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

Paradoxical block of parathormone secretion is mediated by increased activity of G alpha subunits

The paradox of blunted parathormone (PTH) secretion in patients with severe hypomagnesemia has been known for more than 20 years, but the underlying mechanism is not deciphered. We determined the effect of low magnesium on in vitro PTH release and on the signals triggered by activation of the calcium-sensing receptor (CaSR). Analogous to the in vivo situation, PTH release from dispersed parathyroid cells was suppressed under low magnesium. In parallel, the two major signaling pathways responsible for CaSR-triggered block of PTH secretion, the generation of inositol phosphates, and the inhibition of cAMP were enhanced. Desensitization or pertussis toxin-mediated inhibition of CaSR-stimulated signaling suppressed the effect of low magnesium, further confirming that magnesium acts within the axis CaSR-G-protein. However, the magnesium binding site responsible for inhibition of PTH secretion is not identical with the extracellular ion binding site of the CaSR, because the magnesium deficiency-dependent signal enhancement was not altered on CaSR receptor mutants with increased or decreased affinity for calcium and magnesium. By contrast, when the magnesium affinity of the G alpha subunit was decreased, CaSR activation was no longer affected by magnesium. Thus, the paradoxical block of PTH release under magnesium deficiency seems to be mediated through a novel mechanism involving an increase in the activity of G alpha subunits of heterotrimeric G-proteins.

Endotoxin sensitization to kinin B(1) receptor agonist in a non-human primate model: haemodynamic and pro-inflammatory effects

1. Although endotoxaemia induces kinin B(1) receptors in several animal models, this condition is not documented in primates. This study examined the up-regulation of haemodynamic and pro-inflammatory responses to the B(1) agonist des-Arg(10)-kallidin (dKD) in a non-human primate model. 2. Green monkeys (Cercopithecus aethiops St Kitts) received lipopolysaccharide (LPS; 90 microg kg(-1)) or saline intravenously. After 4 h, anaesthetized monkeys were cannulated via the carotid artery to monitor blood pressure changes following intra-arterial injections of dKD or the B(2) agonist bradykinin (BK). Oedema induced by subcutaneous kinin administration was evaluated as the increase in ventral skin folds in anaesthetized monkeys injected with captopril at 4 h to 56 days post-LPS. 3. LPS increased rectal temperature but did not affect blood pressure after 4 h. dKD reduced blood pressure (E(max): 27+/-4 mmHg; EC(50): 130 pmol kg(-1)) and increased heart rate (E(max): 33 b.p.m.) only after LPS. In contrast, the dose-dependent fall in blood pressure with BK was comparable in all groups. The selective B(1) antagonist [Leu(9)]dKD (75 ng kg(-1) min(-1), intravenously) abolished responses to dKD but not BK. 4. dKD injection induced oedema dose-dependently (2.4+/-0.1 mm at 150 nmol) only following LPS (at 4 h to 12 days but not 56 days). In contrast, BK-induced oedema was present and stable in all monkeys. Co-administration of [Leu(9)]dKD (150 nmol) significantly reduced oedema induced by dKD (50 nmol). 5. These results suggest LPS up-regulation of B(1) receptor effects in green monkeys. This non-human primate model may be suitable for testing new, selective B(1) antagonists with therapeutic potential as anti-inflammatory agents.

New drugs in the treatment of heart failure

New therapeutic strategies as well as the development of drugs with more specific targets have been fueled by disappointments in the treatment of adult heart failure. Calcium sensitizers, vesnarinone and angiotensin channel blockers will be addressed in this manuscript. The physiologic and pharmacologic principles that justify their use in the management of heart failure are reviewed. Calcium sensitizers increase myocardial contractility and in part they bypass the adenylyl cyclase cascade, which gives them a more favorable energy profile. Vesnarinone is a quinolinone derivative with ion channel modulation properties, which result in a positive inotropic effect and prolongation of the action potential. In addition vesnarinone has immunomodulatory properties. Angiotensin-converting enzyme inhibitors are the cornerstones for the treatment of heart failure. The discovery of some putative drawbacks to ACE inhibition has challenged this supremacy. Angiotensin receptor blockers have been developed hoping to overcome these deficiencies. Myocardial developmental differences highlight the shortcomings of attempting to extrapolate data on drugs and cellular physiology in adults to children. Studies are needed addressing standards of care, quality of life, morbidity and mortality, neurohumoral activation, its modulation and the consequences of these therapies in pediatric heart failure.

Possible interaction between aspirin and ACE inhibitors: update on unresolved controversy

The widespread use of aspirin and angiotensin converting enzyme (ACE) inhibitors in patients with coronary artery disease contributes significantly to the reduction in morbidity and mortality from this common health problem. These agents are widely and concomitantly used, and they share mechanisms that may interact in negative or positive pathways. Data derived from in vitro preparations, animal studies, human studies, and case-control studies are inconsistent. No study has established firm evidence regarding the safety or adverse effect of aspirin on patients who are on ACE inhibitors. The efficacy and safety of aspirin in combination with ACE inhibitors has been questioned and debated. If a negative interaction does exist, it will affect daily practice in treating patients with coronary artery disease and heart failure. This article reviews the available data regarding the safety of combined aspirin and ACE-inhibitor treatment among patients with ischemic heart disease, to assess the possible interaction between the two drugs and to discuss the significance and implications of the data. (c)2000 by CHF, Inc.

Modulation of (3)H-noradrenaline release by presynaptic opioid, cannabinoid and bradykinin receptors and beta-adrenoceptors in mouse tissues

Release-modulating opioid and cannabinoid (CB) receptors, beta-adrenoceptors and bradykinin receptors at noradrenergic axons were studied in mouse tissues (occipito-parietal cortex, heart atria, vas deferens and spleen) preincubated with (3)H-noradrenaline. Experiments using the OP(1) receptor-selective agonists DPDPE and DSLET, the OP(2)-selective agonists U50488H and U69593, the OP(3)-selective agonist DAMGO, the ORL(1) receptor-selective agonist nociceptin, and a number of selective antagonists showed that the noradrenergic axons innervating the occipito-parietal cortex possess release-inhibiting OP(3) and ORL(1) receptors, those innervating atria OP(1), ORL(1) and possibly OP(3) receptors, and those innervating the vas deferens all four opioid receptor types. Experiments using the non-selective CB agonists WIN 55,212-2 and CP 55,940 and the CB(1)-selective antagonist SR 141716A indicated that the noradrenergic axons of the vas deferens possess release-inhibiting CB(1) receptors. Presynaptic CB receptors were not found in the occipito-parietal cortex, in atria or in the spleen. Experiments using the non-selective beta-adrenoceptor agonist isoprenaline and the beta(2)-selective agonist salbutamol, as well as subtype-selective antagonists, demonstrated the occurrence of release-enhancing beta(2)-adrenoceptors at the sympathetic axons of atria and the spleen, but demonstrated their absence in the occipito-parietal cortex and the vas deferens. Experiments with bradykinin and the B(2)-selective antagonist Hoe 140 showed the operation of release-enhancing B(2) receptors at the sympathetic axons of atria, the vas deferens and the spleen, but showed their absence in the occipito-parietal cortex. The experiments document a number of new presynaptic receptor locations. They confirm and extend the existence of marked tissue and species differences in presynaptic receptors at noradrenergic neurons.

Enhancement of noradrenaline release by angiotensin II and bradykinin in mouse atria: evidence for cross-talk between G(q/11) protein- and G(i/o) protein-coupled receptors

1. The interaction between alpha(2)-autoreceptors and receptors for angiotensin (AT(1)) and bradykinin (B(2)) was studied in mouse isolated atria. The preparations were labelled with [(3)H]-noradrenaline and then superfused with desipramine-containing medium and stimulated electrically. 2. Angiotensin II (10(-11) - 10(-7) M), angiotensin III (10(-10) - 10(-6) M) and bradykinin (10(-11) - 10(-7) M) enhanced the evoked overflow of tritium when preparations were stimulated with conditions that led to marked alpha(2)-autoinhibition (120 pulses at 3 Hz), but not when stimulated with conditions that led to little alpha(2)-autoinhibition (20 pulses at 50 Hz). 3. Blockade of alpha-adrenoceptors by phentolamine (1 or 10 microM) reduced or abolished the effect of angiotensin II and bradykinin on the overflow response to 120 pulses at 3 Hz. 4. Addition of the delta-opioid agonist [D-Ser(2)]-leucine enkephalin-Thr (DSLET, 0.1 microM), or of neuropeptide Y (0.1 microM), together with phentolamine, restored the effect of angiotensin II and bradykinin. 5. The beta-adrenoceptor agonist terbutaline (10(-9) - 10(-4) M) enhanced the evoked overflow of tritium irrespective of the degree of autoinhibition. 6. The experiments show that (i) a marked prejunctional facilitatory effect of angiotensin and bradykinin in mouse isolated atria requires prejunctional alpha(2)-autoinhibition; (ii) in the absence of alpha(2)-autoinhibition, activation of other prejunctional G(i/o) protein-coupled receptors, namely opioid and neuropeptide Y receptors, restores a marked effect of angiotensin II and bradykinin; and (iii) the facilitatory effect of terbutaline is not dependent upon the degree of alpha(2)-autoinhibition. The findings indicate that the major part of the release-enhancing effect elicited through prejunctional G(q/11) protein-coupled receptors is due to disruption of an ongoing, alpha(2)-autoreceptor-triggered G(i/o) protein mediated inhibition.

Angiotensin II receptor blockers in chronic heart failure--not as ELITE as expected!

As with many large-scale long-term outcome trials, more questions have been posed than answered regarding the potential role of angiotensin II receptor blockers as first-line agents in chronic heart failure. Given the present data, in patients with left ventricular systolic dysfunction, ACE inhibitors must remain the treatment of choice, owing to the large body of data supporting their use in this clinical syndrome. However, ARBs seems a reasonable alternative for renin-angiotensin axis blockade in the significant number of heart failure patients who are genuinely intolerant of ACE inhibitors. The pendulum has now swung back in favour of ACE inhibition for chronic heart failure, although one can only await with great expectation the results of the ongoing trials comparing not only angiotensin II receptor blockers with ACE inhibitors but a combination of the two with regards tolerability and survival. Whether this potentially useful class of drugs will ultimately become the cornerstone of heart failure therapy in place of, or in addition to, ACE inhibitors is still in debate, but hopefully we should not have to wait too long for the definitive answers.

Altered baroreflex control of heart rate in bradykinin B2-receptor knockout mice

Recently, we have shown that a knockout mouse strain lacking the bradykinin B2-receptor gene exhibits an accelerated heart rate (HR) under basal conditions, this alteration being associated with mildly elevated blood pressure (BP) levels and ultimately with the development of cardiomyopathy. The goal of the present study was to determine whether genetic disruption of the B2-receptor alters autonomic cardiovascular reflexes to acute or chronic changes in BP. The direct mean BP and HR levels of unrestrained B2 knockout mice (B2-/-) were higher than those of wild type (B2+/+) controls (131 +/- 2 vs. 105 +/- 2 mm Hg and 480 +/- 5 vs. 414 +/- 8 beats/min, P < 0.01 for both comparisons). The difference in HR observed between groups under basal conditions was nullified by the acute administration of propranolol and atropine as well as by hexamethonium; it was attenuated by long-term blockade of angiotensin AT1 receptors. In B2-/- mice, the presence of an alteration in baroreceptor regulation of HR was supported by a reduced gain in the HR responses to acute nitroprusside-induced hypotension or phenylephrine-induced hypertension (slope of the regression line: 0.82 +/- 0.07 vs. 5.58 +/- 0.08 beats/min per mmHg in B2+/+, P < 0.01), as well as by an exaggerated tachycardic response to chronic hypertension induced by clipping of the left renal artery (60 +/- 3 vs. 15 +/- 3 beats/min in B2+/+, P < 0.01). Our findings indicate that disruption of the bradykinin B2-receptor gene is associated with an impaired baroreflex control of HR. The combination of chronically elevated resting HR and impaired baroreflex control could contribute to the development of cardiomyopathy in these animals.

Norepinephrine release and ventricular fibrillation in myocardial ischemia/reperfusion: roles of angiotensin and bradykinin

Exogenous bradykinin (BK), acting at B2-receptors, enhances norepinephrine (NE) release and exacerbates arrhythmias (VF) in myocardial ischemia/reperfusion. Inhibition of BK formation (with serine proteinase inhibitors) alleviates NE release and VF, whereas prevention of BK degradation (with kininase inhibitors) potentiates them. Yet serine proteinase and kininase inhibitors also prevent the formation of angiotensin (AII), a potent NE-release enhancer. Thus we assessed the respective contribution of AII and BK to NE release and VF by using selective B2- and AT1-receptor antagonists. Isolated guinea pig hearts were subjected to 10- and 20-min global ischemia and 45-min reperfusion. NE overflow (pmol/g) was approximately 8 (exocytotic) and approximately 750 (carrier mediated). VF, associated with carrier-mediated NE release, lasted approximately 2 min. The B2-receptor antagonist Hoe 140 (30 nM) affected neither NE overflow nor VF. In contrast, the AT1-receptor antagonist EXP3174 (100 nM) markedly reduced exocytotic and carrier-mediated NE release and shortened VF. When EXP3174 was combined with Hoe 140, NE overflow and VF were decreased even further. Thus in myocardial ischemia, local AII production contributes to NE release and VF via AT1-receptors. Although BK production increases in myocardial ischemia, the effects of BK on adrenergic nerve terminals are uncovered only when BK half-life is prolonged and/or when AII effects are suppressed.

Protective effect of imidaprilat, an angiotensin-converting enzyme inhibitor on *OH generation in rat myocardium

We used a flexibly mounted microdialysis technique to the hearts of rats and examined the protective effect of imidaprilat, an angiotensin-converting enzyme (ACE) inhibitor, on the production of hydroxyl free radical (*OH) generation. A microdialysis probe was implanted into the left ventricular myocardium, and dialysate norepinephrine (NE) concentrations were measured as an index of myocardial interstitial NE levels. Sodium salicylate in Ringer's solution (0.5 nmol/microl/min) was directly infused through a microdialysis probe to detect the generation of *OH reflected by the formation of dihydroxybenzoic acid (DHBA) in rat myocardium. When tyramine (1 mM) was directly infused through the microdialysis probe, the level of NE significantly increased in the dialysate and the level of NE increased by 128 +/- 43%. Imidaprilat (5, 25 and 50 microM) decreased the level of tyramine (1 mM)-induced NE in a concentration-dependent manner. Tyramine clearly produced an increase in *OH formation. In the presence of imidaprilat (50 microM), tyramine failed to increase both 2,3- and 2,5-dihydroxylation. Therefore, the effects of imidaprilat on the *OH generation in the sympathetic nerve blockaded hearts by reserpine treatment were not observed. Moreover, to examine the effect of imidaprilat on *OH formation by ischemia/reperfusion of the myocardium, the heart was subjected to myocardial ischemia for 15 min by occlusion of the left anterior descending coronary artery. When the heart was reperfused, elevation of NE and 2,3- and 2,5-DHBA in imidaprilat (50 microM)-pretreated animals was not observed in the heart dialysate. Imidaprilat 2.5 mg/kg i.p. pretreatment at 5 h before coronary occlusion significantly blunted the rise of serum creatine phosphokinase and improved the electrocardiogram 2 h after coronary occlusion. These results suggest that imidaprilat, an ACE inhibitor, is associated with cardioprotective effect due to the suppression of NE-induced *OH generation.

Crosstalk between Galpha(i)- and Galpha(q)-coupled receptors is mediated by Gbetagamma exchange

Activation of Galpha(i)-coupled receptors often causes enhancement of the inositol phosphate signal triggered by Galpha(q)-coupled receptors. To investigate the mechanism of this synergistic receptor crosstalk, we studied the Galpha(i)-coupled adenosine A(1) and alpha(2C) adrenergic receptors and the Galpha(q)-coupled bradykinin B(2) and a UTP-preferring P2Y receptor. Stimulation of either Galpha(i)-coupled receptor expressed in COS cells increased the potency and the efficacy of inositol phosphate production by bradykinin or UTP. Likewise, overexpression of Gbeta(1)gamma(2) resulted in a similar increase in potency and efficacy of bradykinin or UTP. In contrast, these stimuli did not affect the potency of direct activators of Galpha(q); a truncated Gbeta(3) mutant had no effect on the receptor-generated signals whereas signals generated at the G-protein level were still enhanced. This suggests that the Gbetagamma-mediated signal enhancement occurs at the receptor level. Almost all possible combinations of Gbeta(1-3) with Ggamma(2-7) were equally effective in enhancing the signals of the B(2) and a UTP-preferring P2Y receptor, indicating a very broad specificity of this synergism. The enhancement of the bradykinin signal by (i) Galpha(i)-activating receptor ligands or (ii) cotransfection of Gbetagamma was suppressed when the B(2) receptor was replaced by a B(2)Gbeta(2) fusion protein. Gbetagamma enhanced the B(2) receptor-stimulated activation of G-proteins as determined by GTPgammaS-induced decrease in high affinity agonist binding and by B(2) receptor-enhanced [(35)S]GTPgammaS binding. These findings support the concept that Gbetagamma exchange between Galpha(i)- and Galpha(q)-coupled receptors mediates this type of receptor crosstalk.

Protective effect of imidaprilat, a new angiotensin-converting enzyme inhibitor against 1-methyl-4-phenylpyridinium ion-induced *OH generation in rat striatum

We examined the antioxidant effects of angiotensin-converting enzyme inhibitor on 1-methyl-4-phenylpyridinium ion (MPP+)-induced hydroxyl radical (*OH) formation in extracellular fluid of rat striatum. Rats were anesthetized and sodium salicylate in Ringer's solution (0.5 nmol microl(-1) min(-1) was infused through a microdialysis probe to detect the generation of *OH, as reflected by the non-enzymatic formation of 2,3-dihydroxybenzoic acid (DHBA) in the striatum. MPP+ clearly produced an increase in *OH formation in a concentration-dependent manner. When imidaprilat was infused in MPP+ -pre-treated animals, the formation of dopamine and 2,3-DHBA significantly decreased, as compared with that in the MPP+ -only-treated group. We compared the ability of two non-SH-containing angiotensin-converting enzyme inhibitors (imidaprilat and enalaprilat) with an SH-containing angiotensin-converting enzyme inhibitor (captopril) to scavenge *OH. All three angiotensin-converting enzyme inhibitors were able to scavenge *OH generated by the action of MPP+. However, the changes produced by captopril and enalaprilat were not significant. When dopamine was administered to the MPP+ -pre-treatment group, a marked elevation was observed, showing a positive linear correlation between dopamine and *OH formation (2,3-DHBA) in the dialysate. Moreover, when iron (II) was administered to the MPP+ -pre-treatment group, the same results were obtained: a positive linear correlation (R2 = 0.989) between the release of dopamine and 2,3-DHBA (R2 = 0.989) in the dialysate. When corresponding experiments were performed with imidaprilat-pre-treated animals, the level of 2,3-DHBA decreased. These results suggested that angiotensin-converting enzyme inhibitors may protect against MPP+ -induced *OH formation in the rat striatum.

Endogenous angiotensin II and bradykinin delay and attenuate the hypotension after N-type calcium channel blockade in conscious rabbits

The effects of N-type calcium channel inhibition with omega-conotoxin GVIA (omega-CTX) on cardiovascular parameters and vagally mediated autonomic reflexes and the role of the renin-angiotensin system were assessed in conscious rabbits. Omega-CTX (10 microg/kg, i.v.) resulted in hypotension, tachycardia, and attenuation of the sympathetic and vagal components of the baroreceptor-heart rate reflex (baroreflex). In the control group (no pretreatment), the peak decrease in mean arterial pressure (MAP) of 13 +/- 3 mm Hg from 72 +/- 2 mm Hg occurred after 33 +/- 3 min, with a corresponding tachycardia of 80 +/- 20 beats/min (n = 6). The tachycardia was due to vagal withdrawal, as a similar increase in heart rate (84 +/- 8 beats/min) after omega-CTX was observed after pretreatment with the beta-adrenoceptor antagonist, propranolol (n = 6). Angiotensin-converting enzyme (ACE) inhibition with enalaprilat revealed a larger, more rapid decrease in MAP in response to omega-CTX (-19 +/- 4 mm Hg from 65 +/- 1 mm Hg after 18 +/- 2 min; n = 6) compared with the control group. Similar larger decreases in MAP were also observed in the presence of the AT1-receptor antagonist, losartan, or the bradykinin B2 receptor antagonist, HOE-140 (n = 5-6). Pretreatment with enalaprilat, losartan, or HOE-140 caused a 50% decrease in the reflex tachycardia after omega-CTX compared with that observed in the control group, and omega-CTX caused a greater attenuation of the vagal component of the baroreflex and a decrease in the bradycardia evoked by the Bezold-Jarisch-like reflex. Also, there was a significant decrease in the bradycardia induced by the nasopharyngeal reflex after omega-CTX in the presence of ACE inhibition and HOE-140. Thus in the conscious rabbit, angiotensin II and bradykinin have a role in attenuating and slowing the hypotensive effect of N-type calcium channel inhibition. Vagolytic effects of omega-CTX on the baroreflex are augmented, and on other vagal reflexes are unmasked, via inhibition of the renin-angiotensin system. The complexity and mechanism of the interaction between N-type calcium channels and the renin-angiotensin system remain to be elucidated.

High antagonist potency of GT-2227 and GT-2331, new histamine H3 receptor antagonists, in two functional models

GT-2227 (4-(6-cyclohexylhex-cis-3-enyl)imidazole) and GT-2331 ((1R,2R)-4-(2-(5,5-dimethylhex-1-ynyl)cyclopropyl)imidazole) were developed as new potent histamine H3 receptor antagonists. The functional activity of these ligands on the histamine H3 receptor-mediated inhibition of neurogenic contraction of the guinea-pig jejunum and histamine H3 receptor-mediated inhibition of norepinephrine release from guinea-pig heart synaptosomes were investigated. GT-2227 and GT-2331 both antagonized the inhibitory effects of (R)-alpha-methylhistamine on the contraction induced by electrical field stimulation in the guinea-pig jejunum with pA2 values of 7.9+/-0.1 and 8.5+/-0.03, respectively. In addition, GT-2227 and GT-2331 antagonized the inhibition of norepinephrine release in cardiac synaptosomes by GT-2203 ((1R,2R)-trans-2-(1H-imidazol-4-yl)cyclopropylamine), a histamine H3 receptor agonist. The current results demonstrate the antagonist activity for both GT-2227 and GT-2331 in two functional assays for histamine H3 receptors.

New acetylene based histamine H3 receptor antagonists derived from the marine natural product verongamine

New histamine H3 receptor antagonists were developed using an acetylene moiety as a replacement for the amide-oxime functionality of verongamine 5. Optimization of receptor binding was performed by following aliphatic Topliss tree guidelines. These new H3 ligands demonstrate excellent blood-brain barrier penetration.