The influence of male-specific genes on female muscle fiber types: studies on the sex-reversed (Sxr) mouse.

Previous experiments on the effects of either male castration or injection of androgens have concluded that levels of androgens are responsible for different muscle fiber type proportions between the sexes. However, these conclusions are based on invasive techniques which may involve secondary factors. The sex-reversed (Sxr) mouse is genetically female (X/X) but phenotypically male due to the presence of part of the short-arm of the Y chromosome containing the testis determining gene (Tdy). Serum testosterone in this mouse is in the low normal range and therefore provides a model for investigating the possible control of muscle fiber types by male specific genetic factors. Ten males, ten females, and ten Sxr mice of approximately 60 days of age were used, together with ten males weight-matched to the females. The animals were killed and biceps brachii and soleus muscles removed and prepared for routine muscle histochemistry. Body and muscle weights were similar in the males and Sxr mice and significantly greater than in the females and the weight-matched males. Muscle fiber sizes in biceps brachii reflected the differences in muscle weights and there were no significant differences in fiber type proportions for this muscle. In the soleus muscle, the percentage of slow, oxidative (SO) fibers was higher in the female mice than in any other group. Furthermore, although the fast, oxidative, glycolytic (FOG) fibers were larger in the heavier animals, SO fibers were largest in the female mice.(ABSTRACT TRUNCATED AT 250 WORDS)