Calcium content and exchange in frog skeletal muscle

Calcium content and exchange in frog ELD IV muscle were examined employing the efflux technique. 2. Muscle calcium was found to exchange with four time constants, 21-5 sec 2-7, 32 and 1244 min. 3. All calcium was found to be exchangeable with more than half the total amount residing in an extracellular compartment. 4. Results obtained from ELD IV muscles and single fibres were identical. 5. Muscle calcium content was found to remain constant up to 20 hr in vitro. 6. Extra exchange of calcium occurs upon contraction. This extra exchange appears to occur in the most slowly exchanging component. 7. The data are discussed in relation to morphological and autoradiographic findings and a model of calcium exchange in skeletal muscle is presented.