Influences of testosterone on contractile proteins of the guinea pig temporalis muscle.

The present study outlines the changes in contractile proteins which occur during sexual differentiation of the guinea pig temporalis muscle. This is an androgen sensitive muscle, which in the female progresses from a fetal phenotype to a muscle rich in IIa fibers, and in the male, undergoes a further switch to an almost pure IIb fiber phenotype. We show the IIa to IIb transition in the male involves a change in myosin heavy chain synthesis (Figure 39), a corresponding change in the mobility of native myosin isozymes run on pyrophosphate gels (Figure 43), and changes in the proportions of the alpha and beta subunits of tropomyosin (Figure 41). Myosin light chains appear to be unaltered in this transformation.