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; approximate weight = 300 kg) were administered the beta-adrenergic agonist clenbuterol for 50 d (7 mg.hd-1.d-1), followed by a 78-d withdrawal period. Carcass fatness variables did not differ (P greater than .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 clenbuterol-fed steers relative to controls from 0 to 50 d (P less than .05). After withdrawal these measurements increased no further in the treated steers. Marbling scores were decreased (P less than .05) in clenbuterol-fed steers after 50 d of treatment; this effect persisted after 128 d of withdrawal from treatment. Shear force values were increased 19% (P less than .05) by feeding clenbuterol for 50 d and remained greater (P less than .05) in treated animals after 128 d. Subcutaneous adipocytes in clenbuterol-fed steers were smaller (P less than .05) than those of controls after 50 d, and this effect was still apparent after the 78-d withdrawal period. Rates of lipogenesis did not differ (P less than .05) between treated and control animals at any time. Perirenal (p.r.) adipocytes were smaller (P less than .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 less than .10) and control animals (P less than .05) from 0 to 50 d on trial. Within treated animals there was a significant increase (P less than .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 beta-adrenergic agonist on adipose tissue development were more permanent.