The Vascular Response to Sympathetic Activation is Attenuated in Young Women Following Acute Intermittent Hypoxia

Abstract

INTRODUCTION : Acute intermittent hypoxia (AIH) elicits an increase in sympathetic nervous activity (SNA) that persists following return to room air. This AIH-mediated sympathetic activation is associated with an increase in blood pressure in men, but not women. These data suggest the vascular responsiveness to SNA is attenuated in young women, although mechanisms are unclear. We sought to examine the vascular responsiveness to sympathetic activation before and after AIH in heathy young men and women. We hypothesized the vascular response to sympathetic activation would be preserved following AIH in men. We further hypothesized the vascular response to sympathetic activation would be attenuated in women following AIH, with the greatest effect observed when endogenous estradiol was high. METHODS : Twelve men (25±1 yrs) and nine women (24±1 yrs) participated. Women were studied twice, during the early follicular (EF, Estradiol 28±4 pg/mL) and late follicular (LF, Estradiol 185±51 pg/mL) phases of the menstrual cycle. Before and after 30-min AIH, a 2-min cold pressor test (CPT) was conducted to increase SNA. Forearm blood flow (FBF, venous occlusion plethysmography) and blood pressure (finger photoplethysmography) were measured continuously. FBF was normalized for mean blood pressure (forearm vascular conductance, FVC) and a change in response to CPT was calculated (ΔFVC = Baseline - CPT). RESULTS : Mean blood pressure increased following AIH in men (p=0.03) with no effect in women (p>0.05). In men, the vascular response to sympathetic activation was preserved following AIH in men (ΔFVC -0.5±0.4 vs -0.4±0.6 mL/min/dL/100 mmHg, p=0.85). In contrast, the vascular response to sympathetic activation was attenuated in women following AIH (ΔFVC: EF -1.0±0.3 vs -0.4±0.2 mL/min/dL/100 mmHg; LF -0.8±0.4 vs -0.1±0.2 mL/min/dL/100 mmHg, p<0.01) with no effect of menstrual cycle phase (p=0.99). CONCLUSION : The vascular response to sympathetic activation is preserved in men following AIH, but is attenuated in women and is independent of menstrual cycle phase. Present data highlight sex-related differences in the neurovascular response to AIH and indicate sex differences in the blood pressure response to AIH may be due, in part, to an attenuated vascular response to SNA in women.