Manipulation of medium conditions and differentiation in the rat myogenic cell line L6

Myoblasts of the L6 rat cell line were grown in Ham's F12 nutrient medium containing 10% fetal calf serum (F12 + FCS). Although the cells were confluent by 6 days in culture, fusion was not observed even if cultures were maintained for 10-14 days. At least 80% of the cells in such confluent unfused cultures were in the G1 phase of the cell cycle and less than 5% of the cells in confluent cultures synthesized DNA during a 4-day period. The synthesis of muscle-specific proteins (alpha-actin, beta-tropomyosin, and myosin light chains LC1emb and LC2F) was negligible when compared to fused cultures of L6 cells grown for a similar time in Dulbecco's medium with 10% FCS (DME + FCS). When the unfused cultures were shifted from F12 + FCS to DME + FCS, DNA synthesis could be demonstrated in more than 95% of the cells and fusion occurred, indicating that neither proliferative nor myogenic capacity had been irreversibly lost. Raising the levels of calcium, varying the serum concentration from 0 to 20%, or the addition of medium components (present in DME but reduced or absent in F12) all failed to induce fusion in the L6 cells grown in F12. However, L6 cells will fuse in mixtures of F12 + FCS and DME + FCS. Fusion will also occur if L6 cells are grown at clonal density in F12 + FCS supplemented with calcium. While it has not been possible to determine why F12 + FCS is nonpermissive for L6 cells in confluent mass cultures, the results demonstrate that prolonged residence in the G1 phase of the cell cycle is not a sufficient condition for L6 myoblast differentiation to occur.