INTRODUCTION

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INCREASED DIETARY SODIUM has been associated with the enhanced risk of high blood pressure (4, 12) because increased salt intake induces and/or increases hypertension in several experimental animal models (1, 15, 34, 40). However, the mechanisms of the hypertensinogenic effects of increased dietary sodium have not been completely defined. Several studies (37, 38, 47) have suggested that elevated sodium ingestion is associated with activation of the sympathetic nervous system (SNS), and it has been proposed that increasing dietary salt alters the SNS circuits that regulate SNS control of cardiovascular function (37, 41).

In support of this proposal, recent studies (19, 39) show that animals on a high-salt diet demonstrate enhanced pressor responses to microinjection of several excitatory neurotransmitters into the rostral ventrolateral medulla. These authors suggest that although dietary sodium does not increase blood pressure alone, it may contribute to development of hypertension by potentiating the actions of other hypertensive stimuli on SNS centers. However, in addition to enhancing sympathoexcitatory responses to stimulation of medullary pressor sites, increased dietary sodium also potentiates the depressor responses to both glutamate injections in the nucleus tractus solitarius (19) and aortic nerve stimulation (39) and enhances baroreflex-induced bradycardia (39). Therefore, it appears that both sympathoexcitatory and sympathoinhibitory responses are sensitized by increased dietary sodium. However, the physiological significance of these observations has not been completely defined.

Systemic ANG II has been implicated in chronically increased blood pressure through several mechanisms, including activation of the SNS (25, 48). Although the initial increase in blood pressure during peripheral administration of pressor concentrations of ANG II is due to the direct vasoconstrictor effects of the hormone on vascular smooth muscle, the chronic hypertensive response to longer-term, continuous administration (hours to days) of ANG II is mediated by progressive activation of the SNS (6, 25). Indeed, a predominant mechanism through which ANG II contributes to sustained hypertension is activation of central SNS sympathoexcitatory centers (11). It is possible that increased dietary sodium contributes to hypertension by sensitizing central SNS circuits to the excitatory effects of systemic ANG II. However, the effect of dietary sodium on activation of the SNS in response to peripheral ANG II has not been investigated.

Therefore, the purpose of these studies was to determine whether increased dietary sodium alters the progressive increase in SNS control of blood pressure and heart rate during intravenous infusion of ANG II. To address this question, we used intravenous administration of a hypertensive concentration of ANG II and ganglionic blockade with trimetaphan because these techniques have been previously used to evaluate the contribution of SNS stimulation to the pressor response evoked by ANG II (6, 25).

	MATERIALS AND METHODS

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Animals. Male Sprague-Dawley rats were obtained from a commercial supplier (Charles River) and housed in individual cages with ad libitum access to food and drinking solution. Rats were maintained in a room with a 12:12-h light-dark cycle at 22°C.

Sodium ingestion. Animals were provided either tap water or isotonic saline (Iso) as their sole drinking solution and standard laboratory rat chow. Rats were maintained on this regimen for 3-4 wk before surgery and testing.

Surgical procedure. On the day before testing, animals were anesthetized with 300 mg/kg tribromoethanol (Avertin). A 40-mm polyethylene (PE) catheter (PE-10 cemented into PE-50) filled with heparinized (50 U/ml) Iso was implanted into a femoral artery. In addition, two similar catheters were implanted in a femoral vein. These catheters were used to monitor arterial blood pressure and heart rate, infuse intravenous ANG II, and inject trimetaphan, respectively. The distal ends of the catheters were led subcutaneously to exit between the scapulas. In some animals, the catheters were secured at this point. These animals were then allowed to recover from the anesthetic and were returned to their home cages overnight.

Protocol. On the morning of testing, animals were brought to the laboratory and the arterial catheter was connected to a pressure transducer and MacLab data-acquisition device for continuous monitoring of arterial blood pressure. Heart rate was calculated by the MacLab data-acquisition system from the pressure pulses with the use of an internal rate meter. One femoral vein catheter was attached to a syringe filled with ANG II (5.3 µg/ml isotonic saline; Sigma; St. Louis, MO), which was placed in a syringe pump.

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Statistical analysis. All data are presented as means ± SE. Comparisons between two means were made using Student's t-test. Differences between multiple means were evaluated with a two-factor analysis of variance for repeated measures, followed by a Newman-Keuls a posteriori test.

	RESULTS

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Animals weighed 338 ± 10 g at the time of testing. Furthermore, there were no differences in control blood pressures or heart rates between groups before treatment.

Figure 1 shows blood pressure and heart rate for animals consuming either tap water or Iso before and during 6-h intravenous infusion of ANG II. There were no differences in either variable before the initiation of the infusion period. Furthermore, blood pressure increased to an equivalent degree in tap water-treated and Iso animals during ANG II administration. However, the bradycardia associated with the ANG II-induced pressor response was significantly greater in rats that consumed Iso before being tested.

View larger version (16K): [in this window] [in a new window]   Fig. 1.   Mean arterial blood pressure (A) and heart rate (B) before (Cont) and during intravenous infusion of ANG II in animals consuming tap water (Tap; n = 8) or isotonic saline (Iso; n = 9). *P < 0.05 (individual time points) and P < 0.05 (overall ANOVA) compared with Tap.

The maximum fall in blood pressure and change in heart rate evoked by ganglionic blockade before and during ANG II infusion are illustrated in Fig. 2. The depressor response to trimetaphan before administration of ANG II was significantly smaller in animals that consumed Iso compared with rats that drank tap water. Consequently, the minimum blood pressure during ganglionic blockade was lower in animals that drank tap water (70 ± 3 mmHg) than in animals that drank Iso (85 ± 3 mmHg). The pressor response to ANG II induced the expected decrease in SNS tone in both experimental groups, because at 30 and 60 min of infusion, the fall in blood pressure induced by trimetaphan was significantly smaller than that observed during the control observations. However, after 180 min of ANG II infusion, whereas the magnitude of the depressor effect of ganglionic blockade remained significantly smaller than control in the tap water-treated animals, it was equivalent to control in the animals consuming Iso. Furthermore, after 360-min infusion of ANG II, the fall in blood pressure after trimetaphan was similar to control in both groups, but was significantly smaller in animals consuming Iso compared with tap water-treated rats. Finally, depressor responses during ganglionic blockade after 6 h of ANG II infusion were significantly greater than that observed after 30 min of ANG II administration in animals that drank tap water. This demonstrates a significant increase in the neural component of the pressor response. However, in Iso animals, the depressor response after trimetaphan was similar at all times during ANG II infusion.

View larger version (21K): [in this window] [in a new window]   Fig. 2.   Maximum change in mean arterial blood pressure (A) and heart rate (B) after ganglionic blockade before (Cont) and during intravenous infusion of ANG II in Tap (n = 8) or Iso rats (n = 9). *P < 0.05 and **P < 0.01 compared with Cont; P < 0.05 compared with Tap; §P < 0.05 compared with 30-min Tap.

Heart rate responses after ganglionic blockade are illustrated in Fig. 2B. Before the initiation of the infusion period, trimetaphan produced no significant change in heart rate in either experimental group. However, during 0-6 h of intravenous ANG II administration, ganglionic blockade resulted in a similar tachycardia in both groups of animals.

Figure 3 shows arterial blood pressure and heart rate before, and following 6, 10, 16, and 24 h of ANG II infusion. (Data shown for 6 h of ANG II infusion have been redrawn from Fig. 1.) The increase in blood pressure after 10, 16, and 24 h of ANG II administration was similar between groups and not different from the pressor response observed after 6 h of ANG II infusion.

View larger version (22K): [in this window] [in a new window]   Fig. 3.   Mean arterial blood pressure (A) and heart rate (B) before (Cont) and during infusion of ANG II for 6 h, 10 h (Tap, n = 5; Iso n = 6), 16 h (Tap, n = 5; Iso n = 7), or 24 h (Tap, n = 6; Iso, n = 6). Data for 6 h are redrawn from Fig. 1. *P < 0.05 and **P < 0.01 compared with Cont; P < 0.05 compared with TAP.

While heart rate was significantly reduced in both tap water-treated and Iso animals after 6 h of intravenous ANG II, extending the infusion period to 10 and 16 h resulted in significant increases in heart rate compared with control values. However, the onset and magnitude of the ANG II-induced tachycardia was delayed and attenuated in Iso animals. After 10 h of ANG II infusion, heart rate in tap water-treated rats was significantly greater than control values, whereas heart rate was equivalent to control values in Iso rats, and significantly lower than in tap water-treated animals. Heart rate was significantly greater than control in both groups after 16 h of ANG II infusion, but was significantly smaller in Iso rats compared with tap water-treated animals. Finally, heart rates in both experimental groups were equivalent to their respective control values and to each other after 24 h of ANG II infusion.

Figure 4 illustrates changes in mean arterial blood pressure and heart rate induced by ganglionic blockade after 0.5, 10, 16, and 24 h of ANG II infusion (data shown for 0.5 h ANG II infusion have been redrawn from Fig. 2). Similar to the relationship observed after 6 h of ANG II infusion (Fig. 2), the fall in blood pressure in response to trimetaphan was significantly smaller in Iso, compared with tap water-treated rats after 10 and 16 h of ANG II administration. After 24 h of ANG II infusion, the depressor response to ganglionic blockade was equivalent between groups. Finally, as noted after 6 h of ANG II infusion (Fig. 2), the trimetaphan-induced decrease in blood pressure after 10, 16, and 24 h of ANG II infusion was significantly greater than that observed after 30 min in tap water-treated animals. However, there were no differences in depressor responses at any observation time in Iso rats.

View larger version (18K): [in this window] [in a new window]   Fig. 4.   Maximum change in mean arterial blood pressure (A) and heart rate (B) in response to trimetaphan during infusion of ANG II for 0.5 h (Tap, n = 8; Iso, n = 9), 10 h (Tap, n = 5; Iso, n = 6), 16 h (Tap, n = 5; Iso, n = 7), or 24 h (Tap, n = 6; Iso, n = 6). Data for 0.5 h are redrawn from Fig. 2. P < 0.05 compared with Tap; *P < 0.05 compared with 0.5-h Tap.

The heart rate responses to intravenous trimetaphan were extremely variable following longer infusion times. Because of the large variances, there were no significant differences between groups in the heart rate response to ganglionic blockade or after 10-, 16-, or 24-h intravenous infusion of ANG II.

Finally, intravenous administration of atropine produced equivalent changes in heart rate between tap water- and Iso-treated rats after both 0.5 h (tap: 56 ± 7 beats/min, n = 9; Iso: 70 ± 7 beats/min, n = 8) and 6 h (tap: 14 ± 5 beats/min, n = 6; Iso: 23 ± 13 beats/min, n = 6) of ANG II infusion.

	DISCUSSION

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These studies have demonstrated that moderate increases in dietary sodium alter normal neural control of blood pressure and cardiovascular responses to intravenous infusion of a pressor concentration of ANG II. Specifically, the neural component of arterial blood pressure maintenance is diminished by enhanced sodium ingestion. In addition, recovery of control neural tone following the baroreflex-induced reduction in SNS drive during ANG II infusion occurs significantly faster, but remains smaller in animals consuming increased dietary sodium than in control rats. Furthermore, although the pressor response to intravenous ANG II is not different between groups, the initial bradycardia (0-6 h) associated with intravenous ANG II is significantly enhanced, and the tachycardia resulting from long-term (10-16 h) infusion of ANG II is attenuated by high-sodium diet.

Dietary sodium has been implicated in the development of hypertension for many years (4, 12). However, despite intensive study, the mechanisms mediating the association between salt ingestion and increased blood pressure have not been completely defined. One problem has been that even large increases in dietary salt do not consistently increase blood pressure in normal animals. An intriguing hypothesis has been proposed that even though enhanced dietary sodium is not hypertensive alone, it sensitizes central SNS pressor sites to the effects of other hypertensive agents (19, 39). However, these same studies demonstrated that activation of central sites mediating depressor effects and baroreflex responses are also enhanced by increased dietary sodium. The net effect of enhanced central sympathoexcitatory and sympathoinhibitory responses induced by dietary sodium on blood pressure regulation has not been completely evaluated.

The present studies examined the effects of high-sodium diet on cardiovascular responses to intravenous infusion of a pressor concentration of ANG II. This treatment was selected as a sympathoexcitatory stimulus for several reasons. First, increased systemic ANG II contributes to hypertension through activation of central SNS pathways (11). Second, whereas intravenous ANG II initially produces sympathoinhibition due to baroreflex-mediated withdrawal of SNS tone, the neural component of the pressor response increases over longer-term (hours to days) infusions until the hypertension is predominantly due to increased SNS drive (6, 25, 48). Finally, in addition to sympathoexcitation, which tends to increase arterial blood pressure, long-term intravenous administration of pressor concentrations of ANG II stimulates central centers resulting in sympathoinhibition associated with baroreflex activation (28).

The dose of ANG II (200 ng · kg¹ · min¹) was specifically selected because it produces a rapid, stable increase (40-50 mmHg) in blood pressure initially (<1 h) associated with profound sympathoinhibition, followed by a progressive increase in SNS mediation of the pressor response (25). Indeed, it has been reported that within 24 h the entire increase in blood pressure is due to SNS activation (25). Because the time course of the increasing neural component has been determined in the rat (25), both the latency and magnitude of SNS activation induced by intravenous ANG II infusion can be evaluated. Therefore, infusion of this dose of ANG II provides a documented technique to produce progressive activation the SNS so the effects of increased dietary sodium could be quantified. Although plasma concentrations of ANG II probably exceed those observed under physiological conditions, similar or higher systemic doses of ANG II are routinely used to investigate the central actions of the systemic peptide (25, 29, 42, 43).

Results from these experiments suggest that increased sodium in the diet reduces basal SNS tone to the vasculature. Iso animals exhibited a smaller decrease in control arterial blood pressure during ganglionic blockade than tap water-treated rats (21, 30). These findings are in contrast to other studies (19, 39) reporting that ganglionic blockade produces similar decreases in control blood pressure in animals consuming normal and high-sodium diets. This discrepancy may be due to the use of anesthetized animals in these earlier reports. The results from the present experiments are consistent with investigations reporting that increases in dietary sodium are associated with diminished SNS drive in rats (3, 13) and in humans (21, 30).

It is possible that decreased SNS tone in normal animals on a high-sodium diet may represent a compensatory response to increased cardiac output and/or vascular volume observed in rats (14, 22) and humans (21, 30). However, it is unlikely that animals consuming Iso in the present experiments were volume expanded. Although there are reports (19, 24) of increased plasma and/or blood volume with large increases in dietary sodium (8% diet), other studies (10, 18, 20) report no effects of this diet on vascular volumes in normotensive salt-resistant animals. Furthermore, we could find no studies demonstrating increased fluid volumes in animals consuming a more moderate 4% NaCl diet. The volume of Iso consumed by animals in the current study (40-50 ml/day) represents only a 2.5- to 3-fold increase in normal sodium consumption, which is even less than that observed in rats eating a 4% NaCl diet. Therefore, the animals in the present study were probably not volume expanded as a result of the Iso ingestion alone.

In addition to reduced basal sympathetic tone, the SNS component of the pressor response during 6-16 h of ANG II infusion was significantly smaller in Iso animals than tap water-treated rats. These results indicate that increased dietary sodium did not enhance the sympathoexcitatory response to the circulating peptide. These data are consistent with the proposal that central sympathoinhibitory responses predominate during ANG II infusion in normal rats on high-sodium diet.

It is possible that Iso rats in the 10-, 16-, and 24-h infusion groups might experience some degree of volume expansion due to the sodium-retaining effects of acute ANG II infusion (26, 27). However, this probably does not account for the differences in heart rate or diminished depressor response to trimetaphan in the Iso group. Several studies have reported that acute volume expansion and maintenance on a high-sodium diet either decrease (16, 17) or have no effect (23, 35, 45) on bradycardia and sympathoinhibition during acute loading of the baroreceptors or baroreceptor afferent stimulation. These reported cardiac responses are qualitatively opposite the enhanced bradycardia observed in Iso animals during the initial 6 h of ANG II infusion. Furthermore, if sympathoinhibition were decreased by potential volume expansion due to Iso ingestion and ANG II infusion in the 10-, 16-, and 24-h groups, there should be more trimetaphan-sensitive SNS tone at these time periods. However, ganglionic blockade resulted in a smaller decrease in blood pressure in Iso animals, suggesting less SNS tone, presumably due to greater, prolonged sympathoinhibition.

In addition to decreased vascular SNS tone, data from the present experiments provide evidence that cardiac sympathoinhibition during ANG II infusion is enhanced by Iso ingestion. For example, the baroreflex-induced bradycardia during the initial 6 h of ANG II infusion was significantly greater in animals on a high-salt diet. Because changes in heart rate induced by ganglionic blockade are predominantly due to reduced vagal tone (44), and the tachycardia observed after trimetaphan was similar between Iso and tap water-treated animals, parasympathetic activation appears equivalent between the groups. Furthermore, bradycardia during sustained elevations in blood pressure is increasingly mediated by withdrawal of SNS tone (5, 46). Therefore, the differences in heart rate between Iso and tap water-treated rats during the initial 6 h of ANG II infusion are probably mediated by increased cardiac SNS withdrawal in Iso animals. This proposal is supported by the similar effects of atropine on heart rate in both Iso and tap water-treated rats after 30 and 360 min of ANG II infusion.

Furthermore, the tachycardia resulting from longer-term increases in plasma ANG II (2, 31, 33) was delayed and attenuated by Iso consumption. This tachycardia is mediated by increased SNS drive (36). The failure to demonstrate any tachycardia after 10 h of ANG II infusion, and the significantly smaller heart rate after 16 h in Iso animals compared with tap water rats is consistent with the proposal that increased dietary sodium diminished activation of cardiac SNS circuits during ANG II infusion.

It has been shown that long-term (in hours) intravenous infusion of pressor concentrations of ANG II activates central sites mediating sympathoinhibition and SNS withdrawal (28). Furthermore, increased dietary sodium enhances depressor responses to chemical stimulation of the nucleus tractus solitarius (19) and electrical stimulation of the aortic depressor nerve (39), and increases bradycardia to aortic depressor nerve stimulation (39). These vascular and cardiac responses are consistent with enhanced SNS inhibition. Data from the current study suggest that potentiation of this, or another, central sympathoinhibitory circuit by increased dietary sodium predominates over enhanced responsiveness of sympathoexcitatory centers (19, 39) during systemic ANG II infusion in normal animals.

Previous experiments have demonstrated that hypertension associated with systemic ANG II administration is enhanced with concurrent administration of increased dietary sodium (1, 8, 9). Although not directly tested in all of these earlier studies, the data are consistent with increased SNS tone contributing to the increase in blood pressure (7). However, the present studies found that under these experimental conditions, a moderate increase in sodium ingestion diminishes SNS activation in response to acute intravenous administration of pressor doses of ANG II. The reason(s) for the apparent differences in effects on SNS drive may be related to different durations of ANG II administration (24 h vs. >1 wk), different doses of ANG II (pressor vs. nonpressor), and/or the increase in dietary sodium (3× vs. 6-9×).

Taken together, data from the present studies suggest that increased dietary sodium enhances responses of central sites resulting in vascular and cardiac sympathoinhibition. However, it has also been convincingly demonstrated that increased dietary sodium potentiates pressor responses to stimulation of central sympathoexcitatory sites (19, 39) and that genetically susceptible animals exhibit increased SNS tone when consuming a high-sodium diet (32, 34, 38). Taken together, these data are consistent with the proposition that one component of genetic susceptibility to the hypertensive effects of dietary sodium may be the relative balance of increased sensitivity of central SNS circuits controlling sympathoexcitatory and sympathoinhibitory responses to pressor agents and baroreflex sensitivity. This proposal would suggest that dietary sodium might selectively enhance central sympathoexcitatory centers in genetically susceptible animals, leading to a dose-dependent relationship between sodium ingestion and both onset latency and magnitude of hypertension (38). However, in sodium-resistant animals, increased dietary sodium may selectively potentiate responses of central sympathoinhibitory sites, which increasingly oppose the hypertensinogenic effects of sodium ingestion. It is conceivable that dietary sodium could exceed the ability of central sympathoinhibition to compensate, and increased blood pressure could result even in resistant animals if sodium ingestion was great enough, but to a lesser degree than in susceptible rats.

In previous studies using rats, the pressor response to intravenous infusion of 200 ng · kg¹ · min¹ ANG II was mediated completely by neural mechanisms within 24 h (25). However, in the present experiments, whereas the neural component of the ANG II-induced response increased progressively during longer infusion periods in tap water-treated rats, a significant nonneural contribution remained even after 24 h of ANG II infusion because trimetaphan did not reduce blood pressure to control values. The reason for the differences between these results and previous studies is unknown.

In summary, these studies found that moderate increases in dietary sodium diminish the SNS contribution to basal blood pressure maintenance and to the neural component of the pressor response induced by intravenous ANG II. Furthermore, baroreflex-mediated bradycardia is enhanced, whereas the tachycardia after longer infusions of ANG II is attenuated by increased dietary sodium. These data are consistent with the proposal that a moderate increase in sodium ingestion by normal rats enhances sympathoinhibitory responses during systemic ANG II infusion. These data suggest that one component of genetic susceptibility to the hypertensive effects of dietary sodium may be the relative potentiation of central circuits controlling sympathoexcitation and sympathoinhibition to pressor agents and baroreflex activation.