Eight young healthy male subjects, members of a Himalayan expedition, underwent 24 h Holter monitoring before departure, after 1 and 4 weeks at high altitude (5000 m) and after return to sea level. At high altitude, the circadian reciprocal changes in low and high frequency (LF, HF) were absent, with no significant reduction in the LF to HF ratio over the 24 h; moreover, the proportion of adjacent R-R intervals that differed by more than 50 ms (pNN50) decreased significantly and remained lower after return to sea level. Urine catecholamines increased at high altitude, but only norepinephrine, after 1 week of exposure, rose significantly. Upon return to sea level the density, but not the affinity, of [alpha]2-adrenergic receptors on platelets decreased significantly compared to pre-expedition values. At high altitude increased sympathetic activity was indicated by elevation of urine norepinephrine and by the loss of circadian rhythm in spectral components. The simultaneous reduction of HF and pNN50 demonstrated decreased vagal tone. The persistence of increased sympathetic activity could explain the downregulation of adrenergic receptors after prolonged high altitude exposure.
