Running performance at high running velocities is impaired but $VO_{2max}$ and peripheral endothelial function are preserved in $IL-6^{−/−}$ mice

It has been reported that IL-6 knockout mice ($IL-6^{−/−}$) possess lower endurance capacity than wild type mice (WT), however the underlying mechanism is poorly understood. The aim of the present work was to examine whether reduced endurance running capacity in $IL-6^{−/−}$ mice is linked to impaired maximal oxygen uptake ($V′O_{2max}$), decreased glucose tolerance, endothelial dysfunction or other mechanisms. Maximal running velocity during incremental running to exhaustion was significantly lower in IL-6−/− mice than in WT mice (13.00±0.97 m.min$^{-1}$ vs. 16.89±1.15 m.min$^{-1}$, P<0.02, respectively). Moreover, the time to exhaustion during running at 12 m.min$^{-1}$ in $IL-6^{−/−}$ mice was significantly shorter (P<0.05) than in WT mice. $V′O_{2max}$ in $IL-6^{−/−}$ (n = 20) amounting to 108.3±2.8 ml.kg$^{-1}$.min$^{-1}$ was similar as in WT mice (n = 22) amounting to 113.0±1.8 ml.kg$^{-1}$.min$^{-1}$, (P = 0.16). No difference in maximal COX activity between the $IL-6^{−/−}$ and WT mice in m. soleus and m. gastrocnemius was found. Moreover, no impairment of peripheral endothelial function or glucose tolerance was found in $IL-6^{−/−}$ mice. Surprisingly, plasma lactate concentration during running at 8 m.min−1 as well at maximal running velocity in $IL-6^{−/−}$ mice was significantly lower (P<0.01) than in WT mice. Interestingly, $IL-6^{−/−}$ mice displayed important adaptive mechanisms including significantly lower oxygen cost of running at a given speed accompanied by lower expression of sarcoplasmic reticulum $Ca^{2+}$-ATPase and lower plasma lactate concentrations during running at submaximal and maximal running velocities. In conclusion, impaired endurance running capacity in $IL-6^{−/−}$ mice could not be explained by reduced $V′O_{2max}$, endothelial dysfunction or impaired muscle oxidative capacity. Therefore, our results indicate that IL-6 cannot be regarded as a major regulator of exercise capacity but rather as a modulator of endurance performance. Furthermore, we identified important compensatory mechanism limiting reduced exercise performance in $IL-6^{−/−}$ mice.