Am J Physiol Heart Circ Physiol 313: H782–H787, 2017. First published July 21, 2017; doi:10.1152/ajpheart.00312.2017.
RESEARCH ARTICLE
Integrative Cardiovascular Physiology and Pathophysiology
Muscle sympathetic nerve activity and volume-regulating factors in healthy pregnant and nonpregnant women Nisha Charkoudian,1 Charlotte W. Usselman,2,3 Rachel J. Skow,2,3 Jeffery S. Staab,1 Colleen G. Julian,4 Michael K. Stickland,5 Radha S. Chari,3,5 Rshmi Khurana,3,4 Sandra T. Davidge,3,4 Margie H. Davenport,2,3* and Craig D. Steinback2,3* 1
Submitted 7 June 2017; accepted in final form 18 July 2017
Charkoudian N, Usselman CW, Skow RJ, Staab JS, Julian CG, Stickland MK, Chari RS, Khurana R, Davidge ST, Davenport MH, Steinback CD. Muscle sympathetic nerve activity and volumeregulating factors in healthy pregnant and nonpregnant women. Am J Physiol Heart Circ Physiol 313: H782–H787, 2017. First published July 21, 2017; doi:10.1152/ajpheart.00312.2017.—Healthy, normotensive human pregnancies are associated with striking increases in both plasma volume and vascular sympathetic nerve activity (SNA). In nonpregnant humans, volume-regulatory factors including plasma osmolality, vasopressin, and the renin-angiotensin-aldosterone system have important modulatory effects on control of sympathetic outflow. We hypothesized that pregnancy would be associated with changes in the relationships between SNA (measured as muscle SNA) and volume-regulating factors, including plasma osmolality, plasma renin activity, and arginine vasopressin (AVP). We studied 46 healthy, normotensive young women (23 pregnant and 23 nonpregnant). We measured SNA, arterial pressure, plasma osmolality, plasma renin activity, AVP, and other volume-regulatory factors in resting, semirecumbent posture. Pregnant women had significantly higher resting SNA (38 ⫾ 12 vs. 23 ⫾ 6 bursts/min in nonpregnant women), lower osmolality, and higher plasma renin activity and aldosterone (all P ⬍ 0.05). Group mean values for AVP were not different between groups [4.64 ⫾ 2.57 (nonpregnant) vs. 5.17 ⫾ 2.03 (pregnant), P ⬎ 0.05]. However, regression analysis detected a significant relationship between individual values for SNA and AVP in pregnant (r ⫽ 0.71, P ⬍ 0.05) but not nonpregnant women (r ⫽ 0.04). No relationships were found for other variables. These data suggest that the link between AVP release and resting SNA becomes stronger in pregnancy, which may contribute importantly to blood pressure regulation in healthy women during pregnancy. NEW & NOTEWORTHY Sympathetic nerve activity and blood volume are both elevated during pregnancy, but blood pressure is usually normal. Here, we identified a relationship between vasopressin and sympathetic nerve activity in pregnant but not nonpregnant women. This may provide mechanistic insights into blood pressure regulation in normal pregnancy and in pregnancy-related hypertension. * M. H. Davenport and C. D. Steinback contributed equally to this work. Address for reprint requests and other correspondence: C. D. Steinback, Neurovascular Health Laboratory, Faculty of Physical Education and Recreation, Univ. of Alberta, Edmonton, AB, Canada T6G 2E1 (e-mail: [email protected]). H782
blood volume; autonomic nervous system; blood pressure; vasopressin; plasma osmolality HUMAN PREGNANCY is associated with a marked increase in activity of sympathetic vasoconstrictor nerves. Sympathetic hyperactivity manifests as early as the first trimester (20) and progressively increases throughout gestation (20, 34, 35). Furthermore, plasma volume increases by ~1 liter (⫹ 40%) during normal pregnancy (8). Together, these changes would typically result in a significant increase in blood pressure. Interestingly, most healthy pregnancies are associated with normal or decreased blood pressure (20, 34, 35). The relationship between the changes in plasma volume and blood pressure during pregnancy involves complex interactions among changes in renal function, Na⫹ retention, and systemic vasodilation and is a well-recognized puzzle in integrative physiology (36). In nonpregnant humans and animals, there are important modulatory influences of volume-regulating factors, such as plasma osmolality, arginine vasopressin, and ANG II on control of sympathetic nerve activity (SNA) (3, 11, 13, 15, 16). These influences on sympathetic control of the circulation may defend against blood pressure going too low, such as in dehydration (26). On the other end of the blood pressure spectrum, excessive activation of sympathetic outflow by volume-regulating factors is associated with some forms of chronic hypertension (2). In rats, pregnancy causes striking changes in the central control of sympathetic outflow (22, 23), including alterations in ANG II and vasopressin signaling in the paraventricular nucleus (6). In humans, recent data point to reductions in sympathetic vascular transduction (20, 35) and baroreflex responsiveness (34), both of which may contribute to the maintenance of normal blood pressure in the face of markedly higher SNA. Furthermore, augmented endothelium-dependent vasodilation appears to play a role in maintenance of normotension in pregnancy (33, 37) and may contribute to the decreased vascular transduction of sympathetic vasoconstrictor signals reported by Usselman et al. (35). Because SNA and blood volume increase concurrently during pregnancy, it is likely that the relationships between SNA
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United States Army Research Institute of Environmental Medicine, Natick, Massachusetts; 2Neurovascular Health Laboratory, Program for Pregnancy and Postpartum Health, Faculty of Physical Education and Recreation, Women and Children’s Health Research Institute, University of Alberta, Edmonton, Alberta, Canada; 3Women and Children’s Health Research Institute, University of Alberta, Edmonton, Alberta, Canada; 4Department of Medicine, University of Colorado Denver School of Medicine, Denver, Colorado; and 5Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, Canada
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and various volume-regulating factors are altered compared with those in nonpregnant humans. The goal of the present study was to compare relationships of individual values for muscle SNA (MSNA) and volume-regulatory factors in healthy normotensive pregnant and nonpregnant women. We hypothesized that SNA would be positively related to plasma osmolality, plasma renin activity, vasopressin, and aldosterone in healthy nonpregnant women and that these relationships would be blunted or reversed in pregnant women. METHODS
RESULTS
Participant characteristics are shown in Table 1. Pregnant women were slightly older and had a higher body mass index than nonpregnant women (as expected); prepregnancy body mass index was similar to nonpregnant women. Consistent with previous reports (20, 34, 35), resting MSNA was significantly elevated in pregnant versus nonpregnant women. Average MSNA was 23 ⫾ 6 bursts/min (34 ⫾ 9 bursts/100 heart beats) in nonpregnant women and 38 ⫾ 12 bursts/min (46 ⫾ 13 bursts/100 heart beats) in pregnant women (P ⬍ 0.001 for both). Thus, increased MSNA was not just due to increased HR (i.e., more opportunities for bursts) since burst incidence (bursts/100 heart beats) was also substantially increased in pregnant women. Baroreflex sensitivity was significantly decreased in pregnant women [⫺5.0 ⫾ 1.5 (nonpregnant) vs. ⫺4.0 ⫾ 1.5 (pregnant) bursts·100 heart beats⫺1·mmHg⫺1, P ⫽ 0.014). Systemic hemodynamics. Mean hemodynamic values are shown in Table 2. HR and CO were significantly elevated in pregnant women, whereas arterial pressure was not different between pregnant and nonpregnant women. Accordingly, TPR was lower in pregnant women. Table 1. Participant characteristics Age, yr Height, cm Weight, kg† Body mass index, kg/m2† Prepregnancy body mass index‡ Weeks gestation
Nonpregnant
Pregnant
28 ⫾ 5 166 ⫾ 5 66 ⫾ 13 24 ⫾ 5
31 ⫾ 3* 165 ⫾ 7 73 ⫾ 12* 27 ⫾ 4* 23 ⫾ 3 32 ⫾ 4
All values are means ⫾ SD. *P ⬍ 0.05 vs. the nonpregnant group. †Value at the time of study. ‡Calculated based on recall.
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Subjects. The protocol for this study was reviewed and approved by the University of Alberta Ethics Committee. Twenty-three pregnant (third trimester, 32 ⫾ 4 wk gestation) and 23 nonpregnant healthy young women volunteered to participate in this study and gave their written, informed consent. All subjects were normotensive, had no history of cardiovascular disease, and took no medications other than oral contraceptives (nonpregnant women) or prenatal vitamins (pregnant women). Seven women were not currently using hormonal contraceptives, eight women were using oral contraceptives, five women were using intrauterine devices, and one woman was using Nuvaring. Two women did not provide information. Nonpregnant women were tested in their self-reported follicular phase or placebo phase of oral contraceptives except those using intrauterine devices, who were tested at their convenience. Since pregnancy often leads to insulin resistance, all pregnant women (as per Canadian Healthcare practices) were screened for gestational diabetes and hyperglycemia at ~28 wk gestation. Women reporting a diagnosis of gestational diabetes were excluded from the study. We also analyzed fasting glucose to ensure none of the women had subsequently developed hyperglycemia. Sympathetic reactivity to the cold pressor test (35) (12 pregnant and 12 nonpregnant women) and sympathetic baroreflex function (34) (11 of 12 women in each group from Ref. 35) have been reported previously for a subset of the women included in the present study. The present study focuses on the novel assessment of factors regulating blood volume and their relationship to SNA in a larger group of women, incorporating the originally cited MSNA data plus new MSNA data from 11 pregnant and 11 nonpregnant women. Protocol timeline. Subjects reported to the laboratory after an overnight fast. Height (stadiometer) and weight (laboratory scale) were measured the day of testing and used to calculate body mass index (in kg/m2). Prepregnancy body mass index was calculated using self-reported prepregnancy body weight (questionnaire). Blood samples were drawn from an antecubital vein using aseptic technique for the analysis of plasma osmolality (Posm), Na⫹, K⫹, Cl⫺, glucose, arginine vasopressin, aldosterone, and plasma renin activity. Samples were immediately centrifuged and separated, and plasma was stored at ⫺80°C until analyzed. After blood sampling and a standardized breakfast, volunteers rested semirecumbent during instrumentation for microneurography (described below), measurement of beat-to-beat blood pressure (finger photoplethysmography, Finometer), cardiac output (CO; Modelflow), and heart rate (HR; lead II electrocardiogram). After instrumentation, we recorded MSNA for a period of 7– 8 min of semirecumbent rest. MSNA was recorded via peroneal microneurography as previously described (5, 31). We used microneurography electrodes (UNP35F2T, Frederick Haer), which were 200-m, epoxylite-coated tungsten needles with an impedance of 2 M⍀. The raw sympathetic signal was amplified [preamplifier (1,000 times) and variable-gain, isolated amplifier (10,000 times, model 662C-3, Iowa University Bioengineering)] and bandpass filtered (700 –2,000 Hz). The raw signal was rectified and integrated (decay constant: 0.1s) to produce an integrated neurogram with a characteristic bursting pattern. The microelectrode was placed percutaneously and manipulated using fine movements
until a muscle sympathetic fascicle was identified when taps on the muscle belly or passive muscle stretch evoked mechanoreceptive impulses. MSNA was confirmed when no afferent neural response was evoked by skin stimuli. Data analysis. Bursts of MSNA were detected using a semiautomated peak detection algorithm (Chart 8.1.3, AD Instruments), and bursts were confirmed by a trained observer (C. W. Usselman and C. D. Steinback) based on a stereotypical pulse-synchronous pattern that was observed in both the raw and integrated neurograms (34, 35). SNA was subsequently expressed as integrated burst frequency (bursts/min) and incidence (bursts/100 heart beats). The spontaneous relationship between burst occurrence and diastolic pressure was used to calculate baroreflex control of MSNA, as previously described (34). Total peripheral resistance (TPR) was calculated as mean arterial pressure/CO. SNA and hemodynamic variables are reported as means of 7– 8 min. Vasopressin was measured in extracted plasma by ELISA (74VSPHU-E01.1, ALPCO). Aldosterone (from separated serum) and renin activity (plasma) were analyzed by ELISAs (11-ALDHU-E01 and 11-RENHU-E02, respectively, ALPCO). Osmolality was assessed by a Fiske Associates micro-osmometer (model 210), and electrolytes and glucose were assessed via a Siemens Dimension Xpand Plus Chemistry Analyzer. Group data for neural and hemodynamic variables and blood values are presented as means ⫾ SD and were compared between groups by an unpaired t-test. Individual values for MSNA and volume-regulatory factors were compared within groups using Pearson correlation analysis. For all analyses, P ⬍ 0.05 was accepted as statistically significant.
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Table 2. Systemic hemodynamics in pregnant and nonpregnant groups Blood pressure, mmHg Systolic Diastolic Mean Heart rate, beats/min Cardiac output, l/min Total peripheral resistance, mmHg·l⫺1·min⫺1
Nonpregnant
Pregnant
113 ⫾ 8 68 ⫾ 7 86 ⫾ 7 67 ⫾ 9 5.9 ⫾ 1.2 14.9 ⫾ 2.8
111 ⫾ 12 69 ⫾ 8 85 ⫾ 9 84 ⫾ 10* 7.2 ⫾ 1.5* 12.3 ⫾ 2.6*
All values are means ⫾ SD. *P ⬍ 0.05 vs. the nonpregnant group.
DISCUSSION
The major novel finding from the present study was that resting, semirecumbent values for MSNA in healthy pregnant women were significantly associated with circulating levels of arginine vasopressin. Even more striking was the fact that this
Table 3. Blood measurements in pregnant and nonpregnant groups Nonpregnant
⫺1
⫺1
Plasma renin activity, ng·ml ·h Aldosterone, pg/ml Arginine vasopressin, pg/ml Plasma osmolality, mosmol/kgH2O Na⫹, mmol/l Cl⫺, mmol/l K⫹, mmol/l Glucose, mg/dl
Pregnant
Means ⫾ SD
Correlation BF
Correlation BI
Means ⫾ SD
Correlation BF
Correlation BI
1.26 ⫾ 0.78 212 ⫾ 121 4.64 ⫾ 2.57 300 ⫾ 6 142 ⫾ 2 104 ⫾ 2 3.84 ⫾ 0.33 89 ⫾ 8
⫺0.160 0.346 0.043 ⫺0.016 ⫺0.169 ⫺0.048 0.153 0.051
⫺0.042 0.246 ⫺0.016 0.065 ⫺0.157 ⫺0.126 ⫺0.042 0.053
8.10 ⫾ 8.03* 294 ⫾ 94* 5.17 ⫾ 2.03 290 ⫾ 5* 139 ⫾ 2* 105 ⫾ 2 3.85 ⫾ 0.25 81 ⫾ 8*
0.080 ⫺0.012 0.708† ⫺0.079 ⫺0.315 0.100 0.102 ⫺0.118
0.135 ⫺0.173 0.511† ⫺0.217 ⫺0.351 ⫺0.045 0.046 0.013
All values are means ⫾ SD. BF, burst frequency; BI, burst incidence. *P ⬍ 0.05 vs. the nonpregnant group. †Significant correlation coefficients (r values), P ⬍ 0.05. AJP-Heart Circ Physiol • doi:10.1152/ajpheart.00312.2017 • www.ajpheart.org
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Blood values. Blood values are shown in Table 3 for both groups. Plasma renin activity and aldosterone were significantly higher, and plasma osmolality was significantly lower, in pregnant compared with nonpregnant women. Plasma Na⫹ concentration was slightly, but significantly, lower, whereas K⫹ concentration and Cl⫺ concentration were similar between groups. These electrolyte values are consistent with the expected increases in plasma volume in the pregnant group. Plasma glucose concentration was significantly lower in the pregnant group, although both groups demonstrated fasting glucose levels within the normal range. We note that osmolality values were somewhat higher compared with those normally associated with euhydration, particularly in the nonpregnant group. This may have been due to the fact that subjects reported to the laboratory after an overnight fast. Regression analysis of individual values for MSNA and volume-regulatory factors showed a strong positive relationship between individual values for vasopressin and resting MSNA (burst frequency) in pregnant women (r ⫽ 0.71, P ⬍ 0.001) but not in nonpregnant women (Table 3 and Fig. 1). The relationship for burst incidence was also significant in pregnant women (r ⫽ 0.51, P ⫽ 0.018) but not in nonpregnant women. No further relationships were found between MSNA and other volume-regulatory variables (Table 3) or between vasopressin and cardiovascular variables (mean arterial pressure, TPR, CO, or HR).
relationship was absent in nonpregnant women of similar age and health status, suggesting that the relationship between vascular SNA and vasopressin undergoes a major shift with pregnancy. Our data suggest that vasopressin is an important contributor to the biological variability in SNA (or vice versa) in pregnant women but not in nonpregnant healthy women. In contrast to our original hypothesis, none of the other volumeregulatory factors were related to SNA in either group. These latter findings suggest that the other factors measured are not major contributors to the interindividual variability in resting SNA in healthy young nonpregnant women or in healthy pregnant women at 32 wk of gestation. Several classic studies identified vasopressin as one of the major links between volume regulation and sympathetic neural control of blood pressure (15–17). In nonpregnant rats, vasopressin augments baroreflex-mediated sympatho-inhibition during changes in blood pressure and blood volume via interaction with V1 receptors at the area postrema (16, 17). Heesch et al. (19) proposed that decreased neuronal nitric oxide synthase activity in the paraventricular and supraoptic nuclei of pregnant rats alters control of vasopressin secretion and causes disinhibition of baseline SNA, suggesting a possible mechanism for normal vasopressin levels with increased sympathetic outflow. However, to our knowledge, we are the first to evaluate whether interindividual differences in circulating levels of this hormone are related to SNA in humans. Thus, although the positive correlation between MSNA and circulating levels of vasopressin in the pregnant women in the present experiments does not prove a causal relationship, our results are consistent with a common central nervous system site for the effects of pregnancy on sympathetic outflow and vasopressin secretion. In terms of arterial pressure regulation, both CO (24) and plasma volume (8) are reported to increase substantially during pregnancy, although plasma volume was not measured in the present study. Decreased peripheral vascular resistance appears to offset the increases in CO, such that arterial pressure was not different between groups in the present study. In some cases, arterial pressure has been shown to be lower during pregnancy compared with prepregnancy values (7). This decrease in vascular resistance occurs despite a well-defined increase in basal SNA during pregnancy compared with the nonpregnant state (20, 35). Recent work from Usselman et al. (35) and Jarvis et al. (20) suggests that decreased transduction of SNA to peripheral vascular resistance is part of the mechanism by which total peripheral resistance is decreased in pregnancy.
SYMPATHETIC ACTIVITY AND BLOOD VOLUME REGULATION IN PREGNANCY
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Although multiple factors may contribute to decreased neurovascular transduction, increased nitric oxide-dependent vasodilation in pregnancy likely plays a role (37). Additionally, decreased baroreflex sensitivity, as shown in the present study and previous work (34), likely contributes to the altered blood volume/pressure interactions seen in pregnancy. In nonpregnant humans, normal regulation of blood volume and hydration status involves a careful balance between sympathetic neural vasoconstriction and humoral factors, which affect renal function and/or cause vasoconstriction themselves. On one end of the continuum, during a condition of dehydration in healthy humans, decreases in plasma volume can lead to hypotension and orthostatic intolerance if sympathetic vasoconstriction and/or CO are insufficient to maintain blood pressure and cerebral perfusion (4, 12, 26). On the other end, blood volume expansion with inappropriate levels of SNA and/or renal Na⫹ retention appears to be a significant contributor to certain forms of experimental and human hypertension (1, 21, 25). In this context, the present analysis was designed as an initial evaluation of relationships between sympathetic neural factors and volume-regulatory factors in healthy human pregnancy. Our present data regarding group mean values for plasma osmolality and other volume-regulatory factors are similar to those previously reported in pregnant women compared with nonpregnant controls (9, 10, 18, 32). Decreased osmolality and plasma Na⫹ are consistent with volume expansion, and increases in plasma renin activity and aldosterone contribute to the mechanisms of volume expansion that normally occur with the progression of pregnancy. Experimental considerations. The present study was designed to evaluate neural and hemodynamic values in a crosssectional manner between groups of pregnant and nonpregnant women. There were both strengths and limitations to this approach. On the one hand, we were able to evaluate individual values for neural and hemodynamic variables in large (for this type of study) groups of women. Understanding the dynamics of individual differences has provided insight into other aspects of blood pressure regulation (5). On the other hand, a longitudinal design would give more power to detect changes in
relationships between volume-regulatory and sympathetic neural variables over the time course of gestation. In the present assessment, we used Modelflow analysis to calculate CO and subsequently TPR. Modelflow is a mathematical construct of CO derived from the Finometer pulse waveform, and there have been some concerns about its accuracy for absolute values in some situations (30). We note in this context that Modelflow CO values were validated against Doppler echocardiography in pregnant women (27). Rang et al. (27) found a minimal underestimation of CO using Modelflow in pregnant women in the third trimester, giving us a reasonable level of confidence in our present CO values. Perspectives The relationship we have demonstrated between vasopressin and sympathetic activity in normotensive pregnant women is intriguing when extrapolated to our current understanding of preeclampsia. Preeclampsia is diagnosed by the presence of de novo hypertension and proteinuria during pregnancy. Preeclampsia has also been identified as a state of sympathetic hyperactivity in relation to normotensive pregnancy (14, 29). Recent data have also demonstrated that copeptin (a stable coproduct of vasopressin production) is elevated before the diagnosis of preeclampsia (28, 38) and is highly predictive of the disorder (28). Although the relationship between MSNA and vasopressin has not been assessed in preeclampsia, the correlation we have demonstrated between these variables in normotensive pregnancy suggests a potential mechanistic link. In summary, we report here that normal shifts in volumeregulatory factors with healthy human pregnancy are associated with increases in SNA and maintenance of prepregnancy blood pressure values. Individual variability in SNA was strongly associated with individual variability in vasopressin in pregnant, but not nonpregnant, women, suggesting that a strengthened link between these two regulatory pathways contributes to the altered volume and pressure regulation seen in healthy pregnancy. No relationships were seen with other volume-regulatory factors in either group. It will be of interest
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Fig. 1. Relationships between individual values for muscle sympathetic nerve activity (MSNA) and arginine-vasopressin (AVP) in pregnant women (left) and in nonpregnant women (right). The relationship was significant in pregnant women (r ⫽ 0.71, P ⬍ 0.001) but not in nonpregnant women (r ⫽ 0.04).
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in future studies to evaluate whether SNA shows different relationships with volume-regulatory factors in conditions such as pregnancy-associated hypertension or preeclampsia.
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ACKNOWLEDGMENTS We are grateful to the subjects for patient participation in these studies. 10. GRANTS
DISCLOSURES
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N. Charkoudian and J. S. Staab are employees of the United States Department of Defense. The views, opinions, and/or findings contained in this article are those of the authors and should not be construed as an official United States Department of the Army position, or decision, unless so designated by other official documentation. Approved for public release; distribution unlimited. Citations of commercial organizations and trade names in this report do not constitute an official Department of the Army endorsement or approval of the products or services of these organizations. AUTHOR CONTRIBUTIONS
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C.W.U., M.H.D., and C.D.S. conceived and designed research; C.W.U., J.S.S., M.H.D., and C.D.S. performed experiments; N.C., C.W.U., R.J.S., J.S.S., C.G.J., M.K.S., R.S.C., R.K., S.T.D., M.H.D., and C.D.S. analyzed data; N.C., C.W.U., R.J.S., J.S.S., C.G.J., M.K.S., R.S.C., R.K., S.T.D., M.H.D., and C.D.S. interpreted results of experiments; N.C., C.W.U., R.J.S., J.S.S., C.G.J., M.K.S., R.S.C., R.K., S.T.D., M.H.D., and C.D.S. prepared figures; N.C., C.W.U., R.J.S., J.S.S., C.G.J., M.K.S., R.S.C., R.K., S.T.D., M.H.D., and C.D.S. drafted manuscript; N.C., C.W.U., R.J.S., J.S.S., C.G.J., M.K.S., R.S.C., R.K., S.T.D., M.H.D., and C.D.S. edited and revised manuscript; N.C., C.W.U., R.J.S., J.S.S., C.G.J., M.K.S., R.S.C., R.K., S.T.D., M.H.D., and C.D.S. approved final version of manuscript.
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This work was funded by generous supporters of the Lois Hole Hospital for Women through the Women and Children’s Health Research Institute (WCHRI). Funding for this project was also made possible by the Advancing Women’s Heart Health Initiative supported by Health Canada and the Heart and Stroke Foundation of Canada (to M. H. Davenport). R. J. Skow is supported by the Canadian Institutes for Health Research and WCHRI Doctoral Research Awards. S. T. Davidge is supported by a Tier 1 Canada Research Chair in Maternal and Perinatal Cardiovascular Health.
during pregnancy: a systematic review and meta-analysis. Ultrasound Obstet Gynecol 49: 177–187, 2017. doi:10.1002/uog.17360. Duvekot JJ, Cheriex EC, Pieters FA, Menheere PP, Peeters LH. Early pregnancy changes in hemodynamics and volume homeostasis are consecutive adjustments triggered by a primary fall in systemic vascular tone. Am J Obstet Gynecol 169: 1382–1392, 1993. doi:10. 1016/0002-9378(93)90405-8. Eneroth-Grimfors E, Bevegård S, Nilsson BA, Sätterström G. Effect of exercise on catecholamines and plasma renin activity in pregnant women. Acta Obstet Gynecol Scand 67: 519 –523, 1988. doi:10.3109/00016348809029863. Farquhar WB, Wenner MM, Delaney EP, Prettyman AV, Stillabower ME. Sympathetic neural responses to increased osmolality in humans. Am J Physiol Heart Circ Physiol 291: H2181–H2186, 2006. doi:10.1152/ ajpheart.00191.2006. Fu Q, Witkowski S, Okazaki K, Levine BD. Effects of gender and hypovolemia on sympathetic neural responses to orthostatic stress. Am J Physiol Regul Integr Comp Physiol 289: R109 –R116, 2005. doi:10.1152/ ajpregu.00013.2005. Greaney JL, Ray CA, Prettyman AV, Edwards DG, Farquhar WB. Influence of increased plasma osmolality on sympathetic outflow during apnea. Am J Physiol Regul Integr Comp Physiol 299: R1091–R1096, 2010. doi:10.1152/ajpregu.00341.2010. Greenwood JP, Scott EM, Walker JJ, Stoker JB, Mary DA. The magnitude of sympathetic hyperactivity in pregnancy-induced hypertension and preeclampsia. Am J Hypertens 16: 194 –199, 2003. doi:10.1016/ S0895-7061(02)03256-9. Hasser EM, Bishop VS, Hay M. Interactions between vasopressin and baroreflex control of the sympathetic nervous system. Clin Exp Pharmacol Physiol 24: 102–108, 1997. doi:10.1111/j.1440-1681.1997.tb01791.x. Hasser EM, Cunningham JT, Sullivan MJ, Curtis KS, Blaine EH, Hay M. Area postrema and sympathetic nervous system effects of vasopressin and angiotensin II. Clin Exp Pharmacol Physiol 27: 432–436, 2000. doi:10.1046/j.1440-1681.2000.03261.x. Hasser EM, Undesser KP, Bishop VS. Interaction of vasopressin with area postrema during volume expansion. Am J Physiol Regul Integr Comp Physiol 253: R605–R610, 1987. Heenan AP, Wolfe LA, Davies GA, McGrath MJ. Effects of human pregnancy on fluid regulation responses to short-term exercise. J Appl Physiol 95: 2321–2327, 2003. doi:10.1152/japplphysiol.00984.2002. Heesch CM, Zheng H, Foley CM, Mueller PJ, Hasser EM, Patel KP. Nitric oxide synthase activity and expression are decreased in the paraventricular nucleus of pregnant rats. Brain Res 1251: 140 –150, 2009. doi:10.1016/j.brainres.2008.11.021. Jarvis SS, Shibata S, Bivens TB, Okada Y, Casey BM, Levine BD, Fu Q. Sympathetic activation during early pregnancy in humans. J Physiol 590: 3535–3543, 2012. doi:10.1113/jphysiol.2012.228262. Kawano Y, Yoshida K, Matsuoka H, Omae T. Chronic effects of central and systemic administration of losartan on blood pressure and baroreceptor reflex in spontaneously hypertensive rats. Am J Hypertens 7: 536 –542, 1994. doi:10.1093/ajh/7.6.536. Kvochina L, Hasser EM, Heesch CM. Pregnancy decreases GABAergic inhibition of the hypothalamic paraventricular nucleus. Physiol Behav 97: 171–179, 2009. doi:10.1016/j.physbeh.2009.02.018. Kvochina L, Hasser EM, Heesch CM. Pregnancy increases baroreflexindependent GABAergic inhibition of the RVLM in rats. Am J Physiol Regul Integr Comp Physiol 293: R2295–R2305, 2007. doi:10.1152/ ajpregu.00365.2007. Meah VL, Cockcroft JR, Backx K, Shave R, Stöhr EJ. Cardiac output and related haemodynamics during pregnancy: a series of meta-analyses. Heart 102: 518 –526, 2016. doi:10.1136/heartjnl-2015-308476. Miyajima E, Yamada Y, Yoshida Y, Matsukawa T, Shionoiri H, Tochikubo O, Ishii M. Muscle sympathetic nerve activity in renovascular hypertension and primary aldosteronism. Hypertension 17: 1057–1062, 1991. doi:10.1161/01.HYP.17.6.1057. Rabbitts JA, Strom NA, Sawyer JR, Curry TB, Dietz NM, Roberts SK, Kingsley-Berg SM, Charkoudian N. Influence of endogenous angiotensin II on control of sympathetic nerve activity in human dehydration. J Physiol 587: 5441–5449, 2009. doi:10.1113/jphysiol.2009.176693. Rang S, de Pablo Lapiedra B, van Montfrans GA, Bouma BJ, Wesseling KH, Wolf H. Modelflow: a new method for noninvasive assessment of cardiac output in pregnant women. Am J Obstet Gynecol 196: 235.e1–238.e1, 2007.
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34. Usselman CW, Skow RJ, Matenchuk BA, Chari RS, Julian CG, Stickland MK, Davenport MH, Steinback CD. Sympathetic baroreflex gain in normotensive pregnant women. J Appl Physiol (1985) 119: 468 –474, 2015. doi:10.1152/japplphysiol.00131.2015. 35. Usselman CW, Wakefield PK, Skow RJ, Stickland MK, Chari RS, Julian CG, Steinback CD, Davenport MH. Regulation of sympathetic nerve activity during the cold pressor test in normotensive pregnant and nonpregnant women. Hypertension 66: 858 –864, 2015. doi:10.1161/ HYPERTENSIONAHA.115.05964. 36. West CA, Sasser JM, Baylis C. The enigma of continual plasma volume expansion in pregnancy: critical role of the renin-angiotensin-aldosterone system. Am J Physiol Renal Physiol 311: F1125–F1134, 2016. doi:10. 1152/ajprenal.00129.2016. 37. Williams DJ, Vallance PJ, Neild GH, Spencer JA, Imms FJ. Nitric oxide-mediated vasodilation in human pregnancy. Am J Physiol Heart Circ Physiol 272: H748 –H752, 1997. 38. Yeung EH, Liu A, Mills JL, Zhang C, Männistö T, Lu Z, Tsai MY, Mendola P. Increased levels of copeptin before clinical diagnosis of preelcampsia. Hypertension 64: 1362–1367, 2014. doi:10.1161/ HYPERTENSIONAHA.114.03762.
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28. Santillan MK, Santillan DA, Scroggins SM, Min JY, Sandgren JA, Pearson NA, Leslie KK, Hunter SK, Zamba GK, Gibson-Corley KN, Grobe JL. Vasopressin in preeclampsia: a novel very early human pregnancy biomarker and clinically relevant mouse model. Hypertension 64: 852–859, 2014. doi:10.1161/HYPERTENSIONAHA.114.03848. 29. Schobel HP, Fischer T, Heuszer K, Geiger H, Schmieder RE. Preeclampsia–a state of sympathetic overactivity. N Engl J Med 335: 1480 – 1485, 1996. doi:10.1056/NEJM199611143352002. 30. Shibasaki M, Wilson TE, Bundgaard-Nielsen M, Seifert T, Secher NH, Crandall CG. Modelflow underestimates cardiac output in heatstressed individuals. Am J Physiol Regul Integr Comp Physiol 300: R486 –R491, 2011. doi:10.1152/ajpregu.00505.2010. 31. Sundlöf G, Wallin BG. The variability of muscle nerve sympathetic activity in resting recumbent man. J Physiol 272: 383–397, 1977. doi:10. 1113/jphysiol.1977.sp012050. 32. Tamás P, Worgall S, Sulyok E, Rascher W. Renal electrolyte and water handling in normal pregnancy: possible role of endothelin-1. Eur J Obstet Gynecol Reprod Biol 55: 89 –95, 1994. doi:10.1016/0028-2243(94)90060-4. 33. Tkachenko O, Shchekochikhin D, Schrier RW. Hormones and hemodynamics in pregnancy. Int J Endocrinol Metab 12: e14098, 2014. doi: 10.5812/ijem.14098.
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RESEARCH ARTICLE
Integrative Cardiovascular Physiology and Pathophysiology
Muscle sympathetic nerve activity and volume-regulating factors in healthy pregnant and nonpregnant women Nisha Charkoudian,1 Charlotte W. Usselman,2,3 Rachel J. Skow,2,3 Jeffery S. Staab,1 Colleen G. Julian,4 Michael K. Stickland,5 Radha S. Chari,3,5 Rshmi Khurana,3,4 Sandra T. Davidge,3,4 Margie H. Davenport,2,3* and Craig D. Steinback2,3* 1
Submitted 7 June 2017; accepted in final form 18 July 2017
Charkoudian N, Usselman CW, Skow RJ, Staab JS, Julian CG, Stickland MK, Chari RS, Khurana R, Davidge ST, Davenport MH, Steinback CD. Muscle sympathetic nerve activity and volumeregulating factors in healthy pregnant and nonpregnant women. Am J Physiol Heart Circ Physiol 313: H782–H787, 2017. First published July 21, 2017; doi:10.1152/ajpheart.00312.2017.—Healthy, normotensive human pregnancies are associated with striking increases in both plasma volume and vascular sympathetic nerve activity (SNA). In nonpregnant humans, volume-regulatory factors including plasma osmolality, vasopressin, and the renin-angiotensin-aldosterone system have important modulatory effects on control of sympathetic outflow. We hypothesized that pregnancy would be associated with changes in the relationships between SNA (measured as muscle SNA) and volume-regulating factors, including plasma osmolality, plasma renin activity, and arginine vasopressin (AVP). We studied 46 healthy, normotensive young women (23 pregnant and 23 nonpregnant). We measured SNA, arterial pressure, plasma osmolality, plasma renin activity, AVP, and other volume-regulatory factors in resting, semirecumbent posture. Pregnant women had significantly higher resting SNA (38 ⫾ 12 vs. 23 ⫾ 6 bursts/min in nonpregnant women), lower osmolality, and higher plasma renin activity and aldosterone (all P ⬍ 0.05). Group mean values for AVP were not different between groups [4.64 ⫾ 2.57 (nonpregnant) vs. 5.17 ⫾ 2.03 (pregnant), P ⬎ 0.05]. However, regression analysis detected a significant relationship between individual values for SNA and AVP in pregnant (r ⫽ 0.71, P ⬍ 0.05) but not nonpregnant women (r ⫽ 0.04). No relationships were found for other variables. These data suggest that the link between AVP release and resting SNA becomes stronger in pregnancy, which may contribute importantly to blood pressure regulation in healthy women during pregnancy. NEW & NOTEWORTHY Sympathetic nerve activity and blood volume are both elevated during pregnancy, but blood pressure is usually normal. Here, we identified a relationship between vasopressin and sympathetic nerve activity in pregnant but not nonpregnant women. This may provide mechanistic insights into blood pressure regulation in normal pregnancy and in pregnancy-related hypertension. * M. H. Davenport and C. D. Steinback contributed equally to this work. Address for reprint requests and other correspondence: C. D. Steinback, Neurovascular Health Laboratory, Faculty of Physical Education and Recreation, Univ. of Alberta, Edmonton, AB, Canada T6G 2E1 (e-mail: [email protected]). H782
blood volume; autonomic nervous system; blood pressure; vasopressin; plasma osmolality HUMAN PREGNANCY is associated with a marked increase in activity of sympathetic vasoconstrictor nerves. Sympathetic hyperactivity manifests as early as the first trimester (20) and progressively increases throughout gestation (20, 34, 35). Furthermore, plasma volume increases by ~1 liter (⫹ 40%) during normal pregnancy (8). Together, these changes would typically result in a significant increase in blood pressure. Interestingly, most healthy pregnancies are associated with normal or decreased blood pressure (20, 34, 35). The relationship between the changes in plasma volume and blood pressure during pregnancy involves complex interactions among changes in renal function, Na⫹ retention, and systemic vasodilation and is a well-recognized puzzle in integrative physiology (36). In nonpregnant humans and animals, there are important modulatory influences of volume-regulating factors, such as plasma osmolality, arginine vasopressin, and ANG II on control of sympathetic nerve activity (SNA) (3, 11, 13, 15, 16). These influences on sympathetic control of the circulation may defend against blood pressure going too low, such as in dehydration (26). On the other end of the blood pressure spectrum, excessive activation of sympathetic outflow by volume-regulating factors is associated with some forms of chronic hypertension (2). In rats, pregnancy causes striking changes in the central control of sympathetic outflow (22, 23), including alterations in ANG II and vasopressin signaling in the paraventricular nucleus (6). In humans, recent data point to reductions in sympathetic vascular transduction (20, 35) and baroreflex responsiveness (34), both of which may contribute to the maintenance of normal blood pressure in the face of markedly higher SNA. Furthermore, augmented endothelium-dependent vasodilation appears to play a role in maintenance of normotension in pregnancy (33, 37) and may contribute to the decreased vascular transduction of sympathetic vasoconstrictor signals reported by Usselman et al. (35). Because SNA and blood volume increase concurrently during pregnancy, it is likely that the relationships between SNA
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United States Army Research Institute of Environmental Medicine, Natick, Massachusetts; 2Neurovascular Health Laboratory, Program for Pregnancy and Postpartum Health, Faculty of Physical Education and Recreation, Women and Children’s Health Research Institute, University of Alberta, Edmonton, Alberta, Canada; 3Women and Children’s Health Research Institute, University of Alberta, Edmonton, Alberta, Canada; 4Department of Medicine, University of Colorado Denver School of Medicine, Denver, Colorado; and 5Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, Canada
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SYMPATHETIC ACTIVITY AND BLOOD VOLUME REGULATION IN PREGNANCY
and various volume-regulating factors are altered compared with those in nonpregnant humans. The goal of the present study was to compare relationships of individual values for muscle SNA (MSNA) and volume-regulatory factors in healthy normotensive pregnant and nonpregnant women. We hypothesized that SNA would be positively related to plasma osmolality, plasma renin activity, vasopressin, and aldosterone in healthy nonpregnant women and that these relationships would be blunted or reversed in pregnant women. METHODS
RESULTS
Participant characteristics are shown in Table 1. Pregnant women were slightly older and had a higher body mass index than nonpregnant women (as expected); prepregnancy body mass index was similar to nonpregnant women. Consistent with previous reports (20, 34, 35), resting MSNA was significantly elevated in pregnant versus nonpregnant women. Average MSNA was 23 ⫾ 6 bursts/min (34 ⫾ 9 bursts/100 heart beats) in nonpregnant women and 38 ⫾ 12 bursts/min (46 ⫾ 13 bursts/100 heart beats) in pregnant women (P ⬍ 0.001 for both). Thus, increased MSNA was not just due to increased HR (i.e., more opportunities for bursts) since burst incidence (bursts/100 heart beats) was also substantially increased in pregnant women. Baroreflex sensitivity was significantly decreased in pregnant women [⫺5.0 ⫾ 1.5 (nonpregnant) vs. ⫺4.0 ⫾ 1.5 (pregnant) bursts·100 heart beats⫺1·mmHg⫺1, P ⫽ 0.014). Systemic hemodynamics. Mean hemodynamic values are shown in Table 2. HR and CO were significantly elevated in pregnant women, whereas arterial pressure was not different between pregnant and nonpregnant women. Accordingly, TPR was lower in pregnant women. Table 1. Participant characteristics Age, yr Height, cm Weight, kg† Body mass index, kg/m2† Prepregnancy body mass index‡ Weeks gestation
Nonpregnant
Pregnant
28 ⫾ 5 166 ⫾ 5 66 ⫾ 13 24 ⫾ 5
31 ⫾ 3* 165 ⫾ 7 73 ⫾ 12* 27 ⫾ 4* 23 ⫾ 3 32 ⫾ 4
All values are means ⫾ SD. *P ⬍ 0.05 vs. the nonpregnant group. †Value at the time of study. ‡Calculated based on recall.
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Subjects. The protocol for this study was reviewed and approved by the University of Alberta Ethics Committee. Twenty-three pregnant (third trimester, 32 ⫾ 4 wk gestation) and 23 nonpregnant healthy young women volunteered to participate in this study and gave their written, informed consent. All subjects were normotensive, had no history of cardiovascular disease, and took no medications other than oral contraceptives (nonpregnant women) or prenatal vitamins (pregnant women). Seven women were not currently using hormonal contraceptives, eight women were using oral contraceptives, five women were using intrauterine devices, and one woman was using Nuvaring. Two women did not provide information. Nonpregnant women were tested in their self-reported follicular phase or placebo phase of oral contraceptives except those using intrauterine devices, who were tested at their convenience. Since pregnancy often leads to insulin resistance, all pregnant women (as per Canadian Healthcare practices) were screened for gestational diabetes and hyperglycemia at ~28 wk gestation. Women reporting a diagnosis of gestational diabetes were excluded from the study. We also analyzed fasting glucose to ensure none of the women had subsequently developed hyperglycemia. Sympathetic reactivity to the cold pressor test (35) (12 pregnant and 12 nonpregnant women) and sympathetic baroreflex function (34) (11 of 12 women in each group from Ref. 35) have been reported previously for a subset of the women included in the present study. The present study focuses on the novel assessment of factors regulating blood volume and their relationship to SNA in a larger group of women, incorporating the originally cited MSNA data plus new MSNA data from 11 pregnant and 11 nonpregnant women. Protocol timeline. Subjects reported to the laboratory after an overnight fast. Height (stadiometer) and weight (laboratory scale) were measured the day of testing and used to calculate body mass index (in kg/m2). Prepregnancy body mass index was calculated using self-reported prepregnancy body weight (questionnaire). Blood samples were drawn from an antecubital vein using aseptic technique for the analysis of plasma osmolality (Posm), Na⫹, K⫹, Cl⫺, glucose, arginine vasopressin, aldosterone, and plasma renin activity. Samples were immediately centrifuged and separated, and plasma was stored at ⫺80°C until analyzed. After blood sampling and a standardized breakfast, volunteers rested semirecumbent during instrumentation for microneurography (described below), measurement of beat-to-beat blood pressure (finger photoplethysmography, Finometer), cardiac output (CO; Modelflow), and heart rate (HR; lead II electrocardiogram). After instrumentation, we recorded MSNA for a period of 7– 8 min of semirecumbent rest. MSNA was recorded via peroneal microneurography as previously described (5, 31). We used microneurography electrodes (UNP35F2T, Frederick Haer), which were 200-m, epoxylite-coated tungsten needles with an impedance of 2 M⍀. The raw sympathetic signal was amplified [preamplifier (1,000 times) and variable-gain, isolated amplifier (10,000 times, model 662C-3, Iowa University Bioengineering)] and bandpass filtered (700 –2,000 Hz). The raw signal was rectified and integrated (decay constant: 0.1s) to produce an integrated neurogram with a characteristic bursting pattern. The microelectrode was placed percutaneously and manipulated using fine movements
until a muscle sympathetic fascicle was identified when taps on the muscle belly or passive muscle stretch evoked mechanoreceptive impulses. MSNA was confirmed when no afferent neural response was evoked by skin stimuli. Data analysis. Bursts of MSNA were detected using a semiautomated peak detection algorithm (Chart 8.1.3, AD Instruments), and bursts were confirmed by a trained observer (C. W. Usselman and C. D. Steinback) based on a stereotypical pulse-synchronous pattern that was observed in both the raw and integrated neurograms (34, 35). SNA was subsequently expressed as integrated burst frequency (bursts/min) and incidence (bursts/100 heart beats). The spontaneous relationship between burst occurrence and diastolic pressure was used to calculate baroreflex control of MSNA, as previously described (34). Total peripheral resistance (TPR) was calculated as mean arterial pressure/CO. SNA and hemodynamic variables are reported as means of 7– 8 min. Vasopressin was measured in extracted plasma by ELISA (74VSPHU-E01.1, ALPCO). Aldosterone (from separated serum) and renin activity (plasma) were analyzed by ELISAs (11-ALDHU-E01 and 11-RENHU-E02, respectively, ALPCO). Osmolality was assessed by a Fiske Associates micro-osmometer (model 210), and electrolytes and glucose were assessed via a Siemens Dimension Xpand Plus Chemistry Analyzer. Group data for neural and hemodynamic variables and blood values are presented as means ⫾ SD and were compared between groups by an unpaired t-test. Individual values for MSNA and volume-regulatory factors were compared within groups using Pearson correlation analysis. For all analyses, P ⬍ 0.05 was accepted as statistically significant.
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Table 2. Systemic hemodynamics in pregnant and nonpregnant groups Blood pressure, mmHg Systolic Diastolic Mean Heart rate, beats/min Cardiac output, l/min Total peripheral resistance, mmHg·l⫺1·min⫺1
Nonpregnant
Pregnant
113 ⫾ 8 68 ⫾ 7 86 ⫾ 7 67 ⫾ 9 5.9 ⫾ 1.2 14.9 ⫾ 2.8
111 ⫾ 12 69 ⫾ 8 85 ⫾ 9 84 ⫾ 10* 7.2 ⫾ 1.5* 12.3 ⫾ 2.6*
All values are means ⫾ SD. *P ⬍ 0.05 vs. the nonpregnant group.
DISCUSSION
The major novel finding from the present study was that resting, semirecumbent values for MSNA in healthy pregnant women were significantly associated with circulating levels of arginine vasopressin. Even more striking was the fact that this
Table 3. Blood measurements in pregnant and nonpregnant groups Nonpregnant
⫺1
⫺1
Plasma renin activity, ng·ml ·h Aldosterone, pg/ml Arginine vasopressin, pg/ml Plasma osmolality, mosmol/kgH2O Na⫹, mmol/l Cl⫺, mmol/l K⫹, mmol/l Glucose, mg/dl
Pregnant
Means ⫾ SD
Correlation BF
Correlation BI
Means ⫾ SD
Correlation BF
Correlation BI
1.26 ⫾ 0.78 212 ⫾ 121 4.64 ⫾ 2.57 300 ⫾ 6 142 ⫾ 2 104 ⫾ 2 3.84 ⫾ 0.33 89 ⫾ 8
⫺0.160 0.346 0.043 ⫺0.016 ⫺0.169 ⫺0.048 0.153 0.051
⫺0.042 0.246 ⫺0.016 0.065 ⫺0.157 ⫺0.126 ⫺0.042 0.053
8.10 ⫾ 8.03* 294 ⫾ 94* 5.17 ⫾ 2.03 290 ⫾ 5* 139 ⫾ 2* 105 ⫾ 2 3.85 ⫾ 0.25 81 ⫾ 8*
0.080 ⫺0.012 0.708† ⫺0.079 ⫺0.315 0.100 0.102 ⫺0.118
0.135 ⫺0.173 0.511† ⫺0.217 ⫺0.351 ⫺0.045 0.046 0.013
All values are means ⫾ SD. BF, burst frequency; BI, burst incidence. *P ⬍ 0.05 vs. the nonpregnant group. †Significant correlation coefficients (r values), P ⬍ 0.05. AJP-Heart Circ Physiol • doi:10.1152/ajpheart.00312.2017 • www.ajpheart.org
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Blood values. Blood values are shown in Table 3 for both groups. Plasma renin activity and aldosterone were significantly higher, and plasma osmolality was significantly lower, in pregnant compared with nonpregnant women. Plasma Na⫹ concentration was slightly, but significantly, lower, whereas K⫹ concentration and Cl⫺ concentration were similar between groups. These electrolyte values are consistent with the expected increases in plasma volume in the pregnant group. Plasma glucose concentration was significantly lower in the pregnant group, although both groups demonstrated fasting glucose levels within the normal range. We note that osmolality values were somewhat higher compared with those normally associated with euhydration, particularly in the nonpregnant group. This may have been due to the fact that subjects reported to the laboratory after an overnight fast. Regression analysis of individual values for MSNA and volume-regulatory factors showed a strong positive relationship between individual values for vasopressin and resting MSNA (burst frequency) in pregnant women (r ⫽ 0.71, P ⬍ 0.001) but not in nonpregnant women (Table 3 and Fig. 1). The relationship for burst incidence was also significant in pregnant women (r ⫽ 0.51, P ⫽ 0.018) but not in nonpregnant women. No further relationships were found between MSNA and other volume-regulatory variables (Table 3) or between vasopressin and cardiovascular variables (mean arterial pressure, TPR, CO, or HR).
relationship was absent in nonpregnant women of similar age and health status, suggesting that the relationship between vascular SNA and vasopressin undergoes a major shift with pregnancy. Our data suggest that vasopressin is an important contributor to the biological variability in SNA (or vice versa) in pregnant women but not in nonpregnant healthy women. In contrast to our original hypothesis, none of the other volumeregulatory factors were related to SNA in either group. These latter findings suggest that the other factors measured are not major contributors to the interindividual variability in resting SNA in healthy young nonpregnant women or in healthy pregnant women at 32 wk of gestation. Several classic studies identified vasopressin as one of the major links between volume regulation and sympathetic neural control of blood pressure (15–17). In nonpregnant rats, vasopressin augments baroreflex-mediated sympatho-inhibition during changes in blood pressure and blood volume via interaction with V1 receptors at the area postrema (16, 17). Heesch et al. (19) proposed that decreased neuronal nitric oxide synthase activity in the paraventricular and supraoptic nuclei of pregnant rats alters control of vasopressin secretion and causes disinhibition of baseline SNA, suggesting a possible mechanism for normal vasopressin levels with increased sympathetic outflow. However, to our knowledge, we are the first to evaluate whether interindividual differences in circulating levels of this hormone are related to SNA in humans. Thus, although the positive correlation between MSNA and circulating levels of vasopressin in the pregnant women in the present experiments does not prove a causal relationship, our results are consistent with a common central nervous system site for the effects of pregnancy on sympathetic outflow and vasopressin secretion. In terms of arterial pressure regulation, both CO (24) and plasma volume (8) are reported to increase substantially during pregnancy, although plasma volume was not measured in the present study. Decreased peripheral vascular resistance appears to offset the increases in CO, such that arterial pressure was not different between groups in the present study. In some cases, arterial pressure has been shown to be lower during pregnancy compared with prepregnancy values (7). This decrease in vascular resistance occurs despite a well-defined increase in basal SNA during pregnancy compared with the nonpregnant state (20, 35). Recent work from Usselman et al. (35) and Jarvis et al. (20) suggests that decreased transduction of SNA to peripheral vascular resistance is part of the mechanism by which total peripheral resistance is decreased in pregnancy.
SYMPATHETIC ACTIVITY AND BLOOD VOLUME REGULATION IN PREGNANCY
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Although multiple factors may contribute to decreased neurovascular transduction, increased nitric oxide-dependent vasodilation in pregnancy likely plays a role (37). Additionally, decreased baroreflex sensitivity, as shown in the present study and previous work (34), likely contributes to the altered blood volume/pressure interactions seen in pregnancy. In nonpregnant humans, normal regulation of blood volume and hydration status involves a careful balance between sympathetic neural vasoconstriction and humoral factors, which affect renal function and/or cause vasoconstriction themselves. On one end of the continuum, during a condition of dehydration in healthy humans, decreases in plasma volume can lead to hypotension and orthostatic intolerance if sympathetic vasoconstriction and/or CO are insufficient to maintain blood pressure and cerebral perfusion (4, 12, 26). On the other end, blood volume expansion with inappropriate levels of SNA and/or renal Na⫹ retention appears to be a significant contributor to certain forms of experimental and human hypertension (1, 21, 25). In this context, the present analysis was designed as an initial evaluation of relationships between sympathetic neural factors and volume-regulatory factors in healthy human pregnancy. Our present data regarding group mean values for plasma osmolality and other volume-regulatory factors are similar to those previously reported in pregnant women compared with nonpregnant controls (9, 10, 18, 32). Decreased osmolality and plasma Na⫹ are consistent with volume expansion, and increases in plasma renin activity and aldosterone contribute to the mechanisms of volume expansion that normally occur with the progression of pregnancy. Experimental considerations. The present study was designed to evaluate neural and hemodynamic values in a crosssectional manner between groups of pregnant and nonpregnant women. There were both strengths and limitations to this approach. On the one hand, we were able to evaluate individual values for neural and hemodynamic variables in large (for this type of study) groups of women. Understanding the dynamics of individual differences has provided insight into other aspects of blood pressure regulation (5). On the other hand, a longitudinal design would give more power to detect changes in
relationships between volume-regulatory and sympathetic neural variables over the time course of gestation. In the present assessment, we used Modelflow analysis to calculate CO and subsequently TPR. Modelflow is a mathematical construct of CO derived from the Finometer pulse waveform, and there have been some concerns about its accuracy for absolute values in some situations (30). We note in this context that Modelflow CO values were validated against Doppler echocardiography in pregnant women (27). Rang et al. (27) found a minimal underestimation of CO using Modelflow in pregnant women in the third trimester, giving us a reasonable level of confidence in our present CO values. Perspectives The relationship we have demonstrated between vasopressin and sympathetic activity in normotensive pregnant women is intriguing when extrapolated to our current understanding of preeclampsia. Preeclampsia is diagnosed by the presence of de novo hypertension and proteinuria during pregnancy. Preeclampsia has also been identified as a state of sympathetic hyperactivity in relation to normotensive pregnancy (14, 29). Recent data have also demonstrated that copeptin (a stable coproduct of vasopressin production) is elevated before the diagnosis of preeclampsia (28, 38) and is highly predictive of the disorder (28). Although the relationship between MSNA and vasopressin has not been assessed in preeclampsia, the correlation we have demonstrated between these variables in normotensive pregnancy suggests a potential mechanistic link. In summary, we report here that normal shifts in volumeregulatory factors with healthy human pregnancy are associated with increases in SNA and maintenance of prepregnancy blood pressure values. Individual variability in SNA was strongly associated with individual variability in vasopressin in pregnant, but not nonpregnant, women, suggesting that a strengthened link between these two regulatory pathways contributes to the altered volume and pressure regulation seen in healthy pregnancy. No relationships were seen with other volume-regulatory factors in either group. It will be of interest
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Fig. 1. Relationships between individual values for muscle sympathetic nerve activity (MSNA) and arginine-vasopressin (AVP) in pregnant women (left) and in nonpregnant women (right). The relationship was significant in pregnant women (r ⫽ 0.71, P ⬍ 0.001) but not in nonpregnant women (r ⫽ 0.04).
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in future studies to evaluate whether SNA shows different relationships with volume-regulatory factors in conditions such as pregnancy-associated hypertension or preeclampsia.
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ACKNOWLEDGMENTS We are grateful to the subjects for patient participation in these studies. 10. GRANTS
DISCLOSURES
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N. Charkoudian and J. S. Staab are employees of the United States Department of Defense. The views, opinions, and/or findings contained in this article are those of the authors and should not be construed as an official United States Department of the Army position, or decision, unless so designated by other official documentation. Approved for public release; distribution unlimited. Citations of commercial organizations and trade names in this report do not constitute an official Department of the Army endorsement or approval of the products or services of these organizations. AUTHOR CONTRIBUTIONS
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C.W.U., M.H.D., and C.D.S. conceived and designed research; C.W.U., J.S.S., M.H.D., and C.D.S. performed experiments; N.C., C.W.U., R.J.S., J.S.S., C.G.J., M.K.S., R.S.C., R.K., S.T.D., M.H.D., and C.D.S. analyzed data; N.C., C.W.U., R.J.S., J.S.S., C.G.J., M.K.S., R.S.C., R.K., S.T.D., M.H.D., and C.D.S. interpreted results of experiments; N.C., C.W.U., R.J.S., J.S.S., C.G.J., M.K.S., R.S.C., R.K., S.T.D., M.H.D., and C.D.S. prepared figures; N.C., C.W.U., R.J.S., J.S.S., C.G.J., M.K.S., R.S.C., R.K., S.T.D., M.H.D., and C.D.S. drafted manuscript; N.C., C.W.U., R.J.S., J.S.S., C.G.J., M.K.S., R.S.C., R.K., S.T.D., M.H.D., and C.D.S. edited and revised manuscript; N.C., C.W.U., R.J.S., J.S.S., C.G.J., M.K.S., R.S.C., R.K., S.T.D., M.H.D., and C.D.S. approved final version of manuscript.
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