Responses of calves to treadmill exercise during beta-adrenergic agonist administration

Calves perorally administered the beta-adrenergic agonist (beta-A) clenbuterol for 28 d were studied before, during, and after a 12-min treadmill exercise. During exercise on d 1 of clenbuterol administration, respiratory rate, respiratory minute volume, and heart rate and blood glucose, lactate, and insulin concentrations increased more in beta-A-treated calves than in controls. Oxygen extraction rate and growth hormone concentrations were lower in clenbuterol-treated calves, whereas oxygen consumption, carbon dioxide production, and blood cortisol concentration increased similarly in the absence and presence of the beta-A. After 2 wk of daily clenbuterol administration, respiratory rate and respiratory minute volume during exercise were still higher and oxygen extraction was still lower, whereas all other measures were similar to those in controls. The increased heart rate in response to isoproterenol after 3 wk of clenbuterol administration was reduced markedly in resting but only slightly in exercising animals, whereas heart rate reduction by propranolol during exercise was similar to that in controls. Seven days after withdrawal of clenbuterol, newly administered clenbuterol evoked the same effects as on d 1. In conclusion, there were marked reactions to the first clenbuterol treatment that were in part enhanced during treadmill exercise. After 2 wk of beta-A administration, animals responded much less to the beta-A and changes were not different from those in controls. Resensitization to the beta-A was observed 7 d after its withdrawal.