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.