New Insights into Muscle Fiber Types in the Pig

Abstract

S U M M A R YThe accurate classification of skeletal muscle fiber types according to myosin heavy chain (MyHC) polymorphism remains a difficult task in the pig. Combined myofibrillar ATPase and metabolic enzyme histochemistry, in situ hybridization, and immunocytochemistry were performed on serial transverse sections of pig longissimus (L) and rhomboideus (R) muscles at 100 kg body weight to give a new insight into muscle fiber typing in the pig. Several monoclonal antibodies (MAbs) either specific for a single MyHC (I, IIa, or IIb) or of multiple MyHCs (IIa ϩ IIx or I ϩ IIx ϩ IIb) were used. No monospecific IIx antibody was available for the pig. All three adult Type II isoforms were expressed in the white L muscle, whereas no IIb was observed in the red R muscle, which was confirmed using RNase protection analysis. In most fibers, the distribution of the transcripts closely matched that of the corresponding proteins. When observed, co-expression of MyHCs mostly occured for IIx and IIb in L muscle, and was more common at the protein (11.5%) than at the mRNA (2.2%) level. A minor proportion of myofibers showed a mismatch between MyHC mRNA and protein. According to the type grouping distribution of myofibers encountered in pig muscle, MyHC isoform expression followed the rank order of I → IIa → IIx → IIb from the center to the periphery of the islets, concomitantly with a decrease in oxidative metabolism and an increase in fiber size. The developmental origin and functional significance of the type grouping distribution are discussed.

Differential expression of lipid metabolism-related genes and myosin heavy chain isoform genes in pig muscle tissue leading to different meat quality

“…Usually, the differences in meat quality traits are smaller between Western modern pig breeds because of the long-time similar breeding aims such as fast growth and leanness of the carcass (Ruusunen et al, 2012).The Rongchang pig is one of the local fatty-type pig breeds of the southwest area of China with a white coat color phenotype (Lai et al, 2007), and is especially noted for its early sexual maturity, increased IMF content and high meat quality as other Chinese local pigs. Landrace pigs, a Western pig breed, have been intensively selected for their thin subcutaneous fat layer and high muscularity in the carcass, which has led to deterioration in meat quality (Lefaucheur et al, 2002;Shan et al, 2009). Therefore, these two pig breeds can be used as an ideal comparison study model to investigate the possible mechanisms responsible for the differences in meat quality traits between pig breeds.…”

“…In postnatal pigs, skeletal muscle consists of four MyHC isoforms: MyHCI, MyHCIIa, MyHCIIx and MyHCIIb, with different biochemical and biophysical characteristics such as oxidative and glycolytic capacities, contraction speed, cross-sectional area and TG content (Schiaffino and Reggiani, 1996;Lefaucheur et al, 2002;Lefaucheur, 2010). Muscle fibers are dynamic structures and exhibit high plasticity, and under normal physiological conditions transition among four MyHC isoforms follows an obligatory pathway: MyHCI ↔ MyHCIIa ↔ MyHCIIx ↔ MyHCIIb (Schiaffino and Reggiani, 1994;Pette and Staron, 2000).…”

“…Muscle fibers are dynamic structures and exhibit high plasticity, and under normal physiological conditions transition among four MyHC isoforms follows an obligatory pathway: MyHCI ↔ MyHCIIa ↔ MyHCIIx ↔ MyHCIIb (Schiaffino and Reggiani, 1994;Pette and Staron, 2000). In general, oxidative metabolism decreases in the rank order MyHCI → MyHCIIa → MyHCIIx → MyHCIIb (Lefaucheur et al, 2002), and these metabolic changes are consistent with the transition of MyHC profile toward a faster type (Lefaucheur et al, 2004). It is well-known that intensive selection for faster muscle growth in Western modern pigs has resulted in a shift in muscle fiber toward a higher glycolytic and lower oxidative metabolism (Lefaucheur et al, 2004;Hu et al, 2008;Guo et al, 2011), which is associated with leaner pigs, better feed efficiency and higher growth rate, but gives rise to a deterioration in meat quality (Lefaucheur et al, 2002;Shan et al, 2009).…”

“…The Landrace pigs, one of the Western lean pig breeds, have been intensively selected for a thin subcutaneous fat layer and high muscularity in the carcass, which has led to deterioration in meat quality (Lefaucheur et al, 2002;Shan et al, 2009). Therefore, the underlying molecular mechanisms responsible for the differences in meat quality traits between pig breeds can be investigated using these two pig breeds as the ideal comparison model.…”

“…Therefore, the expression levels of lipid metabolism-related genes in muscle tissue could theoretically contribute to the difference in IMF content, which lead to different meat quality between pig breeds. Moreover, skeletal muscle comprises four types of myofibers, which are encoded by four different myosin heavy chain (MyHC) isoform genes (MyHCI, MyHCIIa, MyHCIIx and MyHCIIb) and have different biochemical and biophysical characteristics such as oxidative and glycolytic capacities, contraction speed, cross-sectional area, glycogen content and TG content (Schiaffino and Reggiani, 1996;Lefaucheur et al, 2002;Lefaucheur, 2010). Thus, the compositions of myofiber types are considered as important influencing factors for meat quality (Franck et al, 2007).…”