EFFECTS OF THYROID HORMONE ON SARCOPLASMIC RETICULUM Ca2+ UPTAKE AND CONTRACTILE PROPERTIES IN RAT SOLEUS MUSCLE / 体力科学

ABSTRACT

The effect of hyperthyroidism on the contractile properties and Ca<SUP>2+</SUP> sequestering abilities by the sarcoplasmic reticulum (SR) in the soleus muscles was examined in rats treated with thyroid hormone (3, 5, 3'-triiodo-L-thyronine, T<SUB>3</SUB>) (300μg/kg body weight) for 3, 7 and 21 days. At the end of a given treatment period, the right or left soleus muscle was mounted isometrically at 30°C, and stimulated directly using supramaximal current intensity. A twitch contraction was elicited by a 1 msec square-wave pulse and a tetanic contraction by 20 Hz stimulation for 600 msec. To evaluate fatigue resistance, muscles were stimulated at 40 Hz for 350 msec with tetani repeated at an interval of 2 sec during a 4-min period. Another soleus muscle was used, for analysis of SR Ca<SUP>2+</SUP> -uptake rate and SR Ca<SUP>2+</SUP> -ATPase activity. Pronounced increases in SR Ca<SUP>2+</SUP> -uptake rate and ATPase activity were observed after T<SUB>3</SUB> treatment periods longer than 6 days. These alterations were accompanied by decreases in twitch and tetanic tension, half-relaxation time, and fatigue resistance. The T<SUB>3</SUB>-treated muscles stimulated at 20 Hz relaxed during the interval between successive stimuli, indicating that the mechanical fusion of tetanic contractions was incomplete. SR Ca<SUP>2+</SUP> uptake rate was significantly correlated both to tetanic tension and to fatigue resistance. These data suggest that there may be a causal relationship between changes in SR Ca<SUP>2+</SUP> uptake and the loss of muscular strength in the hyperthyroid soleus.