Transformation of fast fibres to slow prevented by lack of activity in developing lobster muscle

Abstract

Adult mammalian twitch muscle fibres are extremely plastic, being able to transform from one type to another under the direction of their motoneurones^1–3. The most unequivocal type of fibre transformation is that of fast-twitch to slow-twitch type, either with cross-reinnervation of the fast muscle with a slow nerve^4–6, or with chronic low frequency stimulation of the intact nerve to a fast muscle^7,8. Moreover, direct electrical stimulation of the muscle itself with a tonic pattern is just as effective in converting fast fibres to slow⁹. All these experiments suggest that activity of the muscle itself, imposed by whatever means, is crucial in the transformation of fast to slow fibres^8–10. A similar transformation during development may be responsible for producing the adult diversity of fibre types, as fibres begin by synthesizing myosin of an embryonic type which is replaced by a neonatal type and finally the adult type of fast¹¹ and slow¹², which occurs concurrently with innervation. We report here that during development of the paired claw closer muscle in lobsters, reducing or eliminating activity in one of the muscles by tenotomy or denervation prevents its fast fibres from becoming transformed into slow. Our results therefore suggest a regulatory role for activity in the differentiation of lobster slow fibres.