygen consumption (V̇o_2max): 3.33 ± 0.21 L/min] before and 3.5 h after 40 min of cycling at 70% of V̇o_2max for the measurement of ex vivo PGE₂ production. Muscle strips were incubated in Krebs-Henseleit buffer (control) or supplemented with one of two aspirin concentrations that reflected blood levels after a low (10 µM; typical oral dose: 75-325 mg) or standard (100 µM; typical oral dose: 975-1,000 mg) dose. Low (-22 ± 5%)- and standard (-28 ± 5%)-dose aspirin concentrations both reduced skeletal muscle PGE₂ production, independent of exercise (P < 0.05). There was no difference in PGE₂ suppression between the two doses (P > 0.05). In summary, low-dose aspirin levels are sufficient to inhibit the COX enzyme in skeletal muscle and significantly reduce production of PGE₂, a known regulator of skeletal muscle health. Aerobic exercise does not appear to alter the inhibitory efficacy of aspirin. These findings may have implications for the tens of millions of individuals who chronically consume low-dose aspirin.NEW & NOTEWORTHY This study demonstrated that even low-dose aspirin concentrations can significantly reduce the prostaglandin (PG)E₂/cyclooxygenase (COX) pathway activity in human skeletal muscle and this effect is not altered during the recovery period following aerobic exercise. These findings are noteworthy since aspirin is one of the most commonly consumed drugs in the world and nonaspirin COX-inhibiting drugs have been shown to regulate skeletal muscle health in sedentary and exercise-training individuals.

Ibuprofen does not impair skeletal muscle regeneration upon cardiotoxin-induced injury

Muscle regeneration is regulated through interaction between muscle and immune cells. Studies showed that treatment with supra-physiological doses of Non-Steroidal Anti-Inflammatory Drug (NSAID) abolished inflammatory signaling and impaired muscle recovery. The present study examines the effects of pharmacologically-relevant NSAID treatment on muscle regeneration. C57BL/6 mice were injected in the tibialis anterior (TA) with either PBS or cardiotoxin (CTX). CTX-injected mice received ibuprofen (CTX-IBU) or were untreated (CTX-PLAC). After 2 days, Il-1beta and Il-6 expression was upregulated in the TA of CTX-IBU and CTX-PL vs. PBS. However, Cox-2 expression and macrophage infiltration were higher in CTX-PL vs. PBS, but not in CTX-IBU. At the same time, anabolic markers were higher in CTX-IBU vs. PBS, but not in CTX-PL. Nevertheless, ibuprofen did not affect muscle mass or muscle fiber regeneration. In conclusion, mild ibuprofen doses did not worsen muscle regeneration. There were even signs of a transient improvement in anabolic signaling and attenuation of inflammatory signaling.

High doses of anti-inflammatory drugs compromise muscle strength and hypertrophic adaptations to resistance training in young adults

AIMS

This study tested the hypothesis that high doses of anti-inflammatory drugs would attenuate the adaptive response to resistance training compared with low doses.

METHODS

Healthy men and women (aged 18-35 years) were randomly assigned to daily consumption of ibuprofen (IBU; 1200 mg; n = 15) or acetylsalicylic acid (ASA; 75 mg; n = 16) for 8 weeks. During this period, subjects completed supervised knee-extensor resistance training where one leg was subjected to training with maximal volitional effort in each repetition using a flywheel ergometer (FW), while the other leg performed conventional (work-matched across groups) weight-stack training (WS). Before and after training, muscle volume (MRI) and strength were assessed, and muscle biopsies were analysed for gene and protein expression of muscle growth regulators.

RESULTS

The increase in m. quadriceps volume was similar between FW and WS, yet was (averaged across legs) greater in ASA (7.5%) compared with IBU (3.7%, group difference 34 cm³ ; P = 0.029). In the WS leg, muscle strength improved similarly (11-20%) across groups. In the FW leg, increases (10-23%) in muscle strength were evident in both groups yet they were generally greater (interaction effects P < 0.05) for ASA compared with IBU. While our molecular analysis revealed several training effects, the only group interaction (P < 0.0001) arose from a downregulated mRNA expression of IL-6 in IBU.

CONCLUSION

Maximal over-the-counter doses of ibuprofen attenuate strength and muscle hypertrophic adaptations to 8 weeks of resistance training in young adults. Thus, young individuals using resistance training to maximize muscle growth or strength should avoid excessive intake of anti-inflammatory drugs.

The Role of Inflammation in Age-Related Sarcopenia

Many physiological changes occur with aging. These changes often, directly or indirectly, result in a deterioration of the quality of life and even in a shortening of life expectancy. Besides increased levels of reactive oxygen species, DNA damage and cell apoptosis, another important factor affecting the aging process involves a systemic chronic low-grade inflammation. This condition has already been shown to be interrelated with several (sub)clinical conditions, such as insulin resistance, atherosclerosis and Alzheimer's disease. Recent evidence, however, shows that chronic low-grade inflammation also contributes to the loss of muscle mass, strength and functionality, referred to as sarcopenia, as it affects both muscle protein breakdown and synthesis through several signaling pathways. Classic interventions to counteract age-related muscle wasting mainly focus on resistance training and/or protein supplementation to overcome the anabolic inflexibility from which elderly suffer. Although the elderly benefit from these classic interventions, the therapeutic potential of anti-inflammatory strategies is of great interest, as these might add up to/support the anabolic effect of resistance exercise and/or protein supplementation. In this review, the molecular interaction between inflammation, anabolic sensitivity and muscle protein metabolism in sarcopenic elderly will be addressed.

Aspirin-induced heat stress resistance in chicken myocardial cells can be suppressed by BAPTA-AM in vitro

Our recent studies have displayed the protective functions of aspirin against heat stress (HS) in chicken myocardial cells, and it may be associated with heat shock proteins (HSPs). In this study, we further investigated the potential role of HSPs in the aspirin-induced heat stress resistance. Four of the most important HSPs including HspB1 (Hsp27), Hsp60, Hsp70, and Hsp90 were induced by aspirin pretreatment and were suppressed by BAPTA-AM. When HSPs were induced by aspirin, much slighter HS injury was detected. But more serious damages were observed when HSPs were suppressed by BAPTA-AM than those cells exposed to HS without BAPTA-AM, even the myocardial cells have been treated with aspirin in prior. Comparing to other HSPs, HspB1 presented the largest increase after aspirin treatments, 86-fold higher than the baseline (the level before HS). These findings suggested that multiple HSPs participated in aspirin's anti-heat stress function but HspB1 may contribute the most. Interestingly, during the experiments, we also found that apoptosis rate as well as the oxidative stress indicators (T-SOD and MDA) was not consistently responding to heat stress injury as expected. By selecting from a series of candidates, myocardial cell damage-related enzymes (CK-MB and LDH), cytopathological tests, and necrosis rate (measured by flow cytometry assays) are believed to be reliable indicators to evaluate heat stress injury in chicken's myocardial cells and they will be used in our further investigations.

The skeletal muscle arachidonic acid cascade in health and inflammatory disease

Muscle atrophy and weakness are often observed in patients with chronic inflammatory diseases, and are the major clinical features of the autoimmune myopathies, polymyositis and dermatomyositis. A general understanding of the pathogenesis of muscle atrophy and the impaired muscle function associated with chronic inflammatory diseases has not been clarified. In this context, arachidonic acid metabolites, such as the prostaglandin and leukotriene subfamilies, are of interest because they contribute to immune and nonimmune processes. Accumulating evidence suggests that prostaglandins and leukotrienes are involved in causing muscular pain and inflammation, and also in myogenesis and the repair of muscles. In this Review, we summarize novel findings that implicate prostaglandins and leukotrienes in the muscle atrophy and weakness that occur in inflammatory diseases of the muscles, with a focus on inflammatory myopathies. We discuss the role of the arachidonic acid cascade in skeletal muscle growth and function, and individual metabolites as potential therapeutic targets for the treatment of inflammatory muscle diseases.

The influence of acute resistance exercise on cyclooxygenase-1 and -2 activity and protein levels in human skeletal muscle

This study evaluated the activity and content of cyclooxygenase (COX)-1 and -2 in response to acute resistance exercise (RE) in human skeletal muscle. Previous work suggests that COX-1, but not COX-2, is the primary COX isoform elevated with resistance exercise in human skeletal muscle. COX activity, however, has not been assessed after resistance exercise in humans. It was hypothesized that RE would increase COX-1 but not COX-2 activity. Muscle biopsies were taken from the vastus lateralis of nine young men (25 ± 1 yr) at baseline (preexercise), 4, and 24 h after a single bout of knee extensor RE (three sets of 10 repetitions at 70% of maximum). Tissue lysate was assayed for COX-1 and COX-2 activity. COX-1 and COX-2 protein levels were measured via Western blot analysis. COX-1 activity increased at 4 h (P < 0.05) compared with preexercise, but returned to baseline at 24 h (PRE: 60 ± 10, 4 h: 106 ± 22, 24 h: 72 ± 8 nmol PGH2·g total protein(-1)·min(-1)). COX-2 activity was elevated at 4 and 24 h after RE (P < 0.05, PRE: 51 ± 7, 4 h: 100 ± 19, 24 h: 98 ± 14 nmol PGH2·g total protein(-1)·min(-1)). The protein level of COX-1 was not altered (P > 0.05) with acute RE. In contrast, COX-2 protein levels were nearly 3-fold greater (P > 0.05) at 4 h and 5-fold greater (P = 0.06) at 24 h, compared with preexercise. In conclusion, COX-1 activity increases transiently with exercise independent of COX-1 protein levels. In contrast, both COX-2 activity and protein levels were elevated with exercise, and this elevation persisted to at least 24 h after RE.

Effects of prostaglandins and COX-inhibiting drugs on skeletal muscle adaptations to exercise

It has been ∼40 yr since the discovery that PGs are produced by exercising skeletal muscle and since the discovery that inhibition of PG synthesis is the mechanism of action of what are now known as cyclooxygenase (COX)-inhibiting drugs. Since that time, it has been established that PGs are made during and after aerobic and resistance exercise and have a potent paracrine and autocrine effect on muscle metabolism. Consequently, it has also been determined that orally consumed doses of COX inhibitors can profoundly influence muscle PG synthesis, muscle protein metabolism, and numerous other cellular processes that regulate muscle adaptations to exercise loading. Although data from acute human exercise studies, as well as animal and cell-culture data, would predict that regular consumption of a COX inhibitor during exercise training would dampen the typical muscle adaptations, the chronic data do not support this conjecture. From the studies in young and older individuals, lasting from 1.5 to 4 mo, no interfering effects of COX inhibitors on muscle adaptations to resistance-exercise training have been noted. In fact, in older individuals, a substantial enhancement of muscle mass and strength has been observed. The collective findings of the PG/COX-pathway regulation of skeletal muscle responses and adaptations to exercise are compelling. Considering the discoveries in other areas of COX regulation of health and disease, there is certainly an interesting future of investigation in this re-emerging area, especially as it pertains to older individuals and the condition of sarcopenia, as well as exercise training and performance of individuals of all ages.

Various jobs of proteolytic enzymes in skeletal muscle during unloading: facts and speculations

Skeletal muscles, namely, postural muscles, as soleus, suffer from atrophy under disuse. Muscle atrophy development caused by unloading differs from that induced by denervation or other stimuli. Disuse atrophy is supposed to be the result of shift of protein synthesis/proteolysis balance towards protein degradation increase. Maintaining of the balance involves many systems of synthesis and proteolysis, whose activation leads to muscle adaptation to disuse rather than muscle degeneration. Here, we review recent data on activity of signaling systems involved in muscle atrophy development under unloading and muscle adaptation to the lack of support.

COX-2 inhibitor reduces skeletal muscle hypertrophy in mice

Anti-inflammatory strategies are often used to reduce muscle pain and soreness that can result from high-intensity muscular activity. However, studies indicate that components of the acute inflammatory response may be required for muscle repair and growth. The hypothesis of this study was that cyclooxygenase (COX)-2 activity is required for compensatory hypertrophy of skeletal muscle. We used the synergist ablation model of skeletal muscle hypertrophy, along with the specific COX-2 inhibitor NS-398, to investigate the role of COX-2 in overload-induced muscle growth in mice. COX-2 was expressed in plantaris muscles during compensatory hypertrophy and was localized mainly in or near muscle cell nuclei. Treatment with NS-398 blunted the increases in mass and protein content in overloaded muscles compared with vehicle-treated controls. Additionally, the COX-2 inhibitor decreased activity of the urokinase type plasminogen activator, macrophage accumulation, and cell proliferation, all of which are required for hypertrophy after synergist ablation. Expression of insulin-like growth factor-1 and phosphorylation of Akt, mammalian target of rapamycin, and p70S6K were increased following synergist ablation, but were not affected by NS-398. Additionally, expression of atrogin-1 was reduced during hypertrophy, but was also not affected by NS-398. These results demonstrate that COX-2 activity is required for skeletal muscle hypertrophy, possibly through facilitation of extracellular protease activity, macrophage accumulation, and cell proliferation.

Effects of arachidonic acid supplementation on training adaptations in resistance-trained males

Background

To determine the impact of AA supplementation during resistance training on body composition, training adaptations, and markers of muscle hypertrophy in resistance-trained males.

Methods

In a randomized and double blind manner, 31 resistance-trained male subjects (22.1 ± 5.0 years, 180 ± 0.1 cm, 86.1 ± 13.0 kg, 18.1 ± 6.4% body fat) ingested either a placebo (PLA: 1 g·day^-1 corn oil, n = 16) or AA (AA: 1 g·day^-1 AA, n = 15) while participating in a standardized 4 day·week^-1 resistance training regimen. Fasting blood samples, body composition, bench press one-repetition maximum (1RM), leg press 1RM and Wingate anaerobic capacity sprint tests were completed after 0, 25, and 50 days of supplementation. Percutaneous muscle biopsies were taken from the vastus lateralis on days 0 and 50.

Results

Wingate relative peak power was significantly greater after 50 days of supplementation while the inflammatory cytokine IL-6 was significantly lower after 25 days of supplementation in the AA group. PGE₂ levels tended to be greater in the AA group. However, no statistically significant differences were observed between groups in body composition, strength, anabolic and catabolic hormones, or markers of muscle hypertrophy (i.e. total protein content or MHC type I, IIa, and IIx protein content) and other intramuscular markers (i.e. FP and EP₃ receptor density or MHC type I, IIa, and IIx mRNA expression).

Conclusion

AA supplementation during resistance-training may enhance anaerobic capacity and lessen the inflammatory response to training. However, AA supplementation did not promote statistically greater gains in strength, muscle mass, or influence markers of muscle hypertrophy.

Expression and activity of cyclooxygenase isoforms in skeletal muscles and myocardium of humans and rodents

Conflicting data have been reported on cyclooxygenase (COX)-1 and COX-2 expression and activity in striated muscles, including skeletal muscles and myocardium, in particular it is still unclear whether muscle cells are able to produce prostaglandins (PGs). We characterized the expression and enzymatic activity of COX-1 and COX-2 in the skeletal muscles and in the myocardium of mice, rats and humans. By RT-PCR, COX-1 and COX-2 mRNAs were observed in homogenates of mouse and rat hearts, and in different types of skeletal muscles from all different species. By Western blotting, COX-1 and -2 proteins were detected in skeletal muscles and hearts from rodents, as well as in skeletal muscles from humans. Immunoperoxidase stains showed that COX-1 and -2 were diffusely expressed in the myocytes of different muscles and in the myocardiocytes from all different species. In the presence of arachidonic acid, which is the COX enzymatic substrate, isolated skeletal muscle and heart samples from rodents released predominantly PGE(2). The biosynthesis of PGE(2) was reduced between 50 and 80% (P < 0.05 vs. vehicle) in the presence of either COX-1- or COX-2-selective blockers, demonstrating that both isoforms are enzymatically active. Exogenous PGE(2) added to isolated skeletal muscle preparations from rodents did not affect contraction, whereas it significantly fastened relaxation of a slow type muscle, such as soleus. In conclusion, COX-1 and COX-2 are expressed and enzymatically active in myocytes of skeletal muscles and hearts of rodents and humans. PGE(2) appears to be the main product of COX activity in striated muscles.

Use of nonsteroidal anti-inflammatory drugs following exercise-induced muscle injury

The objective of this article is to examine the use of NSAIDs for attenuating exercise-induced muscle injuries (EIMI), with an emphasis on their safety and usefulness for improving muscle function and reducing soreness. NSAIDs are some of the most widely consumed medications in the world, and NSAID use as therapy for EIMI has increased dramatically over the last 20 years. However, there is a lack of agreement concerning NSAID effectiveness for this purpose. The lack of consensus about the efficacy of NSAID use in relation to EIMI has spawned a recent interest in sports medicine research regarding NSAIDs. The application of a variety of methods used to induce, assess and quantify muscle injury has contributed to the inconsistency among the findings regarding the efficacy of NSAIDs for EIMI. Therefore, future studies should focus on the evaluation of muscle injury and function, with the use of better functional measurement tools and more uniformity in the assessment tools used. However, from review of the current literature, it is concluded that NSAID use for brief periods of time is beneficial for short-term recovery of muscle function and is an important laboratory tool for the study of EIMI.

Expression of a calpastatin transgene slows muscle wasting and obviates changes in myosin isoform expression during murine muscle disuse

Muscle wasting is a prominent feature of several systemic diseases, neurological damage and muscle disuse. The contribution of calpain proteases to muscle wasting in any instance of muscle injury or disease has remained unknown because of the inability to specifically perturb calpain activity in vivo. We have generated a transgenic mouse with muscle-specific overexpression of calpastatin, which is the endogenous inhibitor of calpains, and induced muscle atrophy by unloading hindlimb musculature for 10 days. Expression of the transgene resulted in increases in calpastatin concentration in muscle by 30- to 50-fold, and eliminated all calpain activity that was detectable on zymograms. Muscle fibres in ambulatory, transgenic mice were smaller in diameter, but more numerous, so that muscle mass did not differ between transgenic and non-transgenic mice. This is consistent with the role of the calpain-calpastatin system in muscle cell fusion that has been observed in vitro. Overexpression of calpastatin reduced muscle atrophy by 30 % during the 10 day unloading period. In addition, calpastatin overexpression completely prevented the shift in myofibrillar myosin content from slow to fast isoforms, which normally occurs in muscle unloading. These findings indicate that therapeutics directed toward regulating the calpain-calpastatin system may be beneficial in preventing muscle mass loss in muscle injury and disease.

Effect of indomethacin on the development of eccentric exercise-induced localized sensitive region in the fascia of the rabbit

The effect of indomethacin on the development of delayed onset muscle soreness (DOMS) and localized sensitive region produced by eccentric exercise was examined in lightly anesthetized rabbits (n=12, 2.0-3.3 kg). Repeated eccentric contractions of the gastrocnemius (GS) muscle were made by manual extensions during the tetanic contractions induced by electrical stimulation of the tibial nerve. The development of DOMS was confirmed by evoked reflex EMG in the biceps femoris (BF) muscle elicited by a quantitative manual extension of the GS muscle. The distribution of thresholds for the evoked BF EMG was measured by focal electrical stimulations of the GS muscle. Indomethacin (5 mg/kg in 2% sodium bicarbonate) or a vehicle was injected subcutaneously before, during, and after the exercise (a total of 60 mg/kg in 12 doses). A clear ropy taut band was palpated at the GS muscle on the second day after the exercise and a localized sensitive region for evoked BF EMG was detected at the depth of the fascia of the band in the exercise and vehicle groups, whereas no such phenomena appeared in the control and indomethacin groups. The palpable band and sensitive region disappeared on the seventh day after the exercise. That indomethacin inhibits the development of DOMS and the localized sensitive region suggests that a sensitization of polymodal-type nociceptors in the fascia mediated by prostaglandins is a possible mechanism for the development of DOMS and the localized sensitive region.

Effect of 10-day cast immobilization on sarcoplasmic reticulum calcium regulation in humans

This study investigated the effects of 10-day lower limb cast immobilization on sarcoplasmic reticulum (SR) Ca2+ regulation. Muscle biopsies were analysed in eight healthy females for maximal rates of SR Ca2+ release, Ca2+ uptake and Ca2+ ATPase activity at control, during immobilization at day 3 (IM 3), day 6 (IM 6) and day 10 (IM 10). Quadriceps muscle cross-sectional area (CSA) and 1-repetition maximum (1RM) leg extension strength were measured to determine the extent of muscle size and strength adaptations. Muscle CSA and strength decreased following 10 days of immobilization (11.8 and 41.6%, respectively, P < 0.01). A decrease in SR Ca2+ uptake rate (analysed per g wet wt) was found at IM 3 (13.2%, P=0.05), with a further decrease at IM 10 (19.8% from control, P < 0.01). At IM 10, a decrease in SR Ca2+ uptake rate (per mg protein) also occurred (19.9%, P < 0.01). Sarcoplasmic reticulum Ca2+ ATPase activity and rate of Ca2+ release were not altered with 10 days of immobilization. This study observed a decrease in SR Ca2+ uptake rate, muscular atrophy and strength loss over 10 days of immobilization in humans.

Eye muscle sparing by the muscular dystrophies: lessons to be learned?

The devastating consequences of the various muscular dystrophies are even more obvious when a muscle or muscle group is spared. The study of the exceptional cell or tissue responses may prove to be of considerable value in the analysis of disease mechanisms. The small muscles responsible for eye movements, the extraocular muscles, have functional and morphological characteristics that set them aside from other skeletal muscles. Notably, these muscles are clinically unaffected in Duchenne/Becker, limb-girdle, and congenital muscular dystrophies, pathologies due to a broken mechanical or signaling linkage between the cytoskeleton and the extracellular matrix. Uncovering the strategies used by the extraocular muscles to "naturally" protect themselves in these diseases should contribute to knowledge of both pathogenesis and treatment. We propose that careful investigation of the cellular determinants of extraocular muscle-specific properties may provide insights into how these muscles avoid or adapt to the cascade of events leading to myofiber degeneration in the muscular dystrophies.

Signalling pathways regulating protein turnover in skeletal muscle

The protein content of skeletal muscle is determined by the relative rates of synthesis and degradation which must be regulated coordinately to maintain equilibrium. However, in conditions such as fasting where amino acids are required for gluconeogenesis, or in cancer cachexia, this equilibrium is disrupted and a net loss of protein ensues. This review, utilising studies performed in several situations, summarizes the current state of knowledge on the possible signalling pathways regulating protein turnover in skeletal muscle and highlights areas for future work.

Changes in skeletal muscle biochemistry and histology relative to fiber type in rats with heart failure

One of the primary consequences of left ventricular dysfunction (LVD) after myocardial infarction is a decrement in exercise capacity. Several factors have been hypothesized to account for this decrement, including alterations in skeletal muscle metabolism and aerobic capacity. The purpose of this study was to determine whether LVD-induced alterations in skeletal muscle enzyme activities, fiber composition, and fiber size are 1) generalized in muscles or specific to muscles composed primarily of a given fiber type and 2) related to the severity of the LVD. Female Wistar rats were divided into three groups: sham-operated controls (n = 13) and rats with moderate (n = 10) and severe (n = 7) LVD. LVD was surgically induced by ligating the left main coronary artery and resulted in elevations (P < 0.05) in left ventricular end-diastolic pressure (sham, 5 +/- 1 mmHg; moderate LVD, 11 +/- 1 mmHg; severe LVD, 25 +/- 1 mmHg). Moderate LVD decreased the activities of phosphofructokinase (PFK) and citrate synthase in one muscle composed of type IIB fibers but did not modify fiber composition or size of any muscle studied. However, severe LVD diminished the activity of enzymes involved in terminal and beta-oxidation in muscles composed primarily of type I fibers, type IIA fibers, and type IIB fibers. In addition, severe LVD induced a reduction in the activity of PFK in type IIB muscle, a 10% reduction in the percentage of type IID/X fibers, and a corresponding increase in the portion of type IIB fibers. Atrophy of type I fibers, type IIA fibers, and/or type IIB fibers occurred in soleus and plantaris muscles of rats with severe LVD. These data indicate that rats with severe LVD after myocardial infarction exhibit 1) decrements in mitochondrial enzyme activities independent of muscle fiber composition, 2) a reduction in PFK activity in type IIB muscle, 3) transformation of type IID/X to type IIB fibers, and 4) atrophy of type I, IIA, and IIB fibers.

Evidence of a temperature-sensitive step in the release of prostaglandin E2 in calcium ionophore-stimulated rat muscle

Recent studies have shown that mild hypothermia (32-35 degrees C) confers striking protection against ischemic muscle and neuronal injuries, although the mechanisms are unknown. We previously demonstrated that the release of prostaglandin E2 (PGE2) from metabolically stressed muscles was dependent on calcium and was abolished at or below 35 degrees C. In this study, we examined the temperature response of the release of arachidonic acid (AA) and its cyclooxygenase metabolites, PGE2 and prostaglandin F2 alpha (PGF2 alpha) from rat skeletal muscle in the presence of calcium ionophore A23187, an agent that directly elevates intracellular calcium. Calcium ionophore markedly stimulated the release of AA, PGE2 and PGF2 alpha at 37 degrees C, as expected. Reducing the temperature to 35 degrees C and below sharply decreased PGE2 and PGF2 alpha release but not AA release. The activity of phospholipase A2 stimulated by calcium ionophore was unaffected when temperature of incubation was lowered from 37 to 32 degrees C. The results suggest that reducing temperature from 37 degrees C to 35 degrees C or below inhibits the conversion from free arachidonate to PGs in calcium ionophore-stimulated muscle.

Mechanical stimulation of skeletal muscle increases prostaglandin F2 alpha production, cyclooxygenase activity, and cell growth by a pertussis toxin sensitive mechanism

Repetitive mechanical stimulation of differentiated skeletal muscle in tissue culture increased the long-term production of prostaglandin F2 alpha, an anabolic stimulator of myofiber growth. Within 4 h of initiating mechanical stimulation, the enzymatic activity of cyclooxygenase (prostaglandin GH synthase [PGHS]), a regulatory enzyme in prostaglandin synthesis, was increased 82% (P < .005), and this increase was maintained for at least 24 h. Kinetic analysis of stretch-activated cyclooxygenase activity indicated a two to threefold decrease in the enzyme's Km, with little change in its Vmax. Immunocytochemical analysis of the cell cultures indicated the presence of high levels of the mitogen-inducible isoform of cyclooxygenase (PGHS-2) in the skeletal myofibers compared to the interstitial fibroblasts. While the stretch-induced increase in cyclooxygenase enzymatic activity was not inhibited by tetrodotoxin and therefore was independent of cellular electrical activity, the G protein inhibitor pertussis toxin prevented stretch-induced cyclooxygenase activation. Pertussis toxin also inhibited stretch-induced increases in PGF2 alpha production, phospholipase D activation, and cell growth. It is concluded that stretch of skeletal muscle increases muscle cell growth through a G protein-dependent process involving the activation of cyclooxygenase, an immediate early gene product.

The 20S/26S proteasomal pathway of protein degradation in muscle tissue

Similar to all other eukaryotic cells and tissues muscle tissue contains the proteolytic system of 20S/26S proteasomes with the 20S proteasome existing predominantly in a latent state. Unlike with the mammalian enzyme in vitro transition from the latent to the activated state of the 20S proteasomes isolated from muscle of several fish species and from lobster can be achieved by heat shock. It is very likely that the activated state of the 20S proteasome corresponds to the physiologically active form of the enzyme since only that one is able to attack sarcoplasmic and myofibrillar proteins to any significant extent. As perfusion of rat hindquarters with presumptive low molecular mass activators like free fatty acids does not result in an activation of the muscle proteasome other--possibly protein activators--may serve this purpose in vivo. The 26S proteasome complex may be regarded as such a proteasome/activator complex. The 26S proteasome complex has the ability to degrade protein (-ubiquitin-conjugates) by an ATP-consuming reaction. Since increased amounts of ubiquitinated proteins as well as an enhanced activity of the ATP (-ubiquitin)-dependent proteolytic system have been measured in rat muscle tissue during various catabolic conditions, it is not unlikely that this pathway is responsible for catalysis of muscle protein breakdown.

Calcium and protein kinase C regulation of the glucocorticoid receptor in mouse corticotrope tumor cells

The effect of increasing intracellular free calcium and activating protein kinase C on glucocorticoid receptor (GR) expression was investigated in AtT-20 cells, a mouse corticotrope tumor cell line. Treatment of AtT-20 cells with the calcium ionophore A23187 induced a rapid time- and dose-dependent decrease in [3H]dexamethasone ([3H]DEX) binding when measured in intact cells. Binding fell to 16% of control following 3 h of treatment with 10 microM A23187. In contrast, A23187 did not acutely effect steady state levels of GR mRNA, although levels fell to 76 +/- 1% of control after 8-15 h of treatment. Scatchard analysis of A23187 treated cultures demonstrated a decrease in GR binding capacity but no change in affinity for [3H]DEX. Acute inhibition of protein synthesis with cycloheximide had no effect on [3H]DEX binding, suggesting that the calcium-dependent decrease was not simply due to inhibition of GR protein synthesis. In contrast to the A23187 induced decrease in [3H]DEX binding in intact cells, when binding was measured in cytosol extract from A23187 treated cultures there was no decrease. These data suggest that the A23187 induced decrease in GR binding in whole cells is not due to a decrease in GR protein but reversible conversion of the receptor to a non-binding form. Inducing calcium influx only through L-type voltage-dependent calcium channels with BAY K8644 also decreased [3H]DEX binding at AtT-20 cells, though the effect was less than that induced by A23187. Although activation of protein kinase C with phorbol ester transiently increases intracellular free calcium in AtT-20 cells, when cells were treated for 0.5 to 22 h with phorbol 12-myristate 13-acetate, there was no acute fall in [3H]DEX binding, and only a small decrease following 16 h of treatment. These data demonstrate that sustained increases in intracellular calcium in corticotropes can induce a rapid and marked decrease in GR binding. The mechanism is post-translational and involves the reversible conversion of the receptor to a non-binding form. In addition, the cellular milieu is clearly important in conferring non-binding status on GR since once the cell is disrupted GR binding is restored.

Effect of acetyl-L-carnitine on lipid peroxidation and xanthine oxidase activity in rat skeletal muscle

It has been reported that acetyl-L-carnitine (AcCn) can reduce the degenerative processes in the central nervous system of rats, modify the fluidity of membranes and decrease the accumulation of lipofuscins in neurones. In light of these considerations we have assayed the in vitro effect of acetyl-L-carnitine on spontaneous and induced lipoperoxidation in rat skeletal muscle; in addition, the effect of AcCn on XD/XO ratio was evaluated. The presence of AcCn (10-40 mM) in incubation medium significantly reduced MDA and conjugated diene formation in rat skeletal muscle; moreover, a significant decrease in induced MDA levels was observed when microsomal preparation where incubated in the presence of 10-40 mM AcCn. Since a significant reduction of XO activity was detected in the presence of 10-80 mM AcCn, the reduced lipid peroxidation by AcCn seems to be due to an inhibition of XO activity.

Effect of cell age and phenylhydrazine on the cation transport properties of rabbit erythrocytes

We studied the effect of cell age on the cation transport systems of rabbit erythrocytes by increasing the proportion of circulating young erythrocytes with either repeated bleeding or with phenylhydrazine (PHZ) treatment. We found that when the reticulocyte content of rabbit blood is increased by bleeding (from 1 to 40-50% of the circulating red cells), the response of the various transport pathways differs. The largest increase (fivefold) was found in the activity of K-Cl cotransport, which peaked 3 days after the last bleeding. The Na-K pump activity peaked at a similar time, but the % increase was twofold less than the K-Cl cotransport. There was a very small increase in the activity of the Na-Li exchange, whereas the Na-H exchange reached peak values 10 days after the last bleeding (twofold increase), when activities of K-Cl cotransport and Na-K pump had returned to almost normal levels. In vivo PHZ treatment resulted in anemia and marked reticulocytosis (80-90% of circulating cells). Transport rates were markedly increased (Na-K pump 9.6-fold, Na-H exchange 6.8-fold, Na-Li exchange 2.75-fold; K-Cl cotransport: 10-20-fold). When blood from PHZ-treated rabbits was incubated in vitro for 24-48 hours, red cell volume and K content decreased. This process was associated with a 70% reduction in the activity of the K-Cl cotransport after 24 hours and a 90% reduction after 48 hours. The activity of the other systems also declined and approached baseline values after 48 hours. Loss of transport activity was not affected by 10 microM E-64, whereas 10 mM methylamine reduced the inactivation of the Na-H exchange and of the Na-Li exchange. PHZ treatment of rabbit red cells in vitro resulted in marked increase of the K-Cl cotransport and inhibition of Na-K pump, Na-H exchange, and Na-Li exchange. These effects were abolished by DTT, with the exception of the Na-K pump inhibition, which was DTT insensitive. Thus both cell age and oxidative damage are important determinants of cation transport in rabbit red cells.

Alterations of protein degradation and 2-D protein pattern in muscle cells of MDX and DMD origin

Intracellular protein turnover of MDX, DMD and normal muscle was determined by [35S]methionine pulse-chase experiments and subsequent high resolution 2-D gel electrophoresis. In MDX myotubes intracellular degradation of short-lived and long-lived proteins was markedly increased by a factor of 1,4-2,1. In wildtype the rate of degradation of short-lived proteins was approximately 2.6%/h, whereas in MDX these proteins were degraded by 5.7%/h. Long-lived proteins were degraded in wildtype at a rate of 1.8%/h, and in MDX at a rate of 2.5%/h. Furthermore, we have described a 51.000 Da protein with an IEP of 5.1 (p51/5.1), whose net content is highly and specifically reduced in cultured MDX and DMD muscle cells as well as in isolated MDX muscle fibers. Treatment with calcium-channel blockers Dantrolene and Verapamil inhibited the degradation of p51/5.1 in MDX myotubes by more than 90% in contrast to controls.

Calcium, calmodulin and 3',5'-cyclic nucleotide phosphodiesterase activity in human muscular disorders

3',5'-Cyclic nucleotide phosphodiesterase (PDE) is known to play an important role in the regulation of cyclic nucleotide levels in various tissues including the muscle. Previous studies have estimated the level of this enzyme in several neuromuscular disorders but the results have been variable. Moreover, there was no attempt made to correlate the enzyme levels with the levels of calcium and calmodulin, both of which regulate diverse biological processes including muscle contraction. In the present study we have estimated phosphodiesterase in the muscle of normal controls as well as patients with myotonic (MyD) and Duchenne muscular dystrophy (DMD) and amyotrophic lateral sclerosis (ALS). PDE was found to be increased significantly in all of the diseased muscles as compared to controls (P less than 0.01). But the increase could be coupled with an increase in calcium and calmodulin only in Duchenne dystrophic muscle.

Modulation of prostaglandin E2 synthesis in rat skeletal muscle

The effect of muscle denervation, inhibitors of protein synthesis, G proteins, and sphingolipids on prostaglandin E2 (PGE2) release by rat soleus muscle in vitro was investigated. To assess the effect of muscle denervation, the sciatic nerve in one hindlimb of rats was interrupted, and soleus muscles from the denervated hindlimb and the contralateral sham (control) hindlimb were excised 1-5 days after surgery. Compared with corresponding sham muscles, PGE2 release by denervated muscles was increased 56, 230, and 435% at 1, 3, and 5 days after denervation, respectively. Protein synthesis inhibitors cycloheximide (10 microM) and puromycin (10 microM) lowered PGE2 release by sham and denervated muscles 62-80%. The release of PGE2 by sham and denervated muscles was not altered by pertussis toxin (1 microgram/ml) but was inhibited 30-51% by AlF4-. Addition of 100 microM guanosine 5'-O-(3-thiotriphosphate) to saponin-permeabilized sham and denervated muscles had only a moderate, if any, stimulatory effect on PGE2 release. This effect was not counteracted by 1 mM guanosine 5'-O-(2-thiodiphosphate). Increasing muscle ceramide concentration by incubation with sphingomyelinase (100 mU/ml) increased PGE2 release by sham and denervated muscles 43 and 157%, respectively. Because degradation of ceramides yields sphingosine, the effect of sphingosine was also tested. Sphingosine (25 microM) increased PGE2 release by sham and denervated muscles 139 and 187%, respectively, without affecting muscle viability, as assessed by the release of lactate dehydrogenase. The data indicate that muscle denervation, treatment with sphingomyelinase, and sphingosine stimulate, whereas inhibitors of protein synthesis inhibit PGE2 synthesis by muscle.

Muscle pains and biochemical changes following suxamethonium administration after six pretreatment regimens

The incidence of muscle pains and changes in serum concentrations of potassium, calcium and creatine kinase following suxamethonium were investigated after no pretreatment or pretreatment with intravenous tubocurarine 0.05 mg.kg-1, intravenous chlorpromazine 0.1 mg.kg-1, alphatocopherol (vitamin E) 600 mg in three divided doses orally, aspirin 600 mg orally or intravenous calcium chloride 5 mg.kg-1 in groups of 20 patients each. The incidence of myalgia was reduced significantly by tubocurarine, chlorpromazine and alphatocopherol. However, the increase in creatine kinase was attenuated only in the groups of patients who received tubocurarine and chlorpromazine. The changes in serum potassium and calcium concentrations were within acceptable limits. The intubating conditions were not as good in the patients who received tubocurarine as in the other groups. Effectiveness of chlorpromazine in preventing both the myalgia and the biochemical changes suggests the involvement of phospholipases in the pathogenesis of suxamethonium-induced muscle damage.

Free amino acids during chronic cyclosporine A toxicity in intact and partially nephrectomized rats

The effects of 40 days of treatment with Cyclosporine A (CSA) on plasma and urine free amino acids were investigated in sham-operated (C) and partially nephrectomized (Pnx) female Fischer 344 rats. High Dose CSA (30 mg/kg/day ip) was associated with reduced weight gain, increased plasma urea nitrogen, and hypoproteinemia in C and Pnx animals. These animals also demonstrated increased plasma levels of alanine, markedly reduced levels of tryptophan, and an increase in urinary excretion of methylhistidines. C but not Pnx animals also showed a significant increase in plasma serine and a decrease in plasma taurine. CSA treatment of group C resulted in a progressive aminoaciduria involving substrates of the neutral and acidic renal amino acid transport systems; however, the renal excretion of taurine and beta-alanine by these animals was markedly reduced as compared to vehicle treated controls. High dose CSA exacerbated aminoaciduria in Pnx animals, but in this group, the excretion of beta amino acids was also increased. Our findings demonstrate that chronic CSA toxicity in rodents with normal renal function is characterized by increased muscle protein catabolism, significant reductions in plasma tryptophan, and an apparent decrease in whole body taurine pools. With the exception of the taurine abnormalities. CSA treatment had similar effects on Pnx animals; however, in this group, CSA-induced pathological changes were superimposed on the changes due to renal insufficiency per se. CSA toxicity as identified by the parameters investigated in this study was no more severe in Pnx animals with moderate chronic renal insufficiency than in controls with intact renal function.

Myofibrillar proteinase, cathepsin B, and protein breakdown rates in skeletal muscle from septic rats

Muscle catabolism during sepsis is mainly caused by myofibrillar protein breakdown. The mechanism of this metabolic response is not known. We tested the hypothesis that increased protein breakdown in the extensor digitorum longus (EDL) muscle of septic rats is caused by increased activity of the so-called myofibrillar proteinase, which is a nonlysosomal proteolytic enzyme, and cathepsin B, which is a lysosomal proteinase. Sepsis, induced in male Sprague-Dawley rats (50 to 60 g) by cecal ligation and puncture (CLP), resulted in an approximately 50% increase in myofibrillar proteinase activity and an approximately 30% increase in cathepsin B activity. Concomitantly, both total and myofibrillar protein breakdown rates, measured as release of tyrosine and 3-methylhistidine (3-MH), respectively, by incubated EDL muscles, were substantially elevated. Treatment of septic rats with the mast cell degranulating compound 48/80 or the lysosomal protease inhibitor leupeptin significantly reduced myofibrillar proteinase and cathepsin B activities, but did not affect protein breakdown rates. The results suggest that increased protein breakdown in septic skeletal muscle is associated with, but not caused by, myofibrillar proteinase or cathepsin B activity. The data also support the concept of a mast cell origin of the myofibrillar proteinase activity, but do not suggest an obligatory involvement of mast cell proteinase in increased protein degradation during sepsis.

Effects of prostaglandin E2 and cyclooxygenase inhibitors on clustering and level of nicotinic acetylcholine receptor in mouse myotubes co-cultured with spinal cord explant

The clustering and level of nicotinic acetylcholine receptor (n-AChR) in cultured mouse myotubes are negatively controlled by endogenous phospholipase A2 (PLA2) (Kimura et al., Int. J. Devl. Neurosci. 5, 127-133, 1987). The effects of PLA2-related metabolites, prostaglandins, leukotrienes and platelet-activating factor (PAF) were investigated using fluorescein isothiocyanate-alpha-bungarotoxin. Peak and total fluorescence within a cluster were used as indices of clustering and level of n-AChR, respectively. Prostaglandin E2 (PGE2, 1-10 microM) decreased both indices in a concentration-dependent manner. Aspirin and indomethacin, cyclooxygenase inhibitors, increased the indices at 1.0 microM and 10-30 nM, and decreased them at higher concentrations of 10-30 microM and 0.1-1 microM, respectively. Prostaglandin F2 alpha (PGF2 alpha, 1-10 microM), nordihydroguaiaretic acid (30 microM), a lipoxygenase inhibitor, and PAF (10 microM) had no effect. These results suggest that the control of endogenous PLA2 on the clustering and level of n-AChR is due to PGE2, but not to PGF2 alpha, leukotrienes or PAF.

Different mechanisms of increased proteolysis in atrophy induced by denervation or unweighting of rat soleus muscle

Mechanisms of accelerated proteolysis were compared in denervated and unweighted (by tail-cast suspension) soleus muscles. In vitro and in vivo proteolysis were more rapid and lysosomal latency was lower in denervated than in unweighted muscle. In vitro, lysosomotropic agents (eg, chloroquine, methylamine) did not lessen the increase in proteolysis caused by unweighting, but abolished the difference in proteolysis between denervated and unweighted muscle. Leucine methylester, an indicator of lysosome fragility, lowered latency more in denervated than in unweighted muscle. 3-Methyladenine, which inhibits phagosome formation, increased latency similarly in all muscles tested. Mersalyl, a thiol protease inhibitor, and 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-8), which antagonizes sarcoplasmic reticulum release of Ca2+, reduced accelerated proteolysis caused by unweighting without diminishing the faster proteolysis due to denervation. Calcium ionophore (A23187) increased proteolysis more so in unweighted than control muscles whether or not Ca2+ was present. Different mechanisms of accelerated proteolysis were studied further by treating muscles in vivo for 24 hours with chloroquine or mersalyl. Chloroquine diminished atrophy of the denervated but not the unweighted muscle, whereas mersalyl prevented atrophy of the unweighted but not of the denervated muscle, both by inhibiting in vivo proteolysis. These results suggest that (1) atrophy of denervated, but not of unweighted, soleus muscle involves increased lysosomal proteolysis, possibly caused by greater permeability of the lysosome, and (2) cytosolic proteolysis is important in unweighting atrophy, involving some role of Ca2(+)-dependent proteolysis and/or thiol proteases.

Mechanisms of creatine kinase release from isolated rat skeletal muscles damaged by propylene glycol and ethanol

The organic cosolvents propylene glycol and ethanol are found to cause skeletal muscle damage and creatine kinase release following intramuscular injection. The mechanisms of this organic cosolvent-induced enzyme release have not been elucidated. Cosolvent-induced creatine kinase release was enhanced by the addition of calcium to the incubation medium, and inhibited, albeit modestly, by dibucaine, a nonspecific phospholipase A2 inhibitor. The temporal pattern of creatine kinase release further suggested that cosolvent-induced enzyme release from skeletal muscles may be caused by an intracellular mechanism rather than by a direct solubilization of sarcolemma. This intracellular mechanism may involve the mobilization of calcium.

Effects of cimaterol on muscle protein metabolism and its actions in hypothyroid and hyperthyroid rats

Objectives were to examine mechanisms underlying anabolic actions of cimaterol in skeletal muscle and to evaluate cimaterol's actions in hypothyroid and hyperthyroid rats. In the first study growing rats were fed either a control diet or a diet containing cimaterol for 10 days. In a second study sham-thyroidectomized and thyroidectomized (Tx) rats were assigned to one of 5 treatments: control (sham-Tx), Tx, Tx supplemented with cimaterol, Tx injected with triiodothyronine (T3), and Tx rats injected with T3 and supplemented with cimaterol. Effects of treatments on growth, muscle weights and urinary NT-methylhistidine (NMH) excretion were evaluated in both trials. Muscle was also collected for determinations of DNA, RNA, protein and activities of several proteolytic enzymes. Cimaterol caused muscle hypertrophy and increased urinary NMH excretion. Hence, anabolic actions of cimaterol may result from an increase in myofibrillar protein synthesis which exceeds changes in myofibrillar protein degradation. Urinary NMH excretion was reduced by thyroidectomy and increased in hyperthyroid rats and both hypothyroidism and hyperthyroidism were characterized by myopathy. Cimaterol increased muscle weights in hypothyroid but not in hyperthyroid rats. Therefore, cimaterol's anabolic properties are thyroid hormone-independent and antagonized by excess thyroid hormone. Anabolic actions of cimaterol in hypothyroid rat muscle were attributed to an action on protein synthesis because urinary NMH excretion was not affected by cimaterol but muscle RNA concentration was increased. Activities of cathepsins B, D and L and neutral proteinase were dose-related to thyroid status, however, were unrelated to cimaterol-dependent perturbations in NMH excretion. Control of muscle protein balance by thyroid hormones may involve regulation of these enzymes; however, control of muscle protein degradation by cimaterol is likely directed towards other proteolytic mechanisms or to mechanisms which alter susceptibility of myofibrillar proteins to degradation.

Influence of inhibitors of eicosanoid metabolism on proliferation of rat hepatoma cells and on tumor-host interaction

The influence of eicosanoids on the proliferation of hepatoma (HTC) cells was studied in culture and in tumor-bearing rats. The cells in culture demonstrated a capacity to metabolize arachidonic acid to eicosanoids including thomboxane B2 and the prostaglandins E2 and F2 alpha a. An effect of these eicosanoids on cell proliferation was suggested by the decreased cell division seen with an inhibitor of cyclooxygenase, flurbiprofen. A biphasic effect on the proliferation of HTC cells was observed with increasing concentrations of prostaglandin F2 alpha. These studies were extended to tumor-bearing rats where inhibitory effects on the early stages of tumor growth were seen with flurbiprofen. Bleeding times were decreased in tumor-bearing rats but were restored to control values by treatment with flurbiprofen and an inhibitor of thromboxane synthetase, OKY 046. These drugs and a thromboxane/endoperoxide receptor antagonist, SQ 29, 548, were not observed to have statistically significant effects on isotope-labeled water distribution but they had substantial effects on the maintenance of body weight by tumor-bearing rats. The data suggested that the cachexia of tumor-bearing animals may be mediated at least in part by the action of eicosanoids.

Permeabilisation of the sarcolemma in mouse diaphragm exposed to Bay K 8644 in vitro: time course, dependence on Ca2+ and effects of enzyme inhibitors

Treatment of partially depolarised mouse diaphragm muscle in vitro with the Ca2(+)-channel agonist Bay K 8644 (1 microM) induces permeabilisation of the sarcolemma (visualised by penetration of procion yellow). Procion yellow staining was widespread (74% of fibres) after 2 h of treatment, but was negligible after 60 min, a time at which myofibre breakdown is well advanced and elevation of [Ca2+]i is minimal (Howl and Publicover 1989). Permeabilisation was inhibited in Ca2(+)-free saline, and was much less pronounced in polarised fibres. Inhibitors of free radical generation (particularly OH) afforded considerable protection to the muscle membrane against Bay K 8644-induced membrane permeabilisation. Inhibition of phospholipase A2 and lipoxygenase were also effective, but inhibition of xanthine oxidase (by allopurinol) had little effect. It is concluded that the initial effect of Bay K 8644 treatment is to increase Ca2+ influx through Ca2+ channels at the sarcolemma, and that this action subsequently induces membrane permeabilisation. Membrane damage probably occurs due to free radical generation and activation of phospholipase A2, both resulting from elevation of [Ca2+]i.

Immunochemical study of connectin (titin) in neuromuscular diseases using a monoclonal antibody: connectin is degraded extensively in Duchenne muscular dystrophy

Connectin (also called titin) is a myofibrillar elastic filament which links a thick filament to a neighbouring Z line in a sarcomere and thus contributes significantly to the elastic property of myofibrils. In the present study, the degradation state of connectin in biopsied skeletal muscles from various neuromuscular diseases was investigated by Western blot analysis using a monoclonal antibody which reacts extensively with the degradation products of connectin. In Duchenne muscular dystrophy (DMD), connectin was degraded progressively and relentlessly after 5 years of age. In Becker muscular dystrophy, degradation of connectin was much less than in DMD. Connectin was well preserved in normal controls, and was only minimally degraded in Charcot-Marie-Tooth disease, amyotrophic lateral sclerosis, limb girdle muscular dystrophy and myotonic dystrophy, even when the biopsied muscles showed a similar degree of weakness as those of DMD. The degradation of connectin, even though secondary, is presumed to play an important role in the pathogenesis of myofibrillar degeneration in DMD.

Hyperthermia induced in rabbits by organic calcium antagonists

Verapamil, nifedipine and cinnarizine, when injected intracerebroventricularly (ICV), induced a rise in core temperature related to the dose of the drug and accompanied by vasoconstriction of the ear vascular bed. On the contrary, the calcium channel activator BAY-K-8644, structurally related to nifedipine, elicited a dose-related hypothermic response which was accompanied by vasodilatation. The delay in onset of verapamil-induced hyperthermia was reduced by pretreating the animals with a dose of acetylsalicylic acid (ASA) which antagonized fever induced by E. coli endotoxin. BAY-K-8644 was shown to partially antagonize E. coli endotoxin-induced fever. These findings indicate that neurons responsible for temperature control are a target of organic calcium antagonists and suggest that calcium metabolism is of primary importance in the function of these cells.

Oxidative stress and muscular dystrophy

Oxidative stress may be the fundamental basis of many of the structural, functional and biochemical changes characteristic of the inherited muscular dystrophies in animals and humans. The presence of by-products of oxidative damage, and the compensatory increases in cellular antioxidants, both indicate oxidative stress may be occurring in dystrophic muscle. Changes in the proportions and metabolism of cellular lipids, abnormal functions of cellular membranes, altered activity of membrane-bound enzymes such as the SR Ca2+-ATPase, disturbances in cellular protein turnover and energy production and a variety of other changes all indicate that these inherited muscular dystrophies appear more like the results of oxidative stress to muscle than any other type of underlying muscle disturbance. Particular details of these altered characteristics of dystrophic muscle, in combination with current knowledge on the processes of oxidative damage to cells, may provide some insight into the underlying biochemical defect responsible for the disease, as well as direct research towards the ultimate goal of an effective treatment.