Until recently, muscle fibres were thought to be relatively uniform, with individual fibres containing only one isoform of the contractile protein myosin. But recent evidence has begun to suggest that muscle fibres may be more complex than originally thought; it appears that some mammalian muscle fibres may be comprised of several different myosin isoforms, distributed along the fibre's length. Knowing that some American lobster muscles seem to contain hybrid muscle fibres, Scott Medler, Travis Lilley and Donald Mykles decided to investigate the crustacean's superficial abdominal muscle. Using a combination of cloning techniques and real-time PCR, to quantify the levels of mRNA expressed in the muscle, the team found that many of the muscle fibres contained mixtures of slow-twitch and slow-tonic myosin isoforms. They also found that although some fibres seem to comprise a single myosin protein isoform, the tissue also contained mRNA for another isoform, suggesting that the protein's expression is regulated at the translational level and confirming that some muscle fibres are true hybrids(p. 2755).
Curious as to whether they would find similar distributions of other contractile components in the muscle fibres, the team analysed troponin T,troponin I and tropomyosin distributions, and found that they were also strongly correlated with the myosin isoform expressed in the tissue.
The team emphasise that hybrid muscle fibres can exist anywhere along a continuum, from pure fibres that contain either pure slow-twitch or pure slow-tonic myosin, to fibres containing any combination of the two. They also suspect that the motor neurones that innervate individual fibres may regulate a fibre's heterogeneity, adding that `gradations in neuronal activity may produce a gradation in muscle phenotype'.