ABSTRACT
The objective of the present study was to investigate the effect of outdoor grazing on the expression of genes involved in muscle growth and the nutrient contents of skeletal muscle in steers. Ten Japanese Black steers were divided into two groups: grazing (GR) and concentrate (CT) groups. Crude protein, extractable lipid, moisture, fatty acid, cooking loss and Warner Bratzler shear force in muscle tissue were analyzed. The gene expression of myosin heavy chain (MyHC) isoform (2a, 2x and slow), myostatin, follistatin, peroxisome proliferator-activated receptor γ2 (PPARγ2), CCAAT/enhancer binding protein α (C/EBPα), heat shock protein (HSP) 27 and HSP40 in skeletal muscles was evaluated at the end of fattening. Decreases in MyHC-2a and MyHC-2x (fast-twitch fiber type) expression in the longissimus lumborum (LL) muscle were detected in the GR group compared with the CT group; in contrast, an increase in MyHC-slow (slow-twitch fiber type) expression was shown in the GR group. These results suggest that grazing initiated muscle fiber type conversion to slow-twitch from fast-twitch. A decrease in extractable lipid content was observed in the GR group in the LL and semitendinosus (ST) muscles. Crude protein content in the LL muscle in the GR group was higher than in the CT group. MyHC expression in LL muscle in the GR group was greater than in the CT group. A decrease in myostatin and PPARγ2 gene expression was detected in the GR group compared with the CT group in both muscles. Expression of C/EBPα in LL muscle in the GR group was lower than in the CT group. These results suggest that grazing steers at the end of fattening may lead to an increase in protein content and a decrease in fat accumulation in LL and/or ST muscles by regulation of myostatin, MyHC, PPARγ2 and C/EBPα gene expression.