Renal sympathetic activity in spontaneously hypertensive rats and normotensive controls, as studied by three different methods

Artificial light at night suppresses the day-night cardiovascular variability: evidence from humans and rats

Artificial light at night (ALAN) affects most of the population. Through the retinohypothalamic tract, ALAN modulates the activity of the central circadian oscillator and, consequently, various physiological systems, including the cardiovascular one. We summarised the current knowledge about the effects of ALAN on the cardiovascular system in diurnal and nocturnal animals. Based on published data, ALAN reduces the day-night variability of the blood pressure and heart rate in diurnal and nocturnal animals by increasing the nocturnal values of cardiovascular variables in diurnal animals and decreasing them in nocturnal animals. The effects of ALAN on the cardiovascular system are mainly transmitted through the autonomic nervous system. ALAN is also considered a stress-inducing factor, as glucocorticoid and glucose level changes indicate. Moreover, in nocturnal rats, ALAN increases the pressure response to load. In addition, ALAN induces molecular changes in the heart and blood vessels. Changes in the cardiovascular system significantly depend on the duration of ALAN exposure. To some extent, alterations in physical activity can explain the changes observed in the cardiovascular system after ALAN exposure. Although ALAN acts differently on nocturnal and diurnal animals, we can conclude that both exhibit a weakened circadian coordination among physiological systems, which increases the risk of future cardiovascular complications and reduces the ability to anticipate stress.

Neural control of the circulation during exercise in heart failure with reduced and preserved ejection fraction

Patients with heart failure with reduced (HFrEF) and preserved ejection fraction (HFpEF) exhibit severe exercise intolerance that may be due, in part, to inappropriate cardiovascular and hemodynamic adjustments to exercise. Several neural mechanisms and locally released vasoactive substances work in concert through complex interactions to ensure proper adjustments to meet the metabolic demands of the contracting skeletal muscle. Specifically, accumulating evidence suggests that disease-related alterations in neural mechanisms (e.g., central command, exercise pressor reflex, arterial baroreflex, and cardiopulmonary baroreflex) contribute to heightened sympathetic activation and impaired ability to attenuate sympathetic vasoconstrictor responsiveness that may contribute to reduced skeletal muscle blood flow and severe exercise intolerance in patients with HFrEF. In contrast, little is known regarding these important aspects of physiology in patients with HFpEF, though emerging data reveal heightened sympathetic activation and attenuated skeletal muscle blood flow during exercise in this patient population that may be attributable to dysregulated neural control of the circulation. The overall goal of this review is to provide a brief overview of the current understanding of disease-related alterations in the integrative neural cardiovascular responses to exercise in both HFrEF and HFpEF phenotypes, with a focus on sympathetic nervous system regulation during exercise.

Renal Nerve Activity and Arterial Depressor Responses Induced by Neuromodulation of the Deep Peroneal Nerve in Spontaneously Hypertensive Rats

Hypertension is a main cause of death in the United States with more than 103 million adults affected. While pharmacological treatments are effective, blood pressure (BP) remains uncontrolled in 50–60% of resistant hypertensive subjects. Using a custom-wired miniature electrode, we previously reported that deep peroneal nerve stimulation (DPNS) elicited acute cardiovascular depressor responses in anesthetized spontaneously hypertensive rats (SHRs). Here, we further study this effect by implementing a wireless system and exploring different stimulation parameters to achieve a maximum depressor response. Our results indicate that DPNS consistently induces a reduction in BP and suggests that renal sympathetic nerve activity (RSNA) is altered by this bioelectronic treatment. To test the acute effect of DPNS in awake animals, we developed a novel miniaturized wireless microchannel electrode (w-μCE), with a Z-shaped microchannel through which the target nerves slide and lock into the recording/stimulation chamber. Animals implanted with w-μCE and BP telemetry systems for 3 weeks showed an average BP of 150 ± 14 mmHg, which was reduced significantly by an active DPNS session to 135 ± 8 mmHg (p < 0.04), but not in sham-treated animals. The depressor response in animals with an active w-μCE was progressively returned to baseline levels 14 min later (164 ± 26 mmHg). This depressor response was confirmed in restrained fully awake animals that received DPNS for 10 days, where tail-cuff BP measurements showed that systolic BP in SHR lowered 10% at 1 h and 16% 2 h after the DPNS when compared to the post-implantation baseline. Together, these results support the use of DPN neuromodulation as a possible strategy to lower BP in drug-resistant hypertension.

Glut5 Knockdown in the Nucleus Tractus Solitarii Alleviates Fructose-Induced Hypertension in Rats

BACKGROUND

Several studies have suggested mechanisms whereby excessive fructose intake increases blood pressure (BP). Glucose transporter 5 (GLUT5) is a fructose transporter expressed on enterocytes, and its involvement in the nucleus tractus solitarius (NTS)-modulated increase in BP following fructose intake remains unclear.

OBJECTIVES

Herein, we investigated whether NTS Glut5 knockdown (KD) can alleviate fructose-induced hypertension in rat models.

METHODS

Male Wistar-Kyoto rats (6-8 weeks old; average weight: 230 g) were randomly assigned into 4 groups [control (Con), fructose (Fru), fructose + scrambled (Fru + S), and Fru +  KD]. The Con group rats had ad libitum access to regular water, and the other 3 groups were provided 10% fructose water ad libitum for 4 weeks (2 weeks before lentiviral transfection in the Fru + S and Fru + KD groups). Glut5 short hairpin RNA was delivered into the NTS of rats using a lentivirus system. Fructose-induced hypertension was assessed via the tail-cuff technique, a noninvasive blood pressure measurement approach. GLUT5-associated and other insulin signaling pathways in the NTS of rats were assessed using immunofluorescence and immunoblotting analyses. We evaluated between-group differences using the Mann-Whitney U test or Kruskal-Wallis 1-way ANOVA.

RESULTS

Compared with the Fru + S group, the Fru + KD group had reduced sympathetic nerve hyperactivity (48.8 ± 3.2 bursts/min; P < 0.05), improved central insulin signaling, upregulated protein kinase B (AKT; 3.0-fold) and neuronal NO synthase (nNOS; 2.78-fold) expression, and lowered BP (17 ± 1 mmHg, P < 0.05). Moreover, Glut5 KD restored signaling dependent on adenosine 5'-monophosphate-activated protein kinase and reduced fructose-induced oxidative stress 2.0-fold, and thus decreased NAD(P)H oxidase in p67-phox 1.9-fold within the NTS.

CONCLUSIONS

Fructose-induced reactive oxygen species generates in the NTS of rats through GLUT5 and receptor for advanced glycation end products signaling, thus impairing the AKT-nNOS-NO signaling pathway and ultimately causing hypertension.

Blunted natriuretic response to saline loading in sheep with hypertensive kidney disease following radiofrequency catheter-based renal denervation

Renal sympathetic nerves contribute to renal excretory function during volume expansion. We hypothesized that intact renal innervation is required for excretion of a fluid/electrolyte load in hypertensive chronic kidney disease (CKD) and normotensive healthy settings. Blood pressure, kidney hemodynamic and excretory response to 180 min of isotonic saline loading (0.13 ml/kg/min) were examined in female normotensive (control) and hypertensive CKD sheep at 2 and 11 months after sham (control-intact, CKD-intact) or radiofrequency catheter-based RDN (control-RDN, CKD-RDN) procedure. Basal blood pressure was ~ 7 to 9 mmHg lower at 2, and 11 months in CKD-RDN compared with CKD-intact sheep. Saline loading did not alter glomerular filtration rate in any group. At 2 months, in response to saline loading, total urine and sodium excretion were ~ 40 to 50% less, in control-RDN and CKD-RDN than intact groups. At 11 months, the natriuretic and diuretic response to saline loading were similar between control-intact, control-RDN and CKD-intact groups but sodium excretion was ~ 42% less in CKD-RDN compared with CKD-intact at this time-point. These findings indicate that chronic withdrawal of basal renal sympathetic activity impairs fluid/electrolyte excretion during volume expansion. Clinically, a reduced ability to excrete a saline load following RDN may contribute to disturbances in body fluid balance in hypertensive CKD.

A method to assess heart rate variability in neonate rats: validation in normotensive and hypertensive animals

Several studies have focused on the heart rate variability (HRV) of murine species, while studies discussing HRV in murine neonates and infants remain scarce, since recording hemodynamic signals through invasive methods in small animals has been found to be quite challenging. Thus, this study aimed at describing and validating a novel method to assess HRV in newborn rats. An electrocardiogram (ECG) system was used to determine RR intervals in awake newborns and evaluate HRV in normotensive (Wistar) and hypertensive (SHR) neonate rats. After birth, ECG was recorded in the awake newborns, and they were allowed to rest on a heated surface, restricted only by the weight of the adhesive ECG electrodes. The electrodes were cut and adapted to provide more comfort to the animal, and gently placed on the newborn's skin. RR intervals were recorded over a 30-min period using an ECG system together with LabChart software (4 KHz). Three sequences of 5 min each from the ECG recording period were analyzed in time and frequency domains, using CardioSeries software. ECG data resulted in a clearly interpretable signal that was used to generate an RR interval sequence through time for the analysis of HRV. SHR neonates presented increased cardiac sympathovagal balance compared to Wistar neonates (low frequency/high frequency: 3.85±0.71 vs 0.90±0.09). In conclusion, the ECG setup here described may be used to record RR intervals to assess HRV in neonate rats, thus detecting early impairment of HRV in hypertensive newborns.

Sympathetic Activation in Hypertensive Chronic Kidney Disease - A Stimulus for Cardiac Arrhythmias and Sudden Cardiac Death?

Studies have revealed a robust and independent correlation between chronic kidney disease (CKD) and cardiovascular (CV) events, including death, heart failure, and myocardial infarction. Recent clinical trials extend this range of adverse CV events, including malignant ventricular arrhythmias and sudden cardiac death (SCD). Moreover, other studies point out that cardiac structural and electrophysiological changes are a common occurrence in this population. These processes are likely contributors to the heightened hazard of arrhythmias in CKD population and may be useful indicators to detect patients who are at a higher SCD risk. Sympathetic overactivity is associated with increased CV risk, specifically in the population with CKD, and it is a central feature of the hypertensive state, occurring early in its clinical course. Sympathetic hyperactivity is already evident at the earliest clinical stage of CKD and is directly related to the progression of renal failure, being most pronounced in those with end-stage renal disease. Sympathetic efferent and afferent neural activity in kidney failure is a crucial facilitator for the perpetuation and evolvement of the disease. Here, we will revisit the role of the feedback loop of the sympathetic neural cycle in the context of CKD and how it may aggravate several of the risk factors responsible for causing SCD. Targeting the overactive sympathetic nervous system therapeutically, either pharmacologically or with newly available device-based approaches, may prove to be a pivotal intervention to curb the substantial burden of cardiac arrhythmias and SCD in the high-risk population of patients with CKD.

Interactions between renal vascular resistance and endothelium-derived hyperpolarization in hypertensive rats in vivo

Endothelium derived signaling mechanisms play an important role in regulating vascular tone and endothelial dysfunction is often found in hypertension. Endothelium-derived hyperpolarization (EDH) plays a significant role in smaller renal arteries and arterioles, but its significance in vivo in hypertension is unresolved. The aim of this study was to characterize the EDH-induced renal vasodilation in normotensive and hypertensive rats during acute intrarenal infusion of ACh. Our hypothesis was that the increased renal vascular resistance (RVR) found early in hypertension would significantly correlate with reduced EDH-induced vasodilation. In isoflurane-anesthetized 12-week-old normo- and hypertensive rats blood pressure and renal blood flow (RBF) was measured continuously. RBF responses to acute intrarenal ACh infusions were measured before and after inhibition of NO and prostacyclin. Additionally, RVR was decreased or increased using inhibition or activation of adrenergic receptors or by use of papaverine and angiotensin II. Intrarenal infusion of ACh elicited a larger increase in RBF in hypertensive rats compared to normotensive rats suggesting that endothelial dysfunction is not present in 12-week-old hypertensive rats. The EDH-induced renal vasodilation (after inhibition of NO and prostacyclin) was similar between normo- and hypertensive rats. Reducing RVR by inhibition of α1 -adrenergic receptors significantly increased the renal EDH response in hypertensive rats, but a similar increase was found after activating α-adrenergic receptors using norepinephrine. The results show that renal EDH is present and functional in 12-week-old normo- and hypertensive rats. Interestingly, both activation and inactivation of α1 -adrenergic receptors elicited an increase in the renal EDH-induced vasodilation.

Direct Recording of Cardiac and Renal Sympathetic Nerve Activity Shows Differential Control in Renovascular Hypertension

There is increasing evidence that hypertension is initiated and maintained by elevated sympathetic tone. Increased sympathetic drive to the heart is linked to cardiac hypertrophy in hypertension and worsens prognosis. However, cardiac sympathetic nerve activity (SNA) has not previously been directly recorded in hypertension. We hypothesized that directly recorded cardiac SNA levels would be elevated during hypertension and that baroreflex control of cardiac SNA would be impaired during hypertension. Adult ewes either underwent unilateral renal artery clipping (n=12) or sham surgery (n=15). Two weeks later, electrodes were placed in the contralateral renal and cardiac nerves to record SNA. Baseline levels of SNA and baroreflex control of heart rate and sympathetic drive were examined. Unilateral renal artery clipping induced hypertension (mean arterial pressure 109±2 versus 91±3 mm Hg in shams; P<0.001). The heart rate baroreflex curve was shifted rightward but remained intact. In the hypertensive group, cardiac sympathetic burst incidence (bursts/100 beats) was increased (39±14 versus 25±9 in normotensives; P<0.05), whereas renal sympathetic burst incidence was decreased (69±20 versus 93±8 in normotensives; P<0.01). The renal sympathetic baroreflex curve was shifted rightward and showed increased gain, but there was no change in the cardiac sympathetic baroreflex gain. Renovascular hypertension is associated with differential control of cardiac and renal SNA; baseline cardiac SNA is increased, whereas renal SNA is decreased.

Role of the Sympathetic Nervous System and Its Modulation in Renal Hypertension

The kidneys are densely innervated with renal efferent and afferent nerves to communicate with the central nervous system. Innervation of major structural components of the kidneys, such as blood vessels, tubules, the pelvis, and glomeruli, forms a bidirectional neural network to relay sensory and sympathetic signals to and from the brain. Renal efferent nerves regulate renal blood flow, glomerular filtration rate, tubular reabsorption of sodium and water, as well as release of renin and prostaglandins, all of which contribute to cardiovascular and renal regulation. Renal afferent nerves complete the feedback loop via central autonomic nuclei where the signals are integrated and modulate central sympathetic outflow; thus both types of nerves form integral parts of the self-regulated renorenal reflex loop. Renal sympathetic nerve activity (RSNA) is commonly increased in pathophysiological conditions such as hypertension and chronic- and end-stage renal disease. Increased RSNA raises blood pressure and can contribute to the deterioration of renal function. Attempts have been made to eliminate or interfere with this important link between the brain and the kidneys as a neuromodulatory treatment for these conditions. Catheter-based renal sympathetic denervation has been successfully applied in patients with resistant hypertension and was associated with significant falls in blood pressure and renal protection in most studies performed. The focus of this review is the neural contribution to the control of renal and cardiovascular hemodynamics and renal function in the setting of hypertension and chronic kidney disease, as well as the specific roles of renal efferent and afferent nerves in this scenario and their utility as a therapeutic target.

Dysregulation of Neuronal Ca2+ Channel Linked to Heightened Sympathetic Phenotype in Prohypertensive States

Hypertension is associated with impaired nitric oxide (NO)-cyclic nucleotide (CN)-coupled intracellular calcium (Ca(2+)) homeostasis that enhances cardiac sympathetic neurotransmission. Because neuronal membrane Ca(2+) currents are reduced by NO-activated S-nitrosylation, we tested whether CNs affect membrane channel conductance directly in neurons isolated from the stellate ganglia of spontaneously hypertensive rats (SHRs) and their normotensive controls. Using voltage-clamp and cAMP-protein kinase A (PKA) FRET sensors, we hypothesized that impaired CN regulation provides a direct link to abnormal signaling of neuronal calcium channels in the SHR and that targeting cGMP can restore the channel phenotype. We found significantly larger whole-cell Ca(2+) currents from diseased neurons that were largely mediated by the N-type Ca(2+) channel (Cav2.2). Elevating cGMP restored the SHR Ca(2+) current to levels seen in normal neurons that were not affected by cGMP. cGMP also decreased cAMP levels and PKA activity in diseased neurons. In contrast, cAMP-PKA activity was increased in normal neurons, suggesting differential switching in phosphodiesterase (PDE) activity. PDE2A inhibition enhanced the Ca(2+) current in normal neurons to a conductance similar to that seen in SHR neurons, whereas the inhibitor slightly decreased the current in diseased neurons. Pharmacological evidence supported a switching from cGMP acting via PDE3 in control neurons to PDE2A in SHR neurons in the modulation of the Ca(2+) current. Our data suggest that a disturbance in the regulation of PDE-coupled CNs linked to N-type Ca(2+) channels is an early hallmark of the prohypertensive phenotype associated with intracellular Ca(2+) impairment underpinning sympathetic dysautonomia.

SIGNIFICANCE STATEMENT

Here, we identify dysregulation of cyclic-nucleotide (CN)-linked neuronal Ca(2+) channel activity that could provide the trigger for the enhanced sympathetic neurotransmission observed in the prohypertensive state. Furthermore, we provide evidence that increasing cGMP rescues the channel phenotype and restores ion channel activity to levels seen in normal neurons. We also observed CN cross-talk in sympathetic neurons that may be related to a differential switching in phosphodiesterase activity. The presence of these early molecular changes in asymptomatic, prohypertensive animals could facilitate the identification of novel therapeutic targets with which to modulate intracellular Ca(2+) Turning down the gain of sympathetic hyperresponsiveness in cardiovascular disease associated with sympathetic dysautonomia would have significant therapeutic utility.

Raised arterial blood pressure in neurokinin-1 receptor-deficient mice (NK1R(-/-) ): evidence for a neural rather than a vascular mechanism

NEW FINDINGS

What is the central question of this study? Does genetic ablation of neurokinin-1 receptors alter arterial blood pressure? What is the main finding and its importance? NK1R(-/-) mice have increased mean arterial blood pressure, but without a concomitant change in vascular reactivity. This finding suggests that neurokinin-1 receptors play a role in the neural regulation of blood pressure. Mice with functional ablation of the neurokinin-1 receptor gene, Tacr1, (NK1R(-/-) ) express behavioural abnormalities equivalent to those seen in attention deficit hyperactivity disorder (ADHD). An established model of ADHD is the spontaneously hypertensive rat, which exhibits high blood pressure owing to increased central sympathetic drive. In light of the evidence that the neurokinin-1 receptor (NK1R) also influences cardiovascular haemodynamics, we have investigated whether NK1R(-/-) mice exhibit raised blood pressure. Cardiovascular parameters were recorded for 24 h in conscious mice using radiotelemetry. Vascular function was assessed in mesenteric resistance arteries by wire myography. The NK1R(-/-) mice exhibited a higher blood pressure than wild-type animals throughout the 24 h period. Heart rate and locomotor activity in NK1R(-/-) mice were higher than in wild-type mice during the night period (active phase), consistent with an ADHD-like phenotype, but not during the day. Mesenteric and renal arteries from NK1R(-/-) mice exhibited normal vascular function; the responses to vasoconstrictors (U46619 and phenylephrine) and the endothelium-dependent vasodilator, acetylcholine, were not altered in these animals, suggesting that the NK1R does not regulate vascular tone. Analysis of heart rate variability revealed a higher low-frequency to high-frequency ratio in NK1R(-/-) mice, indicative of increased cardiac sympathetic activity. We propose that the raised blood pressure in NK1R(-/-) mice could be due to a neural mechanism rather than a change in vascular reactivity. Further studies are required to understand this mechanism and to establish whether a subgroup of ADHD patients with polymorphism of the equivalent (TACR1) gene are affected in a similar way.

Differential control of vasomotion by angiotensins in the rostral ventrolateral medulla of hypertensive rats

The central and peripheral renin-angiotensin systems are known for playing a key role in cardiovascular control. In the present study, we evaluated the hemodynamic effects produced by nanoinjections of angiotensin II (Ang II) or angiotensin-(1-7) [Ang-(1-7)] into the rostral ventrolateral medulla (RVLM) of adult male normotensive (Wistar-WT) and spontaneously hypertensive rats (SHR). Animals were anesthetized (urethane 1.2g/kg) and instrumented for recording blood pressure (BP), heart rate (HR) and blood flow (BF) in the femoral, renal or mesenteric arteries. Afterwards, rats were positioned in a stereotaxic and prepared for nanoinjections (100 nl) of saline (NaCl 0.9%), Ang-(1-7) (40 ng) or Ang II (40 ng) into the RVLM. The vascular resistance (VR) was calculated by ΔMAP/ΔBF ratio. In WT, Ang-(1-7) or Ang II caused equipotent pressor effects that were not accompanied by changes in vascular resistance. However, MAP changes were greater in SHR. This strain also showed a concomitant increase in relative vascular resistance (ΔVR/VRbaseline) of renal (0.31 ± 0.07 and 0.3 ± 0.07 vs. 0.02 ± 0.01; Ang-(1-7), Ang II and Saline, respectively) and mesenteric beds (0.3 ± 0.06 and 0.33 ± 0.04 vs. 0.05 ± 0.02; Ang-(1-7), Ang II and saline, respectively). We conclude that Ang II and Ang-(1-7) at the RVLM control the vascular resistance of renal and mesenteric beds during hypertension.

Relationship between plasma levels of vasoactive mediators and optic nerve head circulation shown by laser speckle flowgraphy

PROPOSE

To evaluate relationships between optic nerve head (ONH) circulation by laser speckle flowgraphy (LSFG), and secretion levels of plasma norepinephrine (NE), plasma renin activity (PRA), plasma aldosterone concentration (PAC), and plasma cortisol.

METHOD

Forty subjects were included in the study. The mean blur rates (MBR) throughout the ONH (MBR-all), in the tissue (MBR-tissue), and in vessels (MBR-vessel) were analyzed. In 29 subjects the following parameters were evaluated: plasma NE and ONH circulation in the evening and morning (Δ plasma NE, Δ MBR-all, -tissue, -vessel, and Δ area ratio of blood stream).

RESULTS

Plasma NE was significantly correlated with MBR-all (r = 0.39, P = 0.01) and MBR-vessel (r = 0.51, P = 0.0008). Plasma cortisol was significantly correlated with MBR-vessel (r =0.35, P = 0.03). The PRA (r = 0.31, P = 0.05) and PAC (r = 0.31, P = 0.05) tended to correlate with MBR-vessel. The multiple regression analysis revealed that plasma NE, PAC, and ocular perfusion pressure (OPP) were identified as factors contributing independently to the MBR-vessel (plasma NE: standard regression = 0.48, t-value = 3.12, P = 0.004; PAC: 0.43, 3.10, 0.04; OPP: 0.58, 2.34, 0.03, r = 0.71). Positive correlations between Δ plasma NE and Δ MBR-all (r = 0.46, P = 0.01), Δ MBR-tissue (0.38, 0.04), Δ MBR-vessel (0.41, 0.03), and Δ area ratio of blood stream (0.38, 0.04) were observed.

CONCLUSION

Our results clarified that the measurements of ONH circulation by LSFG is reflecting the plasma secretion levels of vasoactive mediators.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Sympathetic neural activity to the cardiovascular system: integrator of systemic physiology and interindividual characteristics

The sympathetic nervous system is a ubiquitous, integrating controller of myriad physiological functions. In the present article, we review the physiology of sympathetic neural control of cardiovascular function with a focus on integrative mechanisms in humans. Direct measurement of sympathetic neural activity (SNA) in humans can be accomplished using microneurography, most commonly performed in the peroneal (fibular) nerve. In humans, muscle SNA (MSNA) is composed of vasoconstrictor fibers; its best-recognized characteristic is its participation in transient, moment-to-moment control of arterial blood pressure via the arterial baroreflex. This property of MSNA contributes to its typical "bursting" pattern which is strongly linked to the cardiac cycle. Recent evidence suggests that sympathetic neural mechanisms and the baroreflex have important roles in the long term control of blood pressure as well. One of the striking characteristics of MSNA is its large interindividual variability. However, in young, normotensive humans, higher MSNA is not linked to higher blood pressure due to balancing influences of other cardiovascular variables. In men, an inverse relationship between MSNA and cardiac output is a major factor in this balance, whereas in women, beta-adrenergic vasodilation offsets the vasoconstrictor/pressor effects of higher MSNA. As people get older (and in people with hypertension) higher MSNA is more likely to be linked to higher blood pressure. Skin SNA (SSNA) can also be measured in humans, although interpretation of SSNA signals is complicated by multiple types of neurons involved (vasoconstrictor, vasodilator, sudomotor and pilomotor). In addition to blood pressure regulation, the sympathetic nervous system contributes to cardiovascular regulation during numerous other reflexes, including those involved in exercise, thermoregulation, chemoreflex regulation, and responses to mental stress.

Effects of exercise training on cardiovascular adrenergic system

In heart failure (HF), exercise has been shown to modulate cardiac sympathetic hyperactivation which is one of the earliest features of neurohormonal derangement in this syndrome and correlates with adverse outcome. An important molecular alteration related to chronic sympathetic overstimulation in HF is represented by cardiac β-adrenergic receptor (β-AR) dysfunction. It has been demonstrated that exercise reverses β-AR dysfunction by restoring cardiac receptor membrane density and G-protein-dependent adenylyl cyclase activation. In particular, several evidence indicate that exercise reduces levels of cardiac G-protein coupled receptor kinase-2 (GRK2) which is known to be involved in both β1-AR and β2-AR dysregulation in HF. Similar alterations of β-AR system have been described also in the senescent heart. It has also been demonstrated that exercise training restores adrenal GRK2/α-2AR/catecholamine (CA) production axis. At vascular level, exercise shows a therapeutic effect on age-related impairment of vascular reactivity to adrenergic stimulation and restores β-AR-dependent vasodilatation by increasing vascular β-AR responsiveness and reducing endothelial GRK2 activity. Sympathetic nervous system overdrive is thought to account for >50% of all cases of hypertension and a lack of balance between parasympathetic and sympathetic modulation has been observed in hypertensive subjects. Non-pharmacological, lifestyle interventions have been associated with reductions in SNS overactivity and blood pressure in hypertension. Several evidence have highlighted the blood pressure lowering effects of aerobic endurance exercise in patients with hypertension and the significant reduction in sympathetic neural activity has been reported as one of the main mechanisms explaining the favorable effects of exercise on blood pressure control.

The neural regulation of the kidney in hypertension and renal failure

NEW FINDINGS

What is the topic of this review? Reports that bilateral renal denervation in resistant hypertensive patients results in a long-lasting reduction in blood pressure raise the question of the underlying mechanisms involved and how they may be deranged in pathophysiological states of hypertension and renal failure. What advances does it highlight? The renal sensory afferent nerves and efferent sympathetic nerves work together to exert an important control over extracellular fluid volume, hence the level at which blood pressure is set. This article emphasizes that both the afferent and the efferent renal innervation may contribute to the neural dysregulation of the kidney that occurs in chronic renal disease and resistant hypertension. Autonomic control is central to cardiovascular homeostasis, and this is exerted not only at the level of the heart and blood vessels but also at the kidney. At the kidney, the sympathetic neural regulation of renin release and fluid reabsorption may influence fluid balance and, in the longer term, the level at which blood pressure is set. The role of the renal innervation in the regulation of blood pressure has received renewed attention over the past few years, following the reports that bilateral renal denervation of resistant hypertensive patients resulted in a marked reduction in blood pressure, which has been maintained for several years. Such has been the interest that this approach of renal denervation is being applied in other patient groups with diabetes, obesity and renal failure, with the hope that there may be a sustained reduction in blood pressure as well as the amelioration of some aspects of the metabolic syndrome. However, the factors that come into play to cause the rise in blood pressure in these patient groups, particularly the resistant hypertensive patients, are far from clear. Moreover, the mechanisms leading to the fall in blood pressure following renal denervation of resistant hypertensive patients currently elude our understanding and is therefore an area that requires much more investigation to enhance our insight.

Differential inhibition of noradrenaline release mediated by inhibitory A₁-adenosine receptors in the mesenteric vein and artery from normotensive and hypertensive rats

Mesenteric arteries and veins are densely innervated by sympathetic nerves and are crucial in the regulation of peripheral resistance and capacitance, respectively, thus, in the control of blood pressure. Presynaptic adenosine receptors are involved in vascular tonus regulation, by modulating noradrenaline release from vascular postganglionic sympathetic nerve endings. Some studies also suggest that adenosine receptors (AR) may have a role in hypertension. We aim at investigating the role of presynaptic adenosine receptors in mesenteric vessels and establish a relationship between their effects (in mesenteric vessels) and hypertension, using the spontaneously hypertensive rats (SHR) as a model of hypertension. Adenosine receptor-mediated modulation of noradrenaline release was investigated through the effects of selective agonists and antagonists on electrically-evoked [(3)H]-noradrenaline overflow. CPA (A1AR selective agonist: 1-100 nM) inhibited tritium overflow, but the inhibition was lower in SHR mesenteric vessels. IB-MECA (A3AR selective agonist: 1-100 nM) also inhibited tritium overflow but only in WKY mesenteric veins. CGS 21680 (A2AAR selective agonist: up to 100 nM) failed to facilitate noradrenaline release in mesenteric veins, from both strains, but induced a similar facilitation in the mesenteric arteries. NECA (non-selective AR agonist: 1, 3 and 10μM), in the presence of A1 (DPCPX, 20 nM) and A3 (MRS 1523, 1 μM) AR selective antagonists, failed to change tritium overflow. In summary, the modulatory effects mediated by presynaptic adenosine receptors were characterized, for the first time, in mesenteric vessels: a major inhibition exerted by the A1 subtype in both vessels; a slight inhibition mediated by A3 receptors in mesenteric vein; a facilitation mediated by A2A receptors only in mesenteric artery (from both strains). The less efficient prejunctional adenosine receptor mediated inhibitory effects can contribute to an increase of noradrenaline in the synaptic cleft (both in arteries and veins), which might conduce to increased vascular reactivity.

Autonomic regulation of kidney function

The kidneys play a central role in cardiovascular homeostasis by ensuring a balance between the fluid taken in and that lost and excreted during everyday activities. This ensures stability of extracellular fluid volume and maintenance of normal levels of blood pressure. Renal fluid handling is controlled via neural and humoral influences, with the former determining a rapid dynamic response to changing intake of sodium whereas the latter cause a slower longer-term modulation of sodium and water handling. Activity in the renal sympathetic nerves arises from an integration of information from the high and low pressure cardiovascular baroreceptors, the somatosensory and visceral systems as well as the higher cortical centers. Each sensory system provides varying input to the autonomic centers of the hypothalamic and medullary areas of the brain at a level appropriate to the activity being performed. In pathophysiological states, such as hypertension, heart failure and chronic renal disease, there may be an inappropriate sympathoexcitation causing sodium retention which exacerbates the disease process. The contribution of the renal sympathetic nerves to these cardiovascular diseases is beginning to be appreciated with the demonstration that renal denervation of resistant hypertensive patients results in a long-term normalization of blood pressure.

Previous exercise training has a beneficial effect on renal and cardiovascular function in a model of diabetes

Exercise training (ET) is an important intervention for chronic diseases such as diabetes mellitus (DM). However, it is not known whether previous exercise training intervention alters the physiological and medical complications of these diseases. We investigated the effects of previous ET on the progression of renal disease and cardiovascular autonomic control in rats with streptozotocin (STZ)-induced DM. Male Wistar rats were divided into five groups. All groups were followed for 15 weeks. Trained control and trained diabetic rats underwent 10 weeks of exercise training, whereas previously trained diabetic rats underwent 14 weeks of exercise training. Renal function, proteinuria, renal sympathetic nerve activity (RSNA) and the echocardiographic parameters autonomic modulation and baroreflex sensitivity (BRS) were evaluated. In the previously trained group, the urinary albumin/creatinine ratio was reduced compared with the sedentary diabetic and trained diabetic groups (p<0.05). Additionally, RSNA was normalized in the trained diabetic and previously trained diabetic animals (p<0.05). The ejection fraction was increased in the previously trained diabetic animals compared with the diabetic and trained diabetic groups (p<0.05), and the myocardial performance index was improved in the previously trained diabetic group compared with the diabetic and trained diabetic groups (p<0.05). In addition, the previously trained rats had improved heart rate variability and BRS in the tachycardic response and bradycardic response in relation to the diabetic group (p<0.05). This study demonstrates that previous ET improves the functional damage that affects DM. Additionally, our findings suggest that the development of renal and cardiac dysfunction can be minimized by 4 weeks of ET before the induction of DM by STZ.

Neurogenic hypertension and elevated vertebrobasilar arterial resistance: is there a causative link?

There is evidence of sympathetic overdrive in a significant proportion of patients with essential hypertension and an animal model of the condition, the spontaneously hypertensive rat (SHR). The reasons for this remain elusive. However, there is also evidence of narrowing of the arteries supplying the brainstem in the SHR and hypertensive humans. In this review, we discuss the possible role of brainstem hypoperfusion in driving increased sympathetic activity and hypertension.

Increased total volume and dopamine β-hydroxylase immunoreactivity of carotid body in spontaneously hypertensive rats

Under hypertension, it has been reported that the carotid body (CB) is enlarged and noradrenaline (NA) content in CB is increased. Therefore, it is hypothesized that morphological and neurochemical changes in CB are induced in hypertensive animal models. In the present study, we examined the morphological features and dopamine β-hydroxylase (DBH) immunoreactivity in CB of spontaneously hypertensive rats (SHR/Izm) and Wistar Kyoto rats (WKY/Izm). The CB of SHR/Izm was elongated in terms of the cross section of center and was enlarged in the reconstructed images compared with that of WKY/Izm, and the total volume of CB in SHR/Izm (0.048 ± 0.004 mm³) was significantly (p<0.05) increased compared with the value in WKY/Izm (0.032 ± 0.006 mm³). By immunohistochemistry, immunoreactivity for tyrosine hydroxylase in CB was mainly observed in glomus cells and the immunostaining properties were similar between WKY/Izm and SHR/Izm. On the other hand, DBH immunoreactivity was mainly observed in nerve fibers around blood vessels and observed in a few glomus cells in CB of WKY/Izm. The number of glomus cells with strong DBH immunoreactivity was increased in SHR/Izm compared with that in WKY/Izm. In conclusion, the present study exhibited the enlargement of CB as three-dimensional image and revealed the enhanced immunoreactivity for DBH of glomus cells in SHR/Izm. These results suggest that the morphology of CB is affected by the effect of sympathetic nerve and that the signal transduction from CB is regulated by NA in glomus cells under hypertensive conditions.

Time course of the hemodynamic responses to aortic depressor nerve stimulation in conscious spontaneously hypertensive rats

The time to reach the maximum response of arterial pressure, heart rate and vascular resistance (hindquarter and mesenteric) was measured in conscious male spontaneously hypertensive (SHR) and normotensive control rats (NCR; Wistar; 18-22 weeks) subjected to electrical stimulation of the aortic depressor nerve (ADN). The parameters of stimulation were 1 mA intensity and 2 ms pulse length applied for 5 s, using frequencies of 10, 30, and 90 Hz. The time to reach the hemodynamic responses at different frequencies of ADN stimulation was similar for SHR (N = 15) and NCR (N = 14); hypotension = NCR (4194 ± 336 to 3695 ± 463 ms) vs SHR (3475 ± 354 to 4494 ± 300 ms); bradycardia = NCR (1618 ± 152 to 1358 ± 185 ms) vs SHR (1911 ± 323 to 1852 ± 431 ms), and the fall in hindquarter vascular resistance = NCR (6054 ± 486 to 6550 ± 847 ms) vs SHR (4849 ± 918 to 4926 ± 646 ms); mesenteric = NCR (5574 ± 790 to 5752 ± 539 ms) vs SHR (5638 ± 648 to 6777 ± 624 ms). In addition, ADN stimulation produced baroreflex responses characterized by a faster cardiac effect followed by a vascular effect, which together contributed to the decrease in arterial pressure. Therefore, the results indicate that there is no alteration in the conduction of the electrical impulse after the site of baroreceptor mechanical transduction in the baroreflex pathway (central and/or efferent) in conscious SHR compared to NCR.

The sympathetic nervous system and blood pressure in humans: implications for hypertension

A neurogenic component to primary hypertension (hypertension) is now well established. Along with raised vasomotor tone and increased cardiac output, the chronic activation of the sympathetic nervous system in hypertension has a diverse range of pathophysiological consequences independent of any increase in blood pressure. This review provides a perspective on the actions and interactions of angiotensin II, inflammation and vascular dysfunction/brain hypoperfusion in the pathogenesis and progression of neurogenic hypertension. The optimisation of current treatment strategies and the exciting recent developments in the therapeutic targeting of the sympathetic nervous system to control hypertension (for example, catheter-based renal denervation and carotid baroreceptor stimulation) will be outlined.

Elevated vertebrobasilar artery resistance in neonatal spontaneously hypertensive rats

There is a strong correlation between increased vertebral artery resistance and arterial blood pressure in humans. The reasons for this increased resistance at high systemic pressure remain unknown, but may include raised sympathetic activity. With the recent finding that prehypertensive spontaneously hypertensive (PHSH) rats, which have raised sympathetic nerve activity, but a blood pressure comparable to normotensive rat strains, we hypothesized that its vertebrobasilar vascular resistance would already be raised and, as a consequence, would exhibit a more responsive Cushing response (e.g., brain ischemia evoked sympathoexcitation and a pressor response). We report that PHSH rats exhibited a remodeling of the basilar artery (i.e., increased wall thickness and lower lumen-to-wall thickness ratio) that occurred before the onset of hypertension. In a novel in vitro vascularly isolated, arterially perfused brain stem preparation of PHSH rats of 4-5 wk of age, brain stem vascular resistance was raised by ∼35% relative to age- and sex-matched normotensive rats (P < 0.05). In the in situ arterial perfused working heart-brain stem preparation, occlusion of both vertebral arteries in the PHSH rat resulted in a significantly greater increase in sympathetic activity (57 vs. 20%, PHSH vs. control; P < 0.01) that triggered a greater increase in arterial perfusion pressure (8 vs. 3 mmHg, PHSH vs. control; P < 0.01) compared with normotensive rats. These data indicate raised vertebrobasilar artery resistance before the onset of hypertension in the PHSH rat. With the raised responsiveness of the Cushing response in the PHSH rat, we discuss the possibility of brain stem perfusion as a central nervous system determinant of the set point of vasomotor sympathetic tone in the hypertensive condition.

Ovariectomy does not affect the cardiac sympathovagal balance of female SHR but estradiol does

The low incidence of cardiovascular diseases, including hypertension, in premenopausal women has led to the conclusion that ovarian hormones may have a protective effect on the cardiovascular system. We evaluated the effects of ovariectomy and/or estradiol on sympathovagal balance and heart rate variability (HRV) in female spontaneously hypertensive rats (SHR) with tachycardia and compared them to Wistar rats (12 weeks old; N = 8-12). Ovariectomy (OVX) and/or estradiol (10 µg/kg) did not affect basal arterial pressure in either rat strain, but estradiol increased basal heart rate (HR) in OVX SHR (454 ± 18 vs 377 ± 9 bpm). HR changes elicited by methylatropine and propranolol were used to evaluate the sympathovagal balance. Ovariectomy did not affect the cardiac sympathovagal balance of any group, while estradiol increased sympathetic tone in OVX SHR (120 ± 8 vs 56 ± 10 bpm) and sham-operated Wistar rats (57 ± 7 vs 28 ± 4 bpm), and decreased the parasympathetic tone only in OVX SHR (26 ± 7 vs 37 ± 5 bpm). HRV was studied in the frequency domain (Fast Fourier Transformation). Spectra of HR series were examined at low frequency (LF: 0.2-0.75 Hz) and high frequency (HF: 0.75-3 Hz) bands. The power of LF, as well as the LF/HF ratio, was not affected by ovariectomy, but estradiol increased both LF (29 ± 4 vs 18 ± 3 nu in Wistar sham-operated, 26 ± 5 vs 15 ± 3 nu in Wistar OVX, 50 ± 3 vs 38 ± 4 nu in SHR sham-operated, and 51 ± 3 vs 42 ± 3 nu in SHR OVX) and LF/HF (0.48 ± 0.08 vs 0.23 ± 0.03 nu in Wistar sham-operated, 0.41 ± 0.14 vs 0.19 ± 0.05 nu in Wistar OVX, 0.98 ± 0.11 vs 0.63 ± 0.11 nu in SHR sham-operated, and 1.10 ± 0.11 vs 0.78 ± 0.1 nu in SHR OVX). Thus, we suggest that ovariectomy did not affect the cardiac sympathovagal balance of SHR or Wistar rats, while estradiol increased the sympathetic modulation of HR.

Chemoreceptor hypersensitivity, sympathetic excitation, and overexpression of ASIC and TASK channels before the onset of hypertension in SHR

RATIONALE

Increased sympathetic nerve activity has been linked to the pathogenesis of hypertension in humans and animal models. Enhanced peripheral chemoreceptor sensitivity which increases sympathetic nerve activity has been observed in established hypertension but has not been identified as a possible mechanism for initiating an increase in sympathetic nerve activity before the onset of hypertension.

OBJECTIVE

We tested this hypothesis by measuring the pH sensitivity of isolated carotid body glomus cells from young spontaneously hypertensive rats (SHR) before the onset of hypertension and their control normotensive Wistar-Kyoto (WKY) rats.

METHODS AND RESULTS

We found a significant increase in the depolarizing effect of low pH in SHR versus WKY glomus cells which was caused by overexpression of 2 acid-sensing non-voltage-gated channels. One is the amiloride-sensitive acid-sensing sodium channel (ASIC3), which is activated by low pH and the other is the 2-pore domain acid-sensing K(+) channel (TASK1), which is inhibited by low pH and blocked by quinidine. Moreover, we found that the increase in sympathetic nerve activity in response to stimulation of chemoreceptors with sodium cyanide was markedly enhanced in the still normotensive young SHR compared to control WKY rats.

CONCLUSIONS

Our results establish a novel molecular basis for increased chemotransduction that contributes to excessive sympathetic activity before the onset of hypertension.

Electroacupuncture decreases nitric oxide synthesis in the hypothalamus of spontaneously hypertensive rats

Acupuncture-related effects on autonomic function have been explored via biological and neurophysiologic studies. The hypothalamus, known to regulate the autonomic nervous system, is likely affected by acupuncture treatment that modulates sympathetic functions. The aim of this study was to investigate the effect of electroacupuncture at the Jogsamni point (ST36, an acupoint known to modulate autonomic function) on expression of neuronal nitric oxide synthase (nNOS) in the hypothalamus of spontaneously hypertensive rat. Nitric oxide, which is produced by nNOS activity, plays an important role in the regulation of many physiologic processes, including sympathetic activities, in the hypothalamus and other parts of the brain. nNOS expression was assessed by immunohistochemistry of nNOS and histochemistry of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d). The staining intensities of nNOS-positive neurons and NADPH-d-positive neurons were quantitatively assessed using microdensitometry to measure changes in optical density. The results show that electroacupuncture at ST36 reduced the expression and activity of nNOS in the hypothalamus of spontaneously hypertensitive rats. These findings suggest that the electroacupuncture at ST36 results in modulation of the activity of nNOS in the hypothalamus of spontaneously hypertensive rat.

Amplified respiratory-sympathetic coupling in the spontaneously hypertensive rat: does it contribute to hypertension?

Sympathetic nerve activity (SNA) is elevated in established hypertension. We tested the hypothesis that SNA is elevated in neonate and juvenile spontaneously hypertensive (SH) rats prior to the development of hypertension, and that this may be due to augmented respiratory-sympathetic coupling. Using the working heart-brainstem preparation, perfusion pressure, phrenic nerve activity and thoracic (T8) SNA were recorded in male SH rats and normotensive Wistar-Kyoto (WKY) rats at three ages: neonates (postnatal day 9-16), 3 weeks old and 5 weeks old. Perfusion pressure was higher in SH rats at all ages reflecting higher vascular resistance. The amplitude of respiratory-related bursts of SNA was greater in SH rats at all ages (P < 0.05). This was reflected in larger Traube-Hering pressure waves in SH rats (1.4 +/- 0.8 versus 9.8 +/- 1.5 mmHg WKY versus SH rat, 5 weeks old, n = 5 per group, P < 0.01). Recovery from hypocapnic-induced apnoea and reinstatement of Traube-Hering waves produced a significantly greater increase in perfusion pressure in SH rats (P < 0.05). Differences in respiratory-sympathetic coupling in the SH rat were not secondary to changes in central or peripheral chemoreflex sensitivity, nor were they related to altered arterial baroreflex function. We have shown that increased SNA is already present in SH rats in early postnatal life as revealed by augmented respiratory modulation of SNA. This is reflected in an increased magnitude of Traube-Hering waves resulting in elevated perfusion pressure in the SH rat. We suggest that the amplified respiratory-related bursts of SNA seen in the neonate and juvenile SH rat may be causal in the development of their hypertension.

Specific inflammatory condition in nucleus tractus solitarii of the SHR: novel insight for neurogenic hypertension?

Human essential hypertension is a complex polygenic trait with underlying genetic components that remain unknown. Since the brainstem structure--the nucleus of the solitary tract (NTS)--is a pivotal region for regulating the set-point of arterial pressure, we proposed a role for it in the development of primary hypertension. Using microarray and real-time RT-PCR, we have recently identified that some pro-inflammatory molecules, such as junctional adhesion molecule-1 (JAM-1; a leukocyte/platelet adhesion molecule), were over expressed in endothelial cells in the NTS of an animal model of human essential hypertension--the spontaneously hypertensive rat (SHR) compared to normotensive Wistar Kyoto rats (WKY). Adenoviral mediated over expression of JAM-1 in NTS of WKY rats produced both hypertension and localized leukocyte adherence to the microvasculature. With a known effect of leukocyte adhesion causing cytokine release, we predicted differences in the level of gene expression of cytokines in the NTS of SHR relative to WKY. Gene expression of monocyte chemoattractant protein-1 (MCP-1) was higher in the NTS of SHR while inter-leukin-6 (IL-6) was lower in the NTS of SHR compared to the WKY. Because both inflammatory molecules are known to affect neural functions, our data suggest that the microvasculature of NTS of the SHR exhibits a specific inflammatory state. We propose a new hypothesis that as a consequence of enhanced expression of leukocyte adhesion molecules within the microvasculature of NTS there is a specific inflammatory response that leads to cardiovascular autonomic dysfunction contributing to neurogenic hypertension.

Cellular distribution of the renal bumetanide-sensitive Na-K-2Cl cotransporter BSC-1 in the inner stripe of the outer medulla during the development of hypertension in the spontaneously hypertensive rat

1. The renal bumetanide-sensitive Na-K-2Cl cotransporter (BSC-1) is expressed only in the thick ascending limb and selectively traffics from intracellular vesicles (IVs) to apical plasma membranes (PMs), where BSC-1 regulates sodium reabsorption. We showed previously that in kidneys from adult spontaneously hypertensive rats (SHR; model of essential hypertension) total protein expression of BSC-1 was higher compared with kidneys from normotensive Wistar-Kyoto (WKY) rats. However, whether this change is associated with an increased trafficking of BSC-1 from IVs to PMs is unknown. The goal of the present study was to test the hypothesis that the increase in total renal BSC-1 protein expression in SHR is accompanied by an augmented distribution of BSC-1 from IVs to PMs. 2. To test the hypothesis, we obtained renal tissue from the inner stripe of the outer medulla (ISOM; enriched in thick ascending limbs) and isolated IVs and PMs from this tissue by differential centrifugation. Total BSC-1 protein expression in ISOM and BSC-1 protein expression in ISOM IVs and PMs were measured by semiquantitative western blotting in SHR and aged-matched WKY rats at different ages and stages of hypertension. 3. At 5 weeks of age, SHR were prehypertensive (mean arterial blood pressure (MABP) 97 mmHg). At this age, both the total abundance and cellular distribution of BSC-1 were similar in ISOM from SHR and WKY rats. 4. As SHR aged, their hypertension progressed (MABP 137 and 195 mmHg at 8 and 14 weeks of age, respectively). Associated with the increase in MABP was an increase in both steady state protein levels of ISOM BSC-1 and the distribution of ISOM BSC-1 to PMs (four- and sixfold increases at 8 and 14 weeks of age, respectively, compared with age-matched WKY rats; P < 0.001). 5. Using semiquantitative reverse transcription-polymerase chain reaction, BSC-1 mRNA was measured and was found not to differ between SHR and WKY rat ISOM at any age or level of MABP. 6. We conclude that as SHR transition from prehypertensive to established hypertension, there is a marked increase in the total expression of BSC-1 in ISOM that is not related to increases in steady state levels of BSC-1 mRNA and therefore unlikely to be due to changes in either the rate of BSC-1 gene transcription or the stability of BSC-1 mRNA. This suggests changes in either translational efficiency or BSC-1 protein stability in SHR. 7. We also conclude that the age/hypertension-related increase in BSC-1 protein levels in ISOM is accompanied by an equally marked increased trafficking of BSC-1 to PMs in SHR ISOM.

Renal responses to acute reflex activation of renal sympathetic nerve activity and renal denervation in secondary hypertension

We tested whether the responsiveness of the kidney to basal renal sympathetic nerve activity (RSNA) or hypoxia-induced reflex increases in RSNA, is enhanced in angiotensin-dependent hypertension in rabbits. Mean arterial pressure, measured in conscious rabbits, was similarly increased (+16 ± 3 mmHg) 4 wk after clipping the left ( n = 6) or right ( n = 5) renal artery or commencing a subcutaneous ANG II infusion ( n = 9) but was not increased after sham surgery ( n = 10). Under pentobarbital sodium anesthesia, reflex increases in RSNA (51 ± 7%) and whole body norepinephrine spillover (90 ± 17%), and the reductions in glomerular filtration rate (−27 ± 5%), urine flow (−43 ± 7%), sodium excretion (−40 ± 7%), and renal cortical perfusion (−7 ± 3%) produced by hypoxia were similar in normotensive and hypertensive groups. Hypoxia-induced increases in renal norepinephrine spillover tended to be less in hypertensive (1.1 ± 0.5 ng/min) than normotensive (3.7 ± 1.2 ng/min) rabbits, but basal overflow of endogenous and exogenous dihydroxyphenolglycol was greater. Renal plasma renin activity (PRA) overflow increased less in hypertensive (22 ± 29 ng/min) than normotensive rabbits (253 ± 88 ng/min) during hypoxia. Acute renal denervation did not alter renal hemodynamics or excretory function but reduced renal PRA overflow. Renal vascular and excretory responses to reflex increases in RSNA induced by hypoxia are relatively normal in angiotensin-dependent hypertension, possibly due to the combined effects of reduced neural norepinephrine release and increased postjunctional reactivity. In contrast, neurally mediated renin release is attenuated. These findings do not support the hypothesis that enhanced neural control of renal function contributes to maintenance of hypertension associated with activation of the renin-angiotensin system.

Junctional Adhesion Molecule-1 Is Upregulated in Spontaneously Hypertensive Rats

Junctional adhesion molecule-1 (JAM-1) forms part of the tight junction between adjacent endothelial cells. Using microarray technology, we showed previously that JAM-1 was differentially expressed in the brain stem of spontaneously hypertensive rats compared with normotensive Wistar-Kyoto (WKY) rats. In this study, we quantified the expression of JAM-1 in the brain stem of spontaneously hypertensive rats and WKY rats and established whether any differential expression was confined to this region of the brain or was ubiquitous throughout the central nervous system and, indeed, the whole body. Because the nucleus tractus solitarii plays a pivotal role in arterial pressure regulation, we assessed whether JAM-1 in this region affects the chronic regulation of arterial pressure. Real time RT-PCR revealed that JAM-1 mRNA was upregulated in multiple regions of the brain and all of the peripheral vascular beds studied. In the nucleus tractus solitarii, the level of JAM-1 mRNA was significantly higher in both young (3-week-old, prehypertensive) and adult male spontaneously hypertensive rats (15 to 18 weeks old) than that of age-matched WKY rats (fold differences; prehypertensives: 1.01+/-0.06 versus 1.59+/-0.13; n=10; P<0.01; adult: 1.08+/-0.14 versus 2.86+/-0.57; n=10; P<0.01). After adenoviral-mediated expression of JAM-1 in the nucleus tractus solitarii of adult WKY rats (15 weeks old; n=6), systolic pressure was increased from 120+/-4 to 132+/-4 mm Hg (P<0.01). Our data suggest that JAM-1 expression in the spontaneously hypertensive rat is upregulated throughout the body compared with the WKY rat and that this is not secondary to the hypertension. When JAM-1 is expressed in the nucleus tractus solitarii, it raises arterial pressure, suggesting a novel prohypertensive role for this protein within the brain stem.

Role of the renal nerves in γ-aminobutyric acid-induced antihypertensive effect in spontaneously hypertensive rats

The aim of this study was to clarify the role of the renal sympathetic nerves in the gamma-aminobutyric acid (GABA)-induced hypotensive effect in spontaneously hypertensive rats. Male spontaneously hypertensive rats (SHR/Izm) aged 7 weeks were divided into four groups on the basis of diet (containing 0.05% GABA, or GABA-free control diet) and operation (renal sympathetic-denervated or sham-operated) (n = 10, each). Water intake, urine volume and urinary sodium were, or tended to be, slightly higher, while plasma renin activity was significantly lower in the GABA group than the GABA-free control group. GABA inhibited the development of hypertension in sham-operated spontaneously hypertensive rats but not in renal-denervated spontaneously hypertensive rats. Plasma renin activity was significantly higher in sham-operated spontaneously hypertensive rats fed the control diet than in the other three groups. These results suggest that a reduction in the effects induced by the renal nerves may play an important role in the hypotensive effect induced in spontaneously hypertensive rats by chronic dietary administration of GABA.

Sympathetic hyperreactivity to air-jet stress in the chromosome 1 blood pressure quantitative trait locus congenic rats

A chromosome 1 blood pressure quantitative trait locus (QTL) was introgressed from the stroke-prone spontaneously hypertensive rats (SHRSP) to Wistar-Kyoto (WKY) rats. This congenic strain (WKYpch1.0) showed an exaggerated pressor response to both restraint and cold stress. In this study, we evaluated cardiovascular and sympathetic response to an air-jet stress and also examined the role of the brain renin-angiotensin system (RAS) in the stress response of WKYpch1.0. We measured mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) responses to air-jet stress in WKYpch1.0, WKY, and SHRSP. We also examined effects of intracerebroventricular administration of candesartan, an ANG II type 1 receptor blocker, on MAP and HR responses to air-jet stress. Baseline MAP in the WKYpch1.0 and WKY rats were comparable, while it was lower than that in SHRSP rats. Baseline HR did not differ among the strains. In WKYpch1.0, air-jet stress caused greater increase in MAP and RSNA than in WKY. The increase in RSNA was as large as that in SHRSP, whereas the increase in MAP was smaller than in SHRSP. Intracerebroventricular injection of a nondepressor dose of candesartan inhibited the stress-induced pressor response to a greater extent in WKYpch1.0 than in WKY. Intravenous injection of phenylephrine caused a presser effect comparable between WKYpch1.0 and WKY. These results suggest that the chromosome 1 blood pressure QTL congenic rat has a sympathetic hyperreactivity to an air-jet stress, which causes exaggerated pressor responses. The exaggerated response is at least partly mediated by the brain RAS.

Differential expression of nNOS mRNA and protein in the nucleus tractus solitarii of young and aged Wistar–Kyoto and spontaneously hypertensive rats

OBJECTIVE

To examine neuronal nitric oxide synthase (nNOS) mRNA and protein in the nucleus tractus solitarii (NTS) and paraventricular hypothalamic nucleus (PVN) of Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) during their life span.

METHODS

By means of in situ hybridization and immunohistochemistry, we evaluated nNOS mRNA and protein in the NTS and PVN of 15-day- and 1-, 2-, 4-, 8- and 12-month-old SHR and WKY rats.

RESULTS

Two patterns of nNOS expression were observed in two subnuclei of the NTS: medial (NTSm) and central (NTSce). NTSce of the SHR exhibited higher nNOS mRNA and protein expression in all ages analyzed when compared to the age-matched WKY. Increased amounts of nNOS mRNA and protein were seen in the NTSm only during the early life of SHR (15 days to 4 months) when compared to WKY, suggesting a special role of this circuitry before the establishment of hypertension. No changes were seen in nNOS mRNA and protein expression in the PVN of the SHR in comparison to the WKY in all periods. nNOS analysis during the life span showed either a decrease or no change in nNOS mRNA expression in NTS or PVN associated with increased nNOS protein at some analyzed periods, suggesting the differential regulation of nNOS mRNA and protein during aging.

CONCLUSIONS

Data suggest that different NTS subnuclei exhibit nNOS changes in different phases of the life of SHR and this might be important during the development of hypertension in these animals.

Exaggerated response to cold stress in a congenic strain for the quantitative trait locus for blood pressure

OBJECTIVE

Stroke-prone spontaneously hypertensive rats (SHRSP) are known to have sympathetic hyperactivity to various stimuli. In the search for 'intermediate phenotypes' inferring the function of hypertension genes, the present study assessed responsiveness to cold stress in a congenic strain derived from SHRSP/Izm and Wistar-Kyoto/Izm (WKY/Izm).

DESIGN

A congenic strain, WKYpch1.0, was established by 10 generations of backcrossing to transfer the chromosomal fragment between D1Wox29 and D1Arb21 of SHRSP to WKY. This fragment covered the 100:1 confidence interval of the quantitative trait locus (QTL) for blood pressure identified in a previous study. Response to cold stress was studied by exposing rats to 4 degrees C for 4 h. Blood pressure was monitored with telemetry. Urine was collected during the exposure, and urinary concentrations of catecholamines were measured by high-performance liquid chromatography.

RESULTS

Under the cold stress, urinary excretion of norepinephrine (NE) and vanillylmandelic acid (VMA), as well as the plasma level of NE, was significantly greater in WKYpch1.0 than in WKY. The increase in blood pressure during the cold stress was also greater in WKYpch1.0 than in WKY. Further, neonatal chemical sympathectomy using guanethidine abolished the exaggerated response in blood pressure and in urinary excretion of NE and VMA in WKYpch1.0.

CONCLUSION

These results suggested that the QTL region on rat chromosome 1 harbored genes responsible for the exaggerated response of the sympathetic nervous system to the cold stress. The relationship of this with the pathogenesis of hypertension should be elucidated in future studies.

Neural control of renal medullary perfusion

There is strong evidence that the renal medullary circulation plays a key role in long-term blood pressure control. This, and evidence implicating sympathetic overactivity in development of hypertension, provides the need for understanding how sympathetic nerves affect medullary blood flow (MBF). The precise vascular elements that regulate MBF under physiological conditions are unknown, but likely include the outer medullary portions of descending vasa recta and afferent and efferent arterioles of juxtamedullary glomeruli, all of which receive dense sympathetic innervation. Many early studies of the impact of sympathetic drive on MBF were flawed, both because of the methods used for measuring MBF and because single and often intense neural stimuli were tested. Recent studies have established that MBF is less sensitive than cortical blood flow (CBF) to electrical renal nerve stimulation, particularly at low stimulus intensities. Indeed, MBF appears to be refractory to increases in endogenous renal sympathetic nerve activity within the physiological range in all but the most extreme cases. Multiple mechanisms appear to operate in concert to blunt the impact of sympathetic drive on MBF, including counter-regulatory roles of nitric oxide and perhaps even paradoxical angiotensin II-induced vasodilatation. Regional differences in the geometry of glomerular arterioles are also likely to predispose MBF to be less sensitive than CBF to any given vasoconstrictor stimulus. Failure of these mechanisms would promote reductions in MBF in response to physiological activation of the renal nerves, which could, in turn, lead to salt and water retention and hypertension.

Sympathetic modulation of the renal glucose transporter GLUT2 in diabetic rats

We have previously shown that the abolition of renal sympathetic nervous activity (RSNA) can influence cortical GLUT1 expression in diabetic rats. However, no study has examined the effects of nervous activity on expression of GLUT2, the major glucose transporter in proximal renal tubules, which participates in renal glucose handling. The aim of this study was to determine whether sympathetic activity modulates renal GLUT2 content. We studied diabetic and nondiabetic rats with normal, low, or high RSNA. The low-RSNA experiment used four groups of Wistar male rats: Wistar sham-operated, Wistar renal-denervated, Diabetic sham-operated, and Diabetic renal-denervated. The high-RSNA experiment used four groups of Wistar-Kyoto male rats: WKY (control), WKY-Diabetic, SHR (spontaneously hypertensive rats), and SHR-Diabetic. Renal denervation was confirmed by a decrease in intrarenal norepinephrine levels and sympathetic hyperactivity, by measurement of RSNA. Western blotting was used to determine the renal cortical GLUT2 protein content, and 24-h urinary sodium and glucose levels were also evaluated. Compared with controls (Wistar and WKY), diabetes increased the GLUT2 protein content in normal-RSNA Diabetics (47%) and WKY-Diabetics (83%). The renal denervation-induced decrease in RSNA reduced the GLUT2 content in both normal and diabetic rats (-21% and -15%, respectively). Compared to WKY rats, SHR presented elevated RSNA and also showed an increase in renal GLUT2 content (17%). Diabetes caused a major increase in GLUT2 protein (52%) in the SHR. These results demonstrate a direct relationship between RSNA and GLUT2 levels; they also reveal an additive effect of sympathetic hyperactivity and diabetes on GLUT2 expression, suggesting a new mechanism for modulating protein expression in renal tissue.

Brain angiotensin-converting enzyme activity and autonomic regulation in heart failure

Several recent studies suggest an important role for the brain renin-angiotensin system in the pathogenesis of heart failure. Angiotensin-converting enzyme (ACE) activity and binding of angiotensin type 1 (AT1) receptors, which mediate the central effects of ANG II, are increased in heart failure. The present study examined the relationship between brain ACE activity and the autonomic dysregulation characteristic of rats with congestive heart failure. Rats with heart failure (HF) induced by coronary artery ligation and sham-operated control (SHAM) rats were treated with chronic (28 days) third cerebral ventricle [intracerebroventricular (ICV)] or intraperitoneal (IP) infusion of a low dose of the ACE inhibitor enalaprilat (ENL) or vehicle (VEH). VEH-treated HF rats had increased sodium consumption, reduced urine sodium and urine volume, and increased sympathetic nerve activity with impaired baroreflex regulation. These responses were minimized or prevented by ICV ENL started 24 h after coronary ligation. IP ENL at the low dose used in these studies had no beneficial effects on HF rats. Neither IP nor ICV ENL had any substantial effect on the SHAM rats. The findings confirm a critically important contribution of the brain renin-angiotensin system to the pathophysiology of congestive heart failure.

Pulse-synchronous sympathetic burst power as a new index of sympathoexcitation in patients with heart failure

The upper limit of incidence of muscle sympathetic neural bursts can lead to underestimation of sympathetic activity in patients with severe heart failure. This study aimed to evaluate the pulse-synchronous burst power of muscle sympathetic nerve activity (MSNA) as a more specific indicator that could discriminate sympathetic activity in patients with heart failure. In 54 patients with heart failure, the pulse-synchronous burst power at the mean heart rate was quantified by spectral analysis of MSNA. Thirteen patients received a central sympatholytic agent (guanfacine) for 5 days to validate the feasibility of this new index. Both burst incidence and plasma norepinephrine level showed no significant difference between patients in New York Heart Association functional class III (94 ± 6 per 100 heartbeats and 477 ± 219 pg/ml, respectively) and class II (79 ± 14 per 100 heartbeats and 424 ± 268 pg/ml, respectively). In contrast, the burst power was useful for discriminating patients in class III from those in class II (61 ± 8% vs. 39 ± 10%; P < 0.05). Inhibition of sympathetic nerve activity by guanfacine was more sensitively reflected by the change of burst power (−36 ± 25%) than by that of burst incidence (−12 ± 14%; P < 0.001). The sympathetic burst power reflects both burst frequency and amplitude independently of the absolute values and provides a sensitive new index for interindividual comparisons of sympathetic activity in patients with heart failure.

Effect of anaesthetic and rat strain on heart rate responses to simulated haemorrhage

AIM AND METHODS

Haemorrhage is characterized by two distinct responses, sympathoexcitation that evokes tachycardia and supports blood pressure, followed by sympathoinhibition contributing to bradycardia and hypotension. It has been shown that anaesthetics alter the response to haemorrhage and we hypothesized that rat strain may also influence the response. We investigated the effect of simulated haemorrhage on heart rate (HR) responses in three strains of conscious rats, and the effect of three common anaesthetics, by comparing HR responses under anaesthesia to the conscious response. Haemorrhage was simulated by constricting the inferior vena cava. We demonstrate differential effects of anaesthetics, including both maintenance and elimination of HR responses to haemorrhage depending on anaesthetic.

RESULTS

We also show that both phases of the HR response differ in different conscious rat strains, and we have demonstrated a transient increase in HR during the decompensatory phase of haemorrhage, a novel 'second HR peak' with advanced hypotension.

CONCLUSION

Both rat strain and anaesthetic influence HR responses to haemorrhage, and some anaesthetics appear less suitable than others for studies of haemodynamic responses in rats. There was evidence of an additional compensatory mechanism that operates at advanced levels of hypotension in the rat.