Effects of Slaughter Age on Myosin Heavy Chain Isoforms, Muscle Fibers, Fatty Acids, and Meat Quality in Longissimus Thoracis Muscle of Tibetan Sheep

Abstract

Tibetan sheep is one of the dominant livestock at Qinghai-Tibet Plateau, which is the main food source of local people. In order to investigate the effect of slaughter age on meat quality, fatty acid profile and expression of myosin heavy chain (MyHC) isoform genes were analyzed in Tibetan sheep. A total of 24 Tibetan sheep including 4 months old (4 m), 1.5 years old (1.5 y), 3.5 years old (3.5 y), and 6 years old (6 y) were randomly selected. The results indicated that the MyHC IIx and MyHC IIb mRNAs increased with age, whereas MyHC IIa mRNA decreased. MyHC I mRNA was highest at 3.5 y. There were differences in the muscle fiber types of Tibetan sheep at different ages. Intramuscular fat (IMF) was highest at 1.5 y, the pH_45min and pH_24h value of 6 y sheep were lower than the other groups, the shear force increased with age (p < 0.05), and drip loss increased with age (p < 0.01). Tibetan sheep at 1.5 y had lower saturated fatty acid (SFA) contents and higher monounsaturated fatty acid (MUFA) contents (p < 0.05). Different muscle fiber types influence the meat quality and fatty acid composition of Tibetan sheep with increasing age. These results demonstrated the effect of age on meat quality of Tibetan sheep through regulation of expression of the MyHC isoforms which changed the myofiber types, and 1.5 y Tibetan sheep meat was more suitable for a healthy human diet.

Keywords: MyHC isoform; Tibetan sheep; fatty acid; meat quality; muscle fiber.

Figure 1

(A) HE-stained photos of the muscle fibers of Tibetan sheep at different growth stages (×200). (B–D) Show muscle fiber density, area and diameter, respectively. Data are shown as the means ± S.E. Different letters (a–d) indicate significant differences between different growth stages (p < 0.05). 4 m, 4 months old; 1.5 y, 1.5 years old; 3.5 y, 3.5 years old; 6 y, 6 years old.

Figure 2

ATPase staining to determine the type of Tibetan sheep muscle fiber (×100). (A) Type I fiber was stained dark, the type IIa + IIx fiber was unstained, the type IIb fiber was stained gray. (B) Proportions of MyHC I, MyHC IIa + IIx, and MyHC IIb in the LT muscle of Tibetan sheep at different growth stages. (C) The diameter of MyHC I, MyHC IIa + IIx, and MyHC IIb at different growth stages. (D) Fiber density analysis of MyHC I, MyHC IIa + IIx, and MyHC IIb at different growth stages. (E) Fiber area composition of MyHC I, MyHC IIa + IIx, and MyHC IIb at different growth stages. Data are shown as the means ± S.E. Different letters (a,b) indicate significant between different growth stages (p < 0.05).

Figure 3

Effect of slaughter age on muscle fiber type-related gene expressions in Tibetan sheep. The mRNA levels were measured by real-time quantitative PCR. The MyHC I, MyHC IIa, MyHC IIx, and MyHC IIb mRNA levels were normalized to the amount of GAPDH mRNA. Data are shown as the means ± S.E. Different letters (a–d) indicate significant differences between different slaughter age (p < 0.05).