Tissue distribution of clenbuterol in the horse

Clenbuterol Distribution and Residues in Goat Tissues After the Repeated Administration of a Growth-Promoting Dose

This study was conducted to investigate the deposition and depletion process of clenbuterol (CL) in goat tissues, plasma and urine after the repeated administration of a growth-promoting dose. The experiment was conducted in 24 goats (21 treated and 3 controls). Treated animals were administered orally in a dose of 16 µg/kg body mass once daily for 21 consecutive days and randomly sacrificed on days 0.25, 1, 3, 7, 14, 21 and 28 of the withdrawal period. CL in goat tissues was extracted with organic solvents and determined using liquid chromatography tandem mass spectrometry. The depletion rates of tissue differed significantly. The highest concentrations of CL in all tissues are detected on day 0.25 of treatment discontinuation. After administration had been discontinued for 28 days, CL still residues in all tissues, especially, in whole eye, where the concentrations reach 363.29 ± 31.60 μg/kg. These findings confirmed that the whole eye, which are rich in pigment, showed a much higher concentration than any other studied tissue during the withdrawal period.

Effect of long-term oral administration of a low dosage of clenbuterol on body fat percentage in working and nonworking adult horses

OBJECTIVE

To determine the anabolic and lipolytic effects of a low dosage of clenbuterol administered orally in working and nonworking equids.

ANIMALS

8 nonworking horses and 47 polo ponies in active training.

PROCEDURES

Each polo pony continued training and received either clenbuterol (0.8 μg/kg) or an equal volume of corn syrup (placebo) orally twice daily for 21 days, and then was evaluated for another 21-day period. Nonworking horses received clenbuterol or placebo at the same dosage for 21 days in a crossover trial (2 treatments/horse). For working and nonworking horses, percentage body fat (PBF) was estimated before treatment and then 2 and 3 times/wk, respectively. Body weight was measured at intervals.

RESULTS

Full data sets were not available for 8 working horses. For working horses, a significant treatment effect of clenbuterol was detected by day 3 and continued through the last day of treatment; at day 21, the mean change in PBF from baseline following clenbuterol or placebo treatment was -0.80% (representing a 12% decrease in PBF) and -0.32%, respectively. By day 32 through 42 (without treatment), PBF change did not differ between groups. When treated with clenbuterol, the nonworking horses had a similar mean change in PBF from baseline from day 6 onward, which peaked at -0.75% on day 18 (an 8% decrease in PBF). Time and treatment had no significant effect on body weight in either experiment.

CONCLUSIONS AND CLINICAL RELEVANCE

Among the study equids, long-term low-dose clenbuterol administration resulted in significant decreases in body fat with no loss in body weight.

Effects of clenbuterol on horses

Clenbuterol was intended as a treatment for respiratory diseases in horses, but has been used in multiple species, including humans, for its repartitioning of fat to lean effects (free fatty acids are released from adipose tissue to be used by tissues of higher priority). In the horse industry clenbuterol application is restricted to the treatment of chronic obstructive pulmonary disease and reactive airway disease (heaves). Negative effects of clenbuterol exposure include a decrease in maximum oxygen intake and increased muscle fatigue upon exercise. As a result of these and other negative effects, clenbuterol remains strictly controlled by the US Food and Drug Administration.

Effects of clenbuterol administration on serum biochemical, histologic, and echocardiographic measurements of muscle injury in exercising horses

OBJECTIVE

To determine the effects of clenbuterol, at a dosage of up to 3.2 μg/kg for 14 days, PO, on skeletal and cardiac muscle in healthy horses undergoing treadmill exercise.

ANIMALS

12 healthy horses from 3 to 10 years old.

PROCEDURES

Horses were randomly assigned to a control group (n = 6) or clenbuterol group (6) and received either saline (0.9% NaCl) solution or clenbuterol, PO, every 12 hours for 14 days. Horses were subjected to submaximal treadmill exercise daily during treatment. Muscle biopsy specimens were collected before and after treatment for determination of apoptosis. Echocardiographic measurements, serum clenbuterol and cardiac troponin I concentrations, and serum activities of creatine kinase and aspartate aminotransferase were measured before, during, and after treatment. Jugular venous blood samples were collected every 3 days during treatment. Echocardiography was repeated every 7 days after beginning treatment. Response variables were compared between treatment groups and across time periods.

RESULTS

No significant effect of clenbuterol or exercise on response variables was found between treatment and control groups at any time point or within groups over time.

CONCLUSIONS AND CLINICAL RELEVANCE

Results did not reveal any adverse effects of treatment with an approved dose of clenbuterol on equine cardiac or skeletal muscle in the small number of horses tested.

Detection of urine and blood clenbuterol following short-term oral administration in the horse

BACKGROUND AND AIM

The pharmacokinetics of clenbuterol in equine urine and blood was investigated.

MATERIAL AND METHODS

Urine and blood samples were collected following 3-day multiple oral administrations. The samples were examined using enzyme-linked immunosorbent assay and further confirmed by solid phase extraction and capillary electrophoresis.

RESULTS

Urinary clenbuterol was detectable until day 14 after the last dose. The urinary excretion of clenbuterol was characterized by a biphasic pattern. The half-lives of the bi-exponential elimination (t(1/2alpha) and t(1/2beta)) for urinary clenbuterol were about 12.1 and 48 hours. After a single oral administration (4 microg/kg) of clenbuterol, the half-life of serum clenbuterol was approximately 11.4 hours.

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.

Evaluation of cytokine production by equine alveolar macrophages exposed to lipopolysaccharide, Aspergillus fumigatus, and a suspension of hay dust

OBJECTIVE

To evaluate cytokine production by equine alveolar macrophages after exposure to lipopolysaccharide (LPS), Aspergillus fumigatus, and hay dust, and determine the effect of clenbuterol on the cytokine response.

ANIMALS

6 horses.

PROCEDURE

Alveolar macrophages were exposed to PBS solution (negative control), LPS, hyphae and conidia of Aspergillus fumigatus (AF), or a suspension of hay dust (HDS) and incubated for 24 hours at 37 degrees C. Concentrations of tumor necrosis factor (TNF)-alpha and interleukin (IL)-1beta were measured in the supernatant. The procedure was repeated with cells that were concurrently incubated with 0.5 microM clenbuterol.

RESULTS

Exposure to HDS and AF significantly increased production of TNF-alpha by equine alveolar macrophages. The increase in TNF-alpha produced in response to HDS and AF was 5 and 7 times as great, respectively, as the increase measured in response to LPS. The concentration of IL-1beta in the supernatant was significantly increased after exposure of cells to AF. Clenbuterol was effective at inhibiting TNF-alpha production by cells exposed to LPS, HDS, or AF.

CONCLUSIONS AND CLINICAL RELEVANCE

Increased production of TNF-alpha and IL-1 indicated that the pro-inflammatory cytokines produced by alveolar macrophages in response to allergens may play a role in recurrent airway obstruction (RAO) in horses. Equine alveolar macrophages are not only a primary pulmonary defense mechanism but may also influence the pathogenesis of equine RAO. The beta2-adrenoceptor agonist clenbuterol, a drug that is commonly used for treatment of equine RAO, promotes immediate bronchodilation and may also contribute to downward modulation of the inflammatory response.

Dose-dependent apoptotic and necrotic myocyte death induced by the beta2-adrenergic receptor agonist, clenbuterol

We have investigated the dose- and time-dependency of myocyte apoptosis and necrosis induced by the beta2-adrenergic receptor agonist, clenbuterol, with the aim of determining whether myocyte apoptosis and necrosis are two separate processes or a continuum of events. Male Wistar rats were administered subcutaneous injections of clenbuterol, and immunohistochemistry was used to detect myocyte-specific apoptosis and necrosis. Myocyte apoptosis peaked 4 h after, and necrosis 12 h after, clenbuterol administration. In the soleus, peak apoptosis (5.8 +/- 2.0%; P < 0.05) was induced by 10 mug and peak necrosis (7.4 +/- 1.7%; P < 0.05) by 5 mg x kg(-1) clenbuterol. Twelve hours after clenbuterol administration, 73% of damaged myocytes labeled as necrotic, 27% as apoptotic and necrotic, and 0% as purely apoptotic. Administrations of clenbuterol (10 microg x kg(-1)) at 48-h intervals induced cumulative myocyte death over 8 days. These data show that the phenotype of myocyte death is dependent on the magnitude of the insult and the time at which it is investigated. Only very low doses induced apoptosis alone; in most cases apoptotic myocytes lysed and became necrotic and the magnitude of necrosis was greater than that of apoptosis. Thus, it is important to investigate both apoptotic and necrotic myocyte death, contrary to the current trend of only investigating apoptotic cell death.

β2-Adrenergic receptor stimulation in vivo induces apoptosis in the rat heart and soleus muscle

High doses of the β2-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 μg/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 μg/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. β-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 β1-AR, whereas in the soleus direct stimulation of the myocyte β2-AR was involved. These data show that, when administered in vivo, β2-AR stimulation by clenbuterol is detrimental to cardiac and skeletal muscles even at low doses, by inducing apoptosis through β1- and β2-AR, respectively.