Molecular Mechanism Of Porcine Skeletal Muscle Growth And Myofiber Type Distribution

1 The effect of breed and sex on developmental patterns of myofiber type distribution and candidate gene expressionIn present study, Erhualian and Large White pigs were employed to investigate the developmental changes of myofiber type distribution and the expression of candidate genes in LD muscle.1.1 RNA and DNA concentrations and contents in LD muscleTo compare the patterns of skeletal muscle development between lines and sexes, Erhualian and Large White pigs at D3, D20, D45, D90, D120 and D180 were sacrificed and longissimus dorsi (LD) muscles were weighed and sampled for measuring DNA and RNA concentrations. The total contents of RNA and DNA in LD muscle, as well as the RNA/DNA ratio were calculated. The RNA and DNA concentrations, RNA and DNA contents and the RNA/DNA ratio in LD muscle followed similar developmental patterns, independent of lines and sexes. In general, DNA concentrations decreased with age. However, the RNA concentrations fluctuated across the period of investigation. The RNA/DNA ratio showed a consistent tendency of increase throughout the whole period of investigation, despite an obvious stagnation observed from D20 to D45. The total content of RNA and DNA in LD muscle exhibited clear sex and line differences. Males had higher total RNA and DNA content in LD muscle compared with females in Erhualian pigs, while Large White boars accumulated higher total RNA and DNA content in LD muscle than Erhualian boars, which is in agreement with the difference in muscle mass deposition rates.1.2 Development patterns of myofiber type distributionDevelopmental patterns of myofibre type composition in longissimus dorsi (LD) muscle were compared between Erhualian and Large White pigs by determining the ratios of mRNA abundances of four isoforms of MyHC with Multiplex-PCR. The results indicated that: (1) the proportion of MyHC I and 2a fibres were high at 3 days of age, andMyHC 2b fibres were almost not differentiated; (2) from D3 to D20, the composition of myofibres changed dramatically, the MyHC I and 2a fibres decreased significantly (P<0.05), and MyHC 2b fibres increased remarkably (P<0.05), no difference between breeds was observed during early postnatal growth; (3) after D90, the content of MyHC 2b increased rapidly and the strain difference was displayed with significantly higher proportion of MyHC I and 2a fibres in Erhualian pigs compared with Large White pigs at the same age; (4) no sex differences in the developmental pattern of the proportion of MyHC I in Erhualian pigs were found in general, except at D90 and D180, when the proportion of MyHC 2b fibres was higher in females than in males at the same age(P<0.05). The results suggest that the strain differences in the composition of myofibres in LD muscle appear after D90. The remarkably decreased MyHC I fibres and increased MyHC 2b fibres contribute to the fast muscle deposition as well as the accelerated body growth in Large White pigs, while the higher proportion of MyHC I and 2a fibres corresponds to better meat quality in Erhualian pigs.1.3 Development patterns of candidate gene expressionMyostatin, myogenin, myoD, calpain3 and PGC-1 genes mRNA expression in longissimus dorsi (LD) muscle were determined with semi-quantitative RT-PCR in Erhualian and Large White pigs. The results indicated that: (1) myostatin, myogenin, myoD mRNA expression were markedly up-regulated during early postnatal development, around sex maturation, significant gender differences were observed in Erhualian pigs;(2) the strain difference was displayed with significantly lower expression level of calpain3 in Erhualian pigs compared with Large White pigs at D20, D90, D120, but the difference was not observed between Erhualian gilts and boars;(3) compared with Erhualian boars, the PGC-1 expression of Large White pigs exhibited remarkable down-regulation at D20, D90, D120; no sex differences were found before D45, however, at D90 and D180, expression of PGC-1 of Erhualian boars was higher than that of gilts. The up-regulation of myostatin, myogenin and my...