Progeny of high muscling sires have reduced muscle response to adrenaline in sheep.

This study investigated the impact of variation in Australian sheep breeding values (ASBVs) for yearling eye muscle depth (YEMD) within Merino and Poll Dorset sires on intermediary metabolism of progeny. Specifically, the change in the blood concentrations of lactate, non-esterified fatty acids (NEFA) and glucose in response to administration of an exogenous dose of adrenaline was studied. The experiment used 20 Merino and Merino cross Poll Dorset mixed sex sheep. The sires were selected across a range of YEMD ASBVs. The sheep were fitted with indwelling jugular catheters and administered seven levels of adrenaline over a period of 4 days at 4 months of age (0.1, 0.2, 0.4, 0.6, 0.9, 1.2 and 1.6 μg/kg liveweight (LW)) and 16 months of age (0.1, 0.2, 0.6, 1.2, 1.8, 2.4 and 3.0 μg/kg LW). A total of 16 blood samples were collected between -30 min and 130 min relative to administration of the adrenaline challenge and were later measured for the plasma concentrations of lactate, NEFA and glucose. These data were then used to calculate the time to maximum substrate concentration, the maximum concentration and the area under curve (AUC) between 0 and 10 min, thus reflecting the substrate's response to exogenous adrenaline. Selection for muscling led to decreased muscle response due to adrenaline, as indicated by lower maximum concentrations and AUC for lactate. The muscles' response to adrenaline was more prominent at 16 months of age than at 4 months of age. Thus, animals selected for increased muscling have lower levels of glycogenolysis in situations where endogenous adrenaline levels are increased like pre-slaughter. This may minimise the risk of poor meat quality in these animals, as they will express higher muscle concentrations of glycogen at slaughter. Adipose tissue was more sensitive to adrenaline in young lambs from high YEMD sires. This shows that high muscled growing lambs utilise their adipose tissue deposits in times of stress to produce energy. This may explain the phenotypic leanness of these animals. Blood glucose levels that are indicative of liver response to adrenaline decreased with selection for muscling. This response may indicate a potential limiting of glucose that is available within animals selected for muscling, leanness and growth for brain function.