Sex Differences in Sympathetic Vascular Reactivity at High Altitude

A bolus injection of sodium nitroprusside will be given to lower blood pressure and evaluate baroreflex function.

A bolus injection of phenylephrine will be given to raise blood pressure and evaluate baroreflex function immediately following the sodium nitroprusside infusion.

Investigators will give three incremental doses via brachial artery catheter to observe α1-adrenoreceptor mediated vasoconstriction. This will occur three times throughout the protocol.

Investigators will give three incremental doses via brachial artery catheter to observe adrenoreceptor activation. This will occur three times throughout the protocol.

Participants will perform 2 minutes of isometric handgrip at 30% maximal voluntary contraction followed by 2 minutes of post-exercise circulatory occlusion to assess the reactivity of the blood vessels to exercise and the metaboreflex.

Participants will perform 3 minutes of rhythmic handgrip at 25% maximal voluntary contraction to assess the reactivity of blood vessels to dynamic exercise.

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.

Local infusion via brachial artery catheterization will occur continuously for the remainder of the study. Propranolol is a beta-adrenergic antagonist and will allow investigation of the role of beta receptors in responses to various stressors. The above phenylephrine, norepinephrine, isometric handgrip, post-exercise circulatory occlusion, and rhythmic handgrip protocols will be repeated under propranolol infusion.

Local infusion via brachial artery catheterization will occur continuously for the remainder of the study. Phentolamine is an alpha-adrenergic antagonist and will allow investigation the role of alpha receptors in responses to various stressors. It will be continuously infused with propranolol. The above phenylephrine, norepinephrine, isometric handgrip, post-exercise circulatory occlusion, and rhythmic handgrip protocols will be repeated under propranolol infusion.