title: beta2-Adrenergic receptor stimulation in vivo induces apoptosis in the rat heart and soleus muscle.
abstract: High doses of the beta2-adrenergic receptor (AR) agonist clenbuterol can induce necrotic myocyte death in the heart and slow-twitch skeletal muscle of the rat. However, it is not known whether this agent can also induce myocyte apoptosis and whether this would occur at a lower dose than previously reported for myocyte necrosis. Male Wistar rats were given single subcutaneous injections of clenbuterol. Immunohistochemistry was used to detect myocyte-specific apoptosis (detected on cryosections via a caspase 3 antibody and confirmed with annexin V, single-strand DNA labeling, and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling). Myocyte apoptosis was first detected at 2 h and peaked 4 h after clenbuterol administration. The lowest dose of clenbuterol to induce cardiomyocyte apoptosis was 1 microg/kg, with peak apoptosis (0.35 +/- 0.05%; P < 0.05) occurring in response to 5 mg/kg. In the soleus, peak apoptosis (5.8 +/- 2%; P < 0.05) was induced by the lower dose of 10 microg/kg. Cardiomyocyte apoptosis was detected throughout the ventricles, atria, and papillary muscles. However, this damage was most abundant in the left ventricular subendocardium at a point 1.6 mm, that is, approximately one-quarter of the way, from the apex toward the base. beta-AR antagonism (involving propranolol, bisoprolol, or ICI 118551) or reserpine was used to show that clenbuterol-induced myocardial apoptosis was mediated through neuromodulation of the sympathetic system and the cardiomyocyte beta1-AR, whereas in the soleus direct stimulation of the myocyte beta2-AR was involved. These data show that, when administered in vivo, beta2-AR stimulation by clenbuterol is detrimental to cardiac and skeletal muscles even at low doses, by inducing apoptosis through beta1- and beta2-AR, respectively.
authors.affiliation: Research Institute for Sports and Exercise Sciences, Liverpool John Moores Univ., Webster St., Liverpool, L3 2ET, United Kingdom. j.burniston@livjm.ac.uk
authors.forename: Jatin G
authors.initials: JG
authors.lastname: Burniston
authors.forename: Lip-Bun
authors.initials: LB
authors.lastname: Tan
authors.forename: David F
authors.initials: DF
authors.lastname: Goldspink
mesh_terms: D058666:Adrenergic beta-2 Receptor Agonists; D000318:Adrenergic beta-Agonists / Q000008:administration & dosage; D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000502:physiology*; D002976:Clenbuterol / Q000008:administration & dosage*; D004305:Dose-Response Relationship, Drug; D006321:Heart / Q000187:drug effects; D008297:Male; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D018343:Receptors, Adrenergic, beta-2 / Q000378:metabolism*
chemical_list: D058666:Adrenergic beta-2 Receptor Agonists; D000318:Adrenergic beta-Agonists; D018343:Receptors, Adrenergic, beta-2; D002976:Clenbuterol