Free-energy carriers in human cultured muscle cells.

Creatine phosphate (CrP), adenosine triphosphate (ATP), creatine kinase (CK), adenylate kinase (AK), protein, and DNA were quantified in human muscle cell cultures undergoing transition from dividing myoblasts to multinucleate myotubes. CrP is negligible in cultures grown in commonly applied media but increases rapidly when sufficient exogeneous creatine is available. The rise in CrP content precedes the rapid increase of CK and cessation of DNA synthesis found during cell fusion. The addition of creatine has no significant effect on the CK and protein content and on the fusion of the human muscle cells. ATP, AK, and protein increase gradually during proliferation and differentiation. When expressed per milligram of protein, ATP and AK are in the same range as in human skeletal muscle and are not correlated to the cell fusion. In contrast, CrP and CK per milligram protein are much higher in myotubes than in myoblasts but remain much lower than in muscle. These data indicate that only an early differentiation phase has been reached in the muscle cell cultures.