INTERACTIVE EFFECTS OF PITUITARY FUNCTION, GENDER, AND MECHANICAL LOAD ON SKELETAL MUSCLE MASS 533

Abstract

In order to study the relationships between pituitary function, gender, and mechanical load on skeletal muscle mass the predominantly slow-twitch soleus and fast-twitch plantaris were functionally overloaded (FO) in normal and genetically growth hormone (GH) deficient male and female Lewis rats. In normal control rats body mass was 62% greater in male than female rats. Similarly, serum insulin-like growth factor I (IGF-I) concentration was also 74% greater in male rats. Muscle wet weights were greater in male rats, but proportionally (mg/100 g body weight) similar between genders. Though quite variable GH was generally 70-80% less in dwarf rats of both genders. In dwarf male rats serum IGF-I and body mass was decreased ≈40%. Body mass was decreased 24% in dwarf female rats, but IGF-I was not affected. Soleus mass was smallest in dwarf female rats and largest in normal FO male rats(52±2 vs. 163±2 mg, respectively). Utilizing a three-factor ANOVA significant main effects of gender, dwarfism, and FO were found for the soleus muscle (p<0.001). A significant interaction was also detected between dwarfism and gender. In contrast, the effect of FO to increase soleus mass was not influenced by gender or dwarfism. Mass of the fast-twitch plantaris muscle was influenced by interactions between gender, dwarfism, and FO, as well as a significant gender*dwarfism*FO interaction. For example, the plantaris muscle of normal FO male rats was 230% larger than in dwarf female rats (548±26 vs. 166±5 mg, respectively). Collectively, our results indicate that mass of the slow-twitch soleus is influenced by growth and mechanical load, but not through any interaction between load and the endocrine system. However, hypertrophy of the fast-twitch plantaris muscle is potentiated by interactions between mechanical load, pituitary function, and gender.