EXPERIMENTAL STUDY Growth hormone and mild exercise in combination increases markedly muscle mass and tetanic tension in old rats

Objective: A decline of skeletal muscle mass and strength is seen with aging and immobilization. Growth hormone (GH) has been shown to increase muscle mass. In the present study the effects of a combination of mild exercise and GH on skeletal musculature tetanic tension, dry defatted weight (DDW), volume, water, fat and collagen concentrations were investigated in old rats. Design: Recombinant human GH (2.7 mg/kg per day) was injected subcutaneously for 73 days in 21-month-old female rats. Exercised rats ran on a treadmill, 8 m/min for 1 h/day. The in vivo maximal tetanic tension of the calf musculature (m. soleus, m. plantaris, m. gastrocnemius together) was analysed in anaesthetized rats by stimulating the ischiadic nerve. Results: The maximal tetanic tension was increased by 23 % in GH-injected compared to saline-injected rats. Mild exercise GH in combination resulted in a further 18 % increase in maximal tetanic tension. The mild exercise by itself did not in¯uence the maximal tetanic tension signi®cantly when compared with saline injected rats. The GH administration and/or mild exercise did not change skeletal muscle endurance, measured as tetanic tension during 30 s of stimulation. Serum IGF-I concentration was increased twofold in GH-injected rats. Conclusion: The increased muscle mass induced by GH mild exercise was associated with a corresponding increase in maximal tetanic tension. Combination of GH mild exercise resulted in a substantial further increase of muscle mass and maximal tension compared with GH injections alone in these old rats. European Journal of Endocrinology 143 409±41