Mitochondria Content In Rat Ankle Extensor Muscles Following Chronic Endurance Exercise

Abstract

Endurance exercise studies involving animals typically measure muscle mitochondria content to verify intensity of training. Most use one of the three calf ankle extensor muscles, which vary in fiber type. The soleus is all slow twitch fibers (SO) while the plantaris and gastrocnemius are a mixture of fast twitch glycolytic (FG) and fast oxidative glycolytic (FOG) fibers. PURPOSE: To compare mitochondria content changes in the soleus, plantaris, and gastrocnemius in response to a widely used moderate intensity exercise program. METHODS: Male, 8-wk-old, Sprague-Dawley rats were divided into two groups: sedentary (S) and exercised (E) on a treadmill 5 d/wk for 6 wks. Exercise duration and intensity were progressively increased to 1 hr at 30 m/min up a 10.5% incline (75-80% VO2max). Whole muscles plus specific FG and FOG regions of gastrocnemius were homogenized (9-10 for each muscle or fiber type) and cytochrome c oxidase activity, a marker of mitochondria content, was determined using a Clark-type oxygen electrode. RESULTS: E significantly increased cytochrome c oxidase compared to the same S muscle (P<0.05) and the amount of increase was similar for all muscles (P>0.05). On a relative basis (% above mean S value), the increases were 60.7 ± 13.3 (mean ± standard error) in soleus, 72.3 ± 6.8 in plantaris, and 78.7 ± 12.4 in gastrocnemius. On an absolute basis (μMol O2 consumed/min/gram muscle), the increases were 28.5 ± 6.3 in soleus, 35.0 ± 3.3 in plantaris, and 25.7 ± 3.8 in gastrocnemius. E was also greater than S in all fiber types considered (P<0.05), but there were considerable differences in the amount of increase. On a relative basis (%), the increase in FOG fibers (115.5 ± 17.0) was significantly greater (P<0.05) than those in SO (60.7 ± 13.3) or FG (42.8 ± 12.0). On an absolute basis, all fibers differed from each other with FOG increasing the most and FG the least (P<0.05). Specifically, FOG increased by 53.4 ± 7.9, SO by 28.5 ± 6.3, and FG by only 5.1 ± 1.4. CONCLUSION: All three whole muscles increase by similar amounts in response to a moderate intensity (75-80% VO2max) exercise program, thus all are appropriate for verifying training status. Caution is advised when using small portions of plantaris and gastrocnemius because of differences in responses of FOG and FG fibers. Supported by UNCG School of HHS Research Excellence Grant.