Effect of season, supplementation and fasting on glycolytic potential and activity of AMP-activated protein kinase, glycogen phosphorylase and glycogen debranching enzyme in grass-fed steers as determined in Longissimus lumborum muscle

Forty grass fed beef steers close to slaughter weight (500 kg) were used to study the effects of season (one experiment was carried out in autumn and one in summer, same farm, same design), supplementation (grass-fed only = control or flaked corn supplemented = suppl during four weeks before slaughter) and fasting during lairage (Oh or 24 h fasting). The supplementation with flaked corn started with 0.5 kg animal(-1) day(1), fed individually and increasing up to 1% of body weight (approximately 5 kg animal(-1) day(1)) during the first week; this amount was kept constant for three more weeks. The concentrations of muscle glycogen (MGC), glucose-6 phosphate + glucose (G6P + Gluc) and lactate (LA), glycolytic potential (GPot), activity of AMP-activated protein kinase (AMPK), glycogen phosphorylase (GP) and glycogen debranching enzyme (GDE) were determined in M. Longissimus liunborum (LL);pH and postmortem temperature at 0.5 h and 24 h were measured. Biopsies from the LL were taken from each steer at the beginning of each experiment (B1), at 0.5 h (B2) and 24 h postmortem (B3). For each metabolic substrate/product measured in the muscle samples a linear mixed effect model was fitted. GPot, MGC and GP were higher and GDE was lower (P <0.05) in autumn than in summer. Carcass temperature at 0.5 h and 24 h postmortem was lower in autumn than in summer and non -fasted steers had a lower final carcass temperature than those fasted (P <0.05). Supplementation and no fasting were significant (P < 0.05) factors that helped maintaining a higher MGC and GPot in the steers between 81 (on farm biopsy) and 82 (at slaughter); no fasting also helped in increasing GDE activity postmortem (between B2 and B3).The effects of treatments on glycogen reserves and on the activities of the glycolytic enzymes included were not reflected in the ultimate pH of the carcasses, because no differences in terms of mean pH due to any of the factors studied were found (P> 0.05). Perhaps other substrates/enzymes that take part in muscle glycolysis/glycogenolysis not included in this study should be analyzed in future studies; considering the high individual variability observed, intrinsic factors of cattle, like genetics, should be taken into consideration.