SKELETAL-MUSCLE GROWTH AND EXPRESSION OF SKELETAL-MUSCLE ALPHA-ACTIN MESSENGER-RNA AND INSULIN-LIKE GROWTH FACTOR-I MESSENGER-RNA IN PIGS DURING FEEDING AND WITHDRAWAL OF RACTOPAMINE

Sixty crossbred barrows were used to study the effect of ractopamine ( a phenethanolamine/beta-adrenergic agonist) treatment and its withdraw al on muscle growth and on the relative abundance of skeletal muscle a lpha-actin (sk-alpha-actin) mRNA and of liver and longissimus muscle I GF-I mRNA at 4 wk. Ractopamine was fed (20 ppm) for periods of 2, 4, a nd 6 wk (six pigs per group). Additional pigs (four per group) were fe d ractopamine (20 ppm) for 6 wk and then slaughtered 1, 3, and 7 d aft er withdrawal of ractopamine. Ractopamine increased (P <.05) longissim us muscle weight and protein content, although protein concentrations were not different. The increased muscle weight and protein content at tained by feeding ractopamine for 6 wk was retained when ractopamine w as withdrawn. The RNA and DNA concentrations did not change, whereas t otal DNA and RNA content per muscle was 18 and 26.7% greater, respecti vely, in ractopamine-treated pigs at 4 wk, but there were no differenc es at 2 or 6 wk or among the withdrawal groups. The relative abundance of sk-alpha-actin mRNA in the longissimus muscle was 41 and 62% great er (P < .05) in treated animals at 2 and 4 wk but was similar to that in controls at 6 wk and during the withdrawal period. The relative abu ndance of IGF-I mRNA in liver and longissimus muscle was not altered w ith ractopamine treatment for 4 wk. These results indicate that the ra ctopamine-enhanced muscle growth may result from increased myofibrilla r gene expression at the pretranslational level, which is maximal with short-term treatment of ractopamine. It also seems that ractopamine d oes not pretranslationally regulate IGF-I synthesis when ractopamine m aximally induces sk-alpha-actin mRNA abundance.