Renal reflexes in the regulation of blood pressure and sodium excretion

Clarification of hypertension mechanisms provided by the research of central circulatory regulation

Sympathoexcitation, under the regulatory control of the brain, plays a pivotal role in the etiology of hypertension. Within the brainstem, significant structures involved in the modulation of sympathetic nerve activity include the rostral ventrolateral medulla (RVLM), caudal ventrolateral medulla (CVLM), nucleus tractus solitarius (NTS), and paraventricular nucleus (paraventricular). The RVLM, in particular, is recognized as the vasomotor center. Over the past five decades, fundamental investigations on central circulatory regulation have underscored the involvement of nitric oxide (NO), oxidative stress, the renin-angiotensin system, and brain inflammation in regulating the sympathetic nervous system. Notably, numerous significant findings have come to light through chronic experiments conducted in conscious subjects employing radio-telemetry systems, gene transfer techniques, and knockout methodologies. Our research has centered on elucidating the role of NO and angiotensin II type 1 (AT1) receptor-induced oxidative stress within the RVLM and NTS in regulating the sympathetic nervous system. Additionally, we have observed that various orally administered AT1 receptor blockers effectively induce sympathoinhibition by reducing oxidative stress via blockade of the AT1 receptor in the RVLM of hypertensive rats. Recent advances have witnessed the development of several clinical interventions targeting brain mechanisms. Nonetheless, Future and further basic and clinical research are needed.

Oxidative stress in the brain causes hypertension via sympathoexcitation

Activation of the sympathetic nervous system (SNS) has an important role in the pathogenesis of hypertension, and is determined by the brain. Previous many studies have demonstrated that oxidative stress, mainly produced by angiotensin II type 1 (AT(1)) receptor and nicotinamide adenine dinucleotide phosphate (NAD (P) H) oxidase, in the autonomic brain regions was involved in the activation of the SNS of hypertension. In this concept, we have investigated the role of oxidative stress in the rostral ventrolateral medulla (RVLM), which is known as the cardiovascular center in the brainstem, in the activation of the SNS, and demonstrated that AT(1) receptor and NAD (P) H oxidase-induced oxidative stress in the RVLM causes sympathoexcitation in hypertensive rats. The mechanisms in which brain oxidative stress causes sympathoexcitation have been investigated, such as the interactions with nitric oxide (NO), effects on the signal transduction, or inflammations. Interestingly, the environmental factors of high salt intake and high calorie diet may also increase the oxidative stress in the brain, particularly in the RVLM, thereby activating the central sympathetic outflow and increasing the risk of hypertension. Furthermore, several orally administered AT(1) receptor blockers have been found to cause sympathoinhibition via reduction of oxidative stress through the inhibition of central AT(1) receptor. In conclusion, we must consider that AT(1) receptor and the related oxidative stress production in the brain cause the activation of SNS in hypertension, and that AT(1) receptor in the brain could be novel therapeutic target of the treatments for hypertension.

Central mechanisms of abnormal sympathoexcitation in chronic heart failure

It has been recognized that the sympathetic nervous system is abnormally activated in chronic heart failure, and leads to further worsening chronic heart failure. In the treatment of chronic heart failure many clinical studies have already suggested that the inhibition of the abnormal sympathetic hyperactivity by beta blockers is beneficial. It has been classically considered that abnormal sympathetic hyperactivity in chronic heart failure is caused by the enhancement of excitatory inputs including changes in peripheral baroreceptor and chemoreceptor reflexes and chemical mediators that control sympathetic outflow. Recently, the abnormalities in the central regulation of sympathetic nerve activity mediated by brain renin angiotensin system-oxidative stress axis and/or proinflammatory cytokines have been focused. Central renin angiotensin system, proinflammatory cytokines, and the interaction between them have been determined as the target of the sympathoinhibitory treatment in experimental animal models with chronic heart failure. In conclusion, we must recognize that chronic heart failure is a syndrome with an abnormal sympathoexcitation, which is caused by the abnormalities in the central regulation of sympathetic nerve activity.

Oxidative stress in the cardiovascular center has a pivotal role in the sympathetic activation in hypertension

Activation of the sympathetic nervous system has an important role in the pathogenesis of hypertension. However, the precise mechanisms involved are not fully understood. Oxidative stress may be important in hypertension as well as in other cardiovascular disorders. We investigated the role of oxidative stress, particularly in the rostral ventrolateral medulla (RVLM), which is known as the cardiovascular center in the brainstem, in the activation of the sympathetic nervous system in hypertension. We observed that the reactive oxygen species (ROS) production increases in the RVLM in hypertensive rats, thereby enhancing the central sympathetic outflow, which leads to hypertension. Furthermore, the environmental factors of high salt intake and a high-calorie diet may also increase the ROS production in the RVLM, thereby activating the central sympathetic outflow and increasing the risk of hypertension. The activation of the nicotinamide adenine dinucleotide phosphate oxidase via the angiotensin type 1 (AT1) receptors is suggested to be the major source of ROS production, and an altered downstream signaling pathway is involved in the activation of the RVLM neurons, leading to enhanced central sympathetic outflow and hypertension. Thus, the brain AT1 receptors may be novel therapeutic targets, and, in fact, oral treatment with angiotensin receptor blockers has been found to inhibit the central AT1 receptors, despite the blood-brain barrier.

Regional expression of NAD(P)H oxidase and superoxide dismutase in the brain of rats with neurogenic hypertension

BACKGROUND

Single injection of small quantities of phenol into the kidney cortex causes hypertension which is mediated by renal afferent sympathetic pathway activation. This phenomenon can be prevented by superoxide dismutase (SOD) infusion in the lateral ventricle, suggesting the role of superoxide (O(2)(-).) in noradrenergic control of arterial pressure. Since NAD(P)H oxidase is a major source of O(2)(-)., we tested the hypothesis that hypertension in this model may be associated with upregulation of NAD(P)H oxidase in relevant regions of brain.

METHODS

NAD(P)H oxidase subunits, mitochondrial (MnSOD) and cytoplasmic (CuZnSOD) SOD were measured in rats 4 weeks after injection of phenol or saline in the left kidney cortex.

RESULTS

Phenol-injected rats exhibited hypertension, upregulation of gp91(phox), p22(phox), p47(phox) and p67(phox) in the medulla, gp91(phox) and p22(phox) in pons and gp91(phox) in hypothalamus. This was associated with upregulation of MnSOD with little change in CuZnSOD.

CONCLUSIONS

Chronic hypertension in phenol-injected rats is associated with upregulation of NAD(P)H oxidase and hence increased O(2)(-). production capacity in the key regions of the brain involved in regulation of blood pressure. Since reactive oxygen species can intensify central noradrenergic activity, the observed maladaptive changes may contribute to the genesis and maintenance of the associated hypertension.

Oxidative stress mediates the stimulation of sympathetic nerve activity in the phenol renal injury model of hypertension

Renal injury caused by the injection of phenol in the lower pole of one kidney increases blood pressure (BP), norepinephrine secretion from the posterior hypothalamic nuclei (PH), and renal sympathetic nerve activity in the rat. Renal denervation prevents these effects of phenol. We have also demonstrated that noradrenergic traffic in the brain is modulated by NO and interleukin-1beta. In this study, we tested the hypothesis that the increase in sympathetic nervous system (SNS) activity in the phenol renal injury model is because of activation of reactive oxygen species. To this end, first we examined the abundance of several components of reduced nicotinamide-adenine dinucleotide phosphate oxidase (identified as the major source of reactive oxygen species), including gp91phox/Nox2, p22phox, p47phox, and Nox3 using real-time PCR. Second, we evaluated the effects of 2 superoxide dismutase mimetic, tempol (4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl), and superoxide dismutase-polyethylene glycol on central and peripheral SNS activation caused by intrarenal phenol injection. Intrarenal injection of phenol raised BP, NE secretion from the PH, renal sympathetic nerve activity, and the abundance of reduced nicotinamide-adenine dinucleotide phosphate and reduced the abundance of interleukin-1beta and neural-NO synthase mRNA in the PH, paraventricular nuclei, and locus coeruleus compared with control rats. When tempol or superoxide dismutase-polyethylene glycol were infused in the lateral ventricle before phenol, the effects of phenol on BP and SNS activity were abolished. The studies suggest that central activation of the SNS in the phenol-renal injury model is mediated by increased reactive oxygen species in brain nuclei involved in the noradrenergic control of BP.

Static and dynamic responses of renal chemoreceptor neurons to intrapelvic pressure increases in the rat

Multiunit afferent renal nerve activity (ARNA) and single unit activity from R2 chemoreceptors were recorded in anesthetized male Sprague Dawley rats during rapid and graded increases in intrapelvic pressure. Multiunit ARNA in 9 rats was excited by rapid intrapelvic pressure increases to 20 mmHg with non-diuretic urine (285.5 +/- 76.2% above control) but not when isotonic saline was used to fill the pelvis (13.0 +/- 9.0%). Similar responses were recorded from 27 single R2 chemoreceptors. Multiunit ARNA showed a direct, linear relationship with ramp increases in intrapelvic pressure between 0 and 20 mmHg at rates of 0.05, 0.15 and 0.3 mmHg/s. The responses were dynamically linked to intrapelvic pressure and maximum activations showed a positive linear relationship with the rate of pressure increase. Individual R2 chemoreceptor activity showed a similar dynamic relationship with intrapelvic pressure. ARNA was also excited by intrapelvic backflow of diuretic urine (10% expansion of extracellular volume) but the response in impulses/s was depressed by 58.6 +/- 9% in multiunit preparations and 30.1 +/- 10.8% for R2 chemoreceptors. Basal activities were also reduced during diuresis by 49.0 +/- 16% and 36.7 +/- 12.3% for multiunit and single unit preparations, respectively, and the percent increases over background during urine backflow were not different in non-diuresis and diuresis. Both multiunit ARNA and R2 chemoreceptors were also activated by ramp increases in intrapelvic pressure during diuresis, but the dynamic component was lost and responses to each pressure ramp were not different. The similarity in responses between multiunit ARNA and individual R2 chemoreceptors indicates that the multiunit ARNA activation during intrapelvic pressure increases is largely composed of activity from R2 chemoreceptors.