Contractile properties and fibre composition of rat skeletal muscle: effect of mild hyperthyroidism.

The effect of a chronic mild elevation of plasma triiodothyronine levels upon the contractile properties and fibre distribution of skeletal muscle was studied. The isometric twitch characteristics and passive tension-length relationship were studied in the soleus and extensor digitorum longus muscles of anaesthetized rats injected for increasing periods with triiodothyronine. This treatment results in a progressive speeding in the rates of isometric contraction and relaxation of the soleus and in a concomitant decrease in the elasticity of the muscle. There was also a conversion of slow to fast fibre types in the soleus, demonstrated histochemically. It is concluded, in view of differences in the magnitudes and time courses of the various changes, that triiodothyronine increases the rates of contraction and relaxation of existing slow fibres and contraction in the soleus muscle as a whole before any change occurs which is attributable to slow-fast fibre interconversion. The possible mechanisms of the early thyroid hormone effects on muscle are discussed.