Improved tolerance of acute severe hypoxic stress in chronic hypoxic diaphragm is nitric oxide-dependent

Abstract

The effects of chronic hypoxia (CH) on respiratory muscle performance have hardly been investigated, despite clinical relevance. Results from recent studies are indicative of unique adaptive strategies in hypoxic diaphragm. Respiratory muscle tolerance of acute severe hypoxic stress was examined in normoxic and CH diaphragm in the presence and absence of a nitric oxide (NO) synthase inhibitor. We tested the hypothesis that improved tolerance of severe hypoxic stress in CH diaphragm is NO-dependent. Wistar rats were exposed to normoxia (sea-level, n = 6) or CH (ambient pressure = 380 mmHg, n = 6) for 6 weeks. Diaphragm muscle functional properties were determined ex vivo under severe hypoxic conditions (gassed with 95%N2/5% CO2) with and without 1 mM L-N(G)-nitroarginine (L-NNA, nNOS inhibitor). Fatigue tolerance, but not force, was significantly improved in CH diaphragm (p = 0.008). CH exposure did not affect diaphragm muscle fibre oxidative capacity determined from cluster analysis of area-density plots of muscle fibre succinate dehydrogenase activity. Acute NOS inhibition reduced diaphragm peak tetanic force (p = 0.018), irrespective of gas treatment, and completely reversed improved fatigue tolerance of the CH diaphragm. We conclude that CH exposure improves fatigue tolerance during acute severe hypoxic stress in an NO-dependent manner, independent of muscle fibre oxidative capacity.

Fig. 1

a Original representative trace of diaphragm fatigue obtained during repeated muscle stimulation (40 Hz every 2 s for 5 min) during severe hypoxic stress. Inset shows the first and last muscle contraction superimposed to illustrate the altered amplitude and kinetics characteristic of muscle fatigue. b Group data illustrating force potentiation and fatigue in each group over the 5 min of repeated muscle stimulation. Values are mean ± SEM (n = 6 per group). *p < 0.05 one-way ANOVA followed by Newman–Keuls post-hoc test. CH chronic hypoxia, l -NNA N^G-nitro-l-arginine

Fig. 2

Data for diaphragm muscle fatigue index. Values are mean ± SEM (n = 6 per group). Two-way ANOVA revealed a significant effect of CH (p = 0.008) and drug (p = 0.027) treatment. Asterisk indicates significant difference from normoxia; p < 0.05. CH chronic hypoxia, l -NNA N^G-nitro-l-arginine

Fig. 3

a A representative area–density plot for diaphragm fibres histochemically stained for the oxidative enzyme succinate dehydrogenase. b Results from cluster analysis for normoxic and CH diaphragm (n = 6 per group)