Bovine fast-twitch myosin light chain 1: cloning and mRNA amount in muscle of cattle treated with clenbuterol.

The cDNA clone encoding the fast-twitch isoform of myosin light chain 1 (MLC-1f) was isolated from bovine longissimus dorsi muscle and sequenced in M13 and pUC8. An 0.8-kb subclone, produced by digestion of the cDNA with EcoRI, contained the portion of the molecule common to MLC-1f and MLC-3f. The cDNA in pUC8 contained an additional 81 bp upstream of the EcoR I digestion site, which was unique to MLC-1f. The cDNA clone was used to measure MLC-1f mRNA in longissimus dorsi muscle of cattle chronically administered the beta-adrenergic agonist clenbuterol. Treatment with clenbuterol for 50 days increased succinic dehydrogenase negative (type IIB) and positive (types I and IIA) myofiber cross-sectional areas by 25%. After the 50-day treatment period, the amount of MLC-1f mRNA was 90% greater in longissimus dorsi muscle of treated animals than in the initial group. This effect was lost when clenbuterol treatment was withdrawn for a 78-day period, during which time muscle growth in the treated animals stopped completely. We conclude that we have cloned the bovine cDNA for MLC-1f, which has provided additional evidence that beta-adrenergic agonists increase myofibrillar gene expression.

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.