Norepinephrine clearance is increased during acute hypoxemia in humans

Area of Science:

• Physiology
• Cardiovascular Regulation
• Neuroendocrinology

Background:

• Acute hypoxemia typically activates the sympathetic nervous system (SNS).
• However, expected adrenergic vasoconstriction and increased plasma norepinephrine (NE) are often absent.
• This dissociation suggests altered NE metabolism during hypoxemia.

Purpose of the Study:

• To investigate the relationship between sympathetic nervous outflow and plasma NE levels during acute hypoxemia.
• To determine if altered NE metabolism, specifically clearance, explains the attenuated NE rise.
• To examine the impact of hypoxemia on muscle sympathetic nervous activity and forearm vascular resistance.

Main Methods:

• Utilized the [3H]NE infusion technique to measure arterial NE kinetics in six healthy young men.
• Employed peroneal microneurography to assess sympathetic nervous outflow to muscle.
• Measured NE spillover, clearance, and arterial NE concentrations during 25-30 minutes of hypoxemia (O2 saturation 74%).

Main Results:

• Muscle sympathetic nervous activity (MSNA) significantly increased during hypoxemia.
• NE spillover rose, indicating increased NE release from sympathetic nerve terminals.
• Crucially, NE clearance also increased significantly, leading to a blunted rise in arterial NE.
• Forearm blood flow increased, while forearm vascular resistance decreased, suggesting vasodilation localized to skeletal muscle.

Conclusions:

• Acute hypoxemia activates the SNS in humans.
• The expected rise in plasma NE is attenuated due to increased NE clearance, not reduced release.
• This altered NE metabolism contributes to vasodilation in skeletal muscle during hypoxemia.