Intermittent cold exposure causes a muscle-specific shift in the fiber type composition in rats

We examined the effect of long-term intermittent cold exposure on the fiber type composition of the predominantly type I soleus and the predominantly type IIb extensor digitorum longus (EDL) muscles of rats. Cold exposure was accomplished by submerging the rats in shoulder-deep water, maintained at 20 +/- 0.5 degrees C, for 1 h/day, 5 days/wk, for < or = 19 wk. The efficacy of the treatment was tested by subjecting both groups to 20 degrees C water for 45 min while rectal temperature (Tre) and O2 consumption (VO2) were measured. The cold-exposed group displayed a 22% smaller reduction in Tre (P < 0.05) at the end of the exposure and 23% greater VO2 (P < 0.05) during the same period. Fiber type composition was determined using routine histochemical methods for myosin-adenosinetriphosphatase. In the soleus muscle of the cold-exposed rats, the number of type IIa fibers increased 156% (P < 0.05) and the number of type I fibers decreased 24% (P < 0.05). Cold exposure had no significant influence on the fiber type composition of the EDL muscle. Cold exposure resulted in an increase in citrate synthase activity of 20 and 22% in the soleus and EDL muscles, respectively (P < 0.05). The present study demonstrates that intermittent cold exposure induces a type I-to-type IIa transformation in the soleus muscle while having no influence on the EDL muscle.