Marsupial cardiac myosins are similar to those of eutherians in subunit composition and in the correlation of their expression with body size.

Cardiac myosins and their subunit compositions were studied in ten species of marsupial mammals. Using native gel electrophoresis, ventricular myosin in macropodoids showed three isoforms, V(1), V(2) and V(3), and western blots using specific anti-alpha- and anti-beta-cardiac myosin heavy chain (MyHC) antibodies showed their MyHC compositions to be alphaalpha, alphabeta and betabeta, respectively. Atrial myosin showed alphaalpha MyHC composition but differed from V(1) in light chain composition. Small marsupials (Sminthopsis crassicaudata, Antechinus stuartii, Antechinus flavipes) showed virtually pure V(1), while the larger (1-3 kg) Pseudocheirus peregrinus and Trichosurus vulpecula showed virtually pure V(3). The five macropodoids (Bettongia penicillata, Macropus eugenii, Wallabia bicolour, M. rufus and M. giganteus), ranging in body mass from 2 to 66 kg, expressed considerably more alpha-MyHC (22.8%) than expected for their body size. These results show that cardiac myosins in marsupial mammals are substantially the same as their eutherian counterparts in subunit composition and in the correlation of their expression with body size, the latter feature underlies the scaling of resting heart rate and cardiac cross-bridge kinetics with specific metabolic rate. The data from macropodoids further suggest that expression of cardiac myosins in mammals may also be influenced by their metabolic scope.

Myosin isoforms and fibre types in jaw-closing muscles of Australian marsupials.

Myosin heavy chains (MyHCs) and fibre types in the masseter muscle of seven species of Australian marsupials (brushtail and ringtail possums, bettong, bandicoot, dunnart, two species of antechinuses) spanning three orders were studied by native myosin electrophoresis, SDS-PAGE, immunoblotting and immunohistochemistry. We found only two fibre types in the masseter muscles of these animals: (1) masticatory fibres expressing masticatory MyHC, and (2) hybrid alpha/beta fibres that co-express alpha-cardiac and beta-cardiac MyHCs. Masticatory fibres predominate in most species, being appropriate for predation or for chewing tough vegetable matter. The relative abundance of alpha/beta fibres decreased from 60% to 0 in the order: ringtail possum > brushtail possum > bettong > bandicoot > dunnart/antechinus. These variations in masseter fibre type are correlated with decreasing amounts of vegetable matter in the diets of these animals. The results are in contrast to earlier work on masseter fibres of macropodids that expressed alpha-cardiac MyHC almost homogeneously. The fact that the bettong (Family: Potoroidae), which belong to the same marsupial superfamily (Macropodoidea) as kangaroos and wallabies (Family: Macropodidae), has not specialized in the exclusive expression of alpha-cardiac MyHC as members of the latter family suggests that this specialization was of recent phylogenetic origin (30 million years before present).