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 T3 (20 micrograms/100 g bw) for 14 (14 d T3) and 30 (30 d T3) days. Maximum tetanic force (Po) was identical in all groups. During 15 minutes of stimulation with 600 ms pulsetrains of 100 Hz at a rate of 60/min, Po declined by 50%, 54%, and 70% in euthyroid, 14 d T3, and 30 d T3 rats, respectively. The results were similar when indirect or direct stimulation was applied. Force recovered to 80% of Po in all groups within five minutes. Whereas relaxation rate and Ca++ transport activity were increased twofold already after 14 days of T3 treatment, myofibrillar ATPase activity (M-ATPase) was only increased in the 30 d T3 group. The decrease in phosphorylation potential ([ATP]/[ADP]f[Pi]) (PP) during stimulation was similar in euthyroid and 14 d T3 rats, but 50% larger in 30 d T3 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 T3 and 30 d T3 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 T3 group cannot be explained by impaired neuromuscular transmission, nor by shortage of energy supply.(ABSTRACT TRUNCATED AT 250 WORDS)
D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism; D004558:Electric Stimulation; D004734:Energy Metabolism; D006980:Hyperthyroidism / Q000378:metabolism* / Q000503:physiopathology; D008297:Male; D009119:Muscle Contraction; D009132:Muscles / Q000378:metabolism*; D010766:Phosphorylation; D051381:Rats; D011919:Rats, Inbred Strains
D000251:Adenosine Triphosphatases; D002118:Calcium