Therapeutic potential of H(3)-receptor agonists in myocardial infarction

Cardiovascular symptoms in patients with systemic mast cell activation disease

Traditionally, mast cell activation disease (MCAD) has been considered as just one rare (neoplastic) disease, mastocytosis, focused on the mast cell (MC) mediators tryptase and histamine and the suggestive, blatant symptoms of flushing and anaphylaxis. Recently another form of MCAD, the MC activation syndrome, has been recognized featuring inappropriate MC activation with little to no neoplasia and likely much more heterogeneously clonal and far more prevalent than mastocytosis. Increasing expertise and appreciation has been established for the truly very large menagerie of MC mediators and their complex patterns of release, engendering complex, nebulous presentations of chronic and acute illness best characterized as multisystem polymorbidity of generally inflammatory ± allergic theme. We describe the pathogenesis of MCAD with a particular focus on clinical cardiovascular symptoms and the therapeutic options for MC mediator-induced cardiovascular symptoms.

Targeting cardiac mast cells: pharmacological modulation of the local renin-angiotensin system

Enhanced production of angiotensin II and excessive release of norepinephrine in the ischemic heart are major causes of arrhythmias and sudden cardiac death. Mast cell-dependent mechanisms are pivotal in the local formation of angiotensin II and modulation of norepinephrine release in cardiac pathophysiology. Cardiac mast cells increase in number in myocardial ischemia and are located in close proximity to sympathetic neurons expressing angiotensin AT1- and histamine H3-receptors. Once activated, cardiac mast cells release a host of potent pro-inflammatory and pro-fibrotic cytokines, chemokines, preformed mediators (e.g., histamine) and proteases (e.g., renin). In myocardial ischemia, angiotensin II (formed locally from mast cell-derived renin) and histamine (also released from local mast cells) respectively activate AT1- and H3-receptors on sympathetic nerve endings. Stimulation of angiotensin AT1-receptors is arrhythmogenic whereas H3-receptor activation is cardioprotective. It is likely that in ischemia/reperfusion the balance may be tipped toward the deleterious effects of mast cell renin, as demonstrated in mast cell-deficient mice, lacking mast cell renin and histamine in the heart. In these mice, no ventricular fibrillation occurs at reperfusion following ischemia, as opposed to wild-type hearts which all fibrillate. Preventing mast cell degranulation in the heart and inhibiting the activation of a local renin-angiotensin system, hence abolishing its detrimental effects on cardiac rhythmicity, appears to be more significant than the loss of histamine-induced cardioprotection. This suggests that therapeutic targets in the treatment of myocardial ischemia, and potentially congestive heart failure and hypertension, should include prevention of mast cell degranulation, mast cell renin inhibition, local ACE inhibition, ANG II antagonism and H3-receptor activation.

Histamine 3 receptor activation reduces the expression of neuronal angiotensin II type 1 receptors in the heart

In severe myocardial ischemia, histamine 3 (H₃) receptor activation affords cardioprotection by preventing excessive norepinephrine release and arrhythmias; pivotal to this action is the inhibition of neuronal Na⁺/H⁺ exchanger (NHE). Conversely, angiotensin II, formed locally by mast cell-derived renin, stimulates NHE via angiotensin II type 1 (AT₁) receptors, facilitating norepinephrine release and arrhythmias. Thus, ischemic dysfunction may depend on a balance between the NHE-modulating effects of H₃ receptors and AT₁ receptors. The purpose of this investigation was therefore to elucidate the H₃/AT₁ receptor interaction in myocardial ischemia/reperfusion. We found that H₃ receptor blockade with clobenpropit increased norepinephrine overflow and arrhythmias in Langendorff-perfused guinea pig hearts subjected to ischemia/reperfusion. This coincided with increased neuronal AT₁ receptor expression. NHE inhibition with cariporide prevented both increases in norepinephrine release and AT₁ receptor expression. Moreover, norepinephrine release and AT₁ receptor expression were increased by the nitric oxide (NO) synthase inhibitor N(G)-methyl-L-arginine and the protein kinase C activator phorbol myristate acetate. H₃ receptor activation in differentiated sympathetic neuron-like PC12 cells permanently transfected with H₃ receptor cDNA caused a decrease in protein kinase C activity and AT₁ receptor protein abundance. Collectively, our findings suggest that neuronal H₃ receptor activation inhibits NHE by diminishing protein kinase C activity. Reduced NHE activity sequentially causes intracellular acidification, increased NO synthesis, and diminished AT₁ receptor expression. Thus, H₃ receptor-mediated NHE inhibition in ischemia/reperfusion not only opposes the angiotensin II-induced stimulation of NHE in cardiac sympathetic neurons, but also down-regulates AT₁ receptor expression. Cardioprotection ultimately results from the combined attenuation of angiotensin II and norepinephrine effects and alleviation of arrhythmias.

Histamine H3 receptor ligands break ground in a remarkable plethora of therapeutic areas

The neurotransmitter histamine exerts its action through four distinct histamine receptors. The histamine H(1) and H(2) receptor are well established drug targets, whereas the histamine H(4) receptor is undergoing rigorous characterisation at present. The histamine H(3) receptor (H(3)R) is a G(i/o)-protein coupled receptor and is mostly expressed in the CNS. A remarkably large and different array of therapeutic areas, in which ligands for the H(3)R may prove useful, has been identified and a massive research undertaking is underway to substantiate the high expectations for H(3)R ligands. At present, several ligands for the H(3)R are being evaluated in clinical studies. In this review, the many potential therapeutic areas for H(3)R antagonists, inverse agonists and agonists is discussed. Promising medicinal chemistry and toxicological developments, as well as the advancement of several H(3)R ligands into the clinic, will be highlighted. This review also describes the problems that have been overcome and the questions that remain in developing H(3)R-related drugs. Considering the tremendous efforts by industry, it can be expected that the first H(3)R drugs will reach the market soon.

The protective effect of H2-receptor activation against the duration of myocardial hypoxia/reoxygenation-induced ventricular fibrillation in sensitized guinea-pig hearts

Patients with high serum immunoglobulin E levels were reported to be protected against sudden death during acute myocardial infarction. The protection mechanism might be attributed to the facilitation of histamine release from sensitized mast cells; however, this remains to be clarified. In this study, we examined the influence of sensitization on ventricular fibrillation (VF) induced by myocardial hypoxia/reoxygenation (H/R). Guinea pigs were actively sensitized by subcutaneous injection of ovalbumin in Bordetella pertussis vaccine. Hearts isolated from non-sensitized and sensitized guinea pigs were subjected to 30-min hypoxia / 30-min reoxygenation using a Langendorff apparatus. The amount of histamine released in the sensitized guinea-pig hearts was elevated, and the duration of VF was found to be reduced. The treatment with a histamine H2-receptor antagonist inhibited the reduction of VF duration. Treatment of the non-sensitized hearts with the histamine H2-receptor agonist resulted in the decrease of VF duration to the same level as that in the sensitized hearts. In conclusion, these results suggest that the risk of sudden death during myocardial H/R may be attenuated in the sensitized hearts and that histamine H2-receptor activation due to the released histamine may be involved in the protective effect.

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.

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.