Does the Sympathetic Nervous System Adapt to Chronic Altitude Exposure?

During continued exposure to hypobaric hypoxia in acclimatizing lowlanders increasing norepinephrine levels indirectly indicate sympathoexcitation, and in a few subjects serial measurements have suggested some adaptation over time. A few studies have provided direct microneurographic evidence for markedly increased muscle sympathetic nervous activity (MSNA) after 1-50 days of exposure of lowlanders to altitudes of 4100-5260 m above sea level. Only one study has provided two MSNA-measurements over time (10 and 50 days) in altitude (4100 m above sea level) and continued robust sympathoexcitation without adaptation was found in acclimatizing lowlanders. In this study, norepinephrine levels during rest and exercise also remained highly elevated over time. In comparison, acute exposure to hypoxic breathing (FiO2 0.126) at sea level caused no change in sympathetic nervous activity, although the same oxygen saturation in arterial blood (around 90 %) was present during acute (FiO2 0.126) and chronic hypoxic exposure (4100 m above sea level). These findings strongly suggest that the chemoreflex-mechanisms underlying acute hypoxia-induced increases in MSNA are sensitized over time. Collectively, the MSNA data suggests that sensitization of the sympathoexcitatory chemoreflex is evident but not complete within the first 24 h, but is complete after 10 days of altitude exposure. After return from high altitude to sea level the MSNA remains significantly elevated for at least 5 days but completely normalized after 3 months. The few MSNA measurements in high altitude natives have documented high sympathetic activity in all subjects studied. Because serial measurements of MSNA in high altitude natives during sea level exposure are lacking, it is unclear whether the sympathetic nervous system have somehow adapted to lifelong altitude exposure.