The relationship of ornithine decarboxylase activity to proliferation and differentiation of L6 muscle cells

Four different approaches were used to test a postulated relationship between ornithine decarboxylase (ODC) activity and L6 muscle cell proliferation. In the first approach, the level of ODC activity in myoblasts was compared to that of myotubes. The fusion of proliferating myoblasts to form postmitotic myotubes is a unique property of skeletal muscle differentiation which offers special advantages for this study. If ODC activity reflects proliferation, it should be reduced in myotubes. Four hours after the addition of 10% horse serum (HS), ODC activity in myoblasts was 140 times that in myotubes. The small amount of ODC activity measured in myotube cultures could be attributed to remaining myoblasts. In the second approach, cell density was varied to alter the rate of cell division. Density‐dependent inhibition of myoblast proliferation was closely correlated with a decrease of ODC activity. In the third approach, HS concentration was varied to alter the rate of cell division. Through a tenfold change of HS concentration, ODC activity paralleled cell proliferation. In the fourth approach, 1mM putrescine or spermidine was added with 10% HS to myoblasts. Although these treatments prevented the increase in ODC in response to HS, cell division was unimpaired; it seems likely that the polyamines entered the cell and substituted for endogenous polyamines. Mixing experiments indicated the presence of an inhibitor, tentatively identified as ODC antizyme, in homogenates of polyamine‐treated cells. We conclude that ODC activity closely reflects proliferation in muscle cells, although the presence of extracellular polyamines may disrupt this association.