Indirect Evaluation of Sarcoplasmic Reticulum Ca~(2+) Release Function in Skeletal Muscle Strips

SHENG Juan-juan; LI Quan; YU Zhi-bin

Abstract：Objective To set up a simple method to measure the SR Ca~(2+) release function in skeletal muscle strips. Methods The time from peak tension to 75 % relaxation (TR_(75)) during intermittent tetanic contraction (fatigue contraction) of isolated musculus soleus strips was analyzed. It was prolonged rapidly, and then shortened slowly. The ratio of maximal TR_(75) to initial TR_(75)(R-TR_(75)) indicated a balance between SR Ca~(2+) release and uptake. If the SR Ca~(2+) -ATPase (SERCA) activity and inhibition extent of SERCA in fatigue contraction were identical, R-TR_(75) should be an index for SR Ca~(2+) release function. Results Higher stimulation voltage or 5 mmol/L caffeine perfusion which increased the SR Ca~(2+) release channel open probability induced an increase of R-TR_(75) from 2. 5 in control to 3.0. On the contrary, inhibition of SR Ca~(2+) release with 5 mmol/ L magnesium sulfate perfusion significantly decreased R-TR_(75). Inhibition of SR Ca~(2+) release by metabolites during fatigue contraction was recovered slowly. So R-TR_(75) at the second fatigued contraction was reduced after 5 or 10 min of recovery, but no significant difference after 60 min of recovery. During intermittent tetanic contraction R-TR_(75) was increased to be 2. 9 times of normal in unloaded soleus. Conclusion The above results suggest that R-TR_(75) during muscle strip fatigue contraction without any change of SERCA activity can indirectly reflected SR Ca~(2+) release function. In 2-week unloaded soleus, the enhanced R-TR_(75) indicates an increase in SR Ca~(2+) release function.