Abstract: The effects of feeding regimens (pasture and confinement) on meat quality, muscle fiber type composition and key factors regulating muscle fiber type in the Longissimus dorsi muscle of Sunit sheep were investigated. The results showed that carcass mass (P < 0.05)，L* value (P < 0.05), Warner-Bratzler shear force (WBSF) (P < 0.05), pH24 (P < 0.01), the number and area proportion of type IIB muscle fiber (P < 0.01), the diameter of type I, IIA and IIB (P < 0.01), the cross-sectional area of type I (P < 0.05), IIA (P < 0.01) and IIB (P < 0.01), and lactic dehydrogenase (LDH) activity (P < 0.01) in the pasture group were significantly lower than those in the confinement group. However, the number and area proportion of type IIA muscle fiber (P < 0.01), the mRNA expression of the MyHC I and MyHC IIX genes (P < 0.01), the activity of malate dehydrogenase (MDH) and succinate dehydrogenase (SDH) (P < 0.05), and the mRNA expression of the AMPKα2 and COX IV genes (P < 0.05 or 0.01) were significantly higher in the pasture group than those in the confinement group. The mRNA expression of the PGC-1α gene was no significantly different between the two groups (P > 0.05). Overall, the present study indicated that compared to confinement feeding, pasture feeding could augment the relative mRNA expression level of AMPKα2 in the muscle and activate cytochrome c oxidase IV (COX IV) in the mitochondrial inner membrane, thus promoting mitochondrial biogenesis, enhancing oxidative metabolism, inducing the transition of muscle fiber types from glycolyzed to oxidized types, and finally improving meat quality.

Key words: feeding regimens; Sunit sheep; muscle fiber type composition; enzymatic activity; muscle fiber type-related genes