Differential effects of 3,5,3'-triiodothyronine on control and mdx myoblasts and fibroblasts: analysis by flow cytometry.
The possibility of differential effects of triiodothyronine (T3) treatment in vivo on myoblast and fibroblast cell proliferation was examined in control and mdx muscle cultures. Cell isolates were purified in a Percoll gradient, sorted by flow cytometry (light scatter), and characterized as myoblasts and fibroblasts using antiskeletal muscle myosin fluorescence. The two cell types were grown separately or remixed (1:1). Cultures were incubated with or without T3 (10(-9) M) for 19 h. Cells were either exposed to [3H]thymidine for 1 h and DNA prepared for scintillation counts or stained with propidium iodide for cell cycle analysis by flow cytometry. Overall [3H]thymidine uptake per cell was greater in mdx than control cells (mainly fibroblasts and mixed cells) and was decreased by T3 only in myoblast and mixed cultures. Cell cycle data showed that the effects of T3 originated primarily at the G0/G1 phase. There were more mdx than control myoblasts at G0/G1 without T3. After T3 treatment, more control fibroblasts than myoblasts were at G0/G1, but more mdx myoblasts than fibroblasts were at G0/G1. In the absence of T3, there were also fewer mdx than control myoblasts in S. After T3, only the proportion of mdx myoblasts in S phase was reduced. Results are consistent with distinct T3 effects on muscle regeneration in vivo and support the hypothesis that cycling and proliferation of mdx and control myoblasts are differentially modulated by T3. As control and mdx fibroblasts also showed distinct responses to T3, muscle regeneration likely occurs by a complex regulation of gene expression endogenous to specific cell types as well as interactions between cells of different lineage.