Fatigability and recovery of rat soleus muscle in hyperthyroidism

The effect of hyperthyroidism on the fatigue properties of the soleus muscle was investigated in rats treated with T 3 ( 20 μ g 100 g bw ) for 14 (14 d T 3) and 30 (30 d T 3) days. Maximum tetanic force (P o) was identical in all groups. 15 minutes of stimulation with 600 ms pulsetrains of 100 Hz at a rate of 60/min, P o declined by 50%, 54%, and 70% in euthyroid, 14 d T 3, and 30 d T 3 rats, respectively. The results were similar when indirect or direct stimulation was applied. Force recovered to 80% of P o in all groups within five minutes. Whereas relaxation rate and Ca ++ transport activity were increased twofold already after 14 days of T 3 treatment, myofibrillar ATPase activity (M-ATPase) was only increased in the 30 d T 3 group. The decrease in phosphorylation potential ( [ATP] [ADP] f[P i] ) (PP) during stimulation was similar in euthyroid and 14 d T 3 rats, but 50% larger in 30 d T 3 rats. The latter indicated a higher energy consumption, presumably caused by the M-ATPase. Nevertheless, the PP during fatigue was equal in all groups. The decrease in ATP and the increase in lactate content during fatigue were larger in 14 d T 3 and 30 d T 3 rats as compared to euthyroid rats, but did not differ between the two hyperthyroid groups. It is concluded that the higher fatigability in the 30 d T 3 group cannot be explained by impaired neuromuscular transmission, nor by shortage of energy supply. This leaves as the major possibility that excitation-contraction coupling is primarily involved in the thyroid-hormone induced fatigue of slow-twitch muscle.