Mechanical properties of skeletal muscles or motor performance in mammals [1][2][3][4][5][6][7][8][9], frogs [10,11], and fishes [12][13][14][15] are influenced by changes in environmental temperatures. For example, the maximum velocity of shortening and the maximum rate of tension development decreased at a low temperature in the soleus and extensor digitorum longus (EDL) muscles of rats [4]. Further, Faulkner et al. [1] reported that the maximum and sustained powers in mice EDL muscle were reduced by 40 and 62%, respectively, at 25°C compared to those at 35°C, though the isometric force remained constant. On the contrary, the maximum tetanic tension of rat soleus and EDL was increased steadily when the temperature was elevated from 6 to 30-38°C [3]. It is also reported that the average absolute force in skinned fibers of both soleus Japanese Journal of Physiology, 52, 85-93, 2002 Key words: cold exposure of rat, soleus, extensor digitorum longus, contractile properties, plasma hormones.
Abstract:The effects of 20-week cold exposure on contractile properties of soleus and extensor digitorum longus (EDL) muscles and plasma hormone levels were studied in rats. Twenty male Wistar rats (5 week old) were randomly divided into 2 groups (nϭ10 each): cagecontrol and cold-exposed. The rats in the coldexposed group were immersed in shoulder-deep water (ϳ18°C) for 1 h/d, 5 d/week, for 20 weeks. The temperature and humidity of the animal room with 12:12 h light-dark cycle were maintained at ϳ23°C and 55%, respectively. The rats were pair-fed powdered diets. The electromyogram activities in soleus and EDL were elevated by cold exposure. The body weight and absolute soleus wet weight of the cold-exposed group were significantly less than controls at the end of experiment. The one-half relaxation time and contraction time of EDL were significantly longer in the cold-exposed group than in the control group. The rate of twitch tension development, normalized by the maximum twitch tension, in EDL of the cold-exposed group was less than in the control group. Further, the fatigue resistance of EDL, but not of soleus, in response to train stimulation at 10 Hz was improved by cold exposure. The plasma levels of thyroid hormones, 3,5,3Ј-triiodothyronine and thyroxine, were significantly greater in cold-exposed group. Similar changes were also seen in the plasma catecholamine levels in the cold-exposed group ( pϾ0.05). It is suggested that long-term cold exposure causes a shift of the contractile properties of fast-twitch EDL muscle toward the slow-twitch type. The results also indicated that the characteristics of muscles responded more strongly to an increased activity level than to the elevation of plasma hormones.
