Journal of the Autonomic Nervous System, 45 (1993) 1-9 1 © 1993 Elsevier Science Publishers B.V. All rights reserved 0165-1838/93/$06.00

JANS 01423

Naloxone does not prevent vasovagal syncope during simulated orthostasis in humans

M i c h a e l L. S m i t h , M a r k D . C a r l s o n a n d M a r c D . T h a m e s

Departments of Medicine and Biomedical Engineering, Case Western Reserve University and Veterans Affairs Medical Center, Cleveland, Ohio, USA

(Received 14 December 1992) (Revision received 17 February 1993)

(Accepted 2 March 1993)

Key words: Lower body negat ive pressure ; Opio id receptor ; Sympathe t ic activity

Abstract

The mechanism of vasovagal syncope during orthostasis in humans is unknown. Opioid receptors have been implicated in the vasovagal-like responses to hemorrhagic hypotension in conscious animals. We sought to determine if opioid receptor blockade with naloxone (/x receptor antagonist) would prevent or delay the onset of vasovagal syncope in humans. Three protocols were performed in which heart rate, arterial pressure, sympathetic nerve activity, thoracic impedance and forearm vascular resistance were measured during stepwise steady-state increments of lower body negative pressure (LBNP) in nine healthy volunteers. In protocol 1, duplicate trials of LBNP to syncope or -60 mmHg were performed with a 30-45 minute rest period separating the trials. No significant differences in any physiologic responses or cumulative stress tolerance were found. In protocol 2, graded LBNP was repeated after administration of saline or naloxone (0.1 mg/kg) in six subjects in which vasovagal syncope occurred prior to -60 mmHg LBNP. The peak increase of sympathetic nerve activity during LBNP was augmented after naloxone (P = 0.02), but the occurrence of vasovagal syncope was not prevented nor was the cumulative stress tolerated affected (P = 0.42). The heart rate and arterial pressure responses to LBNP were not affected by naloxone. Similarly, in protocol 3, naloxone given just prior to the onset of pre-syncopal symptoms did not alter the physiologic response or the occurrence of vasovagal syncope. These data show that naloxone does not prevent or delay the onset of vasovagal syncope in humans which suggests that /x opioid receptors do not mediate the vasovagal response.

Introduction

Vasovagal syncope is cha rac te r i zed by rapid

onset bradycardia , sympathoinhib i t ion and hy-

po tens ion and is the pr incipal cause of the com-

m o n faint in humans. The mechanism(s) responsi-

ble for vasovagal syncope has not b e e n deter -

Correspondence to: M.L. Smith, Division of Cardiology, Uni- versity Hospitals of Cleveland, 2074 Abington Road, Cleve- land, OH 44106, USA.

mined. In 1944, Barcrof t and col leagues p roposed that the circulatory decompensa t ion associated

with pos t -hemor rhag ic faint ing or ig ina ted in the

hear t [1]. M o r e recently, it has b e e n p roposed

that vasovagal syncope is a mani fes ta t ion of an

inhibitory reflex or ig inat ing f rom m e c h a n o r e c e p -

tors in the left vent r icu lar myocard ium [17,22,24].

However , a recen t study has raised doubts about

the impor tance of this inhibi tory ref lex as the

med ia to r of h u m a n vasovagal syncope. Scher re r et al. r epo r t ed the occur rence of vasovagal re-

sponses observed following ni t ropruss ide infusion

in a cardiac transplant patient [37]. Orthotopic cardiac transplantation results in ventricular den- ervation; therefore, it was suggested that a mech- anism other than an inhibitory reflex originating in the ventricles mediated the cardioinhibitory and sympathoinhibitory responses that were ob- served.

A potential alternative mechanism for vasova- gal syncope involves the action of opioids at their receptors. Opioids modulate cardiovascular func- tion particularly during physiologic and patho- physiologic stresses by modulation of both sympa- thetic and parasympathetic neural activity, either directly or through effects on baroreflex pathways [16]. Several studies have suggested that opioid receptors play a role in cardiovascular control during hemorrhagic hypotension in animals [8,11,34]. Opioid receptor blockade can prevent or reverse sympathoinhibition and hypotension during severe hemorrhage in conscious animals [3,8,11,21,23,32].

In humans, some hemodynamic benefit has been seen with naloxone during septic shock [27]. Administration of exogenous opioids such as morphine often produces bradycardia and hy- potension in humans [18], and similar cardiovas- cular responses are observed during vasovagal syncope. The purpose of this study was to deter- mine if opioid receptors play a role in the occur- rence of vasovagal syncope in humans. We hy- pothesized that opioid receptor blockade would prevent or delay the onset of vasovagal syncope. Vasovagal responses were elicited using graded lower body negative pressure before and after blockade of opioid receptors with intravenous naloxone (0.1 mg/kg).

Materials and Methods

Nine subjects (4 men and 5 women, age = 27 + 2 years) volunteered for this study. Three pro- tocols were performed. First, the stress tolerance (level at which syncopal symptoms occurred) and physiologic responses to duplicate exposures (separated by 30-45 min) to graded lower body negative pressure (LBNP) were studied in seven subjects (3 men and 4 women). Second, stress

tolerance and physiologic responses to LBNP were determined after intravenous administration of sterile saline and after administration of nalox- one (0.1 mg/kg) in six subjects (3 men and 3 women). Third, in two subjects, the same LBNP procedure was repeated with naloxone (0.1 mg/kg) given just prior to the level of LBNP at which vasovagal syncope occurred in the initial trial. Subjects who were studied more than once were studied 2-15 days from the initial study. This study was approved by the Human Research Committee of University Hospitals of Cleveland. Each volunteer gave written informed consent. All studies were performed after subjects had abstained from consumption of caffeine-contain- ing beverages, tobacco, and alcohol for at least 12 h prior to the study.

Measuyemen ts

The electrocardiogram, R-R intervals, thoracic impedance, arterial pressures, forearm blood flow and muscle sympathetic nerve activity were recorded continuously on digital audio tape. Beat-by-beat arterial pressure was determined with a non-invasive photoplethysmographic de- vice placed on the finger (Finapres, Ohmeda, Englewood, CO). Mean thoracic impedance was measured as an index of thoracic blood volume from tape electrodes placed around the lower neck and chest regions. Changes in thoracic impedance have been shown to be a good index of changes in intrathoracic blood volume during simulated orthostasis [4]. Forearm blood flow was measured by venous occlusion plethysmography with a mercury-in-silastic strain gauge placed at a mid-forearm position [40].

Post-ganglionic muscle sympathetic nerve ac- tivity was recorded in five subjects for protocol 2 and in the two subjects for protocol 3. A tungsten microelectrode was inserted into the peroneal nerve near the fibular head. The nerve signal was processed by a preamplifier and an amplifier (Nerve Traffic Analyzer, Model 662C-3, Univer- sity of Iowa Bioengineering, Iowa City, IA) with a total gain of 70 000. Amplified signals were band- pass filtered (700-2000 Hz), rectified, and dis- criminated. Raw nerve signals were integrated by a resistance-capacitance circuit with a time con-

stant of 0.1 s. Muscle sympathetic nerve activity was identified by its characteristic pulse-synchro- nous burst pattern, its relation to cardiac and respiratory activity, and its unresponsiveness to arousal stimuli or skin stroking (which trigger skin, but not muscle sympathetic bursts). Sympa- thetic nerve burst amplitude was normalized dur- ing a control period prior to each trial of LBNP. The mean amplitude (volts) of the bursts occur- ring during each baseline period was given a value of 100 units and all other burst amplitudes during baseline and LBNP were normalized against this standard.

Protocols Two trials of LBNP were performed for each

protocol. For each trial, the level of suction was incremented through stages of - 1 0 mmHg (2 min), - 2 0 mmHg (2 min), - 4 0 mmHg (5 min),

- 50 mmHg (5 min) and - 60 mmHg (5 min). The negative pressure was released at the occurrence of a rapid-onset ( < 10 s) bradycardia (decrease > 20 bpm) and hypotension (decrease > 20 mmHg). In protocol 1, the LBNP procedure was repeated after a 30-45 min rest period. The second run of LBNP was performed at least 20 min after heart rate and arterial pressure had returned to baseline. In protocol 2, each subject received i.v. saline prior to trial 1 and i.v. nalox- one HCI (0.1 mg/kg) prior to trial 2. In protocol 3, the protocol was repeated on a separate day in two subjects in which the naloxone (0.1 mg/kg) was given at the LBNP stage preceding the onset of vasovagal syncope as determined from the initial trial. This was done to approximate the timing of naloxone delivery of animal studies in which naloxone was given at the onset of vasova- gal-like symptoms [23,36]. The cumulative stress

Electrocardiogrom

Arterlol Pressure

Integrated Sympathetic Neurogrom

BQseline ~ ~ i i i • l l lit ', ilqr.:lil~.. ! li~il

HR=63 200 E o

Vosovogel LBNP - 4 0 Syncope

HR=81 HR=86 HR=66

"" / ! ," I . . . . .~ .... .~ ; ,, / t r

goreorm Blood Flow ' " i ' " " " J ' "'" ' "

r'-VR= 18 FVR=37 FVR= 11

l O s

Fig. 1. Sample tracings of electrocardiogram, arterial pressure, sympathetic nerve activity and forearm blood flow f rom one subject during per iods at baseline, LBNP at - 40 mmHg, and at the onse t of vasovagal syncope.

tolerated prior to onset of vasovagal syncope was calculated as the sum of the negative pressure (mmHg) × time (min) at each level of LBNP [20].

Trials for the control and experimental groups were analyzed for differences in response to LBNP by an analysis of variance with repeated measures design. A significant within-subjects t reatment vs. LBNP interaction was used to iden- tify differences between saline and naloxone in the response to LBNP (all P values given below for t reatment effect reflect this within subject interaction). Comparisons were made both for the net response from baseline to each level of LBNP and for the change between consecutive levels. The peak changes in physiologic variables were determined for each trial. The peak changes during LBNP and cumulative stress tolerated were compared between trials using a paired t-test with Bonferroni correction. For all analyses, significance was accepted at the 0.05 level.

Results

Fig. 1 is a sample tracing illustrating responses at rest, LBNP - 4 0 , and during vasovagal syn- cope. LBNP at - 4 0 mmHg is associated with modest tachycardia, sympathoexcitation and vaso- constriction, whereas bradycardia, sympathoinhi- bition and vasodilation occur at the onset of vasovagal syncope. The responses of heart rate, arterial pressure, forearm vascular resistance and mean thoracic impedance for the two trials in protocol 1 are illustrated in Fig. 2. Only the responses prior to the onset of vasovagal syncope are illustrated in this figure. There were no signif- icant differences found for any variable (P > 0.54 for within subject t reatment-LBNP interaction). These data suggest that the hemodynamic stress (i.e. the shift of central blood volume as mea- sured by impedance) and cardiovascular re- sponses (heart rate, forearm vascular resistance and arterial pressure) were reproducible in the two consecutive studies. The occurrence of vaso- vagal syncope also was not different. The cumula- tive stress tolerated was 581 _+ 71 and 564 + 76 mmHg x min, respectively for trials 1 and 2; P = 0.56 (Fig. 3).

Hear t Rate (bpm)

Ar te r ia l P ressu re ( m m H g )

I m p e d a n c e ( o h m s )

F o r e a r m V a s c u l a r

Res i s tance ( u n i t s )

1 O0

80

60

140

100

60 35

30

25

20 60

40

20

• Trial 1 o Trial 2

i f i i i i

I I [ I I [

i i T ( r I

L I i I I I

i i i T i i

I I I I I I

i i i i ~ i

0 -10 -20 30 -40 -50

LBNP (mmHg)

Fig. 2. Mean (+_ S.E.M.) heart rate, arterial pressure, thoracic impedance and forearm vascular resistance responses to pro- gressive LBNP in 2 consecutive trials separated by a 30-min rest period. No significant differences between trials were

found for any variable at any level of LBNP. n = 7.

The data in Figs. 3 and 4 summarize the effect of naloxone on the cumulative stress tolerated and the cardiovascular responses to graded LBNP. The cumulative stress tolerated was not affected by naloxone (445 + 84 vs. 422 _+ 80 min × mmHg before and after naloxone, respectively; P = 0.42). During graded LBNP, the change in thoracic impedance was not different between studies indicating that the physiologic stimulus was similar between the two trials. The increase in sympathetic nerve activity during graded LBNP was significantly greater with naloxone ( P = 0.008 for t rea tment-LBNP interaction). Post hoc analy- sis revealed that the only t reatment difference between successive LBNP levels occurred be- tween baseline and - 1 0 mmHg (P = 0.02; see Fig. 4). The peak responses of sympathetic nerve

- 600

G9 tO

E -400

> ×

0 r -

~ E E ~ - 2 0 0

0

xo '= o

Noloxone

(n=6)

! -6 -6 'C "C

C o n t r o l (n=7)

Fig. 3. Mean (5: S.E.M.) values of cumulative stress tolerated in protocol 1 (Control bars) and protocol 2 (naloxone bars). Cumulative stress was calculated as the sum of the negative pressure ( m m H g ) x time at each LBNP level (min). No signifi-

cant differences were observed.

activity, measured by the change from baseline to the peak response within 1 min of the onset of vasovagal syncope, was significantly greater after naloxone (Table I). Forearm vascular resistance also tended to increase more during graded LBNP with naloxone, although significance was not ob- tained ( P = 0.11). Similarly, the peak forearm

TABLE I

Effect of naloxone on peak responses to LBNP

Saline Naloxone P

Heart Rate (bpm) 24+ 4 27+ 3 0.42 Systolic arterial

p r e s su re (mmHg) - 1 5 + 5 - 1 9 + 5 0.31 Sympathetic nerve

activity (%) 208 + 48 314 _+ 59 0.02 Forearm vascular

resistance (units) 1 8 5 : 3 23_+ 4 0.07

All measures reflect the change from baseline to the peak response measured within 1 min before the onset of vasovagal symptoms, n = 6 (only 5 subjects were studied with sympa- thetic nerve recordings).

Heart Rate (bpm)

Arterial Pressure (mmHg)

Impedance (ohms)

Sympathet ic Nerve Activi ty ( u n i t s / l O s)

Forearm Vascular Resistance

(uni ts)

110

Saline o Naloxone

I I I I I

70

50 I

1 0 0

6 0 I , , , , ,

4 0 / I t I I I

30 t 20 , , ~ ~ f

900 ] 1 I i I

6 0 0 t 3OO

0 1 i i i i

50 A i i J L

30

0 -10 -20 -,.50 -40

LBNP (mmHg) Fig. 4. Mean ( + S.E.M.) heart rate, arterial pressure, thoracic impedance, sympathetic nerve activity and forearm vascular resistance responses to progressive LBNP after intravenous administration of saline or naloxone (0.1 mg/kg) . A 30-min period of rest separated the saline and naloxone trials. A significant t rea tment-LBNP interaction was obtained for sym- pathetic nerve activity (P = 0.008), and post hoc analysis re- vealed a significant difference between 0 and - 1 0 mmHg LBNP (P = 0.02). No other significant t rea tment-LBNP inter-

actions were obtained.

vascular resistance response during LBNP was slightly, but insignificantly (P = 0.07), greater with naloxone present. There were no differences in the arterial pressure or heart rate responses to LBNP ( P - 0.39 and 0.44, respectively). In addi- tion, peak heart rate and arterial pressure re- sponses were unaffected by naloxone, Table I.

6

T A B L E II

t=ffect o f naloxone git~en just prior to syncope on responses to LBNP in 2 subjects

Baseline LBNP Syncope - 40

HR (bpm) Control 66 84 63 Naloxone 62 85 68

SAP (mmHg) Control 131 126 83 Naloxone 128 119 75

SNA (bursts/min) Control 22 62 7 Naloxone 18 68 10

All measures reflect the average of the 2 subjects. Naloxone (0.1 mg/kg) was given after data were collected at LBNP -40. Naloxone did not prevent the occurrence of vasovagal syncope, nor did it alter the physiologic responses associated with the vasovagal event (i.e. bradycardia and sympathoinhibi- tion). HR = heart rate; SAP = systolic arterial pressure; SNA = sympathetic nerve activity.

The extent of cardioinhibition and sympa- thoinhibition during vasovagal syncope was not affected by naloxone. Heart rate increased to a peak of 87 + 5 and 91 + 6 bpm (for saline and naloxone, respectively) before slowing to a nadir of 64 + 7 and 70 + 8 bpm (saline and naloxone, respectively) during syncope. Sympathetic nerve activity increased to a peak of 609 + 52 and 896 + 43 units and decreased to a nadir during syn- cope of 122 + 35 and 97 + 47 units for saline and naloxone, respectively.

In the two subjects who underwent protocol 3, naloxone given just prior to the onset of vasovagal syncope did not change the time of onset of syncope which occurred at a similar level of cu- mulative stress (488 vs. 532 mmHg × min) and at the same level of LBNP ( - 5 0 mmHg) in each subject. The heart rate, arterial pressure and sympathetic nerve activity changes at the onset of vasovagal syncope were similar (Table II).

D i s c u s s i o n

The mechanism of vasovagal syncope during acute reductions of central blood volume in hu- mans is unknown. Previous studies in animals could be interpreted to suggest that vasovagal-like responses can be mediated by opioid receptors

[34]. Our data demonstrate that intravenous naloxone at supra-therapeutic doses (0.1 mg/kg) does not prevent the occurrence or delay the time of onset of vasovagal syncope in humans. In vitro studies have shown that naloxone is primarily a /z-opioid receptor antagonist. Assuming similar receptor specificity with systemic administration of naloxone, these data suggest that /x-opioid receptors do not mediate vasovagal syncope in humans, although a role of other opioid receptors cannot be excluded by our data.

Possible mechanisms of t'aso~'agal syncope Vasovagal syncope is the principal cause of the

common faint in humans, yet the mechanisms responsible for vasovagal syncope are unclear. The most widely accepted mechanism is provoca- tion of a Bezold-Jarisch-like reflex [2]. It has been proposed that this reflex is invoked by vigorous contraction of the ventricles (i.e. augmented con- tractility) when end-systolic volumes are markedly reduced leading to intramyocardial distortion and paradoxic activation of inhibitory cardiac recep- tors located in the left ventricular myocardium [6,22,24]. A recent case report by Scherrer and colleagues suggested that this reflex is not neces- sary for vasovagal-like responses in humans [37]. They observed classic vasovagal-like symptoms of bradycardia (in the innervated recipient atrium), sympathoinhibition and hypotension following in- fusion of nitroprusside in a patient with ortho- topic cardiac transplantation. Cardiac transplan- tation surgery results in an entirely denervated ventricular myocardium; thus, these vasovagal-like symptoms were observed in the absence of vagal afferents originating from the left ventricular my- ocardium. Their findings suggest that mecha- nisms other than an inhibitory reflex originating in the ventricles can mediate vasovagal syncope.

Possible role of opioids in ~'asovagal syncope Based on previous animal studies, release of

endogenous opioids may be an alternative mecha- nism which could mediate a vasovagal response, either independently or as an integral part of a Bezold-Jarisch-like reflex [3,8,34]. Opioid recep- tors have been thought to mediate the paradoxic depressor response to severe hemorrhage in sev-

eral mammalian species./3-Endorphin is released during hypotensive hemorrhage in animals [19] and during vasovagal syncope in humans [26], and naloxone can reverse the fall in arterial pressure during hemorrhagic hypotension in anesthetized and conscious animals [8,11,13,36]. This pressor effect is mediated primarily by increased vasocon- striction via sympathoexcitation or prevention of sympathoinhibition [11,23,35]. The effect of opi- oid blockade on the heart rate response during hemorrhagic hypotension is less pronounced [11,23]. In conscious rabbits, naloxone infusion after hemorrhage causes recovery of arterial pres- sure leading to baroreflex-mediated cardiac slow- ing [31,33]. Opioid blockade has minimal effects in unstressed, normovolemic subjects [16]. Thus, it has been hypothesized that endogenous opioids are normally inactive, but are released during the stress of severe hemorrhage and mediate the vasodepressor response via effects at receptors on structures of the brainstem [16,34].

The role of endogenous opioids in human car- diovascular function has not been studied exten- sively. Rubin and colleagues found that naloxone does not affect the heart rate response to 70 ° tilt, but augments baroreflex control of heart rate during infusion of nitroprusside [30]. Conversely, we found no effect of naloxone on carotid barore- flex control of heart rate as assessed with a ramped neck pressure/suct ion stimulus [39], and these findings were corroborated by Farrell et al. using nitroprusside and phenylephrine to alter the stimulus to the arterial baroreceptors [9]. Foldager and colleagues [12] found no effect of naloxone on the heart rate response of humans to simulated orthostasis with LBNP ( - 4 0 mmHg) and our data support this observation. However, endogenous opioids do appear to impose some tonic modulation of vascular resistance via con- trol of sympathetic nerve activity in humans. Smith and colleagues found that the response of sympa- thetic nerve activity to Valsalva's maneuver is augmented by nal0xone [39]. The stimulus of the Valsalva maneuver is complex and this effect could be mediated by arterial or cardiopulmonary baroreflexes. Recently, Schobel et al. found that the sympathetic nerve response to selective un- loading of cardiopulmonary baroreceptors ( - 5 to

- 1 5 mmHg LBNP) is augmented by naloxone [38]. The sympathetic nerve response to static handgrip exercise also is potentiated [9]. We also observed potent iated sympathoexcitatory re- sponses to LBNP before the onset of vasovagal syncope (see Fig. 4 and Table II). Therefore, opioid receptors appear to modulate some, but not all, reflex mechanisms which control auto- nomic function in humans. Moreover, these ef- fects predominantly affect sympathetic activity, but not parasympathetic activity. It should be noted that these previous data show that opioid receptors can modulate sympathetic nerve activity to muscle vasculature, but the effect on sympa- thetic control in other vascular beds is unknown.

The most important observation of our study was that the occurrence of vasovagal syncope was unaffected by naloxone. The dose used (0.1 mg/kg) is 3-5 times greater than conventional therapeutic doses used to reverse depressant ef- fects of a narcotic overdose. This dose produces blockade of ~ opioid receptors and minimal an- tagonism of 6 and K opioid receptors.

The ~ opioid receptors appear to modulate both sympathetic and parasympathetic efferent activity and attenuate arterial baroreflex function [5,14,28]. During stress situations, ~ opioid recep- tors appear to mediate some excitatory cardiovas- cular effects, but the net effects may vary due to competing effects from different subclasses of /~ receptors [25]. The importance of/.~ receptors in hemorrhagic hypotension in conscious animals is unclear. Generally, naloxone doses greater than 2 m g / k g have been required to elicit reversal of hypotension and sympathoinhibition [34]. Al- though at lower doses naloxone is a reasonably selective ~ receptor antagonist, ~ selectivity is modest at doses > 1 m g /k g which raises the question as to which opioid receptor sub-type is responsible for the naloxone-mediated reversal of hemorrhagic hypotension in animals. It is likely that the high doses of naloxone required in the previous animal studies also produce non-opioid receptor effects. It also is possible that at very high doses there is sufficient antagonism of 6 or K opioid receptors such that these receptors could mediate some effect. Each of these receptors are present in brainstem structures and can modulate

card iovascula r control [10,29]; however, the im- po r t ance of each remains unclear .

Genera l ly , select ive p~ r ecep to r agonis ts are more po t en t than rela t ively select ive 3 or K ago- nists at m o d u l a t i n g ca rd iovascu l a r func t ion [15,25]. A recent s tudy in conscious rabbi ts evalu- a t ed the effects of four op io id r e c e p t o r an tago- nists with varying degrees of selectivity ( fo r /~ , 6 and K recep to r s ) on the dep re s so r responses to caval occlusions p rovoked when ca rd iac ou tpu t was dec reased to ~ 50% of base l ine [7]. An tago - nism of e i t h e r / z or 6 recep to rs could p reven t the dep re s so r responses while select ive an tagonism of K recep to r s with no r -b ina l t o rph imine had no ef- fect. Based on the re la t ive po tency of the antago- nists, it was conc luded that ~ r ecep to r b lockade was most effective in revers ing the dep re s so r ef- fects. W h e t h e r 3 op io id r ecep to r s med ia t e the vasovagal response in humans is unknown and was not t es ted in our s tudies.

Reproducibi l i ty o f L B N P s t imulus Protocol 1 was des igned to d e t e r m i n e the re-

producib i l i ty of the h e m o d y n a m i c responses to g r aded LBNP and of or thos ta t ic t o l e rance as- sessed by consecut ive exposures to LBNP. R e p r o - ducibi l i ty of consecut ive s tudies s e p a r a t e d by less than 1 h has not been s tud ied previously. Light- foot and co l leagues showed that p resyncopa l symptom- l imi ted LBNP is r ep roduc ib l e on con- secutive exposures s e p a r a t e d by th ree days [20]. Mos t invest igat ions involving mul t ip le exposures to L B N P have a l lowed at least 60 min of rest be tween exposures . O u r da ta suggest that the h e m o d y n a m i c and r e f l ex -med ia t ed responses are r ep roduc ib l e when L B N P exposures a re sepa- r a t ed by at least 30 -45 min (Fig. 2). Moreover , the onse t of vasovagal symptoms occur red at a s imilar level of stress (Fig. 3). These da ta demon- s t ra te that a d e q u a t e recovery f rom the first expo- sure occur red and it is unl ikely that the responses dur ing na loxone infusion were af fec ted by the initial exposure to LBNP.

In summary, our da ta d e m o n s t r a t e tha t nalox- one at s u p r a - t h e r a p e u t i c doses p r o d u c e s modes t modu la t i on of the hemodynamic and sympathe t i c responses to s imula ted or thostas is , but does not p reven t or de lay the occur rence of vasovagal syn-

cope in humans. These f indings suggest tha t /x op io id r ecep to r s are not involved in the f rank dep res so r response of vasovagal syncope in hu- mans. These da ta do not exclude the po ten t i a l role of o the r op io id recep to rs in this response .

Acknowledgement

The au thors wish to thank Ms. He len M. Shee- han for her excel lent technica l assistance.

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