Sex Differences In Carotid Baroreflex Control Of Heart Rate At Rest And During Dynamic Exercise In Humans

Abstract

Previous studies in animals have demonstrated that the arterial baroreflex control of heart rate (HR) was greater in female compared to male rodents. In contrast, equivocal results have been reported in humans with several studies reporting that young women, in fact, exhibit a reduced cardiac baroreflex sensitivity compared to young men. In addition, to date, no studies have examined differences in the baroreflex control of HR during exercise in young men and women. PURPOSE: To determine if the carotid baroreflex control of HR differs between young men and women at rest or during dynamic exercise. METHODS: Beat-to-beat HR and blood pressure were measured in nine young women (22±1 yr) and men (22±1 yr) at rest and during steady-state leg cycling at 50% HR reserve. All women were studied during the early follicular phase of the menstrual cycle (days 2-5). Five second pulses of neck suction (NS, -60 Torr) and neck pressure (NP, + 40 Torr) were applied to selectively load and unload the carotid baroreceptors, respectively. In addition, HR variability analyses were employed to investigate time and frequency domain indices of parasympathetic and sympathetic control of HR. RESULTS: At rest and during exercise, HR responses to NS were significantly greater in women than men (rest, D -16±3 women vs. D -8±1 men bpm, P = 0.002; exercise, D -17±3 women vs. D -6±1 men bpm, P = 0.004), whereas responses to NP were similar between groups (rest, D + 7±1 women vs. D + 5±1 men bpm, P = 0.222; exercise, D + 4±1 women vs. D + 2±1 men bpm, P = 0.096). In addition, there were no differences in HR variability measurements between women and men both at rest and during exercise. CONCLUSION: Carotid baroreflex mediated HR responses to a hypertensive challenge are greater in young women both at rest and during exercise. In contrast, the ability of the carotid-cardiac baroreflex to respond to a hypotensive challenge is similar between young women and men. Overall, these preliminary data suggest that sex differences in cardiac baroreflex responses are selective to hypertensive challenges both at rest and during exercise.