High-salt diet elevates baroreceptor pressure thresholds in normal and Dahl rats

Effects of a high salt diet on blood pressure dipping and the implications on hypertension

High blood pressure, also known as hypertension, is a major risk factor for cardiovascular disease. Salt intake has been shown to have a significant impact on BP, but the mechanisms by which it influences the blood pressure dipping pattern, and 24-h blood pressure remains controversial. This literature review aims to both summarize the current evidence on high salt diet induced hypertension and discuss the epidemiological aspects including socioeconomic issues in the United States and abroad. Our review indicates that a high salt diet is associated with a blunted nocturnal blood pressure dipping pattern, which is characterized by a reduced decrease in blood pressure during the nighttime hours. The mechanisms by which high salt intake affects blood pressure dipping patterns are not fully understood, but it is suggested that it may be related to changes in the sympathetic nervous system. Further, we looked at the association between major blood pressure and circadian rhythm regulatory centers in the brain, including the paraventricular nucleus (PVN), suprachiasmatic nucleus (SCN) and nucleus tractus solitarius (nTS). We also discuss the underlying social and economic issues in the United States and around the world. In conclusion, the evidence suggests that a high salt diet is associated with a blunted, non-dipping, or reverse dipping blood pressure pattern, which has been shown to increase the risk of cardiovascular disease. Further research is needed to better understand the underlying mechanisms by which high salt intake influences changes within the central nervous system.

Alpha2B-Adrenergic Receptor Overexpression in the Brain Potentiate Air Pollution-induced Behavior and Blood Pressure Changes

Fine ambient particulate matter (PM2.5) is able to induce sympathetic activation and inflammation in the brain. However, direct evidence demonstrating an essential role of sympathetic activation in PM2.5-associated disease progression is lacking. We assess the contribution of α2B-adrenergic receptor (Adra2b) in air pollution-associated hypertension and behavioral changes in this study. Wild-type mice and Adra2b-transgenic mice overexpressing Adra2b in the brain (Adra2bTg) were exposed to concentrated PM2.5 or filtered air for 3 months via a versatile aerosol concentrator exposure system. Mice were fed with a high salt diet (4.0% NaCl) for 1 week at week 11 of exposure to induce blood pressure elevation. Intra-arterial blood pressure was monitored by radio-telemetry and behavior changes were assessed by open field, light-dark, and prepulse inhibition tests. PM2.5 exposure increased Adra2b in the brain of wild-type mice. Adra2b overexpression enhanced the anxiety-like behavior and high salt diet-induced blood pressure elevation in response to air pollution but not filtered air exposure. Adra2b overexpression induced upregulation of inflammatory genes such as TLR2, TLR4, and IL-6 in the brain exposed to PM2.5. In addition, there were increased frequencies of activated effector T cells and increased expression of oxidative stress-related genes, such as SOD1, NQO1, Nrf2, and Gclm in Adra2bTg mice compared with wild-type mice. Our results provide new evidence of distinct behavioral changes consistent with anxiety and blood pressure elevation in response to high salt intake and air pollution exposure, highlighting the importance of centrally expressed Adra2b in the vulnerability to air pollution exposure.

Burst patterning of hypothalamic paraventricular nucleus-driven sympathetic nerve activity in ANG II-salt hypertension

ANG II-salt hypertension selectively increases splanchnic sympathetic nerve activity (sSNA), but the extent to which this reflects increased respiratory versus cardiac rhythmic bursting is unknown. Here, integrated sSNA was elevated in ANG II-infused rats fed a high-salt (2% NaCl) diet (ANG II-HSD) compared with vehicle-infused rats fed a normal-salt (0.4% NaCl) diet (Veh-NSD; P < 0.01). Increased sSNA was not accompanied by increased inspiratory or expiratory bursting, consistent with no group difference in central inspiratory drive. Consistent with preserved inhibitory baroreflex entrainment of elevated sSNA in ANG II-HSD rats, the time integral ( P < 0.05) and amplitude ( P < 0.01) of cardiac rhythmic sSNA were increased. Consistent with activity of hypothalamic paraventricular nucleus (PVN) neurons supporting basal SNA in ANG II-salt hypertension, inhibition of PVN with the GABA-A receptor agonist muscimol reduced mean arterial pressure (MAP) and integrated sSNA only in the ANG II-HSD group ( P < 0.001). PVN inhibition had no effect on respiratory rhythmic sSNA bursting in either group but reduced cardiac rhythmic sSNA in ANG II-HSD rats only ( P < 0.01). The latter likely reflected reduced inhibitory baroreflex entrainment subsequent to the fall of MAP. Of note is that MAP as well as integrated and rhythmic burst patterns of sSNA were similar in vehicle-infused rats whether they were fed a normal or high-salt diet. Findings indicate that PVN neurons support elevated sSNA in ANG II-HSD rats by driving a tonic component of activity without altering respiratory or cardiac rhythmic bursting. Because sSNA was unchanged in Veh-HSD rats, activation of PVN-driven tonic sSNA appears to require central actions of ANG II. NEW & NOTEWORTHY ANG II-salt hypertension is strongly neurogenic and depends on hypothalamic paraventricular nucleus (PVN)-driven splanchnic sympathetic nerve activity (sSNA). Here, respiratory and cardiac bursts of sSNA were preserved in ANG II-salt rats and unaltered by PVN inhibition, suggesting that PVN neurons drive a tonic component of sSNA rather than modulating dominant patterns of burst discharge.

Neurogenic Pulmonary Edema Induced by Spinal Cord Injury in Spontaneously Hypertensive and Dahl Salt Hypertensive Rats

Neurogenic pulmonary edema (NPE), which is induced by acute spinal cord compression (SCC) under the mild (1.5 %) isoflurane anesthesia, is highly dependent on baroreflex-mediated bradycardia because a deeper (3 %) isoflurane anesthesia or atropine pretreatment completely abolished bradycardia occurrence and NPE development in rats subjected to SCC. The aim of the present study was to evaluate whether hypertension-associated impairment of baroreflex sensitivity might exert some protection against NPE development in hypertensive animals. We therefore studied SCC-induced NPE development in two forms of experimental hypertension – spontaneously hypertensive rats (SHR) and salt hypertensive Dahl rats, which were reported to have reduced baroreflex sensitivity. SCC elicited NPE in both hypertensive models irrespective of their baroreflex sensitivity. It is evident that a moderate impairment of baroreflex sensitivity, which was demonstrated in salt hypertensive Dahl rats, does not exert sufficient protective effects against NPE development.

Identifying physiological origins of baroreflex dysfunction in salt-sensitive hypertension in the Dahl SS rat

Salt-sensitive hypertension is known to be associated with dysfunction of the baroreflex control system in the Dahl salt-sensitive (SS) rat. However, neither the physiological mechanisms nor the genomic regions underlying the baroreflex dysfunction seen in this rat model are definitively known. Here, we have adopted a mathematical modeling approach to investigate the physiological and genetic origins of baroreflex dysfunction in the Dahl SS rat. We have developed a computational model of the overall baroreflex heart rate control system based on known physiological mechanisms to analyze telemetry-based blood pressure and heart rate data from two genetic strains of rat, the SS and consomic SS.13(BN), on low- and high-salt diets. With this approach, physiological parameters are estimated, unmeasured physiological variables related to the baroreflex control system are predicted, and differences in these quantities between the two strains of rat on low- and high-salt diets are detected. Specific findings include: a significant selective impairment in sympathetic gain with high-salt diet in SS rats and a protection from this impairment in SS.13(BN) rats, elevated sympathetic and parasympathetic offsets with high-salt diet in both strains, and an elevated sympathetic tone with high-salt diet in SS but not SS.13(BN) rats. In conclusion, we have associated several important physiological parameters of the baroreflex control system with chromosome 13 and have begun to identify possible physiological mechanisms underlying baroreflex impairment and hypertension in the Dahl SS rat that may be further explored in future experimental and modeling-based investigation.

Limitation of arteriolar myogenic activity by local nitric oxide: segment-specific effect of dietary salt

The purpose of this study was to determine if local nitric oxide (NO) activity attenuates the arteriolar myogenic response in rat spinotrapezius muscle. We also investigated the possibility that hypertension, dietary salt, or their combination can alter any influence of local NO on the myogenic response. Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) fed low-salt (0.45%, LS) or high-salt (7%, HS) diets were enclosed in a ventilated airtight box with the spinotrapezius muscle exteriorized for intravital microscopy. Mean arterial pressure was unaffected by dietary salt in WKY but was significantly higher and augmented by dietary salt in SHR. In all experiments, elevation of microvascular pressure by box pressurization caused a 0-30% decrease in the diameter of large (arcade bridge) arterioles and a 21-27% decrease in the diameter of intermediate (arcade) arterioles. Inhibition of NO synthase with N(G)-monomethyl-L-arginine (L-NMMA) significantly enhanced myogenic responsiveness of arcade bridge arterioles in WKY-LS and SHR-LS but not in WKY-HS and SHR-HS. L-NMMA significantly enhanced the myogenic responsiveness of arcade arterioles in all four groups. Excess L-arginine reversed this effect of L-NMMA in all cases, and arteriolar responsiveness to the NO donor sodium nitroprusside was not different among the four groups. High-salt intake had no effect on the passive distension of arterioles in either strain during box pressurization. We conclude that 1) local NO normally attenuates arteriolar myogenic responsiveness in WKY and SHR, 2) dietary salt impairs local NO activity in arcade bridge arterioles of both strains, and 3) passive arteriolar distensibility is not altered by a high-salt diet in either strain.

Hormones as long-term error signals for the sympathetic nervous system: importance of a new perspective

1. A hormonal-sympathetic reflex model for long-term control of arterial pressure is presented. It is hypothesized that the hormonal-sympathetic reflex regulates arterial pressure during chronic dietary salt loading by decreasing sympathetic tone. This sympathetic response is mediated by an increase in plasma vasopressin (AVP) and a decrease in plasma angiotensin (AngII). 2. Three new models of neurogenic salt-dependent hypertension are presented. All models are theoretically based on an impaired hormonal-sympathetic reflex. 3. In the first model, sympathetic responsiveness is 'clamped' by long-term alpha-adrenergic blockade with prazosin. Prazosin treated rats exhibit marked salt-dependent hypertension despite normal suppression of the renin-angiotensin system. 4. In the second model, the ability of the central nervous system to respond to salt-induced changes in AVP and AngII concentrations was prevented by long-term administration of antagonists selective for the AVP-V1 and AT1. This 'clamp' of the afferent hormonal signal resulted in salt-dependent hypertension identical in magnitude to that observed in prazosin treated rats. 5. In the third model, the long-term arterial pressure responses to increasing dietary salt were examined in sino-aortic denervated (SAD) rats. SAD rats exhibited salt-dependent hypertension, of lesser magnitude than that observed with 'clamped' afferent and efferent pathways of the hormonal-sympathetic reflex. 6. A primary role for hormonal 'error signals' is presented and the impact this perspective has on past and future investigations of central mechanisms of long-term arterial pressure regulation is discussed.

Contribution of potassium channels to the discharge properties of rat aortic baroreceptor sensory endings

The expression of several types of membrane potassium channel at the cell body and central synaptic terminal of the rat aortic arch baroreceptor has been reported by others. It is not known if any of the same channels function at the peripheral sensory terminal of these afferent nerves. Our study examined the effect of three potassium channel blocking agents on the pressure-evoked discharge of such baroreceptors. Thirty-one single unit, regularly discharging baroreceptors were studied using an in vitro aortic arch-aortic nerve preparation. Discharge thresholds and suprathreshold pressure sensitivities were derived from responses of receptors to slowly rising ramps of pressure applied to the aortic arch. Vessel diameter was recorded along with receptor discharge to assess any drug-induced changes in vascular smooth muscle. The blocking agents tested have a range of specificities for classes of potassium channels: tetraethylammonium (TEA), 4-aminopyridine (4-AP) and charybdotoxin. TEA depressed the pressure sensitivity of all baroreceptors tested (n = 3) in a dose-dependent manner. Baroreceptor responses to 4-AP were complex (n = 22) and varied widely across individuals. Three were unaffected by 5 mM 4-AP. Most baroreceptors were generally depressed by 4-AP. Some of the 4-AP effects appeared to be related to actions at vascular smooth muscle. None of the baroreceptors tested (n = 6) was affected by charybdotoxin. The results of selective potassium channel blockade are generally consistent with what would be expected from a sustained depolarization of baroreceptor endings such as has been reported with raising extracellular potassium and probably includes effects of inactivation of other voltage-dependent channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Contrasting actions of cocaine, local anaesthetic and tetrodotoxin on discharge properties of rat aortic baroreceptors

1. Effects of cocaine, lignocaine, benzocaine and tetrodotoxin (TTX) on the simultaneously measured pressure- and diameter-discharge frequency relations of single fibre baroreceptors were compared in rat in vitro aortic arch-aortic nerve preparations. 2. Between 1 and 10 microM, cocaine produced selective increases in the pressure threshold shifting the pressure-response curve without altering the gain or threshold frequency. At near-blocking concentrations, gain was depressed as well. Cocaine experiments were done in nitroprusside (NP, 1 microM). Neither NP or NP with cocaine altered diameter (P > 0.36). 3. Lignocaine (at > 10 microM) and benzocaine (at > 100 microM) shifted pressure-response curves to higher pressures and generally depressed discharge by increasing pressure threshold and decreasing maximum discharge frequency (P < 0.05). Gain decreased and threshold frequency increased at higher concentrations. Diameter was unaffected by lignocaine or benzocaine (P > 0.14). 4. TTX increased thresholds and discharge frequencies at threshold but did not shift pressure-discharge curve locations. This produced superimposable discharge curves with changes occurring as losses of discharge points in the threshold region. Diameter was unaffected by TTX (P > 0.80). 5. The contrasting patterns of effects between TTX and local anaesthetics suggest that blockade of TTX-sensitive sodium channels alone may not be responsible for the effects of cocaine, lignocaine and benzocaine.

Cardiovascular responses to centrally applied sodium chloride solution

In conscious and anaesthetized rats the baroreceptor heart reflex (BHR) was checked before and after i.c.v. application of isotonic or hypertonic (0.6 M; 1.0 M) NaCl solution, artificial cerebrospinal fluid (aCSF) and 1.0 M mannitol solution. The BHR was tested by evaluating the alteration of the inter-beat interval (IBI) in response to an artificial BP rise or drop which had been evoked by i.v. bolus injection of either phenylephrine or sodium nitroprusside. The slope of the correlation function was taken to index the reflex sensitivity. In anaesthetized rats the mean sensitivity of the BHR was 0.6 ms/mm Hg (phenylephrine). l.c.v. administration of isotonic NaCl solution did not change BP, IBI or the BHR sensitivity, whilst i.c.v. infusion of hypertonic NaCl solution increased BP and shortened IBI. The BHR sensitivity was impaired only when 1.0 M NaCl solution was i.c.v. infused by 0.23 ms/mm Hg. In conscious rats the mean sensitivity of the BHR was 1.14 ms/mm Hg (phenylephrine) and 1.35 ms/mm Hg (sodium nitroprusside). In the conscious rats i.c.v. bolus injection of hypertonic NaCl solution increased BP as in anaesthetized rats, however, the IBI was prolonged, whilst 1.0 M mannitol solution and aCSF were without any influence on BP, IBI and BHR. l.c.v. administration of hypertonic NaCl solution reduced the BHR sensitivity by approximately 0.6 ms/mm Hg.

Altered element concentrations in tissues of Dahl salt-sensitive rats

It is recognized that the development of hypertension in Dahl salt-sensitive (DS) rats as compared to Dahl salt-resistant (DR) rats is dependent on the addition of a high percentage of sodium chloride, often 8% to the diet. In this work, blood systolic pressure and the concentrations of many elements in different tissues of DS and DR rats were measured. However, to distinguish the modifications linked to the strain from the modifications owing to excess of sodium intake, no additional Na was included in the diet in all our experiments. Without any addition of sodium chloride to the diet, a statistically significant increase of the systolic blood pressure of DS rats (152 +/- 10 mmHg) in comparison to DR rats (131 +/- 3 mmHg) was observed. The analysis of the concentrations of many elements in different tissues showed no major modifications of sodium concentrations in DS rats as compared to DR rats, but a decrease of calcium in plasma (-9%), brain (-20%), and heart (-7%) and of magnesium in plasma (-13%), kidney (-11%), and bone (-7%). In conclusion, an increased intake of Na is not necessary to obtain a higher systolic blood pressure in DS rats compared to DR rats. Since we did not find noticeable modifications of Na concentration in tissues but modifications of Ca and Mg, we suggest that an alteration of the homeostasis of these two elements may be involved in the development of the hypertension in DS rats.

Sodium depresses arterial baroreceptor reflex function in normotensive humans

Sodium may contribute to the pathogenesis of hypertension by impairing arterial baroreceptor reflex function. The objectives of this study were to 1) determine whether a high sodium diet depresses arterial baroreceptor reflex function in normotensive humans, and 2) determine whether alterations in baroreceptor reflex function are related to changes in arterial compliance. Seventeen normotensive men, aged 30 +/- 2 years, received 10 and 200 meq sodium per day diets, each for 5 days, in a randomized crossover trial. Carotid baroreceptor reflex function was assessed by measuring the blood pressure response to sequential neck suction (0, -10, -20, and -30 mm Hg) and neck pressure (0, +10, +20, and +30 mm Hg). Forearm vascular resistance was determined by venous occlusion plethysmography. Arterial compliance was evaluated by calculating the quotient of the diastolic blood pressure decay time constant and forearm vascular resistance. Blood pressure averaged 124 +/- 3/62 +/- 2 mm Hg on the low sodium diet and 122 +/- 3/60 +/- 2 mm Hg on the high sodium diet (p = NS). Baroreceptor reflex slopes representing the systolic and diastolic blood pressure responses to changes in neck chamber pressure were steeper in the subjects when randomly assigned to low sodium diet than to high sodium diet. Diastolic blood pressure decay time and forearm arterial compliance were similar during low and high sodium intake. We conclude that short-term exposure to a high sodium diet depresses carotid baroreceptor reflex function in normotensive humans. This observation cannot be attributed to changes in the arterial compliance.

Age-dependent salt-induced hypertension in the rat: prevention with DSP-4, a selective noradrenergic neurotoxin

The present study was performed to determine if chronic administration of a high salt diet induces hypertension similarly in young and adult rats and if treatment with DSP-4 alters the development of the hypertension. Three- (young) and ten- (adult) week-old male Sprague-Dawley rats were fed either a standard rat chow diet (0.71% NaCl), a 4% NaCl diet or an 8% NaCl diet for 12 weeks. Systolic blood pressure, using a standard tail-cuff technique, and body weight were recorded weekly during the dietary treatment period. Direct mean arterial pressure, heart rate, heart weight and kidney weight were determined after 12 weeks. Body weight was slightly reduced in young rats on the 8% NaCl diet. A significant increase in blood pressure as well as heart weight was observed only in young rats on the 8% NaCl diet. An increase in kidney weight was observed in both young and adult rats on the 8% NaCl diet. DSP-4 treatment prevented the development of hypertension as well as cardiac hypertrophy in rats fed the high salt diet but had no effect on rats receiving the normal diet. Body and kidney weights were similar in vehicle- and DSP-4-treated rats on the 8% NaCl diet. These results demonstrate that a critical developmental/maturational period exists during which the young rat is susceptible to the hypertensinogenic effects of a high salt diet. An intact central noradrenergic system appears to be necessary for the expression of this enhanced susceptibility and the subsequent development of hypertension.