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J Appl Physiol 104: 1273-1280, 2008. First published January 24, 2008; doi:10.1152/japplphysiol.00954.2006
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Diminished mesenteric vaso- and venoconstriction and elevated plasma ANP and BNP with simulated microgravity

Bradley J. Behnke,1,2 David C. Zawieja,3 Anatoliy A. Gashev,3 Chester A. Ray,4 and Michael D. Delp1,2

1Division of Exercise Physiology, Department of Physiology and Pharmacology, and the Center for Interdisciplinary Research in Cardiovascular Sciences, West Virginia University School of Medicine, Morgantown, West Virginia; 2Department of Applied Physiology and Kinesiology and Center for Exercise Science, University of Florida, Gainesville, Florida; 3Department of Medical Physiology, College of Medicine, Cardiovascular Research Division of Lymphatic Biology, Texas A&M University System Health Science Center, College Station, Texas; and 4Departments of Medicine and Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania

Submitted 29 August 2006 ; accepted in final form 22 January 2008


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Diminished constriction of arteries and veins following exposure to microgravity or bed rest is associated with a reduced ability to augment peripheral vascular resistance (PVR) and stroke volume during orthostasis. We tested the hypothesis that small mesenteric arteries and veins, which are not exposed to large pressure shifts during simulated microgravity via head-down tail suspension (HDT), will exhibit decrements in adrenergic constriction after HDT in rats. Small mesenteric arteries and veins from control (Con; n = 41) and HDT (n = 35) male Sprague-Dawley rats were studied in vitro. Vasoactive responsiveness to norepinephrine (NE) in arteries (10–9 to 10–4 M) and veins (pressure-diameter responses from 2 to 12 cmH2O after incubation in 10–6 or 10–4 M NE) were evaluated. Plasma concentrations of atrial (ANP) and NH2-terminal prohormone brain (NT-proBNP) natriuretic peptides were also measured. In mesenteric arteries, sensitivity and maximal responsiveness to NE were reduced with HDT. In mesenteric veins there was a diminished venoconstriction to NE at any given pressure in HDT. Plasma concentrations of both ANP and NT-proBNP were increased with HDT, and maximal arterial and venous constrictor responses to NE after incubation with 10–7 M ANP or brain natriuretic peptide (BNP) were diminished. These data demonstrate that, in a vascular bed not subjected to large hydrodynamic differences with HDT, both small arteries and veins have a reduced responsiveness to adrenergic stimulation. Elevated levels of circulating ANP or NT-proBNP could adversely affect the ability of these vascular beds to constrict in vivo and conceivably could alter the intrinsic constrictor properties of these vessels with long-term exposure.

arteries; veins; hindlimb unloading

HUMAN CARDIOVASCULAR deconditioning resulting from bed rest or exposure to microgravity induces several adaptations affecting both central [e.g., attenuated baroreflex sensitivity (8) and stroke volume (33)] and peripheral [e.g., altered skeletal muscle vascular reactivity (53)] homeostasis. These perturbations manifest in a diminished capacity to perform and sustain exercise (50), as well as orthostatic intolerance in some individuals (4, 38, 40). Key mechanisms contributing to an altered regulation of mean arterial pressure (MAP) during orthostasis include an inability to augment peripheral vascular resistance (PVR) and stroke volume (4, 40, 57), resulting in reduced cerebral perfusion (2) and, if not corrected, syncope.

Animal models have been used to simulate the effects of microgravity on the cardiovascular system of humans. Head-down tilt (HDT) in rats limits postural and locomotor activity, and HDT elicits many of the hemodynamic alterations that characterize the cardiovascular deconditioning that occurs in humans after prolonged bed rest or exposure to microgravity, including hypovolemia (35), resting and exercising tachycardia (16, 37), altered tissue perfusion (37, 60), and arterial hypotension during head-up tilt (36, 58). The diminished capacity to elevate PVR with simulated microgravity is due, in part, to reductions in vasoconstrictor responsiveness that have been shown to occur in a number of arterial segments, including abdominal and thoracic aorta (11, 13, 46, 47), carotid and femoral arteries (47, 63), and skeletal muscle arterioles (10) from HDT rats. In addition, vasoconstrictor responses of mesenteric arteries to elevations in transmural pressure (34) and norepinephrine (NE) (43) have been reported to be lower in HDT rats. The diminished cardiac output with HDT-induced cardiovascular deconditioning is principally due to a profound reduction in stroke volume (33, 60). Stroke volume is primarily determined by the contractility of the heart and venous filling pressure (48), the latter of which is dependent on central mobilization of venous volume via constriction of large-capacitance peripheral veins. Therefore, diminished adrenergic venoconstriction would result in an increased functional capacitance of veins, thereby attenuating venous filling pressure and reducing stroke volume via the Starling mechanism.

Using the HDT rat as a model of the cardiovascular deconditioning induced by microgravity, previous investigators have suggested that the hyporesponsiveness of the arterial vasculature is primarily the result of changes in the pressure or flow within blood vessels (62). Therefore, this study was undertaken to investigate the effects of simulated microgravity on vaso- and venoconstriction within a vascular bed not exposed to pressure or flow alterations with HDT, i.e., the mesenteric circulation (37). Specifically, we tested the hypothesis that vasoactive responses to adrenergic stimulation will be diminished in vessels from the arterial and venous mesenteric circulation. Furthermore, we hypothesized that circulating levels of atrial (ANP) and brain NH2-terminal prohormone (NT-proBNP) natriuretic peptides, which are sensitive to postural fluid shifts and have the potential to alter the intrinsic vasomotor properties of peripheral arteries and veins, will be elevated in HDT rats. On the basis of findings from the latter set of experiments, we hypothesized that exposure to either ANP or brain natriuretic peptide (BNP) reduces the contractile responses evoked by adrenergic stimulation in arteries and veins in the mesenteric circulation.


   METHODS
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 ABSTRACT
 METHODS
RESULTS
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Materials and Methods

All procedures performed in this study were approved by the Texas A&M University and West Virginia University Animal Care Committees and conformed to the National Institutes of Health (NIH) Guide for the Care and Use of Laboratory Animals.

Animals

Six-month-old male Sprague-Dawley rats were housed in a temperature-controlled (23 ± 2°C) room with a 12:12-h light-dark cycle. Water and rat chow were provided ad libitum. The animals were randomly assigned to either cage control (Con, n = 41) or 14-days head-down tail suspension (HDT; n = 35) groups. The hindlimbs of the HDT group were elevated to an approximate spinal angle of 40–45° via orthopedic tape adhered to the tail, which is a modification of techniques previously described (12, 59, 61). Briefly, the animals were injected with pentobarbital sodium (30 mg/kg ip) to induce anesthesia. While anesthetized, the animal's tail was washed and dried, and a length of breathable nonelastic adhesive tape (Curity Porous tape, Kendall) with a hook attached to the end was placed on the proximal two-thirds of the tail, which allowed the end of the tail to remain unattached. The ends of the adhesive tape were further bonded to the tail with an additional adhesive (Goop) and allowed to dry for 20 min before suspension. Daily inspection of each animal's tail was performed, checking for discoloration of tissue damage from the suspension apparatus. The hook attached to the adhesive tape was connected by a small chain to a swivel apparatus fixed at the top of the cage. The height of the hindlimb elevation was adjusted to prevent the hindlimbs from touching supportive surfaces while the forelimbs maintained contact with the cage floor. This allowed free range of movement around the cage while achieving the desired experimental results. Control animals were individually housed and maintained in a normal cage environment while HDT rats were kept in the head-down position for 14 days. This time period has been shown to be sufficient to induce cephalic fluid shifts (25, 36) and produce cardiovascular alterations in HDT animals (10, 14, 35, 37, 43, 44, 58, 60). After the experimental period, HDT and control animals were weighed and euthanized by injecting 40 mg/kg ip Euthanasia-5 Solution (Henry-Schein, Port Washington, NY) without allowing the hindlimbs of the HDT rats to become weight bearing. The gastrointestinal tract was then carefully dissected free and placed in a 4°C filtered physiological saline solution (PSS) (pH 7.4).

Microvessel Preparation

Under a dissecting microscope, resistance arteries (~200 µm) and veins (~300 µm) from the mesentery were identified, dissected free from surrounding tissue, and transferred to a Lucite vessel chamber containing PSS-albumin solution (pH 7.4) equilibrated with room air and ambient temperature. One end of the microvessel was cannulated with a glass micropipette (~50 µm in tip diameter), and the outside of the vessel was securely tied to the pipette with 11-O ophthalmic suture. Any residual blood within the vessel was then flushed out with PSS solution under low positive pressure (~5 cmH2O). The other end of each vessel was then cannulated with a second pipette and secured with suture. After cannulation, the isolated vessel in the tissue chamber was transferred to the stage of an inverted microscope (Olympus IX71) coupled to a video camera (Panasonic BP310) and videomicrometer (307A, Colorado Video, Boulder, CO). Mesenteric arteries were then pressurized at 108 cmH2O, which corresponds to in vivo arterial pressures in these vessels (18). Mesenteric veins were pressurized at 4 cmH2O, which is similar to that used by Dunbar et al. (15). The vessels were allowed to equilibrate for 1 h at 37°C (PSS solution changed every 15 min) before vasomotor properties were characterized. Internal diameters were measured continuously throughout the experiment using the video caliper.

Experimental Design

Protocol I.   Vasoconstrictor properties of mesenteric resistance arteries were characterized by determining maximal responsiveness and sensitivity to NE. Following cannulation and equilibration in PSS, concentration-response relations were determined by the cumulative addition of NE (10–9 to 10–4 M).

Protocol II.   Venoconstrictor properties of mesenteric veins were characterized by determining pressure-diameter relations before and after exposure to NE. Specifically, following cannulation and equilibration in PSS, intraluminal pressure was lowered to 2 cmH2O and subsequently increased at 2 cmH2O increments to 12 cmH2O. A starting pressure of 2 cmH2O was used because at 0 cmH2O the veins tended to collapse. Vessels were allowed 5 min to equilibrate at a given pressure before luminal diameter was measured. After the active pressure-diameter response was quantified, intraluminal pressure was set at 4 cmH2O, and the vessel was incubated in 10–6 or 10–4 M NE (random order) for 20 min, and the pressure-diameter response was repeated as described above. On completion of all active responses, the bathing solution was replaced with Ca2+-free PSS for 60 min, on which the passive pressure-diameter response (same protocol as the active response) was quantified.

Plasma ANP and NT-proBNP Concentration

Both ANP and NT-proBNP were measured using radioimmunoassay from plasma samples (ALPCO Diagnostics, Salem, NH). Blood was obtained from control (n = 28) and 14-day HDT (n = 26) rats in the morning between 7 AM and 9 AM via puncture of the abdominal aorta and collected in EDTA tubes containing aprotinin (200 kallikrein inhibitory units/ml). The mean intra-assay variance for ANP was 5.2%, with an interassay variance of 6.6%. The sensitivity of the assay was 3.5 pg/ml. The mean intra-assay variance for NT-proBNP was 4.0%, with an interassay variance of 3.8%. The sensitivity of the assay was 4 fmol/ml.

Effects of ANP and BNP on Vascular Responsiveness

In a separate group of 6-mo-old male Sprague-Dawley rats (n = 10), mesenteric arteries and veins were dissected, cannulated, and pressurized as described above. Vasoconstrictor properties of mesenteric resistance arteries and veins were characterized by determining maximal responsiveness and sensitivity to NE before and after incubation in either ANP or BNP. Following cannulation and equilibration in PSS, concentration-response relations were determined by the cumulative addition of NE (10–9 to 10–4 M). After completion of the control response, the PSS bathing solution was replaced every 15 min for 1 h to allow the vessel to reestablish spontaneous tone. Vessels were then incubated either ANP (10–7 M) or BNP (10–7 M) (random order) for 20 min, and the maximal responsiveness and sensitivity to NE were repeated as described above. Preliminary studies were conducted to ensure that vascular responses to NE were not altered by multiple dose-response tests.

Solutions and Drugs

The PSS buffer contained (in mM) 145 NaCl, 4.7 KCl, 1.2 NaH2PO4, 1.17 MgSO4, 2.0 CaCl2, 5.0 glucose, 2.0 pyruvate, 0.02 EDTA, and 3.0 MOPS at a pH of 7.4. Ca2+-free PSS buffer was similar to the PSS buffer except it contained 2 mM EDTA and the CaCl2 was replaced with 2.0 mM NaCl. Concentrated stock solutions of NE were prepared in PSS buffer.

Statistical Analysis

Intraluminal diameters were measured throughout the duration of each experimental protocol. At the end of each experiment the vessel was placed in Ca2+-free bathing solution and rinsed several times over the course of 1 h to obtain a maximal diameter and wall thickness. The development of spontaneous tone is expressed as the percent constriction relative to maximal diameter and calculated as (Dmax – DB)/Dmax x 100, where Dmax is the maximal diameter and DB is the starting baseline diameter. Within veins, pressure-diameter response curves are expressed as intraluminal diameter (µm) as well as the percent change vs. the diameter at a given pressure during the active or passive protocols. Pressure-diameter and concentration-response curves were evaluated using repeated-measures ANOVA with one within factor (intraluminal pressure or concentration) and one between factor (experimental groups). Planned contrasts were conducted at each intraluminal pressure or concentration level to determine whether differences exist between experimental groups (control vs. HDT). To detect differences in sensitivity to NE, EC50 values were designated as the concentration of NE that produces 50% of its maximal response. Student's unpaired t-test was used to determine whether differences in body mass, soleus muscle mass, soleus muscle-to-body weight ratio, developed spontaneous tone, maximal diameter, compliance, and EC50 values were significantly different between groups. All values are presented as the mean ± SE. A value of P < 0.05 was required for significance.


   RESULTS
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 RESULTS
DISCUSSION
 GRANTS
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Soleus Muscle-to-Body Weight Ratio

The HDT rats weighed significantly less than their control counterparts (HDT 381 ± 17 g vs. Con 423 ± 11 g; P < 0.05). There was a significant atrophy of the soleus muscle in the HDT animals compared with controls (HDT 149 ± 12 mg, Con 245 ± 14 mg; P < 0.05), which resulted in a ~35% reduction in the soleus-to-body weight ratio (HDT 0.39 ± 0.01 mg/g, Con 0.58 ± 0.03 mg/g; P < 0.05). The latter is a hallmark of reduced skeletal muscle weight-bearing activity and confirms the efficacy of the HDT procedure (56).

Vessel Characteristics

The maximal intraluminal diameter (determined in Ca2+-free solution) of the mesenteric arteries (at 108 cmH2O) and veins (at 4 cmH2O) was not different between Con and HDT animals (Table 1). In arteries, both groups displayed a similar amount of spontaneous tone (~20%; Table 1). However, at 4 cmH2O the mesenteric veins from control animals exhibited a greater level of spontaneous tone (control 16 ± 6% vs. HDT 2 ± 1%, P < 0.05). HDT did not affect the wall thickness or wall-to-lumen ratio in either mesenteric arteries or veins (Table 1).


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  		Table 1. Mesenteric artery and vein morphology and vasoactive properties in control animals and rats exposed to head-down tilt

 
Mesenteric Artery Vasoconstrictor Responses

NE produced dose-dependent decreases in luminal diameter in mesenteric arteries from both Con and HDT animals (Fig. 1). However, the contractile responses evoked by 10–9 to 10–4 M NE were lower in HDT. The maximal constriction induced by NE was significantly attenuated in arteries from HDT vs. Con (Table 1, Fig. 1). In addition, the sensitivity of mesenteric arteries from HDT to NE was lower than that from Con animals, as indicated by a higher EC50.


Figure 1
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  		Fig. 1. Concentration-response relation of norepinephrine (NE) in mesenteric resistance arteries from control and head-down tail suspension (HDT) animals. Values are means ± SE. Pre, before HDT. *HDT significantly different from that of control (P < 0.05).

 
Mesenteric Vein Constrictor Responses

Lumen diameter increased nearly linearly (R2, Con 0.986 ± 0.006, HDT 0.969 ± 0.007) as a function of intraluminal pressure, and there was a downward shift in the pressure-diameter response curve with exposure to NE (Fig. 2, A and B). However, at any given pressure the percent venoconstriction with both 10–6 and 10–4 M NE was significantly reduced in HDT vs. Con vessels (Fig. 3, A and B).


Figure 2
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  		Fig. 2. Pressure-diameter responses after exposure to NE in small mesenteric veins from control (Con) and HDT animals. A: diameter was smaller in mesenteric veins from control animals at all pressures with NE, and exposure to 10–4 M NE resulted in smaller diameters vs. those at 10–6 M NE (P < 0.05). B: mesenteric veins from HDT animals showed smaller diameters at all pressures with exposure to NE (P < 0.05). Only at 4, 6, and 8 cmH2O was a difference observed between the diameters measured at 10–6 vs. 10–4 M NE (P < 0.05). Values are means ± SE. A and B: *Response different from that without NE (P < 0.05).

 

Figure 3
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  		Fig. 3. Percent constriction of mesenteric veins from control and HDT rats to 10–6 M NE (A) and 10–4 M NE (B). Values are means ± SE. *HDT significantly different from that of control (P < 0.05).

 
Plasma ANP and NT-proBNP Concentrations

Plasma ANP concentration was greater in HDT rats relative to Con animals (Fig. 4A). Likewise, plasma NT-proBNP concentration was elevated in HDT rats (Fig. 4B).


Figure 4
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  		Fig. 4. Plasma atrial natriuretic peptide (ANP; A) and brain NH2-terminal prohormone natriuretic peptide (NT-proBNP; B) concentrations in control (n = 28) and HDT (n = 26) rats. Values are means ± SE. {dagger}P = 0.052 between groups. *P < 0.05 between groups.

 
ANP and BNP Mesenteric Vaso- and Venoconstrictor Responses

Incubation in either ANP or BNP reduced the maximal NE-induced constriction in mesenteric arteries (Fig. 5A) and veins (Fig. 5B). In addition, the attenuation of NE-mediated venoconstriction was greatest in the presence of BNP (Fig. 5B). Contrary to the diminished maximal responsiveness, there were no differences observed in sensitivity to NE (EC50) with ANP or BNP in either mesenteric arteries or veins. Incubation in ANP or BNP had no effect on spontaneous tone developed in either arteries or veins as well (data not shown).


Figure 5
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  		Fig. 5. A: dose-response relations of mesenteric arteries to NE pre- and postincubation in either ANP (10–7 M) or BNP (10–7 M). B: dose-response relations of mesenteric veins to NE pre- and postincubation in either ANP (10–7 M) or BNP (10–7 M). Values are means ± SE. *P < 0.05 vs. control constrictor response at a given concentration of NE. {dagger}P < 0.05 vs. constrictor response after incubation in ANP at a given concentration of NE.

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
This is the first study to demonstrate that cardiovascular deconditioning induced by simulated microgravity causes intrinsic deficits in vasoconstrictor function that are manifest in both arteries and veins of a tissue where arterial pressure and blood flow are not altered by the HDT intervention (37, 59). Pathognomonic of the cardiovascular deconditioning induced by simulated (19, 35) and actual (4, 38, 40) microgravity is an attenuated ability to elevate vascular resistance, including that in the splanchnic circulation (37, 44, 60). As MAP is the product of PVR and cardiac output, alterations in either variable without a compensatory change in the other would adversely affect the maintenance of MAP. Thus the diminished vasoconstriction observed in mesenteric arteries could reduce the ability of the cardiovascular system to increase PVR during orthostatic stress and redirect blood flow from the viscera to active skeletal muscles during exercise. The diminished venoconstriction would compromise peripheral mechanisms to centrally mobilize blood volume to positively affect venous return, atrial filling pressure, and, consequently, stroke volume. Given the large capacitance of the splanchnic vascular bed (49), the results presented herein provide a likely mechanism for the deconditioning-induced inability to mobilize blood volume from this region to support cardiac output and elevate splanchnic vascular resistance in the maintenance of MAP, both of which are critical cardiovascular adjustments to orthostatic and exercise stress.

Results from the present study, as well as those previously published demonstrating compromised vasoconstrictor function of mesenteric lymphatic vessels to HDT (21), give important insight into the mechanism of smooth muscle contractile dysfunction associated with microgravity in large segments of the arterial, venous, and lymphatic circulations, and potentially microgravity-induced smooth muscle contractile dysfunction associated with the uterus (5). For example, it was first proposed by Hargens et al. (24, 26) that headward fluid shifts associated with the removal of gravitational stress or head-down bed rest could alter vascular morphology and consequently vascular function. Indeed, increases in arterial pressure to the head of HDT rats stimulates increases in cerebral artery wall thickness (58, 59) and enhancement of cerebral artery vasoconstrictor function (22, 58, 63). Conversely, decreases in arterial pressure to the hindlimbs of HDT rats induces a thinning of arteriolar wall thickness (12) and a reduction in arteriolar vasoconstrictor function (10). Thus microgravity or simulated microgravity-induced alterations in the mechanical environment of vessels (i.e., vascular transmural pressure and shear stress) can serve as a potent stimulus to alter vascular structure and function. However, arterial, venous, and lymphatic smooth muscle contractile hyporesponsiveness (present study, 21) is unlikely to be the result of altered hydrostatic pressure or intravascular shear stress, since the mesenteric circulation is located at or near the hydrostatic indifference point (46, 59) and exhibits no alterations in blood flow during simulated microgravity (37); the absence of changes in arterial transmural pressure and shear stress are further demonstrated by the lack of change in mesenteric arterial structure with chronic HDT (present study, 34, 59). These results demonstrate that another factor, likely a systemic factor(s), is working to diminish smooth muscle contractile function in the arterial (Fig. 1; Refs. 10, 11, 47), venous (Fig. 3; Ref. 15), and lymphatic (21) circulations, and possibly smooth muscle contractile function throughout the body [e.g., uterine tissues (5)], with one notable exception. The cerebral circulation, where smooth muscle cells are shielded from circulating agents via the blood-brain barrier, is the only vascular bed to consistently show that smooth muscle vasoconstrictor function is not diminished in HDT rats (22, 58, 63). Thus we propose that in addition to microgravity-induced alterations in vascular transmural pressure and shear stress, some circulating substance whose release is also likely associated with headward fluid shifts may stimulate alterations in intrinsic vascular function independent of the abovementioned mechanical influences.

Smooth Muscle Hyporesponsiveness: Role of Natriuretic Peptides

Although alterations in a number of circulating factors accompany headward fluid shifts (7), ANP and BNP are two endogenous hormones released in response to fluid volume shifts which regulate fluid volume via Na+ excretion (3, 9) and have vasoactive properties (45). Within smooth muscle, ANP and BNP selectively bind to the natriuretic peptide receptor Npra (20) and activate guanylyl cyclase to increase intracellular levels of the second messenger cGMP (45). cGMP and its associated cGMP-dependent protein kinase are important modulators of vascular tone (28). Specifically, cGMP-dependent protein kinase decreases cytosolic Ca2+ by activating Ca2+ ATPase, thereby lowering cytosolic Ca2+ in the cell. cGMP also acts as a negative modulator of protein kinase C (PKC) activity, such that long-term exposure of smooth muscle cells to ANP inhibits PKC activity in a dose-dependent manner (31).

ANP and BNP are also both potent inhibitors of ANG II-stimulated ET-1 production (23), the latter of which potentiates the vasoconstrictive action of NE (55). The adrenoreceptor agonist NE binds to a G protein receptor and activates phospholipase C, resulting in the hydrolysis of phosphoinositide and the formation of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). Subsequently, DAG activates PKC (42) and IP3 induces the release of Ca2+ from intracellular stores (1), resulting in smooth muscle contraction. Therefore, we propose that the elevated circulating levels of ANP and BNP associated with simulated microgravity in rats could serve to attenuate smooth muscle cell vasoconstrictor responsiveness to NE by inhibiting both PKC activity (51) and the formation of IP3 (29). Furthermore, smooth muscle cells throughout the body, with the exception of those protected by the blood-brain barrier, may be susceptible to contractile dysfunction via this systemic mechanism.

To our knowledge, the present observation that NT-proBNP is elevated with HDT is the first reported in the literature. However, the effects of HDT on ANP have previously been examined. Our observation that increases in circulating plasma levels of ANP occur during HDT is consistent with the ANP-stimulated elevation in cGMP concentration in the rat choroids plexus with HDT and spaceflight (6). However, others have also reported that plasma ANP concentrations are unchanged (41) or decreased (17) with HDT. The reasons for this discrepancy are unclear but may involve the length of HDT [14 days in the present study vs. 9 days (17)] or the hydration state of the animals.

On the basis of the findings of elevations in ANP and NT-proBNP with HDT (Fig. 4), we tested the hypothesis that acute exposure to either of these peptides would reduce the contractile responses evoked by adrenergic stimulation. Indeed, exposure to either ANP or BNP reduced the maximal responsiveness of mesenteric arteries (Fig. 5A) and veins (Fig. 5B) to NE. Therefore, it is likely that the elevated concentrations of circulating ANP or NT-proBNP found in the present study and after spaceflight (6) contribute to the compromised ability to lower vascular conductance in the mesenteric circulation with postural changes. It should also be noted that these acute effects of ANP and BNP on vessel function will likely differ from those of chronic exposure in vivo, since mesenteric arteries and veins from HDT rats demonstrated an intrinsic hyporesponsiveness to NE with no ANP or BNP in the bathing solution (Figs. 1 and 3). We propose that chronic exposure of smooth muscle cells to ANP, BNP or some other systemic circulating factor associated with HDT impairs their intrinsic contractile function (31).

Mechanisms of Altered Mesenteric Vascular Function

Although not all studies have shown that HDT adversely affects arterial adrenergic vasoconstrictor responsiveness (34), the preponderance of evidence demonstrates that simulated microgravity impairs arterial vasoconstrictor function (10, 11, 13, 27, 43, 47). Likewise, there is not uniform agreement in the literature regarding responses of HDT on the venous circulation. For example, Purdy and colleagues (47) found that simulated microgravity had no effect on femoral veins, whereas Dunbar and colleagues (15) found in large mesenteric veins that simulated microgravity abolished the NE-induced downward shift in the pressure-diameter response. Venular pressure-diameter responses in vivo are regulated by sympathetic activity (54) primarily through {alpha}1-adrenoreceptors (30, 32). HDT-induced decreases in {alpha}1-adrenoreceptors (52) may be partly responsible for the decreased sensitivity to NE observed in veins from several vascular beds [e.g., mesenteric veins (Fig. 3 of present study; Ref. 15); portal vein (39); vena cava (52)]. Therefore, much of the current literature supports the notion that HDT reduces the contractile potential of veins to adrenergic stimuli regardless of an increased passive compliance that may (52) or may not (15) occur.

Ramifications of Diminished Mesenteric Vaso- and Venoconstriction

The effects of diminished arterial and venular contraction could have several functional consequences. Within arteries, the attenuated vasoconstrictor responsiveness of splanchnic tissue could reduce the ability of the cardiovascular system to elevate PVR by as much as 25% during orthostasis (48), which, in accordance with Ohm's law, could diminish the fidelity in which arterial pressure and regional blood flow are regulated. In support of this, rats exposed to simulated microgravity exhibit hypotension during an orthostatic challenge (35, 58). Furthermore, following HDT there is a diminished ability to redistribute cardiac output during exercise such that the splanchnic bed receives a greater proportion of cardiac output than that of control rats (37, 44); this functional overperfusion of the splanchnic bed with exercise could limit the ability to match oxygen consumption to O2 delivery in active skeletal muscles, resulting in premature fatigue. Constriction of peripheral veins is also requisite to augment atrial filling pressure to increase stroke volume (48). Therefore, an attenuated venoconstriction (Figs. 3 and 5B) could diminish venous return and lower venous filling pressure. Indeed, a lower stroke volume has been estimated in HDT rats (33, 60), which would contribute to the reduced maximal oxygen uptake observed in these animals (44).

In conclusion, results from the present study demonstrate that, within the same vascular bed, there is a reduced adrenergic constriction of mesenteric arteries and veins. This adrenergic hyporesponsiveness occurs in a vascular bed where blood flow and arterial pressure alterations do not occur as a result of the HDT intervention (37, 59). Therefore, this suggests that the HDT-induced vascular effects on smooth muscle cells in this region are mediated by a systemic circulating factor(s) rather than altered hydrodynamic mechanical influences. Consistent with this hypothesis is an elevation of plasma ANP and NT-proBNP concentrations in HDT rats (Fig. 4), which can inhibit constrictor responses of mesenteric arteries (Fig. 5A) and veins (Fig. 5B) to adrenergic stimulation. These acute effects or the possible chronic effects of ANP and BNP to alter intrinsic vasomotor properties could occur through several mechanisms, including inhibition of PKC activity and the formation of IP3 in smooth muscle cells. The diminished vaso- and venoconstriction observed (Figs. 1 and 3) would likely affect the ability to augment peripheral vascular resistance and cardiac output during orthostatic and exercise stress. Thus the functional consequences of these vascular alterations may include orthostatic and exercise intolerance commonly associated with actual and simulated microgravity.


   GRANTS
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 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
REFERENCES
 
This study was supported, in part, by National Aeronautics and Space Administration Grants NAG2-1340, NCC2-1166, and NAG9-1034, and National Institutes of Health Grants F32-AG-25622, DC-006459, HL-58503, and P01-HL-077670.


   FOOTNOTES
 

Address for reprint requests and other correspondence: M. D. Delp, Dept. of Applied Physiology and Kinesiology, Univ. of Florida, Gainesville, FL (e-mail: mdelp{at}ufl.edu)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
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 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 

  1. Berridge MJ, Irvine RF. Inositol trisphosphate, a novel second messenger in cellular signal transduction. Nature 312: 315–321, 1984.[CrossRef][Medline]
  2. Bondar RL, Kassam MS, Stein F, Dunphy PT. Cerebrovascular response to standing post spaceflight. Aviat Space Environ Med 64: 430, 1993.
  3. Brenner BM, Ballermann BJ, Gunning ME, Zeidel ML. Diverse biological actions of atrial natriuretic peptide. Physiol Rev 70: 665–699, 1990.[Free Full Text]
  4. Buckey JC Jr, Lane LD, Levine BD, Watenpaugh DE, Wright SJ, Moore WE, Gaffney FA, Blomqvist CG. Orthostatic intolerance after spaceflight. J Appl Physiol 81: 7–18, 1996.[Abstract/Free Full Text]
  5. Burden HW, Poole MC, Zary J, Jeansonne B, Alberts JR. The effects of space flight during gestation on rat uterine smooth muscle. J Gravit Physiol 5: 23–29, 1998.[Medline]
  6. Carcenac C, Herbute S, Masseguin C, Mani-Ponset L, Maurel D, Briggs R, Guell A, Gabrion JB. Hindlimb-suspension and spaceflight both alter cGMP levels in rat choroid plexus. J Gravit Physiol 6: 17–24, 1999.[Medline]
  7. Convertino VA. Conditions of reduced gravity. In: Clinical Autonomic Disorders, edited by Low PA. Philadelphia, PA: Lippincott-Raven, 1997, p. 429–440.
  8. Convertino VA, Doerr DF, Eckberg DL, Fritsch JM, Vernikos-Danellis J. Head-down bed rest impairs vagal baroreflex responses and provokes orthostatic hypotension. J Appl Physiol 68: 1458–1464, 1990.[Abstract/Free Full Text]
  9. de Bold AJ. Atrial natriuretic factor: a hormone produced by the heart. Science 230: 767–770, 1985.[Abstract/Free Full Text]
  10. Delp MD. Myogenic and vasoconstrictor responsiveness of skeletal muscle arterioles is diminished by hindlimb unloading. J Appl Physiol 86: 1178–1184, 1999.[Abstract/Free Full Text]
  11. Delp MD, Brown M, Laughlin MH, Hasser EM. Rat aortic vasoreactivity is altered by old age and hindlimb unloading. J Appl Physiol 78: 2079–2086, 1995.[Abstract/Free Full Text]
  12. Delp MD, Colleran PN, Wilkerson MK, McCurdy MR, Muller-Delp J. Structural and functional remodeling of skeletal muscle microvasculature is induced by simulated microgravity. Am J Physiol Heart Circ Physiol 278: H1866–H1873, 2000.[Abstract/Free Full Text]
  13. Delp MD, Holder-Binkley T, Laughlin MH, Hasser EM. Vasoconstrictor properties of rat aorta are diminished by hindlimb unweighting. J Appl Physiol 75: 2620–2628, 1993.[Abstract/Free Full Text]
  14. Desplanches D, Mayet MH, Sempore B, Frutoso J, Flandrois R. Effect of spontaneous recovery or retraining after hindlimb suspension on aerobic capacity. J Appl Physiol 63: 1739–1743, 1987.[Abstract/Free Full Text]
  15. Dunbar SL, Berkowitz DE, Brooks-Asplund EM, Shoukas AA. The effects of hindlimb unweighting on the capacitance of rat small mesenteric veins. J Appl Physiol 89: 2073–2077, 2000.[Abstract/Free Full Text]
  16. Dunn CD, Johnson PC, Lange RD. Regulation of hematopoiesis in rats exposed to antiorthostatic hypokinetic/hypodynamia. II. Mechanisms of the "anemia". Aviat Space Environ Med 57: 36–44, 1986.[Medline]
  17. Fareh J, Bayard B, Gabrion J, Thibault G, Oliver J, Bouille C, Gauquelin G, Gharib C. Cardiac and plasma atrial natriuretic peptide after 9-day hindlimb suspension in rats. J Appl Physiol 76: 641–649, 1994.[Abstract/Free Full Text]
  18. Fenger-Gron J, Mulvany MJ, Christensen KL. Mesenteric blood pressure profile of conscious, freely moving rats. J Physiol 488: 753–760, 1995.[Abstract/Free Full Text]
  19. Gaffney FA, Nixon JV, Karlsson ES, Campbell W, Dowdey AB, Blomqvist CG. Cardiovascular deconditioning produced by 20 hours of bedrest with head-down tilt (–5 degrees) in middle-aged healthy men. Am J Cardiol 56: 634–638, 1985.[CrossRef][Web of Science][Medline]
  20. Garbers DL. Guanylyl cyclase receptors and their endocrine, paracrine, and autocrine ligands. Cell 71: 1–4, 1992.[CrossRef][Web of Science][Medline]
  21. Gashev AA, Delp MD, Zawieja DC. Inhibition of the active lymph pump by simulated microgravity in rats. Am J Physiol Heart Circ Physiol 290: H2295–H2308, 2006.[Abstract/Free Full Text]
  22. Geary GG, Krause DN, Purdy RE, Duckles SP. Simulated microgravity increases myogenic tone in rat cerebral arteries. J Appl Physiol 85: 1615–1621, 1998.[Abstract/Free Full Text]
  23. Hanehira T, Kohno M, Yoshikawa J. Endothelin production in cultured vascular smooth muscle cells—modulation by the atrial, brain, and C-type natriuretic peptide system. Metabolism 46: 487–493, 1997.[CrossRef][Web of Science][Medline]
  24. Hargens AR. Introduction and historical perspectives. In: Tissue Fluid Pressures and Composition, edited by Hargens AR. Baltimore, MD: Williams and Wilkins, 1981, p. 1–9.
  25. Hargens AR, Steakai J, Johannson C, Tipton CM. Tissue fluid shift, forelimb loading, and tail tension in tail-suspended rats. Physiologist 27: S37–S38, 1984.
  26. Hargens AR, Watenpaugh DE, Breit GA. Control of circulatory function in altered gravitational fields. Physiologist 35: S80–S83, 1992.[Medline]
  27. Hatton DC, Yue Q, Chapman J, Xue H, Dierickx J, Roullet C, Coste S, Roullet JB, McCarron DA. Blood pressure and mesenteric resistance arterial function after spaceflight. J Appl Physiol 92: 13–17, 2002.[Abstract/Free Full Text]
  28. Ignarro LJ, Kadowitz PJ. The pharmacological and physiological role of cyclic GMP in vascular smooth muscle relaxation. Annu Rev Pharmacol Toxicol 25: 171–191, 1985.[CrossRef][Web of Science][Medline]
  29. Khurana ML, Pandey KN. Atrial natriuretic peptide inhibits the phosphoinositide hydrolysis in murine Leydig tumor cells. Mol Cell Biochem 158: 97–105, 1996.[Web of Science][Medline]
  30. Kong JQ, Taylor DA, Fleming WW. Functional distribution and role of alpha-1 adrenoceptor subtypes in the mesenteric vasculature of the rat. J Pharmacol Exp Ther 268: 1153–1159, 1994.[Abstract/Free Full Text]
  31. Kumar R, Cartledge WA, Lincoln TM, Pandey KN. Expression of guanylyl cyclase-A/atrial natriuretic peptide receptor blocks the activation of protein kinase C in vascular smooth muscle cells. Role of cGMP and cGMP-dependent protein kinase. Hypertension 29: 414–421, 1997.[Abstract/Free Full Text]
  32. Leech CJ, Faber JE. Different alpha-adrenoceptor subtypes mediate constriction of arterioles and venules. Am J Physiol Heart Circ Physiol 270: H710–H722, 1996.[Abstract/Free Full Text]
  33. Levine BD, Zuckerman JH, Pawelczyk JA. Cardiac atrophy after bed-rest deconditioning: a nonneural mechanism for orthostatic intolerance. Circulation 96: 517–525, 1997.[Abstract/Free Full Text]
  34. Looft-Wilson RC, Gisolfi CV. Rat small mesenteric artery function after hindlimb suspension. J Appl Physiol 88: 1199–1206, 2000.[Abstract/Free Full Text]
  35. Martel E, Champeroux P, Lacolley P, Richard S, Safar M, Cuche JL. Central hypervolemia in the conscious rat: a model of cardiovascular deconditioning. J Appl Physiol 80: 1390–1396, 1996.[Abstract/Free Full Text]
  36. Maurel D, Ixart G, Barbanel G, Mekaouche M, Assenmacher I. Effects of acute tilt from orthostatic to head-down antiorthostatic restraint and of sustained restraint on the intra-cerebroventricular pressure in rats. Brain Res 736: 165–173, 1996.[CrossRef][Web of Science][Medline]
  37. McDonald KS, Delp MD, Fitts RH. Effect of hindlimb unweighting on tissue blood flow in the rat. J Appl Physiol 72: 2210–2218, 1992.[Abstract/Free Full Text]
  38. Michel EL, Johnston RS, Dietlein LF. Biomedical results of the Skylab Program. Life Sci Space Res 14: 3–18, 1976.[Medline]
  39. Morel JL, Boittin FX, Halet G, Arnaudeau S, Mironneau C, Mironneau J. Effect of a 14-day hindlimb suspension on cytosolic Ca2+ concentration in rat portal vein myocytes. Am J Physiol Heart Circ Physiol 273: H2867–H2875, 1997.[Abstract/Free Full Text]
  40. Mulvagh SL, Charles JB, Riddle JM, Rehbein TL, Bungo MW. Echocardiographic evaluation of the cardiovascular effects of short-duration spaceflight. J Clin Pharmacol 31: 1024–1026, 1991.[Web of Science][Medline]
  41. Neri G, Bova S, Malendowicz LK, Mazzocchi G, Nussdorfer GG. Simulated microgravity impairs aldosterone secretion in rats: possible involvement of adrenomedullin. Am J Physiol Regul Integr Comp Physiol 283: R832–R836, 2002.[Abstract/Free Full Text]
  42. Nishizuka Y. Studies and perspectives of protein kinase C. Science 233: 305–312, 1986.[Abstract/Free Full Text]
  43. Overton JM, Tipton CM. Effect of hindlimb suspension on cardiovascular responses to sympathomimetics and lower body negative pressure. J Appl Physiol 68: 355–362, 1990.[Abstract/Free Full Text]
  44. Overton JM, Woodman CR, Tipton CM. Effect of hindlimb suspension on VO2max and regional blood flow responses to exercise. J Appl Physiol 66: 653–659, 1989.[Abstract/Free Full Text]
  45. Pandey KN. Vascular action and interaction of natriuretic peptide receptors. In: Contemporary Endocrinology: Endocrinology of Vasculature, edited by Sowers JR. Totawa, NJ: Human Press, 1996, p. 255–267.
  46. Papadopoulos A, Delp MD. Effects of hindlimb unweighting on the mechanical and structure properties of the rat abdominal aorta. J Appl Physiol 94: 439–445, 2003.[Abstract/Free Full Text]
  47. Purdy RE, Duckles SP, Krause DN, Rubera KM, Sara D. Effect of simulated microgravity on vascular contractility. J Appl Physiol 85: 1307–1315, 1998.[Abstract/Free Full Text]
  48. Rowell LB. Human Cardiovascular Control. New York: Oxford University Press, 1993.
  49. Rowell LB, Detry JM, Blackmon JR, Wyss C. Importance of the splanchnic vascular bed in human blood pressure regulation. J Appl Physiol 32: 213–220, 1972.[Free Full Text]
  50. Saltin B, Blomqvist G, Mitchell JH, Johnson RL Jr, Wildenthal K, Chapman CB. Response to exercise after bed rest and after training. Circulation 38: VII1–VII78, 1968.[Web of Science][Medline]
  51. Sauro MD, Fitzpatrick DF. Atrial peptides inhibit protein kinase C-mediated contraction in rat aorta. Eur J Pharmacol 180: 373–376, 1990.[CrossRef][Web of Science][Medline]
  52. Sayet I, Neuilly G, Mironneau J, Mironneau C. Influence of spaceflight, hindlimb suspension, and venous occlusion on alpha 1-adrenoceptors in rat vena cava. J Appl Physiol 78: 1882–1888, 1995.[Abstract/Free Full Text]
  53. Shoemaker JK, Hogeman CS, Silber DH, Gray K, Herr M, Sinoway LI. Head-down-tilt bed rest alters forearm vasodilator and vasoconstrictor responses. J Appl Physiol 84: 1756–1762, 1998.[Abstract/Free Full Text]
  54. Shoukas AA, Bohlen HG. Rat venular pressure-diameter relationships are regulated by sympathetic activity. Am J Physiol Heart Circ Physiol 259: H674–H680, 1990.[Abstract/Free Full Text]
  55. Tabuchi Y, Nakamaru M, Rakugi H, Nagano M, Ogihara T. Endothelin enhances adrenergic vasoconstriction in perfused rat mesenteric arteries. Biochem Biophys Res Commun 159: 1304–1308, 1989.[CrossRef][Web of Science][Medline]
  56. Thomason DB, Booth FW. Atrophy of the soleus muscle by hindlimb unweighting. J Appl Physiol 68: 1–12, 1990.[Abstract/Free Full Text]
  57. Watenpaugh DE, Hargens AR. The cardiovascular system in microgravity. In: Handbook of Physiology. Environmental Physiology. Bethesda, MD: Am. Physiol. Soc., 1996, sect. 4, vol. I, chapt. 29, p. 631–674.
  58. Wilkerson MK, Lesniewski LA, Golding EM, Bryan RM Jr, Amin A, Wilson E, Delp MD. Simulated microgravity enhances cerebral artery vasoconstriction and vascular resistance through endothelial nitric oxide mechanism. Am J Physiol Heart Circ Physiol 288: H1652–H1661, 2005.[Abstract/Free Full Text]
  59. Wilkerson MK, Muller-Delp J, Colleran PN, Delp MD. Effects of hindlimb unloading on rat cerebral, splenic, and mesenteric resistance artery morphology. J Appl Physiol 87: 2115–2121, 1999.[Abstract/Free Full Text]
  60. Woodman CR, Sebastian LA, Tipton CM. Influence of simulated microgravity on cardiac output and blood flow distribution during exercise. J Appl Physiol 79: 1762–1768, 1995.[Abstract/Free Full Text]
  61. Wronski TJ, Morey-Holton ER. Skeletal response to simulated weightlessness: a comparison of suspension techniques. Aviat Space Environ Med 58: 63–68, 1987.[Medline]
  62. Zhang LF, Papadopoulos A, Delp MD. Vascular adaptation to microgravity. J Appl Physiol 97: 1584–1587, 2004.[Abstract/Free Full Text]
  63. Zhang LN, Zhang LF, Ma J. Simulated microgravity enhances vasoconstrictor responsiveness of rat basilar artery. J Appl Physiol 90: 2296–2305, 2001.[Abstract/Free Full Text]



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