INTRODUCTION

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HUMANS EXPOSED TO prolonged periods of bedrest or microgravity undergo deconditioning of the cardiovascular system. Microgravity and bedrest result in an immediate central shift of body fluids and a sustained reduction in plasma volume and blood volume (7, 9, 20, 36, 41). Upon resumption of an upright posture or return to gravitational forces, these individuals experience a number of adverse cardiovascular consequences, including resting tachycardia, decreased exercise capacity, and a marked reduction in orthostatic tolerance (3, 5, 6, 17, 23, 24, 36, 41).

Hindlimb unloading (HU) in rodents is an animal model used to simulate cardiovascular deconditioning in humans. The adverse effects on the cardiovascular system due to HU in rats are similar to those experienced by humans after bedrest or microgravity (27, 29, 32). These effects include an initial central shift in fluids, hypovolemia, tachycardia, and reduced exercise capacity (27, 29, 31, 32, 37). Recent evidence also indicates that HU rats experience periods of hypotension and bradycardia during simulated orthostatism (90° head-up rotation; see Ref. 25).

The arterial baroreflexes are important mediators of cardiovascular adjustments to both orthostatic stress and dynamic exercise. Adequate compensation for an orthostatic challenge includes reflex increases in heart rate (HR) and peripheral vascular resistance in an attempt to maintain cardiac output and arterial pressure. The ability to increase peripheral vasoconstriction, elicited primarily through baroreflex-mediated increases in sympathetic nervous system activity, is the most important mechanism utilized to maintain arterial pressure during an orthostatic challenge (34, 43). Dynamic exercise results in increases in HR, cardiac output and arterial pressure, and vasoconstriction in inactive tissue beds. These changes are mediated primarily through increases in sympathetic nervous system activity (2, 34). The arterial baroreflexes contribute importantly to the increase in sympathetic nervous system activity and total peripheral resistance during exercise (15, 30). Because of their important role in cardiovascular adjustments to exercise and to an orthostatic challenge, impairment of baroreflex function could contribute to reductions in both orthostatic tolerance and exercise capacity after cardiovascular deconditioning (19, 34).

Baroreflex control of HR has been examined in human and rodent models of cardiovascular deconditioning (4, 11, 16, 26, 47), with variable results. However, control of peripheral vascular resistance is of primary importance in compensating for an orthostatic challenge and redistributing blood flow during exercise (5, 19, 34). Buckey et al. (5) demonstrated a reduction in the ability of orthostatic-intolerant astronauts to increase peripheral resistance in response to a 10-min stand test after 9-14 days of spaceflight. Therefore, it is reasonable to suggest that cardiovascular deconditioning may alter baroreflex modulation of sympathetic nerve activity (SNA) to the vasculature, thus contributing to orthostatic intolerance and decreased exercise capacity. In addition, changes in baroreflex regulation of the sympathetic nervous system may vary for different vascular beds.

This study was designed to test the hypothesis that cardiovascular deconditioning results in an impairment of arterial baroreflex modulation of sympathetic nervous system activity. It was further hypothesized that HU results in a differential effect on baroreflex control of SNA to the viscera versus skeletal muscle. Therefore, the purpose of this investigation was to determine the effect of HU on baroreflex-mediated changes in HR, renal SNA (RSNA), and lumbar SNA (LSNA) in conscious rats.

	METHODS

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Hindlimb unloading. Male Sprague-Dawley rats (n = 32) obtained from Sasco were randomly assigned to HU or control groups. Both groups were then subdivided into a group for the recording of LSNA and a group for the recording of RSNA. Thus this study contained the following four experimental groups: control LSNA (n = 8), HU LSNA (n = 8), control RSNA (n = 8), and HU RSNA (n = 8). HU rats were acclimated to the unloading procedure by temporarily suspending the hindlimbs for 1-2 h/day for 3 consecutive days before the HU procedure. The hindlimbs of HU rats were then elevated with a harness attached to the proximal two-thirds of the tail by modification of a technique previously described (21). Briefly, two hooks were attached to the tail with moleskin adhesive material. A curved rigid support made of lightweight plastic (X-lite splint; AOA/Kirschner Medical) was placed beneath the tail to allow adequate blood flow. The hooks were connected by a wire to a swivel apparatus at the top of the cage, and the hindlimbs were elevated so there was no contact with supportive surfaces. Rats were maintained in a suspension angle of ~30-35°. A small thoracic cast made from plaster of Paris was applied to reduce lordosis and help prevent the rats from reaching the tail apparatus. Rats received supplemental feedings (~2 g) of peanut butter during the training period and the initial days of suspension as positive reinforcement during the period of adaptation to the unloading procedure. Muscle atrophy and sympathetic nervous system responses of a group of control animals were not altered by similar peanut butter feedings (unpublished observations). Control rats had the thoracic cast applied (n = 14) and were maintained in a normal cage environment. Two animals in the control LSNA group had no cast applied. Animals remained in the HU or control condition for a total of 14 days. This time period was chosen because 1) Spacelab Life Sciences missions have been of similar duration, 2) humans subjected to bedrest exhibit stable changes within this time period (18), and 3) this duration of HU provides stable changes in muscle weight and strength in rats (12, 21, 27, 28). Body weights were recorded before and after the caged control or HU period. During the unloading protocol, the rats were closely monitored several times daily for adequate food and water intake, grooming behavior, and urination and defecation. All rats were housed individually in a temperature-controlled (69-72°F) atmosphere with a 12:12-h light-dark cycle and given rat chow (Purina 5008 rodent chow) and water ad libitum. On the 13th day of the protocol, animals were removed from unloading or cage activity to undergo surgical procedures. HU or control animals were returned to the suspension apparatus or to their cage, respectively, as soon as they had adequately recovered from anesthesia (<1 h). Recovery from anesthesia was assessed by observation of a sternal, alert condition in the animal.

Surgical procedures. All surgical procedures were carried out using aseptic technique under halothane anesthesia. Polyethylene (PE-50 fused to PE-10) catheters were inserted into the aorta and abdominal vena cava via the femoral artery and vein for the measurement of arterial pressure and drug administration, respectively.

Electrodes for recording LSNA were implanted by a modification of a technique previously described (44). Through a midline abdominal incision, the lumbar sympathetic chain was identified immediately caudal to the left kidney. For recording of RSNA, the left kidney was exposed through a retroperitoneal approach, and a sympathetic nerve branch was dissected free (44). In both preparations, two Teflon insulated silver wire electrodes (0.005 in. diameter, 36 gauge; Medwire) threaded through Silastic tubing (0.025 in. ID) were placed around the appropriate isolated sympathetic nerve. Nerves and electrodes were covered with a polyvinylsiloxane gel (Coltene President) that was allowed to harden before closure. A ground wire was sewn to surrounding muscle tissue. Catheters, electrode, and ground wire were routed subcutaneously and exteriorized in the dorsal cervical region. Catheters were filled with heparinized saline (10 U/ml) and capped with an airtight plug. Animals were treated postoperatively with subcutaneous administration of 30 ml saline. They were returned to their cage or unloading apparatus and allowed a 24-h recovery period before any experimental manipulations. At the end of this period, rats were grooming normally and had returned to their normal levels of cage activity.

Experimental procedures. After the 24-h recovery period, HU rats were removed from the suspension apparatus, or control rats were removed from their cage and placed in an experimental cage filled with the animal's own bedding. Animals were removed from unloading and studied in the horizontal position to simulate resumption of upright posture after bedrest or return to a 1-G environment after spaceflight. The experimental cage was placed within a Faraday cage to help reduce electrical noise. The arterial catheter was connected to a pressure transducer for recording of arterial pressure. Mean arterial pressure (MAP) was derived electronically using a low-pass filter. HR was determined with a cardiotachometer, which was triggered from the arterial pressure pulse. For technical reasons, HR could not be reliably determined due to improper triggering by the cardiotachometer in three HU and two control rats.

SNA was amplified 1,000 times using a Grass preamplifier (P511) and filtered using a high-pass frequency level of 30 Hz and a low-pass frequency level of 3 kHz. Action potentials were monitored using a Tektronix oscilloscope and a Grass M8 audio monitor. Nerve activity was rectified and integrated using a root mean square converter with a time constant of 28 ms. The rectified, integrated signal was then electronically averaged, and this mean signal was used as the relative measure of SNA. Background noise was determined when SNA was inhibited to below noise levels by increasing arterial pressure with a bolus dose of phenylephrine (PE, 5-15 µg/kg iv; see Refs. 14, 15, 44).

Experimental protocol. Baseline hemodynamic parameters were recorded for 20-40 min before any experimental manipulations to ensure stabilization of MAP, HR, and LSNA or RSNA. After the equilibration period, arterial baroreflex curves were generated by producing ramp changes in arterial pressure over ~2-3 min. Initially, MAP was increased by infusing PE, an ₁-adrenergic receptor agonist, at increasing rates (2-25 µg · kg¹ · min¹). MAP, HR, and LSNA or RSNA were allowed to return to within 10% of control values (generally within 10 min) before proceeding with the experimental protocol. Arterial pressure then was decreased to 45-55 mmHg within 2-3 min by infusing the vasodilator sodium nitroprusside (SNP) at sequentially increasing rates (10-100 µg · kg¹ · min¹). The rate of change of arterial pressure was held constant by observing the pressure change on the chart recorder and varying the rate of infusion to produce a smooth ramp increase or decrease in pressure. Care was taken to keep the rate of change of arterial pressure similar in all animals at ~1-2 mmHg/s. Volumes infused did not exceed 100 µl. Baroreceptors were always activated first (PE infusion) before unloading (SNP infusion) to minimize any potential effects of reflexly released humoral agents, such as vasopressin or angiotensin II, on baroreflex function.

At the end of the experimental protocol, rats were deeply anesthetized with pentobarbital sodium, and the soleus muscle was removed from the rats and weighed. After muscle removal, rats were euthanized with an overdose of pentobarbital sodium administered through a venous catheter.

Data analysis. HR and SNA were determined at differing levels of MAP during PE and SNP infusion. Data relating changes in HR, LSNA, or RSNA to MAP were fit to a sigmoidal logistic function (22) using a standard software package (Sigma Plot; Jandel Scientific, San Rafael, CA). The equation used for this mathematical model is Parameters (P₁-P₄) that are used to describe basic baroreflex function were generated from data fit to the logistic function. These parameters were 1) the maximum SNA or HR during decreases in arterial pressure (P₁), 2) the coefficient used to calculate the gain as a function of pressure (P₂), 3) the inflection point or MAP at the midpoint of the curve (P₃), and 4) the minimum SNA or HR at an increased arterial pressure (P₄). In addition, the gain (G) of the reflex at each 5-mmHg increment in pressure was calculated for the entire baroreflex curve using the following equation Figure 1 illustrates baroreflex curves relating changes in LSNA to MAP, which were fit to data for a control (Fig. 1A) and HU (Fig. 1B) rat. For each individual animal's fit curve, the four parameters (P₁-P₄) and gain were derived. These parameters and the gain of the baroreflex curve were averaged within a group. The average parameters were then statistically compared (control vs. HU) using independent Student's t-tests. The mean parameters and gain were used to generate an average baroreflex curve for each group. HR, LSNA, RSNA, and gain as a function of MAP were compared using two-way ANOVA. When ANOVA indicated significant primary effects of an intervention or a significant interaction, differences between individual means were assessed by a least-significant difference (LSD) test (40). A probability of P < 0.05 was considered statistically significant. The dotted lines in the figures show values in control animals ± 1 LSD ( = 0.05). If the ANOVA demonstrated significant effects, any points outside the dotted lines should be considered significantly different from casted control group values. All statistical analyses were performed using the Sigma Stat (Jandel Scientific) software package for the IBM.

View larger version (14K): [in this window] [in a new window]   Fig. 1.   Baroreflex curves for a control animal (A) and for a hindlimb-unloaded (HU) animal (B). , Recorded data points; dashed line, fit curve. MAP, mean arterial pressure; LSNA, lumbar sympathetic nerve activity.

Arterial baroreflex curves illustrating the relationship between SNA and MAP were constructed by expressing data as a percentage of baseline or control SNA before changing pressure. Baseline or control SNA was considered to be 100%. This analysis allows for direct evaluation of the animal's ability to reflexly increase or decrease SNA relative to its basal level. In addition, data were expressed relative to the maximal percentage of SNA attained in response to a decrease in pressure. This analysis assesses the level of SNA relative to baroreflex-elicited maximal SNA. In this case, maximum SNA is considered to be 100%.

	RESULTS

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Baseline hemodynamic parameters, muscle weights, and body weights before and after the experimental manipulation of control and HU rats are presented in Table 1. Resting MAP was not altered by HU, whereas HR was significantly higher in the HU animals. There was no statistical difference in the mean body weight of each group, for either the pre- or postexperimental period. However, there was a significant interaction of time and group (control or HU) on body weight. HU rats exhibited a significant loss in body weight (7.8 ± 1.1%) during the 14-day intervention compared with controls (+3.5 ± 2.0%). Soleus muscle weight was significantly less (~43%) in HU rats compared with soleus muscle weight of controls. When expressed relative to body weight, soleus muscle-to-body weight ratio was reduced by 42%. Resting tachycardia (8, 29) and significant atrophy (42) in the soleus muscle confirm the effectiveness of the HU intervention in producing a deconditioned state.

Baroreflex control of HR. The effect of HU on arterial baroreflex modulation of HR is presented in Fig. 2. As indicated in Table 1, HU animals exhibited a significant increase in resting HR and no significant difference in resting MAP. Both groups of rats exhibited reflex bradycardia in response to an increase in MAP and a reflex tachycardia in response to a decrease in MAP. This resulted in characteristic sigmoidal baroreflex curves (Fig. 2A). Figure 2B illustrates the gain of the baroreflex curves as a function of arterial pressure. Although baseline HR was elevated by HU (Table 1), baroreflex control of HR was similar in both groups (Fig. 2). There was no significant difference in the maximum HR or minimum HR during changes in MAP or arterial pressure at the midpoint of the curve (Table 2). The maximum gain and the gain of the baroreflex curves throughout the entire range of MAP were also similar for control and HU animals (Fig. 2B and Table 2).

View larger version (14K): [in this window] [in a new window]   Fig. 2.   A: mean baroreflex curves describing reflex control of heart rate (HR). Both HU (n = 13) and casted control groups (n = 14) exhibited a similar HR response to increases and decreases in MAP. Symbols indicate baseline HR and resting MAP for control () and HU () animals. HU rats exhibited a significantly higher HR at resting MAP compared with control rats, * P < 0.01. B: mean curves illustrating the instantaneous gain of baroreflex control of HR for HU (n = 13) and control (n = 14) rats. Both groups exhibited a similar baroreflex gain throughout the range of MAP.

Baroreflex control of LSNA. The effects of HU on baroreflex control of LSNA are illustrated in Fig. 3. Both groups exhibited characteristic reflex reductions in LSNA in response to an increase in MAP and reflex activation of LSNA in response to a reduced MAP (Fig. 3A). However, there was a significant interaction of group and MAP on LSNA. The ability to increase LSNA from baseline levels in response to a decrease in MAP was diminished in HU rats compared with that of controls (Fig. 3A). The increase in LSNA was significantly attenuated in HU animals over the range of pressures from 40 to 90 mmHg. There was also a significant reduction in maximum LSNA in the HU rats (Table 3). Neither pressure at the midpoint of the baroreflex curves nor the minimum LSNA in response to an increase in MAP was significantly different between groups (Fig. 3A and Table 3). Figure 3B illustrates the gain of the baroreflex curve as a function of arterial pressure. The maximum gain (slope at the inflection point) was significantly lower in HU rats compared with controls (Fig. 3B and Table 3). In addition, there was a significant interaction of group and MAP on gain. Gain was significantly reduced in HU rats over the range of pressures from 65 to 110 mmHg (Fig. 3B).

View larger version (18K): [in this window] [in a new window]   Fig. 3.   A: mean baroreflex curves describing reflex control of LSNA expressed as a percentage of baseline activity. HU rats (n = 8) exhibited a significant attenuation in the ability to increase LSNA in response to a decrease in MAP compared with control rats (n = 8). Symbols indicate %baseline LSNA and resting MAP for control () and HU () animals. B: mean curves illustrating the instantaneous gain of baroreflex control of LSNA for HU (n = 8) and control (n = 8) rats. Dotted lines represent mean values for control animals ± 1 least-significant difference ( = 0.05). Any points outside the dotted lines should be considered significantly different from control animal values.

To assess the level of SNA relative to baroreflex-elicited maximal SNA, data were expressed as a percentage of the maximal response. When data were normalized in this manner, the parameters defining the baroreflex curves were similar for control and HU rats. However, at resting MAP, HU animals exhibited significantly higher LSNA (expressed as percentage of maximal LSNA) compared with control rats (Table 4). Thus the range over which LSNA could be activated in response to a hypotensive challenge was significantly reduced in HU animals.

Baroreflex control of RSNA. The effects of HU on baroreflex control of RSNA are illustrated in Fig. 4. Both groups exhibited characteristic reflex reductions in RSNA in response to an increase in MAP and reflex activation of RSNA in response to reduced MAP. However, the ability to increase RSNA from baseline levels in response to a decrease in MAP was attenuated in HU rats compared with controls (Fig. 4A). This is indicated by a significant interaction of group and MAP on RSNA. The level of RSNA was significantly reduced in HU rats over a pressure range of 40-95 mmHg. In addition, there was a significant decrease in maximum RSNA in HU rats (Table 3). The minimum RSNA in response to an increase in MAP was significantly reduced, whereas MAP at the midpoint of the curve was significantly elevated (Table 3). Figure 4B illustrates the gain of the baroreflex curve as a function of arterial pressure. The maximum gain, or slope at the inflection point, was significantly reduced in HU rats compared with controls (Fig. 4B and Table 3). In addition, the gain of the baroreflex curve was significantly reduced over the range of pressures from 80 through 110 mmHg.

View larger version (18K): [in this window] [in a new window]   Fig. 4.   A: mean baroreflex curves describing reflex control of RSNA expressed as a percentage of baseline activity. HU rats (n = 8) exhibited a significant attenuation in the ability to increase LSNA in response to a decrease in MAP compared with control rats (n = 8). Symbols indicate %baseline RSNA and resting MAP for control () and HU () animals. B: mean curves illustrating the instantaneous gain of baroreflex control of LSNA for HU (n = 8) and control (n = 8) rats. Dotted lines represent mean values for control animals ± 1 LSD ( = 0.05). Any points outside the dotted lines should be considered significantly different from control animal values.

To assess the level of SNA relative to baroreflex-elicited maximum SNA, data were expressed as a percentage of the maximal response. When data were normalized in this manner, only the parameter indicating minimum RSNA was slightly but significantly reduced in HU rats. However, HU animals had a significantly higher RSNA (expressed as a percentage of maximal RSNA) at resting MAP than control rats (Table 4). Thus the range over which RSNA could be activated in response to a decrease in MAP was significantly reduced in HU animals.

	DISCUSSION

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The central hypothesis for this investigation was that HU, an animal model of cardiovascular deconditioning, results in impairment of arterial baroreflex modulation of the sympathetic nervous system. In addition, we hypothesized that arterial baroreflex control of sympathetic outflow to skeletal muscle and viscera are differentially affected. To test these hypotheses, reflex changes in HR and lumbar or renal sympathetic nervous system activity were recorded in response to changes in MAP in rats exposed to 14 days of HU. The major finding of this study was that cardiovascular deconditioning reduced the ability to reflexly increase SNA both to the hindlimb skeletal muscle and kidney in response to decreases in arterial pressure (Figs. 3 and 4). The peak gain of baroreflex control of the sympathetic nervous system (expressed as a percentage of baseline SNA) was significantly reduced. In addition, the gain of the baroreflex curve was significantly reduced in HU rats at resting and below normal arterial pressure for both LSNA and RSNA. This indicates that cardiovascular deconditioning may result in a compromised ability to buffer even small decreases in arterial pressure from a resting level. Baroreflex modulation of HR was unchanged by HU.

Previous work has shown that HU rats do not appropriately redistribute cardiac output during exercise (27, 46). Blood flow to relatively less active tissue, including the duodenum, spleen, and kidney, is higher during exercise in HU rats compared with control animals. This indicates a reduced vasoconstrictor response to exercise in these vascular beds and is associated with blunted norepinephrine turnover (an index of regional sympathetic nervous system activity) in the spleen during exercise (45). In contrast, blood flow to active skeletal muscle is significantly lower in HU rats during heavy exercise (46). This is associated with enhanced norepinephrine depletion in the soleus muscle of these animals (45). These data suggest that the sympathetic nervous system contribution to vasomotor control of the viscera and skeletal muscle may be differentially affected by HU. At least part of the sympathetic nervous system response and the redistribution of cardiac output during exercise are dependent on the arterial baroreflex (15, 19, 30, 34). Therefore, we originally hypothesized that baroreflex control of sympathetic outflow to the skeletal muscle versus kidney would be affected differently by HU. Results indicate that the ability to reflexly increase sympathetic outflow was attenuated similarly to both the skeletal muscle and kidney (Figs. 3 and 4). Thus the attenuation in the ability to increase sympathetic outflow in response to decreased arterial pressure may be a generalized response, occurring at least in skeletal muscle and some visceral tissues. However, the possibility remains that sympathetic outflow during exercise could be differentially affected by cardiovascular deconditioning even though reflex sympathoexcitation to a hypotensive stimulus is reduced.

The present results indicate that the ability to activate the sympathetic nervous system in response to arterial baroreceptor unloading is attenuated after cardiovascular deconditioning in rats. The arterial baroreflex serves as a primary mechanism for the short-term buffering of changes in arterial pressure. This regulatory function is accomplished by increasing or decreasing sympathetic and parasympathetic nerve activity, as required, from the baseline level of activity. In this study, we were interested in the effects of cardiovascular deconditioning on baroreflex regulation of sympathetic outflow in response to changes in arterial pressure. Therefore, SNA was expressed as a percentage of nerve activity at resting arterial pressure. When baroreflex function was evaluated in this manner, HU rats exhibited an attenuated ability to increase RSNA and LSNA in response to decreases in arterial pressure and a reduction in maximum gain of the reflex. More importantly, baroreflex gain was diminished at pressures close to resting values, indicating a reduced capacity to buffer decreases in arterial pressure from baseline levels. Baroreflex-mediated sympathoinhibition in response to increases in arterial pressure appeared to be unaltered. We also expressed RSNA and LSNA as a percentage of maximum levels attained during a decrease in arterial pressure. When the data were expressed in this manner, the parameters describing baroreflex function were not altered by HU. However, the level of RSNA or LSNA at resting arterial pressure was closer to the maximum level attainable, so that the capacity to increase SNA in response to a hypotensive challenge was reduced. Thus, whether SNA was expressed as a percent of baseline activity or a percent of maximum, it appeared that the ability of the arterial baroreflex to activate the sympathetic nervous system in response to decreases in blood pressure was compromised by HU.

It is possible that baseline levels of SNA may be altered by HU. Data from this study do not allow us to compare directly the level of sympathetic activity between control and HU animals. In addition, the effects of cardiovascular deconditioning on indirect indexes of baseline levels of sympathetic outflow are equivocal (7-9, 17). Circulating catecholamine concentrations have been reported to be slightly elevated (17) after spaceflight. However, catecholamine levels appear to be normal in both the supine and sitting positions in humans after bedrest (8). A recent study by Shoemaker and colleagues (39) suggests that sympathetic activity might actually be reduced after bedrest. In addition, norepinephrine depletion rate in heart and soleus muscle appears to be normal at rest after HU, whereas that of the spleen is decreased (45). These studies suggest that sympathetic activity may be similar or even attenuated at rest in control and deconditioned animals and humans. If sympathetic activity is reduced in HU rats, the relative inability to activate the sympathetic nervous system as a function of the original reduced baseline is even more significant. Furthermore, even if resting SNA is elevated after HU, this alone does not account for the reduced ability to reflexly activate the sympathetic nervous system in response to a hypotensive stimulus. Saigusa and Head (35) reported that microinjection of glutamate into the rostral ventrolateral medulla of rats increased baseline SNA. This increase in resting sympathetic outflow was accompanied by a greater maximum sympathetic activity in response to decreases in arterial pressure. In addition, microinjection of angiotensin II into the same region also increased maximum baroreflex-induced sympathoexcitation, without altering baseline SNA. Thus changes in baseline activity of the sympathetic nervous system alone do not necessarily account for changes in the maximum sympathetic response to decreases in arterial pressure. These data, in conjunction with the data from the present study, provide support for the concept that cardiovascular deconditioning specifically results in the decreased capacity of the arterial baroreflex to elicit an appropriate sympathoexcitatory response to a hypotensive stimulus.

Data from the current study indicate that global integrated (i.e., sympathetic and parasympathetically mediated) baroreflex modulation of HR was unaltered by HU (Fig. 2 and Table 2). Several studies have previously investigated the effects of HU in rodents on arterial baroreflex control of HR (4, 16, 26, 31, 47). Martel et al. (26) reported an impairment in baroreflex control of HR. However, this study evaluated baroreflex function after only 24 h of HU (26). Zhang (47) reported an elevated minimum HR plateau and reduced operative range, with no change in gain of the reflex, after 90 days of HU. A study by Brizzee and Walker (4) examined the reflex HR response to increases in arterial pressure produced by bolus doses of PE; reflex tachycardia was not evaluated. These investigators reported that there was a tendency for reflex bradycardia to be depressed after HU (with the animals maintained in the unloaded position), whereas reflex function appeared to be normal after release from suspension. A recent investigation that evaluated reflex responses to both increases and decreases in arterial pressure reported no effect after 14 days of HU on the complete arterial baroreflex curve (16). Thus it appears that, although baroreflex control of SNA is impaired by 2 wk of HU, baroreflex control of HR is unaltered.

Studies examining baroreflex control of HR after cardiovascular deconditioning in humans have indicated either impaired baroreflex function or no change (8, 11, 17). These varied results may relate to differences in methodology used to evaluate the baroreflex. Many of the studies reporting an impairment in baroreflex function used protocols that evaluated primarily the parasympathetic limb of the arterial baroreflex (8, 17). It is possible that the parasympathetic nervous system may be affected differently after cardiovascular deconditioning. In addition, most of these studies specifically evaluated the carotid baroreceptor reflex by changing carotid sinus pressure through neck cuff suction or pressure techniques (8, 17). Crandall and colleagues (11) isolated effects of cardiovascular deconditioning on the carotid baroreceptors from those on aortic baroreceptors. These investigators reported that, although the carotid baroreceptor reflex was attenuated, the aortic baroreceptor reflex was enhanced. This resulted in no change in the overall arterial baroreflex control of HR in humans after cardiovascular deconditioning. Thus it appears that baroreflex control of HR is unchanged after cardiovascular deconditioning in humans and hindimb unloading in rodents.

There are several possible reasons why HU does not result in an alteration in arterial baroreflex control of HR while baroreflex regulation of SNA is attenuated. HR is controlled by both the parasympathetic and sympathetic nervous systems, and it is possible that the two branches of the autonomic nervous system are affected differentially by cardiovascular deconditioning. Therefore, changes in control of sympathetic nervous system activity cannot be adequately evaluated by recording changes in HR. In addition, HR reflects measurement of an end-organ response to autonomic stimuli, and this response may also be altered. This seems especially likely since a recent study by Convertino et al. (10) indicates that, after a period of bedrest, -adrenergic responsiveness of the heart is enhanced. Thus it is possible that, in response to a hypotensive stimulus, the ability to increase cardiac SNA, like LSNA and RSNA, is attenuated in HU animals but that the response of the heart to adrenergic stimuli is enhanced. Together, these two factors may result in no change in the overall tachycardia in response to decreases in arterial pressure.

The mechanism(s) responsible for the attenuation of arterial baroreflex control of SNA to peripheral vascular beds due to HU are unknown. Factors within the arterial baroreflex loop, including changes in afferent signaling or defective central processing of afferent information, might be responsible for the alteration in baroreflex function. Another possibility is that there are no changes in the individual components of the baroreflex loop in HU animals but that other regulatory systems interact with the arterial baroreflexes to cause suppression of the reflex. Possible mechanisms include altered inputs from other central nervous system sites (including vestibular, hypothalamic, and medullary regions), effects of circulating hormones, and other afferent inputs such as skeletal muscle or cardiopulmonary afferents. The cardiopulmonary baroreflex exerts a similar interaction with arterial baroreflex function (1, 33). Activation of cardiopulmonary receptors may occur in response to the initial central shifts in blood volume that accompany HU (37). However, although there is an early increase in central venous pressure that is associated with HU, central venous pressure returns to normal ~24 h after the initial head-down tilt (25, 37). In addition, HU is associated with a sustained hypovolemia (4). Therefore, cardiopulmonary receptors are most likely unloaded when animals are removed from the HU apparatus and placed in the normal posture. If this is the case, cardiopulmonary receptors would not be activated under these conditions and should not result in attenuated arterial baroreflex function. However, chronic changes in cardiopulmonary receptor activation due to HU could lead to alterations in central processing associated with cardiopulmonary afferents and their interaction with the arterial baroreflex arc.

Perspectives

Reductions in maximal oxygen consumption and overall exercise capacity are well documented in humans after prolonged bedrest and microgravity (3, 6, 23, 24, 36, 41) and in rats after HU (27, 32). Previous studies have shown that arterial baroreflexes are critical mediators of the increase in sympathetic drive during exercise (30, 34) and are important in redistributing blood flow away from inactive tissue (19). In the absence of an intact, functioning arterial baroreflex, MAP falls at the onset of exercise in association with a decrease in SNA and HR (15, 19). Thus the depression in baroreflex control of SNA observed in this study may also contribute to the severely diminished exercise capacity after cardiovascular deconditioning.

Previous data indicate changes in vascular reactivity in HU rats. Vasoconstrictor responses of aortic rings were significantly reduced after 14 days of HU (12, 13). Diminished responsiveness of the peripheral vascular system to vasoconstrictor stimuli is a plausible mechanism in itself to explain the effects of cardiovascular deconditioning on the ability to adequately maintain arterial pressure in response to an orthostatic challenge or to appropriately redistribute blood flow away from the viscera during exercise. These results taken with the present data suggest that, during an orthostatic challenge or exercise, sympathetic outflow may be attenuated in addition to a compromised peripheral vascular response to sympathetic stimulation. This would result in an even greater inability of a deconditioned animal to adequately compensate for an orthostatic challenge or to exercise.

In conclusion, orthostatic intolerance and diminished exercise capacity have been well documented in humans after prolonged bedrest or microgravity (3, 6, 20, 23, 24, 41). The present study tested the hypothesis that HU, an animal model of bedrest and microgravity, attenuates baroreflex control of SNA in a differential manner. Our results have demonstrated that there is no change in the arterial baroreflex control of HR after 14 days of HU. However, the ability to reflexly activate both lumbar and RSNA is significantly attenuated, as is the gain of the arterial baroreflex. These results provide support for the concept that an attenuated ability of the arterial baroreflex to reflexly activate sympathetic nervous system activity may contribute to the orthostatic intolerance and reduced exercise capacity after cardiovascular deconditioning.