Early onset of the maximum protein anabolic effect induced by isoproterenol in chick skeletal and cardiac muscle

Isoproterenol induces an increase in muscle fiber size by the proliferation of Pax7-positive cells and in a mTOR-independent mechanism

β-Adrenergic signaling regulates many physiological processes in skeletal muscles. A wealth of evidence has shown that β-agonists can increase skeletal muscle mass in vertebrates. Nevertheless, to date, the specific role of β-adrenergic receptors in different cell phenotypes (myoblasts, fibroblasts, and myotubes) and during the different steps of embryonic skeletal muscle differentiation has not been studied. Therefore, here we address this question through the analysis of embryonic chick primary cultures of skeletal muscle cells during the formation of multinucleated myotubes. We used isoproterenol (ISO), a β-adrenergic receptor agonist, to activate the β-adrenergic signaling and quantified several aspects of muscle differentiation. ISO induced an increase in myoblast proliferation, in the percentage of Pax7-positive myoblasts and in the size of skeletal muscle fibers, suggesting that ISO activates a hyperplasic and hypertrophic muscle response. Interestingly, treatment with ISO did not alter the number of fibroblast cells, suggesting that ISO effects are specific to muscle cells in the case of chick myogenic cell culture. We also show that rapamycin, an inhibitor of the mammalian target of rapamycin signaling pathway, did not prevent the effects of ISO on chick muscle fiber size. The collection of these results provides new insights into the role of β-adrenergic signaling during skeletal muscle proliferation and differentiation and specifically in the regulation of skeletal muscle hyperplasia and hypertrophy.

Isoproterenol hydrochloride augments collagen proliferation and ATPase activity in mice soleus and EDL muscles

Aim of this study is to analyze the effect of chronic administration of beta agonist isoproterenol hydrochloride (60 mg kg(-1) day(-1); 30 days) on soleus (a slow type) and extensor digitorum longus (EDL, a fast type) muscles in young mice. Isoproterenol resulted in significant increase in muscle weight to whole body weight ratio with no increase in hypertrophy index in soleus muscle. A significant increase in noncontractile protein collagen is also observed in both muscles but more prominent in soleus muscle. Collagen proliferation is also analyzed on sodium dodecyle sulphate polyacrylamide gel electrophoresis (SDS-PAGE) of pepsin soluble and Cyanogen Bromide (CN Br) treated pepsin insoluble collagen. Isoproterenol remolded the myofibrillar proteins in both muscles but significant increase in myofibrillar ATPase activity occurred only in soleus muscle. It is concluded that growth stimulatory effect of isoproterenol hydrochloride is more prominent in soleus than FDL muscle. Isoproterenol augmented the proliferation of non-contractile protein collagen in soleus and EDL muscles. The transformation in myofibrillar proteins caused by isoproterenol might lead to an enhancement of contractile performance.

β-Agonists enhance the lactic dehydrogenase (LDH) expression in serum and ventricular myocytes of mice

Beta-agonists have skeletal muscle specific protein anabolic effects and are also known to cause cardiac hypertrophy. Changed total LDH and its isozymic patterns are conveniently employed for the detection of different pathophysiological states of the tissues. The purpose of this study is to confirm total LDH and its isozymic expression in ventricular tissue and serum in mice following oral administration of single but higher dose of isoproterenol (Iso) and clenbuterol (Cl) (100 mg/kg body wt. and 20 mg/kg body wt., respectively), after 4, 8 and 20 hours of drug administration. Mice heart witnessed increased total LDH levels with time. Serum on the other hand showed decline in total LDH concentrations at the initial points of the drug treatment. No doubt, total LDH expression increased towards 20th h post-drug treatment but this increase is mainly due to anaerobic isozymes, i.e. LDH4 and LDH5. The findings of the present study suggest that tissue damage is definitely caused by two beta-agonists after giving single dose for shorter time span (20 hours) and the impact of the damage varies from drug to drug. Increase in total LDH in serum is not due to release from heart but from some other tissues having anaerobic metabolism.

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.

Remodeling of Extracellular Matrix Protein, Collagen by Beta-Adrenoceptor Stimulation and Denervation in Mouse Gastrocnemius Muscle

A chronic administration of isoproterenol hydrochloride (60 mg/kg body weight; 30 days) alters the collagen metabolism in denervated gastrocnemius muscle of mice. Hydroxyproline assay for collagen showed an increase in collagen content by 47%, 44%, and 61% in innervated gastrocnemius + drug, denervated control, and denervated + drug, respectively, in gastrocnemius muscles after 30 days of drug administration. Collagen proliferation is beta-agonist (isoproterenol) specific confirmed with the simultaneous administration of beta-antagonist propranolol (100 mg/kg body weight; 30 days). Van Gieson staining showed heavy collagen proliferation in the epimysium region of the muscle section and adventitia of blood vessels and some specialized regions. However, denervated gastrocnemius muscle represented a heavy collagen proliferation in the endomysium region, which also is probably responsible for extensive collagen proliferation in denervated muscle after drug administration. The SDS-PAGE of pepsin-soluble collagen revealed five bands from origin to the point of migration, gamma, beta1, beta2, alpha1, and alpha2. The SDS-PAGE of CNBr-treated pepsin-insoluble collagen pointed toward the more prominent remodeling of collagen metabolism in the beta-agonist-induced denervated gastrocnemius muscle after drug administration. From the present study, we can conclude that beta-agonist, isoproterenol hydrochloride, augments collagen proliferation in innervated as well as in denervated gastrocnemius muscle.

Beta-adrenoceptor agonist clenbuterol down-regulates matrix metalloproteinase (MMP-9) and results in an impairment of collagen turnover in mice left ventricle

An oral administration of a single dose of beta-adrenoceptor agonist clenbuterol (15 mg/kg body weight) to mice resulted in an increased collagen distribution in the subendocardium and myocardium of the left ventricle. Abundant collagen accumulation is characteristic in myonecrotic regions and around blood vessels. Hydroxyproline assay and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) of pepsin insoluble collagen confirmed this stimulated collagen proliferation. An MMP-activity assay of tissue extract by gelatin in gel zymography demonstrated a significant inhibition of MMP-9 activity in the beta-agonist-treated group. The results suggest that clenbuterol treatment is capable of inducing structural and functional remodeling of the extracellular matrix by down-regulating MMP-9 activity and thereby causing an impairment of collagen turnover. This may lead to changes in the different hemodynamic properties of the tissue, including ventricular compliance.