Mechanisms involved in the blunted nitric oxide-cGMP pathway in hypertensive TGR(mREN2)27 rats

Genetic modification of hypertension by sGCα1

Hypertension is an important modifiable risk factor for coronary heart disease, congestive heart failure, stroke, end-stage renal disease, and peripheral vascular disease, but many of the molecular mechanisms and genetic factors underlying the development of the most common forms of human hypertension remain to be defined. Abundant evidence suggests that nitric oxide (NO) and one of its primary targets, the cyclic guanosine monophosphate (cGMP)-generating enzyme soluble guanylate cyclase (sGC), have a critical role in regulating blood pressure. The availability of murine models of hypertension and the revolution in human genetics research (e.g., genome-wide association studies [GWAS]), resulting in the identification of dozens of genetic loci that affect normal variation in blood pressure and susceptibility to hypertension, provide a unique opportunity to dissect the mechanisms by which NO-cGMP signaling regulates blood pressure and to gain important insights into the pathogenesis of hypertension. In this review, we will give an overview of the current knowledge relating to the role of sGC in the regulation of blood pressure, discussing data obtained from genetically modified mouse models as well as from human genetic studies.

Soluble guanylyl cyclase is a target of angiotensin II-induced nitrosative stress in a hypertensive rat model

Nitric oxide (NO) by activating soluble guanylyl cyclase (sGC) is involved in vascular homeostasis via induction of smooth muscle relaxation. In cardiovascular diseases (CVDs), endothelial dysfunction with altered vascular reactivity is mostly attributed to decreased NO bioavailability via oxidative stress. However, in several studies, relaxation to NO is only partially restored by exogenous NO donors, suggesting sGC impairment. Conflicting results have been reported regarding the nature of this impairment, ranging from decreased expression of one or both subunits of sGC to heme oxidation. We showed that sGC activity is impaired by thiol S-nitrosation. Recently, angiotensin II (ANG II) chronic treatment, which induces hypertension, was shown to generate nitrosative stress in addition to oxidative stress. We hypothesized that S-nitrosation of sGC occurs in ANG II-induced hypertension, thereby leading to desensitization of sGC to NO hence vascular dysfunction. As expected, ANG II infusion increases blood pressure, aorta remodeling, and protein S-nitrosation. Intravital microscopy indicated that cremaster arterioles are resistant to NO-induced vasodilation in vivo in anesthetized ANG II-treated rats. Concomitantly, NO-induced cGMP production decreases, which correlated with S-nitrosation of sGC in hypertensive rats. This study suggests that S-nitrosation of sGC by ANG II contributes to vascular dysfunction. This was confirmed in vitro by using A7r5 smooth muscle cells infected with adenoviruses expressing sGC or cysteine mutants: ANG II decreases NO-stimulated activity in the wild-type but not in one mutant, C516A. This result indicates that cysteine 516 of sGC mediates ANG II-induced desensitization to NO in cells.

Genetic modifiers of hypertension in soluble guanylate cyclase α1-deficient mice

Nitric oxide (NO) plays an essential role in regulating hypertension and blood flow by inducing relaxation of vascular smooth muscle. Male mice deficient in a NO receptor component, the α1 subunit of soluble guanylate cyclase (sGCα1), are prone to hypertension in some, but not all, mouse strains, suggesting that additional genetic factors contribute to the onset of hypertension. Using linkage analyses, we discovered a quantitative trait locus (QTL) on chromosome 1 that was linked to mean arterial pressure (MAP) in the context of sGCα1 deficiency. This region is syntenic with previously identified blood pressure-related QTLs in the human and rat genome and contains the genes coding for renin. Hypertension was associated with increased activity of the renin-angiotensin-aldosterone system (RAAS). Further, we found that RAAS inhibition normalized MAP and improved endothelium-dependent vasorelaxation in sGCα1-deficient mice. These data identify the RAAS as a blood pressure-modifying mechanism in a setting of impaired NO/cGMP signaling.

Inactivation of soluble guanylate cyclase by stoichiometric S-nitrosation

Dysfunction of vascular nitric oxide (NO)/cGMP signaling is believed to contribute essentially to various cardiovascular disorders. Besides synthesis and/or bioavailability of endothelial NO, impaired function of soluble guanylate cyclase (sGC) may play a key role in vascular dysfunction. Based on the proposal that desensitization of sGC through S-nitrosation contributes to vascular NO resistance ( Proc Natl Acad Sci U S A 104: 12312-12317, 2007 ), we exposed purified sGC to dinitrosyl iron complexes (DNICs), known as potent nitrosating agents. In the presence of 2 mM GSH, DNICs stimulated cGMP formation with EC(50) values of 0.1 to 0.5 microM and with an efficacy of 70 to 80% of maximal activity measured with 10 microM 2,2-diethyl-1-nitroso-oxyhydrazine (DEA/NO). In the absence of GSH, the efficacy of DNICs was markedly reduced, and sGC stimulation was counteracted by the inhibition of both basal and DEA/NO-stimulated cGMP formation at higher DNIC concentrations. Inactivation of sGC was slowly reversed in the presence of 2 mM GSH and associated with stoichiometric S-nitrosation of the protein (2.05 +/- 0.18 mol S-nitrosothiol per mol of 143-kDa heterodimer). S-Nitrosoglutathione and sodium nitroprusside caused partial inhibition of DEA/NO-stimulated sGC that was prevented by GSH, whereas nitroglycerin (0.3 mM) had no effect. Our findings indicate that nitrosation of two cysteine residues in sGC heterodimers results in enzyme inactivation. Protection by physiologically relevant concentrations of GSH (10 microM to 3 mM) suggests that S-nitrosation of sGC may contribute to vascular dysfunction in inflammatory disorders associated with nitrosative and oxidative stress and GSH depletion.

Roles of Nitric Oxide and Oxidative Stress in the Regulation of Blood Pressure and Renal Function in Prehypertensive Ren-2 Transgenic Rats

AIMS

The present study was performed to evaluate the role of nitric oxide (NO) and its interaction with superoxide anion (O2-) in the regulation of blood pressure (BP) and renal function during the developmental phase of hypertension in Ren-2 transgenic rats (TGR). The first aim was to compare BP and renal functional responses to acute NO synthase (NOS) inhibition achieved by intravenous (i.v.) infusion of Nomega-nitro-L-arginine-methyl ester (L-NAME) in prehypertensive heterozygous TGR and in transgene-negative Hannover Sprague-Dawley (HanSD) rats. The second aim was to evaluate whether scavenging of O2- by infusion of the superoxide dismutase mimetic tempol increases NO bioavailability which therefore should augment BP and renal functional responses to L-NAME.

METHODS

Rats were anesthetized, prepared for clearance experiments and BP and renal functional responses were evaluated in response to i.v. L-NAME administration (20 microg.100 g(-1).min(-1)) without or with tempol pretreatment (i.v., 300 microg.100 g(-1).min(-1)). In renal cortical tissue, nitrotyrosine protein expression was assessed by immunoblotting as marker of O2- production and urinary 8-epi-PGF(2alpha) excretion as marker of intrarenal oxidative stress was assessed by enzyme immunoassay.

RESULTS

BP, glomerular filtration rate (GFR), renal plasma flow (RPF) and sodium excretion were similar in TGR and HanSD. L-NAME infusion induced greater increases in BP in TGR than in HanSD (+42 +/- 4 vs. +25 +/- 3 mmHg, p < 0.05). In the absence of a significant change in GFR, L-NAME caused similar decreases in RPF (-32 +/- 6 and -25 +/- 4%, p < 0.05) in TGR and HanSD. Despite significantly higher renocortical expression of nitrotyrosine and urinary 8-epi-PGF2alpha excretion in TGR than in HanSD, pretreatment with tempol did not augment the rise in BP and the decrease in RPF induced by L-NAME.

CONCLUSIONS

The greater BP response to L-NAME in TGR suggests that prehypertensive TGR exhibit an enhanced NO activity in the systemic vasculature as compared with HanSD. Despite increased intrarenal oxidative stress in TGR, the dependency of the intrarenal vascular tone on NO appears to be similar in TGR and HanSD. The lack of a compensatory increase in renal NO activity may partially account for the enhanced renal vascular response to ANG II present in TGR.

Acute Effects of Cyclooxygenase-2 Inhibition on Renal Function in Heterozygous Ren-2-Transgenic Rats on Normal or Low Sodium Intake

BACKGROUND/AIMS

Since there are no data available so far on the role of renal cyclooxygenase-2 (COX-2) in hypertensive Ren-2-transgenic rats (TGR), in the present study we evaluated renal cortical COX-2 protein expression and prostaglandin E2 (PGE2) concentrations as well as renal functional responses to acute COX-2 inhibition in male heterozygous TGR and in normotensive Hannover Sprague-Dawley (HanSD) rats fed either a normal-sodium (NS) or a low-sodium (LS) diet.

METHODS

In rats fed either the NS or the LS diet for 12 days and prepared for clearance experiments with left ureteral catheterization, the renal functional responses of the left kidney were evaluated after intrarenal COX-2 inhibition with DuP-697 or NS-398. In renal cortical tissue, COX-2 protein expression was assessed by immunoblotting, and the concentration of PGE2 as a marker of COX-2 activity was determined by enzyme immunoassay. Mean arterial pressure in the right femoral artery was monitored by means of a pressure transducer.

RESULTS

In heterozygous TGR, to our surprise, the LS diet normalized the mean arterial pressure. Despite significantly higher renocortical expression of COX-2 and PGE2 concentrations as well as urinary PGE2 excretion in TGR as compared with HanSD rats kept on the NS diet, selective intrarenal COX-2 inhibition did not influence renal function either in TGR or in HanSD rats. The LS diet increased renocortical COX-2 expression and PGE2 concentrations as well as urinary PGE2 excretion significantly stronger in TGR than in HanSD rats. Regardless of these increases, the intrarenal COX-2 inhibition caused comparable decreases in glomerular filtration rate, in absolute and fractional sodium excretion, as well as in urinary PGE2 excretion in TGR and HanSD rats kept on the LS diet.

CONCLUSIONS

The present data show that a LS diet normalizes the mean arterial pressure in heterozygous male TGR. This first study on the role of renal COX-2 in TGR also demonstrates that COX-2-derived vasodilatory prostanoids do not act as renal compensatory vasodilator and natriuretic substances in this model of hypertension.

Alternative splicing of cGMP-dependent protein kinase I in angiotensin-hypertension: novel mechanism for nitrate tolerance in vascular smooth muscle

Nitrate tolerance (NT) in hypertension is attributed to reduced activity of soluble guanylyl cyclase (sGC). We examined NT in basilar artery vascular smooth muscle cells (VSMCs) from control rats, rats infused with angiotensin II (Ang; 240 microg/kg per hour for 4 days), which were normotensive, and Ang-hypertensive rats (AHR; 240 microg/kg per hour for 28 days). Ca2+-activated K+ (Maxi-K) channels in VSMCs from AHR showed reduced activation by NO donor, consistent with NT. The concentration-response relationship for 8-Br-cGMP was shifted 2.5-fold to the right, indicating that abnormal sGC alone could not account for NT. Inside-out patches from AHR showed normal activation with exogenous cGMP-dependent protein kinase I (cGKI), suggesting no abnormality downstream of cGKI. We hypothesized that the reduction in apparent affinity of 8-Br-cGMP for cGKI in AHR might be due to a change in relative amounts of cGKIalpha versus cGKIbeta, since cGKIbeta is less sensitive to cGMP activators than cGKIalpha. This was substantiated by showing the following in AHR: (1) reduced effect of the cGKIalpha-selective activator 8-APT-cGMP; (2) reduced total cGKI protein (both isoforms), but an increase in cGKIbeta protein in quantitative immunofluorescence and Western blots; (3) similar changes in cGKI isoforms immunoisolated with Maxi-K channels; and (4) a large increase in cGKIbeta mRNA and a decrease in cGKIalpha mRNA in real-time PCR and Northern blots. Upregulation of cytosolic cGKIbeta was evident 4 days after Ang infusion, before development of hypertension. Our data identify a functional role for cGKIbeta in VSMCs previously ascribed exclusively to cGKIalpha. Ang-induced alternative splicing of cGKI represents a novel mechanism for reducing sensitivity to NO/cGMP.

Age-related alterations in soluble guanylyl cyclase and cGMP pathway in spontaneously hypertensive rats

BACKGROUND

Vascular contractility and blood pressure (BP) are regulated by soluble guanylyl cyclase (sGC) and cyclic guanosine monophosphate (cGMP) pathway, which can be influenced by heme oxygenase (HO)-derived carbon monoxide (CO). The age-related changes in sGC/cGMP pathway in tail artery smooth muscle cells (SMCs) in hypertension have not been systematically investigated.

METHODS

In the present study, spontaneously hypertensive rats (SHR) of 4, 8, and 20 weeks old were used. The basal and hemin-modulated levels of sGC and cGMP in tail artery tissues were examined.

RESULTS

Although BP of 20-week SHR was significantly elevated, sGC and cGMP levels were unaltered compared with age-matched Wistar-Kyoto rats (WKY). The levels of sGC and cGMP were significantly lower in 4- and 8-week SHR compared with age-matched WKY although BP of 4-week SHR was normotensive. Hemin administration resulted in a significant decrease in BP in 8-week (158.7 +/- 2.4 versus 123.5 +/- 1.3 mmHg, P < 0.01), but not in pre-hypertensive (4 weeks) or 20-week SHR or WKY at all ages. Coincidently, sGC and cGMP levels in 8-week SHRs were significantly elevated and so did the expression levels of HO-1. Hemin treatment did not increase the cyclic adenosine monophosphate (cAMP) content of tail artery from 8-week SHR. The constitutive HO-2 levels remained unchanged in 8- and 20-week SHR and age-matched WKY.

CONCLUSION

The HO-activity inhibitor, chromium mesoporphyrin, abolished the BP-lowering and HO- stimulating effects of hemin in young SHR. Our results suggest that alteration in sGC/cGMP pathway in vascular SMCs precedes the occurrence of hypertension but returns to normal once hypertension is fully manifested.