ACTIVITY-DEPENDENT INDUCTION OF SLOW MYOSIN GENE-EXPRESSION IN ISOLATED FAST-TWITCH MOUSE MUSCLE

We demonstrate that direct electrical stimulation of isolated fast-twi tch muscle in an organ culture system can induce expression of the slo w myosin heavy chain (beta-MHC) gene, indicative of a phenotype transf ormation. Pairs of extensor digitorum longus (EDL) muscles were isolat ed from adult mice, incubated at resting length in separate chambers, and superfused with the same recirculated media. One muscle was subjec ted to twitch stimulation (5-s trains of 5-Hz pulses at supramaximal v oltage every minute), and force was recorded to assess function. The c ontralateral muscle was incubated without stimulation, to control for effects of the experimental preparation. Both muscles were rapidly fro zen for RNA purification and oligo(T)-primed reverse transcription; se rial studies were carried out to 36 h. Polymerase chain reaction was p erformed utilizing primers specific for cytoplasmic beta-actin (beta-a ctin), a constitutive marker, and beta-MHC, a gene that is either inac tive or expressed at very low levels in control EDL. After 30 h of sti mulation, beta-MHC was consistently detected at a level severalfold hi gher in stimulated EDL than in incubated control EDL when band intensi ties were normalized to those of beta-actin. These results show that s ignals for fiber-specific transformations reside within the muscle and that this shift begins rapidly after induction of continuous stimulat ion.