Molecular impact of clenbuterol and isometric strength training on rat EDL muscles

Abstract

Clenbuterol, a beta2-adrenergic-receptor agonist, is known to provoke muscle hypertrophy and a slow-to-fast phenotype change. A more glycolytic phenotype should be paralleled by changes in muscle glycolytic metabolism. Two groups (n=16 for each) of 3-month-old male Wistar rats (UCL: untrained clenbuterol, and ECL: exercised clenbuterol) received a chronic administration of clenbuterol (2 mg/kg body weight/day). Two other groups of animals (U: untrained and E: exercised), were given a 0.9% NaCl solution instead of clenbuterol. E and ECL animals followed an 8-week progressive isometric force strength-training program. Both clenbuterol administration and training resulted in an increase in extensor digitorum longus (EDL) mass despite the fact that this muscle was indirectly mobilised during isometric force strength training. Clenbuterol and training induced a consistent slow-to-fast phenotype change without drastically increasing specific activities of glycolytic enzymes. Except for GAPDH and hexokinase, modifications in glycolytic-enzyme-specific activities were not explained by transcriptional changes. Lactate dehydrogenase activity was not affected by clenbuterol but was strongly augmented by training. In EDL of ECL rats, both treatments presented an opposite effect compensating each other. GLUT1 mRNA expression was augmented in EDL of UCL and ECL animals, whereas monocarboxylate transporter 1 mRNA amounts were decreased in EDL of UCL rats. Citrate synthase activity was reduced by clenbuterol treatment but remained unchanged in EDL of E animals. Creatine kinase activity was enhanced only by clenbuterol alone. These data show that clenbuterol-induced muscle hypertrophy and slow-to-fast phenotype changes are not associated with a glycolytic-enzyme-activity increase. They also suggest that in EDL isometric force strength training can reverse clenbuterol-induced molecular adaptations.

Clenbuterol, a β2-adrenergic agonist, reciprocally alters PGC-1 alpha and RIP140 and reduces fatty acid and pyruvate oxidation in rat skeletal muscle

“…Chronic administration of clenbuterol induces the transition from slow to fast muscle fiber types (35,49,53,56) and reduces skeletal muscle oxidative capacities (14,66), as indicated by the reductions in citrate synthase (CS) and cytochrome-c oxidase (COX) activities in skeletal muscle (49,53,66). The ability to oxidize fatty acids is likely also impaired, as shown by the clenbuterol-induced downregulation of carnitine palmitoyltransferase (CPT-I) mRNA (58) and skeletal muscle mitochondrial volume (14,66).…”

“…The ability to oxidize fatty acids is likely also impaired, as shown by the clenbuterol-induced downregulation of carnitine palmitoyltransferase (CPT-I) mRNA (58) and skeletal muscle mitochondrial volume (14,66). To compensate, skeletal and cardiac muscle may become more reliant on glycolytic metabolism, as clenbuterol increased GLUT4 protein and glucose transport (14), the activities of glycolytic enzymes (35,49,53), and the rate of glucose oxidation (58). Thus, although there is substantial evidence that clenbuterol markedly alters mitochondrial content and fuel selection in skeletal muscle, on the basis of changes in metabolic enzymes, it remains unclear how these metabolic effects of clenbuterol are mediated.…”

“…Nevertheless, it has been found that the treatment with this clenbuterol provoked deleterious effects on the cardiovascular system (1) and bone mass (12), as well as impairing exercise performance (16,32). The mechanistic bases of the maladaptations induced by clenbuterol in skeletal muscle remain largely unknown.Chronic administration of clenbuterol induces the transition from slow to fast muscle fiber types (35,49,53,56) and reduces skeletal muscle oxidative capacities (14, 66), as indicated by the reductions in citrate synthase (CS) and cytochrome-c oxidase (COX) activities in skeletal muscle (49,53,66). The ability to oxidize fatty acids is likely also impaired, as shown by the clenbuterol-induced downregulation of carnitine palmitoyltransferase (CPT-I) mRNA (58) and skeletal muscle mitochondrial volume (14,66).…”

β-Adrenergic stimulation does not activate p38 MAP kinase or induce PGC-1α in skeletal muscle

“…It was particularly surprising that the ␤-adrenergic agonist used in these studies was the anabolic agent clenbuterol (8,14,27,28,40), which has the well-documented effect of decreasing the mitochondrial content of muscle, causing a shift in muscle fiber type to glycolytic/white/fast twitch, and markedly decreasing endurance exercise capacity (24,29,32,38,39,48). Contrary to the reports by Ezaki's group (27,28,40) Chinsomboon et al (8), and Gerhart-Hines et al (14), our results show that injection of clenbuterol, or of norepinephrine, does not result in increases in PGC-1␣ mRNA or protein or in mitochondrial enzymes.…”

“…In a study to evaluate the possibility that ␤-adrenergic stimulation might mediate the exercise-induced increase in mitochondrial biogenesis, an experiment was performed in 1981 in which rats were injected with large doses of epinephrine daily for 6 wk; the epinephrine treatment had no effect on muscle content of mitochondria (12). Clenbuterol, the ␤ 2 -adrenergic agonist used in the studies reviewed above, is a potent anabolic agent used to stimulate muscle growth in body builders and in animals raised for meat (29,32,38). It is well documented that clenbuterol causes a decrease, not an increase, in muscle mitochondria, with a shift from oxidative to glycolytic muscle fibers (24, 29, 32, 39, 48).…”