title: Alterations in sympathetic neurovascular transduction during acute hypoxia in humans.
abstract: Resting vascular sympathetic outflow is significantly increased during and beyond exposure to acute hypoxia without a parallel increase in either resistance or pressure. This uncoupling may indicate a reduction in the ability of sympathetic outflow to effect vascular responses (sympathetic transduction). However, the effect of hypoxia on sympathetic transduction has not been explored. We hypothesized that transduction would either remain unchanged or be reduced by isocapnic hypoxia. In 11 young healthy individuals, we measured beat-by-beat pressure, multiunit sympathetic nerve activity, and popliteal blood flow velocity at rest and during isometric handgrip exercise to fatigue, before and during isocapnic hypoxia (~80% SpO₂), and derived sympathetic transduction for each subject via a transfer function that reflects Poiseuille's law of flow. During hypoxia, heart rate and sympathetic nerve activity increased, whereas pressure and flow remained unchanged. Both normoxic and hypoxic exercise elicited significant increases in heart rate, pressure, and sympathetic activity, although sympathetic responses to hypoxic exercise were blunted. Hypoxia slightly increased the gain relation between pressure and flow (0.062 ± 0.006 vs. 0.074 ± 0.004 cm·s(-1)·mmHg(-1); P = 0.04), but markedly increased sympathetic transduction (-0.024 ± 0.005 vs. -0.042 ± 0.007 cm·s(-1)·spike(-1); P < 0.01). The pressor response to isometric handgrip was similar during normoxic and hypoxic exercise due to the balance of interactions among the tachycardia, sympathoexcitation, and transduction. This indicates that the ability of sympathetic activity to affect vasoconstriction is enhanced during brief exposure to isocapnic hypoxia, and this appears to offset the potent vasodilatory stimulus of hypoxia.
authors.affiliation: Cardiovascular Research Laboratory, Spaulding Rehabilitation Hospital, Boston, MA, USA. cotan@partners.org
authors.forename: Can Ozan
authors.initials: CO
authors.lastname: Tan
authors.forename: Yu-Chieh
authors.initials: YC
authors.lastname: Tzeng
authors.forename: Jason W
authors.initials: JW
authors.lastname: Hamner
authors.forename: Renaud
authors.initials: R
authors.lastname: Tamisier
authors.forename: J Andrew
authors.initials: JA
authors.lastname: Taylor
mesh_terms: D000208:Acute Disease; D000328:Adult; D000465:Algorithms; D001794:Blood Pressure / Q000502:physiology; D003627:Data Interpretation, Statistical; D005260:Female; D006225:Hand / Q000098:blood supply / Q000502:physiology; D018737:Hand Strength / Q000502:physiology; D006339:Heart Rate / Q000502:physiology; D006439:Hemodynamics / Q000502:physiology; D006801:Humans; D000860:Hypoxia / Q000503:physiopathology*; D007537:Isometric Contraction / Q000502:physiology; D008297:Male; D018763:Muscle Fatigue / Q000502:physiology; D017156:Photoplethysmography; D012039:Regional Blood Flow / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology*; D012867:Skin / Q000098:blood supply; D013564:Sympathetic Nervous System / Q000503:physiopathology*; D055815:Young Adult
keywords: isocapnic hypoxia; muscle sympathetic nerve activity; vascular control