Fiber transformation and replacement in low-frequency stimulated rabbit fast-twitch muscles.
The fast-to-slow conversion of rabbit skeletal muscles by chronic low-frequency (10 Hz, 12 h daily) stimulation involves (1) sequential fast-to-slow fiber-type transitions in the order of type IID-->type IIA-->type I, and (2) the replacement of deteriorating fast-twitch glycolytic fibers by new fibers derived from satellite cells and myotubes. These two processes were analyzed in 30- and 60-day stimulated extensor digitorum longus and tibialis anterior muscles. Fast-to-slow transforming fibers were identified by myofibrillar actomyosin histochemistry as type C fibers and immunohistochemically by their reaction with monoclonal antibodies specific to slow and fast myosin heavy chain isoforms. In situ hybridization of mRNA specific to the myosin heavy chain I isoform identified all fibers expressing slow myosin, i.e., type I and C fibers. The fraction of transforming fibers ranged between 35% and 50% in 30-day stimulated muscles. The percentage of type I fibers (20%) was threefold elevated in extensor digitorum longus muscle, but unaltered (3.5%) in tibialis anterior muscle, suggesting that fast-to-slow fiber conversion was more advanced in the former than in the latter. Fiber replacement was indicated by the finding that the fiber populations of both muscles contained 15% myotubes or small fibers with central nuclei. In situ hybridization revealed that myotubes and small regenerating fibers uniformly expressed myosin heavy chain I mRNA. Similarly, high percentages of slow-myosin-expressing myotubes and small fibers were found in 60-day stimulated muscles.
Schuler
M
M
Fakultät für Biologie, Universität Konstanz, Postfach 5560-M641, D-78434 Konstanz, Germany.
Pette
D
D
D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D004558:Electric Stimulation; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D008297:Male; D018656:Muscle Fibers, Fast-Twitch / Q000201:enzymology / Q000502:physiology* / Q000648:ultrastructure; D018657:Muscle Fibers, Slow-Twitch / Q000201:enzymology / Q000502:physiology* / Q000648:ultrastructure; D018482:Muscle, Skeletal / Q000201:enzymology / Q000502:physiology* / Q000648:ultrastructure; D018995:Myosin Heavy Chains / Q000378:metabolism; D009218:Myosins / Q000378:metabolism; D015347:RNA Probes; D012333:RNA, Messenger / Q000096:biosynthesis; D011817:Rabbits
D015347:RNA Probes; D012333:RNA, Messenger; D000251:Adenosine Triphosphatases; D018995:Myosin Heavy Chains; D009218:Myosins