Fast and slow myosin within single skeletal muscle fibres of adult rabbits

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Abstract
There is good evidence for the coexistence of different myosin types both in developing muscles1–3 and in Purkinje cells from adult chicken hearts4. In skeletal muscle fibres of adult animals, however, coexistence of fast (FM) and slow (SM) myosin has only been demonstrated after long-term electrical stimulation5,6. The term ‘promiscuity’ has recently been coined7 to describe the coexistence of different myosin isoenzymes within a single fibre. Using novel, refined immunological methods we demonstrate here the presence of both FM and SM within single fibres of the musculus tibialis anterior of adult rabbits. Essentially identical results were also obtained with other muscles. Our findings imply that the genes coding for FM and SM can be expressed simultaneously within the same cell throughout an animal's entire life, and not only during development or after artificial electrical stimulation.