Fast and slow myosin in developing muscle fibres.

Slow and fast isoenzymes of myosin coexist in all the fibres of a fast-twitch mammalian muscle during early development. They later become segregated into different populations of fibres. Slow myosin is most abundant when the speed of contraction of the muscle is slow and the fibres are multiply innervated; its synthesis in the majority of the fibres seems to be 'switched off' when the speed of contraction increases and the fibres become innervated by single motoneurones.     

'Square arrays' in the sarcolemma of human skeletal muscle fibres.

Fast twitch fibres of rat and rabbit show rectangular patterns of intramembrane particles in freeze-fracture preparations of the sarcolemma. These 'square arrays' are almost totally absent in the slow twitch coleus muscle of rat. I report here differences in the incidence of square arrays in human fetal and adult muscle and in different fibres within a human muscle. Square arrays probably classify fast and slow twitch fibres in freeze-fracture preparations of mixed muscles.     

Control of tension development in scallop muscle fibres with foreign regulatory light chains

We have shown previously that chemically skinned fibre bundles from scallop muscle can be desensitized to the action of calcium by the removal of the regulatory light chains of myosin and that sensitivity can be restored by the re-addition of scallop light chains. We have now confirmed these results and extended our observations to fibre bundles from which one or both of the regulatory light chains per myosin have been removed (by treatment with EDTA at 7 and 25 degrees C, respectively) and replaced by the corresponding light chains from other species. In the case of a double substitution--where both light chains were replaced--sensitivity was restored completely by light chains from other molluscs and from gizzard muscle; for a single substitution there was restoration of sensitivity by all the light chains tested.     

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

There is good evidence for the coexistence of different myosin types both in developing muscles and in Purkinje cells from adult chicken hearts. In skeletal muscle fibres of adult animals, however, coexistence of fast (FM) and slow (SM) myosin has only been demonstrated after long-term electrical stimulation. The term 'promiscuity' has recently been coined 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.