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Department of Physiology and Pharmacology, The Oregon Health Sciences University, Portland, Oregon 97201
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Acute infusion of hypertonic fluid
increases mean arterial pressure (MAP) in part by elevating nonrenal
sympathetic activity. However, it is not known whether chronic,
physiological increases in osmolality also increase sympathetic
activity. To test this hypothesis, MAP, heart rate (HR), and lumbar
sympathetic nerve activity (LSNA) were measured in conscious, 48-h
water-deprived rats (WD) during a progressive reduction in osmolality
produced by a 2-h systemic infusion (0.12 ml/min) of 5% dextrose in
water (5DW). Water deprivation significantly increased osmolality (308 ± 2 vs. 290 ± 2 mosmol/kgH2O,
P < 0.001), HR (453 ± 7 vs. 421 ± 10 beats/min, P < 0.05), and
LSNA (63.5 ± 1.8 vs. 51.9 ± 3.8% baroreflex maximum,
P < 0.01). Two hours of 5DW infusion
reduced osmolality (
15 ± 5 mosmol/kgH2O), LSNA (23 ± 3% baseline), and MAP (10 ± 1 mmHg). To evaluate
the role of vasopressin in these changes, rats were pretreated with a
V1-vasopressin receptor
antagonist. The antagonist lowered MAP (5 ± 1 mmHg) and
elevated HR (32 ± 7 beats/min) and LSNA (11 ± 3% baseline) in
WD (P < 0.05), but not in
water-replete, rats. 5DW infusion had a similar cumulative effect on
all variables in V1-blocked WD
rats, but had no effect in water-replete rats. Infusion of the same
volume of normal saline in WD rats did not change osmolality, LSNA or
MAP. Together these data indicate that, in dehydrated rats, vasopressin
supports MAP and suppresses LSNA and HR and that physiological changes
in osmolality directly influence sympathetic activity and blood
pressure independently of changes in vasopressin and blood volume.
sodium chloride; vasopressin; V1-vasopressin antagonist; heart rate; conscious rats
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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SEVERAL STUDIES support the notion that increased plasma osmolality is directly linked to increased activity in nonrenal sympathetic nerves. It has been shown that acute intracerebroventricular and intravenous administration of hypertonic saline increases arterial pressure, an effect that is attenuated by ganglionic blockade (7, 10, 13, 17). Increases in plasma norepinephrine levels have also been observed (13, 17, 24). Finally, direct recordings of sympathetic activity, primarily in anesthetized animals, reveal that hypertonic saline increases the activity of splanchnic, adrenal, and lumbar sympathetic nerves while suppressing renal sympathetic activity (4, 17, 21, 28). The latter effect is thought to be due, at least in part, to arterial baroreflex activation (21, 28).
Although the available literature suggests that acute increases in osmolality are sympathoexcitatory in nonrenal sympathetic nerves, it is not known whether chronic, physiological increases in osmolality tonically increase sympathetic outflow. Therefore, this study tested the hypothesis that hypertonicity can produce sustained excitation of nonrenal sympathetic nerves. To test this hypothesis, lumbar sympathetic nerve activity (LSNA) and arterial pressure were measured in dehydrated conscious rats during progressive reductions in osmolality. It was reasoned that if increased osmolality does chronically support sympathetic outflow and blood pressure, then acute decreases in osmolality from elevated levels by water infusion should decrease LSNA and arterial pressure.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Animals
Male Sprague-Dawley rats weighing between 350 and 400 g (Simonson, Gilroy, CA) were housed individually in Plexiglas cages in the Animal Care Unit with ad libitum access to a 1% NaCl diet (Harlan Teklad, Madison, WI) at least 1 wk before surgery. The housing facility was maintained at a constant temperature of 22 ± 2°C with a 12:12-h light-dark cycle.Surgery
Twenty-four hours before the experiment, rats were anesthetized (pentobarbital sodium, 25 mg ip) for catheter and lumbar sympathetic nerve recording electrode placement. Animals were implanted with bilateral femoral arterial catheters (PE-50 heat welded to a length of PE-10) for direct measurement of mean arterial blood pressure (MAP) and arterial blood withdrawal. An additional femoral vein catheter (Tygon, Norton Performance Plastics, Akron, OH) was implanted for infusion of drugs and hydrating solutions. The catheters were tunneled subcutaneously to exit at the nape of the neck, and the femoral incisions were sutured closed. In most rats, a stainless steel, bipolar recording electrode was implanted around a lumbar nerve through a midline abdominal incision to measure nerve activity.The electrode was made from two lengths of single-stranded Teflon-coated, stainless steel wire (bare OD = 0.005 in., A-M Systems, Everett, WA) soldered to a microconnector (Microtech, Boothwyn, PA). The leads were isolated within a length of Silastic tubing extending to the microconnector. The assembly was glued together at the microconnector end with epoxy. The electrode was implanted through a 4-cm midline abdominal incision. First, the microconnector assembly was externalized subcutaneously, along with the vascular catheters, at the nape of the neck, after which the electrode was positioned above the midline using a micromanipulator. The abdominal organs were then retracted, exposing the underlying abdominal aorta and inferior vena cava ~1 cm caudal to the renal artery. The vessels were retracted to the animal's right side, exposing the underlying lumbar nerves. A small (1-2 mm) length of the left lumbar sympathetic nerve was isolated and placed on hooks formed at the end of the two electrode leads. A small loop (000 silk) was tied into the neighboring psoas muscle. The nerve was embedded in a lightweight dental silicon (Bisico, Bielefeld, Germany) that also incorporated the silk such that the loop served as an anchor to keep the electrode in place. The retractors were carefully removed, and the abdominal incision was sutured closed in two layers with the electrode leads coiled within the subcutaneous space. The rats were allowed to recover overnight in their home cage.
Chemicals
Pentobarbital sodium for anesthesia was obtained from Abbott Laboratories, North Chicago, IL. Nitroprusside (Elkins-Sinn, Cherry Hill, NJ) was used for determination of baroreflex-mediated maximum sympathetic nerve activity. The V1-receptor antagonist [1-(
-mercapto-,-cyclopentamethylene proprionic
acid),2-(O-methyl)tyrosine]-Arg-8-arginine vasopressin was obtained from Peninsula Laboratories (Belmont, CA).
Trimethaphan for autonomic ganglion blockade was kindly donated by Dr.
J. Sepinwall from Hoffman-La Roche. Phenylephrine for inhibition of
preganglionic nerve activity was obtained from American Reagent (Shirley, NY).
Data Acquisition
During all experiments, MAP, heart rate (HR), and LSNA were recorded throughout. One arterial catheter was connected to Grass bridge (7P1) and tachograph (7P4) preamplifiers (Grass Instruments, Quincy, MA) for determination of MAP and HR, respectively. The recording electrode microconnector assembly was connected to a Grass differential preamplifier (P511). Raw nerve activity was filtered and amplified (30-10,000 Hz, 20-70,000×). The resulting signal was monitored visually with an oscilloscope (model 2212, Tektronics, Beaverton, OR) and simultaneously fed through a Grass integrator (7P10). The signal was whole wave rectified and integrated with a reset time of 1 s. Together, MAP, HR, and integrated LSNA were recorded on a Grass polygraph (7D). Nerve activity was quantified by averaging the integrated activity just before reset over a 12-s (12 peaks) period during stable periods. For measurement of maximum baroreceptor-mediated increases in activity, averages of peak nerve responses over 3-4 s were measured. Background noise was determined at the end of the experiment by averaging the integrated nerve activity over a 12-s period after elimination of efferent nerve activity with a combined bolus injection of trimethaphan (1.5 mg) and phenylephrine (70 µg). Background noise was subtracted from averaged integrated nerve activity to provide a measurement of LSNA. All measurements of LSNA were normalized (%control) to basal nerve activity determined over a 10-min period before any treatments. Only visibly healthy animals with greater than a 2:1 signal-to-noise ratio (>50% reduction of the signal after ganglionic blockade compared with control levels) were included in the study.Experimental Protocols
Twenty-four hours after surgery, the vascular catheters as well as the nerve electrode leads were connected to the recording equipment between 8:00 and 9:00 AM while the rat rested unrestrained in its home cage. The rat was allowed at least 2 h habituation to the instrumentation. Then one of the following four protocols was performed.Protocol 1. The purpose of this experiment was to test the hypothesis that increases in osmolality chronically stimulate sympathetic outflow. This was done by determining if acute decreases in chronically elevated osmolality are associated with decreased LSNA. Water deprivation began 48 h before the experiment to increase osmolality. Twenty-four hours after removing drinking water, surgery was performed to implant catheters and a nerve electrode. On the next, experimental day, basal measurements were taken and a bolus injection of nitroprusside (70 µg iv) was given for determination of maximal reflex sympathetic activity. Approximately 30 min later, an initial 350 µl blood sample was taken in most experiments for determination of hematocrit, plasma protein, and plasma osmolality. Blood was replaced with isotonic saline. In some rats, the blood sample was used for determination of plasma glucose concentration and/or sodium, potassium, and chloride concentrations. After MAP and nerve activity stabilized after blood withdrawal (5-10 min), a 120 min intravenous infusion of 5% dextrose in water (5DW) was begun (0.12 ml/min) while MAP, HR, and LSNA were monitored continuously. 5DW is isotonic and initially does not affect osmolality or cell volume. However, the dextrose is rapidly metabolized, leaving behind solute-free water, which dilutes the body fluids. Effective dilution of plasma osmolality was documented by taking additional blood samples 30, 60, and 120 min after initiating the infusion. Blood was again replaced with saline.
Protocol 2. Presumably, progressive decreases in chronically elevated osmolality would also suppress increased plasma vasopressin in the water-deprived rats. Vasopressin is recognized to have both sympathoinhibitory and potent vasoconstrictor effects (8, 12). Therefore, this protocol was performed to determine 1) if vasopressin contributed to maintenance of MAP, HR, and LSNA during water deprivation and 2) if a decrease in vasopressin contributed to the decrease in MAP and counteracted the decrease in LSNA during the 5DW infusion in water-deprived rats. In these experiments, responses to V1-receptor antagonist administration were compared in water-deprived and water-replete rats. Rats were pretreated with the vasopressin V1-receptor antagonist (5 µg iv) 30 min before collection of the initial blood sample. This dose of vasopressin antagonist was found to block pressor responses to intravenous injection of a 10 ng dose of arginine vasopressin for at least 2 h after administration. After recovery from the initial blood sampling, 5DW was infused as described in Protocol 1.
Protocol 3. The purpose of this protocol was to determine if 5DW infusion had nonspecific effects on MAP, HR, or LSNA in water-replete rats. In this experiment, rats allowed free access to water were treated with the V1-vasopressin antagonist and were given an infusion of 5DW as described in Protocol 2.
Protocol 4. The purpose of this protocol was to determine if the volume load produced by 5DW infusion contributed to changes in MAP, HR, and LSNA observed during the infusion. Rats were treated precisely as described in Protocol 2, except that isotonic (0.9%) saline was infused (0.12 ml/min) instead of 5DW.
A separate group of rats were implanted with only femoral venous and arterial catheters. A subset of this group also received nonocclusive catheters (3.5 cm of 0.02 in. ID Silastic catheter connected to 0.02 in. ID, 0.06 in. OD Tygon tubing) in the abdominal vena cava for the measurement of central venous pressure (16). These animals were treated as in protocols 1-4, except that some animals were not pretreated with the V1-vasopressin antagonist and the blood withdrawn was used to measure plasma concentrations of sodium, potassium, chloride, and/or glucose.
Analytic Assays
Plasma electrolytes were determined from 150 µl whole blood with a Beckman Lablyte System (model 810, Beckman Instruments, Brea, CA). Plasma osomolality was determined by averaging values of triplicate 20 µl samples of plasma using a micro-osmometer (model 3MO, Advanced Instruments, Norwood, MA). Duplicate hematocrit tubes were filled with ~30 µl arterial blood and spun, and hematocrit was determined with an Adams microhematocrit reader (New York, NY). The tubes were then broken, and the plasma was used for determination of plasma protein with a Hitachi protein refractometer (National Instruments, Baltimore, MA). Glucose was determined with the aid of a COBAS BIO automated spectrophotometer (Roche Analytical Instruments, Somerville, NJ) using Sigma Glucose Trinder reagent and Sigma calibrator serum as a standard.Data Analysis
All data are presented as means ± SE. Two-way ANOVA for repeated measures was used to compare MAP, HR, and LSNA responses to V1-receptor blockade in water-deprived and water-replete animals. Student's t-tests were used to compare control values between water-deprived and water-replete rats (Table 1).
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Two-way ANOVA was used to compare MAP, HR, LSNA, and blood chemistry values during the infusion with the four treatments as the between-group variable and time as the within-group repeated measure (29). Significant interactions were followed up with Tukey Kramer post hoc tests for multiple between- and within-group comparisons (19, 29). Finally, linear regression analysis was used to determine relationships between osmolality and LSNA or MAP (29). All analyses were performed using GB-STAT software (Dynamic Microsystems, Silver Spring, MD). A significance level of P < 0.05 was accepted.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Effects of Water Deprivation on Baseline Parameters
Forty-eight hours of water deprivation increased hematocrit and plasma osmolality as well as plasma sodium and protein concentrations, verifying significant loss of plasma water (Table 1). Plasma concentrations of potassium and chloride were not changed significantly during water deprivation. Arterial pressure tended to be elevated (P = 0.059), while HR and basal LSNA, normalized as percent maximum baroreflex level, were increased in the water-deprived rats.Effects of 5DW Infusion in Water-Deprived Rats
Plasma osmolality, sodium, and chloride progressively decreased over the duration of the 2-h 5DW infusion in water-deprived rats (Fig. 1, Table 2). Plasma protein concentration and hematocrit also decreased, but plasma potassium levels were not altered by the infusion (Table 2). LSNA fell gradually, but the decrease did not reach statistical significance until 80 min after initiating the 5DW infusion (Fig. 1). Arterial pressure decreased rapidly and remained lower, whereas HR rose transiently (Fig. 2). Plasma glucose concentration increased initially but then returned toward control levels (Table 2).
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The effects of decreases in osmolality in rats pretreated with the
vasopressin V1 antagonist are
shown in Figs. 1-4. Administration of the receptor antagonist
decreased MAP and increased both HR and LSNA in water-deprived, but not
water-replete, rats (Fig. 3). Subsequent
infusion of 5DW in water-deprived rats given the V1-receptor antagonist produced
similar decreases in plasma osmolality as in untreated water-deprived
rats, whereas a larger and more rapid decrease in LSNA was observed
(Fig. 1). However, although the initial decrease in LSNA occurred more
quickly, the final levels achieved relative to preantagonist basal
levels were similar whether or not the rats were given the antagonist
(Fig. 1). 5DW infusion produced similar decreases in sodium, chloride,
protein, and hematocrit and increases in glucose in
V1 receptor-blocked and intact
water-deprived animals (Table 2). 5DW infusion reduced MAP in
V1 receptor-blocked,
water-deprived rats, but the fall was delayed in comparison to
V1 receptor-intact rats (Fig. 2). In contrast to intact rats, V1
receptor-antagonized rats showed no change in HR throughout the
infusion (Fig. 2). Figure 4 demonstrates that decreases in both LSNA and MAP from control values obtained immediately before starting the 5DW infusion were highly correlated to
the decrease in osmolality. Collectively, these data illustrate that
decreases in osmolality in water-deprived rats are associated with
decreases in arterial pressure and LSNA, independently of changes in
plasma vasopressin concentration.
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Effects of 5DW Infusion in Water-Replete Rats
5DW infusion did not significantly alter plasma osmolality, sodium, chloride, potassium, LSNA, arterial pressure, or HR in water-replete rats pretreated with a vasopressin V1 antagonist (Fig. 5; Table 2). Hematocrit and plasma protein concentration were also unaffected, but glucose increased transiently (Table 2).
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Effects of Saline Infusion in Water-Deprived Rats
Figure 6 shows the effects of saline infusion in water-deprived rats. Infusion of isotonic saline in V1 receptor-blocked, water-deprived rats produced similar decreases in plasma protein concentration and hematocrit as 5DW infusion in similarly treated animals (Table 2). Nevertheless, the saline volume loading did not produce a significant increase in central venous pressure (2.0 ± 0.6 vs. 1.6 ± 0.6 cmH2O at the end of the infusion;
n = 3;
P > 0.5). A similar insignificant
trend was observed in rats receiving 5DW (2.4 ± 0.7 vs.
2.2 ± 0.6 cmH2O after
120 min of infusion; n = 4;
P > 0.5). In contrast to 5DW infusion, saline did not modify plasma osmolality, sodium, chloride, potassium, or LSNA.
Arterial pressure tended to fall slightly with saline infusion, but the
response was not statistically significant. In contrast to effects
observed during 5DW infusion, HR decreased dramatically during the
saline infusion.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The purpose of this study was to test the hypothesis that long-term increases in osmolality tonically support arterial pressure in part by activation of nonrenal sympathetic nerves. The major new findings are that 1) decreases in osmolality produced by 5DW infusion in water-deprived rats are related to reductions in MAP and LSNA, 2) blockade of vasopressin V1 receptors decreases MAP and increases HR and LSNA in water-deprived rats, and 3) V1 receptor blockade attenuates the decline of MAP, but potentiates the inhibition of LSNA associated with reduced osmolality in water-deprived rats. These are the first data to demonstrate that endogenous, physiological changes in osmolality are sufficient to alter sympathetic tone.
The direct correlation between osmolality and LSNA or arterial pressure in the present study suggests that osmolality may be an important regulator of sympathetic outflow and, consequently, blood pressure. However, correlations do not prove a causal relationship. Therefore, it is important to determine whether factors other than reduced osmolality could account for the decreases in pressure and nerve activity observed with 5DW infusion in water-deprived rats.
One possible confounding factor is vasopressin. Because vasopressin and osmolality are so tightly linked, decreases in osmolality afforded by 5DW infusion very likely suppressed vasopressin secretion. Vasopressin is a potent vasoconstrictor (8, 12); therefore, its decline could have contributed to the hypotension observed during 5DW infusion. Injection of the V1-vasopressin antagonist lowered arterial pressure in water-deprived, but not water-replete, rats, in agreement with many (1, 2, 6), but not all (9, 11), prior studies. Our ability to detect an effect of V1 blockade may be in part because sympathetic nerve recordings require surgery the day before the experiment, and this procedure may have accentuated the dehydration. Nevertheless, these results suggest that a dehydration-induced rise in vasopressin supports MAP. Moreover, in V1-blocked water-deprived rats, the hypotension produced during subsequent 5DW infusion was delayed, suggesting that decreases in vasopressin contributed to the immediate depressor response in unblocked water-deprived rats. However, decreased vasopressin was not responsible for the entire fall, because V1-blocked water-deprived rats eventually showed a decrease in arterial pressure. Thus it appears that both vasopressin- and hypertonicity-induced increases in sympathetic activity contribute to maintenance of arterial pressure during water deprivation.
There is also substantial evidence that exogenous vasopressin inhibits sympathetic outflow via activation of the V1 receptor (8, 12). However, few studies have examined the actions of the endogenous peptide on the sympathetic nervous system. In the present study, V1-receptor antagonist administration increased LSNA in water-deprived rats. Decreases in endogenous vasopressin also appeared to counteract the initial fall in sympathetic activity with 5DW infusion in water-deprived rats, because prior antagonist administration unmasked a precipitous fall in LSNA during the first 20 min of 5DW infusion. The mechanism of the increased LSNA after V1 blockade is unknown, but could be due to loss of a direct inhibitory effect of vasopressin on nerve activity or to hypotension-induced baroreflex stimulation. Nevertheless, these data indicate that vasopressin counteracts the sympathostimulatory effects of high osmolality. Interestingly, the overriding effect in both groups was a reduction in LSNA during 5DW infusion, despite lowered MAP, indicating that osmolality may be a potent regulator of sympathetic activity even in the face of arterial baroreceptor unloading.
Reflex stimulation due to volume loading also could have contributed to the decrease in LSNA observed during 5DW infusion. The rate of 5DW infusion required to significantly decrease osmolality produced significant decreases in hematocrit and plasma protein concentration in water-deprived rats, indicating a substantial volume expansion. However, in a separate group of water-deprived rats, the same volume of saline produced similar decreases in hematocrit and protein but did not alter osmolality or LSNA. Interestingly, neither 5DW nor saline infusion in the present study produced detectable increases in central venous pressure, in agreement with a previous report (3). However, HR was markedly suppressed by saline infusion, suggesting that the volume load activated baroreceptors. These results indicate that changes in volume were not responsible for the fall in LSNA and are consistent with previous evidence that lumbar sympathetic tone is not particularly sensitive to volume expansion (27). Moreover, Oberg and Thoren (23) showed that stimulation of cardiac chemo- and mechanoreceptors has a more profound effect on renal blood flow than on skeletal muscle blood flow. This is logical when considering that reflex control of renal blood flow is more important for regulation of blood volume than is that of skeletal muscle blood flow. Nevertheless, it is not known whether the observed changes in lumbar sympathetic activity reflect changes in sympathetic drive to other vascular beds.
5DW was also infused in water-replete rats to assess its effects independent of changes in osmolality and volume expansion. No significant effects were measured, presumably because of rapid volume excretion and cellular glucose uptake. 5DW was shown to significantly elevate plasma glucose in both water-deprived and water-replete animals. Because increased glucose increases insulin secretion, which can influence sympathetic outflow, the possibility that glucose or insulin contributes to the observed responses needs to be considered. There are two reasons why this possibility is unlikely. First, insulin is sympathoexcitatory (22), and 5DW infusion decreased LSNA. Second, there was no correlation between the changes in glucose and LSNA: glucose levels were highest 30 min after beginning the 5DW infusion, whereas LSNA remained unaffected by the infusion in water-replete rats or reached a minimum at the end of the infusion in dehydrated rats.
Thus the results of multiple control experiments suggest that the decreases in LSNA produced by 5DW infusion in water-deprived rats is due to decreases in osmolality, rather than other nonspecific effects. Collectively, therefore, these findings support the hypothesis that elevated osmolality tonically supports LSNA and arterial pressure. Moreover, the results indicate that acute reductions in osmolality during water deprivation can provide a rapid and potent stimulus to decrease nerve activity and arterial pressure independently of changes in vasopressin or blood volume.
Although abundant prior work has demonstrated that acute intravenous or intracerebroventricular injections of hypertonic saline increase arterial pressure and sympathetic activity, few studies have determined if these effects can be sustained chronically. Two studies (15, 20) demonstrated that chronic intracerebroventricular infusion of hypertonic NaCl increased arterial pressure, but the hypertensive effect was evident only after several days of infusion. Further work implicates a role for sympathoexcitation in the hypertensive response (15, 20). However, the relevance of intracerebroventricular hypertonic saline administration to physiological or even pathophysiological changes in osmolality is debatable. Thus the major aim of the present study was to determine if endogenously produced increases in osmolality tonically enhance sympathetic outflow and arterial pressure. The finding that acute decreases in osmolality are directly and significantly related to decreases in LSNA and arterial pressure in water-deprived, but not water-replete, rats is to our knowledge the first evidence that physiological increases in body fluid osmolality can provide a potent and sustained stimulus to increase sympathetic activity and blood pressure.
An intriguing question is whether the chronic hypertonicity of water deprivation increases sympathetic activity above normal or simply provides a tonic drive for maintenance of normal activity. This question is difficult to address because there is currently no method for directly and quantitatively assessing between-group differences in baseline sympathetic outflow. Nevertheless, indirect evidence in this study that water deprivation elevates sympathetic activity above normal was provided by the observation that basal LSNA, normalized to percent baroreflex-mediated maximum, was significantly increased as was basal HR in dehydrated rats. Increased basal HR has been previously observed in water-deprived rats (11). Further data consistent with increased sympathetic activity are that adrenal mRNA levels for tyrosine hydroxylase, the rate-limiting enzyme involved in the synthesis of norepinephrine, are elevated after dehydration and that the response is prevented by adrenal denervation (5). However, most (9, 18, 26), but not all (25), studies failed to detect an increase in plasma norepinephrine concentration during water deprivation. Thus, unless norepinephrine clearance is increased, it is not presently established that sympathetic activity is increased above normal during water deprivation. Nevertheless, the present results emphasize that chronic increases in osmolality support LSNA, either at normal or elevated levels.
Although decreases in osmolality were associated with decreases in LSNA, no changes in HR were observed in vasopressin-blocked rats. However, saline volume loading produced profound bradycardia. Together, these findings suggest that 5DW may produce conflicting signals to the regulation of HR: the volume load tends to decrease HR, whereas the decrease in osmolality increases HR, resulting in no net change. This interpretation is consistent with prior studies showing a differential effect of osmolality on nerves innervating different organs (4, 28).
The location or nature of receptors at which decreases in osmolality could reduce LSNA and arterial pressure was not investigated in the present study. On the assumption that an osmoreceptor mediates the changes in LSNA and arterial pressure, a prime candidate would be the organum vasculosum of the lamina terminalis, because injection of hypotonic saline in the anteroventral third ventricular region decreases arterial pressure and adrenal nerve activity in anesthetized rats (4). Another possible site is the liver, where osmosensitive cells have been shown to influence arterial pressure, vasopressin, and sympathetic nerve activity (21). Whether changes in osmolality per se or in sodium chloride are sensed by these receptors was also not investigated. Plasma electrolyte measurements revealed that the decrease in osmolality was primarily due to decreases in sodium and chloride, suggesting that the decrease in sodium chloride likely mediated the response in these experiments. However, in a previous study (14), infusion of hypertonic sodium chloride and mannitol produced equivalent increases in arterial pressure, which is consistent with a receptor sensitive to changes in effective osmoles rather than to just sodium and/or chloride alone. Thus further experiments are required to elucidate the characteristics of the receptor involved.
In summary, the present experiments demonstrate that osmolality can provide a sustained and potent stimulus to maintain or elevate LSNA and arterial pressure independently of changes in vasopressin concentration or blood volume. Further work is required to identify the location and nature of receptors that mediate the action of osmolality changes on nerve activity.
Perspectives
Vasopressin secretion and drinking are very sensitive to changes in osmolality: a change of <1% is sufficient to elicit significant changes in urine flow and possibly thirst. In the present study, a 1% decrease in osmolality produced a 5% suppression of LSNA. This potency suggests the hypothesis that osmolality contributes to regulation of sympathetic activity in other physiological or pathophysiological states, such as changes in salt intake and exercise. Nevertheless, whereas the relationship between osmolality and nerve activity is physiologically significant, it is not as potent as the relationship between osmolality and plasma vasopressin concentration. This differential degree of sensitivity was apparent in the water-replete rats given 5DW. In these animals, the water infusion was rapidly excreted (presumably due to decreases in vasopressin) without measurable changes in osmolality. At the same time, however, there were also no detectable decreases in LSNA, suggesting that the immeasurable decreases in osmolality were not sufficient to influence LSNA.| |
ACKNOWLEDGEMENTS |
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The authors gratefully acknowledge the technical contributions of Lisa Welch and suggestions made by Dr. Steve Bealer during preparation of the manuscript.
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FOOTNOTES |
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This study was supported in part by National Heart, Lung, and Blood Institute Grant HL-35872. K. E. Scrogin was supported by National Research Service Award HL-09545.
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. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: V. L. Brooks, Dept. of Physiology and Pharmacology, L-334, The Oregon Health Sciences Univ., 3181 SW Sam Jackson Park Rd., Portland, OR 97201 (E-mail: brooksv{at}ohsu.edu).
Received 25 August 1998; accepted in final form 3 February 1999.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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) or pretreated
(n = 7; 
) with the
V1-vasopressin antagonist. 5DW
infusion was begun at time
zero. LSNA was normalized to values
measured before injection of antagonist. LSNA results from 2 untreated
rats were not included due to low signal-to-noise ratio.
* P < 0.05 compared with
time
zero within group.
