Lactate administration activates the ERK1/2, mTORC1, and AMPK pathways differentially according to skeletal muscle...

Skeletal muscle is described as an endocrine organ, constitutively or intermittently secreting bioactive molecules. The signaling pathways by which these molecules mediate changes in skeletal muscle and regulate interorgan crosstalk are only partly understood. Lactate is widely described as a signaling molecule in different cells, but the role of lactate as a signaling molecule in mature skeletal muscle has not been fully unveiled. The aim of this study was to determine the role of lactate on activation of signaling pathways in adult mouse skeletal muscle. Male mice were injected intraperitoneally with lactate or saline, and tissues were dissected after 40 min. Phosphorylation levels of relevant proteins in muscle were assessed by Western blotting. After lactate administration, we found an increase in p-ERK1/2^{Thr202/Tyr204} (3.5-fold; P = 0.004) and p-p70S6K^Thr389 (1.9-fold; P = 0.01) in quadriceps; and an increase in p-rpS6^Ser235/236 in both quadriceps (6.3-fold; P = 0.01) and EDL (2.3-fold; P = 0.01), without changes in soleus. There was a tendency toward an increase in p-AMPK^Thr172 (1.7-fold; P = 0.08), with a significant increase in p-ACC^Ser79 (1.5-fold; P = 0.04) in soleus, without changes in quadriceps and EDL. These results support the hypothesis that lactate plays a role in the molecular signaling related to hypertrophy and to oxidative metabolism on adult skeletal muscle and suggest that this activation depends on the skeletal muscle type. The mechanisms that underlie the effect of lactate in mature skeletal muscles remain to be established.