Differential effects of diminished oestrogen and androgen levels on development of skeletal muscle fibres in hypogonadal mice.

Androgen and oestrogen hormones influence skeletal muscle size and the characteristics of skeletal muscle fibre types. These effects have typically been assessed by producing acute shortages (castration/ovariectomy) or by hormone supplementation. Little evidence exists, however, on how sex hormone shortages affect muscle development from early stages through to adulthood. Using the hypogonadal mouse model (hpg) we examined the effects of diminished androgen and oestrogen upon muscle size and fibre type composition in murine gastrocnemius and soleus muscles. Hypogonadal male soleus muscle was significantly smaller than normal males, and approximated the normal and hypogonadal females weight and fibre type characteristics. The hypogonadal male gastrocnemius muscle, however, was significantly small in comparison with normal and hypogonadal female gastrocnemius muscles, with the type IIB fibre diameters decreased most markedly. The hypogonadal female soleus muscle approximated the normal female phenotype, but the gastrocnemius muscle was larger than the normal female, approximating the size of the normal male gastrocnemius muscle. Here too, the type IIB fibres showed the most alteration, with greatly increased fibre diameters. Appropriate amounts of androgens were necessary for gender-specific patterns of growth in male muscles, whilst similar amounts of oestrogen were necessary for female gastrocnemius muscle growth, but not for female soleus muscle. Hypogonadism in this murine model generally retards muscle development in males, but has no apparent influence or enhances muscle development in females. Type IIB fibres are most dependent upon sex hormones for appropriate development, but this relationship is muscle-specific.