The influence of sex hormones on cardiac and skeletal muscle function in the MDX mouse model of Duchenne Muscular Dystrophy

[Abstract]: Duchenne Muscular Dystrophy (DMD) is a fatal recessive genetic human disease affecting one in 3500 live male births. DMD is progressive, there is no cure, and patients typically die of respiratory or cardiac failure in their second decade of life. Clinical disease symptoms are exacerbated at the onset of puberty and the physiological basis of this is unknown. The mdx mouse is the preferred experimental animal model of DMD, although aspects of the model remain poorly understood. This dissertation characterises physiological and histological features of the dystrophic mdx mouse in response to manipulations of hormonal status including testosterone treatment, surgical castration, and oestrogen treatment. Sex-specific differences in the mdx were also examined. Furthermore, physiological and histological features of the dystrophic mdx mouse model throughout the mdx lifespan were evaluated. Cardiac muscle contractility, left atrial response to exogenous calcium, and the contractile properties of both fast-twitch (EDL)and slow-twitch (SOL) skeletal muscles were examined in male mdx mice ranging from 14 to 330 days of age. Testosterone treatment produced a nonsignificant trend towards a dose-dependent decrease in both basal and maximal left atrial contractility in the mdx. Surgical castration produced no significant cardiac effects within mouse strains. The mdx castrates had a 45% lower maximum atrial force of contraction than control castrates (p<0.05). Conversely, oestrogen treatment significantly improved cardiac contractility in the mdx. An increase in basal left atrial contractility was evident at doses of 0.08 mg/kg/day (p<0.05) and 0.16 mg/kg/day (p<0.01) and in maximum left atrial contractility at a dose of 0.16 mg/kg/day (p<0.01). Gender studies showed cardiac forces in mdx were not different between males and females at any age tested and that both sexes in mdx had a dampened cardiac responsiveness to exogenous calcium. Skeletal muscle function studies showed that castration produced a 25% increase in mdx EDL specific force generation (p<0.01) and no increase in SOL forces. Oestrogen treatment produced a non-significant trend towards increased EDL forces and a 29% increase in SOL specific force at a dose of 0.16 mg/kg/day (p<0.05). Gender studies revealed no differences between male and female mdx in terms of skeletal muscle force production. Further to the hormonal investigations, lifespan characterisation studies revealed that the mdx mouse showed reduced basal and maximal left atrial contractility specifically at ages 14 and 90 days (p<0.05). Skeletal muscle studies showed that specific tetanic force production was significantly lower than controls at 19 (p<0.05), 21, 23, 27, and 330 days of age (p<0.01) for EDL muscles and at 19, 21, and 23 days of age in SOL muscles (p<0.01). These studies further improve our understanding of the mdx mouse as an experimental model of DMD and emphasises that the model is most appropriate a specific ages for specific muscles. These studies further illustrate that testosterone does not improve cardiac contractility in the mdx mouse but that oestrogen improves both cardiac and skeletal muscle function. Further research is warranted into the potential of oestrogen as a therapeutic agent in the treatment of both cardiac and skeletal musclemanifestations of DMD.