Stimulation of myosin heavy chain synthesis in steady-state muscle cultures by the ionophore, A23187, requires transcription of messenger RNA.

After approximately one week in culture, embryonic chick skeletal muscle cells are at a steady state with respect to myosin heavy chain (MHC) concentration and synthesis rate. Muscle cells normally synthesize MHC at a maximum rate of 2.3 X 10 4 MHC/min/nucleus and contain approximately 3 X 10 7 MHC/nucleus. These cells also contain approximately 3500 copies/nucleus of MHC mRNA associated with polysomes and 1600 copies/nucleus of MHC mRNA localized in the nonpolysomal fraction. To determine if nonpolysomal MHC mRNA in mature muscle cultures could be recruited into active translation complexes when MHC synthesis was stimulated, muscle cultures were treated with the Ca 2+ ionophore, A23187 (0-1 micro M). The MHC synthesis rate was stimulated by 25 to 50% relative to stimulation of the rate of total protein synthesis in the presence of A23187, but this stimulation was blocked when 10 microgram/ml actinomycin D was also present. These results suggest that even though 30% of MHC mRNA is not actively engaged in MHC synthesis in mature muscle cultures, stimulation of MHC synthesis by A23187 results from transcription of new MHC mRNA rather than from utilization of pre-existing mRNA.