Deleterious effects of chronic clenbuterol treatment on endurance and sprint exercise performance in rats

The beta(2)-adrenergic agonist, clenbuterol, has powerful muscle anabolic a nd lipolytic effects and is used by athletes to improve exercise performanc e; however, its use in conjunction with different forms of exercise trainin g has received limited attention. Since previous studies have reported that chronic use of other beta(2)-adrenergic agonists has deleterious effects o n cardiac muscle structure and function, the aim of the present study was t o determine whether chronic clenbuterol administration would reduce the exe rcise capabilities of rats subjected to long-term treadmill sprint running, endurance swimming or voluntary wheel running training. The effect of clen buterol treatment on exercise performance in rats was evaluated in three se parate studies. Different groups of male rats were assigned to an endurance swimming (2 h/day, 5/7 days, 18 weeks) group, a tread mi II sprint running (8 x I min bouts, 1.05 m/s, 20 weeks) group, or a voluntary wheel running (16 weeks) group. In each study, rats were allocated into either a treated group that received clenbuterol (2 mg . kg(-1) . day(-1)) in their drinking water or an untreated control group. In each of the three studies, treated rats exhibited a reduction in exercise performance compared with untreated rats. Treated rats ran similar to 57% less total distance than untreated r ats in the voluntary running programme and were unable to complete the swim ming and sprinting protocols performed by the untreated rats. In each of th e studies, the treated rats exhibited cardiac hypertrophy, with absolute he art mass increased by similar to 19% and heart mass relative to body mass i ncreased by similar to 20%. The hearts of sedentary rats treated with clenb uterol exhibited extensive collagen infiltration surrounding blood vessels and in the wall of the left ventricle. The results indicate strongly that c hronic clenbuterol administration deleteriously affects exercise performanc e in rats, potentially due to alterations in cardiac muscle structure and f unction.