Isometric force and endurance in soleus muscle of thyroid hormone receptor-alpha(1)- or -beta-deficient mice

The specific role of each subtype of thyroid hormone receptor (TR) on skele tal muscle function is unclear. We have therefore studied kinetics of isome tric twitches and tetani as well as fatigue resistance in isolated soleus m uscles of R-alpha(1)- or -beta-deficient mice. The results show 20-40% long er contraction and relaxation times of twitches and tetani in soleus muscle s from TR-alpha(1)-deficient mice compared with their wild-type controls. T R-beta-deficient mice, which have high thyroid hormone levels, were less fa tigue resistant than their wild-type controls, but contraction and relaxati on times were not different. Western blot analyses showed a reduced concent ration of the fast-type sarcoplasmic reticulum Ca2+-ATPase (SERCa1) in TR-a lpha(1)-deficient mice, but no changes were observed in TR-beta-deficient m ice compared with their respective controls. We conclude that in skeletal m uscle, both TR-alpha(1) and TR-beta are required to get a normal thyroid ho rmone response.