Lactate augments intramuscular triglyceride accumulation and mitochondrial biogenesis in rats

Regular exercise induces intramuscular triglyceride accumulation with improved mitochondrial ability, but the mechanism remains unknown. The glycolytic product of exercise, lactate, has long been rec-ognized to suppress lipolysis and promote lipogenesis in adipocytes through inhibition of the cAMP-PKA pathway by activation of the G protein-coupled receptor (GPR81). However, whether lactate results in a similar process in skeletal muscle is unclear. Here, by using intramuscular injection of lactate to the gastrocnemius, the lipid metabolism effects were investigated in rat skeletal muscle. Firstly, the lactate-injection effect was verified by comparing changes in blood lactate levels from injection and exercise (30 min, 31 m/min, treadmill running). After five weeks of lactate intervention, intramuscular triglyceride levels in the gastrocnemius and the proportion of epididymis adipose mass to body weight increased. Chronic intramuscular injection of lactate elevated lactate receptor, GPR81, and reduced cAMP response element-binding (CREB) and P-CREB abundance in the gastrocnemius. Additionally, there was a significant decline in lipolytic-related proteins (AMPK, P-AMPK, P-HSL, CPT-1B, TGF-β2, SDHA) and a significant increase in fat synthesis proteins (SREBP-1C, PPAR-γ). Surprisingly, mitochondrial biomarkers (PGC-1α, CS) were also increased in the gastrocnemius, suggesting that chronic lactate might promote mitochondria biogenesis. Together, these results demonstrated that lactate may play a crucial role in triglyceride storage and mitochondria biogenesis in the skeletal muscle of rat.