Testosterone-induced changes in contractile protein isoforms in the sexually dimorphic temporalis muscle of the guinea pig.

The guinea pig temporalis muscle is sexually dimorphic, classified histochemically as a fast-red muscle in the female, but as a fast-white muscle in the adult male. Since this sexual difference in metabolic properties is related to plasma testosterone levels, we asked if testosterone also affected the contractile protein isoforms. In the newborn guinea pig, both male and female temporalis muscles contained a fast-red isoform of the myosin heavy chain and approximately equal amounts of alpha- and beta-tropomyosins. At puberty, the male began to replace the fast-red isoform with a fast-white isoform of the heavy chain and by 120 days the muscle contained predominantly the fast-white myosin heavy chain. This transition in myosins in the male was accompanied by a shift to greater than 90% alpha-tropomyosin. No changes in myosins or tropomyosins were observed in the female. The changes in the male could be reversed by castration and could be mimicked in the female by the injection of testosterone. Although these same myosins and tropomyosins could be detected in other fast-twitch muscles, postpartum transitions in contractile protein isoforms in those muscles were testosterone-insensitive, and no sexual dimorphism of these proteins was seen in other muscles.