Fig. 6. Analysis of myosin heavy chain (MHC), actin and P75 expression in fast fibers by RT-PCR. (A) Individual reactions were run for each primer set (fast MHC, S₁ MHC or P75) and then products were combined and separated on the same gel. Plasmids containing target sequences (fast MHC, S₁ MHC or P75) were used as controls. (B) Separate reactions were run for actin and separated on a second gel. Deep abdominal flexor muscle (lane a) expressed fast MHC and P75, but actin expression was not detected. Other samples from these muscles expressed actin but at low levels compared with other fibers. Three individual fast fibers from the central region of the cutter claw (lanes b, c and d) expressed both fast and S₁ MHC, P75 and actin. (Inverse images of ethidium bromide-stained gels.)