Clenbuterol diminishes aerobic performance in horses.
OBJECTIVE
The purpose of this 8-wk study was to examine the effect of therapeutic levels of clenbuterol on aerobic performance and hemodynamics associated with exercise.


METHODS
Twenty-three unfit Standardbred mares were divided into four experimental groups, clenbuterol (2.4 microg x kg(-1) body weight twice daily) plus exercise (20 min at 50% O2max; CLENEX; N = 6), clenbuterol only (CLEN; N = 6), exercise only (EX; N = 5), and control (CON; N = 6). All horses performed an incremental exercise test (GXT) to measure maximal oxygen consumption (O2max), blood lactate concentration, total plasma protein concentration, and hematocrit. Plasma volume, heart rate, right ventricular pressure (RVP), and pulmonary artery pressure (PAP) were measured before and after the treatment/training. Each horse also performed an exercise capacity test (ECT) in which they ran at their pretreatment O2max speed until exhausted.


RESULTS
There were no significant changes in blood lactate, total protein, or hematocrit for any group during either the GXT or ECT. CLENEX decreased (P < 0.05) O2max (-6.2%) and velocity to O2max (-10.0%), whereas both CLENEX and CLEN decreased (P < 0.05) in time to exhaustion (-20.5+/-4.7 and -20.9 +/- 5.6%). EX alone increased (P < 0.05) O2max (+6.5%), velocity to O2max (+10.0%), velocity to produces lactate concentration of 4 mmol (+13.5%), and time to exhaustion (+32.3 +/- 15.0%). Plasma volume was altered (P < 0.05) in CLENEX (-10%) and EX (+27%) but not in CLEN. Posttest recovery HR was higher (P < 0.05) at 2 min post-GXT in the CLENEX, CLEN, and CON compared with their pretest values; RVP remained elevated at 2 min of recovery in the CLEN and CON groups; however, in the EX, recovery HR and RVP had returned to pre-GXT levels by 2 min of recovery.


CONCLUSIONS
These data suggest that the combined effect of therapeutic levels of clenbuterol and training decrease aerobic performance and that the resultant reduction in plasma volume may affect improvements in cardiovascular function during recovery normally seen with exercise training.
Kearns
Charles F
CF
Equine Science Center, Department of Animal Science, Rutgers the State University of New Jersey, New Brunswick, NJ 08901-8525, USA.
McKeever
Kenneth H
KH
D000318:Adrenergic beta-Agonists / Q000009:adverse effects*; D000818:Animals; D015415:Biomarkers / Q000097:blood; D001798:Blood Proteins / Q000187:drug effects / Q000378:metabolism; D002976:Clenbuterol / Q000009:adverse effects*; D005080:Exercise Test; D017079:Exercise Tolerance / Q000187:drug effects / Q000502:physiology; D006400:Hematocrit; D006439:Hemodynamics / Q000187:drug effects / Q000502:physiology; D006736:Horses / Q000502:physiology*; D007773:Lactates / Q000097:blood; D023421:Models, Animal; D010101:Oxygen Consumption / Q000187:drug effects / Q000502:physiology; D010805:Physical Conditioning, Animal; D010953:Plasma Volume / Q000187:drug effects / Q000502:physiology; D011597:Psychomotor Performance / Q000187:drug effects*; D012016:Reference Values; D013997:Time Factors
D000318:Adrenergic beta-Agonists; D015415:Biomarkers; D001798:Blood Proteins; D007773:Lactates; D002976:Clenbuterol