The effect of beta-adrenergic blockade on the myopathic changes in experimental hyperthyroidism in rats.

Hyperthyroidism is frequently associated with myopathy. Beta-blocking agents, which are increasingly used in the treatment of hyperthyroidism, have been reported to have a possible beneficial effect on thyrotoxic myopathy. We have studied the myopathic changes in experimental hyperthyroidism in rats and the effect of the beta-blocking agents metoprolol and propranolol on these changes. During experimental hyperthyroidism, conversion of type 1 to type 2 fibres and accumulation of intracellular lipid, increased amounts of mitochondria, and reduced size of muscle fibers were noticed. None of these changes were prevented by metoprolol or propranolol. The results indicate that histochemical and morphological changes in skeletal muscle induced by thyroid hormone are not mediated by beta-receptors. The results of this investigation make dubious a beneficial effect of beta-blocking agents on thyrotoxic myopathy.