2004 Volume 31 Issue 1 Pages 63-69
It is known that heat stress induces expression of heat shock protein 70 (Hsp 70) in myocytes. Hsp 70 plays an important role in chaperoning nascent peptides during translation, and it has been assumed that the elevation of cellular Hsp 70 levels can serve as a countermeasure to attenuate the disuse-induced reduction in protein synthesis. We therefore investigated the effects of heat stress, prolonged stretching, and heat stress combined with prolonged stretching on prevention of disuse muscle atrophy in rat soleus (SOL) muscle. Thirty Wistar rats were divided randomly into the control (Con, n=5), heat stress (Heat, n=5), hindlimb suspension (HS, n=5), HS and heat stress (HS&Heat, n=6), HS and prolonged stretching (HS&Stretch, n=5), and HS and heat stress combined with prolonged stretching (HS&Heat&Stretch, n=4) groups. Disuse muscle atrophy was induced by hindlimb unweighting for 7 days. Each rat in the Heat, HS&Heat, and HS&Heat&Stretch groups was anesthetized, and bilateral hindlimbs were immersed daily in hot water (42 ℃) for 60 min during HS. In HS&Stretch and HS&Heat&Stretch groups, both soleus muscles underwent prolonged stretching daily for 30 min during HS. At the end of the experimental period, the SOL muscles were extracted and weighed. Frozen sections were stained with myofibrillar adenosin triphosphatase (pH 10.5), and muscle fiber diameters of each fiber type were measured. To determine the levels of Hsp 70 in muscular tissue, we performed an enzyme-linked immunosorbent assay (ELISA) and western blotting methods. The weight of the SOL muscles relative to body weight for the HS&Heat, HS&Stretch, and HS&Heat&Stretch groups increased significantly compared with the HS group. The mean muscle fiber diameter of type I and II for the HS&Heat, HS&Stretch, and HS&Heat&Stretch groups increased significantly compared with the HS group. HSP 70 levels were significantly greater in the Heat, HS&Heat, and HS&Heat&Stretch groups compared with the Con, HS, and HS&Stretch groups. Our results suggest that the progression of disuse muscle atrophy can be attenuated by heat or stretch, and this mechanism is related to the expression of Hsp 70 or mechanical stretch in myocytes.
