Reversible Ca2+-induced fast-to-slow transition in primary skeletal muscleculture cells at the mRNA level

1. The adult fast character and a Ca2+-inducible reversible transition from a fast to a slow type of rabbit myotube in a primary culture were demonstr ated at the mRNA level by Northern blot analysis with probes specific for d ifferent myosin heavy chain (MyHC) isoforms and enzymes of energy metabolis m. 2. No non-adult MyHC isoform mRNA was detected after 22 days of culture. Af ter 4 weeks of culture the fast MyHCIId mRNA was strongly expressed while M yHCI mRNA was virtually absent, indicating the fast adult character of the myotubes in the primary skeletal muscle culture. 3. The data show that a fast-to-slow transition occurred in the myotubes at the level of MyHC isoform gene expression after treatment with the Ca2+ io nophore A23187. The effects of ionophore treatment were decreased levels of fast MyHCII mRNA and an augmented expression of the slow MyHCI gene. Chang es in gene expression started very rapidly 1 day after the onset of ionopho re treatment. 4. Levels of citrate synthase mRNA increased and levels of glyceraldehyde 3 -phosphate dehydrogenase mRNA decreased during ionophore treatment. This po ints to a shift from anaerobic to oxidative energy metabolism in the primar y skeletal muscle culture cells at the level of gene expression. 5. Withdrawal of the Ca2+ ionophore led to a return to increased levels of MyHCII mRNA and decreased levels of MyHCI mRNA, indicating a slow-to-fast t ransition in the myotubes and the reversibility of the effect of ionophore on MyHC isoform gene expression.