Rates of Mitochondrial Metabolism of Glucose, Amino Acids, and Fatty Acids by the HEI-OC1 Inner Ear Cell Line

Background: Mitochondrial substrate switching plays an important role in aging. The substrate metabolic rate is closely related to mitochondrial activity, as mitochondria are the primary site for substrate oxidation and ATP production. Different substrates (glucose, amino acids, and fatty acids) enter the mitochondria through distinct pathways and are metabolized at different rates, depending on the energy demand and cellular conditions. However, it remains unclear how the mitochondrial metabolic rate of these substrates affects auditory cellular function. This study aimed to characterize the substrate-dependent mitochondrial respiratory responses of cochlear cells under varying energy supply conditions and metabolic stress, focusing on glucose, amino acids, and fatty acids as representative energy sources. Methods: The oxygen consumption rate (OCR) was measured after substrate addition using an Agilent Seahorse XF24 Flux Analyzer In-House Ear Institute-Organ of Corti 1 (HEI-OC1) cells, and the maximum OCR (MOCR) was determined as part of the mitochondrial stress test. Statistical analyses were performed using analysis of variance (ANOVA). Results: The OCR increased significantly after glutamine (L-Gln) or palmitate addition. The MOCR after L-Gln addition was significantly higher than that after glutamic acid, glycine, and phenylalanine addition. The MOCR after pyruvate addition was significantly higher than that after glucose addition. However, there was no significant increase in the MOCR after fatty acid addition. Conclusions: Glucose is essential for basal metabolism but cannot rapidly meet sudden energy demands. Pyruvate and L-Gln serve as effective substrates for short-term, high-intensity energy demands. Fatty acids increase OCR through mitochondrial uncoupling effects, though their role may be limited in inner ear cells. These findings provide a foundation for exploring metabolic interventions to support cochlear function and hearing health.