Mennens L (Rehabilitation Research Center, Hasselt University, Diepenbeek, Belgium / Human Biology, NUTRIM, Maastricht University Medical Center⁺, Maastricht The Netherlands), Hoebers N, Jocken JWE, Verboven K, Goossens GH (Human Biology, NUTRIM, Maastricht University Medical Center⁺, Maastricht, the Netherlands). lisa.mennens@uhasselt.be

Background: Physical activity is a powerful tool to prevent and reverse type 2 diabetes. It has been shown that myokines released by contracting myocytes affect glucose homeostasis. Reduced oxygen availability during exercise might be a key regulator of these processes. Therefore, we investigated the effects of oxygen level on myokine secretion and metabolic pathways under resting and contractile conditions in primary human myotubes.

Methods: Ex vivo primary human satellite cells were differentiated towards functional myotubes and exposed to different oxygen levels for 3h (1% vs 3% vs 21% O₂) under resting or contractile (Electrical Pulse Stimulation (EPS), 30V, 1Hz, 2ms) conditions. We determined lactate secretion, gene expression and secretion of IL-6, GDF-15, IL-15, SPARC and myonectin, as well as AMPK activation. Data were analyzed using the non-parametric Kruskal-Wallis test, with Bonferroni’s post-hoc test in case of significance.

Results: Both EPS (1.19±0.01mmol/L) and low oxygen availability (1.09±0.03mmol/L) induced a significant increase in lactate secretion compared to resting, non-EPS standard laboratory conditions (21% O₂; 0.99±0.01mmol/L; both p<0.001). Highest lactate secretion was found when cells were exposed to 1% O₂ during EPS (1.33±0.01mmol/L; p<0.001). In comparison to oxygen exposure at 21% O­₂, exposure at 1% O₂ increased IL-6 gene expression (~1.7-fold; p<0.001) and secretion (~1.3-fold; p=0.002), reduced myonectin gene expression (~0.7-fold; p=0.005) and GDF-15 secretion (~0.9-fold; p=0.014), while 3% O₂ increased SPARC secretion (~1.2-fold; p<0.001). Strikingly, EPS during low oxygen exposure further increased gene expression of IL-6 with 20% (p=0.044), GDF-15 with 12% (p<0.001) and IL-15 with 18% (p=0.011), and further increased IL-6 secretion with 14% (p=0.019) compared to non-EPS conditions.

Conclusion: The present findings demonstrate that both contraction and a reduced oxygen availability in the skeletal muscle microenvironment induce a shift towards anaerobic (glycolytic) metabolism and affect myokine expression/secretion.