Hormone phase influences sympathetic responses to high levels of lower body negative pressure in young healthy women

Abstract

We tested the hypothesis that sympathetic responses to baroreceptor unloading may be affected by circulating sex hormones. During lower body negative pressure at −30, −60, and −80 mmHg, muscle sympathetic nerve activity (MSNA), heart rate, and blood pressure were recorded in women who were taking (n = 8) or not taking (n = 9) hormonal contraceptives. All women were tested twice, once during the low-hormone phase (i.e., the early follicular phase of the menstrual cycle and the placebo phase of hormonal contraceptive use), and again during the high-hormone phase (i.e., the midluteal phase of the menstrual cycle and active phase of contraceptive use). During baroreceptor unloading, the reductions in stroke volume and resultant increases in MSNA and total peripheral resistance were greater in high-hormone than low-hormone phases in both groups. When normalized to the fall in stroke volume, increases in MSNA were no longer different between hormone phases. While stroke volume and sympathetic responses were similar between women taking and not taking hormonal contraceptives, mean arterial pressure was maintained during baroreceptor unloading in women not taking hormonal contraceptives but not in women using hormonal contraceptives. These data suggest that differences in sympathetic activation between hormone phases, as elicited by lower body negative pressure, are the result of hormonally mediated changes in the hemodynamic consequences of negative pressure, rather than centrally driven alterations to sympathetic regulation.

the incidence of hypertension is lower (13, 25), and orthostatic intolerance is greater (8, 22, 35), in young premenopausal women compared with similarly aged men. These data, as well as several cross-sectional studies (e.g., 24, 41, 51, 53), imply that circulating sex hormones influence cardiovascular regulation. These data also have led to interest in the effects of sex and sex hormones on the acute regulation of blood pressure.

The tolerance of orthostatic stress (i.e., the prevention of syncope) is contingent upon adequate neurovascular responses to counteract the peripheral pooling of blood. Insufficient increases in muscle sympathetic nerve activity (MSNA) that fail to compensate for reductions in cardiac output have been implicated in the development of presyncope (52). However, the evidence regarding the impact of one’s sex on MSNA responses to orthostasis remains equivocal. Although some studies have determined that MSNA responses are blunted in women relative to men (26, 43, 55), others have reported that MSNA responses are not different between the sexes (14, 15).

Differences in the concentrations of circulating sex hormones in the female participants of the aforementioned studies may contribute to this discrepancy. For example, recent evidence suggests that concentrations of circulating sex hormones exert an influence over the regulation of baseline MSNA (3, 10). Cyclical changes in sex hormones occur across the regular menstrual cycle, and several studies have provided support for a baseline sympathoexcitation during the midluteal (ML; i.e., high estrogen and progesterone) phase of the menstrual cycle relative to the early follicular phase (EF; lower estrogen and progesterone) (3, 32, 33, 38, 47), although not all studies agree (4, 6, 14, 23). Two studies have indicated that sympathetic responses to the orthostasis techniques of head-up tilt (HUT) (14) and lower body negative pressure (LBNP) (6) are elevated in the ML phase relative to EF. Interestingly, these studies report that differences in the MSNA responses between EF and ML appear to be greatest during the most severe stages of baroreceptor unloading (6, 14) and occur in conjunction with a lack of change in sympathetic baroreflex gain across the menstrual cycle (6, 14). This observation implicates a greater orthostatic stress, which occurs in response to the same LBNP or HUT stimulus in the ML phase relative to the EF phase. However, although this change in baroreceptor unloading has been hypothesized to occur (14), menstrual cycle differences in the orthostasis-induced reduction in stroke volume have not yet been observed.

In comparison with studies of the menstrual cycle in eumenorrheic women, less is known regarding sympathetic regulation in women taking hormonal contraceptives (HC), despite at least 20% of women of child-bearing age currently using HC in the United States (37). Moreover, HC use has been associated with small yet significant increases in blood pressures relative to control participants (1, 19, 21), suggesting an effect of HC use on blood pressure regulation. Indeed, phases of HC use have largely been shown not to mirror the menstrual cycle in their effects on sympathetic regulation (5, 32, 34). For instance, placebo and active phases of HC have not been shown to affect baseline MSNA (5, 32, 34), with the exception of a recent study in which we observed a significant increase in baseline MSNA in the active phase of HC use relative to the placebo phase (48). Also, during LBNP, placebo and active HC phases have been associated with similar MSNA responses (5), in contrast to observations across the menstrual cycle in nonusers of HCs (6, 14). However, to the best of our knowledge, no studies have compared the effects of hormone phases on sympathetic responses to simulated orthostasis between users and nonusers of HC.

This study tested the hypothesis that changes in hormone phase, as well as hormone source (i.e., exogenous vs. endogenous), would affect sympathetic responses to moderate and severe simulated orthostatic stress. To enhance applicability of the study in young women, we examined females who were, or were not, using HC. In addition, and to assess main effects of changes in hormone levels, data from women using HC and the women not using HC were pooled. In line with previous studies (5, 32, 34, 48), we took the placebo phase of HC use to represent the low-hormone phase of circulating hormone levels, and the active HC phase to represent the high-hormone phase.

MATERIALS AND METHODS

Participants.

Seventeen women enrolled in the study. Participants were recruited in two groups 1): regular users and 2) nonusers of HC. Participant characteristics are presented in Table 1. The protocols reported here were conducted as part of a larger study, and therefore, baseline data from some of these participants have been reported elsewhere (47, 48). All participants were physically active nonsmokers, free of cardiovascular and respiratory disease, who were not taking any medications (with the exception of users of HC). Of the women using HC, one reported using a patch with the progestin norelgestromin, one was using a triphasic pill containing norgestimate, and the remainder were using monophasic combination pills with either drospirenone (n = 2), desogestrel (n = 1), or levonorgestrel (n = 3) for a minimum of 6 mo. All HC regimes consisted of a placebo or abstinence phase (including removal of the transdermal patch) which was associated with menses, as well as an active phase. In all HCs, the active phase daily doses consisted of 20–30 µg of ethinyl estradiol. All nonusers of HC reported regular menstrual cycles (approximately every 28 days). Participants provided signed consent to the study protocols, which were approved by the Health Sciences Research Ethics Board at The University of Western Ontario, Canada, and conformed to the standards set by the Declaration of Helsinki.

Table 1.

Participant characteristics

	HC	No HC
n	8	9
Age, yr	24 ± 3	24 ± 3
Height, cm	167 ± 3	166 ± 6
Weight, kg	60 ± 4	64 ± 9
BMI, kg/m−2	22 ± 2	23 ± 3

Values are expressed as means ± SD. HC, hormonal contraceptives; BMI, body mass index.

Experimental design.

Each participant visited the laboratory on three occasions. The first was a familiarization visit, during which the participants practiced the LBNP protocol and experienced all of the noninvasive aspects of data acquisition. The order of subsequent hormone phase testing was counterbalanced among the participants. In all participants, tests were scheduled relative to the onset of menses (day 1); placebo (HC users) and EF testing took place between days 1 and 4, and active HC phase and ML testing took place between days 20 and 24. Time of day was kept constant within each participant across test dates. Participants arrived for testing at least 3 h postprandial and had abstained from exercise, caffeine, and alcohol for a minimum of 12 h. In nonusers of HC, an intravenous blood draw was taken from the antecubital vein to assess hormone levels to confirm the target menstrual cycle phases. Indeed, significant increases in 17-β-estradiol and progesterone were observed from EF (151 ± 50 pmol/l and 1.2 ± 0.5 nmol/l, respectively) to ML (638 ± 175 pmol/l and 35.8 ± 9.3 nmol/l).

Participants were positioned supine with their legs and hips sealed in a LBNP chamber. Participants were situated within the box such that the leg not being used for microneurography was used to stabilize the body during LBNP. The LBNP chamber was connected to a vacuum fed through a variable transformer (Staco Energy Products, Dayton, OH), which allowed precise control of suction inside the chamber. Participants were exposed to LBNP at −30, −60, and −80 mmHg. The order of LBNP testing was quasi-random. Specifically, the order of −30 mmHg vs. the severe levels of suction was randomized. However, within the severe levels (−60 and −80 mmHg), −60 mmHg always preceded −80 mmHg to first ensure that all participants could tolerate −60 mmHg. Following recovery from the preceding LBNP stimulus, 5 min of baseline were recorded before each LBNP level to control for any change in the sympathetic or hemodynamic signals that may have occurred over the course of the test session. All LBNP levels were maintained for 3 min, and they were tolerated in all participants except one woman not taking HC, who reported mild nausea following LBNP −60 mmHg; LBNP −80 mmHg was not performed in this participant.

Measures.

Heart rate (HR) was measured through a three-lead electrocardiogram. Blood pressure waveforms were obtained through finger photoplethysmography (Finometer; Finapres Medical Systems, Amsterdam, The Netherlands) and were calibrated to a resting blood pressure, which was the average of three values obtained through manual sphygmomanometry. Cardiac output (Q) was calculated online using the Modelflow algorithm (Finometer), which has been validated for use in orthostatic tests (17). Muscle sympathetic nerve activity was assessed using microneurography at the peroneal nerve (49). Briefly, a tungsten recording electrode with an uninsulated tip was inserted transcutaneously, and an additional electrode was inserted subcutaneously as a reference. Sympathetic recording sites were associated with pulse synchronous bursts of activity, which increased in frequency during end-expiratory apnea and were unaffected by arousal to a loud noise (11). The MSNA signal was amplified (75,000×), band-pass filtered (700–2,000 Hz), rectified, and integrated (0.1 s time constant; model 662C-3; Iowa University, College of Bioengineering, Iowa City, Iowa). The MSNA signal was sampled at 10,000 Hz by an online data acquisition and analysis package (PowerLab /16SP with LabChart 7, ADInstruments, Colorado Springs, CO). Hemodynamic measures were sampled at 1,000 Hz.

Data analysis.

Mean arterial pressure (MAP) was calculated as the mean value of the calibrated brachial blood pressure waveform. Stroke volume (SV) was calculated as Q divided by HR. Total peripheral resistance (TPR) was calculated as MAP divided by Q. MSNA burst amplitudes were normalized within each LBNP condition and associated baseline period by expressing all bursts relative to the largest burst in the preceding baseline period, which was assigned a value of 100. Total MSNA was then calculated as burst frequency (bursts/min) multiplied by mean normalized burst amplitude.

Baseline data were compared using a two-way split-plot ANOVA, which assessed the main effects of HC use (HC vs. no HC) and hormone phase (i.e., low hormones, i.e., placebo phase of HC & EF vs. high hormones, i.e., active phase of HC & ML), as well as possible HC status × hormone phase interactions (Statistical Analysis System, version 9.1.3, SAS Institute, Cary, NC). Similarly, responses to LBNP were assessed using a three-way split-plot ANOVA, which assessed the main effects of HC status, hormone phase, and level of LBNP (−30, −60, and −80 mmHg LBNP). Because of the loss of the MSNA signal or lack of tolerance for LBNP, analyses on LBNP outcomes were performed in seven women taking HC and seven women not taking HC. The Tukey-Kramer correction was applied in all post hoc comparisons. Alpha was set at 0.05.

RESULTS

Baseline.

Hormone phase affected baseline sympathetic activity, such that MSNA was greater in the active and ML phases relative to the placebo and EF phases (P < 0.01; Table 2). Hemodynamic variables (MAP, Q, SV, and TPR) were similar between low- and high-hormone phases, with the exception of HR, which was slightly, yet significantly, elevated in the ML and active phases vs. the EF and placebo phases (P = 0.03). No effect of HC use was observed at baseline, nor were any hormone phase × HC use interactions observed, indicating that the effect of hormone phase on baseline hemodynamics and sympathetic nerve activity did not differ between users and nonusers of HC.

Table 2.

Baseline hemodynamics and muscle sympathetic nerve activity

	HC		No HC		Hormone Phase	HC Use	Phase × HC
	Placebo	Active	EF	ML
MAP, mmHg	92 ± 4	96 ± 9	86 ± 6	84 ± 6	0.5	0.4	0.1
SBP, mmHg	119 ± 10	123 ± 14	114 ± 13	112 ± 11	0.7	0.5	0.2
DBP, mmHg	68 ± 4	70 ± 8	67 ± 9	67 ± 4	0.7	0.2	0.8
PP, mmHg	51 ± 7	53 ± 10	48 ± 18	44 ± 10	0.7	0.6	0.2
HR, Beats/min	64 ± 8	68 ± 13	61 ± 7	64 ± 7	0.03	0.6	0.4
Q, l/min	5.1 ± 0.7	5.6 ± 1.0	4.9 ± 1.0	5.1 ± 1.0	0.2	0.7	0.2
SV, ml	81 ± 13	83 ± 10	80 ± 10	80 ± 14	0.7	0.9	0.3
TPR, mmHg·l−1·min−1	20 ± 4	18 ± 2	18 ± 3	17 ± 5	0.7	0.4	0.9
MSNA burst frequency, bursts/min	11 ± 7	16 ± 8	10 ± 5	14 ± 7	<0.01	0.6	0.6
MSNA burst incidence, bursts/100 heartbeats	16 ± 8	23 ± 9	16 ± 8	21 ± 8	<0.01	0.5	0.8

Data are expressed as means ± SD in women taking hormonal contraceptives (HC; n = 8) during placebo and active phases and in women not taking exogenous hormones (n = 9) during early follicular (EF) and midluteal (ML) phases. Rightmost columns are split-plot ANOVA P values for the main effects of hormone phases (EF/placebo vs. ML/active), groups of women (i.e., women using HC vs. not using HC), and interactions between these effects (Phase × HC). MAP, mean arterial pressure; SBP, systolic blood pressure; DBP, diastolic blood pressure; PP, pulse pressure; HR, heart rate; Q, cardiac output; SV, stroke volume; TPR, total peripheral resistance; MSNA, muscle sympathetic nerve activity.

Sympathetic Responses to LBNP.

LBNP elicited graded elevations in MSNA burst amplitude, burst frequency, and total MSNA (Fig. 1). The relative increases in sympathetic burst frequency and total MSNA were larger in the high-hormone phases across all levels of LBNP. However, increases in burst amplitude were similar between hormone phases. These patterns of sympathetic activation were not different between users and nonusers of HC. Further, we observed no statistical interactions between HC status, hormone phase, or level of LBNP on any of the sympathoexcitatory responses.

Fig. 1.

Muscle sympathetic nerve activity (MSNA) responses to lower body negative pressure (LBNP). A main effect of level of LBNP was observed across all measures of MSNA. Total MSNA and burst frequency responses were greater in the high-hormone phases relative to the low-hormone phases; changes in burst amplitude were not affected by hormone phase. Not pictured are data specific to hormonal contraceptives (HC) and non-HC users as MSNA responses were not significantly different between groups. Interactions between factors were as follows. HC Status × Phase: 0.97 (total MSNA), 0.95 (burst frequency), 0.32 (burst amplitude); HC Status × LBNP: 0.90 (total), 0.81 (frequency), 0.42 (amplitude); Phase × LBNP: 0.59 (total), 0.87 (frequency), 0.48 (amplitude); and HC Status × Phase × LBNP: 0.35 (total), 0.22 (frequency), and 0.66 (amplitude).

Increasing levels of LBNP produced corresponding increases in HR and reductions in SV; these changes were similar between HC users and nonusers (Fig. 2). Regardless of HC use, hormone phase affected LBNP-induced reductions in Q and SV, which were greater in the high-hormone phases than in the low-hormone phases. Compensatory increases in TPR were greater in the high-hormone phases relative to the low-hormone phases, regardless of HC use. While the resultant changes in MAP were not affected by hormone phase, a LBNP × HC status interaction indicated that −80 mmHg LBNP elicited a reduction in MAP only in the users of HC (Fig. 3). No change in MAP was observed across levels of LBNP in women who were not using HC.

Fig. 2.

Hemodynamic responses to LBNP. Graded increases in heart rate (HR) and reductions in stroke volume (SV) were observed across levels of LBNP. Cardiac output (Q) and SV were relatively lower in the high-hormone phases with respect to the low-hormone phases. Total peripheral resistance (TPR) was relatively higher in the high-hormone phases. Not pictured are responses specific to HC users and nonusers because responses were similar between groups. Interactions between factors were as follows. HC Status × Phase: 0.70 (Q), 0.62 (HR), 0.43 (SV), 0.79 (TPR); HC Status × LBNP: 0.55 (Q), 0.62 (HR), 0.99 (SV), 0.71 (TPR); Phase × LBNP: 0.80 (Q), 0.44 (HR), 0.96 (SV), 0.85 (TPR); HC Status × Phase × LBNP: 0.62 (Q), 0.73 (HR), 0.33 (SV), and 0.91 (TPR).

Fig. 3.

Mean arterial pressure (MAP) responses to LBNP. MAP remained unchanged across levels of LBNP in nonusers of HC. Conversely, in users of HC, −80 mmHg LBNP was associated with a reduction in MAP (*P < 0.05 vs. −30 LBNP). Not pictured are responses specific to low- and high-hormone phases because responses were similar between conditions. Additional interactions between factors were as follows: HC Status × Phase: 0.77; Phase × LBNP: 0.68; HC Status × Phase × LBNP: 0.58.

We performed post hoc regression analyses to determine whether larger reductions in SV during LBNP contributed to the exaggerated MSNA responses observed in the high-hormone phases (Fig. 4). Significant, negative relationships were observed between Δ-SV and Δ-total MSNA during both low-hormone (R = −0.7, P < 0.001) and high-hormone phases (R = −0.6, P < 0.001). To compare these slopes between groups, we calculated the Δ-SV and Δ-total MSNA slope in each individual during both the low- and high-hormone phases, and then compared these slopes using a paired t-test. These data indicated that the slopes were not different between hormone phases (P = 0.2).

Fig. 4.

Stroke volume (SV)- MSNA associations during LBNP. The change in SV during LBNP was negatively associated with the change in total MSNA in both low-hormone and high-hormone conditions. The mean slopes of the SV-MSNA relationships were similar between hormone phases (−75 ± 55 AU/ml vs. −110 ± 58 AU/ml, low hormones vs. high hormones; P = 0.2).

DISCUSSION

This study compared sympathetic responses to LBNP during low- and high-hormone phases of the endogenous menstrual cycle and exogenous hormonal contraceptive use. We observed that sympathetic responses to LBNP were not different between regular users and nonusers of HC. However, in all women, sympathetic responses to moderate to high levels of LBNP were elevated in the higher hormone phases relative to the lower hormone phases. Yet, the magnitude of these sympatho-excitatory responses to LBNP was associated with the magnitude of the falls in SV. Therefore, differences in the sympathetic responses in low- vs. high-hormone phases were driven by reflex stimuli related to blood pressure control, rather than a central means of amplification. During the most severe level of LBNP, a significant fall in MAP was observed in the users of HC, which was not observed in nonusers of HC, suggesting a relative reduction in orthostatic tolerance in this group.

In the present study, acute and severe LBNP (−60 and −80 mmHg) was used to evoke large baroreceptor-driven sympathetic responses. Previous studies examining the sympathetic responses to baroreceptor unloading across the menstrual cycle in nonusers of HC used mild to moderate LBNP (−5 to −40 mmHg) (6) and graded upright head-up tilt maintained at 60° to presyncope (14). Both studies observed greater increases in total MSNA in the ML phase than the EF phase. However, the same outcome has not been observed in women taking HC. Specifically, graded LBNP up to −40 mmHg resulted in similar MSNA responses between placebo and active phases of HC use (5). However, in contrast to the present study, the hemodynamic responses to baroreceptor unloading were similar between hormone phases (5, 6, 14). This discrepancy in hemodynamic outcomes may be due to differences in the nature of the orthostatic stimulus. The previous studies made use of either graded LBNP (5, 6), involving levels of suction, which increased in magnitude every 3 min, or head-up tilt for 45 min (or until presyncope) (14). In contrast to these designs, which arguably elicit a gradual and incrementing orthostatic stress, our design involved relatively short and sudden bouts of suction. The duration, time course, and rapidity of the orthostatic stimulus may affect the hemodynamic and sympathetic responses.

During moderate to high levels of LBNP, we observed that both the declines in SV and the increases in MSNA were greater in the high-hormone phases (ML and active HC phase) relative to the low-hormone phases (EF and placebo phase). Subsequent analyses indicated that the increases in total MSNA were related linearly to the magnitude of the reductions in SV in both low- and high-hormone phases. These data are supported by previous studies that have observed inverse relationships between SV and MSNA. Observed under baseline conditions (7, 18), this association appears to express a sex-specific effect because it was not observed in young healthy women (18). On the other hand, strong inverse relationships between SV and MSNA have been observed in both men and women during orthostatic stress (15, 27). Our data reinforce the finding that SV is a primary determinant of reflex increases in MSNA during acute orthostasis. These results also indicate further that SV is a determinant of sex hormone effects on autonomic adjustments to high levels of LBNP.

Importantly, we observed that the mean slopes of these relationships between SV and MSNA were not different between hormone phases, indicating similar baroreceptor-driven control of MSNA across hormone phases. In other words, these data indicate that the exaggerated sympatho-excitation associated with the high-hormone phases was due to greater unloading of the baroreceptors rather than a centrally driven amplification of the efferent sympathetic response. In line with our findings, previous studies examining spontaneous baroreflex sensitivity across the menstrual cycle (6, 14) and across phases of HC use (5) have revealed no differences between hormone phases. Studies using an open-loop assessment of baroreflex sensitivity (the modified Oxford method) have generated mixed results. One study observed that sympathetic baroreflex sensitivity was similar between low- and high-hormone phases of both the menstrual cycle and phases of HC use (32), while other investigations have reported greater sympathetic baroreflex sensitivity in the ML phase of the menstrual cycle relative to the EF phase (33), and increased sympathetic baroreflex sensitivity in the placebo phase of HC use relative to the active phase (34). Thus, further research is required to determine whether baroreflex sensitivity is affected by circulating hormone levels in both women who use and those who do not use HC.

The mechanisms contributing to the larger reductions in SV during high-hormone phases were not studied in the current investigation. However, changes in cardiac function, preload, or afterload may each be affected by sex hormones. Given the differential regulation of sympathetic outflow between cardiac and peripheral vascular targets (39, 40), it is not possible speculate specifically on sympathetic contributions to cardiac contractility using the current data, although others have suggested this possibility. We speculate that decreased preload due to impaired venous return may contribute to the reduced SV during the high-hormone phases, observed here, due to the effect of estrogen on venous compliance and capacitance. This idea has some experimental support. For example, a positive association between circulating estrogen and venous capacitance was observed in normal and ovariectomized rats (50) and pigs (2). Also, progestin components within hormonal contraceptives appear to exert differing effects on venous capacitance (12), suggesting a possible mechanism by which the women using HC may have experienced larger reductions in MAP at the highest levels of LBNP. However, our experimental approach involved the pooling of data between various progestin types, limiting our ability to speculate on this possible mechanism. Also, others report that venous compliance does not change across the menstrual cycle or phases of HC use (29). Arterial compliance is associated with elevations in circulating estrogen across the menstrual cycle (28), perhaps through variations in the estrogen-to-progesterone ratio (20), and according to menopausal status (16, 36). The influence of progestin, and, indeed, HC type, on arterial and venous capacitance during orthostatic challenges warrants further investigation.

In the present study, statistical differences were observed in MAP control between the users and nonusers of HC during LBNP. In women not taking exogenous hormones, we observed a maintenance of MAP across all levels of LBNP. Despite similar TPR responses between the two groups of women, MAP was not maintained during −80 mmHg LBNP in women who were taking hormonal contraceptives at −80 mmHg LBNP. These data are suggestive of an inadequate neurovascular response to orthostatic challenge in women taking HC. Certainly, the vasodilatory effects of estrogen (9, 54), acting through the endothelial nitric oxide synthase pathway (42, 45), may impair vascular reactivity to sympathetic drive during LBNP. The effects of synthetic estrogens, such as those in HCs, may be stronger than those exerted by endogenous estradiol (44), indicating a potential mechanism by which vascular reactivity may be more impaired in women using HCs relative to women in the ML phase of the menstrual cycle. The present data illustrate a potential deleterious impact of HC on blood pressure control, but an effect that may be observed only at high levels of stress.

Limitations.

The type of hormonal contraceptive was not controlled in this study, and although all participants were using combination formulations (i.e., estradiol and progestin), the type of progestin varied among the participants. Given the varying effects that different progestins exert over endothelial function (30, 31, 46), it is unclear to what extent the hemodynamic measures in this study were affected by the range of HC types. Also, this experimental design compared phases in which progesterone or progestins were elevated at the same time as estrogen or ethinyl estradiol. These hormones may exert opposing influences over sympathetic regulation (3), and in the present design, we could not tease apart these separate and perhaps competing influences.

In this study, we relied on the Modelflow algorithm in the Finometer to obtain measures of cardiac output and, therefore, stroke volume. Because of concerns regarding the validity of the absolute values derived by this method, we have refrained from drawing conclusions regarding these values. For all comparisons of orthostatic responses within and between groups, we relied instead on relative values (i.e., responses expressed relative to baseline) of cardiac output and stroke volume.

Perspectives and Significance

These data indicate that neural responses to the vascular consequences of acute orthostasis are graded to the magnitude of the fall in stroke volume, suggesting that the central integration of sympathetic nerve activity is not affected by acute changes in circulating sex hormones. Patterns of hemodynamic and sympathetic responses to graded LBNP are affected by hormone phases, and these patterns are similar between users and nonusers of hormonal contraceptives. The one caveat to these similarities is a fall in mean arterial pressure during the most severe stage of negative pressure in the users of hormonal contraceptives, a response that was not observed in the women who did not use hormonal contraceptives, indicating that orthostatic tolerance may be compromised in women who use hormonal contraceptives.

GRANTS

The authors acknowledge grant support from the Natural Sciences and Engineering Research Council (NSERC) of Canada (K. Shoemaker, principal investigator, Grant 217916–2013). C. W. Usselman was funded by the Ontario Graduate Scholarship Program and the Canadian Institutes of Health Research (CIHR) Frederick Banting and Charles Best Canada Graduate Scholarship (CGS). T. I. Gimon and T. A. Luchyshyn were each funded by the NSERC Undergraduate Student Research Award. C. A. Nielson, T. A. Luchyshyn, and N. S. Coverdale were each funded by CIHR Frederick Banting and Charles Best CGS.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

C.W.U., S.H.M.V.U., and J.K.S. conception and design of research; C.W.U., C.A.N., T.A.L., T.I.G., N.S.C., and J.K.S. performed experiments; C.W.U. and J.K.S. analyzed data; C.W.U., C.A.N., T.A.L., T.I.G., N.S.C., S.H.M.V.U., and J.K.S. interpreted results of experiments; C.W.U. prepared figures; C.W.U. drafted manuscript; C.W.U., C.A.N., T.A.L., T.I.G., N.S.C., S.H.M.V.U., and J.K.S. edited and revised manuscript; C.W.U., C.A.N., T.A.L., T.I.G., N.S.C., S.H.M.V.U., and J.K.S. approved final version of manuscript.