Energy substrate provision in vivo and the changes in muscle pH post mortem

The effects of alpha- and of beta-adrenergic stimulation (both with and without concurrent blockade of lipolysis) on energy substrate provision in vivo, and the subsequent changes in muscle metabolism post mortem, were studied in sheep. Treatment with the alpha-agonist phenylephrine had no effect upon plasma glycerol, free fatty acids, glucose or lactate levels or on muscle metabolism post mortem. Concurrent treatment with an inhibitor of lipolysis, methyl pyrazole carboxylic acid (MPCA), caused a reduction in plasma free fatty acids and glycerol and some decrease in muscle glycogen levels at slaughter. Administration of the beta-agonist, isoproterenol, stimulated an increase in plasma levels of all the energy substrates measured and caused a marked increase in liver glycogen and a decrease in muscle glycogen at slaughter, but no dark cutting. However, when energy from fat depots was blocked by MPCA during beta-stimulation, muscle glycogen was almost exhausted at slaughter and dark cutting was found in all the carcasses. From these experiments it is concluded that dark cutting meat does not result solely from increased adrenergic stimulation, but is dependent upon the availability of energy substrates from muscle, liver and fat energy stores.