EFFECTS OF CHRONIC STIMULATION WITH DIFFERENT IMPULSE PATTERNS ON THEEXPRESSION OF MYOSIN ISOFORMS IN RAT MYOTUBE CULTURES

In order to study maturation and differentiation of aneural myotubes i n vitro, long-term myotube cultures were established from hindlimb mus culature of newborn rats. The developmental state of the myotubes was judged by their myosin heavy chain (HC) patterns. Newly formed myotube s only expressed the embryonic isoform, HCemb, older myotubes expresse d the neonatal isoform HCneo, as well as the fast adult isoforms HCIIb and HCIId. HCIId increased continuously, reaching a relative concentr ation of 47% in 37-day-old cultures. The third fast isoform, HCIIa, wa s not detected and also the slow isoform HCI was absent. Effects of ch ronic (20 days) electrostimulation were studied by exposing the cultur es to various stimulus patterns. Bursts of 250 ms duration at various pulse frequencies were applied at low and high burst frequencies. Alth ough HCemb remained the predominant isoform under all conditions, diff erent stimulus patterns induced specific changes in the patterns of fa st and slow HC isoforms. Bursts of 250 ms duration at 15 Hz, 40 Hz, or 100 Hz, repeated every second or every 4 s, induced the expression of slow myosin, i.e., HCI. Bursts of 250 ms duration at 100 Hz, repeated every 100 s, enhanced the expression of HCIId, but not of HCI. Becaus e slow myosin was induced at high burst frequency with low and high pu lse rates, we suggest that burst frequency rather than pulse frequency has a specifying effect on myosin expression. Our results show that t he basal program of myosin expression during myogenesis in vitro can b e modulated by electrostimulation, suggesting a possible influence of neuromuscular activity on the development of adult fiber types.