The regeneration of a hormone-sensitive muscle (levator ani) in the rat.

In l-month-old male rats the levator ani muscle was crushed and allowed to regenerate. The main questions were whether or not the course of regeneration is similar to that of limb muscles and whether or not the regenerating levator ani muscle is sensitive to the effects of testosterone. Regeneration was followed in three groups of rats: castrated, normal, and testosterone-treated. Histological analysis revealed no detectable differences between regeneration of the levator ani and other muscles. By 31 days both the gross weight and mean cross-sectional area of the muscle fibers in castrated rats were much less than in the other two groups. Testosterone-treated muscles were larger, but not significantly so, than normal regenerating muscles. Twitch and tetanic tensions were lowest in regenerating muscles from castrated rats and highest in those from hormone-treated rats. Both the contraction (time to peak) and half-relaxation times at 7 and 14 days were fastest in hormone-treated muscles and were very slow in castrated rats. By 30 days, the differences between normal and hormone-treated muscles had disappeared. It is concluded that the regenerating levator ani muscle is sensitive to the effects of testosterone.