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Effects of three days of dry immersion on muscle sympathetic nerve activity and arterial blood pressure in humans
Journal of the Autonomic Nervous System 79 (2000) 156–164 www.elsevier.com / locate / jans
Effects of three days of dry immersion on muscle sympathetic nerve activity and arterial blood pressure in humans a,
a ,b
a ,c
a
a
Satoshi Iwase *, Yoshiki Sugiyama , Chihiro Miwa , Atsunori Kamiya , Tadaaki Mano , Yoshinobu Ohira d , Boris Shenkman e , Anatoly I. Egorov e , Inessa B. Kozlovskaya e a
Department of Autonomic Neuroscience, Research Institute of Environmental Medicine, Nagoya University, Furo-cho, Chiksa-ku, Nagoya 461 -8673, Japan b Department of Health and Psychosocial Medicine, Aichi Medical University, Nagakute, Aichi 480 -1195, Japan c Department of Occupational Therapy, School of Health Sciences, Nagoya University School of Medicine, Nagoya University, Higashi-ku, Nagoya 461 -8673, Japan d National Institute of Fitness and Sports, 1 Shiromuzu, Kanoya, Kagoshima 891 -2311, Japan e Institute of Biomedical Problems, State Scientific Center, Khoreshevskoye Shosse 76 A, Moscow 123007, Russia Received 19 August 1999; received in revised form 4 October 1999; accepted 4 October 1999
Abstract The present study was performed to determine how sympathetic function is altered by simulated microgravity, dry immersion for 3 days, and to elucidate the mechanism of post-spaceflight orthostatic intolerance in humans. Six healthy men aged 21–36 years old participated in the study. Before and after the dry immersion, subjects performed head-up tilt (HUT) test to 308 and 608 (5 min each) with recordings of muscle sympathetic nerve activity (MSNA, by microneurography), electrocardiogram, and arterial blood pressure (Finapres). Resting MSNA was increased after dry immersion from 23.763.2 to 40.963.0 bursts / min ( p,0.005) without significant changes in resting heart rate (HR). MSNA responsiveness to orthostasis showed no significant difference but HR response was significantly augmented after dry immersion ( p,0.005). A significant diastolic blood pressure fall at 5th min of 608 HUT was observed in five orthostatic tolerant subjects despite enough MSNA discharge after dry immersion. A subject suffered from presyncope at 2 min after 608 HUT. He showed gradual blood pressure fall 10 s after 608 HUT with initially well-maintained MSNA response and then with a gradually attenuated MSNA, followed by a sudden MSNA withdrawal and abrupt blood pressure drop. In conclusion, dry immersion increased MSNA without changing MSNA response to orthostasis, and resting HR, while increasing the HR response to orthostasis. Analyses of MSNA and blood pressure changes in orthostatic tolerant subjects and a subject with presyncope suggested that not only insufficient vasoconstriction to sympathetic stimuli, but also a central mechanism to induce a sympathetic withdrawal might play a role in the development of orthostatic intolerance after microgravity exposure. 2000 Elsevier Science B.V. All rights reserved. Keywords: Muscle sympathetic nerve activity; Microneurography; Head-up tilt; Orthostatic intolerance
1. Introduction Exposure to microgravity in space has consistently induced cardiovascular deconditioning in crew members during spaceflight. Buckey et al. (1996) reported that 64% of astronauts suffered from orthostatic intolerance after spaceflight, which has also been observed after exposure to ground-based simulated microgravity (Gharib et al., 1992; Rowell, 1993; Lacolley et al., 1993; Fortney et al., 1996; Johansen et al., 1997). In both cases, the mechanisms have *Corresponding author. Tel.: 181-52-789-3883; fax: 181-52-7895047. E-mail address: [email protected] (S. Iwase)
not been well clarified. However, several possible mechanisms have been postulated to explain this phenomenon; i.e. circulatory blood volume loss (Blomqvist and Stone, 1983; Fortney et al., 1996; Watenpaugh and Hargens, 1996), impaired baroreflex sensitivity (Convertino et al., 1990; Fritsch et al., 1994), attenuated peripheral vasoconstriction (Mulvagh et al., 1991; Buckey et al., 1996, Fritsch-Yelle et al., 1996), alterations in autonomic function (Blomqvist and Stone, 1983; Fritsch et al., 1994; Whitson et al., 1995), and / or increased venous compliance enforced by decreased muscle tone (Korijak and Kozlovskaya, 1991; Ploutz-Snyder et al., 1995). The plasma volume loss after microgravity exposure has been suggested to be the main cause of cardiovascular
0165-1838 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0165-1838( 99 )00076-4
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deconditioning. However, restoration of plasma volume by saline ingestion did not completely rectify the orthostatic intolerance (Blomqvist et al., 1980). Recent studies have indicated that attenuated peripheral vasoconstriction might be related to orthostatic hypotension after microgravity exposure (Goldstein et al., 1995), while how vasomotor sympathetic nerve activity is altered after microgravity exposure remains unknown. Direct measurement of sympathetic nerve activity by microneurography would be beneficial for clarification of this issue. Our previous studies suggested that muscle sympathetic nerve activity (MSNA) might be suppressed under short-term microgravity as demonstrated by studies of head-up tilt (HUT) (Iwase et al., 1987, 1991), head-out water immersion (Mano et al., 1985; Saito et al., 1986; Miwa et al., 1996), and parabolic flight (Iwase et al., 1999). Meanwhile, MSNA recording before and after simulated microgravity of longer duration would provide a suitable model of MSNA alterations after exposure to actual microgravity environment in space. Dry immersion is a method of simulation of microgravity as a ground-based experiment by dipping the human body in thermoneutral water covered with waterproof cloth (Shulzhenko and Vil-Vilyams, 1976; Nicogossian, 1993). The principle of dry immersion to simulate microgravity is the same as head-out water immersion in that transmural hydrostatic pressure centralizes the body fluid and buoyancy reduces activities of antigravity muscles (Mano et al., 1985; Saito et al., 1986; Miwa et al., 1996), but longer durations of immersion would be possible for dry immersion from the hygienic point of view (Nicogossian, 1993). Moreover, dry immersion may have more accelerated effects to simulate microgravity on the human body than head-down bedrest (Shulzhenko and Vil-Vilyams, 1976). In the present study, employing the dry immersion method, we compared the sympathetic nerve activity to muscle as well as blood pressure in humans before and after exposure to simulated microgravity. We loaded orthostatic stress using HUT to 308 and 608 before and after dry immersion, to elucidate the mechanism responsible for orthostatic intolerance in humans.
2. Methods
2.1. Subjects Six healthy Russian men aged 21–36 years old participated in the study. Their age, height, and body weight before and after 3 days of dry immersion are listed in Table 1. They were all normotensive without any cardiovascular, pulmonary, or kidney disease. They were informed about the procedure and risks of the experiment, and consent was obtained from each subject. The protocol was approved by the Ethical Committee on Human
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Table 1 Subject
Age
Gender
Height (cm)
Weight (kg)
Post-weight (kg)
Difference (kg)
1 2 3 4 5 6 Mean SD SE
36 35 35 23 36 21 31 7.014 2.864
Male Male Male Male Male Male
173 178 171 180 167 185 175.7 6.6 2.7
66.8 72.1 59.5 88.3 67.9 66.8 70.2 9.72 3.97
67.3 70.1 58.6 87.0 67.3 65.6 69.3 9.48 3.87
0.45 22.07 20.92 21.30 20.57 21.25 20.94 0.84 0.34
Research, National Space Development Agency of Japan, and Ethical Committee of Institute of Biomedical Problems, State Scientific Center of Russia.
2.2. Experimental protocol The experiment was conducted at the Dry Immersion Facility affiliated to Institute of Biomedical Problems, State Scientific Center of Russia, Moscow. The subjects were divided into three groups, and each pair underwent dry immersion for 3 days consecutively. On the 1st experimental day, Subjects 1 and 2 performed pre-immersion tilt test with MSNA recording. On the 2nd experimental day, they were exposed to dry immersion for 72 h. On the 5th experimental day, they finished dry immersion, moved to the tilt bed with remaining in the supine position, and performed the post-immersion tilt test. Subjects 3, 4, 5 and 6 underwent the experiment in the same fashion.
2.3. Dry immersion Dry immersion is a method of simulating microgravity by immersing the human body into thermoneutral water covered with waterproof cloth. Throughout the experiment, the water temperature was maintained at 34.561.08C (Fig. 1), and the subject was immersed in the water with the waterproof cloth floating freely in the thermoneutral water. During the immersion, the subject receives no support, which makes the environment absolutely different from that during head-down bedrest where there exists a support always (Shulzhenko and Vil-Vilyams, 1976). At initiation of the experiment, the subject in supine position was immersed into the water separated by the waterproof cloth. Since the cloth was thin and large enough to render the hydrostatic pressure without loss, the subject received the same effect as the head-out water immersion, although his body was not wet. Dry immersion is thus better from the viewpoint of hygiene as compared with head-out water immersion up to the neck, and enables long exposure for up to two weeks of simulated microgravity. The subjects were allowed to get out of the dry immersion tank for shower and defecation in every morn-
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Fig. 1. Experimental Set-up. MSNA was recorded microneurographically with ECG and blood pressure wave. Orthostatic test was performed by means of HUT before and after dry immersion for 3 days.
ing for 15 min, called morning quarter. All these procedures were carried out in supine position except this morning quarter.
2.4. HUT and measurements of variables Before and after dry immersion, subjects were challenged by orthostatic test with a tilt bed. Briefly, they lay on a bed in the supine position. Postganglionic efferent sympathetic nerve activity innervating the triceps surae muscle was recorded by microneurography. A tungsten microelectrode ([26-05-1, Frederic Haer, Brunswick, ME) with a tip diameter of 1 mm, a shaft diameter of 100 mm, and an impedance of 3–5 MV was inserted manually and percutaneously without anesthesia into the muscle nerve fascicle of the tibial nerve at the popliteal fossa. The sympathetic nerve signals were fed into a high impedance preamplifier (Kohno III, Kohno Instruments, Nagoya, (2,000 in gain) with band-pass filtration of 500–5,000 Hz. The filtered signals were rectified, amplified, and integrated with a time constant of 0.1 sec, and displayed on a digital oscilloscope (VC 6023, Hitachi, Tokyo). MSNA was identified by the criteria described by Mano (1990, 1998): 1) pulse-synchronous spontaneous and rhythmic efferent burst discharges recorded from muscle nerve fascicle; 2) modulation by respiration; 3) increase by a fall and decrease by a rise in systemic blood pressure; and 4) enhancement by maneuvers increasing intrathoracic pressure such as Valsalva’s maneuver. Electrocardiogram (ECG) with bipolar chest leads for heart rate (HR) and blood pressure wave with a Finapres device (Model 2300, Ohmeda, Louisville, CO) were
simultaneously recorded with the finger cuff being fixed at the level of the right atrium. Intermittent measurement of cuff blood pressure was also monitored at the upper arm by an oscillometric method with an autosphygmomanometer (BP-203NP, Nippon Colin Co. Ltd., Komaki, Japan). All variables were stored in a multi-channel digital audiotape (DAT) recorder (PC-216Ax, Sony Precision Technology, Tokyo, Japan) for later off-line analysis. Head-up tilt (HUT) was initiated within 2 hours after termination of dry immersion with the subjects kept in the horizontal supine position. After 15 minutes to obtain baseline measurements, the bed was tilted to 308 (308 HUT), and recordings were made at this position for 5 min, then the bed was tilted up to 608 (608 HUT) with recordings of variables. If the subject exhibited presyncopal symptoms including pallor, cold sweating, nausea, grayout or dizziness, with cardiovascular symptoms satisfying the following criteria such as progressive reduction in systolic blood pressure (SBP) .80 mm Hg, sudden systolic pressure fall .15 mm Hg, or sudden bradycardia .15 min 21 , the tilt was terminated, and the subject was laid down horizontally (Engelke et al., 1996).
2.5. Data analysis Recordings of MSNA, ECG, and blood pressure wave were visualized on a thermal array recorder (Nihon Kohden WS682G) and a thermal pen recorder (NEC Medical Systems, Recti-Horiz). All data were digitized with Spike-2 software (Cambridge Electronic Design Ltd., Cambridge, UK) and a microcomputer (Power Macintosh
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7500 / 100, Apple, Cupertino, CA). MSNA was quantified as the burst number per minute (burst rate) by visual inspection of the full-wave rectified and integrated neurogram traces at the time constant of 0.1 sec with sound monitoring on the thermal pen recorder. MSNA was quantified as burst rate during supine rest (08), 308, and 608 HUT positions. In addition to burst rate, burst incidence was calculated as the burst number per 100 heartbeats (i.e. [burst rate / HR]3(100) in order to assess the central sympathetic outflow. Responsiveness of MSNA burst rate and HR to HUT was quantified as the slope of the regression line between the sine function of HUT angle and burst rate (Iwase et al., 1987, 1991) and HR, respectively, of the designated tilt angles. Data are expressed as means 6S.E., and paired-t test was employed for the comparison of the data between preand post-immersion. SBP and DBP and HR were examined before and after 3 days of dry immersion every 2 min by paired t-test. Statistical analyses were performed using StatView 5.0 (SAS) on a Power Macintosh G3 with p values less than 0.05 considered statistically significant.
3. Results All of the six subjects who participated in the present study completed 3 days of dry immersion and pre- and post-immersion tilt studies. One subject (Subject 1) suffered from presyncopal symptoms 2 min after being tilted up to 608. The data after the presyncopal attack were excluded from the statistical analysis. The subjects showed weight loss of 1.2260.56 kg when Subject 1 was excluded from the data. Only Subject 1 gained 0.45 kg after 3 days of dry immersion.
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3.1. Effects of 3 days of dry immersion on resting MSNA and HR MSNA in the supine resting position was significantly enhanced (Fig. 2) from a burst rate of 23.763.2 (preimmersion) to 40.963.0 (post-immersion) bursts / min ( p5 0.0172, Fig. 3, left). HR was not changed significantly (pre: 64.362.7, vs. post: 68.166.3 min 21 , Fig. 3, right), so that burst incidence was increased from 38.763.8% to 61.866.1% ( p,0.01). Although only the subject 1 gained weight, there were no correlations between body weight loss and percent changes in MSNA or MSNA differences between pre- and post immersion.
3.2. Effects of 3 days of dry immersion on MSNA, HR, and blood pressure responses to HUT MSNA burst rate at 308 was significantly increased from 38.961.4 to 51.562.2 bursts / min, and that at 608 was also significantly increased from 42.862.0 to 57.663.4 bursts / min (Fig. 3, left) after 3 days of dry immersion. HR at 308 and 608 was significantly increased as compared to preimmersion data (67.063.0 to 80.168.1 min 21 at 308, p50.05, and 78.264.4 to 96.7610.8 min 21 at 608, p5 0.035, Fig. 3, right) MSNA responsiveness to HUT exhibited no significant difference between pre- and postimmersion (22.563.5 vs. 19.461.5 bursts min 21 sin[08→908] 21 ). HR response to HUT was significantly augmented after dry immersion (from 15.463.4 to 32.565.2 bursts min 21 sin[08→908] 21 , p,0.005). SBP at supine rest before HUT, 308 HUT, 608 HUT, and recovery rest positions exhibited no significant difference between pre- and post-immersion data. DBP showed a
Fig. 2. Changes in MSNA, ECG, and arterial blood pressure wave before and after dry immersion for 3 days in an orthostatic tolerant subject. Apparent enhancement was observed after dry immersion as compared with the pre-immersion data.
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Fig. 3. Changes in MSNA (left) and ECG (right) during HUT before (s) and after (d) dry immersion for 3 days. MSNA was significantly enhanced at all tilt angles during HUT, while responsiveness to orthostasis exhibited no significant changes after 3 days of dry immersion (left). Resting HR showed no significant changes after dry immersion, while at 308 and 608 heart rate was significantly higher and the responsiveness to orthostasis was augmented after dry immersion for 3 days (right).
significant difference in post-immersion data at 5th min of 608 HUT ( p,0.05) without any significant differences in other time course (Fig. 4). MSNA changes exhibited no significant differences during the time course from the 1st
to 5th min of 608 HUT both in pre- and post-immersion MSNA in spite of a significant fall in diastolic blood pressure after 3 days of dry immersion, although significant differences were observed between pre- and post-
Fig. 4. Changes in SPB and DBP during positional change by HUT before (s) and after (d) dry immersion for 3 days. Effects of 3 days of dry immersion on SBP and DBP exhibited significant differences at the 5th min of 608 HUT in diastolic pressure ( p,0.05).
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immersion MSNA in all over the time course from 1st to 5th min of 608 HUT (Fig. 5).
discharge reappeared in |2 min with gradual recovery of HR and blood pressure.
3.3. Change in HR, blood pressure, and MSNA during presyncopal attack
4. Discussion
Subject 1 suffered from presyncopal attack at 2 min during 608 HUT. His instantaneous HR, arterial blood pressures with Finapres device, and integrated MSNA during this attack are displayed in Fig. 6. There was a gradual blood pressure fall at 10 s after being tilted up to 608 in spite of well-maintained MSNA discharge. Then the MSNA bursts were gradually attenuated during this hypotensive phase with lowered burst amplitude, but without significant change in HR. At 58 s after being tilted up to 608, the blood pressure fell abruptly with symptoms of cold sweating and grayout accompanied by a sudden MSNA withdrawal, but without HR reduction .20 min 21 . The subject was immediately laid down in a horizontal position, and MSNA withdrawal continued for |2 min with relative bradycardia |50 min 21 . The MSNA
The results of the present study can be summarized as follows: after 3 days of dry immersion, (1) five subjects completed the orthostatic test, but one suffered from orthostatic intolerance, (2) resting MSNA in burst rate as well as burst incidence was increased; (3) MSNA response to HUT was not changed; (4) HR response to HUT was augmented; (5) a significant diastolic blood pressure fall was observed in the subjects with good orthostatic tolerance at the 5th min of 608 HUT in spite of unaltered MSNA; and (6) in a presyncopal subject, blood pressure fell followed by gradual decrease to withdrawal of MSNA with a slight but not marked bradycardia during hypotensive attack. These results indicate that even in orthostatic tolerant subjects, reduction in diastolic blood pressure developed with orthostatic positional changes, while responsiveness of vasoconstrictive sympathetic outflow to muscle was not significantly changed and HR responsiveness was significantly augmented. In orthostatic intolerant subject, reduced MSNA response to HUT seems to be crucial to the presyncopal hypotensive attack.
4.1. Dry immersion as a means of simulated microgravity
Fig. 5. Changes in SBP and DBP pressures and MSNA burst rate before (h) and after (d) 3 days of dry immersion. A significant DBP fall was observed at the 5th min of 608 HUT, while MSNA remained constant, exhibiting higher activity than that before 3 days of dry immersion.
Dry immersion seems to be advantageous as a method of microgravity simulation from the viewpoint of accelerated cardiovascular changes and comfort in the initial period just entering the simulated microgravity as well as from the viewpoint of hygiene (Shulzhenko and Vil-Vilyams, 1976; Nicogossian, 1993). The fact that only 3 days of exposure to dry immersion caused the above changes evidenced the accelerated influence of this method on cardiovascular functions. The present results — an increase in MSNA after simulated microgravity — are compatible with those observed previously, after 6 (Mano et al., 1998; Kamiya et al., 1999a), 14 (Kamiya et al., 1999b), and 120 (Mano et al., 1998; Kamiya et al. 1999c) days of head-down bedrest. However, our results were in contrast to those reported by Shoemaker et al. (1998). The difference between our results and those of Shoemaker et al. might have been because their subjects were encouraged to drink 2 l of water per day while in our study subjects were allowed water ad libitum. Accordingly, it is likely that MSNA might be enhanced after simulated microgravity when water intake is organized free. In our previous studies of simulated microgravity, MSNA was suppressed just after entering into the microgravity (Mano et al., 1985; Miwa et al., 1996; Iwase et al., 1999), but MSNA soon recovered to 40-50% of the pre-immersion level within 2 1 / 2 hrs (Saito et al., 1986).
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Fig. 6. Changes in instantaneous HR, blood pressure wave (Finapres) and MSNA (integrated) in a subject with presyncopal attack. The subject showed gradual blood pressure fall at 10 s of 608 HUT with initially well maintained MSNA response, followed by a gradual decrease in MSNA discharge without significant change in HR. Upon presyncopal attack at 58 s of HUT, sudden blood pressure drop was observed simultaneously with sympathetic withdrawal that continued for approximately 2 min, when MSNA gradually reappeared.
Therefore, it is estimated that this recovery might continue for a few days of simulated microgravity, and MSNA might be enhanced after 3 days of dry immersion.
4.2. Effects of 3 days of dry immersion on resting MSNA and responsiveness to HUT MSNA was significantly enhanced after 3 days of dry immersion. One possible mechanism for this sympathoexcitation is circulatory blood volume loss (Blomqvist et al., 1980; Fritsch et al., 1994; Watenpaugh and Hargens, 1996). Since dry immersion induces centralization of body fluid by hydrostatic pressure on the limbs, neural and humoral mechanisms may facilitate diuresis to reduce the intrathoracic blood volume (Gauer and Henry, 1963). Although we could not confirm the increase in intrathoracic blood volume by hydrostatic pressure and subsequent gradual blood volume loss, we assume that the changes in body weight might be attributable largely to dehydration. However, since there was no correlation between body weight loss and MSNA enhancement, not only dehydration but also other factors might contribute for the post-immersion sympathoexcitation. Another possibility of this post-immersion sympathoexcitation would be an attenuated vasoconstrictive response to sympathetic vasomotor activity (Robertson et al., 1994; Convertino et al., 1997). Sensitivity of peripheral a1 -adrenertic receptor might be altered by the simulated microgravity, which in turn enhanced resting MSNA via baroreflex. This at-
tenuated vasomotor response-induced sympathoexcitation was also observed in the experiment that enough dose of a1 -adrenoceptive blocking agent, prazosin hydrochloride enhanced resting MSNA without changing resting heart rate (Iwase et al., 1988). The regression line between the sine function of tilt angle and MSNA burst rate was shifted upward parallelly after dry immersion, suggesting no significant change in MSNA response to HUT. The lack of significant changes in MSNA response to HUT indicates unaltered sensitivity of cardiopulmonary low pressure receptors since MSNA enhancement by orthostatic postural change was reported to be attributable mainly to these receptors (Burke et al., 1977). The parallel upward shift of regression lines was also induced by the administration of enough doses of prazosin hydrochloride (Iwase et al., 1988). This also suggests that exposure to microgravity causes the similar changes in MSNA like a-blocker administration, which attenuates the vasoconstrictive action of MSNA. We have previously reported that the resting MSNA and response to HUT have a close relationship to orthostatic tolerance (Iwase et al., 1994). These relationships must be further investigated in future studies, possibly by dry immersion for longer periods.
4.3. Blood pressure fall in non-fainters at 608 HUT Of total six subjects examined in the present study, one suffered from presyncopal attack, while the other five
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subjects completed each 5-min period of 308 and 608 HUT. However, a significant diastolic blood pressure drop was observed at the end of 608 HUT after 3 days of dry immersion even in the average of nonfainters. In spite of this blood pressure fall at the 5th min of 608 HUT, MSNA in nonfainters showed no significant changes. This suggests that exposure to simulated microgravity employing dry immersion for 3 days induces insufficient response of resistant vessels to vasoconstrictive sympathetic nerve activity to muscles even in orthostatic tolerant subjects. The mechanism responsible for the drop in diastolic blood pressure at 608 HUT after 3 days of dry immersion despite preserved MSNA response as well as augmented HR response to HUT was not yet clear, but there are several possible explanations as follows. First, adaptation to simulated microgravity might reduce the sensitivity of vessels to sympathetic vasoconstrictive outflow through either alteration in a -adrenoreceptor sensitivity of vessels or the sensitivity of the vascular wall to a given stimulus (Robertson et al., 1994; Sayet et al., 1995; Watenpaugh and Hargens, 1996; Convertino et al., 1997). Second, the exposure to simulated microgravity could produce a reduction in the release of norepinephrine from sympathetic terminals (Goldstein et al., 1995). Third, the sympathetic regulation on various vascular beds may differ according to regional differentiation of the adrenergic receptors. Animal studies supported this hypothesis in that decreased sympathetic outflow to the spleen and heart, but increased outflow to the soleus muscle was observed in rats after 14-day tail suspension (Woodman et al., 1997). These factors in combination might be related to the drop in diastolic blood pressure after dry immersion.
4.4. Presyncopal attack at 608 head-up tilt in Subject 1 In addition to this vasoconstrictive failure, insufficient vasoconstricitve response to orthostasis was also evidenced in the presyncopal subject. He exhibited gradually attenuated MSNA discharge following gradual blood pressure decrease without marked bradycarida just before the presyncopal attack. This gradual blood pressure fall without marked bradycardia was often observed in autonomic failure patients (Grubb, 1998) with insufficient vasoconstriction in spite of blood pressure fall. This may be attributable to lack of baroreflex control or primary reduction in the central outflow in vasoconstrictive sympathetic nerve activity. Therefore, although hypotensive response to orthostasis might be due to attenuated peripheral vascular response, the baroreflex and / or central mechanism may also play a role in developing the postflight orthostatic intolerance. The mechanism for the presyncopal attack should be separately considered from the insufficient vasoconstrictive response at 608 HUT in orthostatic tolerant subject. In the attack, the blood pressure dropped abruptly accompanied by MSNA withdrawal without marked bradycardia. This
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also indicates the regional differentiation sympathetic nervous system between muscle and cardiac sympathetic nerve activities. It should be mentioned that this sympathetic withdrawal is not due to the dislocation of the electrode because MSNA re-appeared 2 min after the onset of presyncopal attack, but possibly due to a transient withdrawal of central vasoconstrictor outflow (Wallin and ¨ 1982). Sundlof,
4.5. Conclusion The present study revealed that dry immersion increased MSNA without changing MSNA response to orthostasis, and resting HR, while the response of HR to orthostasis was increased significantly. Insufficient vasoconstriction in spite of enhanced MSNA is estimated to induce blood pressure fall even in orthostatic tolerant subjects after exposure to dry immersion. Analysis of the presyncopal attack suggested that not only insufficient vasoconstriction to sympathetic stimuli, but also a central mechanism to induce a sympathetic withdrawal might play a role in the development of orthostatic intolerance after exposure to microgravity.
Acknowledgements We are grateful to Mr. Michiyuki Kohno for technical assistance and to Dr. Olga L. Vinogradova for helpful comments. This study was supported by a Grant-in Aid for scientific Research (International Scientific Research Program; 07045048) from the Ministry of Education, Science, Sports and Culture of Japan.
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Maisonblanche, P., Duchier, J.L., Levy, B.I., Safar, M.E., 1993. Microgravity and orthostatic intolerance: carotid hemodynamics and peripheral responses. Am. J. Physiol 264, H588–H594. Mano, T., 1990. Sympathetic nerve mechanisms of human adaptation to environment. Findings obtained by recent microneurographic studies. Environ. Med. 34, 1–35. Mano, T., 1998. Microneurographic research on sympathetic nerve responses to environmental stimuli in humans. Jpn. J. Physiol. 48, 99–114. Mano, T., Iwase, S., Yamazaki, Y., Saito, M., 1985. Sympathetic nervous adjustments in man to simulated weightlessness induced by water immersion. J. Univ., Occupational Environ. Health 7, 215–227, suppl. Mano, T., Iwase, S., Kamiya, A., 1998. Sympathetic nerve responses in humans to short and long term simulation of microgravity. J. Gravitational Physiol. 5, P-93–96. Miwa, C., Mano, T., Saito, M., Iwase, S., Matsukawa, T., Sugiyama, Y., Koga, K., 1996. Ageing reduces sympatho-suppressive response to head-out water immersion in humans. Acta. Physiol. Scand. 158, 15–20. Mulvagh, S.L., Charles, J.B., Riddle, J.M., Rehbein, T.L., Bungo, M.W., 1991. Echocardiographic evaluation of the cardiovascular effects of short-duration spaceflight. J. Clin. Pharmacol. 31, 1024–1026. Nicogossian, A.E., 1993. Microgravity simulations and analogues. In: Nicogossian, A.E., Huntoon, C.L., Pool, S.L. (Eds.), Space Physiology and Medicine, 3rd Edn., Chapter 20, Lea and Febiger, Philadelphia, 363–371. Ploutz-Snyder, L.L., Convertino, V.A., Dudley, G.A., 1995. Resistance exercise-induced fluid shifts: change in active muscle size and plasma volume. Am. J. Physiol. 269, R536–R543, Regulatory Integrative Comp. Physiol. 38. Robertson, D., Convertino, V.A., Vernikos, J., 1994. The sympathetic nervous system and physiologic consequences of spaceflight: a hypothesis. Am. J. Med. Sci. 308, 126–132. Rowell, L.B., 1993. Human Cardiovascular Control. New York, NY: Oxford University Press, 3–161. Saito, M., Mano, T., Iwase, S., Koga, K., Matsukawa, T., 1986. Sympathetic nervous responses in man to weightlessness simulated by head-out water immersion. In: Watanabe, S., Mitarai, G., Mori, S. (Eds.), Biological Sciences in Space 1986, My Research, Tokyo, 85–92. Sayet, I., Neuilly, G., Mironneau, J., Mironneau, C., 1995. Influence of spaceflight, hindlimb suspension, and venous occlusion on (1-adrenoreceptors in rat vena cava. J. Appl. Physiol. 78, 1882–1888. Shoemaker, J.K., Hogeman, C.S., Leuenberger, U.A., Herr, M.D., Gray, K., Silber, D.H., Sinoway, L.I., 1998. Sympathetic discharge and vascular resistance after bed rest. J. Appl. Physiol. 84, 612–617. Shulzhenko, F.B., Vil-Vilyams, I.F., 1976. Cardiovascular reactions in 56-day immersion in Combination with counter measures. In: Proceedings of XIth K.E. Tziolkovski readings, section ,,Space Medicine Problems.., 153–159, in Russian. ¨ G., 1982. Sympathetic outflow to muscles during Wallin, B.G., Sundlof, vasovagal syncope. J. Auton. Nerv. Syst. 287–291. Watenpaugh, D.E., Hargens, A.R., 1996. The Cardiovascular system in microgravity. In: Fregly, M.J., Blatteis, C.M. (Eds.), Handbook of physiology: Section 4: Environmental physiology. Vol. I., Chap. 29. Oxford Univ. Press, New York, NY, pp. 631–674. Whitson, P.A., Charles, W.J., Cintron, N.M., 1995. Changes in Sympathoadrenal response to standing in humans after spaceflight. J. Appl. Physiol. 79, 428–433. Woodman, C.R., Kregel, K.C., Tipton, C.M., 1997. Influence of simulated microgravity on the sympathetic response to exercise. Am. J. Physiol. 272, R570–R575, Regulatory Integrative Comp. Physiol. 41.
Effects of three days of dry immersion on muscle sympathetic nerve activity and arterial blood pressure in humans a,
a ,b
a ,c
a
a
Satoshi Iwase *, Yoshiki Sugiyama , Chihiro Miwa , Atsunori Kamiya , Tadaaki Mano , Yoshinobu Ohira d , Boris Shenkman e , Anatoly I. Egorov e , Inessa B. Kozlovskaya e a
Department of Autonomic Neuroscience, Research Institute of Environmental Medicine, Nagoya University, Furo-cho, Chiksa-ku, Nagoya 461 -8673, Japan b Department of Health and Psychosocial Medicine, Aichi Medical University, Nagakute, Aichi 480 -1195, Japan c Department of Occupational Therapy, School of Health Sciences, Nagoya University School of Medicine, Nagoya University, Higashi-ku, Nagoya 461 -8673, Japan d National Institute of Fitness and Sports, 1 Shiromuzu, Kanoya, Kagoshima 891 -2311, Japan e Institute of Biomedical Problems, State Scientific Center, Khoreshevskoye Shosse 76 A, Moscow 123007, Russia Received 19 August 1999; received in revised form 4 October 1999; accepted 4 October 1999
Abstract The present study was performed to determine how sympathetic function is altered by simulated microgravity, dry immersion for 3 days, and to elucidate the mechanism of post-spaceflight orthostatic intolerance in humans. Six healthy men aged 21–36 years old participated in the study. Before and after the dry immersion, subjects performed head-up tilt (HUT) test to 308 and 608 (5 min each) with recordings of muscle sympathetic nerve activity (MSNA, by microneurography), electrocardiogram, and arterial blood pressure (Finapres). Resting MSNA was increased after dry immersion from 23.763.2 to 40.963.0 bursts / min ( p,0.005) without significant changes in resting heart rate (HR). MSNA responsiveness to orthostasis showed no significant difference but HR response was significantly augmented after dry immersion ( p,0.005). A significant diastolic blood pressure fall at 5th min of 608 HUT was observed in five orthostatic tolerant subjects despite enough MSNA discharge after dry immersion. A subject suffered from presyncope at 2 min after 608 HUT. He showed gradual blood pressure fall 10 s after 608 HUT with initially well-maintained MSNA response and then with a gradually attenuated MSNA, followed by a sudden MSNA withdrawal and abrupt blood pressure drop. In conclusion, dry immersion increased MSNA without changing MSNA response to orthostasis, and resting HR, while increasing the HR response to orthostasis. Analyses of MSNA and blood pressure changes in orthostatic tolerant subjects and a subject with presyncope suggested that not only insufficient vasoconstriction to sympathetic stimuli, but also a central mechanism to induce a sympathetic withdrawal might play a role in the development of orthostatic intolerance after microgravity exposure. 2000 Elsevier Science B.V. All rights reserved. Keywords: Muscle sympathetic nerve activity; Microneurography; Head-up tilt; Orthostatic intolerance
1. Introduction Exposure to microgravity in space has consistently induced cardiovascular deconditioning in crew members during spaceflight. Buckey et al. (1996) reported that 64% of astronauts suffered from orthostatic intolerance after spaceflight, which has also been observed after exposure to ground-based simulated microgravity (Gharib et al., 1992; Rowell, 1993; Lacolley et al., 1993; Fortney et al., 1996; Johansen et al., 1997). In both cases, the mechanisms have *Corresponding author. Tel.: 181-52-789-3883; fax: 181-52-7895047. E-mail address: [email protected] (S. Iwase)
not been well clarified. However, several possible mechanisms have been postulated to explain this phenomenon; i.e. circulatory blood volume loss (Blomqvist and Stone, 1983; Fortney et al., 1996; Watenpaugh and Hargens, 1996), impaired baroreflex sensitivity (Convertino et al., 1990; Fritsch et al., 1994), attenuated peripheral vasoconstriction (Mulvagh et al., 1991; Buckey et al., 1996, Fritsch-Yelle et al., 1996), alterations in autonomic function (Blomqvist and Stone, 1983; Fritsch et al., 1994; Whitson et al., 1995), and / or increased venous compliance enforced by decreased muscle tone (Korijak and Kozlovskaya, 1991; Ploutz-Snyder et al., 1995). The plasma volume loss after microgravity exposure has been suggested to be the main cause of cardiovascular
0165-1838 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0165-1838( 99 )00076-4
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deconditioning. However, restoration of plasma volume by saline ingestion did not completely rectify the orthostatic intolerance (Blomqvist et al., 1980). Recent studies have indicated that attenuated peripheral vasoconstriction might be related to orthostatic hypotension after microgravity exposure (Goldstein et al., 1995), while how vasomotor sympathetic nerve activity is altered after microgravity exposure remains unknown. Direct measurement of sympathetic nerve activity by microneurography would be beneficial for clarification of this issue. Our previous studies suggested that muscle sympathetic nerve activity (MSNA) might be suppressed under short-term microgravity as demonstrated by studies of head-up tilt (HUT) (Iwase et al., 1987, 1991), head-out water immersion (Mano et al., 1985; Saito et al., 1986; Miwa et al., 1996), and parabolic flight (Iwase et al., 1999). Meanwhile, MSNA recording before and after simulated microgravity of longer duration would provide a suitable model of MSNA alterations after exposure to actual microgravity environment in space. Dry immersion is a method of simulation of microgravity as a ground-based experiment by dipping the human body in thermoneutral water covered with waterproof cloth (Shulzhenko and Vil-Vilyams, 1976; Nicogossian, 1993). The principle of dry immersion to simulate microgravity is the same as head-out water immersion in that transmural hydrostatic pressure centralizes the body fluid and buoyancy reduces activities of antigravity muscles (Mano et al., 1985; Saito et al., 1986; Miwa et al., 1996), but longer durations of immersion would be possible for dry immersion from the hygienic point of view (Nicogossian, 1993). Moreover, dry immersion may have more accelerated effects to simulate microgravity on the human body than head-down bedrest (Shulzhenko and Vil-Vilyams, 1976). In the present study, employing the dry immersion method, we compared the sympathetic nerve activity to muscle as well as blood pressure in humans before and after exposure to simulated microgravity. We loaded orthostatic stress using HUT to 308 and 608 before and after dry immersion, to elucidate the mechanism responsible for orthostatic intolerance in humans.
2. Methods
2.1. Subjects Six healthy Russian men aged 21–36 years old participated in the study. Their age, height, and body weight before and after 3 days of dry immersion are listed in Table 1. They were all normotensive without any cardiovascular, pulmonary, or kidney disease. They were informed about the procedure and risks of the experiment, and consent was obtained from each subject. The protocol was approved by the Ethical Committee on Human
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Table 1 Subject
Age
Gender
Height (cm)
Weight (kg)
Post-weight (kg)
Difference (kg)
1 2 3 4 5 6 Mean SD SE
36 35 35 23 36 21 31 7.014 2.864
Male Male Male Male Male Male
173 178 171 180 167 185 175.7 6.6 2.7
66.8 72.1 59.5 88.3 67.9 66.8 70.2 9.72 3.97
67.3 70.1 58.6 87.0 67.3 65.6 69.3 9.48 3.87
0.45 22.07 20.92 21.30 20.57 21.25 20.94 0.84 0.34
Research, National Space Development Agency of Japan, and Ethical Committee of Institute of Biomedical Problems, State Scientific Center of Russia.
2.2. Experimental protocol The experiment was conducted at the Dry Immersion Facility affiliated to Institute of Biomedical Problems, State Scientific Center of Russia, Moscow. The subjects were divided into three groups, and each pair underwent dry immersion for 3 days consecutively. On the 1st experimental day, Subjects 1 and 2 performed pre-immersion tilt test with MSNA recording. On the 2nd experimental day, they were exposed to dry immersion for 72 h. On the 5th experimental day, they finished dry immersion, moved to the tilt bed with remaining in the supine position, and performed the post-immersion tilt test. Subjects 3, 4, 5 and 6 underwent the experiment in the same fashion.
2.3. Dry immersion Dry immersion is a method of simulating microgravity by immersing the human body into thermoneutral water covered with waterproof cloth. Throughout the experiment, the water temperature was maintained at 34.561.08C (Fig. 1), and the subject was immersed in the water with the waterproof cloth floating freely in the thermoneutral water. During the immersion, the subject receives no support, which makes the environment absolutely different from that during head-down bedrest where there exists a support always (Shulzhenko and Vil-Vilyams, 1976). At initiation of the experiment, the subject in supine position was immersed into the water separated by the waterproof cloth. Since the cloth was thin and large enough to render the hydrostatic pressure without loss, the subject received the same effect as the head-out water immersion, although his body was not wet. Dry immersion is thus better from the viewpoint of hygiene as compared with head-out water immersion up to the neck, and enables long exposure for up to two weeks of simulated microgravity. The subjects were allowed to get out of the dry immersion tank for shower and defecation in every morn-
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Fig. 1. Experimental Set-up. MSNA was recorded microneurographically with ECG and blood pressure wave. Orthostatic test was performed by means of HUT before and after dry immersion for 3 days.
ing for 15 min, called morning quarter. All these procedures were carried out in supine position except this morning quarter.
2.4. HUT and measurements of variables Before and after dry immersion, subjects were challenged by orthostatic test with a tilt bed. Briefly, they lay on a bed in the supine position. Postganglionic efferent sympathetic nerve activity innervating the triceps surae muscle was recorded by microneurography. A tungsten microelectrode ([26-05-1, Frederic Haer, Brunswick, ME) with a tip diameter of 1 mm, a shaft diameter of 100 mm, and an impedance of 3–5 MV was inserted manually and percutaneously without anesthesia into the muscle nerve fascicle of the tibial nerve at the popliteal fossa. The sympathetic nerve signals were fed into a high impedance preamplifier (Kohno III, Kohno Instruments, Nagoya, (2,000 in gain) with band-pass filtration of 500–5,000 Hz. The filtered signals were rectified, amplified, and integrated with a time constant of 0.1 sec, and displayed on a digital oscilloscope (VC 6023, Hitachi, Tokyo). MSNA was identified by the criteria described by Mano (1990, 1998): 1) pulse-synchronous spontaneous and rhythmic efferent burst discharges recorded from muscle nerve fascicle; 2) modulation by respiration; 3) increase by a fall and decrease by a rise in systemic blood pressure; and 4) enhancement by maneuvers increasing intrathoracic pressure such as Valsalva’s maneuver. Electrocardiogram (ECG) with bipolar chest leads for heart rate (HR) and blood pressure wave with a Finapres device (Model 2300, Ohmeda, Louisville, CO) were
simultaneously recorded with the finger cuff being fixed at the level of the right atrium. Intermittent measurement of cuff blood pressure was also monitored at the upper arm by an oscillometric method with an autosphygmomanometer (BP-203NP, Nippon Colin Co. Ltd., Komaki, Japan). All variables were stored in a multi-channel digital audiotape (DAT) recorder (PC-216Ax, Sony Precision Technology, Tokyo, Japan) for later off-line analysis. Head-up tilt (HUT) was initiated within 2 hours after termination of dry immersion with the subjects kept in the horizontal supine position. After 15 minutes to obtain baseline measurements, the bed was tilted to 308 (308 HUT), and recordings were made at this position for 5 min, then the bed was tilted up to 608 (608 HUT) with recordings of variables. If the subject exhibited presyncopal symptoms including pallor, cold sweating, nausea, grayout or dizziness, with cardiovascular symptoms satisfying the following criteria such as progressive reduction in systolic blood pressure (SBP) .80 mm Hg, sudden systolic pressure fall .15 mm Hg, or sudden bradycardia .15 min 21 , the tilt was terminated, and the subject was laid down horizontally (Engelke et al., 1996).
2.5. Data analysis Recordings of MSNA, ECG, and blood pressure wave were visualized on a thermal array recorder (Nihon Kohden WS682G) and a thermal pen recorder (NEC Medical Systems, Recti-Horiz). All data were digitized with Spike-2 software (Cambridge Electronic Design Ltd., Cambridge, UK) and a microcomputer (Power Macintosh
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7500 / 100, Apple, Cupertino, CA). MSNA was quantified as the burst number per minute (burst rate) by visual inspection of the full-wave rectified and integrated neurogram traces at the time constant of 0.1 sec with sound monitoring on the thermal pen recorder. MSNA was quantified as burst rate during supine rest (08), 308, and 608 HUT positions. In addition to burst rate, burst incidence was calculated as the burst number per 100 heartbeats (i.e. [burst rate / HR]3(100) in order to assess the central sympathetic outflow. Responsiveness of MSNA burst rate and HR to HUT was quantified as the slope of the regression line between the sine function of HUT angle and burst rate (Iwase et al., 1987, 1991) and HR, respectively, of the designated tilt angles. Data are expressed as means 6S.E., and paired-t test was employed for the comparison of the data between preand post-immersion. SBP and DBP and HR were examined before and after 3 days of dry immersion every 2 min by paired t-test. Statistical analyses were performed using StatView 5.0 (SAS) on a Power Macintosh G3 with p values less than 0.05 considered statistically significant.
3. Results All of the six subjects who participated in the present study completed 3 days of dry immersion and pre- and post-immersion tilt studies. One subject (Subject 1) suffered from presyncopal symptoms 2 min after being tilted up to 608. The data after the presyncopal attack were excluded from the statistical analysis. The subjects showed weight loss of 1.2260.56 kg when Subject 1 was excluded from the data. Only Subject 1 gained 0.45 kg after 3 days of dry immersion.
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3.1. Effects of 3 days of dry immersion on resting MSNA and HR MSNA in the supine resting position was significantly enhanced (Fig. 2) from a burst rate of 23.763.2 (preimmersion) to 40.963.0 (post-immersion) bursts / min ( p5 0.0172, Fig. 3, left). HR was not changed significantly (pre: 64.362.7, vs. post: 68.166.3 min 21 , Fig. 3, right), so that burst incidence was increased from 38.763.8% to 61.866.1% ( p,0.01). Although only the subject 1 gained weight, there were no correlations between body weight loss and percent changes in MSNA or MSNA differences between pre- and post immersion.
3.2. Effects of 3 days of dry immersion on MSNA, HR, and blood pressure responses to HUT MSNA burst rate at 308 was significantly increased from 38.961.4 to 51.562.2 bursts / min, and that at 608 was also significantly increased from 42.862.0 to 57.663.4 bursts / min (Fig. 3, left) after 3 days of dry immersion. HR at 308 and 608 was significantly increased as compared to preimmersion data (67.063.0 to 80.168.1 min 21 at 308, p50.05, and 78.264.4 to 96.7610.8 min 21 at 608, p5 0.035, Fig. 3, right) MSNA responsiveness to HUT exhibited no significant difference between pre- and postimmersion (22.563.5 vs. 19.461.5 bursts min 21 sin[08→908] 21 ). HR response to HUT was significantly augmented after dry immersion (from 15.463.4 to 32.565.2 bursts min 21 sin[08→908] 21 , p,0.005). SBP at supine rest before HUT, 308 HUT, 608 HUT, and recovery rest positions exhibited no significant difference between pre- and post-immersion data. DBP showed a
Fig. 2. Changes in MSNA, ECG, and arterial blood pressure wave before and after dry immersion for 3 days in an orthostatic tolerant subject. Apparent enhancement was observed after dry immersion as compared with the pre-immersion data.
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Fig. 3. Changes in MSNA (left) and ECG (right) during HUT before (s) and after (d) dry immersion for 3 days. MSNA was significantly enhanced at all tilt angles during HUT, while responsiveness to orthostasis exhibited no significant changes after 3 days of dry immersion (left). Resting HR showed no significant changes after dry immersion, while at 308 and 608 heart rate was significantly higher and the responsiveness to orthostasis was augmented after dry immersion for 3 days (right).
significant difference in post-immersion data at 5th min of 608 HUT ( p,0.05) without any significant differences in other time course (Fig. 4). MSNA changes exhibited no significant differences during the time course from the 1st
to 5th min of 608 HUT both in pre- and post-immersion MSNA in spite of a significant fall in diastolic blood pressure after 3 days of dry immersion, although significant differences were observed between pre- and post-
Fig. 4. Changes in SPB and DBP during positional change by HUT before (s) and after (d) dry immersion for 3 days. Effects of 3 days of dry immersion on SBP and DBP exhibited significant differences at the 5th min of 608 HUT in diastolic pressure ( p,0.05).
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161
immersion MSNA in all over the time course from 1st to 5th min of 608 HUT (Fig. 5).
discharge reappeared in |2 min with gradual recovery of HR and blood pressure.
3.3. Change in HR, blood pressure, and MSNA during presyncopal attack
4. Discussion
Subject 1 suffered from presyncopal attack at 2 min during 608 HUT. His instantaneous HR, arterial blood pressures with Finapres device, and integrated MSNA during this attack are displayed in Fig. 6. There was a gradual blood pressure fall at 10 s after being tilted up to 608 in spite of well-maintained MSNA discharge. Then the MSNA bursts were gradually attenuated during this hypotensive phase with lowered burst amplitude, but without significant change in HR. At 58 s after being tilted up to 608, the blood pressure fell abruptly with symptoms of cold sweating and grayout accompanied by a sudden MSNA withdrawal, but without HR reduction .20 min 21 . The subject was immediately laid down in a horizontal position, and MSNA withdrawal continued for |2 min with relative bradycardia |50 min 21 . The MSNA
The results of the present study can be summarized as follows: after 3 days of dry immersion, (1) five subjects completed the orthostatic test, but one suffered from orthostatic intolerance, (2) resting MSNA in burst rate as well as burst incidence was increased; (3) MSNA response to HUT was not changed; (4) HR response to HUT was augmented; (5) a significant diastolic blood pressure fall was observed in the subjects with good orthostatic tolerance at the 5th min of 608 HUT in spite of unaltered MSNA; and (6) in a presyncopal subject, blood pressure fell followed by gradual decrease to withdrawal of MSNA with a slight but not marked bradycardia during hypotensive attack. These results indicate that even in orthostatic tolerant subjects, reduction in diastolic blood pressure developed with orthostatic positional changes, while responsiveness of vasoconstrictive sympathetic outflow to muscle was not significantly changed and HR responsiveness was significantly augmented. In orthostatic intolerant subject, reduced MSNA response to HUT seems to be crucial to the presyncopal hypotensive attack.
4.1. Dry immersion as a means of simulated microgravity
Fig. 5. Changes in SBP and DBP pressures and MSNA burst rate before (h) and after (d) 3 days of dry immersion. A significant DBP fall was observed at the 5th min of 608 HUT, while MSNA remained constant, exhibiting higher activity than that before 3 days of dry immersion.
Dry immersion seems to be advantageous as a method of microgravity simulation from the viewpoint of accelerated cardiovascular changes and comfort in the initial period just entering the simulated microgravity as well as from the viewpoint of hygiene (Shulzhenko and Vil-Vilyams, 1976; Nicogossian, 1993). The fact that only 3 days of exposure to dry immersion caused the above changes evidenced the accelerated influence of this method on cardiovascular functions. The present results — an increase in MSNA after simulated microgravity — are compatible with those observed previously, after 6 (Mano et al., 1998; Kamiya et al., 1999a), 14 (Kamiya et al., 1999b), and 120 (Mano et al., 1998; Kamiya et al. 1999c) days of head-down bedrest. However, our results were in contrast to those reported by Shoemaker et al. (1998). The difference between our results and those of Shoemaker et al. might have been because their subjects were encouraged to drink 2 l of water per day while in our study subjects were allowed water ad libitum. Accordingly, it is likely that MSNA might be enhanced after simulated microgravity when water intake is organized free. In our previous studies of simulated microgravity, MSNA was suppressed just after entering into the microgravity (Mano et al., 1985; Miwa et al., 1996; Iwase et al., 1999), but MSNA soon recovered to 40-50% of the pre-immersion level within 2 1 / 2 hrs (Saito et al., 1986).
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Fig. 6. Changes in instantaneous HR, blood pressure wave (Finapres) and MSNA (integrated) in a subject with presyncopal attack. The subject showed gradual blood pressure fall at 10 s of 608 HUT with initially well maintained MSNA response, followed by a gradual decrease in MSNA discharge without significant change in HR. Upon presyncopal attack at 58 s of HUT, sudden blood pressure drop was observed simultaneously with sympathetic withdrawal that continued for approximately 2 min, when MSNA gradually reappeared.
Therefore, it is estimated that this recovery might continue for a few days of simulated microgravity, and MSNA might be enhanced after 3 days of dry immersion.
4.2. Effects of 3 days of dry immersion on resting MSNA and responsiveness to HUT MSNA was significantly enhanced after 3 days of dry immersion. One possible mechanism for this sympathoexcitation is circulatory blood volume loss (Blomqvist et al., 1980; Fritsch et al., 1994; Watenpaugh and Hargens, 1996). Since dry immersion induces centralization of body fluid by hydrostatic pressure on the limbs, neural and humoral mechanisms may facilitate diuresis to reduce the intrathoracic blood volume (Gauer and Henry, 1963). Although we could not confirm the increase in intrathoracic blood volume by hydrostatic pressure and subsequent gradual blood volume loss, we assume that the changes in body weight might be attributable largely to dehydration. However, since there was no correlation between body weight loss and MSNA enhancement, not only dehydration but also other factors might contribute for the post-immersion sympathoexcitation. Another possibility of this post-immersion sympathoexcitation would be an attenuated vasoconstrictive response to sympathetic vasomotor activity (Robertson et al., 1994; Convertino et al., 1997). Sensitivity of peripheral a1 -adrenertic receptor might be altered by the simulated microgravity, which in turn enhanced resting MSNA via baroreflex. This at-
tenuated vasomotor response-induced sympathoexcitation was also observed in the experiment that enough dose of a1 -adrenoceptive blocking agent, prazosin hydrochloride enhanced resting MSNA without changing resting heart rate (Iwase et al., 1988). The regression line between the sine function of tilt angle and MSNA burst rate was shifted upward parallelly after dry immersion, suggesting no significant change in MSNA response to HUT. The lack of significant changes in MSNA response to HUT indicates unaltered sensitivity of cardiopulmonary low pressure receptors since MSNA enhancement by orthostatic postural change was reported to be attributable mainly to these receptors (Burke et al., 1977). The parallel upward shift of regression lines was also induced by the administration of enough doses of prazosin hydrochloride (Iwase et al., 1988). This also suggests that exposure to microgravity causes the similar changes in MSNA like a-blocker administration, which attenuates the vasoconstrictive action of MSNA. We have previously reported that the resting MSNA and response to HUT have a close relationship to orthostatic tolerance (Iwase et al., 1994). These relationships must be further investigated in future studies, possibly by dry immersion for longer periods.
4.3. Blood pressure fall in non-fainters at 608 HUT Of total six subjects examined in the present study, one suffered from presyncopal attack, while the other five
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subjects completed each 5-min period of 308 and 608 HUT. However, a significant diastolic blood pressure drop was observed at the end of 608 HUT after 3 days of dry immersion even in the average of nonfainters. In spite of this blood pressure fall at the 5th min of 608 HUT, MSNA in nonfainters showed no significant changes. This suggests that exposure to simulated microgravity employing dry immersion for 3 days induces insufficient response of resistant vessels to vasoconstrictive sympathetic nerve activity to muscles even in orthostatic tolerant subjects. The mechanism responsible for the drop in diastolic blood pressure at 608 HUT after 3 days of dry immersion despite preserved MSNA response as well as augmented HR response to HUT was not yet clear, but there are several possible explanations as follows. First, adaptation to simulated microgravity might reduce the sensitivity of vessels to sympathetic vasoconstrictive outflow through either alteration in a -adrenoreceptor sensitivity of vessels or the sensitivity of the vascular wall to a given stimulus (Robertson et al., 1994; Sayet et al., 1995; Watenpaugh and Hargens, 1996; Convertino et al., 1997). Second, the exposure to simulated microgravity could produce a reduction in the release of norepinephrine from sympathetic terminals (Goldstein et al., 1995). Third, the sympathetic regulation on various vascular beds may differ according to regional differentiation of the adrenergic receptors. Animal studies supported this hypothesis in that decreased sympathetic outflow to the spleen and heart, but increased outflow to the soleus muscle was observed in rats after 14-day tail suspension (Woodman et al., 1997). These factors in combination might be related to the drop in diastolic blood pressure after dry immersion.
4.4. Presyncopal attack at 608 head-up tilt in Subject 1 In addition to this vasoconstrictive failure, insufficient vasoconstricitve response to orthostasis was also evidenced in the presyncopal subject. He exhibited gradually attenuated MSNA discharge following gradual blood pressure decrease without marked bradycarida just before the presyncopal attack. This gradual blood pressure fall without marked bradycardia was often observed in autonomic failure patients (Grubb, 1998) with insufficient vasoconstriction in spite of blood pressure fall. This may be attributable to lack of baroreflex control or primary reduction in the central outflow in vasoconstrictive sympathetic nerve activity. Therefore, although hypotensive response to orthostasis might be due to attenuated peripheral vascular response, the baroreflex and / or central mechanism may also play a role in developing the postflight orthostatic intolerance. The mechanism for the presyncopal attack should be separately considered from the insufficient vasoconstrictive response at 608 HUT in orthostatic tolerant subject. In the attack, the blood pressure dropped abruptly accompanied by MSNA withdrawal without marked bradycardia. This
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also indicates the regional differentiation sympathetic nervous system between muscle and cardiac sympathetic nerve activities. It should be mentioned that this sympathetic withdrawal is not due to the dislocation of the electrode because MSNA re-appeared 2 min after the onset of presyncopal attack, but possibly due to a transient withdrawal of central vasoconstrictor outflow (Wallin and ¨ 1982). Sundlof,
4.5. Conclusion The present study revealed that dry immersion increased MSNA without changing MSNA response to orthostasis, and resting HR, while the response of HR to orthostasis was increased significantly. Insufficient vasoconstriction in spite of enhanced MSNA is estimated to induce blood pressure fall even in orthostatic tolerant subjects after exposure to dry immersion. Analysis of the presyncopal attack suggested that not only insufficient vasoconstriction to sympathetic stimuli, but also a central mechanism to induce a sympathetic withdrawal might play a role in the development of orthostatic intolerance after exposure to microgravity.
Acknowledgements We are grateful to Mr. Michiyuki Kohno for technical assistance and to Dr. Olga L. Vinogradova for helpful comments. This study was supported by a Grant-in Aid for scientific Research (International Scientific Research Program; 07045048) from the Ministry of Education, Science, Sports and Culture of Japan.
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