Arterial baroreflex regulation of muscle sympathetic nerve activity during hypoxia and hypercapnia in healthy young men and women

Abstract

Background In hypertension augmented vasoconstrictor sympathetic nerve activity (SNA) responses to both hypoxia and hypercapnia have been identified. The arterial baroreflex (ABR) normally restrains SNA, but whether ABR-SNA sensitivity is modified by peripheral, central or combined chemoreflex stimulation (i.e. hypoxia, hypercapnia and hypercapnic hypoxia) in young men and women is incompletely understood. We tested the hypothesis that ABR-SNA sensitivity is attenuated by hypoxia, hypercapnia and combined hypercapnic hypoxia thereby allowing SNA to rise. The existence of sex-differences in ABR-SNA sensitivity was also examined. Methods Ten (5 males) healthy individuals (aged 27±4 yr and BMI 24.2±2.7 kg/m2) volunteered to participate in the study. Muscle SNA (microneurography), ventilation and blood pressure were measured during randomized 5-min breathing trials; room-air, isocapnic hypoxia [10% inspired oxygen (O2)], hypercapnic hyperoxia [7% inspired carbon dioxide (CO2), 50%O2] and hypercapnic hypoxia (7%CO2, 10%O2). ABR-SNA sensitivity was determined as the slope of the linear relationship between SNA burst incidence (i.e., bursts/100 heart beats) and diastolic blood pressure. Results Compared to room-air, isocapnic hypoxia, hypercapnic hyperoxia and hypercapnic hypoxia elevated both muscle SNA (16±5, 20±6, 27±7 and 31±8 bursts/100 heart beats, respectively, p<0.001) and ventilation (7.3±2.5, 9.9±3.6, 21.4±4.3 and 31.1±5.5 L/min, respectively, p<0.001). No between-trial differences were observed in mean blood pressure (p=0.32). ABR-SNA slope was not different during room air, isocapnic hypoxia, hypercapnic hyperoxia and hypercapnic hypoxia (-3.76±1.12, -2.95±1.03, -3.97±2.08 and -2.53±2.24 bursts/100 heart beats/mmHg, respectively, p=0.67). No sex-differences in muscle SNA, ventilation, mean blood pressure and ABR-SNA slope were observed. Discussion These preliminary findings suggest that sympatho-excitation evoked by hypoxia, hypercapnia and hypercapnic hypoxia are similar between men and women and are not attributable to a concurrent reduction in ABR-SNA sensitivity. Thus, elevated SNA evoked by peripheral and central chemoreflexes reflects a primary reflex response of chemoreceptor origin.