Brief exposure to testosterone is sufficient to induce sex differences in the rabbit masseter muscle

in order to evaluate whether testosterone is responsible for inducing long- lasting sex differences in myosin heavy chain gene expression in the rabbit masseter muscle, we castrated young adult animals and administered supra-p hysiologic doses of testosterone for 3 or 6 weeks duration. Biopsies were t aken from the masseter muscles of these animals at the time of castration a nd 3, 6 and 9 weeks later. Both immunoblot and immunohistochemical analyses of the myosin heavy chain (MyHC) isoform content of these muscle samples w ere performed. Exposure to testosterone for either duration resulted in a d ramatic decrease in the content of the cardiac alpha MyHC isoform and a com parable increase in the content of the IIa MyHC isoform. The decrease in th e cardiac alpha MyHC isoform content persisted for as long as 6 weeks after the end of treatment, but the increase in the content of the IIa MyHC isof orm declined to normal during this time. Significant numbers of fibers were found containing both the cardiac alpha (and slow) and the IIa isoforms. S everal fibers were encountered that contained both IIa and, presumably, the IIx isoform, Thus, a brief exposure to testosterone during postnatal matur ation is able to produce a long lasting myosin heavy chain isoform switch t hat is similar in magnitude to that found during normal development. Copyri ght (C) 2001 S. Karger AG, Basel.