Effects of ractopamine and sex on serum metabolites and skeletal muscle gene expression in finishing steers and heifers

We evaluated growth-related responses to ractopamine in steers and heifers. Sixteen Angus steers (512 kg) and 16 Angus heifers (473 kg) housed in individual pens were used in a complete block design. At 90 to 97 d before the experiment, steers were im- planted with 120 mg of trenbolone acetate and 24 mg of estradiol-17β (Component TE-S) and heifers were implanted with 140 mg of trenbolone acetate and 14 mg of estradiol-17β (Component TE-H). Treatments were arranged as a 2 × 2 factorial and included sex (steer vs. heifer) and ractopamine-HCl (0 or 200 mg/d). Cattle were fed a diet based on steam-flaked corn once daily. Blood and LM and biceps femoris (BF) biopsy samples were collected on d 0 (before ractopamine feeding) and after 14 and 28 d of ractopamine feeding. Serum insulin concentrations were not affected by ractopamine or sex. Serum IGF-I concentrations were greater in steers than heifers (P < 0.001), and steers demonstrated greater IGF-I mRNA expression in BF than heifers (P = 0.05). Ractopamine decreased serum IGF-I concentrations in heifers on d 14, but increased serum IGF-I concentra- tions in steers on d 28 (sex × ractopamine × day interac- tion; P = 0.03). Ractopamine did not affect (P ≥ 0.19) mRNA expression of IGF-I, IGFBP-3, or calpastatin in BF or LM. However, ractopamine led to increases in LM expression of IGFBP-5 in heifers, but to decreases in expression in steers (ractopamine × sex interaction; P = 0.04). Ractopamine decreased myosin heavy chain IIA mRNA expression in BF (P = 0.04) but not in LM (P = 0.99). Ractopamine decreased β2-receptor mRNA expression in LM of steers on d 14, but not on d 28; in contrast, expression of β2-receptor mRNA in LM of heifers was not affected by ractopamine (sex × racto- pamine × day interaction; P = 0.03). Although there were a few criteria for which ractopamine led to differ- ences in response between steers and heifers, there were no striking disparities to suggest that the effectiveness of ractopamine would markedly differ between sexes.