A 31P-n.m.r. study of the acute effects of β-blockade on the bioenergetics of skeletal muscle during contraction

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Abstract
1. The effects of beta-adrenoceptor antagonist administration on skeletal muscle contractile performance and bioenergetics in vivo have been investigated during unilateral sciatic nerve stimulation in the rat. 2. Two muscle stimulation protocols have been used: supramaximal stimulation at 4 Hz, or incremental supramaximal stimulation at 1, 2 and 4 Hz. Changes in high-energy phosphate concentrations were followed using 31P-n.m.r., and gastrocnemius muscle twitch characteristics were monitored continuously. 3. Under all conditions investigated, DL-propranolol administration (2.5 mg/kg body wt.) caused a significant decrease in cyclic AMP concentrations in resting and stimulated gastrocnemius muscle, prevented an increase in heart rate upon muscle stimulation, but did not affect plasma glucose, fatty acid or lactate concentrations in comparison with values obtained in control experiments. 4. Administration of DL-propranolol 5 min or 35 min before unilateral stimulation of 4 Hz had no effect on changes in muscle phosphocreatine, ATP or Pi concentrations, intracellular pH or contractile performance. 5. In contrast, animals receiving DL-propranolol 5 min before unilateral stimulation of 1, 2 and 4 Hz showed a significant deterioration in gastrocnemius muscle tension development during 2 and 4 Hz stimulation compared with control animals. Concurrent with this change in contractile performance was a higher muscle concentration of phosphocreatine, a lower concentration of Pi and no significant change in intramuscular pH compared with control experiments. 6. The changes in muscle performance and bioenergetics observed during the incremental stimulation protocol were not observed when D-propranolol was administered and could be completely circumvented by a short period of muscle stimulation of 4 Hz prior to initiation of the incremental stimulation protocol. 7. Mechanisms are discussed which may account for the failure of gastrocnemius muscle to generate the expected force during the incremental stimulation protocol in the presence of beta-blockade.