Effect of clenbuterol on protease activities and protein levels in rat muscle

Acute inhibitory effects of clenbuterol on force, Ca²⁺ transients and action potentials in rat soleus may not involve the β₂-adrenoceptor pathway

1. Clenbuterol, a β(2)-adrenoceptor agonist, can have inhibitory and myotoxic effects on slow-twitch muscles. Clenbuterol is lipophilic and may enter into the intracellular compartment, and because of this, it is likely that clenbuterol will have different effects to classical β(2)-adrenoceptor agonists such as terbutaline. The aim of the present study is to investigate clenbuterol's effect on force, intracellular [Ca(2+)] and electrophysiology, and the role of the β(2)-adrenoceptor pathway in these effects. 2. Simultaneous measurements of isometric force and [Ca(2+)](i) were made from small bundles of rat soleus muscle fibres in which several superficial fibres had been pressure-injected with the fluorescence Ca(2+) indicator Indo-1. The muscle's electrophysiological response was measured using glass intracellular microelectrodes. 3. The most robust effect of clenbuterol was a concentration- (10-50 μmol/L) and frequency-dependent (10-80 Hz) loss of force and [Ca(2+)](i) maintenance during tetanic stimulation of muscle fibres. None of these effects were reduced in the presence of the β(2)-antagonist ICI 118551. 4. In addition clenbuterol had a significant effect on muscle electrophysiology, with action potentials measured during tetanic trains being inhibited in a concentration- and frequency-dependent manner. This response was also unchanged by pre-treatment with the β(2)-antagonist ICI 118551. 5. These results indicate that some of clenbuterol's effects are mediated through a pathway other than the β(2)-adrenoceptors.

Adverse and beneficial functions of proteolytic enzymes in skeletal muscle. An overview

Proteolytic enzymes (proteases) comprise a family of enzymes which hydrolyse protein or peptide substrates in the generalised process of intracellular protein degradation, a process essential for the normal functioning of all cells. Proteases may also have a wide range of additional functions, including metabolic control of physiologically active oligopeptides or precursor protein forms, antigen presentation/recognition by the major histocompatibility complex in the cellular immune response, as well as in digestion, blood clotting, complement activation, etc. In this article, the nomenclature and classification of proteolytic enzymes in skeletal muscle, and their role in normal muscle physiological processes have been reviewed, including exercise, muscle development and ageing. Although proteases play an important role in normal muscle functioning, in pathological situations the enzymes may themselves be regarded as 'toxic agents' in terms of their damaging effects on muscle tissue. Muscle damage resulting from inappropriate activity of proteolytic enzymes in muscle wasting associated with muscular dystrophies, denervation atrophy, inflammatory myopathies, cancer, sepsis, diabetes and alcoholism have been reviewed. In addition, evidence that the adverse effects of drugs known to induce muscle wasting, such as corticosteroids, (or beneficial effects of growth promoting drugs) may be mediated via proteolytic enzymes is also reviewed.

Comparison of proteolytic enzyme activities in adults of insecticide resistant and susceptible strains of the housefly M. domestica L

Intracellular proteolytic activity in a DDT-resistant and a susceptible strain of M. domestica was determined by assaying a comprehensive range of cytoplasmic and lysosomal enzymes. The resistant strain showed significantly higher protease activities in whole body, head, thorax, abdomen and gut homogenates compared to the susceptible strain. The activity of alanyl- and arginyl aminopeptidase from both strains increased substantially after topical treatment with DDT at 1, 2 and 3 h, suggesting an involvement of proteolytic enzymes in the induction of detoxifying enzymes, thus indicating a possible role of the intracellular proteolytic activities in the resistance mechanism.

Effects of clenbuterol on contractile and biochemical properties of skeletal muscle

We investigated the effects of clenbuterol on the muscle mass, contractile properties, myosin phenotype, and bioenergetic enzyme activity in the gastrocnemius (GS)-plantaris (PL)-soleus (SO) muscle complex. Rats were sham-injected or treated with clenbuterol (2 mg.kg-1, subcutaneously) for 14 d. Clenbuterol increased (P < 0.05) body weight and muscle complex weight. Also, clenbuterol treatment resulted in an increase in total muscle force production and maximal shortening velocity (P < 0.05). No difference (P > 0.05) in relative force production (force.g-1 muscle) existed between experimental groups. However, muscle fatigue increased with clenbuterol treatment. Myosin heavy chain (MHC) composition was not altered in the GS or PL muscles, but shifted toward the fast Type II MHC in the SO. Myosin light chain (MLC) composition was not altered in any of the muscles. Clenbuterol caused a decrease in oxidative and glycolytic enzyme activity in the GS and PL, but not the SO. These data suggest that the clenbuterol-induced increase in muscle mass and maximal force generation is due to hypertrophy of both fast and slow fibers. Furthermore, these findings support the notion that beta-agonists may be beneficial in combating conditions that result in muscle wasting and dysfunction.

Effect of prednisone on protease activities and structural protein levels in rat muscles in vivo

To further elucidate the biochemical mechanism by which the corticosteroid prednisone induces differential changes in muscle mass (via altered protein synthesis/degradation rates) in normal or degenerating muscle tissues, we have determined the activity of a range of proteolytic enzyme types, together with levels of muscle structural proteins, in five innervated and denervated muscle types from control and drug treated rats. In both normal and wasting muscles, the activity of many protease types was substantially down-regulated following treatment with prednisone; however, accompanying net decreases in muscle mass were observed (although the structural protein composition of muscles was unaltered following drug treatment). We conclude that whilst overall rates of protein degradation in both normal and degenerating muscle may be reduced (via protease down-regulation) following prednisone treatment, the effect of the latter in reducing protein synthesis rates must be proportionately greater (even in actively degenerating tissue). Thus, the data do not support the hypothesis that the beneficial effect of prednisone in maintaining muscle mass in pathological tissues (e.g., Duchenne muscular dystrophy (DMD)) operates principally via down-regulation of protease action/protein catabolism.

Comparison of structural protein and proteolytic enzyme levels in degenerating and regenerating rat muscle induced by Notechis scutatus venom

To develop a clear understanding of the biochemical mechanism of muscle degeneration and regeneration induced by a single dose of Notechis scutatus scutatus venom, we have correlated changes in the levels of a series of muscle structural proteins and proteolytic enzymes. The degradation of structural proteins post-injection fell into two broad groups; those completely degraded within 3-6 hr (e.g. C- and M-proteins, skelemin), and within 1-2 days (e.g. myosin, actin, troponin), respectively. Similarly, activation of proteases followed two general patterns; those enzymes showing substantially increased activity after 12-24 hr (lysosomal cathepsins, leucyl aminopeptidase) and those enzymes showing decreased activity after 12-24 hr, with substantially increased activity after 3-4 days (mainly cytoplasmic proteases). The data suggest that activation of cathepsins B, L and D and in particular leucyl aminopeptidase, may be responsible for the early stages of structural protein catabolism, and are thus potential therapeutic targets to prevent myonecrosis following envenomation.

Clenbuterol mimics effects of innervation on myogenic regulatory factor expression

The amelioration of denervation atrophy by the beta-adrenoceptor agonist clenbuterol has led to the suggestion that the drug mimics or stimulates production of neurotrophic factors. Neurotrophic factors have profound effects on muscle growth, but the precise mechanisms through which this influence is exerted are unknown. The expression of myoD and myogenin, proteins that in turn regulate the transcription of tissue-specific genes during skeletal muscle differentiation, is controlled by innervation. In muscle undergoing denervation-induced atrophy, myoD and myogenin mRNAs increase. However, this is only partially reversed by electrical activity, thus implicating neurotrophic factors in regulation of these genes. Here we demonstrate that clenbuterol represses myoD and myogenin expression and decreases the levels of acetylcholine receptors in denervated muscles. The data provide the first evidence that the action of clenbuterol is directed through the neural axis.

Effect of protease inhibitors and clenbuterol on the in vitro degradation of dystrophin by endogenous proteases in human skeletal muscle

The in vitro degradation of dystrophin protein by endogenous proteases in human skeletal muscle has been investigated using a tissue homogenate assay system with subsequent protein analysis via SDS polyacrylamide electrophoresis and immunoblotting (using a monoclonal antibody to the central rod region of dystrophin). The rate of dystrophin degradation and nature of the proteolytic fragments formed at pH 5.5 and pH 7.5 (corresponding to the two major protease groups of relevance to intracellular protein catabolism) were broadly similar; incorporation of protease inhibitors in the above system suggested that Ca2+ activated proteinase and cathepsin D are principally responsible for the degradation of dystrophin at pH 7.5 and pH 5.5 respectively. The rate of dystrophin degradation at pH 7.5 was reduced by approximately 20% in the presence of 10(-5) M clenbuterol, a beta-adrenoceptor agonist with therapeutic potential in the treatment of human muscle wasting diseases.

Effect of growth promoters on pig muscle structural protein and proteolytic enzyme levels in vivo and in vitro

To elucidate the biochemical mechanism by which the growth hormone porcine somatotrophin (PST) promotes skeletal muscle growth, we have determined the activity of a comprehensive range of protein catabolizing proteolytic enzymes and structural proteins (determined by analytical electrophoresis, SDS-PAGE) in longissimus dorsi muscle from control and PST treated pigs. There was no significant difference in the levels of muscle structural proteins, or in the activity of muscle proteolytic enzymes at point of slaughter in control or PST treated animals. Similarly, in post-mortem muscle proteolysis time course experiments at pH 5.5 or pH 7.5, there was no significant difference in the rate of structural protein degradation by endogenous muscle proteases (determined via SDS-PAGE) using muscle from control or PST treated animals. In addition, investigation of a range of beta-agonist related drugs (clenbuterol, salbutamol, pirbuterol, fenoterol) showed no effect (10(-4)-10(-8) M) in vitro on the activity of individual protease types in control muscle, or on the degradation rate of muscle structural proteins by endogenous proteases in time course experiments at pH 5.5 or pH 7.5.(ABSTRACT TRUNCATED AT 250 WORDS)