Differential central modulation of the baroreflex by salt loading in normotensive and spontaneously hypertensive rats

Silent Effects of High Salt: Risks Beyond Hypertension and Body's Adaptation to High Salt

Hypertension is a major contributor to heart disease, renal failure, and stroke. High salt is one of the significant risk factors associated with the onset and persistence of hypertension. Experimental and observational studies have confirmed cardiovascular and non-cardiovascular detrimental effects associated with chronic intake of high salt. Because of convenience and present urban lifestyles, consumption of fast food has led to daily salt intake above the recommended level by the World Health Organization. This study provides an understanding of the body regulatory mechanisms that maintain sodium homeostasis under conditions of high salt intake, without health consequences, and how these mechanisms adapt to chronic high salt load, leading to adverse cardiovascular, renal, and non-cardiovascular outcomes. Recent research has identified several mechanisms through which high sodium intake contributes to hypertension. Of them, heightened renin-angiotensin-aldosterone and sympathetic activity associated with impaired pressure diuresis and natriuresis and decreased renal excretory response are reported. Additionally, there is the possibility of endothelial and nitric oxide dysfunction leading to vascular remodeling. These changes raise cardiac output and peripheral vascular resistance. Knowing how these collective mechanisms adapt to chronic intakes of high salt helps develop effective therapeutic policies to fight salt-induced hypertension.

Recent Advances in Understanding Peripheral and Gut Immune Cell-Mediated Salt-Sensitive Hypertension and Nephropathy

Hypertension is the primary modifiable risk factor for cardiovascular, renal, and cerebrovascular diseases and is considered the main contributing factor to morbidity and mortality worldwide. Approximately 50% of hypertensive and 25% of normotensive people exhibit salt sensitivity of blood pressure, which is an independent risk factor for cardiovascular disease. Human and animal studies demonstrate that the immune system plays an important role in the etiology and pathogenesis of salt sensitivity of blood pressure, kidney damage, and vascular diseases. Antigen-presenting and adaptive immune cells are implicated in salt-sensitive hypertension and salt-induced renal and vascular injury. Elevated sodium activates antigen-presenting cells to release proinflammatory cytokines including IL (interleukin) 6, tumor necrosis factor-α, IL-1β, and accumulate isolevuglandin-protein adducts. In turn, these activate T cells release prohypertensive cytokines including IL-17A. Moreover, high-salt intake is associated with gut dysbiosis, leading to inflammation, oxidative stress, and blood pressure elevation but the mechanistic contribution to salt-sensitivity of blood pressure is not clearly understood. Here, we discuss recent advances in research investigating the cause, potential biomarkers, and therapeutic targets for salt-sensitive hypertension as they pertain to the gut microbiome, immunity, and inflammation.

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.

Inflammation and oxidative stress in salt sensitive hypertension; The role of the NLRP3 inflammasome

Salt-sensitivity of blood pressure is an independent risk factor for cardiovascular disease and affects approximately half of the hypertensive population. While the precise mechanisms of salt-sensitivity remain unclear, recent findings on body sodium homeostasis and salt-induced immune cell activation provide new insights into the relationship between high salt intake, inflammation, and hypertension. The immune system, specifically antigen-presenting cells (APCs) and T cells, are directly implicated in salt-induced renal and vascular injury and hypertension. Emerging evidence suggests that oxidative stress and activation of the NLRP3 inflammasome drive high sodium-mediated activation of APCs and T cells and contribute to the development of renal and vascular inflammation and hypertension. In this review, we summarize the recent insights into our understanding of the mechanisms of salt-sensitive hypertension and discuss the role of inflammasome activation as a potential therapeutic target.

Dendritic Cell Epithelial Sodium Channel in Inflammation, Salt-Sensitive Hypertension, and Kidney Damage

Salt-sensitive hypertension is a major risk factor for cardiovascular morbidity and mortality. The pathophysiologic mechanisms leading to different individual BP responses to changes in dietary salt remain elusive. Research in the last two decades revealed that the immune system plays a critical role in the development of hypertension and related end organ damage. Moreover, sodium accumulates nonosmotically in human tissue, including the skin and muscle, shifting the dogma on body sodium balance and its regulation. Emerging evidence suggests that high concentrations of extracellular sodium can directly trigger an inflammatory response in antigen-presenting cells (APCs), leading to hypertension and vascular and renal injury. Importantly, sodium entry into APCs is mediated by the epithelial sodium channel (ENaC). Although the role of the ENaC in renal regulation of sodium excretion and BP is well established, these new findings imply that the ENaC may also exert BP modulatory effects in extrarenal tissue through an immune-dependent pathway. In this review, we discuss the recent advances in our understanding of the pathophysiology of salt-sensitive hypertension with a particular focus on the roles of APCs and the extrarenal ENaC.

Pathophysiology and genetics of salt-sensitive hypertension

Most hypertensive cases are primary and heavily associated with modifiable risk factors like salt intake. Evidence suggests that even small reductions in salt consumption reduce blood pressure in all age groups. In that regard, the ACC/AHA described a distinct set of individuals who exhibit salt-sensitivity, regardless of their hypertensive status. Data has shown that salt-sensitivity is an independent risk factor for cardiovascular events and mortality. However, despite extensive research, the pathogenesis of salt-sensitive hypertension is still unclear and tremendously challenged by its multifactorial etiology, complicated genetic influences, and the unavailability of a diagnostic tool. So far, the important roles of the renin-angiotensin-aldosterone system, sympathetic nervous system, and immune system in the pathogenesis of salt-sensitive hypertension have been studied. In the first part of this review, we focus on how the systems mentioned above are aberrantly regulated in salt-sensitive hypertension. We follow this with an emphasis on genetic variants in those systems that are associated with and/or increase predisposition to salt-sensitivity in humans.

Effect of High-Salt Diet on Age-Related High Blood Pressure and Hypothalamic Redox Signaling

Aim: The aim of this study was to investigate the effect of a high salt (HS) diet on age-related changes in blood pressure (BP) and the possible role played by regulatory central mechanisms. Methods: Young (5 months) and old (27 months) male Fischer 344 × Brown Norway (F344/BN) rats were fed standard chow or 8% HS diet for 12 days. BP and heart rate (HR) were measured by telemetry. Results: Mean arterial BP (MAP) was significantly elevated in old rats during the day and night when compared with young animals. The HS diet further elevated MAP in both age groups, and the increase was more pronounced in the old animals, while HR was not altered by age or HS diet. In addition, cardiovascular responses to restraint stress were diminished in the old when compared with the young and were unchanged with HS diet in either age group. Both age and the HS diet elevated the adrenomedullary mRNA levels of tyrosine hydroxylase, an indicator for sympathoexcitation. HS diet enhanced intracerebroventricular angiotensin II (AngII)-induced BP and HR elevations in both age groups. AngII type 1 receptor mRNA increased significantly in the hypothalamus with age and HS diet. Furthermore, hypothalamic p22phox mRNA and gp91phox protein, subunits of NADPH oxidase, as well as NADPH oxidase activity increased with the HS diet in the old animals, whereas antioxidant enzymes that decreased with age yet remained unaltered with the HS diet. Conclusion: Our findings indicate that sensitivity of BP to HS diet increases with age, and that central AngII-induced pressor responses are diminished in old rats compared with the young both under control conditions and during HS diet treatment. These changes are paralleled by increases in the expression and NADPH oxidase activity in the hypothalamus, possibly leading to central oxidative stress-mediated sympathoexcitation and high BP.

The Role of CNS in the Effects of Salt on Blood Pressure

Sympathetic nerve activity is involved in the pathogenesis of salt-sensitive hypertension. The central nervous system, which regulates sympathetic nerve activity and blood pressure, plays a pivotal role. Central sympathoexcitation is deeply involved in the pathogenesis of salt-sensitive hypertension, although the precise mechanisms have not been fully elucidated because of their complexity. The role of brain oxidative stress in sympathoexcitation has been suggested in some types of hypertensive animal models. We have shown that increased brain oxidative stress may elevate arterial pressure through central sympathoexcitation in salt-sensitive hypertension. Several other factors such as mineralocorticoid receptors, aldosterone, corticosterone, epithelial sodium channels, and angiotensin II also play important roles in central sympathetic activation, some of which can be associated with brain oxidative stress. Furthermore, brain paraventricular nucleus Gαi2-protein-mediated transduction has been recently reported as a candidate for the molecular mechanism countering the development of salt-sensitive hypertension.

How Does Circadian Rhythm Impact Salt Sensitivity of Blood Pressure in Mice? A Study in Two Close C57Bl/6 Substrains

Background

Mouse transgenesis has provided the unique opportunity to investigate mechanisms underlying sodium kidney reabsorption as well as end organ damage. However, understanding mouse background and the experimental conditions effects on phenotypic readouts of engineered mouse lines such as blood pressure presents a challenge. Despite the ability to generate high sodium and chloride plasma levels during high-salt diet, observed changes in blood pressure are not consistent between wild-type background strains and studies.

Methods

The present work was designed in an attempt to determine guidelines in the field of salt-induced hypertension by recording continuously blood pressure by telemetry in mice submitted to different sodium and potassium loaded diets and changing experimental conditions in both C57BL/6N and C57BL/6J mice strain (Normal salt vs. Low salt vs. High-salt/normal potassium vs. High salt/low potassium, standard vs. modified light cycle, Non-invasive tail cuff blood pressure vs. telemetry).

Results

In this study, we have shown that, despite a strong blood pressure (BP) basal difference between C57BL/6N and C57BL/6J mice, High salt/normal potassium diet increases BP and heart rate during the active phase only (dark period) in the same extent in both strains. On the other hand, while potassium level has no effect on salt-induced hypertension in C57BL/6N mice, high-salt/low potassium diet amplifies the effect of the high-salt challenge only in C57BL/6J mice. Indeed, in this condition, salt-induced hypertension can also be detected during light period even though this BP increase is lower compared to the one occurring during the dark period. Finally, from a methodological perspective, light cycle inversion has no effect on this circadian BP phenotype and tail-cuff method is less sensitive than telemetry to detect BP phenotypes due to salt challenges.

Conclusions

Therefore, to carry investigations on salt-induced hypertension in mice, chronic telemetry and studies in the active phase are essential prerequisites.

Increased salt intake during early ontogenesis lead to development of arterial hypertension in salt-resistant Wistar rats

A direct relationship exists between salt consumption and hypertension. Increased sodium intake does not automatically lead to a rise in blood pressure (BP) because of marked intra-individual variability in salt sensitivity. Wistar rats are a salt-resistant strain and increased salt intake in adults does not induce hypertension. Mechanisms regulating BP develop during early ontogenesis and increased sodium consumption by pregnant females leads to an increase in BP of their offspring, but early postnatal stages have not been sufficiently analyzed in salt-resistant strains of rats. The aim of this work was to study the effects of increased salt during early ontogeny on cardiovascular characteristics of Wistar rats. We used 16 control (C; 8 males + 8 females) rats fed with a standard diet (0.2% sodium) and 16 experimental (S; 8 males + 8 females) rats fed with a diet containing 0.8% sodium. BP was measured weekly and plasma renin activity, aldosterone and testosterone concentrations were assayed by radioimmunoassay after the experiment in 16-week-old animals. In the kidney, AT1 receptors were determined by the western blot. BP was higher in the S as compared with the C rats and did not differ between males and females. The relative left ventricle mass was increased in S as compared with C males and no differences were recorded in females. No significant differences between groups were found in hormonal parameters and AT1 receptors. Results indicate that moderately increased salt intake during postnatal ontogeny results in a BP rise even in salt-resistant rats.

The role of CNS in salt-sensitive hypertension

The role of sympathetic nerve activity in hypertension is currently receiving increased attention, because catheter-based renal denervation was recently shown to reduce blood pressure safely in patients with treatment-resistant hypertension. The central nervous system, which regulates sympathetic nerve activity and blood pressure, is pivotal. Central sympathoexcitation has been shown to be deeply involved in the pathogenesis of salt-sensitive hypertension, although its precise mechanisms have not yet been fully elucidated due to their complexity. Recently, a role for brain oxidative stress in sympathoexcitation has been suggested in some hypertensive animal models. We have demonstrated that increased brain oxidative stress may elevate arterial pressure through central sympathoexcitation in salt-sensitive hypertension. Several factors other than oxidative stress have also been shown to play important roles in central sympathetic activation. In the future, strategies may be developed to elicit a sympathetic inhibition by modulating these factors to prevent and manage salt-sensitive hypertension.

The kidney and hypertension: pathogenesis of salt-sensitive hypertension

Salt-sensitive hypertension is closely related with natriuretic capacity of the kidney. Besides several genome-wide research reported candidate gene or gene polymorphism responsible for salt-sensitive hypertension, recently, several new factors for acquired salt-sensitive hypertension are reported. Among them, we have identified that rac1, a small GTPase, activates mineralocorticoid receptor in aldosterone-independent fashion and induces salt-sensitive hypertension in several rodent model. On the other hand, sympathoactivation in the brain and/or kidney regulate sodium handlings in the kidney. Recently it is reported that oxidative stress in the brain or in the kidney may modulate sympathetic tone. Moreover, we reported that β2 adrenoceptor alters histone acetylation and further regulates sodium resorption at distal tubules via activating glucocorticoid receptor. These regulations are to be confirmed in humans and the future, and may open a new door for diagnosis and treatment of salt-sensitive hypertension or moreover preventing development of salt-sensitive hypertension.

Pathophysiology of salt sensitivity hypertension

Dietary salt intake is the most important factor contributing to hypertension, but the salt susceptibility of blood pressure (BP) is different in individual subjects. Although the pathogenesis of salt-sensitive hypertension is heterogeneous, it is mainly attributable to an impaired renal capacity to excrete sodium (Na(+) ). We recently identified two novel mechanisms that impair renal Na(+) -excreting function and result in an increase in BP. First, mineralocorticoid receptor (MR) activation in the kidney, which facilitates distal Na(+) reabsorption through epithelial Na(+) channel activation, causes salt-sensitive hypertension. This mechanism exists not only in models of high-aldosterone hypertension as seen in conditions of obesity or metabolic syndrome, but also in normal- or low-aldosterone type of salt-sensitive hypertension. In the latter, Rac1 activation by salt excess causes MR stimulation. Second, renospecific sympathoactivation may cause an increase in BP under conditions of salt excess. Renal beta2 adrenoceptor stimulation in the kidney leads to decreased transcription of the gene encoding WNK4, a negative regulator of Na(+) reabsorption through Na(+) -Cl (-) cotransporter in the distal convoluted tubules, resulting in salt-dependent hypertension. Abnormalities identified in these two pathways of Na(+) reabsorption in the distal nephron may present therapeutic targets for the treatment of salt-sensitive hypertension.

Cardiovascular and renal manifestations of glutathione depletion induced by buthionine sulfoximine

Oxidative stress contributes to the development of several cardiovascular diseases, including diabetes, renal insufficiency, and arterial hypertension. Animal studies have evidenced the association between higher blood pressure (BP) and increased oxidative stress, and treatment with antioxidants has been shown to reduce BP, while BP reduction due to antihypertensive drugs is associated with reduced oxidative stress. In 2000, it was first reported that oxidative stress and arterial hypertension were produced in normal Sprague-Dawley rats by oral administration of buthionine sulfoximine (BSO), which induces glutathione (GSH) depletion, indicating that oxidative stress may induce hypertension. The contribution of several potential pathogenic factors has been evaluated in the BSO rat model, the prototype of oxidative stress-induced hypertension, including vascular reactivity, endothelium-derived factors, renin-angiotensin system activity, TXA(2)-PGH(2) production, sodium sensitivity, renal dopamine-induced natriuresis, and sympathetic tone. This review summarizes the main factors implicated in the pathogenesis of BSO-induced hypertension and the alterations associated with GSH depletion that are related to renal function or BP control.

Reactive oxygen species and the central nervous system in salt-sensitive hypertension: possible relationship with obesity-induced hypertension

1. There are multiple and complex mechanisms of salt-induced hypertension; however, central sympathoexcitation plays an important role. In addition, the production of reactive oxygen species (ROS) is increased in salt-sensitive hypertensive humans and animals. Thus, we hypothesized that brain ROS overproduction may increase blood pressure (BP) by central sympathostimulation. 2. Recently, we demonstrated that ROS levels were elevated in the hypothalamus of salt-sensitive hypertensive animals. Moreover, intracerebroventricular anti-oxidants suppressed BP and renal sympathetic nerve activity more in salt-sensitive than non-salt-sensitive hypertensive rats. Thus, brain ROS overproduction increased BP through central sympathoexcitation in salt-sensitive hypertension. 3. Salt sensitivity of BP is enhanced in obesity and metabolic syndrome. Interestingly, it is also suggested that, in obesity-induced hypertension models, increases in BP are caused by brain ROS-induced central sympathoexcitation. 4. Recent studies suggest that increased ROS production in the brain and central sympathoexcitation may share a common pathway that increases BP in both salt- and obesity-induced hypertension.

Role of the epithelial sodium channel in salt-sensitive hypertension

The epithelial sodium channel (ENaC) is a heteromeric channel composed of three similar but distinct subunits, α, β and γ. This channel is an end-effector in the rennin-angiotensin-aldosterone system and resides in the apical plasma membrane of the renal cortical collecting ducts, where reabsorption of Na(+) through ENaC is the final renal adjustment step for Na(+) balance. Because of its regulation and function, the ENaC plays a critical role in modulating the homeostasis of Na(+) and thus chronic blood pressure. The development of most forms of hypertension requires an increase in Na(+) and water retention. The role of ENaC in developing high blood pressure is exemplified in the gain-of-function mutations in ENaC that cause Liddle's syndrome, a severe but rare form of inheritable hypertension. The evidence obtained from studies using animal models and in human patients indicates that improper Na(+) retention by the kidney elevates blood pressure and induces salt-sensitive hypertension.

Epigenetic modulation of the renal β-adrenergic-WNK4 pathway in salt-sensitive hypertension

How high salt intake increases blood pressure is a key question in the study of hypertension. Salt intake induces increased renal sympathetic activity resulting in sodium retention. However, the mechanisms underlying the sympathetic control of renal sodium excretion remain unclear. In this study, we found that β(2)-adrenergic receptor (β(2)AR) stimulation led to decreased transcription of the gene encoding WNK4, a regulator of sodium reabsorption. β(2)AR stimulation resulted in cyclic AMP-dependent inhibition of histone deacetylase-8 (HDAC8) activity and increased histone acetylation, leading to binding of the glucocorticoid receptor to a negative glucocorticoid-responsive element in the promoter region. In rat models of salt-sensitive hypertension and sympathetic overactivity, salt loading suppressed renal WNK4 expression, activated the Na(+)-Cl(-) cotransporter and induced salt-dependent hypertension. These findings implicate the epigenetic modulation of WNK4 transcription in the development of salt-sensitive hypertension. The renal β(2)AR-WNK4 pathway may be a therapeutic target for salt-sensitive hypertension.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Role of sympathetic tone in BSO-induced hypertension in mice

BACKGROUND

We investigated the contribution of the sympathetic tone to the hypertension induced by chronic administration of buthionine sulfoximine (BSO) and characterized this model in mice.

METHODS

Three experiments were performed. In experiment I, four groups of CBA-C57 male mice were used: controls and three groups that received oral BSO at 5, 10, or 20 mmol/l. In experiment II, the alpha(1)-adrenergic blocker prazosin was orally administered (10 mg/100 ml) to control and BSO-treated mice. All treatments were maintained for 5 weeks. Body weight (BW), tail blood pressure (BP), and heart rate (HR) were measured weekly. Direct mean arterial pressure (MAP) and morphological, metabolic, plasma, and renal variables were measured at the end of the experiments. In experiment III, the acute response of MAP and HR to the ganglionic blocker pentolinium (10 mg/kg intravenous) was used to further evaluate the sympathetic contribution to BP and HR in control and BSO-treated mice.

RESULTS

BSO produced dose-related increases in BP (control, 115 +/- 0.5; BSO-5, 141 +/- 0.5; BSO-10, 151 +/- 0.9; BSO-20, 163 +/- 1.1 mm Hg) and HR and augmented plasma noradrenaline, brainstem isoprostane levels, and total urinary isoprostane excretion. BSO did not produce cardiac hypertrophy and did not modify metabolic or plasma variables, or creatinine clearance, proteinuria, or renal morphology. Chronic prazosin markedly reduced MAP (control, 101 +/- 4.7; prazosin, 95 +/- 1.29; BSO-10, 130 +/- 2.9; BSO-10 +/- prazosin, 98 +/- 0.9) and HR. Acute pentolinium produced a greater percentage MAP (control, 43 +/- 4.2; BSO-10, 66 +/- 4.5) and HR decrease in BSO-treated mice vs. controls.

CONCLUSION

Sympathetic tone plays a major role in the increased BP and HR of BSO hypertensive mice.

The Renin-Angiotensin System in the Development of Salt-Sensitive Hypertension in Animal Models and Humans

Hypertension is still one of the major causes of death from cardiovascular failure. Increased salt intake may aggravate the rise in blood pressure and the development of consequential damage of the heart, the vessels and other organs. The general necessity of restricted salt intake regardless of blood pressure or salt sensitivity has been a matter of debate over the past decades. This review summarizes the main pathogenic mechanisms of hypertension and salt sensitivity in rat models, particularly in the spontaneously hypertensive rat (SHR), and in patients with essential hypertension (EH). Although SHRs are commonly considered to be salt-resistant, there is much evidence that salt loading may deteriorate blood pressure and cardiovascular function even in these animals. Similarly, EH is not a homogenous disorder - some patients, but not all, exhibit pronounced salt sensitivity. The renin-angiotensin system (RAS) plays a key role in the regulation of blood pressure and salt and fluid homeostasis and thus is one of the main targets of antihypertensive therapy. This review focuses on the contribution of the RAS to the pathogenesis of salt-sensitive hypertension in SHRs and patients with EH.

Differential muscarinic receptor gene expression levels in the ventral medulla of spontaneously hypertensive and Wistar-Kyoto rats: role in sympathetic baroreflex function

OBJECTIVE

We demonstrated previously that central muscarinic cholinergic receptor (mAChR) activation increased splanchnic sympathetic nerve activity and sympathetic baroreflex function via activation of mAChR in the rostral ventrolateral medulla (RVLM), and we found that some RVLM bulbospinal neurons contain muscarinic M2R mRNA. Here, we examined the gene expression, cellular distribution and functional role of muscarinic receptors in the RVLM in spontaneously hypertensive rats (SHR) compared with Wistar-Kyoto (WKY) rats.

METHOD AND RESULTS

Using the sensitive technique of quantitative real time reverse transcriptase-PCR, M2R mRNA level was elevated two-fold (P<0.05) and M4R mRNA was downregulated two-fold (P<0.001), with all other receptors expressed at similar levels, in the rostral ventral medulla of SHR compared with WKY. Bulbospinal, but not catecholaminergic neurons, in the RVLM expressed M2R mRNA (M2RR), and similar numbers were found in the RVLM of SHR and WKY. Could elevated M2R within individual neurons or enhanced presynaptic activity reflects enhanced cholinergic effects in the RVLM? Activation of central mAChR using oxotremorine evoked a larger increase in mean arterial pressure in SHR compared with WKY (P<0.01); however, oxotremorine-induced increases in splanchnic sympathetic nerve activity, and sympathetic baroreflex function were similar in SHR and WKY.

CONCLUSION

These data indicate that enhanced pressor responses in SHR, following centrally mediated mAChR activation, are not associated with RVLM-mediated constriction of the splanchnic circulation or effects on the sympathetic baroreflex, but could reflect modified mAChR gene expression elsewhere. RVLM-dependent splanchnic sympathetic nerve activity effects, evoked by mAChR activation, are not mediated by the differential M2/M4 receptor mRNA levels identified in SHR compared with WKY.

Influence of plasma osmolality on baroreflex control of sympathetic activity

The purpose of this study was to determine if plasma osmolality alters baroreflex control of sympathetic activity when controlling for a change in intravascular volume; we hypothesized that baroreflex control of sympathetic activity would be greater during a hyperosmotic stimulus compared with an isoosmotic stimulus when intravascular volume expansion was matched. Seven healthy subjects (25 +/- 2 yr) completed two intravenous infusions: a hypertonic saline infusion (HSI; 3% NaCl) and, on a separate occasion, an isotonic saline infusion (ISO; 0.9% NaCl), both at a rate of 0.15 ml x kg(-1) x min(-1). To isolate the effect of osmolality, comparisons between HSI and ISO conditions were retrospectively matched based on hematocrit; therefore, baroreflex control of sympathetic outflow was determined at 20 min of a HSI and 40 min of an ISO. Muscle sympathetic outflow (MSNA) was directly measured using the technique of peroneal microneurography; osmolality and blood pressure (Finometer) were assessed. The baroreflex control of sympathetic outflow was estimated by calculating the slope of the relationship between MSNA and diastolic blood pressure during controlled breathing. Plasma osmolality was greater during the HSI compared with the ISO (HSI: 292 +/- 0.9 mosmol/kg and ISO: 289 +/- 0.8 mosmol/kg, P < 0.05). Hematocrits were matched (HSI: 39.1 +/- 1% and ISO: 39.1 +/- 1%, P > 0.40); thus, we were successful in isolating osmolality. The baroreflex control of sympathetic outflow was greater during the HSI compared with the ISO (HSI: -8.3 +/- 1.2 arbitrary units x beat(-1) x mmHg(-1) vs. ISO: -4.0 +/- 0.8 arbitrary units x beat(-1) x mmHg(-1), P = 0.01). In conclusion, when controlling for intravascular volume, increased plasma osmolality enhances baroreflex control of sympathetic activity in humans.

Dynamics of sympathetic baroreflex control of arterial pressure in rats

By a white noise approach, we characterized the dynamics of the sympathetic baroreflex system in 11 halothane-anesthetized rats. We measured sympathetic nerve activity (SNA) and systemic arterial pressure (SAP), while carotid sinus baroreceptor pressure (BRP) was altered randomly. We estimated the transfer functions from BRP to SNA (mechanoneural arc), from SNA to SAP (neuromechanical arc), and from BRP to SAP (total arc). The gain of the mechanoneural arc gradually increased about threefold as the frequency of BRP change increased from 0.01 to 0.8 Hz. In contrast, the gain of the neuromechanical arc rapidly decreased to 0.4% of the steady-state gain as the frequency increased from 0.01 to 1 Hz. Although the total arc also had low-pass characteristics, the rate of attenuation in its gain was significantly slower than that of the neuromechanical arc, reflecting the compensatory effect of the mechanoneural arc for the sluggish response of the neuromechanical arc. We conclude that the quantitative estimation of the baroreflex dynamics is vital for an integrative understanding of baroreflex function in rats.

Carotid-Sinus Baroreflex Modulation of Core and Skin Temperatures in Rats: An Open-Loop Approach

The neural mechanisms of the thermoregulatory control of core and skin temperatures in response to heat and cold stresses have been well clarified. However, it has been unclear whether baroreceptor reflexes are involved in the control of core and skin temperatures. To investigate how the arterial baroreceptor reflex modulates the body temperatures, we examined the effect of pressure changes of carotid sinus baroreceptors on core and skin temperatures in halothane-anesthetized rats. To open the baroreflex loop and control arterial baroreceptor pressure (BRP), we cut vagal and aortic depressor nerves and isolated carotid sinuses. We sequentially altered BRP in 20-mmHg increments from 60 to 180 mmHg and then in 20-mmHg decrements from 180 to 60 mmHg while measuring systemic arterial pressure (SAP), heart rate (HR), and core blood temperature (T(core)) at the aortic arch and skin temperature (T(skin)) at the tail. In response to the incremental change in BRP by 120 mmHg, SAP, HR, and T(core) fell by 90.3 +/- 5.1 mmHg, 60.3 +/- 10.5 beats min(-1), and 0.18 +/- 0.01 degrees C, respectively. T(skin) rose by 0.84 +/- 0.10 degrees C. The maximum rate of change per unit BRP change was -2.1 +/- 0.2 for SAP, -1.5 +/- 0.4 beats min(-1) mmHg(-1) for HR, -0.003 +/- 0.001 degrees C mmHg(-1) for T(core), and 0.011 +/- 0.002 degrees C mmHg(-1) for T(skin). After the administration of hexamethonium or bretylium, these baroreflexogenic responses were completely abolished. We concluded that T(core) and T(skin) are modulated by the arterial baroreceptor reflex.

Down-regulation of basal Fos expression at nucleus tractus solitarii underlies restoration of baroreflex response after antihypertensive treatment in spontaneously hypertensive rats

Antihypertensive therapy not only normalizes the elevated blood pressure but also restores the reduced baroreceptor reflex response associated with hypertension, although the underlying mechanism is not fully understood. We assessed the hypothesis that a reversal of the enhanced basal Fos expression seen during hypertension in nucleus tractus solitarii, the terminal site of baroreceptor afferents, underlies the restoration of baroreceptor reflex sensitivity after antihypertensive treatment. Male adult spontaneously hypertensive or normotensive Wistar-Kyoto rats received for 3 weeks captopril (100 mg/kg/day) added to their drinking water. Evaluated subsequently under pentobarbital anesthesia, captopril-treated spontaneously hypertensive rats exhibited significantly lowered systolic blood pressure and restoration of the sensitivity in baroreceptor reflex control of heart rate to levels comparable with Wistar-Kyoto rats. Reverse transcription-polymerase chain reaction analysis and immunohistochemical evaluation revealed concomitant down-regulation of basal expression in nucleus tractus solitarii of c-fos gene at both mRNA and protein levels. Captopril treatment, on the other hand, elicited no discernible effect on systolic blood pressure, cardiac baroreceptor reflex sensitivity or basal expression of Fos protein at the nucleus tractus solitarii of normotensive Wistar-Kyoto rats. From these findings we suggest that a down-regulation of basal Fos expression in nucleus tractus solitarii may contribute to the restoration of baroreceptor reflex sensitivity in spontaneously hypertensive rats that received antihypertensive treatment such as captopril.

Chromosome 1 blood pressure QTL region influences renal function curve and salt sensitivity in SHR

One or more quantitative trait locus (QTL) for blood pressure (BP) exists on rat chromosome 1, in the vicinity of the Sa gene. The present work examined whether this QTL region: 1) alters pressure-natriuresis relationship and 2) affects the BP response to salt load. Male spontaneously hypertensive rats (SHR), Wistar-Kyoto (WKY) rats, and rats from an SHR congenic strain that contains a WKY chromosome 1 segment spanning the BP QTL region (SHR. WKY-Sa) were used. In an acute study in anesthetized animals, renal function was measured at several levels of renal perfusion pressure. In a chronic study, BP was measured in freely moving rats using telemetry during normal and high sodium intake (2% NaCl as drinking water for 2 wk). WKY rats showed a significantly higher glomerular filtration rate and increased pressure-natriuresis compared with SHR. SHR.WKY-Sa also demonstrated an increased glomerular filtration rate and enhanced pressure-natriuresis, associated with a lower tubular sodium reabsorption, compared with SHR. These modifications were accompanied by a lower basal BP in SHR.WKY-Sa compared with SHR and a markedly reduced BP response to salt load. These findings suggest that the BP QTL(s) present in this region of chromosome 1 influences BP and salt sensitivity, at least partly, by modulating pressure-natriuresis.

Endogenous angiotensin modulates PGE(2)-mediated release of substance P from renal mechanosensory nerve fibers

Increasing renal pelvic pressure increases afferent renal nerve activity (ARNA) by a prostaglandin E2 (PGE2)-mediated release of substance P (SP) from renal pelvic sensory nerves. We examined whether the ARNA responses were modulated by high- and low-sodium diets. Increasing renal pelvic pressure resulted in greater ARNA responses in rats fed a high-sodium than in those fed a low-sodium diet. In rats fed a low-sodium diet, increasing renal pelvic pressure 2.5 and 7.5 mmHg increased ARNA 2 +/- 1 and 13 +/- 1% before and 12 +/- 1 and 22 +/- 2% during renal pelvic perfusion with 0.44 mM losartan. In rats fed a high-sodium diet, similar increases in renal pelvic pressure increased ARNA 10 +/- 1 and 23 +/- 3% before and 1 +/- 1 and 11 +/- 2% during pelvic perfusion with 15 nM ANG II. The PGE2-mediated release of SP from renal pelvic nerves in vitro was enhanced in rats fed a high-sodium diet and suppressed in rats fed a low-sodium diet. The PGE2 concentration required for SP release was 0.03, 0.14, and 3.5 microM in rats fed high-, normal-, and low-sodium diets. In rats fed a low-sodium diet, PGE2 increased renal pelvic SP release from 5 +/- 1 to 6 +/- 1 pg/min without and from 12 +/- 1 to 21 +/- 2 pg/min with losartan in the incubation bath. Losartan had no effect on SP release in rats fed normal- and high-sodium diets. ANG II modulates the responsiveness of renal pelvic mechanosensory nerves by inhibiting PGE2-mediated SP release from renal pelvic nerve fibers.

The interaction of angiotensin II and osmolality in the generation of sympathetic tone during changes in dietary salt intake. An hypothesis

At rest, sympathetic nerves exhibit tonic activity which contributes to arterial pressure maintenance. Significant evidence suggests that the absolute level of sympathetic tone is altered in a number of physiologic and pathophysiologic states. However, the mechanisms by which such changes in sympathetic tone occur are incompletely understood. The purpose of this review is to present evidence that humoral factors are essential in these changes and to detail specifically an hypothesis for the mechanisms that underlie the changes in sympathetic tone that are produced during increases or decreases in dietary salt intake. It is proposed that the net effect of changes in dietary salt on sympathetic activity is determined by the balance between simultaneous and parallel sympathoinhibitory and sympathoexcitatory humoral mechanisms. A key element of the sympathoinhibitory mechanism is the chronic sympathoexcitatory effects of angiotensin II (ANG II). When salt intake increases, ANG II levels fall, and the sympathoexcitatory actions of ANG II are lost. Simultaneously, a sympathoexcitatory pathway is triggered, possibly via increases in osmolality which activate osmoreceptors or sodium receptors. In normal individuals, the sympathoinhibitory effects of increased salt predominate, sympathetic activity decreases, and arterial pressure remains normal despite salt and water retention. However, in subjects with salt-sensitive hypertension, it appears that the sympathoexcitatory effects of salt predominate, possibly due to an inability to adequately suppress the levels or actions of ANG II. The net result, therefore, is an inappropriate increase in sympathetic activity during increased dietary salt which may contribute to the hypertensive process.

Salt sensitivity in genetically hypertensive rats of the Lyon strain

BACKGROUND

Genetically hypertensive (LH) rats of the Lyon strain exhibit a blunted pressure-natriuresis function when compared, in acute conditions, with their normotensive (LN) and low blood pressure (LL) controls. The present work was aimed to determine whether LH rats were salt sensitive in chronic conditions. In addition, a protocol was developed to determine the renal function curve in freely moving rats.

METHODS

Fourteen-week-old rats either untreated or orally treated since weaning with perindopril (3 mg/kg/24 h), an angiotensin-converting enzyme inhibitor, or with valsartan (15 mg/kg/24 h), an angiotensin II subtype 1 receptor antagonist, so as to eliminate the influence of endogenous changes in angiotensin formation were used. Blood pressure (BP) and urinary sodium excretion were measured before, during an oral salt load (2% NaCl in drinking water), and during a two-week aldosterone infusion (50 microg/kg/24 h subcutaneously).

RESULTS

NaCl induced a greater BP increase in untreated LH rats than in LN and LL controls. Perindopril normalized the BP of LH rats but not its elevation during a salt load. Aldosterone slightly increased BP in LH and LL rats either untreated or treated with valsartan. Finally, the combination of telemetric BP measurement with 24-hour urine collection when salt was added to drinking water allowed accurate determination of the slope of the chronic renal function curve in freely moving rats.

CONCLUSION

The present work demonstrates that LH rats are salt sensitive. This characteristic manifests despite the lack of an active renin-angiotensin system and is not explained by a hypersensitivity to aldosterone.

New analytic framework for understanding sympathetic baroreflex control of arterial pressure

The sympathetic baroreflex is an important feedback system in stabilization of arterial pressure. This system can be decomposed into the controlling element (mechanoneural arc) and the controlled element (neuromechanical arc). We hypothesized that the intersection of the two operational curves representing their respective functions on an equilibrium diagram should define the operating point of the arterial baroreflex. Both carotid sinuses were isolated in 16 halothane-anesthetized rats. The vagi and aortic depressor nerves were cut bilaterally. Carotid sinus pressure (CSP) was sequentially altered in 10-mmHg increments from 80 to 160 mmHg while sympathetic efferent nerve activity (SNA) and systemic arterial pressure (SAP) were recorded simultaneously under various hemorrhagic conditions. The mechanoneural arc was characterized by the response of SNA to CSP and the neuromechanical arc by the response of SAP to SNA. We parametrically analyzed the relationship between input and output for each arc using a four-parameter logistic equation model. In baseline states, the two arcs intersected each other at the point at which the instantaneous gain of each arc attained its maximum. Severe hemorrhage lowered the gain and offset of the neuromechanical arc and moved the operating point, whereas the mechanoneural arc remained unchanged. The operating points measured under the closed-loop conditions were indistinguishable from those estimated from the intersections of the two arc curves on the equilibrium diagram. The average root mean square errors of estimate for arterial pressure and SNA were 2 and 3%, respectively. Such an analytic approach could explain a mechanism for the determination of the operating point of the sympathetic baroreflex system and thus helps us integratively understand its function.

New simple methods for isolating baroreceptor regions of carotid sinus and aortic depressor nerves in rats

We developed new methods for isolating in situ baroreceptor regions of carotid sinus and aortic depressor nerves in halothane-anesthetized rats. After ligation of the root of the external carotid artery, the internal carotid and pterygopalatine arteries were embolized with two ball bearings of 0.8 mm in diameter. Bilateral carotid sinus pressures were changed between 60 and 180 mmHg in 20-mmHg steps lasting 1 min each. The sigmoidal steady-state relationship between aortic and carotid sinus pressures in 11 rats indicated the maximum gain of the carotid sinus baroreflex to be -2. 99 +/- 0.75 at 120 +/- 5 mmHg. An in situ isolation of the baroreceptor area of the right aortic depressor nerve could be made by ligation of the innominate, common carotid, and subclavian arteries in 9 rats. Pressure imposed on the subclavian baroreceptor was altered between 40 and 180 mmHg in 20-mmHg steps lasting 1 min each. The sigmoidal steady-state relationship between the aortic depressor nerve activity and imposed pressure showed that the baroreceptor gain peaked at 118 +/- 4 mmHg. We established an easy approach to the rat baroreflex and baroreceptor research.

Renal sympathetic nerve activity in mice: comparison between mice and rats and between normal and endothelin-1 deficient mice

Recently generated knockout mice with disrupted genes encoding endothelin (ET)-1 showed an elevation of arterial blood pressure (AP) and supplied an evidence for intrinsic ET-1 as one of the physiological regulators of systemic AP. Little is yet known, however, why deficiency of ET-1, which was originally found as a potent vasoconstrictor, led to higher AP in these mice. To address this apparent paradox, we first developed a method to measure renal sympathetic nerve activity (RSNA) in mice using rats as reference and successively compared it between normal and ET-1 deficient mice. RSNA was successfully recorded in urethane-anesthetized and artificially ventilated mice by a slight modification of the method used for rats. At basal condition, mean AP (MAP) and RSNA in ET-1 deficient mice (105+/-2 mmHg and 9.71+/-1.49 muVs, n=20) were significantly higher than those in wild-type mice (96+/-2 mmHg and 5. 07+/-0.70 muVs, n=25). Basal heart rate (HR) and baroreflex-control of HR was not significantly different between the two. On the other hand, resting RSNA, RSNA range, and maximum RSNA were significantly greater in ET-1 deficient mice, and thus MAP-RSNA relationship was upwards reset. Hypoxia-induced increase in RSNA was not different between ET-1 deficient (73.4+/-9.4%) and wild-type mice (91.2+/-12.0%), while hypercapnia-induced one was significantly attenuated in ET-1 deficient mice (18.8+/-3.6 vs. 39.1+/-5.2% at 10% CO2). These results indicate that endogenous ET-1 participates in the central chemoreception of CO2 and reflex control of the RSNA. Baroreceptor resetting and normally preserved hypoxia-induced chemoreflex may explain a part of the elevation of AP in ET-1 deficient mice.