DETERMINATION OF CONTRACTILE PARAMETERS OF FIBRES FROM CARNIVORES JAW MUSCLES.

In mammalian genome, the genes coding for various MyHC isoforms are grouped in clusters located in different chromosomes to form 3 subfamilies: a) fast isoforms (2A, 2X, 2B, embryonic, neonatal, extraocular); b) cardiac isoforms (beta/slow and alpha); c) masticatory isoform (M). M-MyHC is a myosin subunit isoform with expression restricted to muscles derived from the first branchial arch, such as jaw closer muscles. Only sparse information is available on the contractile properties of fibres expressing M-MyHC (M fibres). In this study we characterized M fibres in the jaw closer muscles (temporalis and masseter) of two species of domestic carnivores (cat and dog) in comparison with fibres expressing slow or fast (2A, 2X and 2B) isoforms. In each fibre isometric tension (Po) and unloaded shortening velocity (Vo) were determined during maximal calcium activation at 12C. To evaluate temperature effects on isometric tension, a subgroup of fibres were activated at 25C. The results obtained showed that in both species Vo increased regularly from slow to 2A to 2X fibres and that M fibres had Vo value similar to 2A fibres. M fibres showed Po values definitely greater than in any other fibre type both in cat and dog. The diversity in Po value among fibre types was similar at both temperatures and in all fibre types Po was 50% greater at 25C than at 12C. The M myosin offers an optimal contractile performance and its presence is the result of specific embryologically determined expression programs and intense selective pressure related to feeding habits