Diabetes is a major world health problem. Growing evidence from both clinical trials and animal experiments has clearly confirmed that arterial baroreflex dysfunction is a feature of type 1 diabetes, which links to prognosis and mortality of the type 1 diabetic patients. The arterial baroreflex normally regulates the blood pressure and heart rate through sensing changes of arterial vascular tension by the arterial baroreceptors in the aortic arch and carotid sinus. The aortic baroreceptor neuron located in the nodose ganglia is a primary afferent component of the arterial baroreflex. The functional changes of these neurons are involved in the arterial baroreflex dysfunction in the type 1 diabetes. Type 1 diabetes causes the overexpression and hyperactivation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and further reduces cell excitability of the aortic baroreceptor neurons. The alterations of the HCN channels are regulated by angiotensin II-NADPH oxidase-superoxide signaling in the aortic baroreceptor neurons. From the present review, we can understand the possible mechanisms responsible for the attenuated arterial baroreflex in the type 1 diabetes. These findings are beneficial for improving quality of life and prognosis in patients with the type 1 diabetes mellitus.
Baroreflex; Baroreceptor; Ion channels; Angiotensin II; Superoxide; Diabetes
Arterial baroreflex sensitivity is attenuated in chronic heart failure (CHF) state, which is associated with cardiac arrhythmias and sudden cardiac death in the patients with CHF. Our previous study showed that CHF-induced sodium channel dysfunction in the baroreceptor neurons was involved in the blunted baroreflex sensitivity in CHF rats. Mitochondria-derived superoxide overproduction decreased expression and activation of the sodium channels in the baroreceptor neurons from CHF rats. However, the molecular mechanisms responsible for the sodium channel dysfunction in the baroreceptor neurons from CHF rats remain unknown. We tested the involvement of NFκB in the sodium channel dysfunction and evaluated the effects of in-vivo transfection of manganese superoxide dismutase gene and NFκB shRNA on the baroreflex function in CHF rats. CHF was developed at 6–8 weeks after left coronary artery ligation in adult rats. Western bolt and chromatin immunoprecipitation data showed that phosphorylated NFκB p65 and ability of NFκB p65 binding to the sodium channel promoter were increased in the nodose ganglia from CHF rats. In-vivo transfection of adenoviral manganese superoxide dismutase gene or lentiviral NFκB p65 shRNA into the nodose ganglia partially reversed CHF-reduced sodium channel expression and cell excitability in the baroreceptor neurons and improved CHF-blunted arterial baroreflex sensitivity. Additionally, transfection of adenoviral manganese superoxide dismutase also inhibited the augmentation of phosphorylated NFκB p65 in the nodose neurons from CHF rats. The present study suggests that superoxide-NFκB signaling contributes to CHF-induced baroreceptor dysfunction and resultant impairment of baroreflex function.
baroreceptor; baroreflex; heart failure; NFκB; sodium channel; superoxide
Angiotensin II (Ang II)–induced arterial baroreflex dysfunction is associated with superoxide generation in the brain. Exercise training (EX) improves baroreflex function and decreases oxidative stress in cardiovascular diseases linked to elevated central Ang II. The aim of this study was to determine whether previous EX prevents baroreflex impairment caused by central administration of exogenous Ang II via an Ang II–superoxide mechanism. Four groups of rats were used: non-EX artificial cerebrospinal fluid infused, non-EX Ang II infused, EX artificial cerebrospinal fluid infused, and EX Ang II infused. Rats were treadmill trained for 3 to 4 weeks and subjected to intracerebroventricular infusion of Ang II over the last 3 days of EX. Twenty-four hours after the end of EX, the arterial baroreflex was assessed in anesthetized rats. Compared with non-EX artificial cerebrospinal fluid–infused rats, Ang II significantly decreased baroreflex sensitivity (maximum gain: 3.0 ± 0.2% of maximum per millimeter of mercury versus 1.6 ± 0.1% of maximum per millimeter of mercury; P < 0.01), which was abolished by acute intracerebroventricular infusion of the Ang II type 1 receptor antagonist losartan and the reduced nicotinamide-adenine dinucleotide phosphate oxidase inhibitor apocynin. EX prevented the decrease in baroreflex sensitivity and downregulated Ang II type 1 receptor and NADPH oxidase subunit protein expression in the paraventricular nucleus of Ang II–infused rats. Finally, EX decreased superoxide production in the paraventricular nucleus of Ang II–infused rats. These results indicate that EX improves arterial baroreflex function in conditions of high brain Ang II, which is mediated by the central Ang II type 1 receptor and associated with a reduction in central oxidative stress.
exercise; baroreflex; sympathetic nerve activity; reactive oxygen species; AT1 receptor
Voltage-gated sodium (Nav) channels are responsible for initiation and propagation of action potential in the neurons. To explore the mechanisms for chronic heart failure (CHF)-induced baroreflex dysfunction, we measured the expression and current density of Nav channel subunits (Nav1.7, Nav1.8, and Nav1.9) in the aortic baroreceptor neurons and investigated the role of Nav channels on aortic baroreceptor neuron excitability and baroreflex sensitivity in sham and CHF rats. CHF was induced by left coronary artery ligation. The development of CHF (6–8 weeks after the coronary ligation) was confirmed by hemodynamic and morphological characteristics. Immunofluorescent data indicated that Nav1.7 was expressed in A-type (myelinated) and C-type (unmyelinated) nodose neurons but Nav1.8 and Nav1.9 were expressed only in C-type nodose neurons. Real-time RT-PCR and western blot data showed that CHF reduced mRNA and protein expression levels of Nav channels in nodose neurons. In addition, using the whole cell patch-clamp technique, we found that Nav current density and cell excitability of the aortic baroreceptor neurons were lower in CHF rats than that in sham rats. Aortic baroreflex sensitivity was blunted in anesthetized CHF rats, compared with that in sham rats. Furthermore, Nav channel activator (rATX II, 100 nM) significantly enhanced Nav current density and cell excitability of aortic baroreceptor neurons and improved aortic baroreflex sensitivity in CHF rats. These results suggest that reduced expression and activation of the Nav channels is involved in the attenuation of baroreceptor neuron excitability, which subsequently contributes to the impairment of baroreflex in CHF state.
Aortic baroreceptor neuron; Baroreflex; Heart failure; Sodium channel
Status epilepticus (SE) is an acute, prolonged epileptic crisis with a mortality rate of 20–30%; the underlying mechanism is not completely understood. We assessed the hypothesis that brain stem cardiovascular dysregulation occurs during SE because of oxidative stress in rostral ventrolateral medulla (RVLM), a key nucleus of the baroreflex loop; to be ameliorated by brain-derived neurotrophic factor (BDNF) via an antioxidant action.
In a clinically relevant experimental model of temporal lobe SE (TLSE) using Sprague-Dawley rats, sustained hippocampal seizure activity was accompanied by progressive hypotension that was preceded by a reduction in baroreflex-mediated sympathetic vasomotor tone; heart rate and baroreflex-mediated cardiac responses remained unaltered. Biochemical experiments further showed concurrent augmentation of superoxide anion, phosphorylated p47phox subunit of NADPH oxidase and mRNA or protein levels of BDNF, tropomyosin receptor kinase B (TrkB), angiotensin AT1 receptor subtype (AT1R), nitric oxide synthase II (NOS II) or peroxynitrite in RVLM. Whereas pretreatment by microinjection bilaterally into RVLM of a superoxide dismutase mimetic (tempol), a specific antagonist of NADPH oxidase (apocynin) or an AT1R antagonist (losartan) blunted significantly the augmented superoxide anion or phosphorylated p47phox subunit in RVLM, hypotension and the reduced baroreflex-mediated sympathetic vasomotor tone during experimental TLSE, pretreatment with a recombinant human TrkB-Fc fusion protein or an antisense bdnf oligonucleotide significantly potentiated all those events, alongside peroxynitrite. However, none of the pretreatments affected the insignificant changes in heart rate and baroreflex-mediated cardiac responses.
We conclude that formation of peroxynitrite by a reaction between superoxide anion generated by NADPH oxidase in RVLM on activation by AT1R and NOS II-derived NO leads to a reduction in baroreflex-mediated sympathetic vasomotor tone during experimental TLSE; to be ameliorated by the upregulated BDNF/TrkB signaling via inhibition of p47phox phosphorylation. This information offers a new vista in devising therapeutic strategy towards minimizing mortality associated with TLSE.
Hypertensive transgenic (mRen2)27 rats with overexpression of the mRen2 gene have impaired baroreflex sensitivity (BRS) for heart rate control and high NADPH oxidase and kinase-to-phosphatase signaling activity in medullary tissue compared to normotensive Hannover Sprague-Dawley control rats. They also exhibit insulin resistance at a young age. To determine whether blocking angiotensin (Ang) II actions, supplementing Ang-(1-7) or scavenging reactive oxygen species (ROS) in brain differentially alters mean arterial pressure (MAP), BRS or metabolic function, while altering medullary signaling pathways in these animals, we compared intracerebroventricular (ICV) infusions of the AT1 receptor antagonist candesartan (CAN; 4μg/5μL/hr), Ang-(1-7) [0.1μg/5μL/hr], a ROS scavenger tempol (25μg/5μL/hr) or artificial cerebrospinal fluid (aCSF; 5 μL/hr) for 2 weeks. MAP was reduced in CAN treated rats without significantly improving the vagal components of baroreflex function or heart rate variability. In contrast, Ang-(1-7) treatment significantly improved the vagal components of baroreflex function and heart rate variability at a dose that did not significantly lower MAP. Tempol significantly reduced NADPH oxidase activity in brain dorsal medullary tissue, but had no effect on MAP or autonomic function. CAN tended to reduce fat mass, but none of the treatments significantly altered indices of metabolic function or mitogen-activated protein kinase (MAPK) signaling pathways in dorsal medulla. While additional dose response studies are necessary to determine the potential maximal effectiveness of each treatment, the current findings demonstrate that blood pressure and baroreflex function can be essentially normalized independently of medullary NADPH oxidase or MAPK in hypertensive (mRen2)27 rats.
Hypertension; baroreflex function; angiotensin peptides; oxidative stress; brain
Mitochondrial dysfunction is implicated in many cardiovascular diseases, including hypertension, and may be associated with an overactive renin-angiotensin system (RAS). Angiotensin (Ang) II, a potent vasoconstrictor hormone of the RAS, also impairs baroreflex and mitochondrial function. Most deleterious cardiovascular actions of Ang II are thought to be mediated by NADPH-oxidase- (NOX-) derived reactive oxygen species (ROS) that may also stimulate mitochondrial oxidant release and alter redox-sensitive signaling pathways in the brain. Within the RAS, the actions of Ang II are counterbalanced by Ang-(1–7), a vasodilatory peptide known to mitigate against increased oxidant stress. A balance between Ang II and Ang-(1–7) within the brain dorsal medulla contributes to maintenance of normal blood pressure and proper functioning of the arterial baroreceptor reflex for control of heart rate. We propose that Ang-(1–7) may negatively regulate the redox signaling pathways activated by Ang II to maintain normal blood pressure, baroreflex, and mitochondrial function through attenuating ROS (NOX-generated and/or mitochondrial).
In chronic heart failure (CHF), arterial baroreflex function is impaired, in part, by activation of the central renin-angiotensin system. A metabolite of Angiotensin II (Ang II), Ang-(1–7), has been shown to exhibit cardiovascular effects that are in opposition to that of Ang II. However, the action of Ang-(1–7) on sympathetic outflow and baroreflex function is not well understood, especially in CHF. The aim of this study was to determine the effect of intracerebroventricular infusion of Ang-(1–7) on baroreflex control of heart rate (HR) and renal sympathetic nerve activity (RSNA) in conscious rabbits with CHF. We hypothesized that central Ang-(1–7) would improve baroreflex function in CHF. Ang-(1–7) (2 nmol/1 μl/hour) or artificial cerebrospinal fluid (1 μl/hour) was infused by an osmotic mini-pump for 4 days in sham and pacing-induced CHF rabbits (n=3–6/group). Ang-(1–7) treatment had no effects in sham rabbits but reduced HR and increased baroreflex gain (7.4±1.5 bpm/mm Hg vs. 2.5±0.4 bpm/mm Hg, P<0.05) in CHF rabbits. The Ang-(1–7) antagonist A779 (8 nmol/1 μl/hr) blocked the improvement in baroreflex gain in CHF. Baroreflex gain increased in CHF+Ang-(1–7) animals when only the vagus was allowed to modulate baroreflex control by acute treatment with the β-1 antagonist metoprolol, indicating increased vagal tone. Baseline RSNA was significantly lower and baroreflex control of RSNA was enhanced in CHF rabbits receiving Ang-(1–7). These data suggest that augmentation of central Ang-(1–7) inhibits sympathetic outflow and increases vagal outflow in CHF thus contributing to enhanced baroreflex gain in this disease state.
angiotensin-(1–7); heart failure; sympathetic nervous system; baroreflex; vagus nerve; blood pressure; heart rate
Contribution of the vestibular end organ to regulation of arterial pressure was quantitatively compared with the role of baroreceptors in terms of baroreflex sensitivity and c-Fos protein expression in the rostral ventrolateral medulla (RVLM). Baroreflex sensitivity and c-Fos protein expression in the RVLM were measured in conscious rats that had undergone bilateral labyrinthectomy (BL) and/or baroreceptor unloading. BL attenuated baroreflex sensitivity during intravenous infusion of sodium nitroprusside (SNP), but did not significantly affect the sensitivity following infusion of phenylephrine (PE). Baroreflex sensitivity became positive following sinoaortic denervation (SAD) during infusion of PE and attenuated sensitivity during infusion of SNP. Baroreflex sensitivity also became positive following double ablation (BL+SAD) during infusion of PE, and attenuated sensitivity during infusion of SNP. c-Fos protein expression increased significantly in the RVLM in the sham group after SNP administration. However, the BL, SAD, and SAD+BL groups showed significant decreases in c-Fos protein expression compared with that in the sham group. The SAD group showed more reduced c-Fos protein expression than that in the BL group, and the SAD+BL group showed less expression than that in the SAD group. These results suggest that the vestibular system cooperates with baroreceptors to maintain arterial pressure during hypotension but that baroreceptors regulate arterial pressure during both hypotension and hypertension. Additionally, afferent signals for maintaining blood pressure from the vestibular end organs and the baroreceptors may be integrated in the RVLM.
Baroreceptor; Blood pressure; c-Fos protein; Rostral ventrolateral medulla; Vestibular system
The mechanisms by which angiotensin-(1–7) [Ang-(1–7)] exerts its beneficial effects on end-organ damage associated with diabetes and hypertension are not well understood. The purpose of this study was A) to compare the effects of apocynin with Ang-(1–7) on renal vascular dysfunction and NADPH oxidase activity in a combined model of diabetes and hypertension and B) to further determine whether chronic treatment with Ang-(1–7) can modulate renal catalase, and peroxisome proliferator activated receptor-γ (PPAR–γ) levels in streptozotocin induced-diabetes in both normotensive Wistar Kyoto rats (WKY) and in spontaneously hypertensive rats (SHR). Apocynin or Ang-(1–7) treatment for one month starting at the onset of diabetes similarly attenuated elevation of renal NADPH oxidase activity in the diabetic SHR kidney and reduced the degree of proteinuria and hyperglycemia, but had little or modest effect on reducing mean arterial pressure. Both drugs also attenuated the diabetes-induced increase in renal vascular responsiveness to endothelin-1. Induction of diabetes in WKY and SHR animals resulted in significantly reduced renal catalase activity and in PPAR–γ mRNA and protein levels. Treatment with Ang-(1–7) significantly prevented diabetes-induced reduction in catalase activity and the reduction in PPAR–γ mRNA and protein levels in both animal models. Taken together, these data suggest that activation of Ang-(1–7)-mediated signaling could be an effective way to prevent the elevation of NADPH oxidase activity and inhibition of PPAR–γ and catalase activities in diabetes and/or hypertension.
Angiotensin; hypertension; diabetes; kidney
A previous study from this laboratory showed that elevation of endogenous angiotensin II (Ang II) and upregulation of the angiotensin II type 1 (AT1) receptor in the carotid body (CB) are involved in the enhanced peripheral chemoreceptor sensitivity in rabbits with chronic heart failure (CHF). NADPH oxidase-derived superoxide anion mediates the effects of Ang II in many organs. We investigated whether this signaling pathway may mediate the enhanced peripheral chemoreceptor sensitivity induced by Ang II in CHF rabbits.
Methods and results:
By recording single-unit activity from the carotid sinus nerve in isolated preparations, we found that phenylarsine oxide 2 μM (PAO, NADPH oxidase inhibitor) and TEMPOL 1 mM (superoxide dismutase mimetic) significantly decreased not only the Ang II-enhanced CB chemoreceptor responses to different levels of hypoxia in sham rabbits (Δ-12.5 ± 0.8 and Δ-12.8 ± 0.9 imp/s at 40.7 ± 2.3 mm Hg of PO2, and Δ-5.6 ± 0.5 and Δ-5.3 ± 0.4 imp/s at 60.2 ± 3.1 mm Hg of PO2, p<0.05, respectively) but also the CHF-induced elevation of CB chemoreceptor responses to different levels of hypoxia (Δ-13.6 ± 1.1 and Δ-13.7 ± 0.9 imp/s at 40.9 ± 3.1 mm Hg of PO2, and Δ-6.7 ± 1.2 and Δ-6.6 ± 0.8 imp/s at 59.8 ± 3.5 mm Hg of PO2, p<0.05). In addition, mRNA and protein expressions of NADPH oxidase components (gp91phox, p40phox and p47phox) were higher in the CB from CHF rabbits compared to sham rabbits. Furthermore, 100 pM Ang II induced an increase in superoxide production in CB homogenates from sham rabbits, which was similar to that in CB homogenate from CHF rabbits. PAO and Tempol inhibited the Ang II- and CHF-enhanced superoxide anion production.
These results suggest that the enhanced peripheral chemoreceptor sensitivity mediated by Ang II in CHF rabbits occurs via a NADPH oxidase-superoxide signaling pathway.
Angiotensin; Reactive oxygen species; autonomic nervous system; chemoreceptor; heart failure
Central Ang II inhibits baroreflex and plays an important role in the pathogenesis of hypertension. However, the underlying molecular mechanisms are still not fully understood.
Our objective in the present study was to characterize the signal transduction mechanism of PI3-kinase involvement in Ang II-induced stimulation of central neuronal activity in cultured neurons and Ang II-induced inhibition of baroreflex in SHR versus WKY rats.
Methods and Results
Application of Ang II to neurons produced a 42% greater increase in neuronal firing in cells from the SHR than the WKY rat. Whilst in the WKY the Ang II-mediated increase in firing rate was abolished entirely by the PKC inhibitor GF109230, it was necessary to block both PKC and PI3K activity in the SHR. This was associated with an increased ability of Ang II to stimulate NADPH-oxidase-ROS mediated signaling involving phosphorylation of the p47phox subunit of the NADPH oxidase and was dependent on the activation of PI3 Kinase in the SHR. Inhibition of PI3 Kinase resulted in the reduction of levels of p47phox phosphorylation, NADPH oxidase activity, ROS levels and ultimately neuronal activity in cells from the SHR but not the WKY rat. In addition, in working heart-brainstem preparations, inhibition of PKC activity in the NTS in situ abolished the Ang II-mediated depression of cardiac and sympathetic baroreceptor reflex gain in the WKY. In contrast, PKC inhibition in the NTS of SHR only partially reduced the effect of Ang II on the baroreceptor reflex gain.
These observations demonstrate that PI3-Kinase in the cardiovascular brainstem regions of the SHR may be selectively involved in Ang II-mediated signaling that includes a reduction in baroreceptor reflex function, presumably via a NADPH-ROS mediated pathway.
Hypertension; angiotensin II; PI3 kinase; reactive oxygen species; NADPH oxidase
Transgenic (mRen2)27 rats are hypertensive with impaired baroreflex sensitivity for control of heart rate compared to Hannover Sprague-Dawley rats. We assessed blood pressure and baroreflex function in male hemizygous (mRen2)27 rats (30-40 wks of age) instrumented for arterial pressure recordings and receiving into the cisterna magna either an Ang-(1-7) fusion protein or a control fusion protein (CTL-FP). The maximum reduction in mean arterial pressure achieved was -38 ± 7 mm Hg on day 3, accompanied by a 55% enhancement in baroreflex sensitivity in Ang-(1-7) fusion protein-treated rats. Both the high frequency alpha index (HF-α) and heart rate variability increased, suggesting increased parasympathetic tone for cardiac control. The mRNA levels of several components of the renin-angiotensin system in the dorsal medulla were markedly reduced including renin (-80%), neprilysin (-40%) and the AT1a receptor (-40%). However, there was 2 to 3 increase in the mRNA levels of the phosphatases PTP-1b and DUSP1 in the medulla of Ang-(1-7) fusion protein-treated rats. Our finding that replacement of Ang-(1-7) in the brain of (mRen2)27 rats reverses in part the hypertension and baroreflex impairment is consistent with a functional deficit of Ang-(1-7) in this hypertensive strain. We conclude that the increased mRNA expression of phosphatases known to counteract the phosphoinositol 3 kinase (PI3K) and mitogen-activated protein kinases (MAPK), as well as the reduction of renin and AT1a receptor mRNA levels may contribute to the reduction in arterial pressure and improvement in baroreflex sensitivity in response to Ang-(1-7).
Angiotensin-(1-7); hypertension; baroreflex; (mRen2)27; phosphatases
Previous studies showed that a proportion of normotensive Sprague-Dawley rats spontaneously exhibit lower baroreflex sensitivity. However, investigations have not yet been carried out on Wistar rats. We aimed to compare baroreflex sensitivity among rats from the same strain and the same laboratory. Male Wistar normotensive rats (300–400g) were studied. Cannulas were inserted into the abdominal aortic artery through the right femoral artery to measure mean arterial pressure and heart rate. Baroreflex was calculated as the derivative of the variation of heart rate in function of the mean arterial pressure variation (ΔHR/ΔMAP) tested with a depressor dose of sodium nitroprusside (50 µg/kg) and with a pressor dose of phenylephrine (8µg/kg) in the right femoral venous approach through an inserted cannula. We divided the rats into four groups: i) high bradycardic baroreflex, baroreflex gain less than −2 tested with phenylephrine; ii) low bradycardic baroreflex, baroreflex gain between −1 and −2 tested with phenylephrine; iii) high tachycardic baroreflex, baroreflex gain less than −3 tested with sodium nitroprusside; and iv) low tachycardic baroreflex, baroreflex gain between −1 and −3 tested with sodium nitroprusside. Approximately 71% of the rats presented a decrease in bradycardic reflex while around half showed an increase in tachycardic reflex. No significant changes in basal mean arterial pressure and heart rate, tachycardic and bradycardic peak and heart rate range were observed. There was a significant change in baroreflex sensitivity among rats from the same strain and the same laboratory.
baroreceptors; hypertension; autonomic nervous system; parasympathetic nerve; sympathetic nerve.
A subset of normotensive Sprague–Dawley rats show lower baroreflex sensitivity; however, no previous study investigated whether there are differences in baroreflex sensitivity within this subset. Our study compared baroreflex sensitivity among conscious rats of this specific subtype.
Male Wistar Kyoto (WKY) rats (16 weeks old) were studied. Cannulas were inserted into the abdominal aortic artery through the right femoral artery to measure mean arterial pressure (MAP) and heart rate (HR). Baroreflex gain was calculated as the ratio between change in HR and MAP variation (ΔHR/ΔMAP) in response to a depressor dose of sodium nitroprusside (SNP, 50 μg/kg, i.v.) and a pressor dose of phenylephrine (PE, 8 μg/kg, i.v.). Rats were divided into four groups: 1) low bradycardic baroreflex (LB), baroreflex gain (BG) between -1 and -2 bpm/mmHg tested with PE; 2) high bradycardic baroreflex (HB), BG < -2 bpm/mmHg tested with PE; 3) low tachycardic baroreflex (LT), BG between -1 and -2 bpm/mmHg tested with SNP and; 4) high tachycardic baroreflex (HT), BG < -2 bpm/mmHg tested with SNP. Significant differences were considered for p < 0.05.
Approximately 37% of the rats showed a reduced bradycardic peak, bradycardic reflex and decreased bradycardic gain of baroreflex while roughly 23% had a decreased basal HR, tachycardic peak, tachycardic reflex and reduced sympathetic baroreflex gain. No significant alterations were noted with regard to basal MAP.
There is variability regarding baroreflex sensitivity among WKY rats from the same laboratory.
Baroreflex; Rats; Inbred WKY; Sympathetic Nervous System; Parasympathetic Nervous System; Autonomic Nervous System
NADPH oxidases are major sources of superoxide (O2∸) and hydrogen peroxide (H2O2) in vascular cells. Production of these reactive oxygen species (ROS) is essential for cell proliferation and differentiation, while ROS overproduction has been implicated in hypertension and atherosclerosis. It is known that the heme-containing catalytic subunits Nox1 and Nox4 are responsible for oxygen reduction in vascular smooth muscle cells from large arteries. However, the exact mechanism of ROS production by NADPH oxidases is not completely understood. We hypothesized that Nox1 and Nox4 play distinct roles in basal and angiotensin II (AngII)-stimulated production of O2∸ and H2O2. Nox1 and Nox4 expression in rat aortic smooth muscle cells (RASMCs) was selectively reduced by treatment with siNox4 or antisense Nox1 adenovirus. Production of O2∸ and H2O2 in intact RASMCs was analyzed by dihydroethidium and Amplex Red assay. Activity of NADPH oxidases was measured by NADPH-dependent O2∸ and H2O2 production using electron spin resonance (ESR) and 1-hydroxy-3-carboxy-pyrrolidine (CPH) in the membrane fraction in the absence of cytosolic superoxide dismutase. It was found that production of O2∸ by quiescent RASMC NADPH oxidases was five times less than H2O2 production. Stimulation of cells with AngII led to a 2-fold increase of O2∸ production by NADPH oxidases, with a small 15 to 30% increase in H2O2 formation. Depletion of Nox4 in RASMC led to diminished basal H2O2 production, but did not affect O2∸ or H2O2 production stimulated by AngII. In contrast, depletion of Nox1 in RASMC inhibited production of O2∸ and AngII-stimulated H2O2 in the membrane fraction and intact cells. Our data suggest that Nox4 produces mainly H2O2, while Nox1 generates mostly O2∸ that is later converted to H2O2. Therefore, Nox4 is responsible for basal H2O2 production, while O2∸ production in non-stimulated and AngII-stimulated cells depends on Nox1. The difference in the products generated by Nox1 and Nox4 may help to explain the distinct roles of these NADPH oxidases in cell signaling. These findings also provide important insight into the origin of H2O2 in vascular cells, and may partially account for the limited pharmacological effect of antioxidant treatments with O2∸ scavengers that do not affect H2O2.
Neurokinin-1 receptor (NK1-R) expressing neurons are densely distributed throughout the nucleus tractus solitarii (NTS). However, their fundamental role in arterial baroreflex function remains debated. Previously, our group has shown that activation of contraction-sensitive somatic afferents evoke substance P (SP) release in the NTS and resets the arterial baroreflex via activation of a GABAergic NTS circuit. Based on these findings, we hypothesized that modulation of arterial baroreflex function by somatic afferents is mediated by NK1-R dependent inhibition of barosensitive NTS circuits. In the present study, SP-conjugated saporin toxin (SP-SAP) was used to ablate NK1-R expressing NTS neurons. Contraction-sensitive somatic afferents were activated by electrically-evoked muscle contraction and the arterial baroreceptor–heart rate reflex was assessed by constructing reflex curves using a decerebrate, arterially-perfused preparation. Baseline baroreflex sensitivity was significantly attenuated in SP-SAP-treated rats compared with control rats receiving either unconjugated SAP or vehicle. Muscle contraction significantly attenuated baroslope in SAP and vehicle-treated animals and shifted the baroreflex curves to higher systemic pressure. In contrast, somatic afferent stimulation failed to alter baroslope or shift the baroreflex curves in SP-SAP-treated animals. Moreover, when reflex sensitivity was partially restored in SP-SAP animals, somatic stimulation failed to attenuate baroreflex bradycardia. In contrast, SP-SAP and somatic stimulation failed to blunt the reflex bradycardia evoked by the peripheral chemoreflex. Immunohistochemistry revealed that pretreatment with SP-SAP significantly reduced the number of NK1-R expressing neurons in the caudal NTS, while sparing NK1-R expressing neurons rostral to the injection site. This was accompanied by a significant reduction in the number of glutamic acid decarboxylase (GAD67) expressing neurons at equivalent levels of the NTS. These findings indicate that immunolesioning of NK1-R expressing NTS neurons selectively abolishes the depressive effect of somatosensory input on arterial baroreceptor–heart rate reflex function.
nucleus tractus solitarii; neurokinin receptors; substance P; GABA; arterial baroreflex; exercise
Intrauterine growth restriction (IUGR) is associated with impaired vascular function, which contributes to the increased incidence of chronic disease. The aim of this study was to investigate whether aerobic training improves AngII-induced vasoconstriction in IUGR rats. Moreover, we assess the role of superoxide dismutase (SOD) isoforms and NADPH oxidase-derived superoxide anions in this improvement. Female Wistar rats were randomly divided into two groups on day 1 of pregnancy. A control group was fed standard chow ad libitum, and a restricted group was fed 50% of the ad libitum intake throughout gestation. At 8 weeks of age, male offspring from both groups were randomly assigned to 4 experimental groups: sedentary control (SC), trained control (TC), sedentary restricted (SRT), and trained restricted (TRT). The training protocol was performed on a treadmill and consisted of a continuous 60-min session 5 days/week for 10 weeks. Following aerobic training, concentration–response curves to AngII were obtained in endothelium-intact aortic rings. Protein expression of SOD isoforms, AngII receptors and the NADPH oxidase component p47phox was assessed by Western blot analysis. The dihydroethidium was used to evaluate the in situ superoxide levels under basal conditions or in the presence of apocynin, losartan or PD 123,319. Our results indicate that aerobic training can prevent IUGR-associated increases in AngII-dependent vasoconstriction and can restore basal superoxide levels in the aortic rings of TRT rats. Moreover, we observed that aerobic training normalized the increased p47phox protein expression and increased MnSOD and AT2 receptor protein expression in thoracic aortas of SRT rats. In summary, aerobic training can result in an upregulation of antioxidant defense by improved of MnSOD expression and attenuation of NADPH oxidase component p47phox. These effects are accompanied by increased expression of AT2 receptor, which provide positive effects against Ang II–induced superoxide generation, resulting in attenuation of AngII-induced vasoconstriction.
Fibrosis of the glomerulus and the tubulointerstitium occurs in patients with hypertension. Studies have shown that renal oxidative stress appears in hypertensive kidney disease. The potential role of oxidative stress in renal fibrogenesis remains to be elucidated. Herein, we tested the hypothesis that oxidative stress contributes to the development of renal fibrosis during hypertension. Sprague-Dawley rats received angiotensin II (AngII; 9 μg/h s.c.) for 4 weeks with/without co-treatment of antioxidants, apocynin and tempol (120 mg/kg/day each, p.o.). Untreated rats served as controls. Appearance of renal oxidative stress and its effect on the expression of transforming growth factor (TGF)-ß1, population of myofibroblasts, collagen synthesis/degradation and fibrosis in kidneys were examined. Chronic AngII infusion elevated systemic blood pressure (228 ± 6 mm Hg), which was accompanied with extensive renal fibrosis and oxidative stress represented as upregulated NADPH oxidase and suppressed superoxide dismutase (SOD). Co-treatment with antioxidants led to: (1) markedly decreased renal NADPH oxidase; (2) significantly attenuated gene expression of TGF-ß1, type I collagen, and tissue inhibitors of matrix metalloproteinase (TIMP)-I/-II in the kidney; (3) largely reduced population of myofibroblasts in both the cortex and medulla; (4) significantly reduced renal collagen volume, and (5) partially suppressed blood pressure (190 ± 8 mm Hg). Thus, prolonged AngII administration promotes renal oxidative stress, which is associated with hypertensive renal disease. AngII induces renal oxidative stress by increasing NADPH oxidase and reducing SOD in the kidney, which, in turn, upregulates collagen synthesis, while suppressing collagen degradation, thereby promoting the development of fibrosis in kidneys of hypertensive rats.
Hypertension; Angiotensin II; Oxidative stress; Kidney fibrosis
Activation of the dorsal periaqueductal gray (PAG) evokes defense-like behavior including a marked increase in sympathetic drive and resetting of baroreflex function. The goal of this study was to investigate the role of the lateral parabrachial nucleus (LPBN) in mediating dorsal PAG modulation of the arterial baroreflex. Reflex responses were elicited by electrical stimulation of the aortic depressor nerve (ADN) at 5 Hz or 15 Hz in urethane anesthetized rats (n=18). Electrical stimulation of the dorsal PAG at 10 Hz did not alter baseline mean arterial pressure (MAP) but did significantly attenuate baroreflex control of heart rate (HR) evoked by low frequency ADN stimulation. Alternatively, 40 Hz dorsal PAG stimulation increased baseline MAP (43±3 mmHg) and HR (33±3 bpm) and attenuated baroreflex control of HR at both ADN stimulation frequencies. Reflex control of MAP was generally unchanged by dorsal PAG stimulation. Bilateral inhibition of neurons in LPBN area (n=6) with muscimol (0.45 nmol per side) reduced dorsal PAG evoked increases in MAP and HR by 50±4% and 95±4%, respectively, and significantly reduced, but did not completely eliminate dorsal PAG attenuation of the cardiac baroreflex. Bilateral blockade of glutamate receptors in the LPBN area (n=6) with kynurenic acid (1.8 nmol) had a similar effect on dorsal PAG evoked increases in MAP, HR and cardiac baroreflex function. Reflex control of MAP was unchanged with either treatment. These findings suggest that the LPBN area is one of several brainstem regions involved in descending modulation of the cardiac baroreflex function during defensive behavior.
dorsal periaqueductal gray; defense response; lateral parabrachial nucleus and sympathoexcitation
Age-related impairments in baroreflex sensitivity in Sprague-Dawley rats are associated with low solitary tract nucleus content of angiotensin-(1−7). However, transgenic rats with low-brain angiotensinogen resulting from glial overexpression of an antisense oligonucleotide to angiotensinogen (ASrAOGEN) are spared age-related declines in cardiovascular function characteristic of Sprague-Dawley rats. We examine whether cardiovascular and reflex actions of angiotensin-(1−7) persist in the solitary tract nucleus of older (16 to 22 months) ASrAOGEN rats. Baroreflex sensitivity for control of heart rate and chemosensitive vagal afferent activation in response to phenylbiguanide were measured before and after bilateral microinjection of the angiotensin II type 1 receptor antagonist candesartan and angiotensin-(1−7) receptor antagonist (D-Ala7)-angiotensin-(1−7) in urethane/chloralose-anesthetized rats. In older anesthetized ASrAOGEN rats, candesartan had no effect, whereas (D-Ala7)-angiotensin-(1−7) significantly reduced baroreflex sensitivity (1.80±0.43 versus 0.50±0.17 ms/mm Hg). Phenylbiguanide responses were attenuated by injection of candesartan (−79±6 versus −55±12 mm Hg and −277±12 versus −156±27 bpm; P<0.05). In addition, resting blood pressure was reduced by injection of candesartan or (D-Ala7)-angiotensin-(1−7). Within the solitary tract nucleus of older ASrAOGEN rats, it appears that glial angiotensinogen is the main source of angiotensin II attenuation of baroreflex sensitivity; endogenous angiotensin-(1−7) from nonglial sources enhances baroreflex sensitivity; nonglial sources of angiotensin II contribute to chemosensitive vagal afferent activation; and receptors for both peptides modulate resting arterial pressure under anesthesia. These results suggest a novel mechanism for the preservation of baroreflex sensitivity during aging.
transgenic rats; brain renin-angiotensin system; solitary tract nucleus; aging
Postural tachycardia syndrome (POTS) is characterized by excessive orthostatic tachycardia and significant functional disability. We have previously found that POTS patients had increases in plasma angiotensin II (Ang II) twice as high as normal subjects despite normal blood pressures. In this study we assess systemic and renal hemodynamic and functional responses to Ang II infusion in patients with POTS compared with healthy controls.
Methods and Results
Following a 3 day sodium controlled diet, we infused Ang II (3 ng/kg/min) for 1 hour in POTS patients (n=15) and healthy controls (n=13) in the supine position. All study subjects were females with normal blood pressure (BP). Ages were similar for POTS and control subjects (30±2 [mean±SEM] vs. 26±1 years; P=0.11). We measured the changes from baseline mean arterial pressure (MAP), renal plasma flow (RPF), plasma renin activity (PRA), aldosterone, urine sodium and baroreflex sensitivity in both groups.
In response to Ang II infusion, POTS patients had a blunted increase compared with control subjects in MAP (10±1 mmHg vs. 14±1 mmHg; P=0.01), and diastolic BP (9±1 mmHg vs. 13±1 mmHg; P=0.01), but not systolic BP (13±2 mmHg vs. 15±2 mmHg; P=0.40). Renal plasma flow (RPF) decreased similarly with Ang II infusion in POTS patients and controls (−166±20 vs. −181±17 mL/min/1.73 kg/m2; P=0.58). Post-infusion, the decrease in PRA (−0.9±0.2 vs. −0.6±0.2 ng/mL/h; P=0.43) and the increase in aldosterone (17±1 vs. 15±2 pg/ml; P=0.34) were similar in POTS and controls. The decrease in urine sodium excretion was similar in both POTS and controls (−49±12 vs. −60±16 mEq/g Cr; P=0.55). The spontaneous baroreflex sensitivity at baseline was significantly lower in POTS compared to healthy controls (10.1±1.2 vs. 16.8±1.5 ms/mmHg, P=0.003) and it was further reduced with Ang II infusion.
Patients with POTS have blunted vasopressor response to Ang II and impaired baroreflex function. This impaired vasoconstrictive response might be exaggerated with upright posture, and may contribute to the subsequent orthostatic tachycardia that is the hallmark of this disorder.
tachycardia; autonomic nervous system; angiotensin II; aldosterone; renal plasma flow
Early gestation dexamethasone (dex) administration is an ovine model of fetal programming associated with increased coronary reactivity to angiotensin II (Ang II). NADPH oxidase-dependent superoxide production plays an important role in both Ang II signaling and coronary disease. We sought to determine whether early gestation dex-exposure increases coronary reactivity to Ang II by enhancing endothelial NADPH oxidase-dependent superoxide production. Dex (0.28 mg/kg/d for 48 h) was administered to pregnant ewes at 27–28 d gestation. Dex-exposed and control offspring were studied at 4 mo of age. Coronary superoxide production was measured by lucigenin-enhanced chemiluminescence and dihydroethidium fluorescence. Coronary arteries from dex-exposed sheep had significantly enhanced vasoconstriction to Ang II, an effect abolished by either endothelial removal or preincubation with membrane-permeable superoxide dismutase and catalase. Ang II significantly increased endothelial superoxide production and NADPH oxidase activity in coronaries from dex-exposed offspring, but not controls. This programmed alteration in superoxide production was accentuated by PD123319 (AT2 antagonist), but abolished by losartan (AT1 antagonist). In conclusion, early gestation dex-exposure programs coronary reactivity to Ang II by enhancing Ang II-stimulated endothelial superoxide production. This programming effect may predispose to progressive coronary endothelial dysfunction and coronary artery disease.
Baroreflex dysfunction, oxidative stress and inflammation, important hallmarks of hypertension, are attenuated by exercise training. In this study, we investigated the relationships and time-course changes of cardiovascular parameters, pro-inflammatory cytokines and pro-oxidant profiles within the hypothalamic paraventricular nucleus of the spontaneously hypertensive rats (SHR). Basal values and variability of arterial pressure and heart rate and baroreflex sensitivity were measured in trained (T, low-intensity treadmill training) and sedentary (S) SHR at weeks 0, 1, 2, 4 and 8. Paraventricular nucleus was used to determine reactive oxygen species (dihydroethidium oxidation products, HPLC), NADPH oxidase subunits and pro-inflammatory cytokines expression (Real time PCR), p38 MAPK and ERK1/2 expression (Western blotting), NF-κB content (electrophoretic mobility shift assay) and cytokines immunofluorescence. SHR-S vs. WKY-S (Wistar Kyoto rats as time control) showed increased mean arterial pressure (172±3 mmHg), pressure variability and heart rate (358±7 b/min), decreased baroreflex sensitivity and heart rate variability, increased p47phox and reactive oxygen species production, elevated NF-κB activity and increased TNF-α and IL-6 expression within the paraventricular nucleus of hypothalamus. Two weeks of training reversed all hypothalamic changes, reduced ERK1/2 phosphorylation and normalized baroreflex sensitivity (4.04±0.31 vs. 2.31±0.19 b/min/mmHg in SHR-S). These responses were followed by increased vagal component of heart rate variability (1.9-fold) and resting bradycardia (−13%) at the 4th week, and, by reduced vasomotor component of pressure variability (−28%) and decreased mean arterial pressure (−7%) only at the 8th week of training. Our findings indicate that independent of the high pressure levels in SHR, training promptly restores baroreflex function by disrupting the positive feedback between high oxidative stress and increased pro-inflammatory cytokines secretion within the hypothalamic paraventricular nucleus. These early adaptive responses precede the occurrence of training-induced resting bradycardia and blood pressure fall.
Previous human studies have shown that large-artery stiffness contributes to an age-related decrease in cardiovagal baroreflex sensitivity. Whether this is also true with sympathetic baroreflex sensitivity is unknown. We tested the hypothesis that sympathetic baroreflex sensitivity is associated with the stiffness of baroreceptor segments (the carotid artery and the aorta) in elderly individuals, and that sex affects this relationship. Sympathetic baroreflex sensitivity was assessed from the spontaneous changes in beat-by-beat diastolic pressure and corresponding muscle sympathetic nerve activity (microneurography) during supine rest in 30 men [69±1 (mean±SEM) years] and 31 women (68±1 years). Carotid artery stiffness (B-mode ultrasonography) and aortic stiffness (magnetic resonance imaging) were also determined. We found that elderly women had lower sympathetic baroreflex sensitivity than elderly men (–2.33±0.25 vs. –3.32±0.25 bursts·100 beats–1·mmHg–1; P=0.007). β-stiffness indices of the carotid artery and the aorta were greater in elderly women than in men (6.68±0.48 vs. 5.10±0.50 and 4.03±0.47 vs. 2.68±0.42; both P<0.050). Sympathetic baroreflex sensitivity was inversely correlated with carotid artery stiffness in both men and women (r=0.49 and 0.50, both P<0.05), while this relation was shifted in parallel upward (towards a reduced sensitivity) in women with no changes in the slope (0.26 vs. 0.24 a.u.). Sympathetic baroreflex sensitivity and aortic stiffness showed similar trends. Thus, barosensory artery stiffness seems to be one independent determinant of sympathetic baroreflex sensitivity in elderly men and women. The lower sympathetic baroreflex sensitivity in elderly women may predispose them to an increased prevalence of hypertension.
baroreceptors; β-stiffness; muscle sympathetic nerve activity; aging; sex differences