Parasympathetic withdrawal increases heart rate after two weeks at 3,454 m altitude

Chronic hypoxia increases resting heart rate (HR), but the underlying mechanism remains incompletely understood. We investigated the relative contributions of the sympathetic and parasympathetic nervous systems, along with potential non-autonomic mechanisms, by individual and combined pharmacological inhibition of muscarinic and/or β-adrenergic receptors. In seven healthy lowlanders, resting HR was determined at sea level (SL) and after 15–18 days of exposure to 3,454 m high altitude (HA) without drug intervention (CONT) as well as after intravenous administration of either propranolol (PROP), glycopyrrolate (GLYC), or PROP and GLYC in combination (PROP+GLYC). Circulating norepinephrine concentration increased from 0.9 ± 0.4 nmol⁻¹ at SL to 2.7 ± 1.5 nmol⁻¹ at HA (P = 0.03). The effect of HA on HR depended on the type of autonomic inhibition (P = 0.006). Specifically, HR was increased at HA from 64 ± 10 to 74 ± 12 beats min⁻¹ during CONT (P = 0.007) and from 52 ± 4 to 59 ± 5 beats min⁻¹ during PROP (P < 0.001). In contrast, HR was similar between SL and HA during GLYC (110 ± 7 and 112 ± 5 beats min⁻¹, P = 0.28) and PROP+GLYC (83 ± 5 and 85 ± 5 beats min⁻¹, P = 0.25). Our results identify a reduction in cardiac parasympathetic activity as the primary mechanism underlying the elevated HR associated with two weeks of exposure to hypoxia. Unexpectedly, the sympathoactivation at HA that was evidenced by increased circulating norepinephrine concentration had little effect on HR, potentially reflecting down-regulation of cardiac β-adrenergic receptor function in chronic hypoxia. These effects of chronic hypoxia on autonomic control of the heart may concern not only HA dwellers, but also patients with disorders that are associated with hypoxemia.

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