Beef cattle selected for increased muscularity have a reduced muscle response and increased adipose tissue response...

The aim of this experiment was to evaluate the impact of selection for greater muscling on the adrenaline responsiveness of muscle, adipose and liver tissue, as reflected by changes in plasma levels of the intermediary metabolites lactate, non-esterified fatty acids (NEFA) and glucose. This study used 18-month-old steers from an Angus herd visually assessed and selected for divergence in muscling for over 15 years. Ten low muscled (Low), 11 high muscled (High) and 3 high muscled heterozygotes for myostatin mutation (High(Het)) steers were challenged with adrenaline doses ranging between 0.2 to 3.0 μg/kg live weight. For each challenge, 16 blood samples were taken between -30 and 130 min relative to adrenaline administration. Plasma was analysed for NEFA, lactate and glucose concentration and area under curve (AUC) over time was calculated to reflect the tissue responses to adrenaline. Sixteen basal plasma samples from each animal were also assayed for growth hormone. Muscle glycogen and lactate concentration were analysed from four muscle biopsies taken from the semimembranosus, semitendinosus and longissimus thoracis et lumborum of each animal at 14, 90 and 150 days on an ad libitum grain-based diet and at slaughter on day 157. In response to the adrenaline challenges, the High steers had 30% lower lactate AUC than the Low steers at challenges greater than 2 μg/kg live weight, indicating lower muscle responsiveness at the highest adrenaline doses. Aligning with this decrease in muscle response in the High animals were the muscle glycogen concentrations which were 6.1% higher in the High steers. These results suggest that selection for muscling could reduce the incidence of dark, firm, dry meat that is caused by low levels of glycogen at slaughter. At all levels of adrenaline challenge, the High steers had at least 30% greater NEFA AUC, indicating that their adipose tissue was more responsive to adrenaline, resulting in greater lipolysis. In agreement with this response, the High steers had a higher plasma growth hormone concentration, which is likely to have contributed to the increased lipolysis evident in these animals in response to adrenaline. This difference in lipolysis may in part explain the reduced fatness of muscular cattle. There was no effect of selection for muscling on liver responsiveness to adrenaline.