Mitochondrial respiration is fiber‐type specific in skeletal muscles

Abstract

IntroductionMitochondria are the power houses of the cells and they also play critical roles in the cellular oxidative stress and apoptosis. It is known that mitochondrial density and morphology are fiber‐type specific. Based on differential contractile properties of different types of skeletal muscles, we hypothesized that the mitochondria respiration is fiber‐type specific. The aim of the study is to compare and contrast mitochondria respiration between type I and type II muscles.MethodSoleus (predominantly composed of type I fibers) and plantaris (predominantly composed of type II fibers) muscles were taken from adult rats. Mitochondrial respiration was determined in duplicate by a high‐resolution respirometry OROBOROS Oxygraph‐2k (O2k). The mitochondrial substrate‐uncoupler‐inhibitor titration (SUIT) protocol was used to determine mitochondrial oxygen consumption rate (O2 flux) of permeabilized myofibers. Paired t test was used to compare parameters between soleus and plantaris muscles.ResultsWe found that basal mitochondrial respiration (i.e. O2 flux_ complex I plus complex II) and leak of permeabilized muscle fibers was similar between soleus and plantaris muscles. Similarly, mitochondrial coupling control ratio defined by leak divided by basal mitochondrial respiration (i.e. leak/O2 flux_ complex I plus complex II) is similar between soleus and plantaris muscles. Maximal electron transport system capacity (maximal mitochondrial respiration) of soleus muscles was greater than that of plantaris muscles. In addition, mitochondrial respiratory reserve capacity defined by subtracting basal mitochondrial respiration from maximal mitochondrial respiration was greater in soleus muscles than in plantaris muscles.ConclusionBasal mitochondrial respiration and leak are similar between type I and type II muscles. However, mitochondria in type I muscles have greater respiratory reserve compared to that in type II muscles. This fiber‐type specific mitochondrial respiration is likely associated with the differential fuel metabolism and contractile properties of muscle fibers.