The effect of the replacement of calcium by strontium on excitation‐contraction coupling in frog skeletal muscle

1. In frog skeletal muscle strontium can replace calcium in potassium contractures for 5 hr, though it is less effective than Ca. Sr can restore the responsiveness to K after it had been lost in the presence of Mn. 2. Muscles refractory to caffeine following repeated exposure to it in the absence of Ca, recover in part following addition of Sr. 3. The uptake of 85 Sr was increased during mechanical activity, whereas the uptake of 58 Co was not changed. Resting uptake of 58 Co was 3‐4 times greater than that of 85 Sr. 4. Sr fully activated the myofibrillar adenosine triphosphatase (ATP‐ase), though its affinity was about 30 times less than Ca. 5. The sarcoplasmic reticulum took up Sr, though less effectively than Ca.