Effect of postexercise temperature elevation on postexercise glycogen metabolism of isolated mouse soleus muscle.

The effects of temperature elevation after intense repeated contractions on glycogen and energy metabolism as well as contractile function of isolated mouse soleus muscle (slow twitch, oxidative) were investigated. Muscles were stimulated electrically to perform repeated tetanic contractions for 10 min at 25°C, which reduced tetanic force by ~85% and glycogen by 50%. After 120-min recovery at 25°C glycogen was fully restored (~125% of basal), whereas after recovery at 35°C glycogen decreased further (~25% of basal). Glycogen synthase fractional activity averaged 31.8 ± 3.1% (baseline = 33.8 ± 3.4%) after 120-min recovery at 25°C but was increased after recovery at 35°C (63.8 ± 4.8%; < 0.001 vs. 25°C). Phosphorylase fractional and total activities were not affected by the higher temperature. However, recovery at 35°C resulted in a significantly higher content of the phosphorylase substrate inorganic phosphate (~20%; < 0.01 vs. 25°C). Finally, fatigue development during a subsequent bout of repeated contractions at 25°C was similar after 120-min recovery at 25°C and 35°C. These data demonstrate that after intense contractions elevated temperature inhibits glycogen accumulation, likely by increasing the availability of the phosphorylase substrate inorganic phosphate, but has no effect on fatigue development. Thus after heat exposure phosphorylase plays a significant role in glycogen accumulation, and glycogen does not limit muscle performance in isolated mouse soleus muscle after recovery from elevated temperature. Whether elevated temperature affects glycogen biogenesis and contractile performance of isolated slow-twitch muscle is not known. Here we show that after a bout of repeated contractions in isolated mouse soleus muscle at 25°C, increasing muscle temperature during recovery to 35°C blocked glycogen accumulation compared with recovery at 25°C. Surprisingly, during a subsequent bout of repeated contractions at 25°C, the rate of fatigue was not different between groups after recovery at the two temperatures.