Spontaneous cardiovagal baroreflex sensitivity is unaffected by an acute bout of prolonged sitting: no impact of sex, menstrual phase, or oral contraceptive pill phase

Abstract

Effective short-term regulation of arterial blood pressure is an important component of cardiovascular health and is associated with reduced disease risk [1]. Spontaneous fluctuations in systolic blood pressure (SBP) are detected by arterial baroreceptors located in the walls of the aortic arch and carotid sinuses, with increased aortic stiffening associated with reduced cardiovagal baroreflex sensitivity (cvBRS) [2]. Importantly, a single 3-h bout of uninterrupted sitting was found to increase aortic stiffness [3, 4], which may be more pronounced in male [3]. In contrast, Maasakkers et al. [5] observed no change in cvBRS, assessed via a 5-min repeated sit-to-stand protocol (10 s sitting:10 s standing), following 3 h of sitting. Based on these conflicting results, several uncertainties remain regarding the impact of prolonged sitting on vagally mediated blood pressure regulation. Spontaneous cvBRS during shorter durations of sitting (~ 5 min) was shown to be similar between young male and female during the early phase of their oral contraceptive pill (OCP) or natural menstrual (NAT) cycles [6, 7]. Furthermore, cvBRS did not differ among female across the early and late phases within the OCP or NAT groups [6]. Thus, sex, menstrual cycles, and OCP cycles appear to have minimal impact on spontaneous cvBRS during brief periods of sitting, but the influence of prolonged bouts of sitting on cvBRS is unknown. The purpose of this study was to determine the impact that an acute, prolonged bout of uninterrupted sitting had on vagally mediated blood pressure regulation in young male and female during the early and late phases of either OCP or NAT cycles. We hypothesized that cvBRS would be more attenuated in male following 3 h of sitting and aimed to explore the potential influences that NAT and OCP cycles had on cvBRS responses to an acute bout of prolonged sitting. Twenty-seven young, healthy adults (10 males, 9 OCP, 8 NAT) were recruited for the present study (Supplemental Table 1). The OCP group consisted of female using combined, monophasic OCPs. The NAT group consisted of female not using any method of hormonal contraceptive within 12 months of recruitment. The experimental design, prolonged sitting protocol, and the participants’ habitual activity, popliteal endothelial-dependent function, and systemic hemodynamic data have been reported in detail previously [8]. NAT female were assessed during days 1–5 (earlier phase; 3.1 ± 0.5 days) and days 12–14 (later phase; 13.1 ± 0.3 days) post-menstruation, which generally correspond to the earlyand late-follicular phases of the menstrual cycle, respectively. OCP female were assessed during the inactive/placebo pill phase (lower exogenous estrogen; 2.9 ± 0.7 days) and again 12–14 days following withdrawal bleeding (higher exogenous estrogen; 12.8 ± 0.4 days) to align with the testing periods of the NAT participants. All protocols and procedures conformed to the Declaration of Helsinki and were approved by the Dalhousie University Health Sciences Research Ethics Board. Spontaneous cvBRS was determined using at least 5 min of SBP and cardiac interval data at the start and end of the sitting bout to calculate cvBRS. Analyzing cvBRS for each hour (baseline, 1 h, 2 h, 3 h) did not alter any of our results (data not shown). All cvBRS sequences were analyzed using CardioSeries software (version 2.4, Brazil), as described in greater detail elsewhere [9, 10]. Our results were unchanged whether cvBRS for up or down sequences was used (data not shown). The baroreflex effectiveness index (BEI) was calculated as the total number of SBP Myles W. O’Brien and Amera Al-Hinnawi have contributed equally.