Angiotensin II modulates renal sympathetic neurotransmission through nitric oxide in AT2 receptor knockout mice

Renal Sympathetic Nerve-Derived Signaling in Acute and Chronic kidney Diseases

The kidney is innervated by afferent sensory and efferent sympathetic nerve fibers. Norepinephrine (NE) is the primary neurotransmitter for post-ganglionic sympathetic adrenergic nerves, and its signaling, regulated through adrenergic receptors (AR), modulates renal function and pathophysiology under disease conditions. Renal sympathetic overactivity and increased NE level are commonly seen in chronic kidney disease (CKD) and are critical factors in the progression of renal disease. Blockade of sympathetic nerve-derived signaling by renal denervation or AR blockade in clinical and experimental studies demonstrates that renal nerves and its downstream signaling contribute to progression of acute kidney injury (AKI) to CKD and fibrogenesis. This review summarizes our current knowledge of the role of renal sympathetic nerve and adrenergic receptors in AKI, AKI to CKD transition and CKDand provides new insights into the therapeutic potential of intervening in its signaling pathways.

α2A-Adrenoceptors Modulate Renal Sympathetic Neurotransmission and Protect against Hypertensive Kidney Disease

BACKGROUND

Increased nerve activity causes hypertension and kidney disease. Recent studies suggest that renal denervation reduces BP in patients with hypertension. Renal NE release is regulated by prejunctional α2A-adrenoceptors on sympathetic nerves, and α2A-adrenoceptors act as autoreceptors by binding endogenous NE to inhibit its own release. However, the role of α2A-adrenoceptors in the pathogenesis of hypertensive kidney disease is unknown.

METHODS

We investigated effects of α2A-adrenoceptor-regulated renal NE release on the development of angiotensin II-dependent hypertension and kidney disease. In uninephrectomized wild-type and α2A-adrenoceptor-knockout mice, we induced hypertensive kidney disease by infusing AngII for 28 days.

RESULTS

Urinary NE excretion and BP did not differ between normotensive α2A-adrenoceptor-knockout mice and wild-type mice at baseline. However, NE excretion increased during AngII treatment, with the knockout mice displaying NE levels that were significantly higher than those of wild-type mice. Accordingly, the α2A-adrenoceptor-knockout mice exhibited a systolic BP increase, which was about 40 mm Hg higher than that found in wild-type mice, and more extensive kidney damage. In isolated kidneys, AngII-enhanced renal nerve stimulation induced NE release and pressor responses to a greater extent in kidneys from α2A-adrenoceptor-knockout mice. Activation of specific sodium transporters accompanied the exaggerated hypertensive BP response in α2A-adrenoceptor-deficient kidneys. These effects depend on renal nerves, as demonstrated by reduced severity of AngII-mediated hypertension and improved kidney function observed in α2A-adrenoceptor-knockout mice after renal denervation.

CONCLUSIONS

Our findings reveal a protective role of prejunctional inhibitory α2A-adrenoceptors in pathophysiologic conditions with an activated renin-angiotensin system, such as hypertensive kidney disease, and support the concept of sympatholytic therapy as a treatment.

A Clinically Relevant Functional Model of Type-2 Cardio-Renal Syndrome with Paraventricular Changes consequent to Chronic Ischaemic Heart Failure

Cardiorenal syndrome, de novo renal pathology arising secondary to cardiac insufficiency, is clinically recognised but poorly characterised. This study establishes and characterises a valid model representative of Type 2 cardiorenal syndrome. Extensive permanent left ventricular infarction, induced by ligation of the left anterior descending coronary artery in Lewis rats, was confirmed by plasma cardiac troponin I, histology and cardiac haemodynamics. Renal function and morphology was assessed 90-days post-ligation when heart failure had developed. The involvement of the paraventricular nucleus was investigated using markers of inflammation, apoptosis, reactive oxygen species and of angiotensin II involvement. An extensive left ventricular infarct was confirmed following coronary artery ligation, resulting in increased left ventricular weight and compromised left ventricular diastolic function and developed pressure. Glomerular filtration was significantly decreased, fractional excretion of sodium and caspase activities were increased and basement membrane thickening, indicating glomerulosclerosis, was evident. Interestingly, angiotensin II receptor I expression and reactive oxygen species levels in the hypothalamic paraventricular nucleus remained significantly increased at 90-days post-coronary artery ligation, suggesting that these hypothalamic changes may represent a novel, valuable pharmacological target. This model provides conclusive morphological, biochemical and functional evidence of renal injury consequent to heart failure, truly representative of Type-2 cardiorenal syndrome.

Renal sympathetic nerve activation via α2-adrenergic receptors in chronic kidney disease progression

Chronic kidney disease (CKD) is increasing worldwide without an effective therapeutic strategy. Sympathetic nerve activation is implicated in CKD progression, as well as cardiovascular dysfunction. Renal denervation is beneficial for controlling blood pressure (BP) and improving renal function through reduction of sympathetic nerve activity in patients with resistant hypertension and CKD. Sympathetic neurotransmitter norepinephrine (NE) via adrenergic receptor (AR) signaling has been implicated in tissue homeostasis and various disease progressions, including CKD. Increased plasma NE level is a predictor of survival and the incidence of cardiovascular events in patients with end-stage renal disease, as well as future renal injury in subjects with normal BP and renal function. Our recent data demonstrate that NE derived from renal nerves causes renal inflammation and fibrosis progression through alpha-2 adrenergic receptors (α₂-AR) in renal fibrosis models independent of BP. Sympathetic nerve activation-associated molecular mechanisms and signals seem to be critical for the development and progression of CKD, but the exact role of sympathetic nerve activation in CKD progression remains undefined. This review explores the current knowledge of NE-α₂-AR signaling in renal diseases and offers prospective views on developing therapeutic strategies targeting NE-AR signaling in CKD progression.

Molecular Basis of the Brain Renin Angiotensin System in Cardiovascular and Neurologic Disorders: Uncovering a Key Role for the Astroglial Angiotensin Type 1 Receptor AT1R

The central renin angiotensin system (RAS) is one of the most widely investigated cardiovascular systems in the brain. It is implicated in a myriad of cardiovascular diseases. However, studies from the last decade have identified its involvement in several neurologic abnormalities. Understanding the molecular functionality of the various RAS components can thus provide considerable insight into the phenotypic differences and mechanistic drivers of not just cardiovascular but also neurologic disorders. Since activation of one of its primary receptors, the angiotensin type 1 receptor (AT1R), results in an augmentation of oxidative stress and inflammatory cytokines, it becomes essential to investigate not just neuronal RAS but glial RAS as well. Glial cells are key homeostatic regulators in the brain and are critical players in the resolution of overt oxidative stress and neuroinflammation. Designing better and effective therapeutic strategies that target the brain RAS could well hinge on understanding the molecular basis of both neuronal and glial RAS. This review provides a comprehensive overview of the major studies that have investigated the mechanisms and regulation of the brain RAS, and it also provides insight into the potential role of glial AT1Rs in the pathophysiology of cardiovascular and neurologic disorders.

Angiotensin II AT2 Receptors Contribute to Regulate the Sympathoadrenal and Hormonal Reaction to Stress Stimuli

Angiotensin II, through AT1 receptor stimulation, mediates multiple cardiovascular, metabolic, and behavioral functions including the response to stressors. Conversely, the function of Angiotensin II AT2 receptors has not been totally clarified. In adult rodents, AT2 receptor distribution is very limited but it is particularly high in the adrenal medulla. Recent results strongly indicate that AT2 receptors contribute to the regulation of the response to stress stimuli. This occurs in association with AT1 receptors, both receptor types reciprocally influencing their expression and therefore their function. AT2 receptors appear to influence the response to many types of stressors and in all components of the hypothalamic-pituitary-adrenal axis. The molecular mechanisms involved in AT2 receptor activation, the complex interactions with AT1 receptors, and additional factors participating in the control of AT2 receptor regulation and activity in response to stressors are only partially understood. Further research is necessary to close this knowledge gap and to clarify whether AT2 receptor activation may carry the potential of a major translational advance.

Phosphodiesterase 5 inhibition ameliorates angiotensin II-dependent hypertension and renal vascular dysfunction

Changes in renal hemodynamics have a major impact on blood pressure (BP). Angiotensin (Ang) II has been shown to induce vascular dysfunction by interacting with phosphodiesterase (PDE)1 and PDE5. The predominant PDE isoform responsible for renal vascular dysfunction in hypertension is unknown. Here, we measured the effects of PDE5 (sildenafil) or PDE1 (vinpocetine) inhibition on renal blood flow (RBF), BP, and renal vascular function in normotensive and hypertensive mice. During acute short-term Ang II infusion, sildenafil decreased BP and increased RBF in C57BL/6 (WT) mice. In contrast, vinpocetine showed no effect on RBF and BP. Additionally, renal cGMP levels were significantly increased after acute sildenafil but not after vinpocetine infusion, indicating a predominant role of PDE5 in renal vasculature. Furthermore, chronic Ang II infusion (500 ng·kg^-1·min^-1) increased BP and led to impaired NO-dependent vasodilation in kidneys of WT mice. Additional treatment with sildenafil (100 mg·kg^-1·day^-1) attenuated Ang II-dependent hypertension and improved NO-mediated vasodilation. During chronic Ang II infusion, urinary nitrite excretion, a marker for renal NO generation, was increased in WT mice, whereas renal cGMP generation was decreased and restored after sildenafil treatment, suggesting a preserved cGMP signaling after PDE5 inhibition. To investigate the dependency of PDE5 effects on NO/cGMP signaling, we next analyzed eNOS-KO mice, a mouse model characterized by low vascular NO/cGMP levels. In eNOS-KO mice, chronic Ang II infusion increased BP but did not impair NO-mediated vasodilation. Moreover, sildenafil did not influence BP or vascular function in eNOS-KO mice. These results highlight PDE5 as a key regulator of renal hemodynamics in hypertension.

Chronic p38 mitogen-activated protein kinase inhibition improves vascular function and remodeling in angiotensin II-dependent hypertension

Introduction:

An excess of angiotensin II (Ang II) causes hypertension and vascular injury. Activation of mitogen-activated protein kinase p38 (p38-MAPK) plays a substantial role in Ang II-dependent organ damage. Recently, we showed that p38-MAPK activation regulates the pressor response to Ang II. This study evaluates the effect of chronic p38-MAPK inhibition in Ang II-dependent hypertension.

Materials and methods:

C57Bl/6J mice were infused with Ang II for 14 days and either treated with the p38-MAPK inhibitor BIRB796 (50 mg/kg/day) or the vehicle as the control. We assessed vascular function in the aorta and isolated perfused kidneys.

Results:

Chronic p38-MAPK inhibition did not alter blood pressure at the baseline, but attenuated Ang II-induced hypertension significantly (baseline: 122 ± 2 versus 119 ± 4 mmHg; Ang II: 173 ± 3 versus 155 ± 3 mmHg; p < 0.001). In addition, BIRB796 treatment improved vascular remodeling by reducing the aortic media-to-lumen ratio and decreasing the expression of the membrane metalloproteinases (MMP) MMP-1 and MMP-9. Moreover, renal vascular dysfunction induced by chronic Ang II infusion was significantly ameliorated in the BIRP796-treated mice. Acute p38-MAPK inhibition also improved vascular function in the aorta and kidneys of Ang II-treated mice, highlighting the important role of p38-MAPK activation in the pathogenesis of vascular dysfunction.

Conclusions:

Our findings indicated there is an important role for p38-MAPK in regulating blood pressure and vascular injury, and highlighted its potential as a pharmaceutical target.

Angiotensin-(1–7) Modulates Renal Vascular Resistance Through Inhibition of p38 Mitogen-Activated Protein Kinase in Apolipoprotein E–Deficient Mice

Apolipoprotein E–deficient (apoE(−/−)) mice fed on Western diet are characterized by increased vascular resistance and atherosclerosis. Previously, we have shown that chronic angiotensin (Ang)-(1–7) treatment ameliorates endothelial dysfunction in apoE(−/−) mice. However, the mechanism of Ang-(1–7) on vasoconstrictor response to Ang II is unknown. To examine Ang-(1–7) function, we used apoE(−/−) and wild-type mice fed on Western diet that were treated via osmotic minipumps either with Ang-(1–7) (82 μg/kg per hour) or saline for 6 weeks. We show that Ang II–induced renal pressor response was significantly increased in apoE(−/−) compared with wild-type mice. This apoE(−/−)-specific response is attributed to reactive oxygen species–mediated p38 mitogen–activated protein kinase activation and subsequent phosphorylation of myosin light chain (MLC 20 ), causing renal vasoconstriction. Here, we provide evidence that chronic Ang-(1–7) treatment attenuated the renal pressor response to Ang II in apoE(−/−) mice to wild-type levels. Ang-(1–7) treatment significantly decreased renal inducible nicotinamide adenine dinucleotide phosphate subunit p47phox levels and, thus, reactive oxygen species production that in turn causes decreased p38 mitogen-activated protein kinase activity. The latter has been confirmed by administration of a specific p38 mitogen-activated protein kinase inhibitor SB203580 (5 μmol/L), causing a reduced renal pressor response to Ang II in apoE(−/−) but not in apoE(−/−) mice treated with Ang-(1–7). Moreover, Ang-(1–7) treatment had no effect in Mas(−/−)/apoE(−/−) double-knockout mice confirming the specificity of Ang-(1–7) action through the Mas-receptor. In summary, Ang-(1–7) modulates vascular function via Mas-receptor activation that attenuates pressor response to Ang II in apoE(−/−) mice by reducing reactive oxygen species–mediated p38 mitogen-activated protein kinase activity.

Phosphodiesterase 5 attenuates the vasodilatory response in renovascular hypertension

NO/cGMP signaling plays an important role in vascular relaxation and regulation of blood pressure. The key enzyme in the cascade, the NO-stimulated cGMP-forming guanylyl cyclase exists in two enzymatically indistinguishable isoforms (NO-GC1, NO-GC2) with NO-GC1 being the major NO-GC in the vasculature. Here, we studied the NO/cGMP pathway in renal resistance arteries of NO-GC1 KO mice and its role in renovascular hypertension induced by the 2-kidney-1-clip-operation (2K1C). In the NO-GC1 KOs, relaxation of renal vasculature as determined in isolated perfused kidneys was reduced in accordance with the marked reduction of cGMP-forming activity (80%). Noteworthy, increased eNOS-catalyzed NO formation was detected in kidneys of NO-GC1 KOs. Upon the 2K1C operation, NO-GC1 KO mice developed hypertension but the increase in blood pressures was not any higher than in WT. Conversely, operated WT mice showed a reduction of cGMP-dependent relaxation of renal vessels, which was not found in the NO-GC1 KOs. The reduced relaxation in operated WT mice was restored by sildenafil indicating that enhanced PDE5-catalyzed cGMP degradation most likely accounts for the attenuated vascular responsiveness. PDE5 activation depends on allosteric binding of cGMP. Because cGMP levels are lower, the 2K1C-induced vascular changes do not occur in the NO-GC1 KOs. In support of a higher PDE5 activity, sildenafil reduced blood pressure more efficiently in operated WT than NO-GC1 KO mice. All together our data suggest that within renovascular hypertension, cGMP-based PDE5 activation terminates NO/cGMP signaling thereby providing a new molecular basis for further pharmacological interventions.

Chronic treatment with angiotensin-(1-7) improves renal endothelial dysfunction in apolipoproteinE-deficient mice

BACKGROUND AND PURPOSE

ApolipoproteinE-deficient [apoE (-/-)] mice, a model of human atherosclerosis, develop endothelial dysfunction caused by decreased levels of nitric oxide (NO). The endogenous peptide, angiotensin-(1-7) [Ang-(1-7)], acting through its specific GPCR, the Mas receptor, has endothelium-dependent vasodilator properties. Here we have investigated if chronic treatment with Ang-(1-7) improved endothelial dysfunction in apoE (-/-) mice.

EXPERIMENTAL APPROACH

ApoE (-/-) mice fed on a lipid-rich Western diet were divided into three groups and treated via osmotic minipumps with either saline, Ang-(1-7) (82 µg·kg(-1) ·h(-1) ) or the same dose of Ang-(1-7) together with D-Ala-Ang-(1-7) (125 µg·kg(-1) ·h(-1) ) for 6 weeks. Renal vascular function was assessed in isolated perfused kidneys.

KEY RESULTS

Ang-(1-7)-treated apoE (-/-) mice showed improved renal endothelium-dependent vasorelaxation induced by carbachol and increased renal basal cGMP production, compared with untreated apoE (-/-) mice. Tempol, a reactive oxygen species (ROS) scavenger, improved endothelium-dependent vasorelaxation in kidneys of saline-treated apoE (-/-) mice whereas no effect was observed in Ang-(1-7)-treated mice. Chronic treatment with D-Ala-Ang-(1-7), a specific Mas receptor antagonist, abolished the beneficial effects of Ang-(1-7) on endothelium-dependent vasorelaxation. Renal endothelium-independent vasorelaxation showed no differences between treated and untreated mice. ROS production and expression levels of the NAD(P)H oxidase subunits gp91phox and p47phox were reduced in isolated preglomerular arterioles of Ang-(1-7)-treated mice, compared with untreated mice, whereas eNOS expression was increased.

CONCLUSION AND IMPLICATIONS

Chronic infusion of Ang-(1-7) improved renal endothelial function via Mas receptors, in an experimental model of human cardiovascular disease, by increasing levels of endogenous NO.

Regulation of renal sympathetic neurotransmission by renal α(2A)-adrenoceptors is impaired in chronic renal failure

BACKGROUND AND PURPOSE

The mechanisms underlying increased renal noradrenaline in renal failure are still unclear. In this study, the role of α(2A)-adrenoceptors in controlling sympathetic neurotransmission in chronic renal failure was evaluated in a subtotal nephrectomy model. Also, the influence of this receptor subtype on angiotensin II (Ang II)-mediated noradrenaline release was evaluated.

EXPERIMENTAL APPROACH

α(2A)-adrenoceptor-knockout (KO) and wild-type (WT) mice underwent subtotal (5/6) nephrectomy (SNx) or SHAM-operation (SHAM). Kidneys of WT and KO mice were isolated and perfused. Renal nerves were stimulated with platinum electrodes and noradrenaline release was measured by HPLC.

KEY RESULTS

Noradrenaline release induced by renal nerve stimulation (RNS) was significantly increased in WT mice after SNx. RNS-induced noradrenaline release was significantly higher in SHAM-KO compared with SHAM-WT, but no further increase in noradrenaline release could be observed in SNx-KO. α-adrenoceptor antagonists increased RNS-induced noradrenaline release in SHAM-WT but not in SHAM-KO. After SNx, the effect of α₂-adrenoceptor blockade on renal noradrenaline release was attenuated in WT mice. The mRNA expression of α(2A)-adrenoceptors was not altered, but the inhibitory effect of α₂-adrenoceptor agonists on cAMP formation was abolished after SNx. Ang II facilitated RNS-induced noradrenaline release in SHAM-WT but not in SHAM-KO and SNx-WT.

CONCLUSION AND IMPLICATIONS

In our model of renal failure autoregulation of renal sympathetic neurotransmission was impaired. Presynaptic inhibition of noradrenaline release was diminished and the facilitatory effect of presynaptic angiotensin AT₁ receptors on noradrenaline release was markedly decreased in renal failure and depended on functioning α(2A)-adrenoceptors.

RGS4, a GTPase activator, improves renal function in ischemia-reperfusion injury

Acute kidney dysfunction after ischemia-reperfusion injury (IRI) may be a consequence of persistent intrarenal vasoconstriction. Regulators of G-protein signaling (RGSs) are GTPase activators of heterotrimeric G proteins that can regulate vascular tone. RGS4 is expressed in vascular smooth muscle cells in the kidney; however, its protein levels are low in many tissues due to N-end rule-mediated polyubiquitination and proteasomal degradation. Here, we define the role of RGS4 using a mouse model of IRI comparing wild-type (WT) with RGS4-knockout mice. These knockout mice were highly sensitized to the development of renal dysfunction following injury exhibiting reduced renal blood flow as measured by laser-Doppler flowmetry. The kidneys from knockout mice had increased renal vasoconstriction in response to endothelin-1 infusion ex vivo. The intrinsic renal activity of RGS4 was measured following syngeneic kidney transplantation, a model of cold renal IRI. The kidneys transplanted between knockout and WT mice had significantly reduced reperfusion blood flow and increased renal cell death. WT mice administered MG-132 (a proteasomal inhibitor of the N-end rule pathway) resulted in increased renal RGS4 protein and in an inhibition of renal dysfunction after IRI in WT but not in knockout mice. Thus, RGS4 antagonizes the development of renal dysfunction in response to IRI.

AT2 receptor signaling and sympathetic regulation

There is a growing consensus that the balance between Angiotensin Type 1 (AT1R) and Angiotensin Type 2 (AT2R) signaling in many tissues may determine the magnitude and, in some cases the direction, of the biological response. Sympatho-excitation in cardiovascular diseases is mediated by a variety of factors and is, in part, dependent on Angiotensin II signaling in the central nervous system. Recent data have provided evidence that the AT2R can modulate sympatho-excitation in animals with hypertension and heart failure. The evidence for this concept is reviewed and a model is put forward to support the rationale that therapeutic targeting of the central AT2R may be beneficial in the setting of chronic heart failure.

Sympathetic overactivity--the Cinderella of cardiovascular risk factors in dialysis patients

Cardiovascular morbidity and mortality is exceedingly high in patients with chronic renal failure. Sympathetic overactivity is an important pathomechanism contributing to progression of renal disease as well as cardiovascular complications. For more than 30 years it has been known that plasma levels of norepinephrine are elevated in chronic renal failure pointing to increased sympathetic nerve activity. The kidneys are richly innervated by efferent sympathetic and afferent sensory nerves. They participate in many reflex adjustments of renal function. Initially, this finding had not been attributed to increased efferent sympathetic drive, but rather to reduced renal clearance and defective neuronal reuptake of norepinephrine. At this time, however, the evidence for increased sympathetic drive is solid. Interventions to reduce sympathetic overactivity will provide new therapeutic approaches. The available experimental and clinical evidence to suggest such a pathophysiological role of sympathetic overactivity is summarized in this current review.

Nitric oxide and cardiovascular and renal effects

Nitric oxide (NO) has multiple protective effects for regulating the cardiovascular and renal systems. The major functions include endothelium-dependent relaxation, anti-inflammatory effects, as well as antihypertrophic and antithrombotic activities. Many of the activities mediated by NO are systematically antagonized by angiotensin-II (Ang II), a vasconstrictor peptide. Studies described in the review below have demonstrated that the balance between NO and Ang II activities rather than the absolute concentration of each molecule determines their effects on the physiology and pathophysiology of the cardiovascular and renal systems. NO donors have been used for years as therapeutic agents for a range of cardiovascular conditions including angina, myocardial infarction and for the reduction of arterial stiffness. An understanding of the mechanisms underlying the effects of these medications will enable the development of novel therapies to balance the effects of NO in the cardiovascular system.

Newly developed angiotensin II-infused experimental models in vascular biology

Angiotensin II is a major vasoactive peptide in the renin-angiotensin system (RAS). In vitro evidence demonstrates that this peptide can modulate the function of various adhesion molecules, chemokines, cytokines and growth factors, and ultimately contributes to cell proliferation, hypertrophy and inflammation. Moreover, in vivo studies further support that angiotensin II induces several vascular alterations including sustained elevations of blood pressure, enhanced inflammatory response, increased medial thickness of the aortas, and formation of aortic dissection and aneurysms. Thus, it has been a long time that angiotensin II-induced hypertension, atherosclerosis and abdominal aortic aneurysms emerge as important experimental models with respect to vascular biology. Applications of these models to investigate the vascular diseases have dramatically improved our understanding in the pathogenesis of these diseases. However, the pathophysiology of angiotensin II in vivo remains to be determined in many other vascular diseases where angiotensin II has been implicated as the detrimental factor, at least in part due to the limit availability of animal models. Recently some new exciting experimental models based on angiotensin II infusion have been reported to replicate the human diseases, such as postmenopausal hypertension, preeclampsia, vascular remodeling, vascular aging and neovascularization. In this review, we will focus on the rationales and anticipated applications of these newly developed models, with special emphasis placed on those relevant to the vascular biology. We will also discuss the limitations of the method of chronic angiotensin II infusion and additional approaches to overcome these limitations. These experimental models will provide great opportunity for us to investigate the molecular mechanisms of angiotensin II and evaluate therapeutic approaches, particularly to finely tune the potential role of RAS activation in various vascular events using genetically engineered mice.

Endothelial nitric oxide synthase is predominantly involved in angiotensin II modulation of renal vascular resistance and norepinephrine release

Nitric oxide (NO) is mainly generated by endothelial NO synthase (eNOS) or neuronal NOS (nNOS). Recent studies indicate that angiotensin II generates NO release, which modulates renal vascular resistance and sympathetic neurotransmission. Experiments in wild-type [eNOS(+/+) and nNOS(+/+)], eNOS-deficient [eNOS(-/-)], and nNOS-deficient [nNOS(-/-)] mice were performed to determine which NOS isoform is involved. Isolated mice kidneys were perfused with Krebs-Henseleit solution. Endogenous norepinephrine release was measured by HPLC. Angiotensin II dose dependently increased renal vascular resistance in all mice species. EC(50) and maximal pressor responses to angiotensin II were greater in eNOS(-/-) than in nNOS(-/-) and smaller in wild-type mice. The nonselective NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME; 0.3 mM) enhanced angiotensin II-induced pressor responses in nNOS(-/-) and wild-type mice but not in eNOS(-/-) mice. In nNOS(+/+) mice, 7-nitroindazole monosodium salt (7-NINA; 0.3 mM), a selective nNOS inhibitor, enhanced angiotensin II-induced pressor responses slightly. Angiotensin II-enhanced renal nerve stimulation induced norepinephrine release in all species. L-NAME (0.3 mM) reduced angiotensin II-mediated facilitation of norepinephrine release in nNOS(-/-) and wild-type mice but not in eNOS(-/-) mice. 7-NINA failed to modulate norepinephrine release in nNOS(+/+) mice. (4-Chlorophrnylthio)guanosine-3', 5'-cyclic monophosphate (0.1 nM) increased norepinephrine release. mRNA expression of eNOS, nNOS, and inducible NOS did not differ between mice strains. In conclusion, angiotensin II-mediated effects on renal vascular resistance and sympathetic neurotransmission are modulated by NO in mice. These effects are mediated by eNOS and nNOS, but NO derived from eNOS dominates. Only NO derived from eNOS seems to modulate angiotensin II-mediated renal norepinephrine release.

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.

Angiotensin II–nitric oxide interaction in the kidney

PURPOSE OF REVIEW

The balance of angiotensin II and nitric oxide determines the sensitivity of the tubuloglomerular feedback mechanism, renal vascular resistance and filtration rate. Angiotensin II induces nitric oxide release, but the role of angiotensin II receptors here is not fully understood. Further, the angiotensin II-nitric oxide interaction can be modulated by reactive oxygen species. This review focuses on the angiotensin II-nitric oxide interaction and their modulation by reactive oxygen species in the control of renal blood flow.

RECENT FINDINGS

Ideas about the role of angiotensin II type 1 and angiotensin II type 2 receptors are extended by the observation of angiotensin II type 1-mediated nitric oxide release with direct effects on vascular tone, tubuloglomerular feedback and sympathetic neurotransmission. Angiotensin receptors elicit disparate effects on intrarenal circulation. Angiotensin II-nitric oxide interactions are modulated by reactive oxygen species, as shown by angiotensin II type 1-mediated activation of superoxide and depression of antioxidant enzymes leading to reduced nitric oxide concentration - mechanisms that may be also important in angiotensin II-dependent hypertension.

SUMMARY

Recent studies show that angiotensin II stimulates the nitric oxide system via angiotensin II type 1 and angiotensin II type 2 receptors, whereas receptors exert different effects on renal and medullary flow. The interaction via angiotensin II type 1 is modulated by reactive oxygen species.

Alpha(2A)-adrenoceptors regulate sympathetic transmitter release in mice kidneys

BACKGROUND AND PURPOSE

In the present study, a rodent model was used to investigate whether the alpha(2A)-adrenoceptor (alpha(2A)) represents the presynaptic autoinhibitory receptor regulating sympathetic transmitter release in the kidney. Moreover, the potential role of alpha(2A) as a heteroceptor regulating adenosine triphosphate (ATP) release was tested.

EXPERIMENTAL APPROACH

Kidneys from wild-type (WT) and alpha(2A)-knockout (KO) mice were isolated and perfused. Renal nerves were stimulated with platinum-electrodes. Endogenously released noradrenaline (NA) was measured by HPLC. The perfusion pressure was monitored continuously.

KEY RESULTS

Renal nerve stimulation (RNS) induced a frequency (1,2,5,7.5,10,15 Hz)-dependent release of NA in WT mice (994+/-373, 2355+/-541, 6375+/-950, 11626+/-1818, 19138+/-2001 pg NA g(-1) kidney (means+/-s.e.m.)). There was a 2.7-fold (5 Hz) increase of NA release in alpha(2A)-KO mice. In WT animals alpha-adrenoceptor blockade by phentolamine increased RNS-induced NA release in a concentration-dependent manner up to 350% of control. No facilitation by phentolamine was observed in alpha(2A)-KO mice. Pressor responses to 1 Hz and 2 Hz were resistant to alpha(1)-adrenoceptor blockade (0.03 microM prazosin) but abolished by P(2) receptor blockade (5 microM PPADS). Blockade of alpha(2)-adrenoceptors (1 microM rauwolscine) increased these purinergic pressor responses to 296+/-112% (1 Hz) in WT but not in alpha(2A)-KO mice. Exogenous ATP (100 microM) increased basal but not RNS-induced NA release.

CONCLUSIONS AND IMPLICATIONS

alpha(2A)-Adrenoceptor-activation inhibits NA and ATP release from renal sympathetic nerves. Pressor responses to RNS at higher stimulation frequencies (>2 Hz) are mediated by NA. At lower frequencies neuronally released ATP seems to be the predominant transmitter mediating renovascular resistance.