The time course of thyroid-hormone-induced changes in the isotonic and isometric properties of rat soleus muscle.

Rat thyroidectomy resulted in changes in a number of parameters used to characterise the mechanical and histochemical status of skeletal muscle. Thus thyroidectomy resulted in a prolongation of soleus slow-twitch muscle isometric contraction time and half-relaxation time with a reduced maximum velocity of shortening and maximum rate of development of tetanic tension but no significant change in twitch: tetanus ratio i.e. the ratio of twitch force/unit area to tetanic force/unit area. In addition the percentage of IIA fibres was reduced and the percentage of type I fibres increased. Triiodothyronine, administered to hypothyroid rats, brought about a speeding of these parameters again with no change in twitch: tetanus ratio. There was an increase in the percentage of IIA fibres with a concomitant reduction in the percentage of type I fibres. These changes were induced over 18 days and resulted in isotonic and isometric properties almost identical to those of soleus muscles from chronically hyperthyroid rats; speeding could be detected as early as 2 days after triiodothyronine had been given. Consideration is given to the possibility that changes in myosin isoforms and/or the kinetics of changes in intracellular calcium concentration in activation and relaxation could account for the time course of the observed changes.