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.