PSII-B-25 - Age-related changes in growth hormone-insulin like growth factor 1 axis-regulating factors and metabolic hormones in Korean cattle steer

Meat yield is important for beef industry for maximum profits. Enhancing muscle growth by promoting hypertrophy of muscle fiber is required for increasing meat yield. Growth hormone (GH)-insulin like growth factor 1 (IGF1) axis and metabolic hormones have a central role for regulating hypertrophy of muscle fiber. We investigated age-related changes in GH-IGF1 axis-regulating factors and metabolic hormones in beef cattle. Ten Korean cattle steers were used. The longissimus thoracis (LT) samples were biopsied between the 11^th and 12^th rib at 12, 18, and 24 months of age. Steers were slaughtered at 34 month of age and the LT samples were collected. The LT samples were used for histological observation of muscle fiber size and detection of mRNA levels by real-time PCR analysis. Blood was collected at 12, 18, 24, and 34 months of age and used for hormone analyses. The mean muscle fiber size increased (P < 0.01) from 12 to 24 months, and remained unchanged at 34 months. Plasma GH concentrations were unchanged with age, whereas circulating IGF-1 concentrations decreased (P < 0.001) with age. Testosterone and insulin concentrations increased from 12 to 24 months and remained unchanged at 34 months. Leptin concentrations were unchanged from 12 to 24 months and increased at 34 months. The mRNA levels of growth hormone receptor (GHR) and signal transducer and activator of transcription 5a/b (STAT5a/b) decreased (P < 0.01) from 12 to 24 months, whereas mRNA levels of IGF-1 gene increased (P < 0.001) with age. Results indicate that the increased muscle fiber size in LT from 12 to 24 months are characterized with increasing concentrations of circulating testosterone and insulin. Age-dependent changes in muscle fiber hypertrophy were also related with changes in GHR, STAT5a/b, IGF-1 mRNA levels in the LT.