Circadian rhythm of plasma sodium is disrupted in spontaneously hypertensive rats fed a high-NaCl diet

Current perspective on circadian function of the kidney

Behavior and function of living systems are synchronized by the 24-h rotation of the Earth that guides physiology according to time of day. However, when behavior becomes misaligned from the light-dark cycle, such as in rotating shift work, jet lag, and even unusual eating patterns, adverse health consequences such as cardiovascular or cardiometabolic disease can arise. The discovery of cell-autonomous molecular clocks expanded interest in regulatory systems that control circadian physiology including within the kidney, where function varies along a 24-h cycle. Our understanding of the mechanisms for circadian control of physiology is in the early stages, and so the present review provides an overview of what is known and the many gaps in our current understanding. We include a particular focus on the impact of eating behaviors, especially meal timing. A better understanding of the mechanisms guiding circadian function of the kidney is expected to reveal new insights into causes and consequences of a wide range of disorders involving the kidney, including hypertension, obesity, and chronic kidney disease.

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.

Altered Neuronal Discharge in the Organum Vasculosum of the Lamina Terminalis Contributes to Dahl Salt-Sensitive Hypertension

BACKGROUND

Salt-sensitive hypertension in humans and experimental models is associated with higher plasma and cerebrospinal fluid sodium chloride (NaCl) concentrations. Changes in extracellular NaCl concentrations are sensed by specialized neurons in the organum vasculosum of the lamina terminalis (OVLT). Stimulation of OVLT neurons increases sympathetic nerve activity (SNA) and arterial blood pressure (ABP), whereas chronic activation produces hypertension. Therefore, the present study tested whether OVLT neuronal activity was elevated and contributed to SNA and ABP in salt-sensitive hypertension.

METHODS

Male Dahl salt-sensitive (Dahl S) and Dahl salt-resistant (Dahl R) rats were fed 0.1% or 4.0% NaCl diets for 3 to 4 weeks and used for single-unit recordings of OVLT neurons or simultaneous recording of multiple sympathetic nerves during pharmacological inhibition of the OVLT.

RESULTS

Plasma and cerebrospinal fluid Na+ and Cl- concentrations were higher in Dahl S rats fed 4% versus 0.1% or Dahl R rats fed either diet. In vivo single-unit recordings revealed a significantly higher discharge of NaCl-responsive OVLT neurons in Dahl S rats fed 4% versus 0.1% or Dahl R rats. Interestingly, intracarotid infusion of hypertonic NaCl evoked greater increases in OVLT neuronal discharge of Dahl S versus Dahl R rats regardless of NaCl diet. The activity of non-NaCl-responsive OVLT neurons was not different across strain or diets. Finally, inhibition of OVLT neurons by local injection of the gamma-aminobutyric acid agonist muscimol produced a greater decrease in renal SNA, splanchnic SNA, and ABP of Dahl S rats fed 4% versus 0.1% or Dahl R rats.

CONCLUSIONS

A high salt diet activates NaCl-responsive OVLT neurons to increase SNA and ABP in salt-sensitive hypertension.

Daytime Exposure to Blue Light Alters Cardiovascular Circadian Rhythms, Electrolyte Excretion and Melatonin Production

Artificial light is characterized by certain features of its impact on the body in terms of its spectral distribution of power, duration of exposure and intensity. Short waves, perceived as blue light, are the strongest synchronizing agent for the circadian system. In the present work, we investigated the features of the circadian rhythms of blood pressure (BP), heart rate (HR), the excretion of electrolytes and the secretion of melatonin in normotensive (Wistar–Kyoto) and hypertensive (SHR) rats under the action of monochromatic blue light in the daytime period. It was found that the exposure of Wistar–Kyoto rats to monochromatic blue light was accompanied by a significant decrease in nighttime and 24 h systolic BP. The most remarkable changes are characteristic of the HR in SHR rats under monochromatic light. A significant decrease in HR in each time period was found, but the predominance of nighttime over daytime values remained in SHR animals. There was also a significant increase in the mesor of the HR in SHR rats. Additionally, the amplitude of diastolic BP and HR, as well as the range of oscillations in HR, were significantly increased compared with the standard light pattern. In contrast to SHR rats, the regulation of the circadian rhythms in Wistar–Kyoto rats was more flexible and presented more changes, which may be aimed at the adaptation of the body to environmental conditions. For Wistar–Kyoto rats, an increase in the level of excreted electrolytes was observed under the action of monochromatic light, but no similar changes were found in SHR rats. For Wistar–Kyoto rats, a significant decrease in the urine concentration of aMT6s in the daytime and nighttime periods is characteristic, which results in the loss of the circadian rhythm. In SHR rats, there was a significant decrease in the nighttime content of aMT6s in the urine, while the daytime concentration, on the contrary, increased. The obtained data demonstrate that prolonged exposure to monochromatic blue light in the daytime period affects the circadian structure of the rhythms of the cardiovascular system, the rhythm of electrolyte excretion and the production of epiphyseal melatonin in wild-type and hypertensive animals. In SHR rats, the rhythms of BP and HR exhibit a more rigid pattern.

Circadian Control of Sodium and Blood Pressure Regulation

The attention for the control of dietary risk factors involved in the development of hypertension, includes a large effort on dietary salt restrictions. Ample studies show the beneficial role of limiting dietary sodium as a lifestyle modification in the prevention and management of essential hypertension. Not until the past decade or so have studies more specifically investigated diurnal variations in renal electrolyte excretion, which led us to the hypothesis that timing of salt intake may impact cardiovascular health and blood pressure regulation. Cell autonomous molecular clocks as the name implies, function independently to maintain optimum functional rhythmicity in the face of environmental stressors such that cellular homeostasis is maintained at all times. Our understanding of mechanisms influencing diurnal patterns of sodium excretion and blood pressure has expanded with the discovery of the circadian clock genes. In this review, we discuss what is known about circadian regulation of renal sodium handling machinery and its influence on blood pressure regulation, with timing of sodium intake as a potential modulator of the kidney clock.

High-Salt Diet Impairs the Neurons Plasticity and the Neurotransmitters-Related Biological Processes

Salt, commonly known as sodium chloride, is an important ingredient that the body requires in relatively minute quantities. However, consuming too much salt can lead to high blood pressure, heart disease and even disruption of circadian rhythms. The biological process of the circadian rhythm was first studied in Drosophila melanogaster and is well understood. Their locomotor activity gradually increases before the light is switched on and off, a phenomenon called anticipation. In a previous study, we showed that a high-salt diet (HSD) impairs morning anticipation behavior in Drosophila. Here, we found that HSD did not significantly disrupt clock gene oscillation in the heads of flies, nor did it disrupt PERIOD protein oscillation in clock neurons or peripheral tissues. Remarkably, we found that HSD impairs neuronal plasticity in the axonal projections of circadian pacemaker neurons. Interestingly, we showed that increased excitability in PDF neurons mimics HSD, which causes morning anticipation impairment. Moreover, we found that HSD significantly disrupts neurotransmitter-related biological processes in the brain. Taken together, our data show that an HSD affects the multiple functions of neurons and impairs physiological behaviors.

Inhibition of Renin-Angiotensin System from Conception to Young Mature Life Induces Salt-Sensitive Hypertension via Angiotensin II-Induced Sympathetic Overactivity in Adult Male Rats

Previous studies indicate that perinatal compromise of taurine causes cardiovascular disorders in adults via the influence of taurine on renin-angiotensin system (RAS). This study tested whether perinatal inhibition of the RAS would itself alter the adult cardiovascular system in a similar way. Female Sprague-Dawley rats were fed normal rat chow and given water alone (Control) or water containing captopril (400 mg/l) from conception until weaning. Then, the male offspring drank water or water containing captopril until 5 weeks of age followed by normal rat chow and water alone until 7 weeks of age. Thereafter, they drank water alone (Control, Captopril) or 1% NaCl solution (Control+1%, Captopril+1%). At 9 weeks of age, all animals were implanted with femoral arterial and venous catheters. Forty-eight hours later, blood chemistry, glucose tolerance, and hemodynamic parameters were determined in freely moving conscious rats. Then, the same experiments were repeated 2 days after captopril treatment. Body weights, kidney and heart to body weight ratios, fasting and non-fasting blood sugar, glucose tolerance, and heart rates were not significantly different among groups. Further, plasma sodium, mean arterial pressure, and sympathetic activity significantly increased whereas baroreflex sensitivity decreased in Captopril+1% compared to other groups. These changes were normalized by acute captopril treatment and the arterial pressure differences also by acute ganglionic and central adrenergic blockade. The present study suggests that inhibition of the RAS in the early life induces RAS overactivity, leading to salt-sensitive hypertension via sympathetic nervous system overactivity and depressed baroreflex sensitivity in adult male rats.

Predicted effect of circadian clock modulation of NHE3 of a proximal tubule cell on sodium transport

Major renal functions such as renal blood flow, glomerular filtration rate, and urinary excretion are known to exhibit circadian oscillations. However, the underlying mechanisms that govern these variations have yet to be fully elucidated. To better understand the impact of the circadian clock on renal solute and water transport, we have developed a computational model of the renal circadian clock and coupled that model to an epithelial transport model of the proximal convoluted cell of the rat kidney. The activity of the Na⁺-H⁺ exchanger 3 (NHE3) is assumed to be regulated by changes in transcription of the NHE3 mRNA due to regulation by circadian clock proteins. The model predicts the rhythmic oscillations in NHE3 activity, which gives rise to significant daily fluctuations in Na⁺ and water transport of the proximal tubule cell. Additionally, the model predicts that 1) mutation in period 2 (Per2) or cryptochrome 1 (Cry1) preserves the circadian rhythm and modestly raises Na⁺ reabsorption; 2) mutation in Bmal1 or CLOCK eliminates the circadian rhythm and modestly lowers Na⁺ reabsorption; 3) mutation in Rev-Erb or ROR-related orphan receptor (Ror) has minimal impact on the circadian oscillations. The model represents the first step in building a tool set aimed at increasing our understanding of how the molecular clock affects renal ion transport and renal function, which likely has important implications for kidney disease.

The Circadian Clock in the Regulation of Renal Rhythms

Since the kidney is integral to maintenance of fluid and ion homeostasis, and therefore blood pressure regulation, its proper function is paramount. Circadian fluctuations in blood pressure, renal blood flow, glomerular filtration rate, and sodium and water excretion have been documented for decades, if not longer. Recent studies on the role of circadian clock proteins in the regulation of a variety of renal transport genes suggest that the molecular clock in the kidney controls circadian fluctuations in renal function. The circadian clock appears to be a critical regulator of renal function with important implications for the treatment of renal pathologies, which include chronic kidney disease and hypertension. The development, regulation, and mechanism of the kidney clock are reviewed here.

Acute hypernatremia promotes anxiolysis and attenuates stress-induced activation of the hypothalamic-pituitary-adrenal axis in male mice

Previous investigation by our laboratory found that acute hypernatremia potentiates an oxytocinergic tone that inhibits parvocellular neurosecretory neurons in the paraventricular nucleus of the hypothalamus (PVN), attenuates restraint-induced surges in corticosterone (CORT), and reduces anxiety-like behavior in male rats. To investigate the neural mechanisms mediating these effects and extend our findings to a more versatile species, we repeated our studies using laboratory mice. In response to 2.0M NaCl injections, mice had increased plasma sodium concentrations which were associated with a blunted rise in CORT subsequent to restraint challenge relative to 0.15M NaCl injected controls. Immunofluorescent identification of the immediate early gene product Fos found that 2.0M NaCl treatment increased the number of activated neurons producing oxytocin in the PVN. To evaluate the effect of acute hypernatremia on PVN neurons producing corticotropin-releasing hormone (CRH), we used the Cre-lox system to generate mice that produced the red fluorescent protein, tdTomato, in cells that had Cre-recombinase activity driven by CRH gene expression. Analysis of brain tissue from these CRH-reporter mice revealed that 2.0M NaCl treatment caused a dramatic reduction in Fos-positive nuclei specifically in CRH-producing PVN neurons. This altered pattern of activity was predictive of alleviated anxiety-like behavior as mice administered 2.0M NaCl spent more time exploring the open arms of an elevated-plus maze than 0.15M NaCl treated controls. Taken together, these results further implicate an oxytocin-dependent inhibition of CRH neurons in the PVN and demonstrate the impact that slight elevations in plasma sodium have on hypothalamic-pituitary-adrenocortical axis output and anxiety-like behavior.

Neurogenic and sympathoexcitatory actions of NaCl in hypertension

Excess dietary salt intake is a major contributing factor to the pathogenesis of salt-sensitive hypertension. Strong evidence suggests that salt-sensitive hypertension is attributed to renal dysfunction, vascular abnormalities, and activation of the sympathetic nervous system. Indeed, sympathetic nerve transections or interruption of neurotransmission in various brain centers lowers arterial blood pressure (ABP) in many salt-sensitive models. The purpose of this article is to discuss recent evidence that supports a role of plasma or cerebrospinal fluid hypernatremia as a key mediator of sympathoexcitation and elevated ABP. Both experimental and clinical studies using time-controlled sampling have documented that a diet high in salt increases plasma and cerebrospinal fluid sodium concentration. To the extent it has been tested, acute and chronic elevations in sodium concentration activates the sympathetic nervous system in animals and humans. A further understanding of how the central nervous system detects changes in plasma or cerebrospinal fluid sodium concentration may lead to new therapeutic treatment strategies in salt-sensitive hypertension.

How NaCl raises blood pressure: a new paradigm for the pathogenesis of salt-dependent hypertension

Excess dietary salt is a major cause of hypertension. Nevertheless, the specific mechanisms by which salt increases arterial constriction and peripheral vascular resistance, and thereby raises blood pressure (BP), are poorly understood. Here we summarize recent evidence that defines specific molecular links between Na(+) and the elevated vascular resistance that directly produces high BP. In this new paradigm, high dietary salt raises cerebrospinal fluid [Na(+)]. This leads, via the Na(+)-sensing circumventricular organs of the brain, to increased sympathetic nerve activity (SNA), a major trigger of vasoconstriction. Plasma levels of endogenous ouabain (EO), the Na(+) pump ligand, also become elevated. Remarkably, high cerebrospinal fluid [Na(+)]-evoked, locally secreted (hypothalamic) EO participates in a pathway that mediates the sustained increase in SNA. This hypothalamic signaling chain includes aldosterone, epithelial Na(+) channels, EO, ouabain-sensitive α(2) Na(+) pumps, and angiotensin II (ANG II). The EO increases (e.g.) hypothalamic ANG-II type-1 receptor and NADPH oxidase and decreases neuronal nitric oxide synthase protein expression. The aldosterone-epithelial Na(+) channel-EO-α(2) Na(+) pump-ANG-II pathway modulates the activity of brain cardiovascular control centers that regulate the BP set point and induce sustained changes in SNA. In the periphery, the EO secreted by the adrenal cortex directly enhances vasoconstriction via an EO-α(2) Na(+) pump-Na(+)/Ca(2+) exchanger-Ca(2+) signaling pathway. Circulating EO also activates an EO-α(2) Na(+) pump-Src kinase signaling cascade. This increases the expression of the Na(+)/Ca(2+) exchanger-transient receptor potential cation channel Ca(2+) signaling pathway in arterial smooth muscle but decreases the expression of endothelial vasodilator mechanisms. Additionally, EO is a growth factor and may directly participate in the arterial structural remodeling and lumen narrowing that is frequently observed in established hypertension. These several central and peripheral mechanisms are coordinated, in part by EO, to effect and maintain the salt-induced elevation of BP.

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.

Sympathetic nervous system overactivity and its role in the development of cardiovascular disease

This review examines how the sympathetic nervous system plays a major role in the regulation of cardiovascular function over multiple time scales. This is achieved through differential regulation of sympathetic outflow to a variety of organs. This differential control is a product of the topographical organization of the central nervous system and a myriad of afferent inputs. Together this organization produces sympathetic responses tailored to match stimuli. The long-term control of sympathetic nerve activity (SNA) is an area of considerable interest and involves a variety of mediators acting in a quite distinct fashion. These mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity, and a host of humoral factors. A key feature of many cardiovascular diseases is increased SNA. However, rather than there being a generalized increase in SNA, it is organ specific, in particular to the heart and kidneys. These increases in regional SNA are associated with increased mortality. Understanding the regulation of organ-specific SNA is likely to offer new targets for drug therapy. There is a need for the research community to develop better animal models and technologies that reflect the disease progression seen in humans. A particular focus is required on models in which SNA is chronically elevated.

Excess dietary salt intake alters the excitability of central sympathetic networks

The ingestion of excess dietary salt (defined as NaCl) is strongly correlated with cardiovascular disease, morbidity, mortality, and is regarded as a major contributing factor to the pathogenesis of hypertension. Although several mechanisms contribute to the adverse consequences of dietary salt intake, accumulating evidence suggests that dietary salt loading produces neurogenically-mediated increases in total peripheral resistance to raise arterial blood pressure (ABP). Evidence from clinical studies and experimental models clearly establishes a hypertensive effect of dietary salt loading in a subset of individuals who are deemed "salt-sensitive". However, we will discuss and present evidence to develop a novel hypothesis to suggest that while chronic increases in dietary salt intake do not elevate mean ABP in "non-salt-sensitive" animals, dietary salt intake does enhance several sympathetic reflexes thereby predisposing these animals and/or individuals to the development of salt-sensitive hypertension. Additional evidence raises an intriguing hypothesis that these enhanced sympathetic reflexes are largely attributed to the ability of excess dietary salt intake to selectively enhance the excitability of sympathetic-regulatory neurons in the rostral ventrolateral medulla. Insight into the cellular mechanisms by which dietary salt intake alters the responsiveness of RVLM circuits will likely provide a foundation for developing new therapeutic approaches to treat salt-sensitive hypertension. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

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.

Cerebrospinal fluid sodium rhythms

Background

Cerebrospinal fluid (CSF) sodium levels have been reported to rise during episodic migraine. Since migraine frequently starts in early morning or late afternoon, we hypothesized that natural sodium chronobiology may predispose susceptible persons when extracellular CSF sodium increases. Since no mammalian brain sodium rhythms are known, we designed a study of healthy humans to test if cation rhythms exist in CSF.

Methods

Lumbar CSF was collected every ten minutes at 0.1 mL/min for 24 h from six healthy participants. CSF sodium and potassium concentrations were measured by ion chromatography, total protein by fluorescent spectrometry, and osmolarity by freezing point depression. We analyzed cation and protein distributions over the 24 h period and spectral and permutation tests to identify significant rhythms. We applied the False Discovery Rate method to adjust significance levels for multiple tests and Spearman correlations to compare sodium fluctuations with potassium, protein, and osmolarity.

Results

The distribution of sodium varied much more than potassium, and there were statistically significant rhythms at 12 and 1.65 h periods. Curve fitting to the average time course of the mean sodium of all six subjects revealed the lowest sodium levels at 03.20 h and highest at 08.00 h, a second nadir at 09.50 h and a second peak at 18.10 h. Sodium levels were not correlated with potassium or protein concentration, or with osmolarity.

Conclusion

These CSF rhythms are the first reports of sodium chronobiology in the human nervous system. The results are consistent with our hypothesis that rising levels of extracellular sodium may contribute to the timing of migraine onset. The physiological importance of sodium in the nervous system suggests that these rhythms may have additional repercussions on ultradian functions.

Ventral lamina terminalis mediates enhanced cardiovascular responses of rostral ventrolateral medulla neurons during increased dietary salt

Increased dietary salt enhances sympathoexcitatory and sympathoinhibitory responses evoked from the rostral ventrolateral medulla (RVLM). The purpose of the present study was to determine whether neurons of the forebrain lamina terminalis (LT) mediated these changes in the RVLM. Male Sprague-Dawley rats with and without LT lesions were fed normal chow and given access to water or 0.9% NaCl for 14 to 15 days. Unilateral injection of l-glutamate into the RVLM produced significantly larger increases in renal sympathetic nerve activity and arterial blood pressure of sham rats ingesting 0.9% NaCl versus water. However, these differences were not observed between ventral LT-lesioned rats drinking 0.9% NaCl versus water. Similar findings were observed when angiotensin II or gamma-aminobutyric acid was injected into the RVLM. Interestingly, a subset of animals drinking 0.9% but with damage restricted to the organum vasculosum of the lamina terminalis did not show enhanced responses to l-glutamate or gamma-aminobutyric acid. In marked contrast, RVLM injection of l-glutamate or gamma-aminobutyric acid produced exaggerated sympathetic nerve activity and arterial blood pressure responses in animals drinking 0.9% NaCl versus water after an acute ventral LT lesion or chronic lesion of the subfornical organ. Additional experiments demonstrated that plasma sodium concentration and osmolality were increased at night in rats ingesting 0.9% NaCl. These findings suggest that neurons of the ventral LT mediate the ability of increased dietary salt to enhance the responsiveness of RVLM sympathetic neurons.

Role of central nervous system aldosterone synthase and mineralocorticoid receptors in salt-induced hypertension in Dahl salt-sensitive rats

In Dahl salt-sensitive (S) rats, high salt intake increases cerebrospinal fluid (CSF) Na+ concentration ([Na+]) and blood pressure (BP). Intracerebroventricular (ICV) infusion of a mineralocorticoid receptor (MR) blocker prevents the hypertension. To assess the role of aldosterone locally produced in the brain, we evaluated the effects of chronic central blockade with the aldosterone synthase inhibitor FAD286 and the MR blocker spironolactone on changes in aldosterone and corticosterone content in the hypothalamus and the increase in CSF [Na+] and hypertension induced by high salt intake in Dahl S rats. After 4 wk of high salt intake, plasma aldosterone and corticosterone were not changed, but hypothalamic aldosterone increased by ∼35% and corticosterone tended to increase in Dahl S rats, whereas both steroids decreased by ∼65% in Dahl salt-resistant rats. In Dahl S rats fed the high-salt diet, ICV infusion of FAD286 or spironolactone did not affect the increase in CSF [Na+]. ICV infusion of FAD286 prevented the increase in hypothalamic aldosterone and 30 mmHg of the 50-mmHg BP increase induced by high salt intake. ICV infusion of spironolactone fully prevented the salt-induced hypertension. These results suggest that, in Dahl S rats, high salt intake increases aldosterone synthesis in the hypothalamus and aldosterone acts as the main MR agonist activating central pathways contributing to salt-induced hypertension.

Expression of gLTP in sympathetic ganglia of obese Zucker rats in vivo: molecular evidence

Long-term potentiation in sympathetic ganglia (gLTP) is similar to LTP of the hippocampal area CA1 in that its expression involves similar changes in signaling molecules. We have shown previously that the stress-prone, hypertensive obese Zucker rats (OZR) expressed gLTP in sympathetic ganglia and that high blood pressure was reduced by treatment with 5-HT(3) receptor antagonists. In the present study, we present additional electrophysiological evidence for the pre-expression of gLTP in sympathetic ganglia from OZR indicated by failure of repetitive stimulation to express gLTP in isolated superior cervical ganglia (SCG) and inhibition of baseline ganglionic transmission by a 5-HT(3) receptor antagonist. We have also investigated the role of key signaling molecules in the expression of gLTP in the hypertensive OZR. Immunoblot analysis showed a significant increase in the levels of phosphorylated (P-)CaMKII and protein kinase C gamma (PKCgamma) in SCG from OZR. The ratio of P-CaMKII to the total CaMKII was markedly increased in OZR ganglia, suggesting increased phosphorylation of this molecule. Additionally, there was a significant decrease in the levels of calcineurin in ganglia. Furthermore, the neural nitric oxide synthase and hemeoxygenase II, which are essential for the expression of gLTP, were significantly elevated in OZR ganglia. The present findings confirm that ganglia from OZR have expressed gLTP and that synaptic plasticity in sympathetic ganglia may involve a molecular cascade similar to that of LTP of the brain hippocampal area CA1.

The central role of the brain in salt-sensitive hypertension

PURPOSE OF REVIEW

To integrate recent studies showing that abnormal Na transport in the central nervous system plays a pivotal role in genetic models of salt-sensitive hypertension.

RECENT FINDINGS

Na transport-regulating mechanisms classically considered to reflect renal control of the blood pressure, i.e. aldosterone-mineralocorticoid receptors-epithelial sodium channels-Na/K-ATPase, have now been demonstrated to be present in the central nervous system contributing to regulation of cerebrospinal fluid [Na] by the choroid plexus and to neuronal responsiveness to cerebrospinal fluid/brain [Na]. Dysfunction of either or both can activate central nervous system pathways involving 'ouabain' and angiotensin type 1 receptor stimulation. The latter causes sympathetic hyperactivity and adrenal release of marinobufagenin - a digitalis-like inhibitor of the alpha1 Na/K-ATPase isoform - both contributing to hypertension on high salt intake. Conversely, specific central nervous system blockade of mineralocorticoid receptors or epithelial sodium channels prevents the development of hypertension on high salt intake, irrespective of the presence of a 'salt-sensitive kidney'. Variants in the coding regions of some of the genes involved in Na transport have been identified, but sodium sensitivity may be mainly determined by abnormal regulation of expression, pointing to primary abnormalities in regulation of transcription.

SUMMARY

Looking beyond the kidney is providing new insights into mechanisms contributing to salt-sensitive hypertension, which will help to dissect the genetic factors involved and to discover novel strategies to prevent and treat salt-sensitive hypertension.

Forebrain osmotic regulation of the sympathetic nervous system

1. Accumulating evidence in both humans and animals indicates that acute increases in plasma osmolality elevate sympathetic nerve activity (SNA). In addition, plasma hyperosmolality (or hypernatraemia) can produce sustained increases in SNA and arterial blood pressure (ABP) through stimulation of forebrain osmoreceptors. 2. Although an abundance of information exists regarding the osmoregulatory circuits for thirst and secretion of antidiuretic hormone, much less is known about those pathways and synaptic mechanisms linking osmotic perturbations and SNA. To date, the available evidence suggests that osmosensitive sites within the forebrain lamina terminalis, such as the organum vasculosum of the lamina terminalis, are key elements that link plasma hypertonicity to elevated SNA. 3. The major efferent target of osmosensitive regions in the forebrain lamina terminalis is the hypothalamic paraventricular nucleus (PVH). Evidence from a number of studies indicates that the PVH contributes to both acute and chronic osmotically driven increases in SNA. In turn, PVH neurons increase SNA through a direct vasopressinergic spinal pathway and/or a glutamatergic pathway to bulbospinal sympathetic neurons of the rostral ventrolateral medulla. 4. Future studies are needed to: (i) define the contribution of various osmosensitive regions of the forebrain lamina terminalis to acute and chronic osmotically driven increases in SNA; (ii) identify the cellular mechanisms and neural circuitry linking plasma osmolality and SNA; and (iii) define whether such mechanisms contribute to elevated SNA in salt-sensitive hypertension.

Role of long-term potentiation of sympathetic ganglia (gLTP) in hypertension

Ganglionic long-term potentiation (gLTP) is an activity-dependent sustained increase in the synaptic efficacy of the nicotinic pathway that has been demonstrated in autonomic ganglia. Sustained enhancement in ganglionic transmission as in chronic mental stress may affect the activity of autonomic functions, including blood pressure and heart rate. An increase in sympathetic activity associated with psychosocial stress and stress-prone conditions such as obesity and aging could result in in vivo expression of gLTP leading to hypertension of a neural origin. Recent reports indicated that the prevention of the expression of gLTP in animal models of hypertension prevented or reduced high blood pressure. Although stress-induced hypertension normalizes within a few days of stress relief, prolonged mild-moderate hypertension may contribute to atherosclerotic cardiovascular diseases. The relation between hypertension and enhanced ganglionic transmission as a result of in vivo expression of gLTP is discussed in this review.

Time course of synergistic interaction between DOCA and salt on blood pressure: roles of vasopressin and hepatic osmoreceptors

In DOCA-salt rats, the time course of the synergistic interaction between osmolality and DOCA to produce hypertension is unknown. Therefore, in rats 2 wk after implantation of subcutaneous silicone pellets containing DOCA (65 mg) or no drug (sham), we determined blood pressure (BP) and heart rate (HR) responses, using telemetric pressure transducers, during 2 wk of excess salt ingestion (1% NaCl in drinking water). BP was unaltered in sham rats after increased salt, but in DOCA rats BP increased within 4 h. The initial hypertension of 30–35 mmHg stabilized within 2 days, followed ∼5 days later by a further increment of ∼30 mmHg. HR first decreased during the dark phase; the second phase was linked to an abrupt increase in HR and BP variability and decreased HR variability. Pressor responses to acute intravenous hypertonic saline infusion were doubled in DOCA-treated rats via vasopressin and nonvasopressin mechanisms. Only in DOCA-treated rats, portal vein hypertonic saline infusion increased BP, which was prevented by V1 vasopressin blockade. After 2 wk of DOCA-salt, oral ingestion of water rapidly decreased BP. Intraportal infusion of water did not lower BP in DOCA-salt rats, suggesting that hepatic osmoreceptors were not involved. In summary, the hypertension of DOCA-treated rats consuming excess salt exhibits multiple phases and can be rapidly reversed. Hypertonicity-induced vasopressin and nonvasopressin pressor mechanisms that are augmented by DOCA, and hepatic osmoreceptors may contribute to the initial developmental phase. With time, combined DOCA-salt induces marked changes in the regulation of the autonomic nervous system, which may favor hypertension development.

Translation of salt retention to central activation of the sympathetic nervous system in hypertension

1. Increased dietary salt increases blood pressure in many hypertensive individuals, producing salt-sensitive hypertension (SSH). The cause is unknown, but a major component appears to be activation of the sympathetic nervous system. The purpose of this short review is to present one hypothesis to explain how increased dietary salt increases sympathetic activity in SSH. 2. It is proposed that increased salt intake causes salt retention and raises plasma sodium chloride (NaCl) concentrations, which activate sodium/osmoreceptors to trigger sympathoexcitation. Moreover, we suggest that small and often undetectable increases in osmolality can drive significant sympathoexcitation, because the gain of the relationship between osmolality and increased sympathetic activity is enhanced. Multiple factors may contribute to this facilitation, including inappropriately elevated levels of angiotensin II or aldosterone, changes in gene expression or synaptic plasticity and increased sodium concentrations in cerebrospinal fluid. 3. Future studies are required to delineate the brain sites and mechanisms of action and interaction of osmolality and these amplification factors to elicit sustained sympathoexcitation in SSH.

Links between dietary salt intake, renal salt handling, blood pressure, and cardiovascular diseases

Epidemiological, migration, intervention, and genetic studies in humans and animals provide very strong evidence of a causal link between high salt intake and high blood pressure. The mechanisms by which dietary salt increases arterial pressure are not fully understood, but they seem related to the inability of the kidneys to excrete large amounts of salt. From an evolutionary viewpoint, the human species is adapted to ingest and excrete <1 g of salt per day, at least 10 times less than the average values currently observed in industrialized and urbanized countries. Independent of the rise in blood pressure, dietary salt also increases cardiac left ventricular mass, arterial thickness and stiffness, the incidence of strokes, and the severity of cardiac failure. Thus chronic exposure to a high-salt diet appears to be a major factor involved in the frequent occurrence of hypertension and cardiovascular diseases in human populations.

Circadian clock genes directly regulate expression of the Na+/H+ exchanger NHE3 in the kidney

BACKGROUND

Daily rhythms in mammalian physiology are generated by a transcription/translation feedback loop orchestrated by a set of clock genes. However, little is known about the molecular cascade from the clock gene oscillators to cellular function.

METHODS

The mRNA expression profiles of NHE3 and clock genes were examined in mice and rat kidneys. First, luciferase assays followed by a site directed mutagenesis of an E-box sequence were used to assess the CLOCK:BMAL1-transactivated NHE3 promoter activity. A direct binding of CLOCK:BMAL1 heterodimers to an E-box sequences of NHE3 promoter was confirmed by electrophoretic mobility shift assay (EMSA).

RESULTS

We present evidence that renal tubular NHE3, the Na(+)/H(+) exchanger critical for systemic electrolyte and acid-base homeostasis, is a clock-controlled gene regulated directly by CLOCK:BMAL1 heterodimers in kidneys. NHE3 mRNA level in rat kidney displayed circadian kinetics, and this circadian expression was severely blunted in homozygous CRY1/2 double-deficient mice, suggesting that the transcriptional machinery of peripheral clocks in renal tubular cells directly regulates the circadian expression of NHE3. By analyzing the 5' upstream region of the NHE3 gene, we found an E box critical for the transcription of NHE3 via the CLOCK:BMAL1-driven circadian oscillator. The circadian expression of NHE3 mRNA was reflected by oscillating protein levels in the proximal tubules of the rat kidney.

CONCLUSION

NHE3 should represent an output gene of the peripheral oscillators in kidney, which is regulated directly by CLOCK:BMAL1 heterodimers.

Increased osmolality of conscious water-deprived rats supports arterial pressure and sympathetic activity via a brain action

To test the hypothesis that high osmolality acts in the brain to chronically support mean arterial pressure (MAP) and lumbar sympathetic nerve activity (LSNA), the osmolality of blood perfusing the brain was reduced in conscious water-deprived and water-replete rats by infusion of hypotonic fluid via bilateral nonoccluding intracarotid catheters. In water-deprived rats, the intracarotid hypotonic infusion, estimated to lower osmolality by approximately 2%, decreased MAP by 9+/-1 mmHg and LSNA to 86+/-7% of control; heart increased by 25+/-8 beats per minute (bpm) (all P<0.05). MAP, LSNA, and heart rate did not change when the hypotonic fluid was infused intravenously. The intracarotid hypotonic fluid infusion was also ineffective in water-replete rats. Prior treatment with a V1 vasopressin antagonist did not alter the subsequent hypotensive and tachycardic effects of intracarotid hypotonic fluid infusion in water-deprived rats. In summary, acute decreases in osmolality of the carotid blood of water-deprived, but not water-replete, rats decreases MAP and LSNA and increases heart rate. These data support the hypothesis that the elevated osmolality induced by water deprivation acts via a region perfused by the carotid arteries, presumably the brain, to tonically increase MAP and LSNA and suppress heart rate.

Increases in CSF [Na+] precede the increases in blood pressure in Dahl S rats and SHR on a high-salt diet

In Dahl salt-sensitive (S) and salt-resistant (R) rats, and spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats, at 5-6 wk of age, a cannula was placed in the cisterna magna, and cerebrospinal fluid (CSF) was withdrawn continuously at 75 microl/12 h. CSF was collected as day- and nighttime samples from rats on a regular salt intake (0.6% Na+; R-Na) and then on a high salt intake (8% Na+; H-Na). In separate groups of rats, the abdominal aorta was cannulated and blood pressure (BP) and heart rate (HR) measured at 10 AM and 10 PM, with rats first on R-Na and then on H-Na. On H-Na, CSF [Na+] started to increase in the daytime of day 2 in Dahl S rats and of day 3 in SHR. BP and HR did not rise until day 3 in Dahl S rats and day 4 in SHR. In Dahl R and WKY rats, high salt did not change CSF [Na+], BP, or HR. In a third set of Dahl S rats, sampling of both CSF and BP was performed in each individual rat. Again, significant increases in CSF [Na+] were observed 1-2 days earlier than the increases in BP and HR. In a fourth set of Dahl S rats, BP and HR were recorded continuously by means of radiotelemetry for 5 days on R-Na and 8 days on H-Na. On H-Na, BP (but not HR) increased first in the nighttime of day 2. In another set of Dahl S rats, intracerebroventricular infusion of antibody Fab fragments binding ouabain-like compounds (OLC) with high affinity prevented the increase in BP and HR by H-Na but further increased CSF [Na+]. Finally, in Wistar rats on H-Na, intracerebroventricular infusion of ouabain increased BP and HR but decreased CSF [Na+]. Thus, in both Dahl S and SHR on H-Na, increases in CSF [Na+] preceded the increases in BP and HR, consistent with a primary role of increased CSF [Na+] in the salt-induced hypertension. An increase in brain OLC in response to the initial increase in CSF [Na+] appears to attenuate further increases in CSF [Na+] but at the "expense" of sympathoexcitation and hypertension.

Excitatory amino acids in rostral ventrolateral medulla support blood pressure during water deprivation in rats

Water deprivation is associated with regional increases in sympathetic tone, but whether this is mediated by changes in brain stem regulation of sympathetic activity is unknown. Therefore, this study tested the hypothesis that water deprivation increases excitatory amino acid (EAA) drive of the rostral ventrolateral medulla (RVLM), by determining whether bilateral microinjection of kynurenate (Kyn; 2.7 nmol) into the RVLM decreases arterial pressure more in water-deprived than water-replete rats. Plasma osmolality was increased in 48-h water-deprived rats (313 ± 1 mosmol/kgH2O; P < 0.05) compared with 24-h water-deprived rats (306 ± 2 mosmol/kgH2O) and water-replete animals (300 ± 2 mosmol/kgH2O). Kyn decreased arterial pressure by 28.1 ± 5.2 mmHg ( P < 0.01) in 48-h water-deprived rats but had no effect in water-replete rats (–5.9 ± 1.3 mmHg). Variable depressor effects were observed in 24-h water-deprived animals (–12.5 ± 2.4 mmHg, not significant); however, in all rats the Kyn depressor response was strongly correlated to the osmolality level ( P < 0.01; r2 = 0.47). The pressor responses to unilateral microinjection of increasing doses (0.1, 0.5, 1.0, and 5.0 nmol) of glutamate were enhanced ( P < 0.05) during water deprivation, but the pressor responses to intravenous phenylephrine injection were smaller ( P < 0.05). These data suggest that water deprivation increases EAA drive to the RVLM, in part by increasing responsiveness of the RVLM to EAA such as glutamate.

The role of the nervous system in hypertension

The central nervous system plays an important role in the minute-to-minute regulation of arterial pressure, but its contribution to chronic regulation of arterial pressure is less clear. A nervous system role in essential hypertension in humans has been postulated for decades, but conclusive data on the relationship has been lacking. However, several lines of evidence in animal models and in humans suggest that the sympathetic nervous system is a primary contributor to the development and maintenance of some forms of essential hypertension. The primary final common pathway for the nervous system's contribution to hypertension is the sympathetic nervous system. Sympathetic nervous system overactivity may result from either inappropriately elevated sympathetic drive from brain centers, an increase in synaptically released neurotransmitters in the periphery, or amplification of the neurotransmitter signal at the target tissue. This review examines recent evidence for the central and peripheral nervous systems' roles in hypertension, and considers recent findings in this area that suggest that sex steroids and circadian rhythms are important considerations in the nervous system's regulation of arterial pressure.