Troponin isoform dependent pH dependence of the Ca²⁺-activated myofibrillar atpase activity of avian slow and fast skeletal muscles

The effects of acidic pH were examined on the Ca²⁺-sensitive ATPase activity of chicken fast and slow skeletal muscle myofibrils. In both types of myofibrils the Ca²⁺-sensitivity declined on decreasing pH from 7.4 to 6.2, but the slow skeletal muscle myofibrils were found to exhibit a Ca²⁺-sensitive ATPase activity which was more resistant to acidic pH compared to the fast skeletal muscle myofibrils. The results obtained by using a novel method for replacing whole troponin complex in myofibrils provide the first evidence that troponin is responsible for this higher resistibility of slow skeletal muscle myofibrils to acidosis.