Hypoxemia raises muscle sympathetic activity but not norepinephrine in resting humans

The experimental objective was to determine whether moderate to severe hypoxemia increases skeletal muscle sympathetic nervous activitiy (MSNA) in resting humans without increasing venous plasma concentrations of norepinephrine (NE) and epinephrine (E). In nine healthy subjects (20-34 yr), we measured MSNA (peronal nerve), venous plasma levels of NE and E, arterial blood pressure, heart rate, and end-tidal O₂ and CO₂ before (control) and during breathing of 1) 12% O₂ for 20 min, 2) 10% O₂ for 20 min, and 3) 8% O₂ for 10 min - in random order. MSNA increased above control in five, six, and all nine subjects during 12, 10 and 8% O₂, respectively (P < 0.01), but only after delays of 12 (12% O₂) and 4 min (8 and 10% O₂). MSNA (total activity) rose 83 ± 20, 260 ± 146, and 298 ± 109% (SE) above control by the final minute of breathing 12, 10, and 8% O₂, respectively. NE did not rise above control at any level of hypoxemia; E rose slightly (P < 0.05) at one time only with both 10 and 8% O₂. Individual changes in MSNA during hypoxemia were unrelated to elevations in heart rate or decrements in blood pressure and end-tidal CO₂ - neither of which always fell. We conclude that in contrast to some other sympathoexcitatory stimuli such as exercise or cold stress, moderate to severe hypoxemia increases leg MSNA without raising plasma NE in resting humans.