Menstrual cycle elicits divergent forearm vascular responses to vestibular activation in humans

Is the Pannexin-1 Channel a Mechanism Underlying Hypertension in Humans? a Translational Study of Human Hypertension

BACKGROUND

In preclinical models, the pannexin-1 channel has been shown to be involved in blood pressure regulation through an effect on peripheral vascular resistance. Pannexin-1 releases ATP, which can activate constrictive purinergic receptors on the smooth muscle cells. Pannexin-1 opening is proposed to be mediated by α-adrenergic receptors to potentiate sympathetic constriction. This positions pannexin-1 as a putative pharmacological target in blood pressure regulation in humans. The aim was to provide the first translational evidence for a role of pannexin-1 in essential hypertension in humans by use of an advanced invasive mechanistic approach.

METHODS

Middle-aged stage-1 hypertensive (n=13; 135.7±6.4 over 83.7±3.7 mm Hg) and normotensive men (n=12; 117.3±5.7 over 72.2±3.5 mm Hg) were included. Blood pressure and leg vascular resistance were determined during femoral arterial infusion of tyramine (α-adrenergic receptor stimulation), sodium nitroprusside, and acetylcholine. Measurements were made during control conditions and with pannexin-1 blockade (3000 mg probenecid). Expression of purinergic- and α-adrenergic receptors in skeletal muscle biopsies was determined by Western blot.

RESULTS

The changes in leg vascular resistance in response to tyramine (+289% versus +222%), sodium nitroprusside (-82% versus -78%) and acetylcholine (-40% versus -44%) infusion were not different between the 2 groups (P>0.05) and pannexin-1 blockade did not alter these variables (P>0.05). Expression of pannexin-1 and of purinergic- and α-adrenergic receptors was not different between the 2 groups (P>0.05).

CONCLUSIONS

Contrary to our hypothesis, the data demonstrate that pannexin-1 does not contribute to the elevated blood pressure in essential hypertension, a finding, which also opposes that reported in preclinical models.

Assessing static and dynamic sympathetic transduction using microneurography

The relationship between sympathetic nerve activity and the vasculature has been of great interest due to its potential role in various cardiovascular-related diseases. This relationship, termed "sympathetic transduction," has been quantified using several different laboratory and analytical techniques. The most common method is to assess the association between relative changes in muscle sympathetic nerve activity, measured via microneurography, and physiological outcomes (e.g., blood pressure, total peripheral resistance, blood flow, etc.) in response to a sympathetic stressor (e.g., exercise, cold stress, orthostatic stress). This approach, however, comes with its own caveats. For instance, elevations in blood pressure and heart rate during a sympathetic stressor can have an independent impact on muscle sympathetic nerve activity. Another assessment of sympathetic transduction was developed by Wallin and Nerhed in 1982, where alterations in blood pressure and heart rate were assessed immediately following bursts of muscle sympathetic nerve activity at rest. This approach has since been characterized and further innovated by others, including the breakdown of consecutive burst sequences (e.g., singlet, doublet, triplet, and quadruplet), and burst height (quartile analysis) on specific vascular outcomes (e.g., blood pressure, blood flow, vascular resistance). The purpose of this review is to provide an overview of the literature that has assessed sympathetic transduction using microneurography and various sympathetic stressors (static sympathetic transduction) and using the same or similar approach established by Wallin and Nerhed at rest (dynamic neurovascular transduction). Herein, we discuss the overlapping literature between these two methodologies and highlight the key physiological questions that remain.

Influence of sex, menstrual cycle, and oral contraceptives on cerebrovascular resistance and cardiorespiratory function during Valsalva or standing

Women experience orthostatic intolerance more than men, and they experience faintness more in the early follicular [i.e., low-hormone (LH)] than luteal [i.e., high-hormone (HH)] phase of the menstrual cycle. Men (n = 13, 25.8 ± 1.8 yr old) and women in the LH (days 2-5; placebo) and HH (days 18-24; high dose) phases of the menstrual cycle with (OC; n = 14, 22.0 ± 0.8 yr old) or without (NOC; n = 12, 21.8 ± 0.5 yr old) oral contraceptive (OC) use underwent the Valsalva maneuver and a supine-sit-stand protocol. Blood pressure, normalized stroke volume [stroke volume index (SVi)], cardiac output index, heart rate, end-tidal CO₂, and middle cerebral artery (MCA) blood flow velocity were measured. When subjected to the Valsalva maneuver, all women had a greater increase in diastolic and mean MCA blood flow velocity than men (P ≤ 0.065), with no significant effect of menstrual cycle phase or OC use. When subjected to the supine-sit-stand protocol, men had lower MCA blood flow velocity (P < 0.038) than all women, and SVi was higher in men than in the NOC group in all postures (P < 0.011) and in the OC group in the LH phase of the menstrual cycle during standing (P = 0.010). Only men experienced higher resistance index (P < 0.001) and pulsatility index (P < 0.001) with standing. The OC group had lower end-tidal CO₂ (P = 0.002) than the NOC group (P = 0.030) and men (P ≤ 0.067). SVi (P = 0.004) and cardiac output index (P = 0.008) were higher in the OC than NOC group. A tendency toward a lower mean MCA blood flow velocity (P = 0.058) and higher SVi (P = 0.059) and pulsatility index (P = 0.058) was noted in the HH than LH phase. Mean arterial pressure was higher in the OC than NOC group in the LH phase (P = 0.049) and lower in the HH than LH phase (P = 0.014). Our results indicate that cycling estrogens/progestins can influence ventilatory, cardiovascular, and/or cerebrovascular physiology.NEW & NOTEWORTHY We have found sex differences in the cerebrovascular response to the Valsalva maneuver and standing. Men have greater cerebral vasoconstriction (or women have greater cerebral vasodilation) during late phase II of the Valsalva maneuver, and the cerebrovascular resistance index increases in men, but not in women, during standing. Furthermore, our findings indicate that both the menstrual cycle phase and oral contraceptive use can influence cardiovascular function both at rest and during active standing.

Role of the ovarian cycle on neural cardiovascular control in sleep-deprived women

The midluteal (ML) phase of the ovarian cycle is often sympathoexcitatory compared with the early follicular (EF) phase. We recently reported that 24-h total sleep deprivation (TSD) augmented cardiovascular reactivity in both men and women, but that sex differences existed in resting muscle sympathetic nerve activity (MSNA) responses to TSD. In the present study, we hypothesized increased resting MSNA and augmented cardiovascular reactivity to acute laboratory stressors during the ML phase in sleep-deprived women. Heart rate (HR), mean arterial pressure (MAP), forearm vascular conductance (FVC), and MSNA were measured in 14 eumenorrheic women (age, 20 ± 1 yr) during 10 min supine rest, 5 min mental stress (MS) trial, and 2 min cold pressor test (CPT) trial. Subjects were tested twice after TSD: once during EF phase and once during ML phase (randomized, crossover design). Estradiol (29 ± 2 vs. 63 ± 8 pg/ml, P = 0.001) and progesterone (1.6 ± 0.2 vs. 4.4 ± 0.7 ng/ml, P = 0.002) were elevated during the ML phase. Resting supine MAP (75 ± 2 vs. 72 ± 1 mmHg, P = 0.042) was lower during the ML phase. In contrast, resting supine HR, MSNA, and FVC were not significantly different between EF and ML phases. MAP, HR and FVC reactivity to MS were not statistically different between the EF and ML phases. Similarly, MAP and HR reactivity to CPT were not different between the ovarian phases. Contrary to our original hypothesis, the ML phase was not associated with sympathoexcitation or exaggerated cardiovascular reactivity in sleep-deprived premenopausal women. However, our data reveal elevated resting blood pressure during the EF phase in sleep-deprived women.

Vestibulo-sympathetic responses

Evidence accumulated over 30 years, from experiments on animals and human subjects, has conclusively demonstrated that inputs from the vestibular otolith organs contribute to the control of blood pressure during movement and changes in posture. This review considers the effects of gravity on the body axis, and the consequences of postural changes on blood distribution in the body. It then separately considers findings collected in experiments on animals and human subjects demonstrating that the vestibular system regulates blood distribution in the body during movement. Vestibulosympathetic reflexes differ from responses triggered by unloading of cardiovascular receptors such as baroreceptors and cardiopulmonary receptors, as they can be elicited before a change in blood distribution occurs in the body. Dissimilarities in the expression of vestibulosympathetic reflexes in humans and animals are also described. In particular, there is evidence from experiments in animals, but not humans, that vestibulosympathetic reflexes are patterned, and differ between body regions. Results from neurophysiological and neuroanatomical studies in animals are discussed that identify the neurons that mediate vestibulosympathetic responses, which include cells in the caudal aspect of the vestibular nucleus complex, interneurons in the lateral medullary reticular formation, and bulbospinal neurons in the rostral ventrolateral medulla. Recent findings showing that cognition can modify the gain of vestibulosympathetic responses are also presented, and neural pathways that could mediate adaptive plasticity in the responses are proposed, including connections of the posterior cerebellar vermis with the vestibular nuclei and brainstem nuclei that regulate blood pressure.

Ovarian cycle and sympathoexcitation in premenopausal women

The influence of the ovarian cycle on muscle sympathetic nerve activity (MSNA) remains controversial. Some studies report an increase of resting MSNA during the mid luteal (ML) phase of the ovarian cycle compared with the early follicular phase, whereas other studies do not. These inconsistent findings may be attributable, in part, to the variable surges in estradiol and progesterone. We tested the hypothesis that the degree of sympathoexcitation during the ML phase (ΔMSNA) is associated with changes in estradiol (ΔE(2)) and progesterone (ΔP). Multiple regression analysis of data from previous studies with complete recordings of mean arterial pressure, MSNA, E(2), and P during both early follicular and ML phases were available from 30 eumenorrheic women (age, 28 ± 1 years; body mass index, 23 ± 0 kg/m(2)). ML phase increased E(2) (37 ± 2 to 117 ± 9 pg/mL; P<0.001), P (1 ± 0 to 11 ± 1 ng/mL; P<0.001), and MSNA (12 ± 1 to 15 ± 1 bursts/min; P=0.02), but did not alter mean arterial pressure (83 ± 2 to 83 ± 2 mm Hg; P=0.91). ΔMSNA was correlated with ΔE(2) (r=-0.50, P=0.003) and ΔE(2)/ΔP (r=-0.52, P=0.002) but not ΔP (r=0.21, P=0.13). There was no association between Δmean arterial pressure and ΔE(2) (r=-0.13, P=0.49), ΔP (r=-0.04, P=0.83), or ΔE(2)/ΔP (r<0.01, P=0.98). In conclusion, sympathoexcitation during the ML phase of the ovarian cycle seems to be dependent, in part, on the degree of sex steroid surges. This dynamic interaction among E(2), P, and MSNA likely explains previously reported inconsistencies in the field; it remains possible that other sex steroids, such as testosterone, might explain further variance.

Collateralization of projections from the rostral ventrolateral medulla to the rostral and caudal thoracic spinal cord in felines

Stimulation of vestibular receptors elicits distinct changes in blood flow to the forelimb and hindlimb, showing that the nervous system has the capacity to produce changes in sympathetic outflow which are specific for a particular region of the body. However, it is unclear whether the rostral ventrolateral medulla (RVLM), the primary region of the brainstem that regulates sympathetic outflow to vascular smooth muscle, has the appropriate connectivity with sympathetic preganglionic neurons to generate anatomically patterned responses. To make this determination, the retrograde fluorescent tracer Fast Blue was injected into the T(4) spinal cord segment of cats, which regulates upper body blood flow, whereas Fluoro-Ruby was injected into the T(10) segment to label projections to a region of the spinal cord that regulates lower body blood flow. More neurons were single-labeled by a particular tracer (92 %) than were double labeled by both tracers (8 %), supporting the notion that the RVLM can regulate sympathetic outflow from a limited number of spinal cord segments. Since a large fraction of RVLM neurons that control sympathetic outflow in rodents contain epinephrine, we additionally determined whether the tracer-labeled cells were immunopositive for the enzyme tyrosine hydroxylase (TH), which participates in the synthesis of catecholamines. Double labeling by the two tracers injected into the spinal cord was more common for TH-immunopositive neurons than for the general population of RVLM neurons: 19 % of the TH-positive cells contained both Fast Blue and Fluoro-Ruby, 30 % contained one of the tracers, and 51 % were not labeled by either tracer. Furthermore, many spinally projecting neurons in close proximity to the RVLM catecholaminergic neurons (41 % of the population) were not immunopositive for TH, suggesting that feline RVLM is neurochemically heterogeneous.

Lack of effect of ovarian cycle and oral contraceptives on baroreceptor and nonbaroreceptor control of sympathetic nerve activity in healthy women

Endogenous and exogenous female hormones regulate sympathetic nerve activity (SNA) in animal models, but their impact in humans is controversial. The purpose of this study is to investigate the effects of the ovarian cycle and oral contraceptive pills (OCPs) on SNA. We hypothesized that the effects of endogenous hormones were baroreflex (BR)-mediated and that these cyclical changes in BR control were blunted by OCPs. Furthermore, we hypothesized that the nocturnal fall in blood pressure (BP) ("dipping"), which is sympathetically mediated, also varied with the ovarian cycle. In 23 healthy females (13 OCP users, 10 age-matched, no OCPs), SNA was recorded (microneurography) at rest, during BR activation/deactivation, and cold pressor test (CPT) during low and high hormonal phases. Furthermore, 24-h BP monitoring was performed during low and high hormonal phases. SNA was lower during the low vs. high hormone phase in non-OCP users (17.3 ± 2.4 vs. 25.4 ± 3.2 bursts/min, P < 0.001) but was not different between phases in OCP users [15.5 ± 1.7 vs. 16.6 ± 2.0 bursts/min, P = not significant (NS)]. BR control of SNA was not different during the hormone phases in either group [SNA (total activity/min) mean slope %change from baseline, no OCP users, low vs. high hormone phase 35.4 ± 6.2 vs. 29.6 ± 3.4%, P = NS and OCP users, low vs. high hormone phase 35.7 ± 3.9 vs. 33.5 ± 3.5%, P = NS]. SNA activation during CPT was not impacted by hormonal phase or OCP use. Finally, nondipping was not different between OCP users and nonusers, although there was a trend for nondipping to occur more frequently in the OCP users. SNA varies during the ovarian cycle in women in the absence of OCPs. This modulation cannot be attributed to cyclical changes in the BR sensitivity.

Differential vasoactive effects of oestrogen, oestrogen receptor agonists and selective oestrogen receptor modulators in rat middle cerebral artery

Cerebrovascular disorders are less common in pre-menopausal than post-menopausal women and in females than males. This protection may be due, in part at least, to direct effects of oestrogens on blood vessels. Oestrogen's vasodilatory mechanisms have been reported to be via the endothelium, vascular smooth muscle and extracellular matrix, depending on the vascular bed studied. Herein we investigated the vasoactive effects of oestrogen, oestrogen receptor (ER) and GPR30 agonists and selective ER modulators (SERMs) in the rat middle cerebral artery(MCA), an artery affected in focal ischaemia. MCAs isolated from male Sprague Dawley rats were mounted on a wire myograph. Concentration response curves were constructed to 17β-oestradiol, ERα agonist-PPT, ERβ agonist-DPN, GPR30 agonist-G1 and novel SERMs (LY362321 and LY2120310) in pre-constricted vessels, in the presence and absence of endothelium, blocking agents for nitric oxide synthase (L-NAME), classic ER antagonist (ICI182,780) or plasma membrane specific ERα (ERα-36) antibody. 17β-oestradiol induced rapid vasorelaxation of the MCA which was not affected by endothelium removal, L-NAME or ICI182,780. Vasorelaxation was mimicked by PPT, DPN and G1 but not by the SERMs. Using ERα-36 antibody, effects of oestrogen were partially blocked. PPT had a greater vasorelaxation, while DPN and G1 had a lesser effect than 17β-oestradiol. These findings indicate that activation of plasma membrane bound ERα, β and GPR30 elicits rapid, endothelial-nitric oxide-independent relaxation of the rat MCA.

Sex differences in the modulation of vasomotor sympathetic outflow during static handgrip exercise in healthy young humans

Sex differences in sympathetic neural control during static exercise in humans are few and the findings are inconsistent. We hypothesized women would have an attenuated vasomotor sympathetic response to static exercise, which would be further reduced during the high sex hormone [midluteal (ML)] vs. the low hormone phase [early follicular (EF)]. We measured heart rate (HR), blood pressure (BP), and muscle sympathetic nerve activity (MSNA) in 11 women and 10 men during a cold pressor test (CPT) and static handgrip to fatigue with 2 min of postexercise circulatory arrest (PECA). HR increased during handgrip, reached its peak at fatigue, and was comparable between sexes. BP increased during handgrip and PECA where men had larger increases from baseline. Mean ± SD MSNA burst frequency (BF) during handgrip and PECA was lower in women (EF, P < 0.05), as was ΔMSNA-BF smaller (main effect, both P < 0.01). ΔTotal activity was higher in men at fatigue (EF: 632 ± 418 vs. ML: 598 ± 342 vs. men: 1,025 ± 416 a.u./min, P < 0.001 for EF and ML vs. men) and during PECA (EF: 354 ± 321 vs. ML: 341 ± 199 vs. men: 599 ± 327 a.u./min, P < 0.05 for EF and ML vs. men). During CPT, HR and MSNA responses were similar between sexes and hormone phases, confirming that central integration and the sympathetic efferent pathway was comparable between the sexes and across hormone phases. Women demonstrated a blunted metaboreflex, unaffected by sex hormones, which may be due to differences in muscle mass or fiber type and, therefore, metabolic stimulation of group IV afferents.

Responses of neurons in the rostral ventrolateral medulla to whole body rotations: comparisons in decerebrate and conscious cats

The responses to vestibular stimulation of brain stem neurons that regulate sympathetic outflow and blood flow have been studied extensively in decerebrate preparations, but not in conscious animals. In the present study, we compared the responses of neurons in the rostral ventrolateral medulla (RVLM), a principal region of the brain stem involved in the regulation of blood pressure, to whole body rotations of conscious and decerebrate cats. In both preparations, RVLM neurons exhibited similar levels of spontaneous activity (median of ∼17 spikes/s). The firing of about half of the RVLM neurons recorded in decerebrate cats was modulated by rotations; these cells were activated by vertical tilts in a variety of directions, with response characteristics suggesting that their labyrinthine inputs originated in otolith organs. The activity of over one-third of RVLM neurons in decerebrate animals was altered by stimulation of baroreceptors; RVLM units with and without baroreceptor signals had similar responses to rotations. In contrast, only 6% of RVLM neurons studied in conscious cats exhibited cardiac-related activity, and the firing of just 1% of the cells was modulated by rotations. These data suggest that the brain stem circuitry mediating vestibulosympathetic reflexes is highly sensitive to changes in body position in space but that the responses to vestibular stimuli of neurons in the pathway are suppressed by higher brain centers in conscious animals. The findings also raise the possibility that autonomic responses to a variety of inputs, including those from the inner ear, could be gated according to behavioral context and attenuated when they are not necessary.

Role of the rostral ventrolateral medulla (RVLM) in the patterning of vestibular system influences on sympathetic nervous system outflow to the upper and lower body

Research on animal models as well as human subjects has demonstrated that the vestibular system contributes to regulating the distribution of blood in the body through effects on the sympathetic nervous system. Elimination of vestibular inputs results in increased blood flow to the hindlimbs during vestibular stimulation, because it attenuates the increase in vascular resistance that ordinarily occurs in the lower body during head-up tilts. Additionally, the changes in vascular resistance produced by vestibular stimulation differ between body regions. Electrical stimulation of vestibular afferents produces an inhibition of most hindlimb vasoconstrictor fibers and a decrease in hindlimb vascular resistance, but an initial excitation of most upper body vasoconstrictor fibers accompanied by an increase in upper body vascular resistance. The present study tested the hypothesis that neurons in the principal vasomotor region of the brainstem, the rostral ventrolateral medulla (RVLM), whose projections extended past the T10 segment, to spinal levels containing sympathetic preganglionic neurons regulating lower body blood flow, respond differently to electrical stimulation of the vestibular nerve than RVLM neurons whose axons terminate rostral to T10. Contrary to our hypothesis, the majority of RVLM neurons were excited by vestibular stimulation, despite their level of projection in the spinal cord. These findings indicate that the RVLM is not solely responsible for establishing the patterning of vestibular-sympathetic responses. This patterning apparently requires the integration by spinal circuitry of labyrinthine signals transmitted from the brainstem, likely from regions in addition to the RVLM.

Neurovascular responses to mental stress in prehypertensive humans

Neurovascular responses to mental stress have been linked to several cardiovascular diseases, including hypertension. Mean arterial pressure (MAP), muscle sympathetic nerve activity (MSNA), and forearm vascular responses to mental stress are well documented in normotensive (NT) subjects, but responses in prehypertensive (PHT) subjects remain unclear. We tested the hypothesis that PHT would elicit a more dramatic increase of MAP during mental stress via augmented MSNA and blunted forearm vascular conductance (FVC). We examined 17 PHT (systolic 120-139 and/or diastolic 80-89 mmHg; 22 ± 1 yr) and 18 NT (systolic < 120 and diastolic < 80 mmHg; 23 ± 2 yr) subjects. Heart rate, MAP, MSNA, FVC, and calf vascular conductance were measured during 5 min of baseline and 5 min of mental stress (mental arithmetic). Mental stress increased MAP and FVC in both groups, but the increases in MAP were augmented (Δ 10 ± 1 vs. Δ14 ± 1 mmHg; P < 0.05), and the increases in FVC were blunted (Δ95 ± 14 vs. Δ37 ± 8%; P < 0.001) in PHT subjects. Mental stress elicited similar increases in MSNA (Δ7 ± 2 vs. Δ6 ± 2 bursts/min), heart rate (Δ21 ± 3 vs. Δ18 ± 3 beats/min), and calf vascular conductance (Δ29 ± 10 vs. Δ19 ± 5%) in NT and PHT subjects, respectively. In conclusion, mental stress elicits an augmented pressor response in PHT subjects. This augmentation appears to be associated with altered forearm vascular, but not MSNA, responses to mental stress.