Beneficial Effects of Soluble Guanylyl Cyclase Stimulation and Activation in Sickle Cell Disease Are Amplified by Hydroxyurea: In Vitro and In Vivo Studies

The complex pathophysiology of sickle cell anemia (SCA) involves intravascular hemolytic processes and recurrent vaso-occlusion, driven by chronic vascular inflammation, which result in the disease’s severe clinical complications, including recurrent painful vaso-occlusive episodes. Hydroxyurea, the only drug frequently used for SCA therapy, is a cytostatic agent, although it appears to exert nitric oxide/soluble guanylyl cyclase (sGC) modulating activity. As new drugs that can complement or replace the use of hydroxyurea are sought to further reduce vaso-occlusive episode frequency in SCA, we investigated the effects of the sGC agonists BAY 60-2770 (sGC activator) and BAY 41-2272 (sGC stimulator) in the presence or absence of hydroxyurea on SCA vaso-occlusive mechanisms and cell recruitment both ex vivo and in vivo. These agents significantly reduced stimulated human SCA neutrophil adhesive properties ex vivo in association with the inhibition of surface β2-integrin activation. A single administration of BAY 60-2770 or BAY 41-2272 decreased tumor necrosis factor cytokine–induced leukocyte recruitment in a mouse model of SCA vaso-occlusion. Importantly, the in vivo actions of both agonists were significantly potentiated by the coadministration of hydroxyurea. Erythroid cell fetal hemoglobin (HbF) elevation is also a major goal for SCA therapy. BAY 41-2272 but not BAY 60-2770 at the concentrations employed significantly induced γ-globin gene transcription in association with HbF production in cultured erythroleukemic cells. In conclusion, sGC agonist drugs could represent a promising approach as therapy for SCA, for use either as stand-alone treatments or in combination with hydroxyurea.

The sGC stimulator riociguat inhibits platelet function in washed platelets but not in whole blood

BACKGROUND AND PURPOSE

Stimulation of soluble guanylyl cyclase (sGC) is a valuable therapeutic strategy for the treatment of several cardiovascular diseases. The sGC stimulator riociguat has been approved for the treatment of two forms of pulmonary hypertension. Platelets contain large amounts of sGC and play a key role in the regulation of haemostasis. Therefore, we investigated the effects of riociguat on platelet function.

EXPERIMENTAL APPROACH

The effect of riociguat treatment on human platelet activation and aggregation was investigated. The sGC-specific effects of riociguat were determined by comparing wild-type and platelet-specific sGC-knockout mice.

KEY RESULTS

Riociguat induced cGMP synthesis and subsequent PKG activation in human platelets, suggesting that the inhibitory effects are mediated by cGMP signalling. This finding was confirmed when sGC-knockout platelets were not inhibited by riociguat. In washed human platelets, 100 nM riociguat reduced ADP-induced GPIIb/IIIa activation, while a 10-fold higher concentration was required to reduce convulxin-stimulated GPIIb/IIIa activation. Riociguat inhibited ADP-induced platelet shape change and aggregation, while ATP-induced shape change remained unaffected. However, in PRP and whole blood, 50-100 μM riociguat was required to inhibit platelet activation and aggregation. Riociguat in combination with iloprost significantly inhibited platelet aggregation, even in whole blood.

CONCLUSIONS AND IMPLICATIONS

Riociguat inhibits platelet activation in whole blood only at concentrations above 50 μM, while the plasma concentrations in riociguat-treated patients are 150 to 500 nM. This finding indicates that riociguat treatment does not affect platelet function in patients. Nevertheless, the possibility that riociguat acts synergistically with iloprost to inhibit platelet activation should be considered.

Distinct molecular requirements for activation or stabilization of soluble guanylyl cyclase upon haem oxidation-induced degradation

BACKGROUND AND PURPOSE

In endothelial dysfunction, signalling by nitric oxide (NO) is impaired because of the oxidation and subsequent loss of the soluble guanylyl cyclase (sGC) haem. The sGC activator 4-[((4-carboxybutyl){2-[(4-phenethylbenzyl)oxy]phenethyl}amino)methyl[benzoic]acid (BAY 58-2667) is a haem-mimetic able to bind with high affinity to sGC when the native haem (the NO binding site) is removed and it also protects sGC from ubiquitin-triggered degradation. Here we investigate whether this protection is a unique feature of BAY 58-2667 or a general characteristic of haem-site ligands such as the haem-independent sGC activator 5-chloro-2-(5-chloro-thiophene-2-sulphonylamino-N-(4-(morpholine-4-sulphonyl)-phenyl)-benzamide sodium salt (HMR 1766), the haem-mimetic Zn-protoporphyrin IX (Zn-PPIX) or the haem-dependent sGC stimulator 5-cyclopropyl-2-[1-(2-fluoro-benzyl)-1H-pyrazolo[3,4-b]pyridin-3-yl]-pyrimidin-4-ylamine (BAY 41-2272).

EXPERIMENTAL APPROACH

The sGC inhibitor 1H-(1,2,4)-oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) was used to induce oxidation-induced degradation of sGC. Activity and protein levels of sGC were measured in a Chinese hamster ovary cell line as well as in primary porcine endothelial cells. Cells expressing mutant sGC were used to elucidate the molecular mechanism underlying the effects observed.

KEY RESULTS

Oxidation-induced sGC degradation was prevented by BAY 58-2667 and Zn-PPIX in both cell types. In contrast, the structurally unrelated sGC activator, HMR 1766, and the sGC stimulator, BAY 41-2272, did not protect. Similarly, the constitutively haem-free sGC mutant beta(1)H105F was stabilized by BAY 58-2667 and Zn-PPIX.

CONCLUSIONS

The ability of BAY 58-2667 not only to activate but also to stabilize oxidized/haem-free sGC represents a unique example of bimodal target interaction and distinguishes this structural class from non-stabilizing sGC activators and sGC stimulators such as HMR 1766 and BAY 41-2272, respectively.

Cardiovascular effects of a novel potent and highly selective azaindole-based inhibitor of Rho-kinase

BACKGROUND AND PURPOSE

Rho-kinase (ROCK) has been implicated in the pathophysiology of altered vasoregulation leading to hypertension. Here we describe the pharmacological characterization of a potent, highly selective and orally active ROCK inhibitor, the derivative of a class of azaindoles, azaindole 1 (6-chloro-N4-{3,5-difluoro-4-[(3-methyl-1H-pyrrolo[2,3-b]pyridin-4-yl)oxy]-phenyl}pyrimidine-2,4-diamine).

EXPERIMENTAL APPROACH

Pharmacological characterization of azaindole 1 was performed with human recombinant ROCK in vitro. Vasodilator activity was determined using isolated vessels in vitro and different animal models in vivo.

KEY RESULTS

This compound inhibited the ROCK-1 and ROCK-2 isoenzymes with IC50 s of 0.6 and 1.1 nM in an ATP-competitive manner. Although ATP-competitive, azaindole 1 was inactive against 89 kinases (IC50>10 microM) and showed only weak activity against an additional 21 different kinases (IC50=1-10 microM). Only the kinases TRK und FLT3 were inhibited by azaindole 1 in the sub-micromolar range, albeit with IC50 values of 252 and 303 nM, respectively. In vivo, azaindole 1 lowered blood pressure dose-dependently after i.v. administration in anaesthetized normotensive rats. In conscious normotensive and spontaneously hypertensive rats azaindole 1 induced a dose-dependent decrease in blood pressure after oral administration without inducing a significant reflex increase in heart rate. In anaesthetized dogs, azaindole 1 induced vasodilatation with a moderately elevated heart rate.

CONCLUSIONS AND IMPLICATIONS

Azaindole 1 is representative of a new class of selective and potent ROCK inhibitors and is a valuable tool for the elucidation of the role of ROCK in the cardiovascular system.

BAY 41-2272: a stimulator of soluble guanylyl cyclase induces nitric oxide-dependent penile erection in vivo

OBJECTIVES

To determine the effectiveness of BAY 41-2272 on penile erections in an in vivo rabbit model. The nitric oxide (NO)-dependent increase of intracellular cyclic guanosine monophosphate (cGMP) by cGMP-phosphodiesterase (PDE5) inhibition has been shown to be an effective mechanism in the treatment of erectile dysfunction. Direct, NO-independent stimulation of soluble guanylyl cyclase should also lead to elevated cGMP levels in tissues and could be an attractive alternative therapeutic option for the treatment of erectile dysfunction. BAY 41-2272 is a novel non-NO-based direct stimulator of soluble guanylyl cyclase that activates purified enzyme in a synergistic fashion with NO.

METHODS

BAY 41-2272 was administered to conscious rabbits intravenously (IV) and orally (PO). Erection was assessed in a time-dependent manner by measuring the length of the uncovered penile mucosa. Erections were evaluated in the absence and presence of NO (with intravenous sodium nitroprusside [SNP] as the NO donor).

RESULTS

BAY 41-2272 only induced weak penile erections in conscious rabbits after IV (1 mg/kg) and PO (10 mg/kg) administration in the absence of an NO donor. However, the efficacy of BAY 41-2272 was potentiated by the simultaneous administration of SNP. Through simultaneous SNP administration, the effective doses of BAY 41-2272 were reduced significantly (minimal effective dose 0.1 mg/kg IV and 1 mg/kg PO).

CONCLUSIONS

The results of this study clearly demonstrated the effect of BAY 41-2272 on penile erection in the conscious rabbit model after PO and IV administration. The time-course and onset of erection was concurrent with the stimulation by exogenous NO (SNP), suggesting that this new pharmacologic mechanism of soluble guanylyl cyclase stimulation could be used in the treatment of erectile dysfunction. Because the effect is increased by SNP, it can be expected that BAY 41-2272 would have enhanced activity during sexual arousal, when NO is produced endogenously.

Effects of in vivo nitroglycerin treatment on activity and expression of the guanylyl cyclase and cGMP-dependent protein kinase and their downstream target vasodilator-stimulated phosphoprotein in aorta

BACKGROUND

Chronic in vivo treatment with nitroglycerin (NTG) induces tolerance to nitrates and cross-tolerance to nitrovasodilators and endothelium-derived nitric oxide (NO). We previously identified increased vascular superoxide formation and reduced NO bioavailability as one causal mechanism. It is still controversial whether intracellular downstream signaling to nitrovasodilator-derived NO is affected as well.

METHODS AND RESULTS

We therefore studied the effects of 3-day NTG treatment of rats and rabbits on activity and expression of the immediate NO target soluble guanylyl cyclase (sGC) and on the cGMP-activated protein kinase I (cGK-I). Tolerance was induced either by chronic NTG infusion via osmotic minipumps (rats) or by NTG patches (rabbits). Western blot analysis, semiquantitative reverse transcription-polymerase chain reaction, and Northern blot analysis revealed significant and comparable increases in the expression of sGC alpha(1) and beta(1) subunit protein and mRNA. Studies with the oxidative fluorescent dye hydroethidine revealed an increase in superoxide in the endothelium and smooth muscle. Stimulation with NADH increased superoxide signals in both layers. Although cGK-I expression in response to low-dose NTG was not changed, a strong reduction in vasodilator-stimulated phosphoprotein (VASP) serine239 phosphorylation (specific substrate of cGK-I) was observed in tolerant tissue from rats and rabbits. Concomitant in vivo and in vitro treatment with vitamin C improved tolerance, reduced oxidative stress, and improved P-VASP.

CONCLUSIONS

We therefore conclude that increased expression of sGC in the setting of tolerance reflects a chronic inhibition rather than an induction of the sGC-cGK-I pathway and may be mediated at least in part by increased vascular superoxide.

NO-independent stimulators of soluble guanylate cyclase

SARs around a novel type of guanylate cyclase stimulator which act by a mechanism different from classical NO-donors are described. Several pyrazolopyridinylpyrimidines are shown to relax aortic rings and revealed a long-lasting blood pressure lowering effect in rats after oral application.

NO-independent regulatory site on soluble guanylate cyclase

Nitric oxide (NO) is a widespread, potent, biological mediator that has many physiological and pathophysiological roles. Research in the field of NO appears to have followed a straightforward path, and the findings have been progressive: NO and cyclic GMP are involved in vasodilatation; glycerol trinitrate relaxes vascular smooth muscles by bioconversion to NO; mammalian cells synthesize NO; and last, NO mediates vasodilatation by stimulating the soluble guanylate cyclase (sGC), a heterodimeric (alpha/beta) haem protein that converts GTP to cGMP2-4. Here we report the discovery of a regulatory site on sGC. Using photoaffinity labelling, we have identified the cysteine 238 and cysteine 243 region in the alpha1-subunit of sGC as the target for a new type of sGC stimulator. Moreover, we present a pyrazolopyridine, BAY 41-2272, that potently stimulates sGC through this site by a mechanism that is independent of NO. This results in antiplatelet activity, a strong decrease in blood pressure and an increase in survival in a low-NO rat model of hypertension, and as such may offer an approach for treating cardiovascular diseases.

The vasodilator-stimulated phosphoprotein (VASP): target of YC-1 and nitric oxide effects in human and rat platelets

The effects of the different types of soluble guanylate cyclase (sGC) stimulators on the phosphorylation status of vasodilator-stimulated phosphoprotein (VASP) in both human and rat platelets were studied under in vitro and in vivo conditions. sGC-dependent VASP phosphorylation (at Ser(239) and Ser(157)) both by the new direct sGC stimulator YC-1 and by NO donors was examined by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS/PAGE) with different antibodies. One antibody, which recognizes VASP independent of its phosphorylation state, was used to detect the mobility shift of VASP caused by Ser(157) phosphorylation. The other antibody was specifically directed against VASP phosphorylated at Ser(239), the cGMP-dependent protein kinase (PKG) preferred phosphorylation site of VASP. In vitro YC-1 increased both VASP phosphorylation and cyclic guanosine monophosphate (cGMP) levels as did the NO donors 2-(N,N-diethylamino)-diazenolate-2-oxide (DEA/NO) and sodium nitroprusside (SNP). The combination of both types induced a synergistic effect in both VASP phosphorylation and cGMP increase. In rat platelets, similar effects could be shown in vitro. In vivo we observed a significant increase in cGMP and a distinct effect on VASP phosphorylation in rat platelets 1 h after oral administration of YC-1. These biochemical alterations are supported by a significant prolongation in rat-tail bleeding time. Direct stimulators of sGC like YC-1 are on the one hand direct potent stimulators of the cGMP/PKG/VASP pathway in platelets and on the other hand synergize with NO, the physiologic stimulator of sGC. Therefore YC-1-like substances are interesting tools for the development of new cardiovascular drugs with vasodilatory and antithrombotic properties.

Generation and characterization of a stable soluble guanylate cyclase-overexpressing CHO cell line

A stably transfected soluble guanylate cyclase (sGC, alpha1 and beta1 subunits of the rat lung enzyme)-overexpressing CHO cell line was generated for the characterization of different types of activators of the soluble guanylate cyclase. Polyclonal antibodies directed against both subunits of the rat enzyme were used to detect both subunits in the cytosol of the transfected CHO cells. We studied the effects of different nitric oxide (NO) donors like SNP and DEA/NO and, in particular, the direct, NO-independent stimulator of the soluble guanylate cyclase 3-(5'-hydroxymethyl-2'furyl)-1-benzyl indazole (YC-1), on intracellular guanosine 3',5'-cyclic monophosphate (cGMP) production. DEA/NO (0.01-3 microM), SNP (1-10 microM), and YC-1 (1-10 microM) induced a concentration-dependent intracellular cGMP increase with maximal effects of 16-fold (3 microM DEA/NO), 8-fold (10 microM SNP), and 6-fold (10 microM YC-1) stimulation compared to controls, respectively. In addition, a synergistic effect of the combination of the NO donor and YC-1 could be observed with a maximal stimulation of 64-fold by SNP (10 microM) and YC-1 (10 microM). 1H-(1,2,4)-Oxadiazolo-(4,3-a)-6-bromo-quinoxazin-1-one (ODQ, 10 microM), a potent and selective inhibitor of sGC, inhibited both the single effects of NO donors [DEA/NO (3 microM), 77%; SNP (3 microM), 83%] and YC-1 [YC-1 (3 microM), 82%], but moreover the synergistic effects between NO donors and YC-1 [DEA/NO (3 microM) + YC-1 (3 microM), 81%; SNP (3 microM) + YC-1 (3 microM),89%] on intracellular cGMP production. In summary,we have generated a simple, sensitive, and useful bioassay method to characterize all types of sGC activators on the cellular level without the need of primary cell culture, several transfections, or purifying enzyme from biological materials.

Increased NADH-oxidase-mediated superoxide production in the early stages of atherosclerosis: evidence for involvement of the renin-angiotensin system

BACKGROUND

Angiotensin II activates NAD(P)H-dependent oxidases via AT1-receptor stimulation, the most important vascular source of superoxide (O2*-). The AT1 receptor is upregulated in vitro by low-density lipoprotein. The present study was designed to test whether hypercholesterolemia is associated with increased NAD(P)H-dependent vascular O2*- production and whether AT1-receptor blockade may inhibit this oxidase and in parallel improve endothelial dysfunction.

METHODS AND RESULTS

Vascular responses were determined by isometric tension studies, and relative rates of vascular O2*- production were determined by use of chemiluminescence with lucigenin, a cypridina luciferin analogue, and electron spin resonance studies. AT1-receptor mRNA was quantified by Northern analysis, and AT1-receptor density was measured by radioligand binding assays. Hypercholesterolemia was associated with impaired endothelium-dependent vasodilation and increased O2*- production in intact vessels. In vessel homogenates, we found a significant activation of NADH-driven O2*- production in both models of hyperlipidemia. Treatment of cholesterol-fed animals with the AT1-receptor antagonist Bay 10-6734 improved endothelial dysfunction, normalized vascular O2*- and NADH-oxidase activity, decreased macrophage infiltration, and reduced early plaque formation. In the setting of hypercholesterolemia, the aortic AT1 receptor mRNA was upregulated to 166+/-11%, accompanied by a comparable increase in AT1-receptor density.

CONCLUSIONS

Hypercholesterolemia is associated with AT1-receptor upregulation, endothelial dysfunction, and increased NADH-dependent vascular O2*- production. The improvement of endothelial dysfunction, inhibition of the oxidase, and reduction of early plaque formation by an AT1-receptor antagonist suggests a crucial role of angiotensin II-mediated O2*- production in the early stage of atherosclerosis.

Purified soluble guanylyl cyclase expressed in a baculovirus/Sf9 system: stimulation by YC-1, nitric oxide, and carbon monoxide

Soluble guanylyl cyclase (sGC) is the main receptor for nitric oxide, a messenger molecule with multiple clinical implications. Understanding the activation of sGC is an important step for establishing new therapeutic principles. We have now overexpressed sGC in a baculovirus/Sf9 system optimized for high protein yields to facilitate spectral and kinetic studies of the activation mechanisms of this enzyme. It was expressed in a batch fermenter using a defined mixture of viruses encoding the alpha and beta1 subunits of the rat lung enzyme. The expressed enzyme was purified from the cytosolic fraction by anion exchange chromatography, hydroxyapatite chromatography, and size exclusion chromatography. By use of this new method 2.5 l culture yielded about 1 mg of apparently homogeneous sGC with a content of about one heme per heterodimer without the need of a heme reconstitution step. The enzyme did not contain stoichiometric amounts of copper. The basal activities of the purified enzyme were 153 and 1259 nmol min(-1) mg(-1) in the presence of Mg2+ and Mn2+, respectively. The nitric oxide releasing agent 2-(N,N-diethylamino)-diazenolate-2-oxide (DEA/NO) stimulated the enzyme 160-fold with Mg2+, whereas the NO-independent activator 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1) induced an increase in the activity of 101-fold at a concentration of 300 microM. The combination of DEA/NO (10 microM) and YC-1 (100 microM) elicited a dose-dependent synergistic stimulation with a maximum of a 792-fold increase over the basal activity in the presence of Mg2+, resulting in a specific activity of 121 micromol min(-1) mg(-1). The synergistic stimulation of DEA/NO and YC-1 was attenuated by the sGC inhibitor 1H-(1,2,4)oxadiazole(4,3-a)quinoxalin-1-one (ODQ) (10 microM) by 94%. In a different experimental setup a saturated carbon monoxide solution in the absence of ambient oxygen or NO stimulated the enzyme 15-fold in the absence and 1260-fold in the presence of YC-1 compared to an argon control. The heme spectra of the enzyme showed a shift of the Soret peak from 432 to 399 and 424 nm in the presence of DEA/NO or carbon monoxide, respectively. The heme spectra were not affected by YC-1 in the absence or in the presence of DEA/NO or of carbon monoxide, which reflects the fact that YC-1 does not interact directly with the heme group of the enzyme. In summary, this study shows that our expression/purification procedure is suitable for producing large amounts of highly pure sGC which contains one heme per heterodimer without a reconstitution step. The activator experiments show that in a synergistic stimulation with YC-1 sGC can be activated maximally both by nitric oxide and by carbon monoxide and that YC-1 does not directly act via heme. The described method should help to facilitate the investigation of the new therapeutic principle of NO-independent guanylyl cyclase activators.

Effects of angiotensin II type 1 receptor blockade and angiotensin-converting enzyme inhibition on cardiac beta-adrenergic signal transduction

Inhibition of the renin-angiotensin system has been shown to improve symptoms and prognosis in heart failure. We compared the effects of inhibition of angiotensin-converting enzyme or blockade of angiotensin II type 1 (AT1) receptors in a model with renin-induced hypertension that is known to exhibit similar changes in sympathetic activation and beta-adrenergic desensitization, as observed in heart failure. Treatment with captopril (100 mg/kg of feed) or the AT1-antagonist Bay 10-6734 (100 mg/kg of feed) was performed in transgenic rats harboring the mouse renin 2d gene [TG(mREN2)27]. Neuropeptide Y and angiotensin II levels, adenylyl cyclase activity, beta-adrenergic receptors, G(salpha), and G(ialpha) were investigated. TG(mREN2)27 showed a depletion of myocardial neuropeptide Y stores and an increase in myocardial angiotensin II concentrations. Isoprenaline- and guanylylimidodiphosphate-stimulated adenylyl cyclase activities and beta-adrenergic receptor density were reduced, whereas the catalyst and G(salpha)-function were unchanged. G(ialpha) protein and mRNA concentrations were increased. All alterations were normalized by both treatments. Systolic left ventricular pressures, plasma atrial natriuretic peptide, and myocardial steady state atrial natriuretic peptide mRNA concentrations and heart weights were similarly reduced by both treatments. Sympathetic neuroeffector defects are similarly reversed by angiotensin-converting enzyme inhibition or AT1 antagonism. The data support the concept that pharmacological interventions in the myocardial renin-angiotensin system significantly reverse local sympathetic neuroeffector defects. This could be important for the beneficial effects of these agents.

Positive inotropic effect of exogenous and endogenous NO in hypertrophic rat hearts

1. Recent evidence suggests that nitric oxide (NO) modulates the contractile force of isolated cardiomyocytes in a biphasic manner. We sought to examine whether myocardial hypertrophy induced by long-term hypertension changes the effects of NO on myocardial contractility. 2. We used constant flow perfused non-paced Langendorff preparations of hearts of 3 months old Wistar rats (WIS, n = 23) and of stroke-prone spontaneously hypertensive rats (SHR) at the age of 10 months (SHR10, n = 16) and 15 months (SHR15, n = 8). Changes of left ventricular peak pressure (LVP), +dP/dt(max), -dP/dt(max), coronary perfusion pressure (CPP) and heart rate (HR) were recorded after infusion of noradrenaline (NA, 0.1 micromol l(-1)), glyceryl trinitrate (GTN, 1-100 micromol l(-1)), S-nitroso-N-acetyl-D,L-penicillamine (SNAP, 1-10 micromol l(-1)) and N(omega)-nitro-L-arginine (L-NOARG, 0.1-1 mmol l(-1)). 3. Long-term hypertension induced myocardial hypertrophy and an abnormal response to NA. The relative heart weight (in mg kg(-1)) increased from 2.95 +/- 0.04 (WIS) to 6.67 +/- 0.34 (SHR15), while the increase in +dP/dt(max) induced by NA was absent in SHR15. Hearts of SHR10 showed an intermediate response. 4. Both SNAP and GTN significantly increased LVP, +dP/dt(max) and -dP/dt(max) in hearts of WIS and of SHR. In WIS but not in SHR10, SNAP also increased HR. In SHR10 the lowest concentration of SNAP (1 micromol l(-1)) showed no effect on contractility but a significantly diminished reduction of CPP suggesting inactivation of extracellularly released NO in the coronary circulation of SHR. 5. L-NOARG significantly reduced contractility in hearts of WIS and of SHR to a similar extent. At a concentration of 1 mmol l(-1) L-NOARG also reduced HR. 6. These results suggests that positive inotropic effects of exogenous and endogenous NO are not changed in hypertension induced myocardial hypertrophy.

Angiotensin receptor antagonism and angiotensin converting enzyme inhibition improve diastolic dysfunction and Ca(2+)-ATPase expression in the sarcoplasmic reticulum in hypertensive cardiomyopathy

BACKGROUND

Hypertensive cardiomyopathy is a major risk factor for the development of chronic heart failure.

OBJECTIVE

To investigate whether treatment with an angiotensin converting enzyme inhibitor (ACEI) or with an angiotensin type 1 receptor antagonist (AT1-RA) is sufficient to prevent the development of hypertensive cardiomyopathy and cardiac contractile dysfunction. Special emphasis was placed on the effects of both treatments on sarcoplasmic reticulum Ca(2+)-ATPase (SERCA 2a) gene expression as a major cause of impaired diastolic cardiac relaxation.

METHODS AND RESULTS

Eight-week-old rats harboring the mouse renin 2d gene [TG(mREN2)27] were treated for 8 weeks with 100 mg/kg captopril (Cap) in their food and 100 mg/kg of the AT1-RA Bay 10-6734 (Bay) in their food. Untreated TG(mREN2)27 and Sprague-Dawley rats (SDR) were used as controls. Both treatment regimens normalized the left ventricular weight, which was increased significantly (P < 0.001) in TG(mREN2)27. Both treatments normalized the left ventricular end-systolic and end-diastolic pressures, which were significantly (P < 0.001) higher in TG(mREN2)27 than they were in SDR, and they improved the velocity of the decrease in pressure [P < 0.05, Bay and Cap versus TG(mREN2)27]. Decreased left ventricular SERCA 2a mRNA and protein levels and increased atrial natriuretic peptide messenger RNA levels were normalized by Bay and Cap treatments (P < 0.05, Bay and Cap versus TG(mREN2)27, by Northern and Western blotting). According to radioimmunoassay and an enzyme assay, respectively, Bay, but not Cap, increased plasma angiotensin I concentrations and the renin activity above normal levels (P < 0.05), whereas myocardial angiotensin II concentrations (determined by radioimmunoassay), which were significantly (P < 0.05) increased in TG(mREN2)27, were normalized equally by Bay and Cap.

CONCLUSIONS

In renin-induced hypertensive cardiomyopathy, left ventricular diastolic dysfunction occurs at the stage of compensated myocardial hypertrophy. The decreased left ventricular relaxation velocity might be due to reduced SERCA 2a gene expression. In this model of hypertensive cardiomyopathy, AT1-RA and ACEI treatments are similarly effective at reducing the arterial pressure, preventing myocardial hypertrophy and diastolic contractile dysfunction. Normalization of SERCA 2a gene expression, either by AT1-RA or by ACEI treatment, might contribute to the improvement in diastolic function.

Long-term blockade of the angiotensin II receptor in renin transgenic rats, salt-loaded Dahl rats, and stroke-prone spontaneously hypertensive rats

These studies were designed to investigate the protective effects of the new angiotensin II receptors antagonist BAY 10-6734 (6-n-butyl-4-methoxycarbonyl-2-oxo-1[(2'-(1H-tetrazol-5-yl) -3-fluorobiphenyl-4-yl)methyl] 1,2-dihydropyridine, CAS 156001-18-2) on haemodynamic, hormonal, renal, and structural parameters in renin transgenic rats (TGR(mRen2)27), salt-loaded Dahl S and R rats, and salt-loaded stroke-prone spontaneously hypertensive rats (SHR-SP) in long-term trials. Study 1: In SHR-SP the development of blood pressure, cardiac hypertrophy, and the deleterious effects of salt loading on kidney structure and kidney function was prevented by BAY 10-6734. Study 2: In salt-loaded Dahl S rats with a suppressed plasma renin activity treatment with BAY 10-6734 did not delay the increase in blood pressure but prevented cardiac hypertrophy and the increase in plasma ANP (Atrial natriuretic peptide). Study 3: TGR develop malignant hypertension associated with cardiac hypertrophy, elevated left-ventricular end-diastolic pressure and increased plasma ANP. After 6 weeks of treatment with BAY 10-6734 (30 mg/kg p.o. bid) cardiac pump function was improved and cardiac hypertrophy was reversed in this angiotension dependent form of hypertension. The beneficial effects of BAY 10-6734 in these different animal hypertension models are also emphasized by a reduction in mortality.

Effect of YC-1, an NO-independent, superoxide-sensitive stimulator of soluble guanylyl cyclase, on smooth muscle responsiveness to nitrovasodilators

1. We studied the effects of 3-(5'-hydroxymethyl-2'furyl)-1-benzyl indazole (YC-1) on the activity of purified soluble guanylyl cyclase (sGC), the formation of guanosine-3':5' cyclic monophosphate (cyclic GMP) in vascular smooth muscle cells (VSMC), and on the tone of rabbit isolated aortic rings preconstricted by phenylephrine (PE). In addition, we assessed the combined effect of YC-1, and either NO donors, or superoxide anions on these parameters. 2. YC-1 elicited a direct concentration-dependent activation of sGC (EC50 18.6 +/- 2.0 microM), which was rapid in onset and quickly reversible upon dilution. YC-1 altered the enzyme kinetics with respect to GTP by decreasing KM and increasing Vmax. Activation of sGC by a combination of sodium nitroprusside (SNP) and YC-1 was superadditive at low and less than additive at high concentrations, indicating a synergistic activation of the enzyme by both agents. A specific inhibitor of sGC, 1H-(1,2,4)-oxdiazolo-(4,3-a)-6-bromo-quinoxazin-1-one (NS 2028), abolished activation of the enzyme by either compound. 3. YC-1 induced a concentration-dependent increase in intracellular cyclic GMP levels in rat cultured aortic VSMC, which was completely inhibited by NS 2028. YC-1 applied at the same concentration as SNP elicited 2.5 fold higher cyclic GMP formation. Cyclic GMP-increases in response to SNP and YC-1 were additive. 4. YC-1 relaxed preconstricted endothelium-denuded rabbit aortic rings in a concentration-dependent manner (50% at 20 microM) and markedly increased cyclic GMP levels. Relaxations were inhibited by NS 2028. A concentration of YC-1 (3 microM), which elicited only minor effects on relaxation and cyclic GMP, increased the vasodilator potency of SNP and nitroglycerin (NTG) by 10 fold and markedly enhanced SNP- and NTG-induced cyclic GMP formation. 5. Basal and YC-1-stimulated sGC activity was sensitive to inhibition by superoxide (O-2) generated by xanthine/xanthine oxidase, and was protected from this inhibition by superoxide dismutase (SOD). YC-1-stimulated sGC was also sensitive to inhibition by endogenously generated (O-2 in rat preconstricted endothelium-denuded aortic rings. Relaxation to YC-1 was significantly attenuated in aortae from spontaneously hypertensive rats (SHR), which generated O-2 at a higher rate than aortae from normotensive Wistar Kyoto rats (WKY). SOD restored the vasodilator responsiveness of SHR rings to YC-1. 6. In conclusion, these results indicate that YC-1 is an NO-independent, O-2-sensitive, direct activator of sGC in VSMC and exerts vasorelaxation by increasing intracellular cyclic GMP levels. The additive or even synergistic responses to NO-donors and YC-1 in cultured VSMC and isolated aortic rings apparently reflect the direct synergistic action of YC-1 and NO on the sGC. The synergism revealed in this in vitro study suggests that low doses of YC-1 may be of therapeutic value by permitting the reduction of nitrovasodilator dosage.

Neutral endopeptidase inhibition potentiates the effects of natriuretic peptides in renin transgenic rats

The influence of neutral endopeptidase (NEP) inhibition with (S)-thiorphan on the hormonal, renal, and blood-pressure-lowering effects of an infusion of atrial (ANP), brain (BNP), and C-type natriuretic peptide (CNP) was evaluated in hypertensive transgenic rats (TGR) harboring an additional mouse renin gene (TGR(m(Ren2)27)). These TGR possess an activated natriuretic peptide system as compared with Sprague-Dawley rats (SDR), used in this study as control. (S)-Thiorphan significantly decreased blood pressure in anesthetized TGR but not in anesthetized SDR during the 60-min infusion period. Exogenously administered ANP decreased blood pressure in SDR with no significant effects in TGR after 60 min. In contrast, BNP infusion significantly decreased blood pressure in TGR, while changes in SDR were not significant. The blood pressure was further decreased after combined infusion of ANP and BNP with (S)-thiorphan in TGR. No effect on blood pressure was registered during infusion of CNP in either experimental group. The plasma levels of ANP, BNP, and cGMP were higher in TGR than in SDR, whereas plasma renin activity was lower. Co-administration of ANP, BNP, or CNP with the NEP inhibitor (S)-thiorphan potentiated the plasma ANP, BNP, and cGMP. Infusion of ANP alone did not affect BNP plasma levels of TGR and vice versa. In contrast, CNP infusion increased ANP plasma levels in both TGR and SDR. Renal excretion of sodium and cGMP increased after infusion of (S)-thiorphan and ANP or BNP in both TGR and SDR. The combination of ANP and (S)-thiorphan had a slightly greater effect on urinary excretion of sodium and cGMP in TGR than either compound alone, but the effects were more pronounced in SDR than in TGR. Finally, infusion of CNP alone and in combination with (S)-thiorphan influenced the excretion of sodium and cyclic GMP only slightly. These results indicate that inhibition of neutral endopeptidase by (S)-thiorphan potentiates the hemodynamic and renal effects of natriuretic peptides ANP and BNP, and to some extent those of CNP, in hypertensive TGR and normotensive SDR. In contrast to ANP and BNP, infusion of CNP had no effect on the blood pressure in anesthetized TGR or SDR. Inhibition of NEP therefore seems to be a promising way to potentiate endogenous levels of natriuretic peptides, which may be of therapeutic benefit in cardiovascular diseases such as hypertension or heart failure.

Cardiorenal consequences of dual angiotensin converting enzyme and neutral endopeptidase 24.11 inhibition in transgenic rats with an extra renin gene

The cardiovascular consequences of mixed angiotensin converting enzyme and neutral endopeptidase (ACE/NEP) inhibition with alatriopril/alatrioprilat were compared with the consequences of endopeptidase (NEP) inhibition alone with (S)-thiorphan/ecadotril by determining the acute effects of the compounds on hemodynamic, hormonal, and renal parameters in hypertensive transgenic rats harboring an additional mouse renin gene (TGR(mRen2)27). Infusion of alatrioprilat and (S)-thiorphan in anesthetized TGR decreased blood pressure in a dose-dependent manner, but heart rate remained unchanged. The renal excretion of water, sodium, and cGMP also increased dose-dependently, with nearly the same maximal effects after infusion of (S)-thiorphan and alatrioprilat. At the end of infusion, plasma ANP and cGMP were elevated both after (S)-thiorphan and after alatrioprilat, whereas plasma renin activity increased only after alatrioprilat. The ACE inhibition effect was studied in ganglion-blocked rats receiving a continous infusion of angiotensin I. Alatrioprilat decreased the mean blood pressure dose-dependently, but about 30 times higher concentrations were needed to produce the same effects as the ACE inhibitor captopril. At a dose of 30 mg/kg p.o., ecadotril, the orally active prodrug of (S)-thiorphan, decreased the systolic blood pressure in conscious TGR by 22 mmHg for 6 h, whereas alatriopril (100 mg/kg p.o.) also reduced the systolic pressure in these rats with a maximal reduction of 22 mmHg. In addition, ecadotril and alatriopril significantly increased the urinary excretion of sodium. In contrast, ACE inhibition with captopril decreased the excretion of sodium dose-dependently in conscious TGR. In conclusion, combined ACE/NEP inhibition produced a comparable lowering of blood pressure and improvement in renal function as those with NEP inhibition in TGR. Dual ACE/NEP inhibition may therefore be useful in cardiovascular conditions such as hypertension or heart failure.

Renal and antihypertensive effects of neutral endopeptidase inhibition in transgenic rats with an extra renin gene

The cardiovascular consequences of neutral endopeptidase (NEP) inhibition with the NEP inhibitor ecadotril were evaluated by determining acute and long-term effects of the compound on hemodynamic, hormonal, renal, and structural parameters in hypertensive transgenic rats harboring a mouse renin gene (TGR (m(Ren2)27) and in normotensive controls (Sprague-Dawley rats, SDR). Acute administration of ecadotril (10 and 30 mg/kg, orally) produced a dose-dependent decrease in systolic blood pressure with a maximal effect of -23 mm Hg between 2 and 4 h after oral administration. The NEP activity in plasma was significantly inhibited and the plasma levels of atrial (ANP) and brain (BNP) natriuretic peptides and their second messenger, cyclic GMP, were distinctly raised after oral administration. In addition, ecadotril (10 and 30 mg/kg, orally) produced a dose-dependent increase in the urinary excretion of sodium and cyclic GMP. These effects were more pronounced in TGR (mRen2)27 than in the normotensive SDR without an activated natriuretic peptide system. In the long-term study, the systolic pressure in control TG (m(Ren2)27) rats increased from 213 +/- 5 to 255 +/- 7 mm Hg, whereas, in animals treated with ecadotril (30 mg/kg, orally twice daily), the blood pressure increased only from 213 +/- 5 to 227 +/- 6 mm Hg during the observation period of 13 weeks. The increases in heart weight and in kidney weight were also delayed. At the end of the study, cyclic GMP was elevated and ANP tended to be higher, whereas plasma renin activity had decreased. These data indicate a beneficial pharmacological profile of neutral endopeptidase inhibition that could prove useful in the treatment of cardiovascular diseases like hypertension.

Role of neutral endopeptidase 24.11 in AV fistular rat model of heart failure

OBJECTIVE

The aortovenocaval fistular (AVF) rat represents a model of heart failure caused by increased cardiac volume overload and reduced renal function. Both circulating vasoconstrictors like the renin-angiotensin-aldosterone system and vasodilators like atrial and brain natriuretic peptides (ANP and BNP) are activated in this animal model of heart failure. In addition, neutral endopeptidase 24.11 (NEP) in plasma and urine is elevated in AVF rats. In the present investigation we examined the renal and hormonal effects of the NEP inhibitor, ecadotril, in acute and chronic studies in rats with an aortovenocaval fistula (AVF).

METHODS

Sprague Dawley rats (350-430 g) were prepared by introducing a shunt between abdominal aorta and the vena cava.

RESULTS

Acute administration of the neutral endopeptidase inhibitor, ecadotril (30 mg/kg p.o.), significantly improved the reduced renal excretion of sodium in AVF rats (83 +/- 10 to 145 +/- 14 mumol/kg/h, P < 0.01) but had no significant effect in sham-operated rats. However, neutral endopeptidase activity in urine was significantly decreased after ecadotril in both groups. Plasma ANP was increased after ecadotril only in AVF rats (275 +/- 83 to 748 +/- 187 pg/ml, P < 0.05), whereas the increase in plasma BNP was not statistically significant. After 4 weeks of observation the ANP and BNP plasma levels, renin activity (PRA), angiotensin I, and neutral endopeptidase activity were significantly higher in AVF rats than in sham-operated rats. Four weeks on ecadotril (30 mg/kg p.o., b.i.d.) increased plasma ANP (245 +/- 48 as opposed to 450 +/- 77 pg/ml, P < 0.05) and decreased PRA (11.3 +/- 1.5 as opposed to 6.8 +/- 1.2 ng/ml/h, P < 0.005) in AVF rats. Plasma NEP activity was inhibited in both groups. Ventricle weight was significantly higher in AVF rats than in sham-operated controls, and ecadotril treatment over 4 weeks decreased ventricular hypertrophy to a slight extent.

CONCLUSION

These results indicate that in the AVF rat model of heart failure the neutral endopeptidase inhibitor, ecadotril, improves the reduced kidney function in AVF rats by raising natriuretic peptides in plasma and probably in urine. NEP inhibition with ecadotril could therefore offer useful therapeutic possibilities in the treatment of heart failure.

Prolonged endothelin blockade prevents hypertension and cardiac hypertrophy in stroke-prone spontaneously hypertensive rats

The cardiovascular consequences of endothelin (ET) blockade with the ETA-receptor antagonist FR 139317 were evaluated by determining the long-term effects of the drug on hemodynamic, hormonal, renal and structural parameters in stroke-prone spontaneously hypertensive rats (SHR-SP). Young SHR-SP on a high-sodium diet develop malignant hypertension accompanied by renovascular and cerebrovascular lesions. In control SHR-SP the systolic blood pressure increased from 196 +/- 3 to 260 +/- 4 mm Hg, whereas in animals treated with FR 139317 (20 mg/kg intraperitoneally, twice daily) it increased only from 196 +/- 4 to 212 +/- 3 mm Hg during a treatment period of 6 weeks. There was also an increase in heart weight. At the end of the experiment the plasma levels of atrial natriuretic peptide and brain natriuretic peptide were significantly lower in the group treated with FR 139317 than in the controls. The endothelin plasma levels were significantly higher and the plasma renin activity was lower in the group treated with the endothelin receptor antagonist. These data indicate that endothelin is involved in the maintenance of high blood pressure and cardiac hypertrophy in malignant hypertension, as exemplified by SHR-SP.

Interaction of a neutral endopeptidase inhibitor with an ANP-C receptor ligand in anesthetized dogs

Inhibition of important degradative pathways of atrial natriuretic peptide (ANP) in vivo could be a valuable therapeutic tool for regulating endogenous levels of ANP. The aim was to investigate the in vivo effects of both blockade of atrial natriuretic peptide clearance receptor and inhibition of neutral endopeptidase 24.11, an enzyme shown to be involved in ANP breakdown. Therefore, we infused a specific neutral endopeptidase inhibitor ((S)-thiorphan) and an ANP-C receptor ligand (AP 811) alone or in combination into anaesthetized beagle dogs. Compared with vehicle controls, coadministration of (S)-thiorphan and AP 811 (100 micrograms/kg/min and 10 micrograms/kg/min, resp.) had greater effects on endocrine and renal parameters than administration of either substance alone. Coadministration of both compounds increased urinary excretion of volume and sodium, cGMP and ANP. We found also increased plasma cGMP, plasma ANP and decreased plasma renin activity. No effects were observed with respect to blood pressure, left ventricular pressure or heart rate during the infusion period of 2 h. We conclude from these investigations, that blocking both degrading pathways of ANP with the ANP-C receptor ligand AP 811 and the neutral endopeptidase inhibitor (S)-thiorphan is more effective than inhibition of either system alone. Such a combination might therefore be a useful therapeutic tool in cardiovascular diseases.

Prolonged inhibition of neutral endopeptidase 24.11 by sinorphan in stroke-prone spontaneously hypertensive rats

The cardiovascular consequences of inhibition of the neutral endopeptidase 24.11 (NEP) with the orally active NEP inhibitor sinorphan were evaluated by determining long-term effects of the drug on hemodynamic, hormonal and structural parameters in stroke-prone spontaneously hypertensive rats (SHR-SP). Systolic blood pressure increased in young SHR-SP from 194 +/- 2 to 266 +/- 7 mmHg, whereas in sinorphan (30 mg/kg p.o. bid) treated animals systolic blood pressure increased only from 193 +/- 4 to 229 +/- 4 mmHg during the treatment period of 9 weeks. The increase in relative heart weight was also delayed. Plasma ANP was higher in the sinorphan group than in the controls. The results of a second study demonstrate a substantial improvement of cardiac pump function and ventricular hypertrophy in old SHR-SP with compromised cardiac function by long-term inhibition of NEP. Thirteen-month-old SHR-SP were treated with sinorphan (30 mg/kg p.o. bid) for two weeks. At the end of experiment, the increase in ANP plasma levels did not reach statistical significance, whereas plasma cGMP was higher in sinorphan treated animals than in controls. Left ventricular end-diastolic pressure was markedly elevated in controls and significantly lower in sinorphan treated animals. In addition, sinorphan reduced cardiac hypertrophy in these old SHR-SP. In conclusion, the results of the present studies demonstrate that long-term NEP inhibition with sinorphan has inhibitory effects on malignant hypertension and associated cardiac hypertrophy in young SHR-SP on a high-sodium diet. NEP inhibition substantially improves cardiac pump function and reduces ventricular hypertrophy of old SHR-SP with compromised cardiac function.

Renal effects of captopril and nitrendipine in transgenic rats with an extra renin gene

We investigated the acute effects of captopril and nitrendipine on renal function and sodium excretion in hypertensive, male, heterozygous transgenic rats harboring a mouse renin gene [TGR (mRen-2)27]. Both drugs reduced blood pressure dose dependently in conscious transgenic rats. The oral ED20 for captopril was 0.5 mg/kg and 2.7 mg/kg for nitrendipine. In orally salt-loaded (20 mL/kg saline) transgenic rats captopril (0.3 to 3.0 mg/kg) reduced sodium excretion by approximately 90% in the 6 hours after administration, whereas equally antihypertensive doses of nitrendipine increased sodium excretion by approximately 100%. The antinatriuretic effect of captopril was accompanied by a reduction in creatinine clearance and a decrease in the excretion of cyclic GMP. In orally water-loaded (20 mL/kg water) transgenic rats captopril also reduced sodium excretion by more than 90%, and nitrendipine slightly increased sodium excretion. In control Sprague-Dawley rats the effects were opposite; namely, captopril tended to increase natriuresis, and nitrendipine caused a small but distinct decrease in sodium excretion. Intravenous captopril in anesthetized transgenic rats caused an antinatriuresis with a decrease in inulin clearance but not in Sprague-Dawley rats. To control for non-renin-related effects of captopril, we gave transgenic rats oral losartan. Losartan also decreased urinary sodium excretion. The results suggest a role for the renin-angiotensin system in the maintenance of glomerular filtration rate and sodium excretion in transgenic TGR (mRen-2)27 rats.

Effects of nimodipine on the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced depletions in the biogenic amine levels in mice

In view of the calcium hypothesis the effect of nimodipine (Bay e 9736, CAS 66085-59-4) on the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced depletions in dopamine and serotonin were investigated in C57-BL/6 mice. Oral treatment with nimodipine (5 mg/kg and 20 mg/kg b.i.d., respectively, for 9 days) prior to, during and following administration of MPTP appeared to attenuate MPTP-induced neurochemical changes in a dose-related manner. The results suggest that nimodipine reduces MPTP-induced damages, especially in the serotoninergic system, through its calcium antagonistic effects.

Inhibition by atrial natriuretic peptide of endothelin-1-stimulated proliferation of vascular smooth-muscle cells

The effect of endothelin-1 (ET-1) on proliferation of aortic vascular smooth-muscle cells (VSMCs) from spontaneously hypertensive (SHRs) and normotensive Wistar-Kyoto (WKY) rats was assessed by the measurement of [3H]-thymidine incorporation into DNA. ET-1 stimulated DNA synthesis in a concentration-dependent manner. Half-maximal stimulation occurred at a concentration of 7 x 10(-11) M. Three separate administrations of ET-1 to the cell cultures resulted in a half-maximal stimulation at 3 x 10(-12) M in of VSCMs from SHRs. VSMCs from SHRs responded to a far greater extent compared with WKY rats. The stimulatory effect of ET-1 was significantly attenuated by atrial natriuretic peptide (ANP). Repeated administration of ANP led to exacerbation of the inhibitory effect. Serum-stimulated DNA synthesis was not influenced by ANP. The proliferative action of ET-1 and the inhibition by ANP are discussed with respect to the development of vascular disease in atherosclerosis and hypertension.

Autoradiographic localization of [125I]-C-ANP compared to [125I]-ANP in rat tissue

Whole-body autoradiography demonstrated the different distribution of [125I]-C-ANP and [125I]-ANP to rat tissues. Highest enrichment of radioactivity of both labelled peptides was found in the kidney. In some organs we found remarkable differences between [125I]-ANP and [125I]-C-ANP. In the kidney cortex, especially in the glomeruli, as well as in the endocardium, the zona glomerulosa and the medulla of the adrenal gland, where high levels of radioactivity after [125I]-ANP administration were detected, no or just few radioactivity was found after administration of [125I]-C-ANP. On the other hand in the kidney papilla and the outer subcortical medulla, characteristic blackening was found after [125I]-C-ANP administration. Those differences might be important for the understanding of pharmacological actions of ANP analogues.

Endogenous atrial natriuretic factor is involved in the natriuresis following sodium loading in rats with chronic heart failure

STUDY OBJECTIVE

Plasma levels of atrial natriuretic factor are increased in chronic heart failure; however, it is still controversial whether these raised levels contribute to the diuresis and natriuresis in this setting. To address this issue the potential contribution of endogenous atrial natriuretic factor in the renal excretion of a moderate oral sodium load in a rat model of chronic heart failure was studied.

DESIGN

A monoclonal antibody against atrial natriuretic factor was used for specific antagonisation of its in vivo effects. Animals were subjected to oral sodium loading (30 ml.kg-1 0.9% NaCl, 2.5% dextrose) at baseline, immediately after, and 5 d after injection of monoclonal antibody or control solvent.

EXPERIMENTAL MATERIAL

Sham operated rats and rats with chronic heart failure due to myocardial infarction (infarct size 35(SEM 4)% of left ventricle) were studied 4-5 weeks after surgery.

MEASUREMENTS AND MAIN RESULTS

The renal excretion of cyclic guanosine monophosphate (cGMP), which represents a specific marker for the activation of the atrial natriuretic factor system, was markedly increased in infarcted rats, at 17.9(SEM 3.4) vs 5.8(1.2) nmol.kg-1, p less than 0.01. Atrial natriuretic factor antibody given immediately before sodium loading reduced the natriuretic response (0-4 h period) in infarcted rats from 1270(171) to 805(76) mumol.kg-1 (p less than 0.01) but not in sham operated animals. Similarly, the excretion of cGMP was only decreased by atrial natriuretic factor antibody in infarcted rats, from 29.8(6.3) to 20.7(3.7) nmol.kg-1. The reduction in sodium and cGMP excretion in infarcted rats was confirmed with a purified antibody preparation.

CONCLUSIONS

Endogenous atrial natriuretic factor appears to be involved in the natriuresis following a moderate oral volume load in chronic heart failure. Thus the raised concentrations found in chronic heart failure may contribute to the regulation of urinary sodium excretion under these conditions despite the fact that the diuretic effects of exogenous atrial natriuretic factor are attenuated in chronic heart failure.

Autoradiographic localization of 125I-big endothelin-1 in rat tissues

Endothelin-1 (ET-1), a potent vasoconstrictor with a characteristically long-acting activity in vitro and in vivo, is thought to be generated in endothelial cells from a less active intermediate, big endothelin-1 (big ET-1). In addition to ET-1, big ET-1 is also present in the circulation. The autoradiographic localization of 125I-big ET-1 has been studied after intravenous administration in rat tissues. Highest enrichment of radioactivity was found in the kidney cortex. Compared to blood levels, enrichment of radioactivity is also detected, especially in the vascular wall of the aorta. Comparing the radioactivity pattern of ET-1 and big ET-1, a nearly identical tissue distribution is observed, with the exception of the relative enrichment in the lung, the endocardium and the zona glomerulosa.

125I-endothelin-1 and 125I-big endothelin-1 in rat tissues: autoradiographic localization and receptor binding

The potent vasoconstrictor peptide, endothelin-1 (ET-1), which exhibits a characteristically long-acting activity in vitro and in vivo, is thought to be generated in endothelial cells from a less active intermediate, big endothelin-1 (big ET-1). In addition to ET-1, big ET-1 is also present in the circulation. The autoradiographic localization of 125I-big ET-1 and 125I-ET-1 has been studied after intravenous administration in rat tissues. Highest enrichment of radioactivity was found in the kidney cortex for both peptides. Compared to blood levels, enrichment of radioactivity is also detected, in the vascular wall of the aorta. Comparing the radioactivity pattern of ET-1 and big ET-1, a nearly identical tissue distribution is observed, with the exception of the relative enrichment in the lung and the zona glomerulosa after administration of ET-1. Both radioligands show a specific and saturable binding to lung and kidney membranes. In the case of lung tissue, Ki values are 10(-10) M for endothelin-1 and 10(-8) M for big endothelin-1. This difference in affinities may account for the lack of binding of big endothelin-1 to lung tissue.

Characterization of Bay U 3405, a novel thromboxane A2/endoperoxide receptor antagonist

The thromboxane A2-receptor antagonistic properties of Bay U 3405 [(3R)-3-(4-fluorophenylsulfonamido)-1,2,3,4-tetrahydro-9-carbaz o-lepropanoic acid] have been evaluated in various pharmacologic models. Bay U 3405 specifically inhibits platelet aggregation induced by U 46619, collagen, platelet-activating factor, and the second wave of ADP (IC50 0.5, 0.07, 0.3, 0.19 microM) in human plasma. The plasma phase of ADP-induced aggregation is not affected. U 46619-induced platelet aggregation is competitively antagonized (pA2 = 6.3). In humans, ex vivo platelet aggregation is inhibited after oral application of 2 or 50 mg Bay U 3405. Bay U 3405 also specifically and competitively antagonizes U 46619-induced contractions of rabbit aortic rings (pA2 = 7.4). In vivo, Bay U 3405 protects rabbits dose dependently from arachidonic acid or collagen-induced thromboembolism (ED50 1-3 mg/kg p.o). Chronic administration of Bay U 3405 to stroke-prone spontaneously hypertensive rats reduces stroke-related mortality and diminishes the occurrence of cerebral hemorrhages. From these results, we conclude that Bay U 3405 is an orally active, selective, and competitive thromboxane A2-receptor antagonist that may be beneficial in the treatment of cardiovascular or cerebrovascular diseases.

Therapy of diseased stroke-prone spontaneously hypertensive rats with nimodipine

We investigated the therapeutic effect of nimodipine or parathyroidectomy in old, diseased stroke-prone spontaneously hypertensive rats by observing 98 male 1-year-old rats over 5 months. After stroke had occurred, the rats were divided into three groups: 1) parathyroidectomy, 2) nimodipine, and 3) controls. In the nimodipine group, the rats survived longer than those in the other groups. Blood pressure of the controls did not differ from the nimodipine-treated and parathyroidectomy animals. The increase in calcium content of brain and kidney tissues and of plasma renin activity, urea, and creatinine was attenuated by nimodipine or parathyroidectomy. The histology of the kidneys revealed widespread fibrinoid necrosis of arteries in all rats. In the nimodipine-treated or parathyroidectomy groups, healing of the lesions was detectable. Cerebral lesions were mainly characterized by fibrinoid necrosis. Nimodipine-treated as well as parathyroidectomied animals showed significantly fewer hypertensive cerebral lesions. In old, diseased stroke-prone spontaneously hypertensive rats, therapy with nimodipine or parathyroidectomy increased their survival rate. The cerebrovascular and renovascular lesions of treated animals were attenuated, and morphologic signs of healing were observed. Reduction of calcium overload by nimodipine or parathyroidectomy, even in an advanced stage of disease, had a therapeutic effect.

Role of endogenous ANP on endocrine function investigated with a monoclonal antibody

Substantial volume expansion in conscious rats induces a strong natriuresis, cyclic GMP excretion, increase in cyclic GMP in plasma and kidney tissue, decrease in plasma renin activity and plasma aldosterone concentration. These effects are directly related to an increase in plasma levels of atrial natriuretic peptides. The renal response and the changes in plasma and kidney cyclic GMP, plasma renin activity and aldosterone could be totally blocked by simultaneous administration of monoclonal antibodies directed against ANP. From this study it seems to be clear that the rise in cyclic GMP and the inhibition of the renin-aldosterone system is not a direct effect of volume expansion but is specifically mediated by the released ANP. The great importance of ANP in acute volume expansion made us wonder about the role of ANP in chronic volume expansion and under basal conditions without volume loading. Chronic volume loading was induced pharmacologically by the sodium retaining vasodilatator minoxidil. Under both chronic volume expansion and basal conditions the neutralization of the circulation ANP by antibody administration leads to reduced plasma cyclic GMP levels. No alterations in urinary sodium excretion, plasma renin activity and plasma aldosterone concentration could be observed: In conclusion, the monoclonal antibody directed against ANP is a useful tool for the investigation of the physiological role of endogenous ANP.

Endothelin receptors in cultured renal epithelial cells

We demonstrated the presence of specific binding sites for endothelin in the renal epithelial cell lines. MDCK and LLC-PK1. Endothelin binding induced mobilisation of intracellular calcium, as shown by an increase in 45Ca2+ efflux. This suggests a direct effect of endothelin on renal reabsorption in addition to the effects on the renal vasculature.

Influence of calcium antagonists on renal function and secondary hyperparathyroidism in acute renal failure in rats

Glycerol-induced acute renal failure (ARF) is characterized by an increase in serum creatinine, urea, and phosphate concentration, and severe impairment of creatinine clearance. Secondary hyperparathyroidism develops rapidly during ARF. The calcium antagonist nisoldipine clearly improves renal function, which becomes evident by an improvement of creatinine clearance and attenuation of the increase of serum creatinine, urea, and phosphate concentrations. Further secondary hyperparathyroidism is ameliorated by nisoldipine treatment. In spite of normalization of the hypocalcemia in ARF by nisoldipine, weak hyperparathyroidism persists, suggesting that hypocalcemia is not exclusively responsible for elevated parathyroid hormone serum levels in ARF.

Effects of nisoldipine on atrial natriuretic peptides, blood pressure and cardiac hypertrophy in Dahl rats

The role of the calcium antagonist nisoldipine and the arteriolar vasodilator minoxidil on plasma levels of atrial natriuretic peptides (ANP), systolic blood pressure and heart weight was estimated in inbred Dahl salt sensitive (S) rats and inbred Dahl resistant (R) rats in long-term experiments. S rats develop quickly malignant hypertension, cardiac hypertrophy and have increased ANP plasma levels when fed a high salt diet (8% NaCl), while R rats on a high salt stay normotensive. In S rats 5 weeks on a high salt diet, therapeutic treatment with nisoldipine for 5 weeks not only decreased blood pressure but also produced a regression in cardiac hypertrophy and a reduction in elevated ANP plasma levels in comparison to the untreated salt-loaded S controls. Similar results were achieved in a preventive trial. In contrast with nisoldipine, therapeutic treatment with minoxidil in salt-loaded S rats lead to no reduction in cardiac hypertrophy and produced an additional increase in plasma ANP despite a reduction in blood pressure. The increase in plasma ANP level in this model of hypertension and its modulation by antihypertensive treatment with a calcium antagonist or an arteriolar vasodilator show that the changes in ANP plasma levels are probably secondary to hypertensive disease and the associated cardiac volume overload.

Different effects of ANP and nitroprusside on cyclic GMP extrusion of isolated aorta

Atrial natriuretic peptide (ANP) and sodium nitroprusside have potent vasodilator effects on the noradrenaline-precontracted isolated rabbit aorta. A distinct elevation of cyclic GMP in the aortic tissue was observed after both vasodilators. In contrast to sodium nitroprusside, ANP-(5-28) induced a dose-dependent cyclic GMP extrusion from the tissue into the medium. Thus, release of cyclic GMP appears to be specific for activation of particulate guanylate cyclase and provides a mechanism in addition to synthesis and degradation by which the cells can regulate their internal concentrations of cyclic GMP.

Autoradiographic localization of 125J-endothelin in rat tissues

A potent peptidergic vasoconstrictor in vitro and in vivo termed endothelin has been isolated from the supernatant of cultured endothelial cells. The autoradiographic localization of 125J-endothelin has been studied after intravenous administration in rat tissues. Highest enrichment of radioactivity was found in kidney and lung. Activity was also detected, especially in vascular wall of the aorta and adrenal gland.

Autoradiographic localization of [125I]endothelin-1 and [125I]atrial natriuretic peptide in rat tissue: a comparative study

The autoradiographic localization of [125I]endothelin-1 (ET-1) and [125I]atrial natriuretic peptide (ANP) after i.v. administration has been investigated in rats. Labeled peptides are rapidly distributed to tissues and peripheral organs. After administration of [125I]ET-1 (1-21), the highest enrichment of radioactivity was found in the lung, kidney, liver, adrenal gland, and heart. After administration of [125I]ANP (1-28), the highest levels of radioactivity could be observed in the kidney, adrenal gland, and endocardium. Compared to blood levels in both cases, a relative enrichment of radioactivity is also found in the vascular wall of the aorta. For ANP, no distribution of radioactivity in the lung could be observed. Other organs, especially the kidney and the adrenal gland, showed a similar distribution pattern with respect to substructures.

Endothelin stimulates release of atrial natriuretic peptides in vitro and in vivo

The effect of endothelin (END) on the release of atrial natriuretic peptides (ANP) was studied in isolated rat atria and in conscious rats. END stimulates the ANP release in vitro in a dose-dependent manner. An increase in ANP plasma levels and cyclic GMP plasma levels was also observed in conscious rats after injection of END. When a monoclonal antibody directed against ANP was injected together with END the increase in cyclic GMP was completely blocked. From this study it is concluded that END is a potent secretagogue for ANP both in vitro and in vivo.

Modulation by chronic nifedipine of plasma atrial natriuretic peptide, cell Na+ transport and plasma volume in rats with renal hypertension

The influence of a 6-week treatment of female Wistar rats with one-kidney, one clip (1-K, 1 C) renal hypertension, with the calcium antagonist nifedipine on plasma volume, red cell Na+ handling and plasma atrial natriuretic peptide immunoreactivity (ANP-IR) was studied. Measurements were performed at 2 and 6 weeks after surgery. In 1-K, 1 C rats plasma volume was increased and red cell Na+ pump activity was reduced only after 2 weeks. Blood pressure, heart weights and plasma ANP were massively increased after both 2 and 6 weeks. 1-K, 1 C-rats treated with nifedipine had normal plasma volume, plasma ANP, and red cell Na+ pump activity in comparison with sham-operated rats. Increases in blood pressure and heart weights were attenuated. It is concluded that 1-K, 1 C hypertension in the rat is associated with cardiomegaly, rise in plasma ANP, initial hypervolemia and depression of the membrane Na+ pump. Nifedipine prevents the occurrence of hypervolemia and secondary phenomena such as rises in plasma ANP and cardiomegaly. This may play an important contributory role in the prevention of pathological sequelae.

4,4-Diphenylpiperidine derivates and their sila analogues. A comparative study of their interaction with neural receptor biding sites and synaptosomal monoamine uptake

The potential anti-Parkinson drugs 1-R-4,4-diphenylpiperidines and 1-R-4,4-diphenyl-4-sila-piperidines (R = H, CH3, i-propyl and t-butyl) were evaluated for their neuroreceptor affinity with respect to their structure-activity relationship. In these compounds substitution of the central carbon at position 4 by a silicon leads to more lipophilic substances. While the binding of these compounds to dopamine, serotonin and gamma-aminobutyric acid/benzodiazepine receptors is relatively non-specific, the binding to the mu- and delta-subtypes of opiate receptors and to the 1-methyl-4-phenyl-1,2,3,6-tetra-hydropyridine receptor binding site show probably pharmacologically relevant effects. In almost all cases the sila-compounds have a slightly higher receptor affinity than the corresponding carbon-compounds. The studies on the uptake sites for the biogenic amines noradrenaline, dopamine and serotonin, on the other hand, reveal some considerable differences between the carbon- and silicon-containing analogues. The 4,4-diphenyl-4-sila-piperidine has much stronger uptake inhibiting properties for noradrenaline and serotonin than the corresponding carbon compound.