Smooth muscle cell responsiveness to nitrovasodilators in hypertensive and normotensive rats

Regulation of soluble guanylyl cyclase redox state by hydrogen sulfide

Soluble guanylate cyclase (sGC) is a receptor for nitric oxide (NO). Binding of NO to ferrous (Fe(2+)) heme increases its catalytic activity, leading to the production of cGMP from GTP. Hydrogen sulfide (H2S) is a signaling molecule that exerts both direct and indirect anti-oxidant effects. In the present, study we aimed to determine whether H2S could regulate sGC redox state and affect its responsiveness to NO-releasing agents and sGC activators. Using cultured rat aortic smooth muscle cells, we observed that treatment with H2S augmented the response to the NO donor DEA/NO, while attenuating the response to the heme-independent activator BAY58-2667 that targets oxidized sGC. Similarly, overexpression of H2S-synthesizing enzyme cystathionine-γ lyase reduced the ability of BAY58-2667 to promote cGMP accumulation. In experiments with phenylephrine-constricted mouse aortic rings, treatment with rotenone (a compound that increases ROS production), caused a rightward shift of the DEA/NO concentration-response curve, an effect partially restored by H2S. When rings were pre-treated with H2S, the concentration-response curve to BAY 58-2667 shifted to the right. Using purified recombinant human sGC, we observed that treatment with H2S converted ferric to ferrous sGC enhancing NO-donor-stimulated sGC activity and reducing BAY 58-2667-triggered cGMP formation. The present study identified an additional mechanism of cross-talk between the NO and H2S pathways at the level of redox regulation of sGC. Our results provide evidence that H2S reduces sGC heme Fe, thus, facilitating NO-mediated cellular signaling events.

Phosphinodithioate and Phosphoramidodithioate Hydrogen Sulfide Donors

Hydrogen sulfide is rapidly emerging as a key physiological mediator and potential therapeutic tool in numerous areas such as acute and chronic inflammation, neurodegenerative and cardiovascular disease, diabetes, obesity and cancer. However, the vast majority of the published studies have employed crude sulfide salts such as sodium hydrosulfide (NaSH) and sodium sulfide (Na2S) as H2S "donors" to generate H2S. Although these salts are cheap, readily available and easy to use, H2S generated from them occurs as an instantaneous and pH-dependent dissociation, whereas endogenous H2S synthesis from the enzymes cystathionine γ-lyase, cystathionine-β-synthase and 3-mercaptopyruvate sulfurtransferase is a slow and sustained process. Furthermore, sulfide salts are frequently used at concentrations (e.g. 100 μM to 10 mM) far in excess of the levels of H2S reported in vivo (nM to low μM). For the therapeutic potential of H2S is to be properly harnessed, pharmacological agents which generate H2S in a physiological manner and deliver physiologically relevant concentrations are needed. The phosphorodithioate GYY4137 has been proposed as "slow-release" H2S donors and has shown promising efficacy in cellular and animal model diseases such as hypertension, sepsis, atherosclerosis, neonatal lung injury and cancer. However, H2S generation from GYY4137 is inefficient necessitating its use at high concentrations/doses. However, structural modification of the phosphorodithioate core has led to compounds (e.g. AP67 and AP105) with accelerated rates of H2S generation and enhanced biological activity. In this review, the therapeutic potential and limitations of GYY4137 and related phosphorodithioate derivatives are discussed.

Nitroxyl donors retain their depressor effects in hypertension

Nitroxyl (HNO), the redox congener of nitric oxide, has numerous vasoprotective actions including an ability to induce vasodilation and inhibit platelet aggregation. Given HNO is resistant to scavenging by superoxide and does not develop tolerance, we hypothesised that HNO would retain its in vivo vasodilatory action in the setting of hypertension. The in vitro and in vivo vasodilator properties of the HNO donors Angeli's salt (AS) and isopropylamine/NONOate (IPA/NO) were compared with the NO donor diethylamine/NONOate (DEA/NO) in spontaneously hypertensive rats (SHR) and normotensive [Wistar-Kyoto (WKY) rats]. AS (10, 50, and 200 μg/kg), IPA/NO (10, 50, and 200 μg/kg), and DEA/NO (1, 5, and 20 μg/kg) caused dose-dependent depressor responses in conscious WKY rats of similar magnitude. Depressor responses to AS and IPA/NO were significantly attenuated (P < 0.01) after infusion of the HNO scavenger N-acetyl-l-cysteine (NAC), confirming that AS and IPA/NO function as HNO donors in vivo. In contrast, responses to DEA/NO were unchanged following NAC infusion. Depressor responses to AS and IPA/NO in conscious SHR retained their sensitivity to the inhibitory effects of NAC (P < 0.01), yet those to DEA/NO in SHR were significantly (P < 0.05) enhanced following NAC infusion. Importantly, depressor responses to AS, IPA/NO, and DEA/NO were preserved in hypertension and vasorelaxation to AS and DEA/NO, in isolated aorta, unchanged in SHR as compared with WKY rats. This study has shown for the first time that HNO donors exert antihypertensive effects in vivo and may, therefore, offer a therapeutic alternative to traditional nitrovasodilators in the treatment of cardiovascular disorders such as hypertension.

cGMP-dependent protein kinase contributes to hydrogen sulfide-stimulated vasorelaxation

A growing body of evidence suggests that hydrogen sulfide (H₂S) is a signaling molecule in mammalian cells. In the cardiovascular system, H₂S enhances vasodilation and angiogenesis. H₂S-induced vasodilation is hypothesized to occur through ATP-sensitive potassium channels (K_ATP); however, we recently demonstrated that it also increases cGMP levels in tissues. Herein, we studied the involvement of cGMP-dependent protein kinase-I in H₂S-induced vasorelaxation. The effect of H₂S on vessel tone was studied in phenylephrine-contracted aortic rings with or without endothelium. cGMP levels were determined in cultured cells or isolated vessel by enzyme immunoassay. Pretreatment of aortic rings with sildenafil attenuated NaHS-induced relaxation, confirming previous findings that H₂S is a phosphodiesterase inhibitor. In addition, vascular tissue levels of cGMP in cystathionine gamma lyase knockouts were lower than those in wild-type control mice. Treatment of aortic rings with NaHS, a fast releasing H₂S donor, enhanced phosphorylation of vasodilator-stimulated phosphoprotein in a time-dependent manner, suggesting that cGMP-dependent protein kinase (PKG) is activated after exposure to H₂S. Incubation of aortic rings with a PKG-I inhibitor (DT-2) attenuated NaHS-stimulated relaxation. Interestingly, vasodilatory responses to a slowly releasing H₂S donor (GYY 4137) were unaffected by DT-2, suggesting that this donor dilates mouse aorta through PKG-independent pathways. Dilatory responses to NaHS and L-cysteine (a substrate for H₂S production) were reduced in vessels of PKG-I knockout mice (PKG-I−/−). Moreover, glibenclamide inhibited NaHS-induced vasorelaxation in vessels from wild-type animals, but not PKG-I−/−, suggesting that there is a cross-talk between K_ATP and PKG. Our results confirm the role of cGMP in the vascular responses to NaHS and demonstrate that genetic deletion of PKG-I attenuates NaHS and L-cysteine-stimulated vasodilation.

Protein kinase G phosphorylates soluble guanylyl cyclase on serine 64 and inhibits its activity

OBJECTIVE

Binding of nitric oxide (NO) to soluble guanylyl cyclase (sGC) leads to increased cGMP synthesis that activates cGMP-dependent protein kinase (PKG). Herein, we tested whether sGC activity is regulated by PKG.

METHODS AND RESULTS

Overexpression of a constitutively active form of PKG (DeltaPKG) stimulated (32)P incorporation into the alpha1 subunit. Serine to alanine mutation of putative sites revealed that Ser64 is the main phosphorylation site for PKG. Using a phospho-specific antibody we observed that endogenous sGC phosphorylation on Ser 64 increases in cells and tissues exposed to NO, in a PKG-inhibitable manner. Wild-type (wt) sGC coexpressed with DeltaPKG exhibited lower basal and NO-stimulated cGMP accumulation, whereas the S64A alpha1/beta1 sGC was resistant to the PKG-induced reduction in activity. Using purified sGC we observed that the S64D alpha1 phosphomimetic /beta1 dimer exhibited lower Vmax; moreover, the decrease in Km after NO stimulation was less pronounced in S64D alpha1/beta1 compared to wild-type sGC. Expression of a phosphorylation-deficient sGC showed enhanced responsiveness to endothelium-derived NO, reduced desensitization to acute NO exposure, and allowed for greater VASP phosphorylation.

CONCLUSIONS

We conclude that PKG phosphorylates sGC on Ser64 of the alpha1 subunit and that phosphorylation inhibits sGC activity, establishing a negative feedback loop.

Mechanisms and biomarkers of cardiovascular injury induced by phosphodiesterase inhibitor III SK&F 95654 in the spontaneously hypertensive rat

The cardiovascular injury of the type III selective PDE inhibitor SK&F 95654 was investigated in SHR. Twenty-four hours after a single sc injection of 100 or 200 mg/kg of the drug, rats exhibited cardiomyocyte necrosis and apoptosis, interstitial inflammation, hemorrhage and edema, as well as mesenteric arterial hemorrhage and necrosis, periarteritis, EC and VSMC apoptosis, EC activation, and MC activation and degranulation. Elevated serum levels of cTnT and decreased cTnT immunoperoxidase staining on cardiomyocytes were detected in the drug-treated rats. Serum levels of alpha2-macroglobulin and IL-6 were significantly elevated following drug treatment. NMR spectral patterns of urine samples are significantly different between the drug-treated and control rats. These results indicate that measurement of serum cTnT, acute phase proteins, and cytokines as well as metabonomic urine profiles may serve as potential biomarkers for drug-induced cardiovascular injury in rats. Increased expression of CD63 on MC (tissue biomarker of MC), of nitrotyrosine on MC and EC (an indirect indicator of NO in vivo), and of iNOS on MC and EC (source of NO) suggest that NO produced by activated and degranulated MC as well as activated EC play an important role in SK&F 95654-induced mesenteric vascular injury.

Vanadate is a potent activator of endothelial nitric-oxide synthase: evidence for the role of the serine/threonine kinase Akt and the 90-kDa heat shock protein

We investigated the molecular mechanisms of sodium vanadate (vanadate)-induced nitric oxide (NO) production. Exposure of bovine lung microvascular cells (BLMVEC) to vanadate increased the release of biologically active NO in endothelium/smooth muscle cocultures, as measured by the accumulation of its surrogate marker, cGMP. This release was sensitive to NO synthase (NOS) inhibition and was greater than that observed with ionomycin. Although calcium chelators (BAPTA, EGTA) inhibited basal and ionomycin-induced NO production, they failed to inhibit vanadate-induced NO release. Moreover, in the absence of calcium/calmodulin, cell lysates from vanadate-treated cells exhibited greater NOS activity compared with control cells. Vanadate activates the phosphoinositide3-kinase (PI3-K)/Akt pathway, which is known to increase endothelial NOS (eNOS) activity by direct phosphorylation of Ser-1179. Treatment of BLMVEC with vanadate resulted in phosphorylation of both Akt and endothelial NOS. In addition, wortmannin, a PI3-K inhibitor, blocked both the vanadate-induced phosphorylation of eNOS and the increase in cGMP accumulation. Similarly, adenovirus-mediated gene transfer of an activation deficient form of Akt (AA-Akt) blocked the release of NO brought about by vanadate. To further investigate the mechanism of action of vanadate, eNOS was immunoprecipitated and its association with proteins that alter eNOS activity was tested. Immunoblots demonstrated that the eNOS-caveolin interaction remained unaffected by vanadate, whereas vanadate promoted recruitment of the 90-kDa heat shock protein (hsp90) to eNOS. We conclude that vanadate causes NO release via a mechanism that involves Akt-induced eNOS phosphorylation and increased binding of the activator protein hsp90 to eNOS.

Nitric Oxide Synthase in Spontaneously Hypertensive Rats

Since its discovery by Furchgott and Zawadzki in 1980 [18], endothelium-derived relaxing factor (EDRF) has been shown to play a central role in the cardiovascular system [10]. The endothelial product is chemically equivalent to nitric oxide (NO) [23, 40] or a biochemical congener thereof [48]. Fifteen years ago, this small, simple and highly toxic molecule was known as a lengthy list of environmental pollutants found in unsavory haunts such as smoke and smog, and even as destroyer of ozone, suspected carcinogen, and precursor of acid rain. In addition, NO seems an unlikely biological jack of all trades for most of the body's functions are regulated by extraordinarily large and complex proteins and compounds. But over the past decade, diverse lines of evidence have converged to show that this sometime poison is a fundamental player in the everyday business of the human body.

High salt intake impairs vascular nitric oxide/cyclic guanosine monophosphate system in spontaneously hypertensive rats

In aortas of spontaneously hypertensive rats (SHRs), excessive dietary salt causes down-regulation of soluble guanylate cyclase (sGC) followed by decreased cyclic GMP production, which leads to impairment of the vascular relaxation response to nitric oxide (NO). The present study aimed to elucidate whether this impaired NO/cyclic GMP system results secondarily from increased blood pressure or from an effect of the salt itself. The antihypertensive drug nifedipine was used on 4-week-old SHRs that received a normal-salt diet or a high-salt diet for 4 weeks. Treatment with nifedipine (30 mg/kg/day, p.o.) reduced the increased blood pressure of SHRs fed the high-salt diet to the level of SHRs fed the normal-salt diet. In aortic rings from SHRs fed the high-salt diet, not only endothelium-dependent relaxations but also endothelium-independent relaxations were significantly impaired. However, these impairments were not alleviated by treatment with nifedipine. Furthermore, nifedipine did not prevent the increase in protein levels of endothelial NO synthase and the decrease in the protein levels of sGC in aortas from SHRs fed the high-salt diet. These alterations by high salt intake were restored after replacement with the normal-salt diet for 4 additional weeks. These results indicate that in SHRs given excessive dietary salt, normalization of salt intake but not blood pressure reduction can ameliorate alterations in the NO/cyclic GMP system. High salt intake may directly affect the vascular smooth muscle and cause impairment of the relaxation response to NO.

Human vascular endothelial cells contain membrane binding sites for estradiol, which mediate rapid intracellular signaling

Estrogen induces both rapid and delayed effects on the cardiovascular system. The early effects take place within minutes (e.g., changes in vasomotor tone) and are mediated through rapid intracellular signaling pathways; whereas the delayed effects (e.g., remodeling or lipid alterations) require hours to days to occur and require transcriptional effects with subsequent modulation of protein expression. To study the acute effects of 17beta-estradiol (E2) treatment on vascular function, we have investigated the rapid (on the order of minutes) effects of E2 treatment on intracellular signaling in human endothelial cells (EC). Our previous data have shown that E2 induces rapid release of NO from and activation of guanylate cyclase in human EC. In this study, we demonstrate that E2 also activates mitogen-activated protein kinase (extracellular signal-related kinase) signaling within minutes in EC. We hypothesized that this effect might be mediated by estrogen receptors (ER) localized to the cell surface. Our data show that membrane-impermeant forms of E2 also activate EC mitogen-activated protein kinase as well as stimulate cGMP production and NO release. The ER antagonist ICI 182,780 blocks this effect. Using confocal microscopy and flow cytometric analysis, we demonstrate that EC contain surface binding sites for E2, detectable by cell-impermeant ligand binding and equally with an anti-ERalpha antibody. Immunoreactive bands of 66 and 45 kDa are detectable with an anti-ERalpha mAb in human EC, and their individual presence correlates functionally with E2-stimulated genomic and rapid nongenomic responses, respectively. Membrane ERs may provide key molecular switches in these novel, rapid signaling pathways induced by E2 in EC.

Molecular aspects of soluble guanylyl cyclase regulation

Soluble guanylyl cyclase (sGC) is a heterodimeric enzyme (comprised of alpha and beta subunits) that generates the intracellular second messenger cyclic guanosine monophosphate (cGMP) from guanosine triphosphate (GTP). cGMP is subsequently important for the regulation of protein kinases, ion channels, and phosphodiesterases. Since recent evidence has demonstrated that heterodimerization of the alpha/beta subunits is essential for basal and stimulated enzymatic activity, the existence of several types of isoforms for each of the two subunits, along with their varying degrees of expression in different tissues, implies that multiple regulatory mechanisms exist for sGC. Yet, progress in studying and clarifying the regulatory processes that can alter sGC expression and activity has only slowly started being elucidated. In the following paper, we elaborate on sGC structure, function, and distribution along with recently described signaling pathways that modulate sGC gene expression.

Estrogen stimulates heat shock protein 90 binding to endothelial nitric oxide synthase in human vascular endothelial cells. Effects on calcium sensitivity and NO release

Estradiol (E(2)) causes endothelium-dependent vasodilation, mediated, in part, by enhanced nitric oxide (NO) release. We have previously shown that E(2)-induced activation of endothelial nitric oxide synthase (eNOS) reduces its calcium dependence. This pathway of eNOS activation is unique to a limited number of stimuli, including shear stress, the response to which is herbimycin-inhibitable. Consistent with this, herbimycin and geldanamycin pretreatment of human umbilical vein endothelial cells (HUVEC) abrogated E(2)-stimulated NO release and cGMP production, respectively. These benzoquinone ansamycins are potent inhibitors of Hsp90 function, which has recently been shown to play a role in stimulus-dependent eNOS activation. As in response to shear, E(2) induced an Hsp90-eNOS association, peaking at 30 min and completely inhibited by the conventional estrogen receptor antagonist ICI 182,780. These findings suggest that Hsp90 plays an important role in the rapid, estrogen receptor-mediated modulation of eNOS activation by estrogen.

Effect of chronic treatment with perindopril on endothelium-dependent relaxation of aorta and carotid artery in SHRSP

Endothelium-dependent relaxation of aorta and carotid artery from stroke-prone spontaneously hypertensive rats (SHRSP) and the effect of chronic treatment of SHRSP with perindopril, an angiotensin converting enzyme inhibitor, on endothelium-dependent relaxation were studied. Endothelium-dependent relaxation was induced by acetylcholine (ACh) in preparations of SHRSP and normotensive Wistar Kyoto rats (WKY) precontracted with noradrenaline. The ACh-induced relaxation in both preparations was abolished by L-nitroarginine. The ACh-induced relaxation was impaired in preparations from SHRSP and contraction was observed at high concentrations of ACh. In the presence of indomethacin, impairment of endothelium-dependent relaxation in SHRSP was minimized and the contraction was inhibited. The relaxation with sodium nitroprusside did not differ between the preparations from WKY and SHRSP. Treatment of SHRSP with perindopril (2 mg/kg/day) for 6 weeks decreased systolic blood pressure and improved the ACh-induced relaxation of aorta and carotid artery. The treatment inhibited the contraction by higher concentrations of ACh in the presence of L-nitroarginine. These results indicate that the impairment of endothelium-dependent relaxation in aorta and carotid artery of SHRSP may be caused by the reduced availability of nitric oxide. The perindopril-treatment may prevent these changes in SHRSP.

Effect of antioxidant treatments on nitrate tolerance development in normotensive and hypertensive rats

OBJECTIVES

To investigate the effect of chronic antioxidant treatments on the development of nitrate tolerance in spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats by evaluating (i) coronary vascular reactivity, (ii) lipid peroxidation (malondialdehyde), and (iii) peroxynitrite formation (3-nitrotyrosine).

METHODS

Tolerance was induced in 16-week-old male SHR and WKY, by 4 days of continuous treatment with nitroglycerin patches. Two groups were orally pre-treated (2-weeks) with antioxidants: N-acetyl-L-cysteine (NAC) or melatonin. Effects of serotonin (5-HT) and sodium nitroprusside (SNP) perfusion were tested in isolated Langendorff-perfused hearts. 3-Nitrotyrosine levels were measured in coronary sinus effluent and malondialdehyde in plasma.

RESULTS

Nitrate tolerance reduced SNP-induced dilation in both strains. This alteration was differently improved by antioxidants: melatonin was effective in SHR, whereas NAC was effective in WKY. Tolerance also reduced 5-HT-mediated vasodilation in WKY, which was reversed by both antioxidants. By contrast, nitrate tolerance enhanced the vasoconstriction to 5-HT in SHR and both antioxidants prevented this response. Furthermore, tolerance was associated with higher malondialdehyde levels in both strains and with higher 3-nitrotyrosine levels in SHR. These changes were reversed by both antioxidants.

CONCLUSIONS

A participation of oxidative stress was suggested during nitrate tolerance development, since antioxidants prevented the increase in lipid peroxidation and improved vascular responses to SNP and 5HT. Differential effects of antioxidants on SNP-induced vasodilation in SHR and WKY may suggest distinct mechanisms of tolerance development in hearts from hypertensive and normotensive rats. An increased peroxynitrite generation, expressed by higher 3-nitrotyrosine levels, could contribute to nitrate tolerance in the coronary circulation of SHR.

Aging and Chronic Hypertension Decrease Expression of Rat Aortic Soluble Guanylyl Cyclase

Abstract —We analyzed the influence of aging and genetic hypertension on the function and expression of soluble guanylyl cyclase (sGC) in the aortas of prehypertensive and old spontaneously hypertensive rats (SHR) as well as in age-matched normotensive Wistar-Kyoto rats (WKY). The expression of heterodimeric sGC (α 1 and β 1 ) was assessed at the mRNA and protein level, and its function was assessed by the relaxant responses of phenylephrine-contracted endothelium-denuded aortic rings to the nitric oxide (NO) donor sodium nitroprusside. The vasodilator potency of sodium nitroprusside was significantly reduced ( P <0.05) with age (3- to 6-fold increase in the EC 50 in old WKY and SHR compared with their young counterparts) as well as with hypertension (3-fold increase in old SHR compared with age-matched WKY), whereas the vasodilator potency of sodium nitroprusside did not differ between young SHR and WKY. A similar influence of aging and hypertension on NO-stimulated GC activity was revealed at the GC expression level: Whereas the β 1 protein content was similar in young rats of both strains, old WKY exhibited 60% lower and old SHR exhibited 80% lower β 1 subunit protein compared with young rats ( P <0.05). Moreover, the abundance of α 1 and β 1 mRNA (assessed by reverse transcriptase—polymerase chain reaction) was similar in young rats but was 2.5-fold (α 1 ) and 4.3-fold (β 1 ) lower in old SHR compared with old WKY. In conclusion, our findings show that both aging and hypertension decrease sGC expression and its NO-dependent activation in aortic tissue. Downregulation of sGC may therefore contribute to arterial dysfunction in senescence and chronic hypertension.

Effects of superoxide on signaling pathways in smooth muscle cells from rats

The effects of hypoxanthine and xanthine oxidase-induced superoxide anion were evaluated on various signal transduction pathways in aortic smooth muscle cells (SMCs) from spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Superoxide increased inositol 1,4,5-tris-phosphate (IP(3)) formation in a concentration- and time-dependent manner in both strains but more markedly in SMCs from SHR. Various antioxidants significantly decreased the superoxide-induced IP(3) formation in both strains. In addition, tyrosine kinase inhibitors, genistein and tyrphostin A25, inhibited the superoxide-induced IP(3) formation more markedly in SHR than in WKY. Moreover, superoxide decreased the basal level of cGMP to a greater extent in SHR and also suppressed the rise in cGMP induced by S-nitroso-N-acetylpenicillamine. In addition, the superoxide-induced increase in IP(3) formation was significantly inhibited by guanylyl cyclase stimulator S-nitroso-N-acetylpenicillamine but was potentiated by ODQ (a guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one) and KT5823 (a cGMP-dependent protein kinase inhibitor), with a greater effect in SHR. Finally, the superoxide-enhanced IP(3) formation was not accompanied by simultaneous changes in cAMP levels, and inhibition of the adenylyl cyclase pathway did not modify the superoxide-induced IP(3) formation. Our results thus demonstrate a stimulatory effect of superoxide on IP(3) formation, mediated by the tyrosine kinase-coupled phospholipase C(gamma) activity, and an inhibitory effect of superoxide on cGMP formation in vascular SMCs. The increased reactivity of the phospholipase C pathway and the decreased cross inhibition of the IP(3) pathway by cGMP in the presence of superoxide may underlie the altered functions of vascular SMCs in SHR.

Effect of nitro-L-arginine on electrical and mechanical responses to acetylcholine in the superior mesenteric artery from stroke-prone hypertensive rat

1. High salt diet is known to aggravate the vascular pathology in spontaneously hypertensive stroke-prone rats (SHR-SP). The aim of the present study was to assess the involvement of endothelial dysfunction in this effect. Contractile tension and membrane potential were simultaneously recorded in superior mesenteric artery rings of untreated and NaCl-loaded (1% NaCl in the drinking water) SHR-SP and normotensive Wistar Kyoto rats (WKY). 2. In unstimulated artery, hyperpolarization evoked by acetylcholine was not different in WKY and in NaCl-loaded WKY; it was reduced in SHR-SP and further reduced in NaCl-loaded SHR-SP. Hyperpolarization was unaffected by N(omega)-nitro-L-arginine (L-NA) but was abolished in high-KCl solution. 3. In noradrenaline-stimulated artery, ACh-evoked hyperpolarization and relaxation were not different in WKY and in SHR-SP. NaCl-treatment did not affect the responses to ACh in WKY but decreased maximum relaxation in SHR-SP from 93+/-2% to 72+/-7% of the contraction. In WKY, in NaCl-loaded WKY and in SHR-SP, L-NA similarly shifted the concentration-relaxation curve to ACh to the right and depressed its maximum but L-NA did not affect the hyperpolarization to ACh. In NaCl-loaded SHR-SP, L-NA blunted the effects of ACh on membrane potential and on contraction. 4. The NO donor SNAP abolished the depolarization and the contraction evoked by noradrenaline with the same potency in WKY and in untreated SHR-SP but was more potent in NaCl-loaded SHR-SP. 5. In KCl-contracted arteries the relaxations to ACh were not different in WKY and SHR-SP but NaCl-loaded SHR-SP were more sensitive to ACh. 6. The results showed that NaCl-rich diet markedly reduced the L-NA-resistant responses to ACh and increased the sensitivity to NO in SHR-SP.

Trafficking of endothelial nitric-oxide synthase in living cells. Quantitative evidence supporting the role of palmitoylation as a kinetic trapping mechanism limiting membrane diffusion

To examine endothelial nitric-oxide synthase (eNOS) trafficking in living endothelial cells, the eNOS-deficient endothelial cell line ECV304 was stably transfected with an eNOS-green fluorescent protein (GFP) fusion construct and characterized by functional, biochemical, and microscopic analysis. eNOS-GFP was colocalized with Golgi and plasma membrane markers and produced NO in response to agonist challenge. Localization in the plasma membrane was dependent on the palmitoylation state, since the palmitoylation mutant of eNOS (C15S/C26S eNOS-GFP) was excluded from the plasma membrane and was concentrated in a diffuse perinuclear pattern. Fluorescence recovery after photobleaching (FRAP) revealed eNOS-GFP in the perinuclear region moving 3 times faster than the plasmalemmal pool, suggesting that protein-lipid or protein-protein interactions are different in these two cellular domains. FRAP of the palmitoylation mutant was two times faster than that of wild-type eNOS-GFP, indicating that palmitoylation was influencing the rate of trafficking. Interestingly, FRAP of C15S/C26S eNOS-GFP but not wild-type eNOS-GFP fit a model of protein diffusion in a lipid bilayer. These data suggest that the regulation of eNOS trafficking within the plasma membrane and Golgi are probably different mechanisms and not due to simple diffusion of the protein in a lipid bilayer.

Decreased flow-induced dilation and increased production of cGMP in spontaneously hypertensive rats

-We investigated flow (shear stress)- and agonist-induced cGMP release in mesenteric vascular beds of spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY). The mesenteric vascular bed was perfused in situ with Tyrode's solution. Vascular relaxation and cGMP release in the perfusate were determined on stimulation by flow or by acetylcholine (0.1 micromol/L) or sodium nitroprusside (0.1 mmol/L). Flow-induced release of cGMP was significantly greater in SHR than in WKY (P<0.01), despite a lower flow-induced dilation in SHR. In both strains, NG-nitro-L-arginine methyl ester (L-NAME) completely inhibited cGMP release in response to flow (P<0.001), although flow-induced dilation was not affected by L-NAME in SHR. Moreover, the activity of the constitutive nitric oxide synthase (NOS) was significantly greater in SHR (82+/-3.5 fmol/min) than in WKY (66+/-3.5 fmol/min; P<0.05) and was associated with increased expression of endothelial NOS mRNA in SHR. Sodium nitroprusside induced larger increases in cGMP release in SHR (3593+/-304 fmol/min) than in WKY (2467+/-302 fmol/min; P<0.05). The release of cGMP in response to acetylcholine was significantly lower in SHR (292+/-80 fmol/min) than in WKY (798+/-218 fmol/min; P<0.05) in parallel with smaller acetylcholine-induced relaxation in SHR. Despite increased cGMP production in response to flow and NOS activity, flow-induced dilation was decreased in SHR, suggesting an upregulation of the NO/cGMP pathway to compensate for the increased vascular tone in SHR.

Nitroglycerin-induced aortic relaxation mediated by calcium-activated potassium channel is markedly diminished in hypertensive rats

Nitroglycerin (NTG), a nitric oxide (NO) donor, is considered to relax vascular smooth muscle by stimulating soluble guanylate cyclase, which in turn increases cyclic GMP (cGMP) level. Recently it became evident that NO-induced vasodilatation is also mediated by stimulating Ca-activated K (K(Ca)) channels directly and/or indirectly through cGMP. We, therefore, tried to investigate the possible involvement or the alteration of K(Ca) channels in the mechanism of vasodilation induced by NTG in physiological and pathological conditions. Using rings prepared from thoracic aortas of spontaneously hypertensive rats (SHR) and those of age-matched Wistar-Kyoto rats (WKY), we studied changes in isometric tension of the rings in response to NTG to evaluate effects of a soluble guanylate cyclase inhibitor methylene blue (MB), and a specific blocker of K(Ca) channel charybdotoxin (CTX). Rings from WKY and SHR precontracted with norepinephrine showed similar aortic relaxation to NTG. MB markedly suppressed the NTG-induced relaxation in both strains, leaving about 30% of MB-resistant relaxation. CTX nearly completely eliminated this MB-resistant relaxation in WHY but did not affect this relaxation in SHR. These results suggest that NTG-induced vasorelaxation is mediated through i) cGMP-dependent and ii) cGM P-independent K(Ca) channel involving mechanisms, the latter may be diminished or virtually eliminated in hypertensive state.

Enhanced involvement of endothelin in the haemodynamic sequelae of endotoxaemia in conscious, hypertensive, transgenic ((mRen-2)27) rats

1. Age-matched (3-4 months old) male, heterozygous, hypertensive, transgenic ((mRen-2)27) rats (abbreviated to TG rats) and the normotensive control animals (homozygous, Hannover Sprague-Dawley rats (abbreviated to SD rats), were chronically instrumented for the assessment of regional haemodynamic responses to continuous lipopolysaccharide (LPS) infusion (150 microg kg(-1) h(-1), i.v.) 2. The early (1-2 h) hypotension in SD rats (-11+/-3 mmHg; n=7) was significantly less than that in TG rats (-35+/-3 mmHg; n=8), but by 24 h mean arterial blood pressure (MAP) in both strains of rat was not different from the pre-LPS value (SD rats: baseline, 108+/-3 mmHg; 24 h LPS, 112+/-4 mmHg; TG rats: baseline, 171+/-2 mmHg; 24 h LPS, 169+/-3 mmHg). At this stage in the SD rats there was a renal vasodilatation (delta vascular conductance, 29+/-10 [kHz mmHg(-1)]10(3)) but not in TG rats (delta vascular conductance 2+/-3[kHz mmHg(-1)]10(3)). 3. Co-infusion of LPS and the non-selective endothelin receptor antagonist, SB 209670 (600 microg kg(-1) bolus, 600 microg kg(-1) h(-1)) between 24 and 31 h in SD rats caused a fall in MAP of 16+/-2 mmHg accompanied by hindquarters vasodilatation (delta vascular conductance 11+/-3 (kHz mmHg(-1))10(3)). In TG rats, under the same conditions, the fall in MAP was -60+/-6 mmHg, and there were renal, mesenteric and hindquarters vasodilatations (delta vascular conductance, 23+/-5, 32+/-7, and 14+/-4 (kHz mmHg(-1))10(3), respectively). All effects, except the hindquarters vasodilatation, were greater in TG than in SD rats. 4. In TG rats infused with LPS alone for 31 h, between 24 and 31 h the fall in MAP was -17+/-4 mmHg, and the changes in renal, mesenteric and hindquarters vascular conductances were 5+/-3, -4+/-5, and 12+/-4 (kHz mmHg(-1)10(3), respectively. 5. Administration of the angiotensin (AT1)-receptor antagonist, losartan (10 mg kg(-1), i.v.) following co-infusion of LPS and SB 209670 between 24 and 31 h caused similar falls in MAP in SD and TG rats (-12+/-3 and -14+/-4 mmHg, respectively). 6. These results, together with previous findings, are consistent with a relative enhancement of the contribution of endothelin to the maintenance of cardiovascular status in endotoxaemic TG rats, particularly through a mesenteric vasoconstrictor action.

Cross-tolerance between endogenous nitric oxide and exogenous nitric oxide donors

It is still unclear whether cross-tolerance develops between endogenously produced nitric oxide and exogenous nitric oxide donors. Thus, cGMP accumulation was determined in cultured aortic smooth muscle cells exposed to a nitric oxide source. Exposure of human, rat, rabbit, porcine or bovine smooth muscle cells to sodium nitroprusside led to a time- and concentration-dependent development of tolerance. In rat aortic smooth muscle cells, cross-tolerance developed between the sodium nitroprusside and S-nitroso-N-acetylpenicillamine, but not between sodium nitroprusside and atriopeptin. In addition, when rat aortic smooth muscle cells were treated with endotoxin or interleukin-1beta, they displayed lower sodium nitroprusside-induced cGMP accumulation as compared to control cells. When rat aortic smooth muscle cells were exposed to sodium nitroprusside for 12 h they displayed a decreased ability to accumulate cGMP in response to endothelium-derived nitric oxide released from bovine aortic endothelial cells. In addition, co-cultures of rat aortic smooth muscle cells with bovine aortic endothelial cells showed an L-nitroarginine methylester-sensitive decrease in sodium nitroprusside-induced cGMP accumulation compared to single rat aortic smooth muscle cell cultures. We conclude that cross-tolerance between endothelium-derived nitric oxide and exogenously applied nitric oxide donors occurs in vitro.

Vasodilator dysfunction in aged spontaneously hypertensive rats: changes in NO synthase III and soluble guanylyl cyclase expression, and in superoxide anion production

OBJECTIVE/METHODS

Genetic hypertension is associated with an apparent endothelial dysfunction and impaired endothelium-dependent vasodilatation in response to increased flow and receptor-dependent agonists. However, the link between impaired vasodilatation and nitric oxide (NO) synthase expression is still unclear. In the present study, dilator responses were determined in the aorta and coronary circulation of 16 month old spontaneously hypertensive (SHR) and Wistar Kyoto rats (WKY). Changes in vascular reactivity were compared with alterations in superoxide anion production as well as endothelial NO synthase (NOS III) and soluble guanylyl cyclase expression.

RESULTS

In the isolated perfused heart both the bradykinin- and sodium nitroprusside-induced vasodilator responses were attenuated in SHR compared to WKY. Western blot analysis revealed a parallel reduction in NOS III expression in coronary microvascular endothelial cells from SHR. Superoxide anion production in aortae from SHR was markedly elevated over that of aortae from WKY, and was almost completely abolished by pretreatment with superoxide dismutase. Superoxide dismutase induced similar relaxations in phenylephrine-preconstricted aortic rings from both SHR and WKY, but failed to restore the attenuated acetylcholine- and sodium nitroprusside-induced relaxations in SHR. No difference in NOS III expression was detected in the aortae from either strain whereas soluble guanylyl cyclase expression was markedly decreased in SHR.

CONCLUSIONS

These results demonstrate that NOS III expression in different tissues is differentially affected by hypertension. Moreover, although an elevated superoxide anion production is apparent in the aorta, a reduced soluble guanylyl cyclase expression appears to account for the observed vasodilator dysfunction in SHR.

Zn2+ inhibits nitric oxide formation in response to lipopolysaccharides: implication in its anti-inflammatory activity

There is compelling evidence to indicate an anti-inflammatory action of Zn2+. Most inflammatory diseases are associated with an increase of the inducible form of nitric oxide (NO) synthase. Additionally, inflammatory mediators such as histamine or bradykinin stimulate the constitutive NO synthase. Thus, the present study was undertaken to investigate whether Zn2+ inhibits production of inducible NO synthase and/or constitutive NO synthase activity to produce NO. Lipopolysaccharide, 5 mg/kg i.v., administered to Zn2+-deficient (ZD) rats, rats supplemented with Zn2+ sulfate (ZG), 10 mg/kg s.c., or controls resulted in a significant reduction of their serum Zn2+. The levels of N(G)-nitro-L-arginine methylester (L-NAME)-sensitive cyclic GMP (cGMP) in aortas isolated from ZD or ZG were significantly lower than those obtained from control animals. Zinc (100-150 microM) produced a dose-dependent inhibition of lipopolysaccharide or interleukin-1beta-induced NO formation in isolated rat aortic smooth muscle cells. Compared to cyclohexamide or actinomycin-D, the time course of inhibition of NO formation by 150 microM Zn2+ did not suggest an effect of Zn2+ on inducible NO synthase protein synthesis. Moreover, Zn2+ (150 microM) significantly reduced the rate of conversion of [3H]arginine to [3H]citrulline in lung homogenates from lipopolysaccharide-treated rats. Incubation of rat aortic smooth muscle cells and bovine pulmonary artery endothelial cell co-cultures with Zn2+ (150 microM) caused a significant reduction in basal and bradykinin- or A-23187-induced formation of cGMP. Thus, our results indicate that Zn2+ is capable of inhibiting lipopolysaccharide- or interleukin-1beta-induced NO formation as well as NO formation by constitutive NO synthase basally or in response to bradykinin or A-23187, and may explain the reported anti-inflammatory activity of Zn2+.

Nitric oxide production contributes to the angiogenic properties of vascular endothelial growth factor in human endothelial cells

Vascular endothelial growth factor (VEGF) is a regulator of vasculogenesis and angiogenesis. To investigate the role of nitric oxide (NO) in VEGF-induced proliferation and in vitro angiogenesis, human umbilical vein endothelial cells (HUVEC) were used. VEGF stimulated the growth of HUVEC in an NO-dependent manner. In addition, VEGF promoted the NO-dependent formation of network-like structures in HUVEC cultured in three dimensional (3D) collagen gels. Exposure of cells to VEGF led to a concentration-dependent increase in cGMP levels, an indicator of NO production, that was inhibited by nitro-L-arginine methyl ester. VEGF-stimulated NO production required activation of tyrosine kinases and increases in intracellular calcium, since tyrosine kinase inhibitors and calcium chelators attenuated VEGF-induced NO release. Moreover, two chemically distinct phosphoinositide 3 kinase (PI-3K) inhibitors attenuated NO release after VEGF stimulation. In addition, HUVEC incubated with VEGF for 24 h showed an increase in the amount of endothelial NO synthase (eNOS) protein and the release of NO. In summary, both short- and long-term exposure of human EC to VEGF stimulates the release of biologically active NO. While long-term exposure increases eNOS protein levels, short-term stimulation with VEGF promotes NO release through mechanisms involving tyrosine and PI-3K kinases, suggesting that NO mediates aspects of VEGF signaling required for EC proliferation and organization in vitro.

17 beta-estradiol regulation of human endothelial cell basal nitric oxide release, independent of cytosolic Ca2+ mobilization

Estradiol retards the development of atherosclerosis. Animal models have suggested that NO may be a critical effector molecule in this cardiovascular protection. In this study, female human umbilical vein endothelial cells (HUVECs) were propagated in phenol red-free gonadal hormone-free medium and pretreated with 17 beta-estradiol (E2). Reduced NO2- and NO3- (NOx) concentration, determined by chemiluminescence, demonstrated a rapid increase in basal HUVEC NO release in response to physiological concentrations of E2. The estrogen receptor (ER) antagonist ICI 164,384 inhibited the augmented NO release, demonstrating an ER-mediated component of this response. Because endothelial NO synthase (eNOS) activity is largely regulated by cytosolic Ca2+, relative [Ca2+]i in response to E2 was determined in a fluorometric assay. E2 did not promote HUVEC Ca2+ fluxes. Furthermore, eNOS activity in E2-pretreated endothelial whole-cell lysates was not dependent on additional Ca2+. Despite involving the ER, this is a nongenomic effect E2, as demonstrated by maintained responses in transcriptionally inhibited cells and by the rapidly (10 minutes) of cGMP formation in an NO bioassay. We demonstrate, for the first time, that independent of cytosolic Ca2+ mobilization, there is augmentation of eNOS activity with a resultant increase in HUVEC basal NO release in response to short-term estradiol exposure. Implications for the cardiovascular protective role of estrogen are discussed.

Sex-related differences in the response of spontaneously hypertensive rats to angiotensin-converting enzyme inhibitor

We have compared the endothelium-dependent responses of thoracic aortic rings obtained from age-matched male and female SHR in order to explore gender differences in the effectiveness of antihypertensive drug therapy in correcting the endothelial dysfunction found in these animals. For this, concentration-effect curves to acetylcholine and sodium nitroprusside were obtained using aortic rings with and without endothelium isolated from male and female rats which had or had not been pre-treated with enalapril for 72 h (acute) or 15 d (chronic). The maximal responses achieved and the EC50s were determined. The blood pressure of male and female spontaneously hypertensive rats (SHR) decreased to normal levels within 72 h of initiating treatment with enalapril and remained normal during the remainder of the treatment period (15 d). However, enalapril was not effective in restoring a normal blood pressure in all of the male and female SHR. Female SHR were more responsive to enalapril after both acute and chronic treatment (70% of the females and 45% of the males became normotensive). Enalapril corrected the decreased response to acetylcholine in male but not in female SHR. An increased sensitivity to sodium nitroprusside, an endothelium-independent vasodilator, was observed after acute or chronic treatment with enalapril in aortic rings with endothelium from male SHR. Indomethacin restored the decreased response to acetylcholine in aortic rings from enalapril-treated females and potentiated the response to acetylcholine in aortic rings from treated male SHR. We conclude that: a) there are significant differences in the responses of male and female SHR to enalapril, b) the imbalance in endothelium-dependent relaxing and contracting factors in SHR is corrected by enalapril in male but not in female SHR, c) correction of the endothelial dysfunction probably occurs independently of the normalization of blood pressure levels and appears to be gender-dependent.

Cardiovascular effects of a novel, potent and selective phosphodiesterase 5 inhibitor, DMPPO: in vitro and in vivo characterization

1. The aim of this study was to investigate the cardiovascular effects of a novel, potent and specific phosphodiesterase 5 (PDE 5) inhibitor, 1,3 dimethyl-6-(2-propoxy-5-methane sulphonylamidophenyl)-pyrazolo[3,4-d]pyrimidin-4-(5H)-one (DMPPO) in phenylephrine-precontracted rat aortic rings and different in vivo rat preparations. 2. DMPPO elicited a concentration-dependent relaxation of rat aortic rings with functional endothelium. DMPPO-induced relaxation was abolished by endothelium removal or pretreatment with the soluble guanylate cyclase inhibitor, methylene blue (10 microM). 3. In aortic rings without endothelium, the potency (pD2= -log10 EC50) of atrial natriuretic peptide (ANP) to induce relaxation increased from 8.13 +/- 0.05 in the absence of DMPPO, to 8.32 +/- 0.05 and 8.52 +/- 0.08 in the presence of 30 nM and 100 nM DMPPO, respectively. Similarly, the potency of sodium nitroprusside (SNP) in inducing relaxation increased from 7.38 +/- 0.07 in the absence of the PDE 5 inhibitor to 8.07 +/- 0.11 and 8.15 +/- 0.08 in the presence of 30 nM and 100 nM DMPPO, respectively. In contrast, relaxation to the adenylate cyclase activator, forskolin, was unchanged by DMPPO (100 nM). 4. In rings without endothelium, DMPPO (100 nM) increased by 2.5 fold intracellular levels of guanosine 3':5'-cyclic monophosphate (cyclic GMP). Moreover, DMPPO (100 nM) potentiated the increases in cyclic GMP levels induced by ANP (30 nM) by 3 fold and SNP (30 nM) by 2.7 fold. Adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels were not modified by DMPPO. 5. In anaesthetized normotensive or spontaneously hypertensive rats (SHR), DMPPO (2 and 5 mg kg-1, i.v.) lowered blood pressure without affecting heart rate. Similarly, in conscious SHR, orally administered DMPPO (5 mg kg-1) induced a 25 mmHg decrease in blood pressure for at least 7 h without modifying heart rate. Meanwhile, urinary cyclic GMP was increased by 50% whereas cyclic AMP remained unchanged. 6. In normotensive anaesthetized rats, sodium nitroprusside (SNP) (i.v. bolus) induced a decrease in blood pressure which rapidly returned to baseline. In DMPPO (1 mg kg-1, i.v.)-treated rats, the hypotensive effects of SNP (10 to 100 micrograms kg-1) were prolonged over time whereas the peak effect was unchanged. 7. In pithed rats, phenylephrine (i.v. bolus) induced dose-dependent increases in blood pressure. Pretreatment with DMPPO (5 mg kg-1, i.v.) partially inhibited the pressor response to phenylephrine (0.3 to 100 micrograms kg-1). 8. In conclusion, the potent and selective PDE 5 inhibitor, DMPPO, produces relaxation in isolated vessels in the presence of a cyclic GMP drive and reduces blood pressure of intact animals. Its high oral bioavailability and long duration of action should make it a useful tool to study the role of cyclic GMP in various biological systems.

Benidipine improves endothelial function in renal resistance arteries of hypertensive rats

We studied the effects of long-term antihypertensive treatment on endothelial function in renal resistance arteries from spontaneously hypertensive rats (SHR). Wistar-Kyoto rats (WKY) were used as a normotensive reference. Adult SHR were treated with benidipine (a calcium antagonist) or ecarazine (a vasodilator) for 10 weeks; the drugs caused similar reductions in blood pressure. Changes in isometric tension of rings prepared from the third-order branches of the renal arteries were recorded. Endothelium-dependent relaxations induced by acetylcholine in rings contracted with norepinephrine were smaller in SHR than in WKY. The impaired relaxation was improved by benidipine treatment, but ecarazine had no significant effect. In vitro treatment with meclofenamic acid, a cyclooxygenase inhibitor, did not alter the differences in the relaxations. In the presence of meclofenamic acid, N omega-nitro-L-arginine methyl ester slightly reduced the relaxations; the relaxation was smaller in SHR than in WKY and was not affected by benidipine treatment. In rings contracted with 40 mmol/L. KCI, the relaxations induced by acetylcholine in the presence of meclofenamic acid were smaller than those in rings contracted with norepinephrine. The relaxation was smaller in SHR than in WKY but was normalized by benidipine treatment. Thus, acetylcholine relaxes rat renal resistance arteries by releasing nitric oxide and endothelium-derived hyperpolarizing factor from the endothelium, which is impaired in SHR. Long-term benidipine treatment improves the impaired relaxation in SHR by enhancing nitric oxide-mediated relaxation.

cGMP accumulation and gene expression of soluble guanylate cyclase in human vascular tissue

Gene expression of soluble guanylate cyclase (sGC) and cGMP accumulation in response to sodium nitroprusside (SNP) were studied in cultured human vascular cells and freshly harvested vascular tissue. As revealed by reverse transcriptase-polymerase chain reaction, cultured smooth muscle and endothelial cells, as well as freshly isolated human vascular tissue, express mRNA for the alpha 3 and beta 3 subunits but not for the alpha 2 and beta 3 subunits is evident even in the absence of increased cGMP accumulation in response to SNP. cGMP accumulation in human cells cultured from different vascular beds typically increased two- to five-fold (maximum of 11.4-fold) over baseline following stimulation with 100 microM SNP. Bovine, murine, canine, and avian vascular smooth muscle cells accumulated similar or lower amounts of cGMP than human cells, whereas porcine, rat, and rabbit smooth muscle cells accumulated greater amounts of cGMP. In freshly harvested human vessels, cGMP accumulation in response to SNP was found to increase fifteen-fold over baseline. In contrast to the SNP-induced cGMP accumulation, cGMP levels in response to particulate guanylate cyclase activator atriopeptin II were equal to or greater in cultured human cells than in fresh human vascular tissue. We conclude that human vascular cells (fresh and cultured) express the mRNA for both a large (alpha 3) and a small (beta 3) sGC subunit and that fresh human cells are more sensitive to SNP stimulation.

Antihypertensive therapy and arterial function in experimental hypertension

1. Alterations in the function of the endothelium and arterial smooth muscle may be important in the establishment of hypertension. Thus, the possible favorable influences of blood pressure-lowering agents on vascular responsiveness may be important in the chronic antihypertensive actions of these compounds. 2. A number of reports have suggested that ACE inhibitors can improve arterial function in hypertension, whereas the knowledge about the vascular effects of other antihypertensive drugs, like beta-blockers, calcium channel blockers, and diuretics remains rather limited. 3. In this article, the effects of antihypertensive therapy on arterial function in human and experimental hypertension are reviewed.

Nitric oxide release from kidneys of hypertensive rats treated with imidapril

To examine whether endothelial dysfunction in hypertension is reversible or not, we studied the effects of imidapril, an angiotensin-converting enzyme inhibitor, on nitric oxide release in stroke-prone spontaneously hypertensive rats (SHR) and deoxycorticosterone acetate (DOCA)-salt hypertensive rats. After a 4-week treatment with imidapril (1 or 10 mg/d SC) or vehicle, acetylcholine-induced vasodilation and nitric oxide release in the isolated kidneys were determined. Nitric oxide release was measured by a chemiluminescense assay. Imidapril lowered blood pressure in stroke-prone SHR in a dose-dependent manner. Untreated stroke-prone SHR exhibited significantly attenuated responses to acetylcholine (10(-8) mol/L) of both renal perfusion pressure (stroke-prone SHE 42 +/- 4% versus Wistar-Kyoto rats [WKY] 58 +/- 4% [mean +/- SE], P < .01) and nitric oxide release (stroke-prone SHR +7.6 +/- 2.1 versus WKY +29.7 +/- 9.7 fmol/min per gram of kidney wt, P < .01). Imidapril at 10 mg/d significantly increased acetylcholine-induced renal vasodilation and nitric oxide release in stroke-prone SHR (renal perfusion pressure, 56 +/- 3%; nitric oxide release, +27.1 +/- 6.4 fmol/min per gram of kidney wt; both P < .01 versus stroke-prone SHR treated with vehicle). On the other hand, imidapril neither decreased blood pressure nor changed nitric oxide release induced by acetylcholine in DOCA-salt hypertensive rats. Staining for endothelial nitric oxide synthase and brain nitric oxide synthase was clearly detected in the kidneys of both stroke-prone SHR and WKY, whereas staining intensity was weaker in DOCA-salt hypertensive rats. Inducible nitric oxide synthase immunoreactivity was barely noticeable in any type of rat. Thus, imidapril restored endothelial damage by pressure-dependent mechanisms. Most of the nitric oxide detected in the perfusate seemed to be derived from constitutive nitric oxide synthase.

Effect of magnesium on the function of the rabbit corpus cavernosum

Contraction and relaxation of the smooth muscle, including the corpus cavernosum, are mediated by changes in the intracellular concentration of calcium. Since magnesium modulates the movement of calcium, it can modify the function of the erectile tissue. We designed this study to investigate the effects of magnesium in doses ranging from 5 to 30 mM on the function of the rabbit corpus cavernosum in vitro. The resting tension of tissue strips was significantly reduced by exposure to a solution high in magnesium (5-30 mM). The contractile response to field stimulation under resting conditions, and the contraction to phenylephrine, were significantly decreased by magnesium (5-30 mM). There were no differences in the contractile strength of the corpus cavernosum to KCl. Although the relaxation induced by field stimulation under preincubation with 200 microM phenylephrine was abolished in the presence of 30 mM magnesium, there were no differences at a concentration of 5 mM or of 10 mM magnesium. The relaxation induced by sodium nitroprusside under precontraction with 200 microM phenylephrine was further increased by magnesium dose dependently. A high concentration of magnesium (30 mM) enhanced both bethanechol-induced and ATP-induced relaxations under precontraction with phenylephrine. Our study demonstrated that magnesium reduced the receptor-mediated contraction of the rabbit corpus cavernosum and enhanced the relaxation of this tissue induced by sodium nitroprusside, bethanechol, and ATP.

Mechanisms of tolerance to sodium nitroprusside in rat cultured aortic smooth muscle cells

1. While exposure of smooth muscle cells to sodium nitroprusside (SNP) leads to the development of tolerance to soluble guanylate cyclase (sGC) activation, the mechanisms responsible for this phenomenon in intact cells remain unclear. In the present study, possible mechanisms of tolerance were investigated in a cell culture model where sGC activity was estimated from the accumulation of cyclic GMP in response to 10 microM SNP over a 15 min period in the presence of a phosphodiesterase (PDE) inhibitor. 2. Pretreatment of rat aortic smooth muscle cells with 10-500 microM SNP led to a dose-dependent downregulation of cyclic GMP accumulation upon subsequent SNP stimulation. This effect was evident as early as 2 h following incubation with 10 microM SNP, reached a plateau at 4 h and was blocked by co-incubation with 30 microM oxyhaemoglobin. 3. Pretreatment of smooth muscle cells with the PDE inhibitor, zaprinast, resulted in downregulation of the SNP-induced cyclic GMP accumulation in a time- and concentration-dependent manner, that was first evident after 12 h. Moreover, while the zaprinast-induced downregulation of cyclic GMP accumulation was completely inhibited by the protein kinase A (PKA) inhibitor, H89, tolerance to SNP was partially reversed by H89. 4. beta 1 sGC steady state mRNA levels of S-nitroso N-acetylpenicillamine (SNAP)- or 8Br-cyclic GMP-pretreated cells were unchanged, as indicated by Northern blot analysis. However, Western blot analysis revealed that alpha 1 protein levels were decreased in zaprinast, but not in SNP, SNAP or 8Br-cyclic GMP pretreated cells. 5. While thiol depletion did not prevent the development of tolerance, pretreatment of cells with SNP in the presence of reducing agents partially or completely restored the ability of cells to respond to SNP. 6. We conclude that tolerance to SNP results from two distinct mechanisms: an early onset, NO-mediated event that is reversed by reducing agents and a more delayed, PKA-sensitive process that is mediated through increases in cyclic GMP and a decrease in sGC protein levels.

Physiological role of large, Ca2+-activated K+ channels in human glomerular mesangial cells

1. Contraction assays and patch clamp methods were used to determine the role of K+ channels in the regulation of contractile tone of human mesangial cells (MC) in culture. 2. MC contraction was induced by vasoconstrictor agents, such as angiotensin II (AngII; 100 nmol/L) and glybenclamide (Glyb), but not by iberiotoxin (IbTX), a blocker of large Ca2+-activated K+ channels (BK(Ca)). These results suggest that Glyb-sensitive K+ channels, but not BK(Ca) channels, were active at rest. 3. In the presence of 100 nmol/L IbTX, contraction by AngII was slightly, but not significantly, enhanced, indicating that BK(Ca) has a minimal role as a negative feedback regulator of contraction. Nitroprusside (NP; 100 mu mol/L) a nitric oxide (NO) donor, atrial natriuretic peptide (ANP; 1.0 mu mol/L) and db-cGMP (10 mu mol/L) attenuated AngII-induced contraction in the absence, but not in the presence, of IbTX, suggesting that BK(Ca) channels were activated by cGMP. 4. In patch clamp experiments, three distinct K+-selective channels of 9, 65 and 150 pS (outward currents) were found in excised, inside-out patches. The 150 pS channel was completely inhibited by 100 nmol/L IbTX and displayed voltage- and calcium-dependent gating qualitatively similar to BK(Ca) in other cell types. 5. In cell attached (CA) patches, the response of BK(Ca) to bath AngII (100 nmol/L) was relatively minor in control solutions, but was considerably greater in the presence of db-cGMP. 6. In excised patches, Mg-ATP (1 mmol/L) plus db-cGMP (1 mu mol/L) activated BK(Ca) in the absence, but not the presence, of the non-specific kinase inhibitor, staurosporine. 7. Separate experiments showed that BK(Ca) were also activated by arachidonic acid and high ambient glucose concentrations. 8. These results indicate that: (i) resting MC tone is sensitive to glybenclamide and apamin; and (ii) the role of BK(Ca) as a negative feedback regulator of contraction is minimal under normal conditions but is markedly enhanced by cGMP-stimulating relaxants and arachidonic acid.