Selection for muscling reduces muscle response to adrenaline

Carcass lean meat yield is a key profit driver within the sheep and beef industries, and is improved by selection for muscling. This selection causes muscle hypertrophy which is associated with greater proportions of fast-gylcolytic type IIX myofibres in both cattle and sheep (Wegner el al., 2000; Greenwood el aI., 2007). Muscle tissue that is high in type IIX myofibres will also have increased gylcolytic and gylcogenolytic capacity (Wegner el al, 2000), which is likely to result us a greater potential for stress/adrenaline induced muscle glycogen depletion. This has great relevance to industry, as low muscle glycogen at slaughter will result in high ultimate pH (>5.7) carcases leading to dark firm dry (DFD) meat. However, in contradiction to this theory Martin el al. (2004) found more muscle glycogen in sheep selected for muscling. This was most evident when energy intake was high, implicating greater insulin sensitivity which may counteract the hypothesised greater adrenaline response. Therefore a link between selection for muscling and response to stress/adrenaline remains to be confirmed. In this study, we tested the hypothesis that selection for muscling will increase the muscle response to adrenaline.