Kinin B1 receptor participates in the control of cardiac function in mice

Kinin B1 receptor: a potential therapeutic target in sepsis-induced vascular hyperpermeability

Background

In sepsis, the endothelial barrier becomes incompetent, with the leaking of plasma into interstitial tissues. VE-cadherin, an adherens junction protein, is the gatekeeper of endothelial cohesion. Kinins, released during sepsis, induce vascular leakage and vasodilation. They act via two G-protein coupled receptors: B1 (B1R) and B2 (B2R). B1R is inducible in the presence of pro-inflammatory cytokines, endotoxins or after tissue injury. It acts at a later stage of sepsis and elicits a sustained inflammatory response. The aim of our study was to investigate the relationships between B1R and VE-cadherin destabilization in vivo in a later phase of sepsis.

Methods

Experimental, prospective study in a university research laboratory. We used a polymicrobial model of septic shock by cecal ligation and puncture in C57BL6 male mice or C57BL6 male mice that received a specific B1R antagonist (R-954). We studied the influence of B1R on sepsis-induced vascular permeability 30 h after surgery for several organs, and VE-cadherin expression in the lung and kidneys by injecting R-954 just before surgery. The 96-h survival was determined in mice without treatment or in animals receiving R-954 as a “prophylactic” regimen (a subcutaneous injection of 200 µg/kg, prior to CLP and 24 h after CLP), or as a “curative” regimen (injection of 100 µg/kg at H6, H24 and H48 post-surgery).

Results

B1R inactivation helps to maintain MAP above 65 mmHg but induces different permeability profiles depending on whether or not organ perfusion is autoregulated. In our model, VE-cadherin was destabilized in vivo during septic shock. At a late stage of sepsis, the B1R blockade reduced the VE-cadherin disruption by limiting eNOS activation. The survival rate for mice that received R-954 after sepsis induction was higher than in animals that received an antagonist as a prophylactic treatment.

Conclusions

B1R antagonizing reduced mortality in our model of murine septic shock by limiting the vascular permeability induced by VE-cadherin destabilization through maintenance of the macrohemodynamics, consequently limiting organ dysfunctions.

Cardiac morphofunctional characteristics of transgenic rats with overexpression of the bradykinin B1 receptor in the endothelium

Our aim was to evaluate whether endothelial overexpressing of the bradykinin B1 receptor could be associated with altered left ventricular and myocardial performance. Echocardiography and hemodynamic were employed to assess left ventricular morphology and function in Sprague Dawley transgenic rats overexpressing the endothelial bradykinin B1 receptor (Tie2B1 rats). The myocardial inotropism was evaluated on papillary muscles contracting in vitro. In Tie2B1 animals, an enlarged left ventricular cavity and lower fractional shortening coupled with a lower rate of pressure change values indicated depressed left ventricular performance. Papillary muscle mechanics revealed that both Tie2B1 and wild-type rat groups had the same contractile capacities under basal conditions; however, in transgenic animals, there was accentuated inotropism due to post-pause potentiation. Following treatment with the Arg(9)-BK agonist, Tie2B1 papillary muscles displayed a reduction in myocardial inotropism. Endothelial B1 receptor overexpression has expanded the LV cavity and worsened its function. There was an exacerbated response of papillary muscle in vitro to a prolonged resting pause, and the use of a B1 receptor agonist impairs myocardial inotropism.

ÓXIDO NÍTRICO E DINÂMICA DE CA2+ EM CARDIOMIÓCITOS: INFLUÊNCIA DA CAPACIDADE DE EXERCÍCIO

RESUMO Introdução: A capacidade intrínseca para o exercício aeróbico está relacionada com o inotropismo cardíaco. Por outro lado, a participação do óxido nítrico (NO) como mensageiro intracelular sobre a dinâmica do Ca2+ ainda permanece desconhecida em ratos com diferentes capacidades intrínsecas para o exercício. Objetivo: Avaliar se o NO modula diferentemente o transiente intracelular de Ca2+ e liberações espontâneas de Ca2+(sparks) em cardiomiócitos de ratos com diferentes capacidades intrínsecas para o exercício. Métodos: Ratos machos Wistar foram selecionados como desempenho padrão (DP) e alto desempenho (AD), de acordo com a capacidade de exercício até a fadiga, mensurada através de teste de esforço progressivo em esteira. Os cardiomiócitos dos ratos foram utilizados para determinar o transiente intracelular de Ca2+ e Ca2+sparks em microscópio confocal. Para estimar a contribuição do NO foi utilizado o inibidor das sínteses do NO (L-NAME, 100 µM). Os dados foram analisados através de ANOVA two-way seguido do pós-teste de Tukey e apresentados como médias ± EPM. Resultados: Os cardiomiócitos de ratos AD exibiram aumentos na amplitude do transiente de Ca2+ em comparação aos DP. Entretanto, o L-NAME aumentou a amplitude do transiente de Ca2+ somente em ratos DP. Não foram encontradas diferenças na constante de tempo de decaimento do transiente de Ca2+ (t) em cardiomiócitos de ratos com DP e AP, contudo, a administração do L-NAME diminuiu o t em cardiomiócitos em ambos os grupos. cardiomiócitos de ratos AD apresentaram menor amplitude e frequência de Ca2+sparks em comparação ao grupo DP. A administração de L-NAME aumentou a amplitude de Ca2+sparks em cardiomiócitos do grupo AD. Conclusão: O NO modula o transiente de Ca2+ e as sparks de Ca2+ em cardiomiócitos de ratos com diferentes capacidades intrínsecas para o exercício.

Protein restriction after weaning modifies the calcium kinetics and induces cardiomyocyte contractile dysfunction in rats

Protein restriction (PR) is associated with cardiovascular diseases. The purpose of this study was to investigate the effects on single ventricular cardiomyocyte contractile function of a short-term PR after weaning. Male Fischer rats that were 28 days old were randomly divided into a control group (CG, n = 16) and a protein-restricted group (PRG, n = 16). After weaning, CG and PRG animals received isocaloric diets containing 15 and 6% protein, respectively, for 35 days. Biometric parameters were then measured, and the hearts were removed for the analysis of contractile function and calcium transient in isolated cardiomyocytes of the left ventricule (LV), and the quantification of calcium and collagen fibers in LV myocardium. PRG animals had lower body weight (BW) and LV weight (LVW), an increased LVW to BW ratio and a higher proportion of collagen fibers than CG animals. PRG animals exhibited reduced tissue levels of calcium, reduced the length, width and volume of cardiomyocytes and their sarcomere length compared to CG animals. Cardiomyocytes from PRG animals had a lower amplitude of shortening, a slower time to the peak of shortening and a longer time to half-relaxation than those from the CG. Cardiomyocytes from PRG animals also presented a lower peak of calcium transient and a longer calcium transient decay time than CG animals. Taken together, the results indicate that short-term PR after weaning induces a marked structural remodeling of the myocardium parenchyma and stroma that coexists with contractile dysfunctions in single LV cardiomyocytes of rats, which is probably associated with pathological changes of the intracellular calcium kinetics, rather than inadequate available amounts of this mineral in cardiac tissue.

Antioxidant/oxidant status and cardiac function in bradykinin B(1)- and B(2)-receptor null mice

Kinin-vasoactive peptides activate two G-protein-coupled receptors (R), B(1)R (inducible) and B(2)R (constitutive). Their complex role in cardiovascular diseases could be related to differential actions on oxidative stress. This study investigated impacts of B(1)R or B(2)R gene deletion in mice on the cardiac function and plasma antioxidant and oxidant status. Echocardiography-Doppler was performed in B(1)R (B(1)R(-/-)) and B(2)R (B(2)R(-/-)) deficient and wild type (WT) adult male mice. No functional alteration was observed in B(2)R(-/-) hearts. B(1)R(-/-) mice had significantly lowered fractional shortening and increased isovolumetric contraction time. The diastolic E and A waves velocity ratio was similar in all mice groups. Thus B(1)R(-/-) mice provide a model of moderate systolic dysfunction, whereas B(2)R(-/-) mice displayed a normal cardiac phenotype. Plasma antioxidant capacity (ORAC) was significantly decreased in both B(1)R(-/-) and B(2)R(-/-) mice whereas the vitamin C levels were decreased in B(2)R(-/-) mice only. Plasma ascorbyl free radical was significantly higher in B(1)R(-/-) compared to WT and B(2)R(-/-) mice. Therefore, the oxidative stress index, ascorbyl free radical to vitamin C ratio, was increased in both B(1)R(-/-) and B(2)R(-/-) mice. Hence, B(1)R and B(2)R deficiency are associated with increased oxidative stress, but there is a differential imbalance between free radical production and antioxidant defense. The interrelationship between the differential B(1)R and B(2)R roles in oxidative stress and cardiovascular diseases remain to be investigated.

Effects of severe caloric restriction from birth on the hearts of adult rats

There has been increasing evidence suggesting that a severe caloric restriction (SCR) (above 40%) has beneficial effects on the hearts of rats. However, most of the reports have focused on the effects of SCR that started in adulthood. We investigated the consequences of SCR on the hearts of rats subjected to SCR since birth (CR50). From birth to the age of 3 months, CR50 rats were fed 50% of the food that the ad libitum group (AL) was fed. Thereafter, a maximal aerobic test was performed to indirectly evaluate global cardiovascular function. Indices of contractility (+dT/dt) and relaxation (−dT/dt) were analyzed in isolated heart preparation, and cardiomyocyte diameter, number, density, and myocardium collagen content were obtained through histologic analysis. Ventricular myocytes were isolated, using standard methods to evaluate phosphorylated AKT levels, and Ca2+ handling was evaluated with a combination of Western blot analysis, intracellular Ca2+ imaging, and confocal microscopy. CR50 rats exhibited increased aerobic performance and cardiac function, as shown by the increase in ±dT/dt. Despite the smaller cardiomyocyte diameter, CR50 rats had an increased heart−body weight ratio, increased cardiomyocyte density and number, and similar levels of myocardium collagen content, compared with AL rats. AKT was hyperphosphorylated in cardiomyocytes from CR50 rats, and there were no significant differences in Ca2+ transient and SERCA2 levels in cardiomyocytes between CR50 and AL rats. Collectively, these observations reveal the beneficial effects of a 50% caloric restriction on the hearts of adult rats restricted since birth, which might involve cardiomyocyte AKT signaling.

Aqueous fraction from Costus spiralis (Jacq.) Roscoe leaf reduces contractility by impairing the calcium inward current in the mammalian myocardium

ETHNOPHARMACOLOGICAL RELEVANCE

Brazilian folk medicine uses infusion of Costus spiralis leaf to help people to treat arterial hypertension and syndromes of cardiac hyperexcitability.

AIM OF THE STUDY

Evaluate the aqueous fraction (AqF) effect on atrial contractility and investigate its mechanism of action.

MATERIALS AND METHODS

The AqF effect on the cardiac contractility was studied on isolated electrically driven guinea pig left atria. Atropine and tetraethylammonium (TEA) were employed to investigate whether potassium contributes for the inotropic mechanism of the AqF. The role of calcium in this effect was also studied. This was done by analysing the AqF effect on the Bowditch's phenomenon, as well as by studying whether it could interfere with the concentration-effect curve for CaCl(2), isoproterenol, and BAY K8644. Mice isolated cardiomyocytes were submitted to a whole-cell patch-clamp technique in order to evaluate whether the L-type calcium current participates on the AqF effect. Furthermore, the intracellular calcium transient was studied by confocal fluorescence microscopy.

RESULTS

AqF depressed the atrial contractile force. It was the most potent fraction from C. spiralis leaf (EC(50)=305 ± 41 mg/l) (crude extract: EC(50)=712 ± 41; ethyl acetate: EC(50)=788 ± 121; chloroform: EC(50)=8,948 ± 1,346 mg/l). Sodium and potassium content in the AqF was 0.15 mM and 1.91 mM, respectively. Phytochemical analysis revealed phenols, tannins, flavones, xanthones, flavonoids, flavonols, flavononols, flavonones, and saponins. Experiments with atropine and TEA showed that potassium does not participate of the inotropic mechanism of AqF. However, this fraction decreased the force overshoot characteristic of the Bowditch's phenomenon, and shifted the concentration-response curve for CaCl(2) (EC(50) from 1.12 ± 0.07 to 7.23 ± 0.47 mM) indicating that calcium currents participate on its mechanism of action. Results obtained with isoproterenol (1-1,000 pM) and BAY K8644 (5-2000nM) showed that AqF abolished the inotropic effect of these substances. On cardiomyocytes, 48mg/l AqF reduced (∼23%) the L-type calcium current density from -6.3 ± 0.3 to -4.9 ± 0.2 A/F (n=5 cells, p<0.05) and reduced the intracellular calcium transient (∼20%, 4.7 ± 1.2 a.u., n=42 cells to 3.7 ± 1.00 a.u., n=35 cells, p<0.05). However, the decay time of the fluorescence was not changed (control: 860 ± 32 ms, n=42 cells; AqF: 876 ± 26 ms, n=35 cells, p>0.05).

CONCLUSIONS

The AqF of C. spiralis leaf depresses myocardial contractility by reducing the L-type calcium current and by decreasing the intracellular calcium transient. Despite the lack of data on the therapeutic dose of AqF used in folk medicine, our results support, at least in part, the traditional use of this plant to treat cardiac disorders.

Curine, a bisbenzylisoquinoline alkaloid, blocks L-type Ca²⁺ channels and decreases intracellular Ca²⁺ transients in A7r5 cells

Curine is a novel bisbenzylisoquinoline alkaloid that has previously been reported as a vasodilator. The underlying mechanism(s) of the vasodilator effect of curine remains to be characterized. In this study, we investigated the cellular mechanism that is responsible for the vasodilator effect of curine in the rat aorta. The vasorelaxant activity of curine was recorded using a myograph. Ca(2+) currents in A7r5 cells were measured using the whole-cell patch-clamp technique. Intracellular Ca(2+) transients were determined using confocal microscopy. In a concentration-dependent manner, curine inhibited contractions elicited by high extracellular K(+) and Bay K8644 in the rat aorta and reduced the rise in the intracellular Ca(2+) concentration induced by membrane depolarization in response to an increase in extracellular K(+) concentration in vascular smooth muscle cells. Moreover, curine decreased the peak amplitude of L-type Ca(2+) currents (I(Ca,L)) in a concentration-dependent manner without changing the characteristics of the current density vs. voltage relationship and the steady-state activation of I(Ca,L). Furthermore, curine shifted the steady-state inactivation curve of I(Ca,L) toward more hyperpolarized membrane potentials. None of the following modified the effect of curine on I(Ca,L) amplitude: 3-isobutyl-1-methylxanthine, an inhibitor of phosphodiesterases; dibutyryl cyclic AMP, an activator of protein kinase A (PKA); or 8-Br-cyclic GMP, an activator of protein kinase G (PKG). Our results showed that curine inhibited the L-type voltage-dependent Ca(2+) current in rat aorta smooth muscle cells, which caused a decrease in intracellular global Ca(2+) transients that led to vasorelaxation.

Cardiotoxic effects of Loxosceles intermedia spider venom and the recombinant venom toxin rLiD1

Loxosceles spider bites cause many human injuries worldwide. Injections in mice of whole Loxosceles (L.) intermedia venom or a recombinant toxin (rLiD1) produce systemic symptoms similar to those detected in envenomed humans. This animal model was used to characterize the effects of Loxosceles intermedia venom in cardiac tissues. L. intermedia antigens were detected by ELISA in kidney, heart, lung and liver of experimentally envenomed mice. In addition, rLiD1 binding to cardiomyocytes was demonstrated by immunofluorescence and confocal microscopy. Furthermore, isolated perfused heart preparations and ventricular cardiomyocytes from envenomed mice showed heart function impairment, and a significant increase of I(Ca,L) density and intracellular Ca(2+) transients, respectively. Thus, L. intermedia spider venom, as shown through the use of the recombinant toxin rLiD1, causes cardiotoxic effects and a protein from the sphingomyelinase D family plays a key role in heart dysfunction. Thus, L. intermedia spider venom and the Loxtox rLiD1 play a key role in heart dysfunction.

Cardiac oxidative stress is involved in heart failure induced by thiamine deprivation in rats

Thiamine is an important cofactor of metabolic enzymes, and its deficiency leads to cardiovascular dysfunction. First, we characterized the metabolic status measuring resting oxygen consumption rate and lactate blood concentration after 35 days of thiamine deficiency (TD). The results pointed to a decrease in resting oxygen consumption and a twofold increase in blood lactate. Confocal microscopy showed that intracellular superoxide (approximately 40%) and H(2)O(2) (2.5 times) contents had been increased. In addition, biochemical activities and protein expression of SOD, glutathione peroxidase, and catalase were evaluated in hearts isolated from rats submitted to thiamine deprivation. No difference in SOD activity was detected, but protein levels were found to be increased. Catalase activity increased 2.1 times in TD hearts. The observed gain in activity was attended by an increased catalase protein level. However, a marked decrease in glutathione peroxidase activity (control 435.3 + or - 28.6 vs. TD 199.4 + or - 30.2 nmol NADPH x min(-1) x ml(-1)) was paralleled by a diminution in the protein levels. Compared with control hearts, we did observe a greater proportion of apoptotic myocytes by TdT-mediated dUTP nick end labeling (TUNEL) and caspase-3 reactivity techniques. These results indicate that during TD, reactive oxygen species (ROS) production may be enhanced as a consequence of the installed acidosis. The perturbation in the cardiac myocytes redox balance was responsible for the increase in apoptosis.

Attenuation of isoproterenol-induced cardiac fibrosis in transgenic rats harboring an angiotensin-(1-7)-producing fusion protein in the heart

Objective: It has been shown that Ang-(1-7) has cardioprotective actions. To directly investigate the effects of Ang-(1-7) specifically in the heart, we generated and characterized transgenic (TG) rats which express an Ang-(1-7)-producing fusion protein driven by the α-MHC promoter.

Methods and Results: After microinjection of the transgene into fertilized rat zygotes, we obtained four different transgenic lines. Homozygous animals were analyzed with regard to the expression profile of the transgene by ribonuclease protection assay. Transgene expression was detected mainly in the heart with weak or no expression in other organs. Heterozygous TG(hA-1-7)L7301 rats presented a significant increase in cardiac Ang-(1-7) concentration compared with control rats (17.1±2.1 versus 3.9±1.4 pg/mg protein in SD rats). Radiotelemetry analysis revealed that TG rats presented no significant changes in blood pressure and heart rate compared with normal rats. Overexpression of Ang-(1-7) in the heart produced slight improvement in resting cardiac function (+ dT/dt: 81530±1305.0 versus 77470±345.5 g/s bpm in SD rats, p < 0.05), which was in keeping with the enhanced [Ca2+] handling observed in cardiomyocytes of TG rats. TG(hA-1-7)L7301 rats also showed a greater capacity to withstand stress since TG rats showed a less pronounced deposition of collagen type III and fibronectin induced by isoproterenol treatment in the subendocardial area than in corresponding controls. In addition, hearts from TG rats showed reduced incidence and duration of reperfusion arrhythmias in comparison with SD rats.

Conclusion: These results indicate that Ang-(1-7) has blood pressure-independent, antifibrotic effects, acting directly in the heart.

Angiotensin-(1-7) prevents cardiomyocyte pathological remodeling through a nitric oxide/guanosine 3',5'-cyclic monophosphate-dependent pathway

The renin-angiotensin (Ang) system plays a pivotal role in the pathogenesis of cardiovascular disease, with Ang II being the major effector of this system. Multiple lines of evidence have shown that Ang-(1-7) exerts cardioprotective effects in the heart by counterregulating Ang II actions. The questions that remain are how and where Ang-(1-7) exerts its effects. By using a combination of molecular biology, confocal microscopy, and a transgenic rat model with increased levels of circulating Ang-(1-7) (TGR[A1-7]3292), we evaluated the signaling pathways involved in Ang-(1-7) cardioprotection against Ang II-induced pathological remodeling in ventricular cardiomyocytes. Rats were infused with Ang II for 2 weeks. We found that ventricular myocytes from TGR(A1-7)3292 rats are protected from Ang II pathological remodeling characterized by Ca(2+) signaling dysfunction, hypertrophic fetal gene expression, glycogen synthase kinase 3beta inactivation, and nuclear factor of activated T-cells nuclear accumulation. Moreover, cardiomyocytes from TGR(A1-7)3292 rats infused with Ang II presented increased expression levels of neuronal NO synthase. To provide a signaling pathway involved in the beneficial effects of Ang-(1-7), we treated neonatal cardiomyocytes with Ang-(1-7) and Ang II for 36 hours. Treatment of cardiomyocytes with Ang-(1-7) prevented Ang II-induced hypertrophy by modulating calcineurin/nuclear factor of activated T-cell signaling cascade. Importantly, antihypertrophic effects of Ang-(1-7) on Ang II-treated cardiomyocytes were prevented by N(G)-nitro-l-arginine methyl ester and 1H-1,2,4oxadiazolo4,2-aquinoxalin-1-one, suggesting that these effects are mediated by NO/cGMP. Taken together, these data reveal a key role for NO/cGMP as a mediator of Ang-(1-7) beneficial effects in cardiac cells.

Exercise capacity is related to calcium transients in ventricular cardiomyocytes

The aim of the present study was to evaluate the Ca2+ handling and contractility properties of cardiomyocytes isolated from rats with high intrinsic aerobic exercise capacity. Standard-performance (SP) and high-performance (HP) rats were categorized with a treadmill progressive exercise test according to the exercise time to fatigue (TTF). The SP group included rats with TTF between 16.63 and 46.57 min, and the HP group included rats with TTF >46.57 min. Isolated ventricular cardiomyocytes were dissociated from the hearts of SP and HP rats, and intracellular global Ca2+ ([Ca2+]i) transients were measured. The [Ca2+]i transient peak was increased in the HP group relative to the SP group (5.54 ± 0.31 vs. 4.18 ± 0.12 F/F0; P ≤ 0.05) and was positively correlated with the TTF attained during the progressive test ( r = 0.81). We also performed contractility measurements in isolated cardiomyocytes and found higher amplitude of contraction in the HP group compared with the SP group (6.7 ± 0.2 vs. 6.0 ± 0.3% resting cell length; P ≤ 0.05). To reinforce the intrinsic differences between SP and HP rats, we performed Western blot experiments and observed increased expression of sarco(endo)plasmic reticulum Ca2+-ATPase type 2a (1.30 ± 0.07 vs. 1.74 ± 0.18 arbitrary units; P ≤ 0.05) and ryanodine receptor type 2 (1.86 ± 0.13 vs. 3.57 ± 0.12 arbitrary units; P ≤ 0.05) in HP rats. In summary, our data showed important intrinsic differences in cardiomyocyte properties that could explain some of the divergence observed in rats with high intrinsic aerobic exercise capacity.

Cardiac structural changes and electrical remodeling in a thiamine-deficiency model in rats

AIMS

Thiamine is an important cofactor present in many biochemical reactions, and its deprivation can lead to heart dysfunction. Little is known about the influence of thiamine deprivation on the electrophysiological behavior of the isolated heart cells and information about thiamine deficiency in heart morphology is controversial. Thus, we decided to investigate the major repolarizing conductances and their influence in the action potential (AP) waveform as well as the changes in the heart structure in a set of thiamine deficiency in rats.

MAIN METHODS

Using the patch-clamp technique, we investigated inward (I(K1)) and outward K(+) currents (I(to)), T-type and L-type Ca(2+) currents and APs. To evaluate heart morphology we used hematoxylin and eosin in transversal heart sections.

KEY FINDINGS

Thiamine deficiency caused a marked decrease in left ventricle thickness, cardiomyocyte number, cell length and width, and membrane capacitance. When evaluating I(to) we did not find difference in current amplitude; however an acceleration of I(to) inactivation was observed. I(K1) showed a reduction in the amplitude and slope conductance, which implicated a less negative resting membrane potential in cardiac myocytes isolated from thiamine-deficient rats. We did not find any difference in L-type Ca(2+) current density. T-type Ca(2+) current was not observed. In addition, we did not observe significant changes in AP repolarization.

SIGNIFICANCE

Based on our study we can conclude that thiamine deficiency causes heart hypotrophy and not heart hypertrophy. Moreover, we provided evidence that there is no major electrical remodeling during thiamine deficiency, a feature of heart failure models.

Bradykinin inhibits the transient outward K+ current in mouse Schwann cells via the cAMP/PKA pathway

Bradykinin (BK) is an endogenous peptide with diverse biological actions and is considered to be an important mediator of the inflammatory response in both the peripheral and the central nervous systems. BK has attracted recent interest as a potential mediator of K(+) conductance, Cl(-) channels, and Ca(2+)-activated K(+) channels. However, few reports have associated BK with the voltage-gated K(+) current. In this study, we demonstrated that BK suppressed the transient outward potassium current (I(A)) in mouse Schwann cells using whole cell recording techniques. At a concentration of 0.1 muM to 5 muM, BK reversibly inhibited I(A) in a dose-dependent manner with the modulation of steady-state activation and inactivation properties. The effect of BK on I(A) current was abolished after preincubation with a B(2) receptor antagonist but could not be eliminated by B(1) receptor antagonist. Intracellular application of GTP-gammaS induced an irreversible decrease in I(A), and the inhibition of G(s) using NF449 provoked a gradual augmentation in I(A) and eliminated the BK-induced effect on I(A,) while the G(i)/(o) antagonist NF023 did not. The application of forskolin or dibutyryl-cAMP mimicked the inhibitory effect of BK on I(A) and abolished the BK-induced effect on I(A). H-89, an inhibitor of PKA, augmented I(A) amplitude and completely eliminated the BK-induced inhibitory effect on I(A). In contrast, activation of PKC by PMA augmented I(A) amplitude. A cAMP assay revealed that BK significantly increased intracellular cAMP level. It is therefore concluded that BK inhibits the I(A) current in Schwann cells by cAMP/PKA-dependent pathways via activation of the B(2) receptor.

Neuronal nitric oxide synthase-derived hydrogen peroxide is a major endothelium-dependent relaxing factor

Endothelium-dependent vasorelaxation in large vessels is mainly attributed to Nomega-nitro-L-arginine methyl ester (L-NAME)-sensitive endothelial nitric oxide (NO) synthase (eNOS)-derived NO production. Endothelium-derived hyperpolarizing factor (EDHF) is the component of endothelium-dependent relaxations that resists full blockade of NO synthases (NOS) and cyclooxygenases. H2O2 has been proposed as an EDHF in resistance vessels. In this work we propose that in mice aorta neuronal (n)NOS-derived H2O2 accounts for a large proportion of endothelium-dependent ACh-induced relaxation. In mice aorta rings, ACh-induced relaxation was inhibited by L-NAME and Nomega-nitro-L-arginine (L-NNA), two nonselective inhibitors of NOS, and attenuated by selective inhibition of nNOS with L-ArgNO2-L-Dbu-NH2 2TFA (L-ArgNO2-L-Dbu) and 1-(2-trifluoromethylphehyl)imidazole (TRIM). The relaxation induced by ACh was associated with enhanced H2O2 production in endothelial cells that was prevented by the addition of L-NAME, L-NNA, L-ArgNO2-L-Dbu, TRIM, and removal of the endothelium. The addition of catalase, an enzyme that degrades H2O2, reduced ACh-dependent relaxation and abolished ACh-induced H2O2 production. RT-PCR experiments showed the presence of mRNA for eNOS and nNOS but not inducible NOS in mice aorta. The constitutive expression of nNOS was confirmed by Western blot analysis in endothelium-containing vessels but not in endothelium-denuded vessels. Immunohistochemistry data confirmed the localization of nNOS in the vascular endothelium. Antisense knockdown of nNOS decreased both ACh-dependent relaxation and ACh-induced H2O2 production. Antisense knockdown of eNOS decreased ACh-induced relaxation but not H2O2 production. Residual relaxation in eNOS knockdown mouse aorta was further inhibited by the selective inhibition of nNOS with L-ArgNO2-L-Dbu. In conclusion, these results show that nNOS is constitutively expressed in the endothelium of mouse aorta and that nNOS-derived H2O2 is a major endothelium-dependent relaxing factor. Hence, in the mouse aorta, the effects of nonselective NOS inhibitors cannot be solely ascribed to NO release and action without considering the coparticipation of H2O2 in mediating vasodilatation.