Bradykinin B2-receptor activation augments norepinephrine exocytosis from cardiac sympathetic nerve endings. Mediation by autocrine/paracrine mechanisms

Role of the Renin-Angiotensin-Aldosterone System in Dystrophin-Deficient Cardiomyopathy

Dystrophin-deficient cardiomyopathy (DDC) is currently the leading cause of death in patients with dystrophinopathies. Targeting myocardial fibrosis (MF) has become a major therapeutic goal in order to prevent the occurrence of DDC. We aimed to review and summarize the current evidence about the role of the renin-angiotensin-aldosterone system (RAAS) in the development and perpetuation of MF in DCC. We conducted a comprehensive search of peer-reviewed English literature on PubMed about this subject. We found increasing preclinical evidence from studies in animal models during the last 20 years pointing out a central role of RAAS in the development of MF in DDC. Local tissue RAAS acts directly mainly through its main fibrotic component angiotensin II (ANG2) and its transducer receptor (AT1R) and downstream TGF-b pathway. Additionally, it modulates the actions of most of the remaining pro-fibrotic factors involved in DDC. Despite limited clinical evidence, RAAS blockade constitutes the most studied, available and promising therapeutic strategy against MF and DDC. Conclusion: Based on the evidence reviewed, it would be recommendable to start RAAS blockade therapy through angiotensin converter enzyme inhibitors (ACEI) or AT1R blockers (ARBs) alone or in combination with mineralocorticoid receptor antagonists (MRa) at the youngest age after the diagnosis of dystrophinopathies, in order to delay the occurrence or slow the progression of MF, even before the detection of any cardiovascular alteration.

The orphan receptor GPR139 signals via Gq/11 to oppose opioid effects

The interplay between G protein-coupled receptors (GPCRs) is critical for controlling neuronal activity that shapes neuromodulatory outcomes. Recent evidence indicates that the orphan receptor GPR139 influences opioid modulation of key brain circuits by opposing the actions of the µ-opioid receptor (MOR). However, the function of GPR139 and its signaling mechanisms are poorly understood. In this study, we report that GPR139 activates multiple heterotrimeric G proteins, including members of the G_q/11 and G_i/o families. Using a panel of reporter assays in reconstituted HEK293T/17 cells, we found that GPR139 functions via the G_q/11 pathway and thereby distinctly regulates cellular effector systems, including stimulation of cAMP production and inhibition of G protein inward rectifying potassium (GIRK) channels. Electrophysiological recordings from medial habenular neurons revealed that GPR139 signaling via G_q/11 is necessary and sufficient for counteracting MOR-mediated inhibition of neuronal firing. These results uncover a mechanistic interplay between GPCRs involved in controlling opioidergic neuromodulation in the brain.

Interactions between the Cyclooxygenase Metabolic Pathway and the Renin-Angiotensin-Aldosterone Systems: Their Effect on Cardiovascular Risk, from Theory to the Clinical Practice

Coronary artery disease (CAD) and stroke are the most common and serious long-term complications of hypertension. Acetylsalicylic acid (ASA) significantly reduces their incidence and cardiovascular mortality. The RAAS activation plays an important role in pathogenesis of CVD, resulting in increased vascular resistance, proliferation of vascular-smooth-muscle-cells, and cardiac hypertrophy. Drugs acting on the renin-angiotensin-aldosterone system (RAAS) are demonstrated to reduce cardiovascular events in population with cardiovascular disease (CVD). The cyclooxygenase inhibitors limit the beneficial effect of RAAS-inhibitors, which in turn may be important in subjects with hypertension, CAD, and congestive heart failure. These observations apply to most of nonsteroidal anti-inflammatory drugs and ASA at high doses. Nevertheless, there is no strong evidence confirming presence of similar effects of cardioprotective ASA doses. The benefit of combined therapy with low-doses of ASA is-in some cases-significantly higher than that of monotherapy. So far, the significance of ASA in optimizing the pharmacotherapy remains not fully established. A better understanding of its influence on the particular CVD should contribute to more precise identification of patients in whom benefits of ASA outweigh the complication risk. This brief review summarizes the data regarding usefulness and safety of the ASA combination with drugs acting directly on the RAAS.

Delayed coronary reperfusion is ineffective at impeding the dynamic increase in cardiac efferent sympathetic nerve activity following myocardial ischemia

Acute myocardial infarction (MI) is associated with an adverse and sustained increase in cardiac sympathetic nerve activity (SNA), triggering potentially fatal ventricular arrhythmias. While myocardial reperfusion undoubtedly improves patient prognosis, it remains unknown whether reperfusion therapy also attenuates the dangerous increase in SNA. This study aimed to investigate the effect of time-dependent coronary reperfusion therapy on cardiac SNA following acute MI. Electrophysiological recordings of cardiac efferent SNA were performed in urethane-anaesthetized rats following ligation of the left anterior descending coronary artery (i.e., MI) for either 15 or 45 min, followed by 'early' or 'delayed' reperfusion, respectively. Another group of rats had permanent ischemia with no reperfusion. Forty-five minutes of ischemia induced a 55 % increase in efferent SNA. Subsequent 'delayed' reperfusion was ineffective at ameliorating further increases in SNA (maximal 153 % increase), so that MI-induced increases in SNA mirrored that observed in rats with permanent MI. Although SNA did not increase during 15 min of ischemia, it did significantly increase, albeit delayed, during the subsequent reperfusion period (max. 75 % increase). Importantly, however, this increase in SNA, which tended to be lower in the 'early'-reperfusion group, was matched with a lower incidence of arrhythmias and mortality rate, compared to the 'delayed'-reperfusion and permanent-MI groups. These results highlight that 'prompt' coronary reperfusion, before SNA becomes activated, may provide a crucial window of opportunity for improving outcome. Further research is essential to identify the mechanisms that underpin, not only sympathetic activation, but also importantly sympathetic deactivation as a potential therapeutic target for MI.

Dysregulation of Norepinephrine Release in the Absence of Functional Synaptotagmin 7

Synaptotagmin 7 (Syt7) is expressed in cardiac sympathetic nerve terminals where norepinephrine (NE) is released in both Ca(2+)-dependent exocytosis and Ca(2+)-independent norepinephrine transporter (NET)-mediated overflow. The role of Syt7 in the regulation of NE release from cardiac sympathetic nerve terminals is tested by employing a Syt7 knock-in mouse line that expresses a non-functional mutant form of Syt7. In cardiac sympathetic nerve terminals prepared from these Syt7 knock-in mice, the Ca(2+)-dependent component of NE release was diminished. However, these terminals displayed upregulated function of NET (∼130% of controls) and a significant increase in Ca(2+)-independent NE overflow (∼140% of controls), which is greater than the Ca(2+)-dependent component of NE exocytosis occurring in wild-type controls. Consistent with a significant increase in NE overflow, the Syt7 knock-in mice showed significantly higher blood pressures compared to those of littermate wild-type and heterozygous mice. Our results indicate that the lack of functional Syt7 dysregulates NE release from cardiac sympathetic nerve terminals.

The Cardioprotective Effects of Late-Phase Remote Preconditioning of Trauma Depends on Neurogenic Pathways and the Activation of PKC and NF-κB (But Not iNOS) in Mice

BACKGROUND

A superficial abdominal surgical incision elicits cardioprotection against cardiac ischemia-reperfusion (I/R) injury in mice. This process, called remote preconditioning of trauma (RPCT), has both an early and a late phase. Previous investigations have demonstrated that early RPCT reduces cardiac infarct size by 80% to 85%. We evaluated the cardioprotective and molecular mechanisms of late-phase RPCT in a murine I/R injury model.

METHODS

Wild-type mice, bradykinin (BK) 2 receptor knockout mice, 3M transgenic mice (nuclear factor κB [NF-κb] repressor inhibitor of nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor alpha [IκBα((S32A, S36A, Y42F))]), and inducible nitric oxide synthase (iNOS) knockout mice were analyzed using a previously established I/R injury model. A noninvasive abdominal surgical incision was made 24 hours prior to I/R injury and the infarct size was determined at 24 hours post-I/R injury.

RESULTS

The results indicated that a strong cardioprotective effect occurred during late-phase RPCT (58.42% ± 1.89% sham vs 29.41% ± 4.00% late RPCT, mean area of the infarct divided by the mean area of the risk region; P ≤ .05; n = 10). Furthermore, pharmacological intervention revealed the involvement of neurogenic signaling in the beneficial effects of late RPCT via sensory and sympathetic thoracic nerves. Pharmacological experiments in transgenic mice-implicated BK receptors, β-adrenergic receptors, protein kinase C, and NF-κB but not iNOS signaling in the cardioprotective effects of late RPCT.

CONCLUSION

Late RPCT significantly decreased myocardial infarct size via neurogenic transmission and various other signaling pathways. This protective mechanism differentiates late and early RPCT. This study describes a new cardiac I/R injury prevention method and refines the concept of RPCT.

β-blockade abolishes the augmented cardiac tPA release induced by transactivation of heterodimerised bradykinin receptor-2 and β2-adrenergic receptor in vivo

Bradykinin (BK) receptor-2 (B2R) and β2-adrenergic receptor (β2AR) have been shown to form heterodimers in vitro. However, in vivo proofs of the functional effects of B2R-β2AR heterodimerisation are missing. Both BK and adrenergic stimulation are known inducers of tPA release. Our goal was to demonstrate the existence of B2R-β2AR heterodimerisation in myocardium and to define its functional effect on cardiac release of tPA in vivo. We further investigated the effects of a non-selective β-blocker on this receptor interplay. To investigate functional effects of B2R-β2AR heterodimerisation (i. e. BK transactivation of β2AR) in vivo, we induced serial electrical stimulation of cardiac sympathetic nerves (SS) in normal pigs that underwent concomitant BK infusion. Both SS and BK alone induced increases in cardiac tPA release. Importantly, despite B2R desensitisation, simultaneous BK infusion and SS (BK+SS) was characterised by 2.3 ± 0.3-fold enhanced tPA release compared to SS alone. When β-blockade (propranolol) was introduced prior to BK+SS, tPA release was inhibited. A persistent B2R-β2AR heterodimer was confirmed in BK-stimulated and non-stimulated left ventricular myocardium by immunoprecipitation studies and under non-reducing gel conditions. All together, these results strongly suggest BK transactivation of β2AR leading to enhanced β2AR-mediated release of tPA. Importantly, non-selective β-blockade inhibits both SS-induced release of tPA and the functional effects of B2R-β2AR heterodimerisation in vivo, which may have important clinical implications.

Natriuretic peptide-induced catecholamine release from cardiac sympathetic neurons: inhibition by histamine H3 and H4 receptor activation

We reported previously that natriuretic peptides, including brain natriuretic peptide (BNP), promote norepinephrine release from cardiac sympathetic nerves and dopamine release from differentiated pheochromocytoma PC12 cells. These proexocytotic effects are mediated by an increase in intracellular calcium secondary to cAMP/protein kinase A (PKA) activation caused by a protein kinase G (PKG)-mediated inhibition of phosphodiesterase type 3 (PDE3). The purpose of the present study was to search for novel means to prevent the proadrenergic effects of natriuretic peptides. For this, we focused our attention on neuronal inhibitory Gα(i/o)-coupled histamine H(3) and H(4) receptors. Our findings show that activation of neuronal H(3) and H(4) receptors inhibits the release of catecholamines elicited by BNP in cardiac synaptosomes and differentiated PC12 cells. This effect results from a decrease in intracellular Ca(2+) due to reduced intracellular cAMP/PKA activity, caused by H(3) and H(4) receptor-mediated PKG inhibition and consequent PDE3-induced increase in cAMP metabolism. Indeed, selective H(3) and H(4) receptor agonists each synergized with a PKG inhibitor and a PDE3 activator in attenuating BNP-induced norepinephrine release from cardiac sympathetic nerve endings. This indicates that PKG inhibition and PDE3 stimulation are pivotal for the H(3) and H(4) receptor-mediated attenuation of BNP-induced catecholamine release. Cardiac sympathetic overstimulation is characteristic of advanced heart failure, which was recently found not to be improved by the administration of recombinant BNP (nesiritide), despite the predicated beneficial effects of natriuretic peptides. Because excessive catecholamine release is likely to offset the desirable effects of natriuretic peptides, our findings suggest novel means to alleviate their adverse effects and improve their therapeutic potential.

Aldehyde dehydrogenase type 2 activation by adenosine and histamine inhibits ischemic norepinephrine release in cardiac sympathetic neurons: mediation by protein kinase Cε

During myocardial ischemia/reperfusion, lipid peroxidation leads to the formation of toxic aldehydes that contribute to ischemic dysfunction. Mitochondrial aldehyde dehydrogenase type 2 (ALDH2) alleviates ischemic heart damage and reperfusion arrhythmias via aldehyde detoxification. Because excessive norepinephrine release in the heart is a pivotal arrhythmogenic mechanism, we hypothesized that neuronal ALDH2 activation might diminish ischemic norepinephrine release. Incubation of cardiac sympathetic nerve endings with acetaldehyde, at concentrations achieved in myocardial ischemia, caused a concentration-dependent increase in norepinephrine release. A major increase in norepinephrine release also occurred when sympathetic nerve endings were incubated in hypoxic conditions. ALDH2 activation substantially reduced acetaldehyde- and hypoxia-induced norepinephrine release, an action prevented by inhibition of ALDH2 or protein kinase Cε (PKCε). Selective activation of G(i/o)-coupled adenosine A(1), A(3), or histamine H(3) receptors markedly inhibited both acetaldehyde- and hypoxia-induced norepinephrine release. These effects were also abolished by PKCε and/or ALDH2 inhibition. Moreover, A(1)-, A(3)-, or H(3)-receptor activation increased ALDH2 activity in a sympathetic neuron model (differentiated PC12 cells stably transfected with H(3) receptors). This action was prevented by the inhibition of PKCε and ALDH2. Our findings suggest the existence in sympathetic neurons of a protective pathway initiated by A(1)-, A(3)-, and H(3)-receptor activation by adenosine and histamine released in close proximity of these terminals. This pathway comprises the sequential activation of PKCε and ALDH2, culminating in aldehyde detoxification and inhibition of hypoxic norepinephrine release. Thus, pharmacological activation of PKCε and ALDH2 in cardiac sympathetic nerves may have significant protective effects by alleviating norepinephrine-induced life-threatening arrhythmias that characterize myocardial ischemia/reperfusion.

Interaction between sensory C-fibers and cardiac mast cells in ischemia/reperfusion: activation of a local renin-angiotensin system culminating in severe arrhythmic dysfunction

Renin, the rate-limiting enzyme in the activation of the renin-angiotensin system (RAS), is synthesized and stored in cardiac mast cells. In ischemia/reperfusion, cardiac sensory nerves release neuropeptides such as substance P that, by degranulating mast cells, might promote renin release, thus activating a local RAS and ultimately inducing cardiac dysfunction. We tested this hypothesis in whole hearts ex vivo, in cardiac nerve terminals in vitro, and in cultured mast cells. We found that substance P-containing nerves are juxtaposed to renin-containing cardiac mast cells. Chemical stimulation of these nerves elicited substance P release that was accompanied by renin release, with the latter being preventable by mast cell stabilization or blockade of substance P receptors. Substance P caused degranulation of mast cells in culture and elicited renin release, and both of these were prevented by substance P receptor blockade. Ischemia/reperfusion in ex vivo hearts caused the release of substance P, which was associated with an increase in renin and norepinephrine overflow and with sustained reperfusion arrhythmias; substance P receptor blockade prevented these changes. Substance P, norepinephrine, and renin were also released by acetaldehyde, a known product of ischemia/reperfusion, from cardiac synaptosomes and cultured mast cells, respectively. Collectively, our findings indicate that an important link exists in the heart between sensory nerves and renin-containing mast cells; substance P released from sensory nerves plays a significant role in the release of mast cell renin in ischemia/reperfusion and in the activation of a local cardiac RAS. This culminates in angiotensin production, norepinephrine release, and arrhythmic cardiac dysfunction.

Peripheral nociception associated with surgical incision elicits remote nonischemic cardioprotection via neurogenic activation of protein kinase C signaling

BACKGROUND

Although remote ischemic stimuli have been shown to elicit cardioprotection against ischemia/reperfusion injury, there is little known about the effects of nonischemic stimuli. We previously described a remote cardioprotective effect of nonischemic surgical trauma (abdominal incision) called remote preconditioning of trauma (RPCT). In the present study, we elucidate mechanisms underlying this phenomenon.

METHODS AND RESULTS

We used a murine model of myocardial infarction to evaluate ischemia/reperfusion injury, and either abdominal surgical incision, or application of topical capsaicin, to elicit cardioprotection. We show that the cardioprotective effect of RPCT is initiated by skin nociception, and requires neurogenic signaling involving spinal nerves and activation of cardiac sensory and sympathetic nerves. Our results demonstrate bradykinin-dependent activation and repression, respectively, of PKCepsilon and PKCdelta in myocardium after RPCT, and we show involvement of the K(ATP) channels in cardioprotection. Finally, we show that topical application of capsaicin, which selectively activates C sensory fibers in the skin, mimics the cardioprotective effect of RPCT against myocardial infarction.

CONCLUSIONS

Nontraumatic nociceptive preconditioning represents a novel therapeutic strategy for cardioprotection with great potential clinical utility.

Histamine in Macaca mulatto monkey cardiac sympathetic nerve system: a morphological and functional assessment

Our previous study demonstrated the co-localization of histamine with norepinephrine (NE) within superior cervical ganglia (SCG), and the release of histamine from sympathetic nerve endings of guinea pig evoked by stimulations. We have now further investigated that whether the histamine can be synthesized, stored and released from the sympathetic nerve systems of Macaca mulatto monkey, and investigated the modulation of the sympathetic endogenous histamine release through histamine H(3) receptor in the monkey cardiac sympathetic nerve system. Double-labeled immunofluorescence technique was applied to investigate co-localization of histamine and NE in SCG of Macaca mulatto monkey. The cardiac sympathetic nerve terminals (synaptosomes) of Macaca mulatto monkey was prepared and depolarized with 50 mmol/L K(+). Histamine released from synaptosomes was detected by spectrofluorometer and regulations of histamine release through Ca(2+), Ca(2+)-channel blockers, H(3)-receptor agonist (R)-alpha-methylhistamine and histamine H(3)-receptor antagonist, thioperamide were observed. Co-localization of histamine and NE was identified within the same neuron of SCG. Release of histamine was Ca(2+)-dependent and inhibited by N-type Ca(2+)-channel blocker omega-conotoxin, but not affected by the L-type Ca(2+)-channel blocker lacidipine. Compound 48/80, a mast cell releaser, did not affect cardiac synaptosome histamine exocytosis. Cardiac synaptosome histamine release was augmented by the enhanced synthesis of histamine or the inhibition of histamine metabolism. Histamine H(3)-receptor activation by (R)-alpha-methylhistamine inhibited high K(+)-evoked histamine release and thioperamide blocked the effects of (R)-alpha-methylhistamine. These results firstly showed that histamine co-existed with NE within sympathetic neurons of monkey and the exocytosis of histamine from sympathetic terminals could be regulated by presynaptic histamine H(3) receptors. Sympathetic histamine may act as a neurotransmitter to modulate sympathetic neurotransmission.

Renin: at the heart of the mast cell

Cardiac mast cells proliferate in cardiovascular diseases. In myocardial ischemia, mast cell mediators contribute to coronary vasoconstriction, arrhythmias, leukocyte recruitment, and tissue injury and repair. Arrhythmic dysfunction, coronary vasoconstriction, and contractile failure are also characteristic of cardiac anaphylaxis. In coronary atherosclerosis, mast cell mediators facilitate cholesterol accumulation and plaque destabilization. In cardiac failure, mast cell chymase causes myocyte apoptosis and fibroblast proliferation, leading to ventricular dysfunction. Chymase and tryptase also contribute to fibrosis in cardiomyopathies and myocarditis. In addition, mast cell tumor necrosis factor-alpha promotes myocardial remodeling. Cardiac remodeling and hypertrophy in end-stage hypertension are also induced by mast cell mediators and proteases. We recently discovered that cardiac mast cells contain and release renin, which initiates local angiotensin formation. Angiotensin causes coronary vasoconstriction, arrhythmias, fibrosis, apoptosis, and endothelin release, all demonstrated mechanisms of mast-cell-associated cardiac disease. The effects of angiotensin are further amplified by the release of norepinephrine from cardiac sympathetic nerves. Our discovery of renin in cardiac mast cells and its release in pathophysiological conditions uncovers an important new pathway in the development of mast-cell-associated heart diseases. Several steps in this novel pathway may constitute future therapeutic targets.

Modulation of sympathetic activity by tissue plasminogen activator is independent of plasminogen and urokinase

Sympathetic neurons synthesize, transport, and release tissue-type plasminogen activators (t-PAs) and urinary-type plasminogen activators (u-PAs). We reported that t-PA enhances sympathetic neurotransmission and exacerbates reperfusion arrhythmias. We have now assessed the role of u-PA and plasminogen. Neurogenic contractile responses to electrical field stimulation (EFS) were determined in vasa deferentia (VD) from mice lacking t-PA (t-PA(-/-)), plasminogen activator inhibitor-1 (PAI-1(-/-)), plasminogen (plgn(-/-)), u-PA (u-PA(-/-)), and wild-type (WT) controls. Similar levels of t-PA were present in VD and cardiac synaptosomes of WT, PAI-1(-/-), plgn(-/-), and u-PA(-/-) mice, whereas t-PA was undetectable in t-PA(-/-) tissues. EFS responses were potentiated and attenuated in VD from PAI-1(-/-) and t-PA(-/-) mice, respectively, but indistinguishable from WT responses in VD from plgn(-/-) and u-PA(-/-) mice. Moreover, t-PA inhibition with t-PA(stop) decreased EFS response in WT mice, whereas u-PA(stop) did not. VD responses to ATP, norepinephrine, and K(+) in t-PA(-/-), PAI-1(-/-), plgn(-/-), and u-PA(-/-) mice were similar to those in WT, whereas t-PA(stop) did not modify VD responses to norepinephrine in WT, t-PA(-/-), and PAI-1(-/-) mice, indicating a prejunctional site of action for t-PA-induced potentiation of sympathetic neurotransmission. Indeed, K(+)-induced norepinephrine exocytosis from cardiac synaptosomes was potentiated in PAI-1(-/-), attenuated in t-PA(-/-) and not different from WT in u-PA(-/-) and plgn(-/-) mice. Likewise, ATP exocytosis was decreased in t-PA(-/-) and attenuated by t-PA(stop) in WT mice. Thus, t-PA-induced enhancement of sympathetic neurotransmission is a prejunctional event associated with increased transmitter exocytosis and independent of u-PA and plasminogen availability. This novel t-PA action may be a potential therapeutic target in hyperadrenergic states.

Histamine H3-receptor signaling in cardiac sympathetic nerves: Identification of a novel MAPK-PLA2-COX-PGE2-EP3R pathway

We hypothesized that the histamine H(3)-receptor (H(3)R)-mediated attenuation of norepinephrine (NE) exocytosis from cardiac sympathetic nerves results not only from a Galpha(i)-mediated inhibition of the adenylyl cyclase-cAMP-PKA pathway, but also from a Gbetagamma(i)-mediated activation of the MAPK-PLA(2) cascade, culminating in the formation of an arachidonate metabolite with anti-exocytotic characteristics (e.g., PGE(2)). We report that in Langendorff-perfused guinea-pig hearts and isolated sympathetic nerve endings (cardiac synaptosomes), H(3)R-mediated attenuation of K(+)-induced NE exocytosis was prevented by MAPK and PLA(2) inhibitors, and by cyclooxygenase and EP(3)-receptor (EP(3)R) antagonists. Moreover, H(3)R activation resulted in MAPK phosphorylation in H(3)R-transfected SH-SY5Y neuroblastoma cells, and in PLA(2) activation and PGE(2) production in cardiac synaptosomes; H(3)R-induced MAPK phosphorylation was prevented by an anti-betagamma peptide. Synergism between H(3)R and EP(3)R agonists (i.e., imetit and sulprostone, respectively) suggested that PGE(2) may be a downstream effector of the anti-exocytotic effect of H(3)R activation. Furthermore, the anti-exocytotic effect of imetit and sulprostone was potentiated by the N-type Ca(2+)-channel antagonist omega-conotoxin GVIA, and prevented by an anti-Gbetagamma peptide. Our findings imply that an EP(3)R Gbetagamma(i)-induced decrease in Ca(2+) influx through N-type Ca(2+)-channels is involved in the PGE(2)/EP(3)R-mediated attenuation of NE exocytosis elicited by H(3)R activation. Conceivably, activation of the Gbetagamma(i) subunit of H(3)R and EP(3)R may also inhibit Ca(2+) entry directly, independent of MAPK intervention. As heart failure, myocardial ischemia and arrhythmic dysfunction are associated with excessive local NE release, attenuation of NE release by H(3)R activation is cardioprotective. Accordingly, this novel H(3)R signaling pathway may ultimately bear therapeutic significance in hyper-adrenergic states.

Bradykinin B(2) receptor does not contribute to blood pressure lowering during AT(1) receptor blockade

This study tested the hypothesis that endogenous bradykinin contributes to the effects of angiotensin AT(1) receptor blockade in humans. The effect of the bradykinin B(2) receptor antagonist d-Arg-Arg-Pro-Hyp-Gly-Thi-Ser-d-Tic-Oic-Arg (HOE-140) (18 microg/kg/h i.v. for 6 h) on hemodynamic and endocrine responses to acute and chronic (1-month) treatment with valsartan (160 mg/day) was determined in 13 normotensive and 12 hypertensive salt-deplete subjects. Acute valsartan increased plasma renin activity (PRA) from 5.3 +/- 9.9 to 15.6 +/- 19.8 ng of angiotensin (Ang) I/ml/h (P < 0.001) and decreased aldosterone from 18.3 +/- 10.5 to 12.0 +/- 9.6 ng/dl (P < 0.001). Chronic valsartan significantly increased baseline PRA (10.5 +/- 15.5 ng of Ang I/ml/h; P = 0.004) but did not affect baseline angiotensin-converting enzyme activity or aldosterone. HOE-140 tended to increase the PRA response to valsartan, and it attenuated the decrease in aldosterone following chronic valsartan (P = 0.03). Acute valsartan decreased mean arterial pressure 12.7 +/- 6.9% (from 100.2 +/- 8.4 to 87.5 +/- 9.8 mm Hg in hypertensives and from 82.4 +/- 8.6 to 70.3 +/- 8.4 mm Hg in normotensives). HOE-140 did not affect the blood pressure response to either acute (effect of valsartan, P < 0.001; effect of HOE-140, P = 0.98) or chronic (valsartan, P = 0.01; HOE-140, P = 0.84) valsartan. Plasma cGMP was increased significantly during chronic valsartan (P = 0.048) through a bradykinin receptor-independent mechanism (effect of HOE-140, P = 0.13). Both acute (P < 0.001) and chronic (P < 0.001) valsartan increased heart rate. HOE-140 augmented the heart rate response to chronic valsartan (P = 0.04). These data suggest that endogenous bradykinin does not contribute significantly to the blood pressure-lowering effect of valsartan through its B(2) receptor.

Interstitial K+ concentration in active muscle after myocardial infarction

Previous work demonstrated that Na(+)-K(+) pump activity within skeletal muscle is attenuated in myocardial infarction (MI). This may lead to enhanced interstitial K(+) concentration ([K(+)](o)) in the muscle. We tested the hypothesis that [K(+)](o) rises with muscle contraction and that, in rats with MI, the rate of rise in [K(+)](o) is greater than it is in control animals. Microdialysis probes were inserted in the skeletal muscle of six healthy control and six MI rats. The ends of the probes were then attached to the K(+) electrodes, and [K(+)](o) was continuously measured. Muscle contraction was induced by electrical stimulation of the sciatic nerves for 1 min. Stimulation at 1 and 3 Hz increased muscle [K(+)](o) by 14.2% and 44.7% in controls and by 22.9% and 62.8% in MI rats (P < 0.05 vs. controls), respectively. When ouabain, an inhibitor of Na(+)-K(+) pump, was added to the perfusate, muscle [K(+)](o) rose significantly. This effect of ouabain was significantly attenuated in MI animals. In conclusion, when compared with that in control animals, an increase of [K(+)](o) in exercising muscle is augmented in MI rats, likely due to an attenuation of Na(+)-K(+) pump activity.

The plasminogen activator system modulates sympathetic nerve function

Sympathetic neurons synthesize and release tissue plasminogen activator (t-PA). We investigated whether t-PA modulates sympathetic activity. t-PA inhibition markedly reduced contraction of the guinea pig vas deferens to electrical field stimulation (EFS) and norepinephrine (NE) exocytosis from cardiac synaptosomes. Recombinant t-PA (rt-PA) induced exocytotic and carrier-mediated NE release from cardiac synaptosomes and cultured neuroblastoma cells; this was a plasmin-independent effect but was potentiated by a fibrinogen cleavage product. Notably, hearts from t-PA–null mice released much less NE upon EFS than their wild-type (WT) controls (i.e., a 76.5% decrease; P < 0.01), whereas hearts from plasminogen activator inhibitor-1 (PAI-1)–null mice released much more NE (i.e., a 275% increase; P < 0.05). Furthermore, vasa deferentia from t-PA–null mice were hyporesponsive to EFS (P < 0.0001) but were normalized by the addition of rt-PA. In contrast, vasa from PAI-1–null mice were much more responsive (P < 0.05). Coronary NE overflow from hearts subjected to ischemia/reperfusion was much smaller in t-PA–null than in WT control mice (P < 0.01). Furthermore, reperfusion arrhythmias were significantly reduced (P < 0.05) in t-PA–null hearts. Thus, t-PA enhances NE release from sympathetic nerves and contributes to cardiac arrhythmias in ischemia/reperfusion. Because the risk of arrhythmias and sudden cardiac death is increased in hyperadrenergic conditions, targeting the NE-releasing effect of t-PA may have valuable therapeutic potential.

Cardiac mast cell-derived renin promotes local angiotensin formation, norepinephrine release, and arrhythmias in ischemia/reperfusion

Having identified renin in cardiac mast cells, we assessed whether its release leads to cardiac dysfunction. In Langendorff-perfused guinea pig hearts, mast cell degranulation with compound 48/80 released Ang I-forming activity. This activity was blocked by the selective renin inhibitor BILA2157, indicating that renin was responsible for Ang I formation. Local generation of cardiac Ang II from mast cell-derived renin also elicited norepinephrine release from isolated sympathetic nerve terminals. This action was mediated by Ang II-type 1 (AT1) receptors. In 2 models of ischemia/reperfusion using Langendorff-perfused guinea pig and mouse hearts, a significant coronary spillover of renin and norepinephrine was observed. In both models, this was accompanied by ventricular fibrillation. Mast cell stabilization with cromolyn or lodoxamide markedly reduced active renin overflow and attenuated both norepinephrine release and arrhythmias. Similar cardioprotection was observed in guinea pig hearts treated with BILA2157 or the AT1 receptor antagonist EXP3174. Renin overflow and arrhythmias in ischemia/reperfusion were much less prominent in hearts of mast cell-deficient mice than in control hearts. Thus, mast cell-derived renin is pivotal for activating a cardiac renin-angiotensin system leading to excessive norepinephrine release in ischemia/reperfusion. Mast cell-derived renin may be a useful therapeutic target for hyperadrenergic dysfunctions, such as arrhythmias, sudden cardiac death, myocardial ischemia, and congestive heart failure.

Aging augments interstitial K+concentrations in active muscle of rats

Skeletal muscle performance declines with advancing age, and the underlying mechanism is not completely understood. A large body of convincing evidence has demonstrated a crucial role for interstitial K+concentration ([K+]o) in modulating contractile function of skeletal muscle. The present study tested the hypothesis that during muscle contraction there is a greater accumulation of [K+]oin aged compared with adult skeletal muscle. Twitch muscle contraction was induced by electrical stimulation of the sciatic nerves of 8- and 32-mo-old Fischer 344 × Brown Norway rats. Levels of [K+]owere measured continuously by a microdialysis technique with the probes inserted into the gastrocnemius muscle. Stimulation at 1, 3, and 5 Hz elevated muscle [K+]oby 52, 64, and 88% in adult rats, and by 78, 98, and 104% in aged rats, respectively, and the increase was significantly higher in aged than in adult rats. Recovery for [K+]o, as measured by the time for [K+]oto recover by 20 and 50% from peak response after stimulation, was slower in aged rats. Ouabain (5 mM), a specific inhibitor of the Na+-K+pump, was added in the perfusate to inhibit the reuptake of K+into the cells to assess the role of the pump in the overall K+balance. Ouabain elevated muscle [K+]oat rest, and the effect was significantly attenuated in aged animals. The present data demonstrated an augmented [K+]oin aged skeletal muscle compared with adult skeletal muscle, and the data suggested that an alteration in the function of the Na+-K+pump may contribute, in part, to the deficiency in K+balance in skeletal muscle of aged rats.

Norepinephrine reduces heat responses of cutaneous C-fiber nociceptors in Sprague–Dawley rats in vitro

Nociceptors are excited or sensitized by many inflammatory mediators as well as by elevation of tissue temperature. We have shown that there is a facilitatory synergistic interaction between norepinephrine (NE) and bradykinin (BK) on cutaneous C-fiber nociceptors in normal Lewis rats. These interactions may play an important role in the mechanism of sympathetically maintained pain. In the present experiment, using skin-saphenous nerve in vitro preparations, we tested the effect of NE on the activity of nociceptive fibers and their response to heat in normal Sprague-Dawley rats. For comparison with the previous data on Lewis rats, we also examined the effect of NE on BK response. NE (10(-5) or 10(-6) M) did not excite nociceptive fibers before repeated heat stimuli or BK superfusion (10(-5) or 10(-6) M) to the receptive field. In contrast, after a few applications of heat or BK, NE excited 20-43% of nociceptive fibers to similar magnitudes. We also found that NE sensitized subsequent BK responses, but somewhat unexpectedly that it suppressed subsequent heat responses. This occurred regardless of the presence or absence of NE-induced excitation. These results suggest different mechanisms of NE modification to the BK and heat responses of cutaneous C-fiber nociceptors.

Mast cells: a unique source of renin

In addition to the traditional renin-angiotensin system, a great deal of evidence favors the existence of numerous independent tissue-specific renin-angiotensin systems. We report that mast cells are an additional source of renin and constitute a unique extrarenal renin-angiotensin system. We use renin-specific antibodies to demonstrate that cardiac mast cells contain renin. Extending this observation to the human mast cell line HMC-1, we show that these mast cells also express renin. The HMC-1 renin RT-PCR product is 100% homologous to Homo sapiens renin. HMC-1 cells also contain renin protein, as demonstrated both by immunoblot and immunocytochemical analyses. Renin released from HMC-1 cells is active; furthermore, HMC-1 cells are able to synthesize renin. It is known that, in the heart, mast cells are found in the interstitium in close proximity to nerves and myocytes, which both express angiotensin II receptors. Inasmuch as myocardial interstitium contains angiotensinogen and angiotensin-converting enzyme, and because we were able to detect renin only in mast cells, we postulate that the release of renin from cardiac mast cells is the pivotal event triggering local formation of angiotensin II. Because of the ubiquity of mast cells, our results represent a unique paradigm for understanding local renin-angiotensin systems, not just in the heart, but in all tissues. Our findings provide a rationale for targeting mast cells in conjunction with renin-angiotensin system inhibitors in the management of angiotensin II-related dysfunctions.

Histamine H3-receptor-induced attenuation of norepinephrine exocytosis: a decreased protein kinase a activity mediates a reduction in intracellular calcium

We had reported that activation of presynaptic histamine H(3)-receptors inhibits norepinephrine exocytosis from depolarized cardiac sympathetic nerve endings, an action associated with a marked decrease in intraneuronal Ca(2+) that we ascribed to a decreased Ca(2+) influx. An H(3)-receptor-mediated inhibition of cAMP-dependent phosphorylation of Ca(2+) channels could cause a sequential attenuation of Ca(2+) influx, intraneuronal Ca(2+) and norepinephrine exocytosis. We tested this hypothesis in sympathetic nerve endings (cardiac synaptosomes) expressing native H(3)-receptors and in human neuroblastoma SH-SY5Y cells transfected with H(3)-receptors. Norepinephrine exocytosis was elicited by K(+) or by stimulation of adenylyl cyclase with forskolin. H(3)-receptor activation markedly attenuated the K(+)- and forskolin-induced norepinephrine exocytosis; pretreatment with pertussis toxin prevented this effect. Similar to forskolin, 8-bromo-cAMP elicited norepinephrine exocytosis but, unlike forskolin, it was unaffected by H(3)-receptor activation, demonstrating that inhibition of adenylyl cyclase is a pivotal step in the H(3)-receptor transductional cascade. Indeed, we found that H(3)-receptor activation attenuated norepinephrine exocytosis concomitantly with a decrease in intracellular cAMP and PKA activity in SH-SY5Y-H(3) cells. Moreover, pharmacological PKA inhibition acted synergistically with H(3)-receptor activation to reduce K(+)-induced peak intracellular Ca(2+) in SH-SY5Y-H(3) cells and norepinephrine exocytosis in cardiac synaptosomes. Furthermore, H(3)-receptor activation synergized with N- and L-type Ca(2+) channel blockers to reduce norepinephrine exocytosis in cardiac synaptosomes. Our findings suggest that the H(3)-receptor-mediated inhibition of norepinephrine exocytosis from cardiac sympathetic nerves results sequentially from H(3)-receptor-G(i)/G(o) coupling, inhibition of adenylyl cyclase activity, and decreased cAMP formation, leading to diminished PKA activity, and thus, decreased Ca(2+) influx through voltage-operated Ca(2+) channels.

Interactions of bradykinin and norepinephrine on rat cutaneous nociceptors in both normal and inflamed conditions in vitro

Many inflammatory chemical mediators excite or sensitize nociceptors, which had led some researchers to believe that they may interact with each other to maintain a persistent painful state. We examined how the excitatory mediators norepinephrine (NE) and bradykinin (BK) interact, using single fiber recordings from cutaneous nociceptors. We observed that NE augmented the BK-induced response in both control and adjuvant-inflamed rats in a way different from NE-induced excitation in inflamed animals only. BK also tended to augment the NE-induced response (examined only in inflamed rats). Our results provide the first evidence that BK and NE synergistically interact on nociceptors.

Coupling of angiotensin II AT1 receptors to neuronal NHE activity and carrier-mediated norepinephrine release in myocardial ischemia

In ischemia, cardiac sympathetic nerve endings (cSNE) release excessive amounts of norepinephrine (NE) via the nonexocytotic Na(+)-dependent NE transporter (NET). NET, normally responsible for NE reuptake into cSNE, reverses in myocardial ischemia, releasing pathological amounts of NE. This carrier-mediated NE release can be triggered by elevated intracellular Na(+) levels in the axoplasm. The fact that ischemia activates the intracellular pH regulatory Na(+)/H(+) exchanger (NHE) in cSNE is pivotal in increasing intraneuronal Na(+) and thus activating carrier-mediated NE release. Angiotensin (ANG) II levels are also significantly elevated in the ischemic heart. However, the effects of ANG II on cSNE, which express the ANG II receptor, AT(1)R, are poorly understood. We hypothesized that ANG II-induced AT(1)R activation in cSNE may be positively coupled to NHE activity and thereby facilitate the pathological release of NE associated with myocardial ischemia. We tested this hypothesis in a cSNE model, human neuroblastoma cells stably transfected with rat recombinant AT(1A) receptor (SH-SY5Y-AT(1A)). SH-SY5Y-AT(1A) constitutively expresses amiloride-sensitive NHE and the NET. NHE activity was assayed in BCECF-loaded SH-SY5Y-AT(1A) as the rate of the Na(+)-dependent alkalinization in response to an acute acidosis. ANG II activation of AT(1)R markedly increased NHE activity in SH-SY5Y-AT(1A) via a Ca(2+)-dependent pathway and promoted carrier-mediated NE release. In addition, in guinea pig cSNE expressing native AT(1)R, ANG II elicited carrier-mediated NE release. In SH-SY5Y-AT(1A) and cSNE, amiloride inhibited the ANG II-mediated release of NE. Our results provide a link between AT(1)R and NHE in cSNE, which can exacerbate carrier-mediated NE release during protracted myocardial ischemia.

Ectonucleotidase in sympathetic nerve endings modulates ATP and norepinephrine exocytosis in myocardial ischemia

We recently reported that ATP, coreleased with norepinephrine (NE) from cardiac sympathetic nerves, increases NE exocytosis via a positive feedback mechanism. A neuronal ectonucleotidase (E-NTPDase) metabolizes the released ATP, decreasing NE exocytosis. Excessive NE release in myocardial ischemia exacerbates cardiac dysfunction. Thus, we studied whether the ATP-mediated autocrine amplification of NE release is operative in ischemia and, if so, whether it can be modulated by E-NTPDase and its recombinant equivalent, solCD39. Isolated, guinea pig hearts underwent 10- or 20-min ischemic episodes, wherein NE was released by exocytosis and reversal of the NE transporter, respectively. Furthermore, to restrict the role of E-NTPDase to transmitter ATP, sympathetic nerve endings were isolated (cardiac synaptosomes) and subjected to increasing periods of ischemia. Availability of released ATP at the nerve terminals was either increased via E-NTPDase inhibition or diminished by enhancing ATP hydrolysis with solCD39. P2X receptor blockade with PPADS was used to attenuate the effects of released ATP. We found that, in short-term ischemia (but, as anticipated, not in protracted ischemia, where NE release is carrier-mediated), ATP exocytosis was linearly correlated with that of NE. This indicates that by limiting the availability of ATP at sympathetic terminals, E-NTPDase effectively attenuates NE exocytosis in myocardial ischemia. Our findings suggest a key role for neuronal E-NTPDase in the control of adrenergic function in the ischemic heart. Because excessive NE release is an established cause of dysfunction in ischemic heart disease, solCD39 may offer a novel therapeutic approach to myocardial ischemia and its consequences.

Metabolic control of excessive extracellular nucleotide accumulation by CD39/ecto-nucleotidase-1: implications for ischemic vascular diseases

Platelets are responsible for maintaining vascular integrity. In thrombocytopenic states, vascular permeability and fragility increase, presumably due to the absence of this platelet function. Chemical or physical injury to a blood vessel induces platelet activation and platelet recruitment. This is beneficial for the arrest of bleeding (hemostasis), but when an atherosclerotic plaque is ulcerated or fissured, it becomes an agonist for vascular occlusion (thrombosis). Experiments in the late 1980s cumulatively indicated that endothelial cell CD39-an ecto-ADPase-reduced platelet reactivity to most agonists, even in the absence of prostacyclin or nitric oxide. As discussed herein, CD39 rapidly and preferentially metabolizes ATP and ADP released from activated platelets to AMP, thereby drastically reducing or even abolishing platelet aggregation and recruitment. Since ADP is the final common agonist for platelet recruitment and thrombus formation, this finding highlights the significance of CD39. A recombinant, soluble form of human CD39, solCD39, has enzymatic and biological properties identical to the full-length form of the molecule and strongly inhibits human platelet aggregation induced by ADP, collagen, arachidonate, or TRAP (thrombin receptor agonist peptide). In sympathetic nerve endings isolated from guinea pig hearts, where neuronal ATP enhances norepinephrine exocytosis, solCD39 markedly attenuated norepinephrine release. This suggests that NTPDase (nucleoside triphosphate diphosphohydrolase) could exert a cardioprotective action by reducing ATP-mediated norepinephrine release, thereby offering a novel therapeutic approach to myocardial ischemia and its consequences. In a murine model of stroke, driven by excessive platelet recruitment, solCD39 reduced the sequelae of stroke, without an increase in intracerebral hemorrhage. CD39 null mice, generated by deletion of apyrase-conserved regions 2 to 4, exhibited a decrease in postischemic perfusion and an increase in cerebral infarct volume when compared with controls. "Reconstitution" of CD39 null mice with solCD39 reversed these changes. We hypothesize that solCD39 has potential as a novel therapeutic agent for thrombotic diatheses.

Norepinephrine release from the ischemic heart is greatly enhanced in mice lacking histamine H3 receptors

We previously reported that histamine H(3) receptors (H(3)Rs) are present in cardiac sympathetic nerve endings (cSNE) of animals and humans, where they attenuate norepinephrine (NE) release in normal and hyperadrenergic states, such as myocardial ischemia. The recent creation of a transgenic line of mice lacking H(3)R provided us with the opportunity to assess the relevance of H(3)R in the ischemic heart. We isolated SNE from hearts of wild-type (H(3)R(+/+)) and knockout (H(3)R(-/-)) mice and found that basal NE release from H(3)R(-/-) cSNE was approximately 60% greater than that from H(3)R(+/+) cSNE. NE exocytosis evoked by K(+)-induced depolarization of cSNE from H(3)R(+/+) mice was attenuated by activation of either H(3)R or adenosine A(1) receptors (A(1)R). In contrast, NE release from cSNE of H(3)R(-/-) was unaffected by H(3)R agonists, but it was still attenuated by A(1)R activation. When isolated mouse hearts were subjected to ischemia for 20 min, NE overflow into the coronaries was 2-fold greater in the H(3)R(-/-) hearts than in those from H(3)R(+/+) mice. Furthermore, whereas stimulation of H(3)R or A(1)R reduced ischemic NE overflow from H(3)R(+/+) hearts by 50%, only A(1)R, but not H(3)R activation, reduced NE release in H(3)R(-/-). Our data demonstrate that NE release from cSNE can be modulated by various heteroinhibitory receptors (e.g., H(3)R and A(1)R) and that H(3)Rs are particularly important in modulating NE release in myocardial ischemia. Inasmuch as excessive NE release is clinically recognized as a major cause of arrhythmic cardiac dysfunction, our findings reveal a significant cardioprotective role of H(3)R on cSNE.

Bradykinin B1 and B2 receptors differentially regulate cardiac Na+-H+ exchanger, Na+-Ca2+ exchanger and Na+-HCO3- symporter

Bradykinin B(1) and B(2) receptors are up-regulated in the infarcted myocardium, and both receptors are involved in the regulation of intracellular pH and Ca(2+). The present study investigated the role of bradykinin B(1) and B(2) receptors in the regulation of Na(+)-H(+) exchanger (NHE-1), Na(+)-Ca(2+) exchanger (NCE-1) and Na(+)-HCO(3)(-) symporter (NBC-1) in the infarcted myocardium. NHE-1, NCE-1 and NBC-1 mRNA expression was determined by Northern blot analysis and the protein levels by Western blot analysis. Measurements were performed 1, 7 and 14 days after induction of myocardial infarction. Localization of NHE-1, NCE-1 and NBC-1 within the myocardium was studied using confocal microscopy. Cardiac morphology was measured in picrosiris-red-stained hearts. Rats were treated with placebo, the bradykinin B(2) receptor antagonist icatibant (0.5 mg/kg/day) or the bradykinin B(1) receptor antagonist des-Arg(9)-[Leu(8)]bradykinin (1 mg/kg/day). Treatment was started 1 week prior to surgery and continued until 1, 7 and 14 days post infarction. NHE-1, NCE-1 and NBC-1 mRNA expression and protein levels were increased 1 day and reached maximum values on day 7 post infarction. NHE-1 was localized in the plasma membrane, NCE-1 in the membrane of the sarcoplasmatic reticulum and NBC-1 near the Z-line. Icatibant reduced NHE-1 and inhibited NCE-1 mRNA- and protein up-regulation, while des-Arg(9)-[Leu(8)]bradykinin had no effect on NHE-1 and NCE-1 expression and translation. Transcriptional and translational up-regulation of NBC-1 was unaffected by the bradykinin B(1) and B(2) receptor antagonists. Icatibant, but not des-Arg(9)-[Leu(8)]bradykinin, limited infarct size and reduced left ventricular dilation, septal thickening and interstitial fibrosis post infarction. Bradykinin B(2) receptors are involved in transcriptional and translational regulation of NHE-1 and NCE-1 in the ischemic myocardium. Chronic B(2) receptor blockade might exert an anti-ischemic effect via limitation of NHE-1-mediated acidosis and NCE-1-mediated Ca(2+)-overload.

Activation of a renin-angiotensin system in ischemic cardiac sympathetic nerve endings and its association with norepinephrine release

We had reported that in the ischemic heart, locally formed bradykinin (BK) and angiotensin II (Ang II) activate B2- and AT1-receptors on sympathetic nerve terminals (SNE), promoting reversal of the norepinephrine (NE) transporter in an outward direction (i.e., carrier-mediated NE release). Although both BK and Ang II contribute to ischemic NE release, Ang II is likely to play a more important role. Since BK is formed by ischemic SNE, we questioned whether cardiac SNE also contribute to local Ang II formation, in addition to being a target of Ang II. SNE were isolated from surgical specimens of human right atrium and incubated in ischemic conditions. These SNE released large amounts of endogenous NE via a carrier-mediated mechanism, as evidenced by the inhibitory effect of desipramine on this process. Moreover, two renin inhibitors, pepstatin-A and BILA 2157 BS, the ACE inhibitor enalaprilat and the AT1-receptor antagonist EXP3174 prevented ischemic NE release. Western blot analysis revealed the presence of renin in cardiac SNE. Renin abundance increased more than three-fold during ischemia. Thus, renin is present in cardiac SNE and is activated during ischemia, eventually culminating in Ang II formation, stimulation of AT1-receptors and carrier-mediated NE release. Our findings uncover a novel autocrine mechanism, by which Ang II, formed at SNE in myocardial ischemia, elicits carrier-mediated NE release by activating prejuntional AT1-receptors.

Safety and efficacy of valsartan versus enalapril in heart failure patients

Although a cornerstone in the treatment of heart failure, angiotensin-converting enzyme inhibitors are under-used, partly due to side effects. If proven at least similarly efficacious to angiotensin-converting enzyme inhibitors, angiotensin-receptor blockers may replace them due to their superior tolerability. We aimed to compare the efficacy and safety of valsartan and enalapril in heart failure patients stabilised on an angiotensin-converting enzyme inhibitor. We randomised 141 patients (mean 68 years, 74% males) with stable mild/moderate heart failure and left ventricular ejection fraction 0.45 or less, to valsartan 160 mg q.d. (n=70) or enalapril 10 mg b.i.d. (n=71) for 12 weeks. Changes in 6-min-walk test (primary efficacy variable), patients' wellbeing and left ventricular size and function did not differ significantly between the treatment groups. Valsartan was significantly non-inferior to enalapril in walk test distance change: least-square means treatment difference +1.12 m (95% confidence interval -21.9 to 24.1), non-inferiority P<0.001. Left ventricular size (P<0.001) and function (P=0.048) improved significantly only in the valsartan group. Fewer patients experienced adverse events in the valsartan group (50%) than in the enalapril group (63%), although statistically non-significant. Valsartan is similarly efficacious and safe to enalapril in patients with stable, mild/moderate heart failure, previously stabilised on an angiotensin-converting enzyme inhibitor and directly switched to study medication.

Effect of ACE-inhibitor ramiprilat and AT1-receptor antagonist candesartan on cardiac norepinephrine release: comparison between ischemic and nonischemic conditions

ACE-inhibitors and AT -receptor antagonists may exert part of their pharmacological actions by interference with angiotensin-and/or bradykinin-mediated prejunctional stimulation of cardiac norepinephrine release. As endogenous formation of angiotensin and bradykinin is increased in ischemia, we investigated the effects of the ACE-inhibitor ramiprilat and the AT -receptor antagonist candesartan on cardiac norepinephrine release in isolated perfused rat hearts, under nonischemic and stop-flow conditions. Exocytotic release of endogenous norepinephrine was induced by electrical field stimulation and measured by HPLC. Paired stimulations were applied in each heart to obtain an intraindividual comparison of the effect of the pharmacological agent on norepinephrine release with the release under baseline conditions. The ACE-inhibitor ramiprilat (0.1-10 nM) and the AT -receptor antagonist candesartan (1-100 nM) were studied during normal flow or in the fourth minute of stop-flow. Under nonischemic conditions, the ACE-inhibitor slightly reduced norepinephrine release at the highest concentration, while the AT -receptor antagonist did not influence norepinephrine release in normoxia. Conversely, both substances significantly increased norepinephrine release during ischemia. Augmentation of norepinephrine release in ischemia by ramiprilat and candesartan was blocked by the bradykinin B -receptor antagonist HOE 140 and, in case of candesartan, by the AT -receptor antagonist PD 123319. The ACE-inhibitor ramiprilat and AT -receptor antagonist candesartan enhance cardiac norepinephrine release selectively in ischemia by stimulating presynaptic bradykinin B -receptors. Regarding the AT -receptor antagonist, AT -receptor activation is also involved in bradykinin-mediated prejunctional stimulation.

Ischemia promotes renin activation and angiotensin formation in sympathetic nerve terminals isolated from the human heart: contribution to carrier-mediated norepinephrine release

We recently reported that in the ischemic human heart, locally formed angiotensin II activates angiotensin II type 1 (AT(1)) receptors on sympathetic nerve terminals, promoting reversal of the norepinephrine transporter in an outward direction (i.e., carrier-mediated norepinephrine release). The purpose of this study was to assess whether cardiac sympathetic nerve endings contribute to local angiotensin II formation, in addition to being a target of angiotensin II. To this end, we isolated sympathetic nerve endings (cardiac synaptosomes) from surgical specimens of human right atrium and incubated them in ischemic conditions (95% N(2,) sodium dithionite, and no glucose for 70 min). These synaptosomes released large amounts of endogenous norepinephrine via a carrier-mediated mechanism, as evidenced by the inhibitory effect of desipramine on this process. Norepinephrine release was further enhanced by preincubation of synaptosomes with angiotensinogen and was prevented by two renin inhibitors, pepstatin-A and BILA 2157BS, as well as by the angiotensin-converting enzyme inhibitor enalaprilat and the AT(1) receptor antagonist EXP 3174 [2-N-butyl-4-chloro-1-[2'-(1H-tetrazol-5-yl)biphenyl-4-yl] methyl]imidazole-5-carboxylic acid]. Western blot analysis revealed the presence of renin in cardiac sympathetic nerve terminals; renin abundance increased ~3-fold during ischemia. Thus, renin is rapidly activated during ischemia in cardiac sympathetic nerve terminals, and this process eventually culminates in angiotensin II formation, stimulation of AT(1) receptors, and carrier-mediated norepinephrine release. Our findings uncover a novel autocrine/paracrine mechanism whereby angiotensin II, formed at adrenergic nerve endings in myocardial ischemia, elicits carrier-mediated norepinephrine release by activating adjacent AT(1) receptors.

Magnesium and the parathyroid

The serum levels of parathyroid hormone and magnesium depend on each other in a complex manner. The secretion of parathyroid hormone by the parathyroid is physiologically controlled by the serum calcium level, but magnesium can exert similar effects. While low levels of magnesium stimulate parathyroid hormone secretion, very low serum concentrations induce a paradoxical block. This block leads to clinically relevant hypocalcemia in severely hypomagnesiemic patients. The mechanism of this effect has recently been traced to an activation of the alpha-subunits of heterotrimeric G-proteins. This activation mimicks activation of the calcium sensing receptor and thus causes inhibition of parathyroid hormone secretion. In addition to the effects of magnesium on parathyroid hormone secretion, parathyroid hormone in turn regulates magnesium homeostasis by modulating renal magnesium reabsorption. The distal convoluted tubule is of crucial importance for parathyroid hormone-regulated magnesium homeostasis.

Role of hydroxyl radical formation in neurotoxicity as revealed by in vivo free radical trapping

Reactive oxygen species have been implicated in dopaminergic toxicity caused by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and iron. Although MPTP produces a parkinsonian syndrome after its conversion to 1-methyl-4-phenylpyridine (MPP(+)) by type B monoamine oxidase (MAO-B) in the brain, the etiology of this disease remains obscure. MPP(+) is a highly potent dopaminbergic-releasing agents and dopamine (DA) autoxidation catalyzed by iron and oxidative stress may be involved in the pathogenesis of Parkinson's disease. Neuromelanine synthesis from DA produce highly reactive free radicals. Although the controversy possible neurotoxin and/or neuroprotective roles of nitric oxide (NO) was discussed, NO contributes to oxidative injury to brain neurons in vivo. An environmental estrogen-like chemical also related to MPP(+)-induced *OH generation. This review describes actual mechanism of the free radicals formation by dialysis studies of in vivo free radical trapping in the pathogenesis of neurodegenerative disorders, including in the Parkinson's disease, Alzheimer disease and traumatic brain injuries.

Cardiovascular effects of peptide kinin B2 receptor antagonists in rats

Bradykinin (BK) is a vasoactive peptide reputed to play an important role in cardiovascular homeostasis. In this study, we describe the cardiovascular changes (mean blood pressure (BP) and heart rate (HR)) induced by the i.v. administration (left jugular vein) of two selective kinin B2 receptor antagonist, namely icatibant (0.1-1 micromol/kg as a bolus) and MEN1 1270 (0.1-1 micromol/kg as a bolus or 1 micromol/kg infused in 15 or 60 min), in urethane-anaesthetized or conscious rats with an indwelling catheter implanted in the right carotid artery for BP measurements. In conscious rats, icatibant at 0.1 or 0.3 micromol/kg did not change BP but at 0.1 micromol/kg increased HR at 30 min from administration. MEN1 1270 at 0.1 or 0.3 micromol/kg induced a dose-related increase in BP and a concomitant bradycardia (significant at 0.3 micromol/kg) lasting for 5 or 30 min, respectively. Icatibant at 1 micromol/kg induced a slight (P < 0.05) increase in BP that resolved in 5 min and a biphasic tachycardia (peaks at 30 and 90 min from administration). MEN1 1270 at 1 micromol/kg induced a triphasic change in HR (tachycardia in the first 5 min, bradycardia at 30 min, and tachycardia at 90 and 120 min) and a biphasic change in BP (hypotension at 15 min and hypertension at 30 min). The i.v. infusion of MEN1 1270 (1 micromol/kg in 15 or 60 min) produced hypertension, whereas HR was increased only following the 15-min infusion. In urethane-anaesthetized rats, both icatibant and MEN1 1270 (0.1 micromol/kg as a bolus) increased BP and the onset for this effect was correlated with the time course of the antagonism of BK-induced hypotension, where the effect of MEN1 1270 was more rapid than that of icatibant. These results indicate that kinin B2 receptor antagonists can induce acute cardiovascular effects, and the reason for the different haemodynamic profile between icatibant and MEN1 1270 could be putatively attributed to kinetic characteristics.

EctoNucleotidase in cardiac sympathetic nerve endings modulates ATP-mediated feedback of norepinephrine release

ATP, coreleased with norepinephrine, affects adrenergic transmission by acting on purinoceptors at sympathetic nerve endings. Ectonucleotidases terminate the actions of ATP. Previously, we had preliminary evidence for ectonucleotidase activity in cardiac sympathetic nerve terminals. Therefore, we investigated whether this ectonucleotidase might influence norepinephrine release in the heart. Sympathetic nerve endings isolated from guinea pig heart (cardiac synaptosomes) were rich in Ca(2+)-dependent ectonucleotidase activity, as measured by metabolism of exogenously added radiolabeled ATP or ADP. By its inhibitor profile, ectonucleotidase resembled ectonucleoside triphosphate diphosphohydrolase 1 (E-NTPDase1). Exogenous ATP elicited concentration-dependent norepinephrine release from cardiac synaptosomes (EC(50) 0.96 microM). This release was antagonized by the P2X receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) (10 microM) and potentiated by the P2Y receptor antagonist 2'-deoxy-N(6)-methyladenosine-3',5'-diphosphate (MRS 2179) (30 nM). Norepinephrine release promoted by ATP was also potentiated by the nucleotidase inhibitor 6-N,N-diethyl-beta-gamma-dibromomethylene-D-adenosine-5'-triphosphate (ARL67156) (30 microM) and blocked by a recombinant, soluble form of human E-NTPDase1 (solCD39). In contrast, ARL67156 had no effect on norepinephrine release induced by the nonhydrolyzable analog, alpha, beta-methyleneadenosine-5'-triphosphate (alpha,beta-MeATP). Depolarization of cardiac synaptosomes with K(+) elicited release of endogenous norepinephrine. This was attenuated by PPADS and solCD39 and potentiated by MRS 2179 and ARL67156. Importantly, our results demonstrate that facilitation of ATP-induced norepinephrine release from cardiac sympathetic nerves is a composite of two autocrine components: positive, mediated by P2X receptors, and negative, mediated by P2Y receptors. Modulation of norepinephrine release by coreleased ATP is terminated by endogenous as well as exogenous ectonucleotidase. We propose that ectonucleotidase control of norepinephrine release should provide cardiac protection in hyperadrenergic states such as myocardial ischemia.

Decreased intracellular calcium mediates the histamine H3-receptor-induced attenuation of norepinephrine exocytosis from cardiac sympathetic nerve endings

Activation of presynatic histamine H(3) receptors (H(3)R) down-regulates norepinephrine exocytosis from cardiac sympathetic nerve terminals, in both normal and ischemic conditions. Analogous to the effects of alpha(2)-adrenoceptors, which also act prejunctionally to inhibit norepinephrine release, H(3)R-mediated antiexocytotic effects could result from a decreased Ca(2+) influx into nerve endings. We tested this hypothesis in sympathetic nerve terminals isolated from guinea pig heart (cardiac synaptosomes) and in a model human neuronal cell line (SH-SY5Y), which we stably transfected with human H(3)R cDNA (SH-SY5Y-H(3)). We found that reducing Ca(2+) influx in response to membrane depolarization by inhibiting N-type Ca(2+) channels with omega-conotoxin (omega-CTX) greatly attenuated the exocytosis of [(3)H]norepinephrine from both SH-SY5Y and SH-SY5Y-H(3) cells, as well as the exocytosis of endogenous norepinephrine from cardiac synaptosomes. Similar to omega-CTX, activation of H(3)R with the selective H(3)R-agonist imetit also reduced both the rise in intracellular Ca(2+) concentration (Ca(i)) and norepinephrine exocytosis in response to membrane depolarization. The selective H(3)R antagonist thioperamide prevented this effect of imetit. In the parent SH-SY5Y cells lacking H(3)R, imetit affected neither the rise in Ca(i) nor [(3)H]norepinephrine exocytosis, demonstrating that the presence of H(3)R is a prerequisite for a decrease in Ca(i) in response to imetit and that H(3)R activation modulates norepinephrine exocytosis by limiting the magnitude of the increase in Ca(i). Inasmuch as excessive norepinephrine exocytosis is a leading cause of cardiac dysfunction and arrhythmias during acute myocardial ischemia, attenuation of norepinephrine release by H(3)R agonists may offer a novel therapeutic approach to this condition.

Nitric oxide and depolarization induce hydroxyl radical generation

Nitric oxide (NO) contributes to the extracellular potassium-ion concentration ([K+]o)-induced hydroxyl radical (*OH) generation. Cytotoxic free radicals such as peroxinitrite (ONOO-) and *OH may also be implicated in NO-mediated cell injury. NO is synthesized from L-arginine by NO synthase (NOS). NOS activation was induced by K+ depolarization. Oxidative modification of low-density lipoprotein (LDL) is thought to contribute to the production of oxygen derived-free radicals. However, LDL oxidation may be related to noradrenaline-induced *OH generation, but LDL oxidation may be unrelated to *OH generation via NOS activation. Abnormal levels of extracellular free dopamine (DA) and/or intraneuronal Ca2+ triggered by 1-methyl-4-phenylpyridinium ion (MPP+) may be detrimental to the functioning of dopaminergic nerve terminals in the striatum. Although [K+]o-induced depolarization enhances the formation of *OH product due to MPP+, the *OH generation via NOS activation may be unrelated to the DA-induced *OH generation. Depolarization enhances the formation of *OH products via NOS activation.

Aspirin impairs reverse myocardial remodeling in patients with heart failure treated with beta-blockers

OBJECTIVES

We hypothesized that aspirin (ASA) might alter the beneficial effect of beta-blockers on left ventricular ejection fraction (LVEF) in patients with chronic heart failure.

BACKGROUND

Aspirin blunts the vasodilation caused by both angiotensin-converting enzyme (ACE) inhibitors and beta-blockers in hypertensive patients and in patients with heart failure. Several studies suggest that ASA also blunts some of beneficial effects of ACE inhibitors on mortality in patients with heart failure. To our knowledge, there have been no data evaluating the possible interaction of ASA and beta-blockers on left ventricular remodeling in patients with heart failure.

METHODS

We retrospectively evaluated patients entered into the Multicenter Oral Carvedilol Heart failure Assessment (MOCHA) trial, a 6-month, double-blind, randomized, placebo-controlled, multicenter, dose-response evaluation of carvedilol in patients with chronic stable symptomatic heart failure. Multivariate analysis was performed to determine if aspirin independently influenced the improvement in LVEF.

RESULTS

Over all randomized patients (n = 293), LVEF improved 8.2 +/- 0.8 ejection fraction (EF) units in ASA nonusers and 4.5 +/- 0.7 EF units in ASA users (p = 0.005). In subjects randomized to treatment with carvedilol (n = 231), LVEF improved 9.5 +/- 0.9 EF units in ASA nonusers and 5.8 +/- 0.8 EF units in ASA users (p = 0.02). In subjects randomized to treatment with placebo (n = 62), LVEF improved 2.8 +/- 1.2 EF units in ASA nonusers and 0.5 +/- 1.4 EF units in ASA users (p = 0.20). Aspirin did not significantly affect the heart rate or systolic blood pressure response in either the placebo or carvedilol groups. The effect of ASA became more significant on multivariate analysis. The change in LVEF was also influenced by carvedilol dose, etiology of heart failure, baseline heart rate, EF and coumadin use. The detrimental effect of ASA on the improvement in LVEF was dose-related and was present in both placebo and carvedilol groups, although the effect was statistically significant only in the much larger carvedilol group.

CONCLUSIONS

Aspirin significantly affects the changes in LVEF over time in patients with heart failure and systolic dysfunction treated with carvedilol. The specific mechanism(s) underlying this interaction are unknown and further studies are needed to provide additional understanding of the molecular basis of factors influencing reverse remodeling in patients with heart failure.

Negative inotropic effect of bradykinin in porcine isolated atrial trabeculae: role of nitric oxide

OBJECTIVES

To investigate whether bradykinin affects cardiac contractility independently of its effects on coronary flow and noradrenaline release, and whether such inotropic effects, if present, are mediated via nitric oxide (NO).

METHODS

Right atrial trabeculae were obtained from 35 pigs, suspended in organ baths and attached to isometric transducers. Resting tension was set at approximately 750 mg and tissues were paced at 1.5 Hz. Tissue viability was checked by constructing a concentration response curve (CRC) to noradrenaline. Next, CRCs were constructed to bradykinin, either under baseline conditions or after pre-stimulation with the positive inotropic agent forskolin (1 or 10 micromol/l), in the absence or presence of the bradykinin type 2 (B2) receptor antagonist D-Arg [Hyp3-Thi5, d-Tic7, Oic8]-bradykinin (Hoe 140) (1 micromol/l), the NO synthase inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME) (100 micromol/l) and/or the NO scavenger hydroxocobalamin (200 micromol/l).

RESULTS

Bradykinin exerted a negative inotropic effect, both with and without forskolin pre-stimulation, reducing contractility by maximally 22 +/- 3.6% (mean +/- SEM) and 23 +/- 3.6%, respectively (pEC50 8.37 +/- 0.23 and 8.62 +/- 0.22, respectively). L-NAME reduced this effect in pre-stimulated, but not in unstimulated, trabeculae. Hoe 140 and hydroxocobalamin fully blocked the inotropic effect of bradykinin.

CONCLUSIONS

Bradykinin induces a modest negative inotropic effect in porcine atrial trabeculae that is mediated via B2 receptors and NO. The inconsistent results obtained with L-NAME suggest that it depends on NO synthesized de novo and/or NO from storage sites.

Presynaptic regulation of cardiac norepinephrine release in ischemia

In myocardial ischemia presynaptic regulation of norepinephrine release may be altered either by ischemic effects on presynaptic receptor signaling or by ischemia-evoked accumulation of endogenous agonists. Because presynaptic receptors are targets of several drugs. such alterations may have pharmacotherapeutic implications. We investigated the effect of brief ischemic periods on presynaptic regulation of norepinephrine release by alpha2-adrenoceptors, beta2-adrenoceptors, adenosine A1-, angiotensin AT1-, and bradykinin B2-receptors in isolated perfused rat hearts. Exocytotic norepinephrine release was evoked by electrical field stimulation. Paired stimulations were performed to compare the pharmacologic intervention (S2) with the release under baseline conditions (S1), and the effects of receptor agonists and antagonists were compared under nonischemic and stop-flow conditions. In summary. during brief myocardial ischemia, presynaptic modulation of norepinephrine release is differentially regulated. Autoinhibitory alpha2-adrenoceptors lose their activity, whereas stimulatory beta2-adrenoceptors are sensitized. Inhibitory adenosine A1-receptors gain importance during ischemia owing to endogenous adenosine formation. Bradykinin- and angiotensin-mediated stimulation of norepinephrine release is not affected under ischemic conditions.

Effect of long-term hormone replacement therapy on angiotensin-converting enzyme activity and bradykinin in postmenopausal women with essential hypertension and normotensive postmenopausal women

OBJECTIVE

Hormone replacement therapy (HRT) reduces the incidence of cardiovascular disease in postmenopausal women. Three-month short-term HRT in postmenopausal women with essential hypertension increased the plasma concentrations ofbradykinin with decreased serum angiotensin-converting enzyme (ACE) activity, which may be partially responsible for the cardioprotective effects of HRT. The objective was to determine whether 12-month long-term HRT in postmenopausal women with essential hypertension would maintain the decreased ACE activity and increased bradykinin levels and whether long-term HRT would increase the plasma bradykinin concentrations of normotensive postmenopausal women who had shown no significant changes in the 3-month HRT study, despite their decreased serum ACE activity.

DESIGN

Twenty hypertensive and 15 normotensive postmenopausal women were treated with conjugated estrogens (0.625 mg/day) and medroxyprogesterone (2.5 mg/day) for 12 months. Twenty hypertensive and 15 normotensive postmenopausal women were used as controls. The controls were not treated with HRT. Serum ACE activity and plasma bradykinin concentrations were measured at baseline and at 12 months.

RESULTS

Long-term HRT in both hypertensive and normotensive postmenopausal women significantly decreased serum ACE activity from 15.5+/-0.7 IU/L and 16.0+/-0.9 IU/L, respectively, at baseline to 13.3+/-0.5 IU/L and 14.2+/-0.9 IU/L, respectively, 12 months after the start of HRT (p < 0.05 and p < 0.05, respectively). Long-term HRT in both hypertensive and normotensive postmenopausal women also significantly increased plasma bradykinin concentrations from 22.1+/-4.4 pg/mL and 19.2+/-3.0 pg/mL, respectively, at baseline to 86.7+/-21.2 pg/mL and 73.5+/-23.0 pg/mL, respectively, 12 months after the start of HRT (p < 0.01 and p < 0.05, respectively). No significant changes in serum ACE activity or plasma bradykinin concentrations were observed in the control groups.

CONCLUSIONS

Long-term HRT in hypertensive and normotensive postmenopausal women decreases their serum ACE activity and increases their plasma bradykinin concentrations. Thus, maintenance of elevated bradykinin with decreased serum ACE activity by HRT may be useful in reducing the risk of cardiovascular disease in both hypertensive and normotensive postmenopausal women.

Effect of fluvastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, on nitric oxide-induced hydroxyl radical generation in the rat heart

We examined the effect of fluvastatin, a 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, on the production of hydroxyl radical (*OH) generation via nitric oxide synthase (NOS) activation by an in vivo microdialysis technique. The microdialysis probe was implanted in the left ventricular myocardium of anesthetized rats and tissue was perfused with Ringer's solution through the microdialysis probe at a rate of 1 microl/min. Sodium salicylate in Ringer's solution (0.5 nmol/microl/min) was infused directly through a microdialysis probe to detect the generation of *OH. Induction of [K(+)](o) (70 mM) or tyramine (1 mM), significantly increased the formation of *OH trapped as 2,3-dihydroxybenzoic acid (DHBA). The application of N(G)-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor, significantly decreased the K(+) depolarization-induced *OH formation, but the effect of tyramine significantly increased the level of 2,3-DHBA. When fluvastatin (100 microM), an inhibitor of low-density lipoprotein (LDL) oxidation, was administered to L-NAME-pretreated animals, both KCl and tyramine failed to increase the level of 2,3-DHBA formation. The effect of fluvastatin may be unrelated to K(+) depolarization-induced *OH generation. To examine the effect of fluvastatin on ischemic/reperfused rat myocardium, the heart was subjected to myocardial ischemia for 15 min by occlusion of the left anterior descending coronary artery (LAD). When the heart was reperfused, a marked elevation of the level of 2,3-DHBA was observed. However, in the presence of fluvastatin (100 microM), the elevation of 2,3-DHBA was not observed in ischemia/reperfused rat heart. Fluvastatin, orally at a dose of 3 mg/kg/day for 4 weeks, significantly blunted the rise of serum creatine phosphokinase and improved the electrocardiogram 2 h after coronary occlusion. These results suggest that fluvastatin is associated with a cardioprotective effect due to the suppression of noradrenaline-induced *OH generation by inhibiting LDL oxidation in the heart.