Molecular Mechanism Of Myofiber Type Distribution And Transformation In Skeletal Muscle Of Pigs

Three experiments for this thesis were conducted to study the molecular mechanism of myofiber type distribution and transformation in skeletal muscle of pigs.1.Effects of breed,body weight,and nutrition on myofiber type distribution and transformation in pigs:This experiment was conducted to study the developmental changes of fiber size and myofiber types in longissimus dorsi(LD) muscle of Rongchang and Duroc×Landrace×Yorkshire(DLY) pigs under different dietary nutritioal condition by determining fiber diameter,density,and the ratios of mRNA abundance of four isoforms of myosin heavy chain(MyHC I,2a,2b and 2x) with semi-quantitative RT-PCR.The results showed that:(1) Fiber diameter increased with BW,especially from 10 kg to 35 kg BW,it increased by more than 50%.Fiber density decreased with BW.It decreased by more than 100%from 10 kg to 35 kg BW,and aider 80 kg BW,no significant change was observed.Fiber density had a significant negative correlation with fiber diameter.(2) From 10 kg to 20 kg BW,the composition of myofiber types in LD muscle of both breeds changed dramatically with significant decrease of MyHCⅠand 2x fibers but ramarkabl increase of MyHC 2a fiber.(3) From 20 kg to 120 kg BW,the developmental changes of myofibers in LD muscle varied with breed and myofiber type.Except MyHC 2b fiber,the developmental patterns of the other three myofiber types were different between the two breeds(4)There was no difference in fiber size and the proportion of four myofiber types in LD muscle during 10～50 kg BW between the two breeds,but at 80 kg BW,Rongchang pigs showed significantly smaller fiber size,lower proportion of MyHC 2b fiber and higher ratio of MyHC 2a fiber compared with DLY pigs.(5) Dietary nutitional levels had no effect on fiber diameter,density,and the composition of myofiber types in LD muscle of both breeds.The results suggested that the developmental changes and composition of myofiber types in LD muscle varied with pig breed.The breed difference of the composition appeared at 80 kg,and RC pigs showed significantly lower proportion of MyHC 2b fiber, which might explain for their better meat quality. 2.Effects of breed,and body weight on some gene expressions related to fiber growth in pigs:The second experiment was conducted to study the developmental changes of MyoD,CaN and PGC-1 mRNA in LD muscle of Rongchang and DLY pigs. The results showed that:(1) MyoD mRNA levels in LD muscle of Rongchang pigs increased with BW,reaching the highest at 20 kg BW,and then decreased.However,in DLY pigs,there were no significant changes in MyoD mRNA levels during the period of 10～120 kg BW.At 20 kg BW,MyoD mRNA level in Rongchang pigs was markedly higher than that in DLY pigs.(2) There was no difference in CaN mRNA of LD muscle in Rongchang pigs untill 80 kg BW,and then dramatical decrease.While in DLY pigs,no significant changes were observed before 50 kg BW,and there was then marked decrease. (3) PGC-1 mRNA levels in LD muscle of Rongchang pigs were kept relatively high before 80 kg BW,following by a marked decline.However,in DLY pigs,PGC-1 mRNA levels were kept relatively high before 20 kg BW,and then decreased significantly.Rongchang pigs showed higher levels of PGC-1 mRNA at 50 kg and 80 kg BW compared to DLY pigs. These results suggested that developmental patterns of these gene expressions observed in this experiment varied with breed and parameter.MyoD,CaN and PGC-1 might play an important role in the expression of MyHC genes and the transformation of muscle fiber types,but the mechanisms need further studies.3.Effects of conjugated linoleic acids(CLA) on the distribution and transformation of myofiber types of pigs in vitro:The third experiment was conducted in vitro to study the effects of different CLA levels(0,50,100,150,200μg/mL) on the distribution and transformation of myofiber types in skeletal muscle cells of pigs after 4,8 and 12 d of culture time by determining the ratios of mRNA abundance of four isoforms of myosin heavy chain(MyHC I,2a,2b and 2x) with semi-quantitative RT-PCR.The results showed that:(1) The distribution of myofiber types in skeletal muscle cells changed markedly with culture time.On d 8, MyHC I and 2a fibers decreased clearly,while MyHC 2b fibers increased dramatically. On d 12,MyHC 2b fiber was increased markedly,hut the other three types of muscle fibers were decreased significantly.(2) 50μg/mL CLA had no effect on the composition of myofiber types from d 4 to d 12.(3) The effects of 100μg/mL CLA on the composition of myofiber types appeared mainly on d 12,with an up-regulation of MyHC I and 2a fibers, and a down-regulation of MyHC 2x and 2b fibers.(4) The compositions of myofiber types from d 4 to d 12 were significantly changed by 150～200μg/mL CLA with increase of the proportions of MyHC I and 2a fibers and decrease of the ratio of MyHC 2b fiber.These results suggested that the compositions of myofiber types in skeletal muscle cells were influenced by the addition of CLA,which depended on the CLA dose and culture time.The effects of CLA on the composition of myofiber mainly were represented by increasing proportions of MyHC I and 2a fibers and decreasing the ratios of MyH 2b and 2x fibers, which could explain partially why CLA improved pork quality.