Regulation of isometric contraction in skeletal muscle

The purpose of this study was to determine whether differences in isometric twitch contraction times of skeletal muscles are related more closely to myosin adenosine triphosphatase (ATPase) activity or to the ability of the sarcotubules to accumulate Ca²⁺. The isometric contraction time was observed to be shorter in the crureus muscle than in the soleus muscle of the rabbit. These muscles were found to have similar myosin ATPase activities, but sarcotubules isolated from crureus had a faster rate of Ca²⁺ uptake than soleus sarcotubules. Furthermore, the yield of sarcotubules was 41% higher from crureus. Likewise, the isometric contraction time of rat soleus decreased during the first 3 weeks of life, but no change was observed in myosin ATPase activity. However, sarcotubular uptake of Ca²⁺ increased as did the yield of sarcotubules (44% higher). It is suggested that the differences in isometric contraction time between skeletal muscles are related more closely to the sarcotubular Ca²⁺ uptake than to the activity of myosin ATPase.