Glycogen depletion patterns in myofibres of cattle during stress

The response of the three major types of fibre in beef M. longissimus dorsi during stress and recovery was examined with respect to cellular glycogen content. Two different forms of stress were induced in Friesian bulls, by mixing with strangers for a 5h period or by subcutaneous injections of adrenaline. Muscle biopsy samples were taken during stress and recovery for biochemical and histochemical analysis of glycogen content. Muscle glycogen concentration fell to 45% of the resting value during mixing stress and to 37% of the resting value after adrenaline treatment. Slow-twitch and fast-twitch fibre types responded differently to stress, and the differences were reversed in the two forms of stress studied. Mixing stress caused a greater loss of glycogen from the two fast fibre types than from the slow type while adrenaline caused a much greater loss of glycogen from the slow fibres than from the two fast fibre types. The results demonstrate that glycogen is selectively depleted in muscle fibres in response to stress and that the pattern of depletion is different in adrenaline versus mixing stress.