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Lean weight is related to muscle fiber number. Muscle fiber formation (myogenesis) occurs only during embryonic development when it is under the control of the MyoD gene family consisting of myogenin, MyoD1, myf-5, and myf-6. Myogenin has a central position within the MyoD gene family because myogenin expression abrogates myoblast proliferation potential and regulates the differentiation of single nucleated myoblasts into multinucleated myofibers. Thus, myogenin genotype could be related to variation in the number of muscle fibers formed, leading to variation in muscle mass and, thus, lean weight. A polymorphism at the porcine myogenin locus was associated with birth weight, growth rate, lean weight at 200 d, and backfat thickness. Yorkshire pigs from two commercial lines were genotyped, and crosses between heterozygous pigs and heterozygous and homozygous pigs were made. Resulting litters were genotyped, and phenotypic data were collected. Significant differences were found between the two homozygous myogenin genotypes for birth weight, growth rate, and lean weight, but not for backfat thickness. Variation at the myogenin locus explained 4% of the total phenotypic variation in birth weight, growth rate, and carcass weight, and 5.8% of the total variation in lean weight. We conclude that myogenin genotype influences porcine growth rate and muscle mass.

This experiment was conducted to investigate the effects of feeding a protein-free diet on mRNA levels of the calpain system in skeletal muscle of growing pigs during a 15-d feeding trial. Twenty crossbred barrows were divided into two dietary treatments: control or protein-free diet (mean initial weight for both groups: 38.3 kg). Daily diets were provided at 2.5 times energy for maintenance (twice a day). On d 0, 3, and 14, biopsies were taken from longissimus muscle between the third and fourth ribs (d 0 and 3) and between the fourth and fifth rib (d 14). On d 15, animals were slaughtered and longissimus muscles were dissected and analyzed for calpastatin, and mu- and m-calpain activity. From biopsies, mRNA level of skeletal muscle calpain, mu- and m-calpain, and calpastatin were measured using reversed transcription PCR. Subsequently, PCR products were quantified using ELISA. Feeding the protein-free diet lowered growth rate to almost zero. Only total level of mRNA of mu-calpain on d 14 was influenced by dietary treatments, being lower for the protein-free group than for the control group (P < .05). However, proteolytic activities were not different between treatments. Total RNA concentration in longissimus muscle decreased during the experiment for both treatments, but on d 14 this was more pronounced for the protein-free than for the control group (P < .05). If mRNA levels were corrected for this change, specific mRNA level on d 14 of skeletal muscle calpain and mu-calpain were lower (P < .05) for the protein-free than for the control group. These data suggest that activity of the components of the calpain system are differentially regulated.