This study was performed to evaluate breast muscle development in chicken genotypes divergently selected for muscularity. In the first experiment, 2 commercial broiler lines (a high breast yield, HBY, and a normal breast yield broiler strain-cross, NBY) and a Leghorn line were grown up to 35 d to evaluate BW, breast weight, and breast yield. At 7 and 21 d of age, pectoralis muscle was used to estimate myofiber density (MFD, number of myofibers per mm2) and total apparent myofiber number (MFN). In the second experiment, the ontogeny of myostatin was determined from broiler- and Leghorn-type chick embryos, at embryonic days 1 to 20 (E1 to E20), using reverse transcription (RT)-PCR. As expected, the Leghorn line had lower BW, breast weight, and breast yield than broiler lines. The HBY line showed higher breast yield at all ages evaluated, but lower BW at 21 and 35 d than the NBY line. The Leghorn line had 45% higher MFD than broilers, which indicates an increased cross-sectional area of the myofibers in broiler lines. No MFD difference was observed between the broiler strains (P > 0.05). The myofiber number of broilers was more than twice that of Leghorns and HBY had 10% higher MFN than the NBY line. Myofiber number was correlated to BW (r = 0.58), breast weight (r = 0.58), and breast yield (r = 0.69). Conversely, MFD showed negative correlation with BW, breast weight, and breast yield (r = -0.85, -0.83, and -0.88, respectively). No effect of genotype or interaction between genotype and embryonic age was observed for myostatin expression. This study showed that broilers have higher MFN in the breast muscles than Leghorn-type chickens, and that high breast yield of broiler strains may be due to increased MFN. Higher muscularity of broilers, as compared with Leghorns, was not attributed to lower expression of myostatin during embryonic development.

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A 6-h continuous infusion of L-[U-14C]tyrosine was used to estimate fractional protein synthesis rates (FSR) in the longissimus dorsi (LD), semitendinosus (ST) and brachialis (BR) muscles of intact male pigs weighing 22 or 45 kg. The FSR was approximately 20% lower for pigs at 45 kg (4.2 vs. 5.2%/d, 5.2 vs. 6.4%/d and 5.1 vs. 6.4%/d for the LD, ST and BR muscles, respectively) compared with pigs at 22 kg. Fractional protein accretion or growth rates (FGR) were estimated over a 2-wk period. At 22 kg, FGR for the LD, ST and BR were 0.7, 2.4 and 1.7%/d, respectively; at 45 kg, FGR for these muscles were 1.7, 1.9 and 0.7%/d. Fractional protein breakdown rates (FBR) derived by difference (FBR = FSR - FGR) were 44, 16 and 8% lower for the LD, ST and BR muscles, respectively, at 45 kg compared with those at 22 kg. Therefore, it is suggested that muscle growth rate is modulated by alterations in FBR. By using the fractional rates found in these muscles to determine total-muscle protein synthesized or degraded in pigs at 22 and 45 kg, the proportion of protein retained was approximately 28% of that synthesized by the pigs at each weight. Since individual skeletal muscles of the pigs differed in protein turnover rates during postnatal growth and development, selection of a muscle(s) for turnover rates in growth studies is critical.

Eleven Landrace pigs (six boars and five gilts, 50 kg) representing lines selected for three generations for maximum weight at 200 d of age were compared to eight pigs (four boars and four gilts, 50 kg) representing contemporary randomly selected Landrace controls to determine the effect of selection for growth on the metabolic clearance rate (MCR) and plasma concentrations of porcine growth hormone (GH). To estimate MCR of GH, the disappearance of a bolus of porcine GH was monitored over 120 min following its i.v. injection. Blood samples also were collected every 15 min over a 6-h period before injecting GH to determine baseline and overall mean GH concentrations, mean peak amplitude and number of GH secretory episodes. Boars exhibited greater overall mean GH concentrations (4.80 vs 3.11 ng/ml; P less than .05) and had greater maximum GH concentrations associated with secretory episodes (16.11 vs 10.80 ng/ml; P less than .05) than did gilts. There were no differences between boars and pigs exhibited greater baseline GH concentrations (2.04 vs 1.25 ng/ml; P less than .01) than did those from the unselected Landrace line. Selected and control pigs exhibited similar (P greater than .15) overall mean concentrations of GH, frequency of secretory episodes, amplitude of GH peaks and MCR. These data demonstrate that pigs selected for heavier weight at 200 d of age had greater basal plasma GH concentrations than did unselected control pigs.

Thirty-six neonatal pigs were randomly assigned to the following treatment groups: sham implanted gonadally intact males (B), sham-implanted castrated males (C), or castrated

To determine effects of clenbuterol (CB) on muscle protein turnover and growth hormone (GH) secretion, 16 crossbred wether lambs (14.4 kg) were randomized into two groups designated to receive daily oral boluses of gelatin capsules containing corn starch with either 0 (control, CTL) or 1.87 mg/kg body weight CB for either 14 (n = 8) or 28 d (n = 8). This calculates to be approximately 40 mg CB/kg diet. Lambs had ad libitum access to a 16% crude protein corn-soy diet and feed consumption (FC) was measured. After 14 and 28 d, lambs were slaughtered and semitendinosus (ST), longissimus (LD) and brachialis (BR) muscles were exercised, weighed and analyzed for protein (TP) content. For 6 h prior to slaughter of 28-d lambs, 2.5 microCi L-[U-14C]tyrosine/kg was infused intravenously, blood was sampled and plasma was analyzed for specific radioactivity of tyrosine. Plasma GH concentrations were assessed by radioimmunoassay. No differences due to treatment were found in FC, rate of gain or GH concentrations. Semitendinosus and BR weights of control lambs at 14 d did not differ between treatments. At 28 d, ST and BR weights of control lambs (58.8 and 18.5 g, respectively) were less (P less than .10) than those of lambs treated with CB (74.3 and 23.1 g, respectively). The TP per ST and BR at 28 d for control lambs was 71.5 and 85.1% (P less than .10) that of muscles of lambs treated with CB. Fractional protein synthesis rates (FSR) of the BR (9.4 vs 6.1%/d) and total protein synthesized in ST muscle per day (1.4 vs .8 g) were elevated (P less than .10) in lambs treated with CB compared to controls. These data suggest that the increased fractional accretion rate observed in lambs treated with CB for 28 d was caused by increased FSR.

All‐male populations of channel catfish, Ictalurus punelatus, were produced by feeding a diet containing trenbolone acetate to swim‐up fry for 60 d. This hormone was effective in producing 100% males at doses of 50 to 150 mg/kg of diet. Fish that received 0 or 1 mg/kg of diet did not differ from the expected 1:1 sex ratio. Survival of control fish and fish fed diets containing trenbolone acetate were similar (P > 0.05) at the end of the treatment period and after a 60‐d grow‐out period. Our results represent the first report of successful masculinization of channel catfish with a synthetic anabolic androgenic steroid.