Effect of clenbuterol on sarcoplasmic reticulum function in single skinned mammalian skeletal muscle fibers

The beta2 -adrenergic receptor - a re-emerging target to combat obesity and induce leanness?

Treatment of obesity with repurposed or novel drugs is an expanding research field. One approach is to target beta₂ -adrenergic receptors because they regulate the metabolism and phenotype of adipose and skeletal muscle tissue. Several observations support a role for the beta₂ -adrenergic receptor in obesity. Specific human beta₂ -adrenergic receptor polymorphisms are associated with body composition and obesity, for which the Gln27Glu polymorphism is associated with obesity, while the Arg16Gly polymorphism is associated with lean mass in men and the development of obesity in specific populations. Individuals with obesity also have lower abundance of beta₂ -adrenergic receptors in adipose tissue and are less sensitive to catecholamines. In addition, studies in livestock and rodents demonstrate that selective beta₂ -agonists induce a so-called 'repartitioning effect' characterized by muscle accretion and reduced fat deposition. In humans, beta₂ -agonists dose-dependently increase resting metabolic rate by 10-50%. And like that observed in other mammals, only a few weeks of treatment with beta₂ -agonists increases muscle mass and reduces fat mass in young healthy individuals. Beta₂ -agonists also exert beneficial effects on body composition when used concomitantly with training and act additively to increase muscle strength and mass during periods with resistance training. Thus, the beta₂ -adrenergic receptor seems like an attractive target in the development of anti-obesity drugs. However, future studies need to verify the long-term efficacy and safety of beta₂ -agonists in individuals with obesity, particularly in those with comorbidities.

The molecular basis for load-induced skeletal muscle hypertrophy

In a mature (weight neutral) animal, an increase in muscle mass only occurs when the muscle is loaded sufficiently to cause an increase in myofibrillar protein balance. A tight relationship between muscle hypertrophy, acute increases in protein balance, and the activity of the mechanistic target of rapamycin complex 1 (mTORC1) was demonstrated 15 years ago. Since then, our understanding of the signals that regulate load-induced hypertrophy has evolved considerably. For example, we now know that mechanical load activates mTORC1 in the same way as growth factors, by moving TSC2 (a primary inhibitor of mTORC1) away from its target (the mTORC activator) Rheb. However, the kinase that phosphorylates and moves TSC2 is different in the two processes. Similarly, we have learned that a distinct pathway exists whereby amino acids activate mTORC1 by moving it to Rheb. While mTORC1 remains at the forefront of load-induced hypertrophy, the importance of other pathways that regulate muscle mass are becoming clearer. Myostatin, is best known for its control of developmental muscle size. However, new mechanisms to explain how loading regulates this process are suggesting that it could play an important role in hypertrophic muscle growth as well. Last, new mechanisms are highlighted for how β2 receptor agonists could be involved in load-induced muscle growth and why these agents are being developed as non-exercise-based therapies for muscle atrophy. Overall, the results highlight how studying the mechanism of load-induced skeletal muscle mass is leading the development of pharmaceutical interventions to promote muscle growth in those unwilling or unable to perform resistance exercise.

Effects of chronic administration of clenbuterol on contractile properties and calcium homeostasis in rat extensor digitorum longus muscle

Clenbuterol, a β₂-agonist, induces skeletal muscle hypertrophy and a shift from slow-oxidative to fast-glycolytic muscle fiber type profile. However, the cellular mechanisms of the effects of chronic clenbuterol administration on skeletal muscle are not completely understood. As the intracellular Ca²⁺ concentration must be finely regulated in many cellular processes, the aim of this study was to investigate the effects of chronic clenbuterol treatment on force, fatigue, intracellular calcium (Ca²⁺) homeostasis and Ca²⁺-dependent proteolysis in fast-twitch skeletal muscles (the extensor digitorum longus, EDL, muscle), as they are more sensitive to clenbuterol-induced hypertrophy. Male Wistar rats were chronically treated with 4 mg.kg⁻¹ clenbuterol or saline vehicle (controls) for 21 days. Confocal microscopy was used to evaluate sarcoplasmic reticulum Ca²⁺ load, Ca²⁺ -transient amplitude and Ca²⁺ spark properties. EDL muscles from clenbuterol-treated animals displayed hypertrophy, a shift from slow to fast fiber type profile and increased absolute force, while the relative force remained unchanged and resistance to fatigue decreased compared to control muscles from rats treated with saline vehicle. Compared to control animals, clenbuterol treatment decreased Ca²⁺-transient amplitude, Ca²⁺ spark amplitude and frequency and the sarcoplasmic reticulum Ca²⁺ load was markedly reduced. Conversely, calpain activity was increased by clenbuterol chronic treatment. These results indicate that chronic treatment with clenbuterol impairs Ca²⁺ homeostasis and this could contribute to the remodeling and functional impairment of fast-twitch skeletal muscle.

Calpastatin overexpression in the skeletal muscle of mice prevents clenbuterol-induced muscle hypertrophy and phenotypic shift

Accumulating evidence suggests that the calpain/calpastatin system is involved in skeletal muscle remodelling induced by β(2) -adrenoceptor agonist treatment. In addition to other pathways, the Akt/mammalian target of rapamycin (mTOR) pathway, controlling protein synthesis, and the calcium/calmodulin-dependent protein kinase 2 (CamK2) and AMP-activated protein kinase (AMPK) pathways, recently identified as calpain substrates, could be relevant in β(2) -adrenoceptor agonist-induced skeletal muscle remodelling. In the present study we investigated muscle hypertrophy and phenotypic shifts, as well as the molecular response of components of the Akt/mTOR pathway (i.e. Akt, eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), ribosomal protein S6 (rpS6), CamK2 and AMPK), in response to calpastatin overexpression in the skeletal muscle of mice treated with 1 mg/kg per day clenbuterol for 21 days. Using gene electrotransfer of a calpastatin expression vector into the tibialis anterior of adult mice, we found that calpastatin overexpression attenuates muscle hypertrophy and phenotypic shifts induced by clenbuterol treatment. At the molecular level, calpastatin overexpression markedly decreased calpain activity, but was ineffective in altering the phosphorylation of Akt, 4E-BP1 and rpS6. In contrast, calpastatin overexpression increased the protein expression of both total AMPK and total CamK2. In conclusion, the results support the contention that the calpain/calpastatin system plays a crucial role in skeletal muscle hypertrophy and phenotypic shifts under chronic clenbuterol treatment, with AMPK and CamK2 probably playing a minor role. Moreover, the calpastatin-induced inhibition of hypertrophy under clenbuterol treatment was not related to a decreased mTOR-dependent initiation of protein translation.

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.

Nesprin-1 and actin contribute to nuclear and cytoskeletal defects in lamin A/C-deficient cardiomyopathy

Lamin A/C mutations are the most common cause of familial dilated cardiomyopathy (DCM) but the pathogenetic mechanisms are incompletely understood. Nesprins are spectrin repeat-containing proteins that interact with lamin A/C and are components of the linker-of-nucleoskeleton-and-cytoskeleton (LINC) complex that connects the nuclear envelope to the actin cytoskeleton. Our aim was to determine whether changes in nesprin-1 and actin might contribute to DCM in homozygous Lmna knockout (Lmna(-/-)) mice. Here we find that Lmna(-/-) cardiomyocytes have altered nuclear envelope morphology, disorganization of nesprin-1 and heterogeneity in the distribution of nuclear and cytoskeletal actin. Functional interactions of nesprin-1 with nuclear G-actin and with the cytoskeletal γ-actin, α-cardiac actin and α-smooth muscle actin (α-SMA) isoforms were shown by immunoprecipitation and Western blotting. At 4-6 weeks of age, Lmna(-/-) mice had normal levels of γ-actin and α-cardiac actin, but α-SMA expression was increased by 50%. In contrast to the predominant vascular distribution of α-SMA in WT ventricular sections, α-SMA had a diffuse staining pattern in Lmna(-/-) sections. Osmotic swelling studies showed enhanced radial swelling in Lmna(-/-) cardiomyocytes indicative of cytoskeletal instability. The distensibility of Lmna(-/-) cardiomyocytes with osmotic stress was reduced by addition of α-SMA-specific fusion peptide. Our findings support a model in which uncoupling of the nucleus and cytoskeleton associated with disruption of the LINC complex promotes mechanical instability and defective force transmission in cardiomyocytes. Changes in the distribution and expression patterns of nuclear and cytoskeletal actin suggest that diverse transcriptional and structural defects may also contribute to DCM in Lmna(-/-) mice.

Clenbuterol and formoterol decrease force production in isolated intact mouse skeletal muscle fiber bundles through a beta2-adrenoceptor-independent mechanism

Although the acute actions of short-acting β(2)-adrenoceptor agonists on force production in isolated mammalian skeletal muscle fibers have been the subject of a number of previous studies, those of long-acting β(2)-adrenoceptor agonists have never been investigated. Also, little is known about the cellular signal transduction events mediating their actions. Therefore, the primary aim of this study was to investigate the acute effects of treatment of mouse fast- and slow-twitch muscle fiber bundles with clenbuterol, formoterol, and salbutamol. Both clenbuterol and salbutamol increased the levels of cAMP in both fiber types, and this effect was reversed by ICI-118551. On the other hand, clenbuterol and formoterol decreased force production in both fiber types. They also increased the phosphorylation of phospholamban and β(2)-adrenoceptors in slow-twitch fiber bundles, and their effects were insensitive to propranolol, ICI-118551, and 14-22 amide. In contrast, salbutamol increased force production in both fiber types. It also increased the phosphorylation of β(2)-adrenoceptors in slow-twitch fibers only, but it had no effect on the phosphorylation of phospholamban in either fiber type. These effects were reversed by propranolol and ICI-118551 but not by 14-22 amide. Instead, 14-22 amide further potentiated the effects of salbutamol on force. In summary, long- and short-acting β(2)-adrenoceptor agonists have opposite effects on force production in isolated intact mouse skeletal muscle fiber bundles. From these results, we suggest that the acute actions of short-acting β(2)-adrenoceptor agonists on force production in mammalian skeletal muscles are mediated through the β(2)-adrenoceptor, whereas those of long-acting β(2)-adrenoceptor agonists are not.

Differential roles of NHERF1, NHERF2, and PDZK1 in regulating CFTR-mediated intestinal anion secretion in mice

The epithelial anion channel CFTR interacts with multiple PDZ domain-containing proteins. Heterologous expression studies have demonstrated that the Na+/H+ exchanger regulatory factors, NHERF1, NHERF2, and PDZK1 (NHERF3), modulate CFTR membrane retention, conductivity, and interactions with other transporters. To study their biological roles in vivo, we investigated CFTR-dependent duodenal HCO3- secretion in mouse models of Nherf1, Nherf2, and Pdzk1 loss of function. We found that Nherf1 ablation strongly reduced basal as well as forskolin-stimulated (FSK-stimulated) HCO3- secretory rates and blocked beta2-adrenergic receptor (beta2-AR) stimulation. Conversely, Nherf2-/- mice displayed augmented FSK-stimulated HCO3- secretion. Furthermore, although lysophosphatidic acid (LPA) inhibited FSK-stimulated HCO3- secretion in WT mice, this effect was lost in Nherf2-/- mice. Pdzk1 ablation reduced basal, but not FSK-stimulated, HCO3- secretion. In addition, laser microdissection and quantitative PCR revealed that the beta2-AR and the type 2 LPA receptor were expressed together with CFTR in duodenal crypts and that colocalization of the beta2-AR and CFTR was reduced in the Nherf1-/- mice. These data suggest that the NHERF proteins differentially modulate duodenal HCO3- secretion: while NHERF1 is an obligatory linker for beta2-AR stimulation of CFTR, NHERF2 confers inhibitory signals by coupling the LPA receptor to CFTR.

Anabolic effects of a non-myotoxic dose of the beta2-adrenergic receptor agonist clenbuterol on rat plantaris muscle

Previous investigations of the effects of clenbuterol have used suprapharmacological doses that induce myocyte death, alter muscle phenotype, and do not approximate the proposed therapeutic dose for humans. Recently, we reported that smaller doses of clenbuterol induce muscle growth without causing myocyte death. In the present study we used histochemical and proteomic techniques to investigate the molecular effects of this dose. Male Wistar rats (n = 6, per group) were infused with saline or 10 microg/kg/day clenbuterol via subcutaneously implanted osmotic pumps. After 14 days the animals' plantaris muscles were isolated for histochemical and proteomic analyses. Clenbuterol induced significant muscle growth with concomitant protein accretion and preferential hypertrophy of fast oxidative glycolytic fibers. Clenbuterol reduced the optical density of mitochondrial staining in fast fibers by 20% and the glycogen content of the muscle by 30%. Differential analysis of two-dimensional gels showed that heat shock protein 72 and beta-enolase increased, whereas aldolase A, phosphogylcerate mutase, and adenylate kinase decreased. Only heat shock protein 72 has previously been investigated in clenbuterol-treated muscles. The clenbuterol-induced increase in muscle growth was concomitant with qualitative changes in the muscle's proteome that need to be considered when proposing therapeutic uses for this agent.

Ultrastructural Findings for the Mitochondrial Subpopulation of Mice Skeletal Muscle after Adrenergic Stimulation by Clenbuterol

Clenbuterol, a beta-adrenoceptor agonist, has been reported to induce skeletal muscle hypertrophy. However, it has also been known to reduce aerobic exercise performance and to deleteriously affect endurance and sprint exercise performance in rats. In the present study, the chronic administration of clenbuterol (2 mg/kg body weight; 30 days) resulted in various ultrastructural changes in three different types of muscles, gastrocnemius, a mixed-fiber type; anterior latissimus dorsi (ALD), a predominantly fast-twitch type; and diaphragm, a largely oxidative-type. The most prominent changes included mitochondrial swelling, matricular vesiculation in mitochondria, mitochondrial hyperplasia, sarcoplasmic vesiculation, and intermyofibrillar dilations. An increase in the cross-sectional area of both the subsarcolemmal (170, 167, and 79%) and the intermyofibrillar (129, 134, and 84%) mitochondria is noticed in the gastrocnemius, ALD, and diaphragm, respectively. The ultramicroscopic and morphometric results suggest drug-induced defects in contractile and oxidative activities.

Dose-dependent separation of the hypertrophic and myotoxic effects of the beta(2)-adrenergic receptor agonist clenbuterol in rat striated muscles

Muscle growth in response to large doses (milligrams per kilogram) of beta(2)-adrenergic receptor agonists has been reported consistently. However, such doses may also induce myocyte death in the heart and skeletal muscles and hence may not be safe doses for humans. We report the hypertrophic and myotoxic effects of different doses of clenbuterol. Rats were infused with clenbuterol (range, 1 microg to 1 mg.kg(-1)) for 14 days. Muscle protein content, myofiber cross-sectional area, and myocyte death were then investigated. Infusions of >or=10 microg.kg(-1).d(-1) of clenbuterol significantly (P<0.05) increased the protein content of the heart (12%-15%), soleus (12%), plantaris (18%-29%), and tibialis anterior (11%-22%) muscles, with concomitant myofiber hypertrophy. Larger doses (100 microg or 1 mg) induced significant (P<0.05) myocyte death in the soleus (peak 0.2+/-0.1% apoptosis), diaphragm (peak 0.15+/-0.1% apoptosis), and plantaris (peak 0.3+/-0.05% necrosis), and significantly increased the area fraction of collagen in the myocardium. These data show that the low dose of 10 microg.kg(-1).d(-1) can be used in rats to investigate the anabolic effects of clenbuterol in the absence of myocyte death.

A functional comparison of the venom of three Australian jellyfish—Chironex fleckeri, Chiropsalmus sp., and Carybdea xaymacana—on cytosolic Ca2+, haemolysis and Artemia sp. lethality

Cnidarian venoms produce a wide spectrum of envenoming syndromes in humans ranging from minor local irritation to death. Here, the effects of Chironex fleckeri, Chiropsalmus sp., and Carybdea xaymacana venoms on ventricular myocyte cytosolic Ca2+, haemolysis and Artemia sp. lethality are compared for the first time. All three venoms caused a large, irreversible elevation of cytosolic Ca2+ in myocytes as measured using the Ca2+ sensitive fluorescent probe Indo-1. The L-type Ca2+ channel antagonist verapamil had no effect on Ca2+ influx whilst La3+, a non-specific channel and pore blocker, inhibited the effect. Haemolytic activity was observed for all venoms, with C. xaymacana venom displaying the greatest activity. These activities are consistent with the presence of a pore-forming toxin existing in the venoms which has been demonstrated by transmission electron microscopy in the case of C. fleckeri. The venom of C. fleckeri was found to be more lethal against Artemia sp. than the venom of the other species, consistent with the order of known human toxicities. This suggests that the observed lytic effects may not underlie the lethal effects of the venom, and raises the question of how such potent activities are dealt with by envenomed humans.

Effect of indomethacin on force responses and sarcoplasmic reticulum function in skinned skeletal muscle fibers and cytosolic [Ca2+] in myotubes

In this study, the effects of phospholipase A2 (PLA2) inhibitors on excitation-contraction coupling (ECC) and sarcoplasmic reticulum (SR) function were examined in skinned extensor digitorum longus (EDL) muscle fibers of the rat. The nonspecific PLA2 inhibitor indomethacin (200 microM) significantly increased the peak (approximately 2-fold, P = 0.02) and the width (approximately 6-fold, P = 0.008) of depolarization-induced force responses (DIFRs) elicited in the fibers (n = 4). Exposure of the skinned EDL fibers to indomethacin (200 microM) (n = 7) and another PLA2 inhibitor quinacrine (200 microM) (n = 5) resulted in the return of large DIFRs after use-dependent rundown. However, aristolochic acid (100 microM), an inhibitor of secretory PLA2, failed to return DIFRs after rundown. Indomethacin did not protect against the loss of DIFRs induced by exposure to elevated myofibrillar [Ca2+]. Indomethacin (200 microM) produced a small but significant increase in the Ca2+ sensitivity of the contractile apparatus of skinned EDL fibers and the maximum force production. Indomethacin (200 microM) also had significant effects on SR function, increasing SR Ca2+ loading in the skinned fibers (117.2 +/- 3.0% of controls, P = 0.0008, n = 8) and inducing intracellular Ca2+ release in isolated intact flexor digitorum brevis (FDB) fibers (n = 7) and C2C12 myotubes (n = 6). These data suggest that intracellular PLA2 may be an important modulator of ECC in skeletal muscle.

MHC-based fiber type and E-C coupling characteristics in mechanically skinned muscle fibers of the rat

In this study, we investigated whether the previously established differences between fast- and slow-twitch single skeletal muscle fibers of the rat, in terms of myosin heavy chain (MHC) isoform composition and contractile function, are also detectable in excitation-contraction (E-C) coupling. We compared the contractile responsiveness of electrophoretically typed, mechanically skinned single fibers from the soleus (Sol), the extensor digitorum longus (EDL), and the white region of the sternomastoid (SM) muscle to t-system depolarization-induced activation. The quantitative parameters assessed were the amplitude of the maximum depolarization-induced force response (DIFR(max); normalized to the maximum Ca(2+)-activated force in that fiber) and the number of responses elicited until the force declined by 75% of DIFR(max) (R-D(75%)). The mean DIFR(max) values for type IIB EDL and type IIB SM fibers were not statistically different, and both were greater than the mean DIFR(max) for type I Sol fibers. The mean R-D(75%) for type IIB EDL fibers was greater than that for type I Sol fibers as well as type IIB SM fibers. These data suggest that E-C coupling characteristics of mechanically skinned rat single muscle fibers are related to MHC-based fiber type and the muscle of origin.

Characterization of adrenoceptor involvement in skeletal and cardiac myotoxicity Induced by sympathomimetic agents: toward a new bioassay for beta-blockers

Excessive levels of catecholamines have long been known to be cardiotoxic, but less well known are their toxic effects on skeletal muscle. By using an antimyosin monoclonal antibody and quantitative methods to measure the extent of myocyte necrosis, and by employing modulators of adrenoceptors (ARs), including clenbuterol, bupranolol, propranolol, bisoprolol, atenolol, ICI-118551, phenoxybenzamine, prazosin, and yohimbine, the involvement of ARs in isoproterenol-induced myotoxicity was characterized. In the myocardium, the toxic effects were predominantly mediated via the beta(1)-ARs. In the soleus muscle, it was almost solely via the beta(2)-ARs. Myotoxicity was also observed in the myocardium when challenged with the beta(2)-AR agonist clenbuterol. This was found to be mediated via sympathetic presynaptic beta(2)-ARs, leading to enhanced release of norepinephrine. This effect was abolished by prior treatment with reserpine. The skeletal muscle was found to be more sensitive to the myotoxic effects than cardiac muscle at lower doses of beta-AR agonists. These experiments introduce a new way of assaying beta-AR antagonists by classifying them according to their ability to prevent catecholamine-induced myotoxicity. Further research along these lines may deepen understanding of which beta-blockers work best in heart failure therapy.

The effect of chelerythrine on depolarization-induced force responses in skinned fast skeletal muscle fibres of the rat

1 We examined the effect of the protein kinase C (PKC) inhibitor chelerythrine on depolarization-induced force responses (DIFRs) and sarcoplasmic reticulum (SR) function in single, mechanically skinned skeletal muscle fibres of the rat. 2 In this study, the DIFRs in the skinned fibres normally underwent an irreversible loss of excitation-contraction coupling (ECC) after 10-15 responses. Chelerythrine (12 micro M) was shown to restore ECC in these fibres. Restored force responses were similar in peak (control 50.8+/-6.4%, chelerythrine 56.9+/-12.4% of maximum force, P=0.42, n=21), but significantly broadened compared to initial control responses (full-width at half maximum, control; 3.7+/-0.3 s, chelerythrine; 13.3+/-1.1 s, P<0.001). Early exposure to chelerythrine prevented run-down of DIFRs. Chelerythrine also induced spontaneous force responses in some fibres. 3 The PKC inhibitors calphostin C and staurosporine did not restore ECC, and the PKC activator phorbol 12-myristate 13-acetate did not promote loss of ECC in the skinned fibres. 4 Chelerythrine significantly increased SR Ca(2+) loading by 8.4+/-1.7% (P=0.02, n=9) and SR Ca(2+) release by at least 14.1+/-2.7% (P=0.004, n=11) in the skinned fibres. 5 Chelerythrine had no significant effect on maximum force production or the [Ca(2+)] producing half maximal activation of the myofilaments. However, chelerythrine did have a small effect on the slope of the force-Ca(2+) relationship (P=0.02, n=10). 6 Chelerythrine reverses the use-dependent loss of excitation-contraction coupling in skinend skeletal muscle fibres by a PKC independent pathway. Chelerythrine may be an important pharmacological probe for examining the mechanisms of contraction-induced muscle injury.

Myotoxic effects of clenbuterol in the rat heart and soleus muscle

Myocyte-specific necrosis in the heart and soleus muscle of adult male Wistar rats was investigated in response to a single subcutaneous injection of the anabolic beta(2)-adrenergic receptor agonist clenbuterol. Necrosis was immunohistochemically detected by administration of a myosin antibody 1 h before the clenbuterol challenge and quantified by using image analysis. Clenbuterol-induced myocyte necrosis occurred against a background of zero damage in control muscles. In the heart, the clenbuterol-induced necrosis was not uniform, being more abundant in the left subendocardium and peaking 2.4 mm from the apex. After position (2.4 mm from the apex), dose (5 mg clenbuterol/kg), and sampling time (12 h) were optimized, maximum cardiomyocyte necrosis was found to be 1.0 +/- 0.2%. In response to the same parameters (i.e., 5 mg of clenbuterol and sampled at 12 h), skeletal myocyte necrosis was 4.4 +/- 0.8% in the soleus. These data show significant myocyte-specific necrosis in the heart and skeletal muscle of the rat. Such irreversible damage in the heart suggests that clenbuterol may be damaging to long-term health.

Chronic clenbuterol administration negatively alters cardiac function

PURPOSE

Chronic administration of pharmacological levels of beta2-agonists have been shown to have toxic effects on the heart; however, no data exist on cardiac function after chronic clenbuterol administration. The purpose of this study was to examine the effect of therapeutic levels of clenbuterol on cardiac performance.

METHODS

Twenty unfit Standardbred mares were divided into four experimental groups: clenbuterol (2.4 microg.kg(-1) twice daily 5 d.wk(-1)) plus exercise (20 min at 50% .VO(2max)) (CLENEX; N = 6), clenbuterol (CLEN; N = 6), exercise (EX; N = 4), and control (CON; N = 4). M-mode and two-dimensional echocardiography (2.5-MHz sector scanner transducer) were used to measure cardiac size and function before and immediately after an incremental exercise test, before and after 8 wk of drug and/or exercise treatments.

RESULTS

After treatment, CLENEX and CLEN demonstrated significantly higher left ventricular internal dimension (LVD) at end diastole (+23.7 +/- 4.8%; +25.6 +/- 4.1%), LVD at end systole (+29.2 +/- 8.7%; +40.1 +/- 7.9%), interventricular septal wall thickness (IVS) at end diastole (+28.9 +/- 11.0%; +30.7 +/- 7.0%), IVS at end systole (+29.2 +/- 8.7%; +40.1 +/- 7.9%), and left ventricular posterior wall systolic thickness (+43.1 +/- 14.%; +45.8 +/- 14.1%). CLENEX and CLEN had significantly increased aortic root dimensions (+29.9 +/- 6.1%; +24.0 +/- 1.7%), suggesting increased risk of aortic rupture.

CONCLUSION

Taken together, these data indicate that chronic clenbuterol administration may negatively alter cardiac function.

Hydrogen peroxide increases depolarization-induced contraction of mechanically skinned slow twitch fibres from rat skeletal muscles

The effect of exogenous hydrogen peroxide (H(2)O(2)) on excitation-contraction (E-C) coupling and sarcoplasmic reticulum (SR) function was compared in mechanically skinned slow twitch fibres (prepared from the soleus muscles) and fast twitch fibres (prepared from the extensor digitorum longus; EDL muscles) of adult rats. Equilibration (5 min) with 1 mM H(2)O(2) diminished the ability of the Ca(2+)-depleted SR to reload Ca(2+) in both slow (P < 0.01) and fast twitch fibres (P < 0.05) compared to control. Under conditions when all Ca(2+) uptake was prevented, 1 mM H(2)O(2) increased SR Ca(2+) "leak" in fast twitch fibres by 24 +/- 5 % (P < 0.05), but leak was not altered in slow twitch fibres. Treatment with 1 mM H(2)O(2) also increased the peak force of low [caffeine] contracture by approximately 45% in both fibre types compared to control (P < 0.01), which could be partly reversed following treatment with 10 mM dithiothreitol (DTT). The changes in SR function caused by 1 mM H(2)O(2) were associated with an approximately 65% increase in the peak height of depolarization-induced contractile response (DICR) in slow twitch fibres, compared to control (no H(2)O(2); P < 0.05). In contrast, peak contractile force of fast twitch fibres was not altered by 1 mM H(2)O(2) treatment. Equilibration with 5 mM H(2)O(2) induced a spontaneous force response in both slow and fast twitch fibres, which could be partly reversed by 2 min treatment with 10 mM DTT. Peak DICR was also increased approximately 40% by 5 mM H(2)O(2) in slow twitch fibres compared to control (no H(2)O(2); P < 0.05). Our results indicate that exogenous H(2)O(2) increases depolarization-induced contraction of mechanically skinned slow but not fast twitch fibres. The increase in depolarization-induced contraction in slow twitch fibres might be mediated by an increased SR Ca(2+) release during contraction and/or an increase in Ca(2+) sensitivity.

Effect of taurine on sarcoplasmic reticulum function and force in skinned fast-twitch skeletal muscle fibres of the rat

We examined the effect of taurine on depolarisation-induced force responses and sarcoplasmic reticulum (SR) function in mechanically skinned skeletal muscle fibres from the extensor digitorum longus (EDL) of the rat. Taurine (20 mM) produced a small but significant (P < 0.01) decrease in the sensitivity of the contractile apparatus to Ca(2+) (increase in the [Ca(2+)] corresponding to 50 % of maximum force of about 7 %; n = 10) and in maximum force (92.0 +/- 1.0 % of controls) in the skinned fibres. Taurine had no statistically significant effect on the slope of the force-pCa curve. Depolarisation-induced force responses in the skinned fibres were markedly increased in peak value by 20 mM taurine, to 120.8 +/- 5.3 % of control measurements (P = 0.0006, n = 27). Taurine (20 mM) significantly increased the SR Ca(2+) accumulation in the skinned fibres by 34.6 +/- 9.3 % compared to control conditions (measured by comparing the integral of caffeine contractures in fibres previously loaded with Ca(2+) in the absence or presence of taurine; P = 0.0014, n = 10). Taurine (20 mM) also increased both the peak and rate of rise of caffeine-induced force responses in the fibres by 29.2 +/- 9.7 % (P = 0.0298, n = 6) and 27.6 +/- 8.9 % (P = 0.037), respectively, compared with controls. This study shows that taurine is a modulator of contractile function in mammalian skeletal muscle. Taurine may increase the size of depolarisation-induced force responses by augmenting SR Ca(2+) accumulation and release.

Preservation of denervated muscle form and function by clenbuterol in a rat model of peripheral nerve injury

The effects of clenbuterol in preserving the form and function of muscle after unilateral sciatic nerve division and epineural repair were investigated in a rat model. The drug (a beta2-adrenoceptor agonist) was administered daily for six weeks by gastric gavage (10 microg/kg body weight), interrupted every 5 days by a 2 day omission of dosing to avoid drug desensitization. Clenbuterol reduced the loss of wet weight, total protein, muscle fibre cross sectional area and (in part) contractile forces in denervated hindlimb muscles, with most effects lasting until reinnervation. The effects were dependent on muscle type, with slow-twitch oxidative muscle (soleus) and mixed-fibre (gastrocnemius) showing greater sensitivity to the drug than fast-twitch muscle (extensor digitorum longus). Anabolic effects on the contralateral innervated muscles tended to be small. The results suggest a potential for the adjuvant use of selective beta -adrenoceptor agonists in the management of peripheral nerve injuries in humans.

Effects of beta(2)-agonist clenbuterol on biochemical and contractile properties of unloaded soleus fibers of rat

The effects of clenbuterol beta(2)-agonist administration were investigated in normal and atrophied [15-day hindlimb-unloaded (HU)] rat soleus muscles. We showed that clenbuterol had a specific effect on muscle tissue, since it reduces soleus atrophy induced by HU. The study of Ca(2+) activation properties of single skinned fibers revealed that clenbuterol partly prevented the decrease in maximal tension after HU, with a preferential effect on fast-twitch fibers. Clenbuterol improved the Ca(2+) sensitivity in slow- and fast-twitch fibers by shifting the tension-pCa relationship toward lower Ca(2+) concentrations, but this effect was more marked after HU than in normal conditions. Whole muscle electrophoresis indicated slow-to-fast transitions of the myosin heavy chain isoforms for unloaded and for clenbuterol-treated soleus. The coupling of the two latter conditions did not, however, increase these phenotypical transformations. Our findings indicated that clenbuterol had an anabolic action and a beta(2)-adrenergic effect on muscle fibers and appeared to counteract some effects of unloading disuse conditions.

An intact renin-angiotensin system is a prerequisite for normal renal development

All components of the renin-angiotensin system (RAS) are highly expressed in the developing kidney in a pattern that suggests a role for angiotensin II in renal development In support of this notion, pharmacological interruption of angiotensin II type-1 (AT1) receptor-mediated effects in animals with an ongoing nephrogenesis produces specific renal abnormalities characterized by papillary atrophy, abnormal wall thickening of intrarenal arterioles, tubular atrophy associated with expansion of the interstitium, and a marked impairment in urinary concentrating ability. Similar changes in renal morphology and function also develop in mice with targeted inactivation of the genes that encode angiotensinogen, angiotensin converting enzyme, or both AT1 receptor isoforms simultaneously. Taken together, these results clearly indicate that an intact signalling through AT1 receptors is a prerequisite for normal renal development In a recent study, an increased incidence of congenital anomalies of the kidney and urinary tract was detected in mice deficient in the angiotensin II type-2 receptor, suggesting that this receptor subtype is also involved in the development of the genitourinary tract The present report mainly reviews the renal abnormalities that have been induced by blocking the RAS pharmacologically or by gene targeting in experimental animal models. In addition, pathogenetic mechanisms and clinical implications are discussed.