Sex Differences in Sympathetic Activity and Vascular Reactivity During Acute and Chronic Hypoxia.
Condition: Vasodilation; Vasoconstriction
MeSH terms: Hypoxia
Study type: Interventional

Summary: 
      This study aims to investigate sex differences in blood pressure control associated with
      exposure to acute hypoxia (low oxygen), and short term acclimatization to hypoxia at high
      altitude.
    

Detailed description: 
      About 200 million people worldwide live at high altitudes. Millions of others travel to high
      altitude every year for work or travel. The low oxygen (or hypoxia) experienced at high
      altitudes causes considerable physiological stress, and the body adapts in order to maintain
      adequate oxygen delivery to its tissues. One of these adaptations is an increase in the
      activity of the sympathetic nervous system. This system is responsible for the "fight or
      flight" response. Sympathetic activity regulates the size of the blood vessels, which will
      affect blood pressure. Previous studies suggest that males and females regulate their blood
      pressure in different ways. Females tend to have lower resting sympathetic activity, and less
      vasoconstriction in response to stressors than males. Blood pressure changes are also
      influenced by levels of sex hormones; for example, estrogen may cause opposing vasodilation.
      Therefore, it is important to understand the differences in how males and females respond to
      low oxygen.

      The main purpose of this study is to examine the differences between males and females in
      control of blood vessels during exposure to low oxygen. To study this question, we will test
      vasoreactivity to set physiological stressors in both males and females. Participants will be
      recruited at the Canadian sites, and we will test them while they breathe low oxygen for a
      short amount of time at low altitude. We will also perform assessments on the same
      participants during a two week stay at high altitude at White Mountain, CA, which is at an
      altitude of 3,800m. These data will have implications in the basic understanding of
      differences between male and female physiology. Specifically, to males and females living or
      travelling to high altitude.
    

Intervention name(s): Phenylephrine Hydrochloride; Cold Pressor Test; Isocapnic Acute Hypoxia; Hyperoxia
Intervention type(s): Drug; Other; Other; Other
Intervention description(s): Series of three incremental bolus injections to observe α1-adrenoreceptor mediated vasoconstriction.; Standardized sympathetic stressor involving submersion of the hand in ice-cold water for 3-minutes, aiming to elicit endogenous neurotransmitter release and blood pressure increases.; During low altitude assessment, participants will be exposed to isocapnic hypoxia equivalent to 3,800m to assess responses to acute exposure.; During high altitude assessments, participants will breathe an oxygen concentration comparable to their end-tidal values at low altitude for the assessment of the influences of acclimatization.
Other intervention name(s): Neo-Synephrine
Intervention MeSH terms: Phenylephrine; Oxymetazoline; Synephrine
Allocation: Non-Randomized
Intervention model: Crossover Assignment
Intervention model description: Participants will partake in all three arms of the study, which include: 1) low altitude, 2) early acclimatization, and 3) late acclimatization to high altitude assessments.
Masking: None (Open Label)
Primary purpose: Basic Science