Male rats were trained to escape from radiant heat of infrared lamp (250W) by pressing a bar that turned a lamp off for 8 sec. To determine effects of repetitive exercise on this heat-escape behavior rats were either subjected to a 4-week physical training program in which they were forced to swim in agitating water of 36t or 38°C for 1 hour each day or were not trained (non-exercised controls) . After the program in 36°C water, the bar-pressing rate during the test period decreased markedly compared with that before the training period. Temperatures of the tail-skin and the environment in the test box increased to significantly higher levels in the trained rats than those before the training period, while the rectal temperature in the trained rats remained at the same level to that in the pretraining period. When a 4-week physical training program was completed in the same manner but using 38t water, no differences in the heat-escape activity and the extents of temperatures concurrently measured were obtained between those before and after the training period in the trained rats or controls.
The significant reduction of heat-escape activity in rats with the repetitive exercise for 4-week in the 36t water is a result of adaptive changes in the autonomic thermoregulation due to the repetitive exercise itself.

1. The effect of thyroidectomy on histochemical properties of the extensor digitorum longus and soleus muscles was studied in rats. Two or 13 months after thyroidectomy animals were killed and muscle sections of 10μm thickness were stained for succinate dehydrogenase and myosin adenosine triphosphatase activities.
2. No significant difference of the body weight was found between the thyroidectomized and sham-operated control groups. There was also no significant difference in the muscle weight between the two groups except that 13 months after thyroidectomy the soleus muscle weight was significantly lower in the thyroidectomized group than in the sham-operated control group.
3. The measurement of muscle fiber diameter revealed that fast-twitch oxidative glycolytic fibers of the soleus muscle were smaller in the thyroidectomized group than in the sham-operated control group two or 13 months after thyroidectomy.
4. The percentage of fast-twitch fibers decreased significantly after thyroidectomy with a concomitant increase in the percentage of slow-twitch fibers in both the extensor digitorum longus and soleus muscles. This finding suggests the involvement of thyroid hormone in determining a phenotypic expression of muscle fiber type.
5. The results show that thyroidectomy-induced changes differ not only between the extensor digitorum longus and soleus muscles but also between same types of fibers from the two muscles. The results also indicate that thyroidectomy-induced changes depend on the period after thyroidectomy.

Postnatal changes of myosin light chain patterns and enzyme activities were examined in the fast extensor digitorum longus and slow soleus muscles of Wistar strain rats aged 5, 10 and 20 weeks. Sex differences in each parameter were also studied.
The results were summarized as follows:
1) The extensor digitorum longus muscle contained mostly the fast light chains (fLC 1, fLC 2, fLC 3) . However, small amounts of the slow light chains (sLC 1, sLC 2) were also found. There were no postnatal changes in myosin light chain patterns in the extensor digitorum longus muscle.
2) The soleus muscle contained slow and fast light chains (sLC 1, sLC 2, fLC 1, fLC 2, fLC 3) in 5 weeks of age, slow light chains being more predominant than fast light chains. There were an increase in the percentage of slow light chains and a concomitant decrease in the percentage of fast light chains with age. FLC 3 which was found at 5 weeks of age did not be detected in the soleus muscle from the 20-week-old rat.
3) PFK activity decreased significantly with age in the soleus muscle, whereas there was no postnatal changes in PFK activity in the extensor digitorum longus muscle.
4) There was a significant increase in SDH activity during growth in both the extensor digitorum longus and soleus muscles with the exception of the extensor digitorum longus muscle from the male rat.
5) The enzyme activity ratio (PFK/SDH) significantly decreased with age in all muscles with an exception of the extensor digitolum longus muscle from the male rat.
6) In the soleus muscle an increase in the relative amount of slow type myosin light chains and a decrease in the enzyme activity ratio (PFK/SDH) occurred coincidentally during postnatal growth.
7) In most cases no significant sex differences existed in myosin light chain pattern and enzyme activity examined.

Sex difference in enzyme activities of the skeletal muscle were examined in rats aged 10 and 35 weeks. At 10 and 35 weeks of age, animals were anesthetized with ether and weighed. M, gastrocnemius, m.extensor digitorum longus and m.soleus were removed from both legs under pentobarbital anethesia and weighed. These muscles were used for the determination of myosin ATPase activity, phosphofruktokinase (PFK) activity, succinate dehydrogenase (SDH) activity and non-collagenous protein (NCP) content.
The results were summarized as follows:
1) Thirty-five week-old rats had heavier body and muscle weights than 10-weekold rats in both sexes and males had significantly heavier body and muscle weights than females at both 10 and 35 weeks of age.
2) Similar tendency was observed with regard to total NCP content. Furthermore, it was found that total NCP content was positively correrated to muscle weight (r=0.871, r=0.909 and r=0.871 in m. gastrocnemius, m.extensor digitorum longus and m.soleus, respectively) . However, no significant difference in NCP content per wet weight was found between both sexes and between different age groups.
3) Myosin ATPase activity tended to be lower at 35 weeks of age than at 10 weeks of age, the tendency being the most prominent in the gastrocnemius muscle of male rat (p<0.05) . However, no significant sex difference in myosin ATPase activity was observed in three muscle examined in both age groups.
4) Although not statistically significant, mean PFK activity was slightly lower in 35-week-old rats than in 10-week-old rats, and there was no sex difference in PFK activity at both weeks of age.
5) SDH activity was significantly lower in 35-week-old rats than in 10-week-old rats except that there was no significant age difference in the gastrocnemius muscle of males. There was no significant sex difference in SDH activity in both age groups with an exception of the extensor digitorum longus muscle from 35-week-old rats, where males had significantly higher SDH activity than females.

Myosin light chain patterns were examined in single fibers from the extensor digitorum longus (EDL) and soleus muscles of Wistar strain rats. The fiber type composition of both muscles was also determined to estimate the relationship between myosin light chain patterns and histochemical fiber types.
The results were summarized as follows ;
1. All of single fibers from the EDL muscle was typical fast type which contained fast light chains only (fLC 1·fLC 2·fLC 3), except that there was one mixed type fiber cotaining both fast and slow light chains (fLC 1·fLC 2·sLC 1·sLC 2) .
2. There were two myosin light chain patterns in single fibers from the soleus muscle. One was typical slow type which contained slow light chains only (sLC1·sLC 2) and the other, mixed type in which fast and slow light chains coexisted. Mixed type fibers were divided into eight groups based on the light chain composition. The percentage occurence of typical slow type and mixed type fibers was 35.4% and 64.6%, respectively.
3. The average percentage of type II fibers was 94.6% in the EDL muscle 5.4% in the soleus muscle.
4. These results suggest that both fast and slow skeletal muscles contain mixed type fibers. The results further imply that in slow skeletal muscle, myosin light chain pattern of mixed type occurs not only in type II but also in type I fibers.