Primary cultures of embryonic chick pectoral skeletal muscle were used to study calcium regulation of myoblast fusion to form multinucleated myotubes. Using atomic absorption spectrometry to measure total cellular calcium and the 45Ca-exchange method to determine free cellular Ca++, our data suggest that only the free cellular calcium changes significantly during development under conditions permissive for myotube formation (0.9 mM external Ca++). Increases in calcium uptake occurred before and toward the end of the period of fusion with the amount approximating 2 to 4 pmol per cell in mass cultures. If the medium [Ca++] is decreased to 0.04 mM, as determined with a calcium electrode, a fusion-block is produced and free cell Ca++ decreased 5- to 10-fold. Removal of the fusion-block by increasing medium [Ca++] results in a release of the fusion-block and an increase in cellular Ca++ to approximately 1 pmol per cell during fusion, and higher thereafter. Cation ionophore A23187 produced transient increases in cellular calcium and stimulated myoblast fusion and the final extent of myotube formation only when added at the onset of culture. Results suggest that transient increased calcium uptake alone is insufficient for fusion because critical cellular content in conjunction with permissive amounts of medium [Ca++] must exist. The latter suggests further that cell surface Ca++ was also critical.

No abstract

No abstract

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.