CALCIUM AND THE CONTROL OF MUSCLE-SPECIFIC CREATINE-KINASE ACCUMULATION DURING SKELETAL-MUSCLE DIFFERENTIATION INVITRO

A polyacrylamide gel separation method for creatine kinase (CPK) isoenzymes is described, and its use to determine muscle-specific CPK (M-CPK) levels in skeletal muscle cultures is illustrated. In cultures in which cell fusion has been prevented by very low Ca2+ concentrations, the increases in M-CPK after 96 hr are similar to those in control cultures. Slightly higher concentrations of Ca2+, however, inhibit both cell fusion and M-CPK accumulation. As the calcium concentration is gradually increased further, cell fusion is permitted, followed, at even higher Ca2+ levels, by M-CPK accumulation. These effects can be obtained both by adding EGTA to the culture medium and by using Ca2+-free culture medium and varying the Ca2+ concentration directly. The latter method has the advantage that deleterious effects of EGTA on cell attachment and cell numbers do not occur, even at the lowest Ca2+ concentrations. By revealing dramatic effects on CPK levels of small changes in external Ca2+ concentrations, these observations may resolve conflicting data in the literature on the question of whether cell fusion is a prerequisite for muscle-specific protein synthesis. Possible mechanisms for the two effects of Ca2+ on CPK specific activity (permissive at very low, but inhibitory at intermediate, concentrations) are considered, including membrane mediation, mediation by changes in ionized cytoplasmic Ca2+ levels, and possible involvement of cyclic nucleotides. © 1979.