Increased vascular α1-adrenergic receptor sensitivity in older adults with posttraumatic stress disorder

Abstract

Posttraumatic stress disorder (PTSD) is an independent risk factor for the development of hypertension and cardiovascular disease. Patients with PTSD have heightened blood pressure and sympathetic nervous system reactivity; however, it is unclear if patients with PTSD have exaggerated vasoconstriction in response to sympathetic nerve activation that could also contribute to increased blood pressure reactivity. Therefore, we hypothesized that patients with PTSD have increased sensitivity of vascular α₁-adrenergic receptors (α1ARs), the major mediators of vasoconstriction in response to release of norepinephrine at sympathetic nerve terminals. To assess vascular α1AR sensitivity, we measured the degree of venoconstriction in a dorsal hand vein in response to exponentially increasing doses of the selective α1AR agonist, phenylephrine (PE), in 9 patients with PTSD (age = 59 ± 2 yr) and 10 age-matched controls (age = 60 ± 1 yr). Individual dose-response curves were generated to determine the dose of PE that induces 50% of maximal venoconstriction (i.e., PE ED₅₀) reflective of vascular α1AR sensitivity. In support of our hypothesis, PE ED₅₀ values were lower in PTSD compared with controls (245 ± 54 ng/min vs. 1,995 ± 459 ng/min, P = 0.012), indicating increased vascular α₁AR sensitivity in PTSD. The PTSD group also had an increase in slope of rise in venoconstriction, indicative of an altered venoconstrictive reactivity to PE compared with controls (19.8% ± 1.2% vs. 15.1% ± 1.2%, P = 0.009). Heightened vascular α₁AR sensitivity in PTSD may contribute to augmented vasoconstriction and blood pressure reactivity to sympathoexcitation and to increased cardiovascular disease risk in this patient population.

INTRODUCTION

Posttraumatic stress disorder (PTSD) is a debilitating mental health condition that is highly prevalent in both the general population and military veterans (16, 19, 30). In addition to psychological symptoms such as flashbacks, nightmares, and hypervigilance, PTSD is independently associated with an increased risk of hypertension and cardiovascular (CV) disease (6, 10). Earlier studies have shown that post-9/11 veterans with PTSD have a 59% higher chance of developing hypertension (10), and Vietnam veterans with PTSD have a 2.2-fold greater risk of CV mortality (6). Furthermore, increasing PTSD symptom severity is correlated with a stepwise increase in the risk of CV disease, demonstrating a link between PTSD symptomatology and future CV risk (20). Despite epidemiological evidence that links PTSD symptoms with future risk of hypertension and CV disease, the underlying mechanisms have not been fully elucidated.

One potential mechanism that may contribute to future CV risk in PTSD is overactivation of the sympathetic nervous system (SNS). Earlier studies that used both direct and indirect measures of SNS activity have suggested that PTSD is characterized by chronic overactivation of the SNS (7, 13, 14, 26, 28). Patients with PTSD have been shown to have higher resting blood pressure and heart rates, decreased heart rate variability, and increased plasma and urine catecholamine levels, all suggestive of heightened SNS activation (5, 7). Our previous work using direct measurements of muscle sympathetic nerve activity (MSNA) demonstrated that post-9/11 combat veterans with PTSD have exaggerated hemodynamic and SNS responses during mental stress (28). SNS overactivation can contribute to subsequent increased risk of hypertension and CV disease both by increasing blood pressure and via deleterious effects on the vasculature and end organs that are independent of blood pressure (4, 12, 32).

Although we have previously shown that sympathetic nerve activity is elevated in PTSD, it remains unclear whether patients with PTSD have heightened vasoconstriction in response to sympathetic nerve activation. The vascular α1-adrenergic receptors (α1ARs) are the primary mediators of vasoconstriction in response to release of norepinephrine (NE) from sympathetic nerve terminals. Augmented vascular α1AR sensitivity could lead to a greater degree of vasoconstriction in response to the same degree of SNS activation, thereby leading to a greater increase in total peripheral resistance and blood pressure during sympathoexcitation. Heightened vascular α1AR sensitivity could also contribute to exaggerated increases in blood pressure in response to SNS activation during mental stress as observed in PTSD (28). Other populations characterized by increased CV risk such as chronic kidney disease (CKD) have also been characterized by both heightened sympathetic nerve activity and heightened vascular α1AR sensitivity (1, 33). The goal of this study is to determine if patients with PTSD have heightened vascular α1AR sensitivity. Increased vascular α1AR sensitivity in PTSD, in combination with known increases in sympathetic nerve reactivity, could have an additive effect on risk on hypertension and CV disease in this population.

METHODS

Ethical approval.

This study was approved by the Emory University Institutional Review Board and the Atlanta Veterans Affairs (VA) Health Care System Research and Development Committee. Written, informed consent was obtained before study participation for all study participants. All study procedures met the standards of the Declaration of Helsinki.

Participants.

Both controls and participants with PTSD were recruited as part of a previous study (33) involving patients receiving their usual health care at the Atlanta VA Hospital. Study participation was advertised through flyers placed at the VA hospital, and interested participants were screened before being accepted into the study. Participants with and without a diagnosis of PTSD were identified by a review of medical charts; participants with PTSD had PTSD diagnosed by an Atlanta VA Psychiatrist or Psychologist using established Diagnostic and Statistical Manual of Mental Disorders (DSM) criteria. All participants with PTSD had a current diagnosis of PTSD at the time of the study, whereas control participants did not have a current or previous diagnosis of PTSD. Participant ages ranged from 50 to 71, and all female participants were postmenopausal. Exclusion criteria for all study participants included uncontrolled hypertension (BP > 160/90 mmHg), heart disease, vascular disease, alcohol use >2 drinks/day, illicit drug use within the past 12 mo, use of clonidine, neuropathy, and pregnancy.

Experimental protocol.

Experimental methods were conducted as previously described (33). On the day of data collection, each participant arrived at the human physiology laboratory at the Atlanta VA hospital having fasted and abstained from alcohol and caffeine for at least 12 h and exercise for at least 24 h prior. Participants took all medications as prescribed, with the exception of α- and β-blockers, which were held for 24 h before experimentation. Seated blood pressure was recorded in triplicate by an automated blood pressure device (Omron, Hem907XL, Hoffman Estates, IL) after 5 min of quiet rest. A blood sample was collected from an antecubital vein for assessment of a basic metabolic panel. The participant then moved to a semireclined hospital bed. A 24-gauge intravenous catheter was placed into a relatively linear dorsal hand vein and the hand was placed above the heart level to ensure complete emptying of hand veins at rest. Normal saline (0.9% NaCl; NS) was continuously infused through the catheter with a mini-syringe infusion pump (Harvard Apparatus, Holliston, MA) at 0.6 mL/min. A linear-variable differential transformer (LVDT: model MHR 100; Shaevitz) with a movable central core was placed on the same dorsal hand vein ∼1 cm distal to the tip of the catheter and a blood pressure cuff was placed around the upper arm. The LVDT was connected to a data acquisition system (PowerLab, AD Instruments, Bela Vista, NSW, Australia) and venous distension was measured through linear displacement of the LVDT core that was converted to a change in voltage that could be visualized and then quantified offline (Labchart 7, AD Instruments). After 30 min of rest for the vein to recover after catheterization, the upper arm cuff was inflated to 50 mmHg to induce venous filling for 3 min during NS infusion to record maximum venodilation. Three measurements were taken during NS infusion to establish baseline maximum venodilation. After baseline measures, exponentially increasing doses of phenylephrine (PE) at a constant volume of 0.6 mL/min were then infused through the catheter for 7 min/dose. The doses administered were the following (in ng/min): 15, 25, 50, 125, 375, 750, 1,500, 3,000, 6,000, and 12,000. These doses were selected based on optimal concentrations found to elicit a venoconstrictive response in the majority of participants in previous studies (2, 33). The upper arm cuff was inflated during the last 3 min of PE infusion for each dose, and the degree of venous distension during each dose was measured. Ten doses of PE were administered in total, unless maximal venoconstriction was achieved before the last dose was given, determined by a plateauing or lack of further venoconstriction in two consecutive doses. Because of the sensitivity of the assay, not all doses result in a measurable data point if the participant moved during the experiment. Participant data were included in the analysis if the participant received at least five doses with measurable results.

Dorsal hand veins have been shown to yield consistent and reliable PE-induced reductions in venous distension with the LVDT technique (31). Importantly, PE-induced venoconstriction significantly correlates with PE-induced systemic increases in mean arterial blood pressure, suggesting that venous and arterial responses to PE are similar (37). The LVDT technique is characterized by low intrasubject, diurnal, and day-to-day variability (3).

Data analysis.

Individual dose-response curves were generated by expressing vascular distension as a % reduction from baseline maximum vasodilation and plotted against logPE dose rates. Dose-response curves were analyzed using a four-variable sigmoid dose-response model using Sigma Plot 13 (Systat Software). PE ED₅₀ values were derived as the dose of PE that produced 50% of individual maximal venoconstriction. ED₅₀ reflects sensitivity to PE (i.e., α₁-AR sensitivity), with lower values representing increased sensitivity. Individual curves were constrained to a minimum venoconstriction of 0% and a maximum venoconstriction (E_max) of 100% as previously described (33). Since an absolute value for maximal venoconstriction (V_max) cannot be determined when curves are constrained to 100%, a mixed-effects model analysis was performed to assess vasoconstrictive reactivity between groups as a secondary outcome.

Statistics.

Demographic data between groups were compared via unpaired, two-tailed, t tests. Data normality was assessed with the Shapiro–Wilk test. For the primary analysis, unpaired, two-tailed t tests were used to compare ED₅₀ values between PTSD and control groups. For the secondary analysis of vasoconstrictive reactivity, a random-intercept mixed-effects model was used to compare responses over time and between groups using SAS-JMP Pro 15 (SAS Institute). Only dose and dose × group were included as fixed effects in the model because we assumed that there would be no group difference at the baseline level (no dose). All data are given as means ± SE and exact P values are reported. P values <0.05 were considered statistically significant for all analyses.

RESULTS

Participants.

Nine participants with PTSD and 10 age-matched controls were enrolled. These data were also used as control group data in a prior study of patients with CKD (33). After confirmation of data normality, data collected from one control participant were excluded as an outlier according to multivariate robust outlier analysis with Mahalanobis distances (35). Results are reported for n = 9 PTSD and n = 9 controls. Participant demographic data are shown in Table 1. There were no significant differences in age, sex, body mass index, blood pressure, heart rate, race, comorbidities, smoking, or medication use between groups (Table 1).

Table 1.

Baseline characteristics

Characteristic	CON	PTSD	P
n	9	9
Age, yr	60 ± 1	59 ± 2	0.66
Sex (M/F)	8/1	5/4	0.13
Body mass index, kg/m3	29 ± 2	29 ± 2	0.90
Systolic blood pressure, mmHg	134 ± 6	123 ± 3	0.12
Diastolic blood pressure, mmHg	84 ± 3	77 ± 2	0.12
Mean arterial pressure, mmHg	101 ± 4	92 ± 2	0.38
Heart rate, beats/min	67 ± 5	68 ± 4	0.90
Race, n (%)			0.33
Black	9 (100)	8 (90)
White	0 (0)	1 (11)
Hypertension, n (%)	6 (67)	7 (78)	0.62
Diabetes mellitus, n (%)	1 (11)	2 (22)	0.55
Smoking, n (%)	4 (44)	3 (33)	0.77
Antihypertensive medications, n (%)
Calcium channel blockers	1 (11)	3 (33)	0.28
ACE inhibitors/ARBs	5 (56)	2 (22)	0.16
Diuretics	1 (11)	4 (44)	0.13
β-Blockers	2 (22)	2 (22)	1.00
α-Blockers	0 (0)	1 (11)	0.3
SSRIs, n (%)	3 (33)	3 (33)	1.00
Depression	3 (33)	5 (56)	0.37

Values are means $\pm$ SE or as otherwise indicated. ACE, angiotensin-converting enzyme; ARB, angiotensin receptor blocker; CON, control; SSRI, selective serotonin reuptake inhibitor; PTSD, posttraumatic stress disorder. Participants took all antihypertensive medications as prescribed, with the exception of β-blockers and α-blockers.

Vascular α₁-adrenergic receptor sensitivity.

All participants showed a physiological response characteristic of PE-induced vasoconstriction, with increasing venoconstriction of the dorsal hand vein in response to increasing doses of PE. Representative dose-response curves from a PTSD and a control participant are shown in Fig. 1. Mean PE ED₅₀ values were lower (P = 0.012) in the PTSD group (logPE ED₅₀ 2.39 ± 0.22 ng/min or PE ED₅₀ 245 ± 54 ng/min) compared with the control group (logPE ED₅₀ 3.30 ± 0.23 ng/min or PE ED₅₀ 1,995 ± 459 ng/min) as shown in Fig. 2, demonstrating higher vascular α₁AR sensitivity in PTSD. LogPE ED₅₀ values ranged between 2.33 and 4.25 (214–17,783 ng/min) in the control group and between 1.46 and 3.13 (29–1,349 ng/min) in the PTSD group.

Fig. 1.

Phenylephrine (PE) dose-response curves plotted as a function of dorsal hand vein vasoconstriction in individual representative control (CON) (A) and participants with posttraumatic stress disorder (PTSD) (B) with logPE ED₅₀ values of 2.82 (660 ng/mL) and 2.44 (275 ng/mL), respectively.

Fig. 2.

Mean phenylephrine logED₅₀ values between control (CON, n = 9) and posttraumatic stress disorder (PTSD, n = 9) groups. Unpaired, two-tailed t tests compared mean logED₅₀ values. *P = 0.012. CON, control; PTSD, posttraumatic stress disorder.

Vasoconstrictive reactivity.

As the maximum vasoconstrictive capacity (E_max) was constrained to 100%, absolute mean values of E_max from individual dose-response curves are not reported. However, we assessed vasoconstrictive reactivity by using a linear mixed-model analysis of the cumulative data for each group to assess the rate of rise in venoconstriction in response to PE as a measure of vasoconstrictive reactivity. The PTSD group had an increased slope of rise in venoconstriction in response to increasing doses of PE compared with controls [19.8 ± 1.2 vs. 15.1% ± 1.2%/log (ng/min), P = 0.009], demonstrating higher vasoconstrictive reactivity and greater rate of vasoconstriction for a given dose of PE.

DISCUSSION

This investigation compared vascular α₁AR sensitivity between patients with PTSD and age- and comorbidity-matched controls. Although prior reports suggest that patients with PTSD have increased SNS activity and reactivity (13, 14, 28), we have now demonstrated for the first time that patients with PTSD exhibit greater vascular α₁AR sensitivity, as evidenced by a lower ED₅₀ for the selective α₁AR agonist PE. This combination of heightened sympathetic nerve activity with an augmented adrenergic receptor sensitivity at the level of the vasculature may have an additive effect on CV disease risk in patients with PTSD.

PTSD is a highly prevalent chronic mental health condition that occurs after exposure to a traumatic event. The lifetime prevalence of PTSD is around 6.8% in the general population, and PTSD affects up to 20% of post-9/11 military veterans and over 40% of impoverished, urban population (16, 30). Symptoms of PTSD include intrusive thoughts, nightmares, flashbacks, and hypervigilance that lead to significant social, occupational, and interpersonal dysfunction. In addition, multiple studies have shown that PTSD is independently associated with increased risk of CV disease, mortality, and incident hypertension (5–8, 20). Data from the US National Comorbidity Survey showed that patients with PTSD were at 2.9-fold greater risk for developing hypertension (22). In a prospective study of over 4,000 Vietnam veterans without CV disease at baseline, those with PTSD had a 2.2-fold greater risk of CV mortality determined using a multivariate model that adjusted for comorbid conditions, lifestyle, and combat exposure (4). World War II Prisoners of War (POWs) with PTSD compared with POWs without PTSD had a 60% greater risk of developing vascular disease, 25% greater risk of hypertension, and 19% greater risk of chronic heart disease (21). In addition, CV risk increases with the severity of PTSD symptoms. For example, in a large prospective study of over 1,900 patients with PTSD, men had a 26% increased risk for nonfatal myocardial infarction and fatal coronary heart disease for every standard deviation increase in symptom level (23). Despite compelling epidemiological evidence that PTSD is significantly and independently associated with increased hypertension and CV risk, the mechanisms underlying this association remain unclear.

Earlier studies suggest that patients with PTSD have increased SNS activity and reactivity that may contribute to long-term risk of CV disease and hypertension. Some studies (15, 25), although not all (28), have shown that resting blood pressure and heart rate are elevated in patients with PTSD. Meta-analysis has revealed that both plasma and urinary NE, indirect markers of SNS activity, are consistently elevated in PTSD (7, 26). Our prior work using direct measures of SNS activity showed that although there were no differences in resting SNS activity, MSNA and blood pressure reactivity to mental stress was augmented in post-9/11 veterans with PTSD (28). These patients with PTSD were also found to have dysregulation of the SNS characterized by blunted cardiac and sympathetic baroreflex sensitivity at rest and during mental stress that could contribute to increased SNS and blood pressure reactivity and is also independently associated with increased CV disease risk (28).

In addition to the known increases in sympathetic nerve reactivity in PTSD, we currently show that the degree of vasoconstriction in response to SNS activation is augmented via heightened vascular α₁AR sensitivity. Increased α₁AR sensitivity, in conjunction with heightened sympathetic nerve activation, could also contribute to the increased blood pressure reactivity during mental stress observed in PTSD. Heightened sympathetic nerve activity coupled with increased α₁AR sensitivity goes against the paradigm that receptors are downregulated when ligand levels are high. However, this scenario has been observed in other populations characterized by increased CV disease risk. For example, black adults have been shown to have heightened sympathetic reactivity to stress (9) and exaggerated vasoconstriction in response to bursts of sympathetic nerve activity, i.e., neurovascular transduction (34, 39), that contribute to increased hypertension and CV risk. Adefurin and colleagues (2) showed that black adults also have increased vascular α₁AR sensitivity and increased vasoconstrictive capacity compared with white individuals. Similarly, patients with CKD are at significantly greater risk of CV disease and have increased resting sympathetic activity (18, 27), heightened SNS reactivity (29), and increased vascular α₁AR sensitivity that contribute to increased neurovascular transduction in this patient population (33).

Potential mechanisms that could contribute to enhanced vascular α₁AR sensitivity in PTSD remain untested but could include increased α₁AR density, altered expression of α₁AR subtypes, modulation by circulating factors such as inflammation and oxidative stress, and alterations in molecular signaling mechanisms leading to greater degree of smooth muscle contraction with receptor binding. Earlier studies have shown that patients with PTSD have chronic low-grade inflammation and oxidative stress that increases with worsening PTSD symptoms (13, 24), whereas previous reports in experimental models have shown that free radicals may potentiate α-adrenergic vasoconstriction in spontaneously hypertensive rats (17). Whether oxidative stress might modulate α₁AR activity in PTSD remains unknown. In addition, in hypertensive animal models and patients, vascular α₁AR subtype expression has been shown to be altered, with upregulation of α_1DAR and downregulation of α_1AAR and α_1BAR subtypes (36). PE as used in the current study is an agonist of all three α₁AR subtypes; whether there is differential distribution of vascular α₁AR subtypes in PTSD that could potentially alter the overall vasoconstrictive response to sympathetic activation remains unknown. Prazosin, an α₁AR antagonist, is often used for the treatment of PTSD symptoms; however, it is unclear whether use of prazosin through either adrenergic blockade and/or improvement in PTSD symptoms may be associated with improvements in future risk of hypertension and CV disease in PTSD. Future work should seek to elucidate the precise mechanisms contributing to enhanced vascular α₁AR sensitivity in PTSD for the potential development of targeted pharmaceutical interventions.

Special considerations need to be taken in the interpretation of our findings. A potential limitation of our study is that our protocol directly measured α₁AR sensitivity from veins, rather than from arteries, or specifically, resistance arterioles. However, earlier studies have shown that the distribution and characteristics of α₁AR are similar between veins and arteries (38) and hand vein PE-sensitivity correlates with pressor responses to systemic infusions of PE (37). In addition, sympathetic innervation of veins is important for modulating venous capacitance and blood volume and therefore plays a major role in blood pressure regulation. A substantial proportion of both PTSD and control groups had comorbid hypertension. However, there was no difference in the proportion of hypertensives or in antihypertensive medication use between groups. In addition, vascular α₁AR sensitivity has been shown to be similar between hypertension and normotension (11). Therefore, the difference in vascular α₁AR between groups is likely secondary to PTSD. Moreover, PTSD diagnosis was ascertained through a review of the medical chart. We did not confirm the diagnosis of PTSD within the PTSD group or the lack of PTSD in the controls using the Clinician Administered PTSD Scale (CAPS). Therefore, some of the controls may have included those with subthreshold PTSD, and we are unable to correlate PTSD symptom severity or subtypes with vascular adrenergic sensitivity. Additionally, it is known that racial differences may exist when considering α1AR sensitivity. Although all of the participants in the control group in the current study are black, there was one white participant in the PTSD group. Analysis with the omission of this participant revealed that the mean logED₅₀ for the PTSD group did not change (2.31 ± 1.22 from 2.39 ± 0.22) and our findings are still significant (P = 0.01). Finally, a subset of both PTSD and control groups had comorbid depression and were treated with selective serotonin reuptake inhibitors (SSRIs), although there were no differences in the proportion of these patients between groups. Although the current study is not powered to perform subgroup analyses, future work should investigate the potential impact of comorbid depression and SSRI use on α₁AR sensitivity in PTSD.

In summary, our findings indicate that patients with PTSD have lower PE ED₅₀ indicative of heightened vascular α₁AR sensitivity as well as a greater degree of vasoconstriction for a given dose of PE compared with controls without PTSD. Our results pave the way for future studies on mechanisms contributing to heightened α₁AR sensitivity to further elucidate the cellular processes involved in stress-induced CV disease risk. Future work should seek to elucidate the precise mechanisms contributing to heightened vascular α₁AR-sensitivity in PTSD for the potential development of targeted pharmaceutical interventions.

Perspectives and Significance

PTSD is an independent risk factor for the development of CV disease and mortality. Our findings provide new mechanistic insight into PTSD-induced CV risk as increased vascular α₁AR sensitivity may contribute, at least in part, to the changes in neurocirculatory regulation in this population. Additionally, increased vascular α₁AR sensitivity combined with heightened sympathetic reactivity may augment CV disease risk in PTSD. Future work should further investigate vascular α₁AR regulation in PTSD to inform the development of pharmacological treatment targets to mitigate CV risk in patients with PTSD.

GRANTS

This work was supported by National Institutes of Health (NIH) R01 HL135183, R61 AT10457, T32DK007656, and HL140145; Merit Review Award number I01CX001065 from the US Department of Veterans Affairs (VA) Clinical Sciences Research and Development Program; Department of Veterans Affairs, Veterans Health Administration, Office of Research and Development and the Clinical Studies Center of the Atlanta VA Health Care System, Decatur, Georgia; and Foundation for Atlanta Veterans Education and Research (FAVER). C. L. Hartwig was supported by the NIH 5K12GM000680 IRACDA Fellowship.

DISCLOSURES

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

AUTHOR CONTRIBUTIONS

J.P. conceived and designed research; J.D.S., D.G.M., and J.P. performed experiments; C.L.H., J.D.S., J.J., Y.H., and J.P. analyzed data; C.L.H., J.D.S., C.M.S., and J.P. interpreted results of experiments; C.L.H. and J.D.S. prepared figures; C.L.H. drafted manuscript; C.L.H., J.D.S., J.J., Y.H., D.G.M., C.M.S., S.P., and J.P. edited and revised manuscript; C.L.H., J.D.S., J.J., Y.H., D.G.M., C.M.S., S.P., and J.P. approved final version of manuscript.