EFFECTS OF THYROID HORMONE ON SARCOPLASMIC RETICULUM Ca^<2+> UPTAKE AND CONTRACTILE PROPERTIES IN RAT SOLEUS MUSCLE
Ca^<2+> uptake rate
The effect of hyperthyroidism on the contractile properties and Ca^<2+> sequestering abilities by the sarcoplasmic reticulum (SR) in the soleus muscles was examined in rats treated with thyroid hormone (3, 5, 3'-triiodcrL-thyronine, T_3) (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℃, 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 350msec with tetani repeated at an interval of 2 sec during a 4-min period. Another soleus muscle was used for analysis of SR Ca^<2+> -uptake rate and SR Ca^<2+> -ATPase activity. Pronounced increases in SR Ca^<2+> -uptake rate and ATPase activity were observed after T_3 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_<3-> 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^<2+> 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^<2+> uptake and the loss of muscular strength in the hyperthyroid soleus.
体力科学 : Japanese Journal of Physical Fitness and Sports Medicine
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Graduate School of Integrated Arts and Sciences