Effects of chronic nitric oxide synthase inhibition on $VO_{2max}$ and exercise capacity in mice

Acute inhibition of NOS by L-NAME ($N^{\omega}$-nitro-L-arginine methyl ester) is known to decrease maximal oxygen consumption ($V'O_{2max}$) and impair maximal exercise capacity, whereas the effects of chronic L-NAME treatment on $V'O_{2max}$ and exercise performance have not been studied so far. In this study, we analysed the effect of L-NAME treatment, (LN2 and LN12, respectively) on $V'O_{2max}$ and exercise capacity (in maximal incremental running and prolonged sub-maximal incremental running tests), systemic NO bioavailability (plasma nitrite ($NO_{2}^{-}$) and nitrate ($NO_{3}^{-}$)) and prostacyclin ($PGI_{2}$) production in C57BL6/J mice. Mice treated with L-NAME for 2 weeks (LN2) displayed higher $V'O_{2max}$ and better running capacity than age-matched control mice. In LN2 mice, NO bioavailability was preserved, as evidenced by maintained $NO_{2}^{-}$ plasma concentration. $PGI_{2}$ production was activated (increased 6-keto-$PGF_{1\alpha}$ plasma concentration) and the number of circulating erythrocytes (RBC) and haemoglobin concentration were increased. In mice treated with L-NAME for 12 weeks (LN12), NO bioavailability was decreased (lower $NO_{2}^{-}$ plasma concentration), and 6-keto-$PGF_{1\alpha}$ plasma concentration and RBC number were not elevated compared to age-matched control mice. However, LN12 mice still performed better during the maximal incremental running test despite having lower $V'O_{2max}$. Interestingly, the LN12 mice showed poorer running capacity during the prolonged sub-maximal incremental running test. To conclude, short-term (2 weeks) but not long-term (12 weeks) treatment with L-NAME activated robust compensatory mechanisms involving preservation of NO2- plasma concentration, overproduction of $PGI_{2}$ and increased number of RBCs, which might explain the fully preserved exercise capacity despite the inhibition of NOS.