Dose-dependent modulation of mitochondrial physiology and nutrient secretion in Sertoli cells by l-carnitine

l-Carnitine is a ubiquitous quaternary amine, essential for the mitochondrial β-oxidation of long-chain fatty acids, and a dietary supplement widely consumed by athletes due to its purported ergogenic and recovery-enhancing properties. Beyond its metabolic role in muscle tissue, l-Carnitine is highly concentrated in the male reproductive tract, suggesting a critical function in spermatogenesis. In the testis, Sertoli cells (SCs) act as essential "nurse cells," providing structural, nutritional, and immunomodulatory support to developing germ cells. This study elucidates the effects of l-Carnitine on SC viability, proliferation, redox state, and mitochondrial physiology, using an in vitro model (TM4 cell line) exposed to a range of l-Carnitine concentrations (0.005, 0.05, 0.5, and 5 mM) for 24 h. We demonstrate that a concentration mirroring plasma levels in athletes (0.05 mM) significantly enhances SC proliferation and metabolic viability, while selectively upregulating the expression of ATP synthase (Complex V). Conversely, supra-physiological doses (0.5 mM) induce a significant metabolic shift in the SC exometabolome, characterized by a marked reduction in the production of lactate, alanine, and acetate. This suggests a possible disruption of the metabolic support provided to germ cells. At the highest concentration (5 mM), l-Carnitine exhibits overt cytotoxicity, as evidenced by significant lactate dehydrogenase (LDH) leakage. Furthermore, l-Carnitine acts as a potent antioxidant, reducing endogenous reactive oxygen species (ROS) and mitigating oxidative damage to proteins and lipids. These findings provide mechanistic insights into how l-Carnitine use modulates the bioenergetic landscape of the seminiferous epithelium, suggesting that while physiological supplementation may bolster male fertility by improving SC health, excessive concentrations could paradoxically impair germ cell nutrition.