Effects of chronic hypoxemia on the afferent nerve activities from skeletal muscle

An acute reduction of the oxygen supply to contracting muscles not only aff ects their metabolism but also modifies their sensorimotor control through changes in afferent discharge of the group I and group III-IV nerve fibers, the latter playing a pivotal role in the protective mechanisms against mus cle fatigue. The effects of chronic hypoxemia on the muscle sensitivity are totally unknown. In the present study, group I fibers (mechanosensory affe rents) and group III-IV fibers (mechanosensory and chemosensory afferents) from the anterior tibial muscle were recorded in normoxemic and chronic hyp oxemic rats. Hypoxemic rats breathed for 45 d a gas mixture containing 9.5 to 10% O-2 in N-2. The data were compared. with those obtained in normoxemi c animals of the same age. To activate the different muscle afferents, we u sed different test agents, including electrically induced fatigue (EIF), KC l lactic acid injections, as well as tendon vibrations. The conduction velo city of all nerve fibers was significantly (p < 0.01) higher in hypoxemic r ats than in the normoxemic group. Chronic hypoxemia significantly depressed the response of the group III-IV muscle afferents to KCI injections and ev en abolished their response to lactic acid and EIF. However, the response t o tendon vibrations of the group I afferents was similar in hypoxemic and n ormoxemic rats. These results suggest that chronic hypoxemia markedly alter s the chemosensitivity of the group III-IV muscle afferents, which may expl ain the higher fatigability of hypoxemic subjects.