Adipose tissue cellularity and muscle growth in young steers fed the beta-adrenergic agonist clenbuterol for 50 days and after 78 days of withdrawal.

Angus steers (n = 40; approdate weight = 300 kg) were administered the p-adrenergic agonist clenbuterol for 50 d (7 mg.hd-'.ct*), followed by a 78d withdrawal period. Carcass fatness variables did not differ (P > .05) between treated and Control animals either after 50 d or after 128 d Weights of the 9-10-11th rib longissimus muscle were 25% larger, and longissimus cross-sectional areas were 28% greater, in clenbuteml-fed steers relative to controls from 0 to 50 d (P .05) between treated and control animals at any time. Perirenal (p.r.) adipoqtes were smaller (P < .05) in treated animals after 50 d, but this effect disappeared by the end of the experiment. There was no indication of a bimodal distribution of smaller S.C. or p.r. adipocytes in either of the treatment groups. Apparent hyperplasia of S.C. adipocytes occurred in the area of the 9-10-11th rib in both treated (P c .lo) and control animals (P < .05) from 0 to 50 d on trial. Within treated animals there was a significant increase (P c .05) in total adipocytes in this depot during the withdrawal period. Although the effects of clenbuterol on muscle growth generally were reversed after 78 d, the effects of the &adrenergic agonist on adipose tissue development were more permanent.