Markers of mitochondrial function and oxidative metabolism in skeletal muscle do not display intrinsic circadian regulation in female mice

Mitochondria are key regulators of metabolism and ATP supply in skeletal muscle, while circadian rhythms influence many physiological processes. However, whether mitochondrial function is intrinsically regulated in a circadian manner in mouse skeletal muscle is inadequately understood. Accordingly, we measured postabsorptive transcript abundance of markers of mitochondrial autophagy, dynamics, and metabolism [extensor digitorum longus (EDL), soleus, gastrocnemius], protein abundance of electron transport chain complexes (EDL and soleus), enzymatic activity of succinate dehydrogenase (tibialis anterior and plantaris), and maximal mitochondrial respiration (tibialis anterior) in different skeletal muscles from female C57BL/6NJ mice at four zeitgeber times: 1, 7, 13, and 19. Our findings demonstrate that markers of mitochondrial function and oxidative metabolism do not display intrinsic time-of-day regulation at the gene, protein, enzymatic, or functional level. The core-clock genes <i>Bmal1</i> and <i>Dbp</i> exhibited intrinsic circadian rhythmicity in skeletal muscle (i.e., EDL, soleus, gastrocnemius) and circadian amplitude varied by muscle type. These findings demonstrate that female mouse skeletal muscle does not display circadian regulation of markers of mitochondrial function or oxidative metabolism over 24 h.<b>NEW & NOTEWORTHY</b> The prominent hypothesis in the field of skeletal muscle chronobiology is that markers of skeletal muscle mitochondrial function are intrinsically time-of-day regulated. However, this has not yet been directly tested. Thus, we measured several markers of mitochondrial function in five muscles from female mice, and maximal uncoupled respiration, at four timepoints across a 24-h day. Our findings indicate that markers of mitochondrial function are not intrinsically regulated by time-of-day in female mouse muscle.