Human histamine H2 receptors can initiate cardiac arrhythmias in a transgenic mouse

Novel insights on the biology and immunologic effects of histamine: A road map for allergists and mast cell biologists

Histamine (C5H9N3, molecular weight 111.15 g/mol) is a well-studied endogenous biogenic amine composed of an imidazole ring attached to an ethylamine side chain. It has a limited half-life of a few minutes within tissues and in circulation. Several cell types including mast cells (MCs), basophils, platelets, histaminergic neurons, and enterochromaffin cells produce varying amounts of histamine using histidine decarboxylase. However, only MCs and basophils have complex mechanisms to pack and store histamine in granules along with other mediators using serglycin and its carried glycosaminoglycan side chains. Relatively low granule pH (∼5.5) supports the binding of stored histamine to heparin, whereas exposure to neutral pH after degranulation weakens the binding and histamine becomes liberated. Histamine exerts multifaceted regulatory biofunctions by engaging its 4 types of heptahelical G protein-coupled receptors (H1R-H4R), which have different expression profiles and functions. MCs express H1R, H2R, and H4R, which gives them a dual role in histamine biology as producers and responsive target cells. Histamine plays a role in a variety of physiologic and pathologic processes such as cell proliferation, differentiation, hematopoiesis, vascular permeability, embryogenesis, tissue regeneration, and wound healing. The emergence of histamine receptor-deficient mouse models and the development of multiple histamine receptor agonists and antagonists have helped researchers better understand these physiologic and pathogenic functions of histamine. We review the biology of histamine with a focus on immunologic aspects and the role of histamine in allergy and MC biology.

Stimulation of histamine H1-receptors produces a positive inotropic effect in the human atrium

There is a controversy whether histamine H1-receptor activation raises or lowers or does not affect contractility in the human heart. Therefore, we studied stimulation of H1-receptors in isolated electrically stimulated (one beat per second) human atrial preparations (HAP). For comparison, we measured force of contraction in left atrial preparations (LA) from mice with overexpression of the histamine H1-receptor in the heart (H1-TG). We detected the messenger ribonucleic acid (mRNA) expression of human histamine H1-receptors in HAP. In LA from H1-TG, each cumulatively applied concentration of histamine and a dual H1/H2-agonist called 2-(2-thiazolyl)-ethylamine (ThEA) caused a time-dependent initial negative inotropic effect followed over time by a lasting positive inotropic effect. Both effects were concentration-dependent in LA from H1-TG. After 100 µM cimetidine, 10 µM histamine exercised a positive inotropic effect in HAP that was diminished by 10 µM mepyramine, an H1-receptor antagonist. The concentrations of mepyramine and cimetidine used here are based on the work of others and our own work (e.g., Guo et al. J Cardiovasc Pharmacol. 6:1210-5 1984, Rayo Abella et al. J Pharmacol Exp Ther. 389:174-185 2024). Similarly, we observed that ThEA (10 µM, 30 µM, 100 µM cumulatively applied) induced a concentration- and time-dependent positive inotropic effect in HAP. In HAP, we detected never negative inotropic effects to either histamine or ThEA. The positive inotropic effects to ThEA in HAP were reduced by mepyramine. The positive inotropic effects of ThEA in LA from H1-TG and in HAP were not accompanied by reductions in the time of tension relaxation. We conclude that stimulation of histamine H1-receptors only increases and does not decrease force of contraction in the HAP in our patients.

Cardiovascular toxicity in antitumor therapy: biological and therapeutic insights

The evolution of antitumor therapies has significantly improved cancer prognosis but has concurrently resulted in cardiovascular toxicities. Understanding the biological mechanisms behind these toxicities is crucial for effective management. Immunotherapy-related cardiovascular toxicities are primarily mediated by immune cells and secreted cytokines. Chemotherapy may cause cardiovascular damage through autophagy disruption and mitochondrial dysfunction. Targeted therapies can induce toxicity through endothelin-1 (ET-1) production and cardiac signaling disruption. Radiotherapy may lead to cardiomyopathy and myocardial fibrosis by affecting endothelial cells, triggering inflammatory responses and accelerating atherosclerosis. This review provides insights into these mechanisms and strategies, aiming to enhance the clinical prevention and treatment of cardiovascular toxicities.

Further investigations on the influence of protein phosphatases on the signaling of muscarinic receptors in the atria of mouse hearts

The vagal regulation of cardiac function involves acetylcholine (ACh) receptor activation followed by negative chronotropic and negative as well as positive inotropic effects. The resulting signaling pathways may include Gi/o protein-coupled reduction in adenylyl cyclase (AC) activity, direct Gi/o protein-coupled activation of ACh-activated potassium current (IKACh), inhibition of L-type calcium ion channels, and/or the activation of protein phosphatases. Here, we studied the role of the protein phosphatases 1 (PP1) and 2A (PP2A) for muscarinic receptor signaling in isolated atrial preparations of transgenic mice with cardiomyocyte-specific overexpression of either the catalytic subunit of PP2A (PP2A-TG) or the inhibitor-2 (I2) of PP1 (I2-TG) or in double transgenic mice overexpressing both PP2A and I2 (DT). In mouse left atrial preparations, carbachol (CCh), cumulatively applied (1 nM-10 µM), exerted at low concentrations a negative inotropic effect followed by a positive inotropic effect at higher concentrations. This biphasic effect was noted with CCh alone as well as when CCh was added after β-adrenergic pre-stimulation with isoprenaline (1 µM). Whereas the response to stimulation of β-adrenoceptors or adenosine receptors (used as controls) was changed in PP2A-TG, the response to CCh was unaffected in atrial preparations from all transgenic models studied here. Therefore, the present data tentatively indicate that neither PP2A nor PP1, but possibly other protein phosphatases, is involved in the muscarinic receptor-induced inotropic and chronotropic effects in the mouse heart.

Initial Characterization of a Transgenic Mouse with Overexpression of the Human H1-Histamine Receptor on the Heart

There is a debate on whether H1-histamine receptors can alter contractility in the mammalian heart. We studied here a new transgenic mouse model where we increased genetically the cardiac level of the H1-histamine receptor. We wanted to know if histamine could augment or decrease contractile parameters in mice with cardiac-specific overexpression of human H1-histamine receptors (H1-TG) and compared these findings with those in littermate wild-type mice (WT). In H1-TG mice, we studied the presence of H1-histamine receptors by autoradiography of the atrium and ventricle using [3H]mepyramine. The messenger RNA for human H1-histamine receptors was present in the heart from H1-TG and absent from WT. Using in situ hybridization, we noted mRNA for the human H1-histamine receptor in cardiac cells from H1-TG. We noted that histamine (1 nM-10 µM) in paced (1 Hz) left atrial preparations from H1-TG, exerted at each concentration of histamine initially reduced force of contraction and then raised contractile force. Likewise, in spontaneously beating left atrial preparations from H1-TG, we noted that histamine led to a transient reduction in the spontaneous beating rate followed by an augmentation in the beating rate. The negative inotropic and chronotropic and the positive inotropic effects on histamine in isolated atrial muscle strips from H1-TG were attenuated by the H1-histamine receptor antagonist mepyramine. Histamine failed to exert an increased force or reduce the heartbeat in atrial preparations from WT. We concluded that stimulation of H1-histamine-receptors can decrease and then augment contractile force in the mammalian heart and stimulation of H1-histamine receptors exerts a negative chronotropic effect. SIGNIFICANCE STATEMENT: We made novel transgenic mice with cardiomyocyte-specific high expressional levels of the human H1-histamine receptor to contribute to the clarification of the controversy on whether H1-histamine receptors increase or decrease contractility and beating rate in the mammalian heart. From our data, we conclude that stimulation of H1-histamine receptors first decrease and then raise contractile force in the mammalian heart but exert solely negative chronotropic effects.

Effects of hallucinogenic drugs on the human heart

Hallucinogenic drugs are used because they have effects on the central nervous system. Their hallucinogenic effects probably occur via stimulation of serotonin receptors, namely, 5-HT2A-serotonin receptors in the brain. However, a close study reveals that they also act on the heart, possibly increasing the force of contraction and beating rate and may lead to arrhythmias. Here, we will review the inotropic and chronotropic actions of bufotenin, psilocin, psilocybin, lysergic acid diethylamide (LSD), ergotamine, ergometrine, N,N-dimethyltryptamine, and 5-methoxy-N,N-dimethyltryptamine in the human heart.

Ergometrine stimulates histamine H2 receptors in the isolated human atrium

Ergometrine (6aR,9R)-N-((S)-1-hydroxypropan-2-yl)-7-methyl-4,6,6a,7,8,9-hexa-hydro-indolo-[4,3-fg]chinolin-9-carboxamide or lysergide acid β-ethanolamide or ergonovine) activates several types of serotonin and histamine receptors in the animal heart. We thus examined whether ergometrine can activate human serotonin 5-HT4 receptors (h5-HT4R) and/or human histamine H2 receptors (hH2R) in the heart of transgenic mice and/or in the human isolated atrium. Force of contraction or beating rates were studied in electrically stimulated left atrial or spontaneously beating right atrial preparations or spontaneously beating isolated retrogradely perfused hearts (Langendorff setup) of mice with cardiac specific overexpression of the h5-HT4R (5-HT4-TG) or of mice with cardiac specific overexpression of the hH2R (H2-TG) or in electrically stimulated human right atrial preparations obtained during cardiac surgery. Western blots to assess phospholamban (PLB) phosphorylation on serine 16 were performed. Ergometrine exerted concentration- and time-dependent positive inotropic effects and positive chronotropic effects in atrial preparations starting at 0.3 µM and reaching a plateau at 10 µM in H2-TGs (n = 7). This was accompanied by an increase in PLB phosphorylation at serine 16. Ergometrine up 10 µM failed to increase force of contraction in left atrial preparations from 5-HT4-TGs (n = 5). Ten micrometer ergometrine increased the force of contraction in isolated retrogradely perfused spontaneously beating heart preparations (Langendorff setup) from H2-TG but not 5-HT4-TG. In the presence of the phosphodiesterase inhibitor cilostamide (1 µM), ergometrine at 10 µM exerted positive inotropic effects in isolated electrically stimulated human right atrial preparations, obtained during cardiac surgery, and these effects were eliminated by 10 µM of the H2R antagonist cimetidine but not by 10 µM of the 5-HT4R antagonist tropisetron. Furthermore, ergometrine showed binding to human histamine H2 receptors (at 100 µM and 1 mM) using HEK cells in a recombinant expression system (pKi < 4.5, n = 3). In conclusion, we suggest that ergometrine is an agonist at cardiac human H2Rs.

Temperature alters the inotropic, chronotropic and proarrhythmic effects of histamine in atrial muscle preparations from humans and H2-receptor overexpressing mice

We investigated whether hypothermia and hyperthermia can alter the efficacy and potency of histamine at increasing the force of cardiac contractions in mice that overexpress the human H2 receptor only in their cardiac myocytes (labelled H2-TG). Contractile studies were performed in an organ bath on isolated, electrically driven (1 Hz) left atrial preparations and spontaneously beating right atrial preparations from H2-TG mice and wild-type (WT) littermate control mice. The basal beating rate in the right atrial preparations from H2-TG mice was lowered by hypothermia (23 °C) and elevated by hyperthermia (42 °C). Furthermore, the efficacy of histamine (0.01-100 µM) at exerting positive inotropic effects was more severely attenuated in the left and right H2-TG mouse atria under hypothermia and hyperthermia than under normothermia (37 °C). Similarly, the inotropic response to histamine was attenuated under hypothermia and hyperthermia in isolated electrically stimulated (1 Hz) right atrial preparations obtained from humans undergoing cardiac surgery. The phosphorylation state of phospholamban at serine 16 at 23 °C was inferior to that at 37 °C in left atrial preparations from H2-TG mice in the presence of 10 µM histamine. In contrast, in human atrial preparations, the phosphorylation state of phospholamban at serine 16 in the presence of 100 µM histamine was lower at 42 °C than at 37 °C. Finally, under hyperthermia, we recorded more and longer lasting arrhythmias in right atrial preparations from H2-TG mice than in those from WT mice. We conclude that the inotropic effects of histamine in H2-TG mice and in human atrial preparations, as well as the chronotropic effects of histamine in H2-TG mice, are temperature dependent. Furthermore, we observed that, even without stimulation of the H2 receptors by exogenous agonists, temperature elevation can increase arrhythmias in isolated right atrial preparations from H2-TG mice. We propose that H2 receptors play a role in hyperthermia-induced supraventricular arrhythmias in human patients.

Function and Role of Histamine H1 Receptor in the Mammalian Heart

Histamine can change the force of cardiac contraction and alter the beating rate in mammals, including humans. However, striking species and regional differences have been observed. Depending on the species and the cardiac region (atrium versus ventricle) studied, the contractile, chronotropic, dromotropic, and bathmotropic effects of histamine vary. Histamine is present and is produced in the mammalian heart. Thus, histamine may exert autocrine or paracrine effects in the mammalian heart. Histamine uses at least four heptahelical receptors: H1, H2, H3 and H4. Depending on the species and region studied, cardiomyocytes express only histamine H1 or only histamine H2 receptors or both. These receptors are not necessarily functional concerning contractility. We have considerable knowledge of the cardiac expression and function of histamine H2 receptors. In contrast, we have a poor understanding of the cardiac role of the histamine H1 receptor. Therefore, we address the structure, signal transduction, and expressional regulation of the histamine H1 receptor with an eye on its cardiac role. We point out signal transduction and the role of the histamine H1 receptor in various animal species. This review aims to identify gaps in our knowledge of cardiac histamine H1 receptors. We highlight where the published research shows disagreements and requires a new approach. Moreover, we show that diseases alter the expression and functional effects of histamine H1 receptors in the heart. We found that antidepressive drugs and neuroleptic drugs might act as antagonists of cardiac histamine H1 receptors, and believe that histamine H1 receptors in the heart might be attractive targets for drug therapy. The authors believe that a better understanding of the role of histamine H1 receptors in the human heart might be clinically relevant for improving drug therapy.

The potential mechanism of histamine-inducing cardiopulmonary inflammation and apoptosis in a novel oral model of rat intoxication

Histamine (HIS) is a potent vasodilator that contributes to anaphylactic reactions. Our investigation aims to study the possible toxic impact of repeated oral administration of histamine on the target organs of HIS poisoning (lung & heart) in rats as a model of scombroid poisoning. We used 15 rats that were separated into three groups with 5 rats in each. All rats received the treatments orally for 14 days as follows; (1): distilled water, (2) HIS at a dosage level of 250 mg/kg BWT daily and (3) HIS at a dosage level of 1750 mg/kg BWT weekly. Our results revealed that the consumption of HIS either daily or weekly could cause marked cardiopulmonary toxicity in rats. HIS can trigger inflammatory reactions in the cardiopulmonary tissues and induce oxidative stress damage along with apoptosis of such organs. HIS was markedly increase the MDA levels and decrease the CAT and GSH activity in both lung and heart tissues. The main pathological lesion observed is inflammation which was confirmed by immunohistochemistry and demonstrated strong iNOS and TNF-α protein expressions. Cardiac muscles showed extensive degeneration and necrosis and displayed strong casp-3 protein expression. Additionally, all HIS receiving groups noticed marked elevation of the pulmonary transcription levels of Cox2, TNF-α, and IL1β along with substantial elevation of casp-3 and bax genes and downregulation of Bcl2 gene in the cardiac tissue. We concluded that the oral administration of HIS either daily or weekly can induce cardiopulmonary toxicity via the upregulation of proinflammatory cytokines resulting in ROS overgeneration and inducing both oxidative stress and apoptosis.

Natural Taxanes: From Plant Composition to Human Pharmacology and Toxicity

Biologically active taxanes, present in small- to medium-sized evergreen conifers of various Taxus species, are widely used for their antioxidant, antimicrobial and anti-inflammatory effects, but mostly for their antitumour effects used in the treatment of solid tumours of the breast, ovary, lung, bladder, prostate, oesophagus and melanoma. More of the substances found in Taxus plant extracts have medical potential. Therefore, at the beginning of this review, we describe the methods of isolation, identification and determination of taxanes in different plant parts. One of the most important taxanes is paclitaxel, for which we summarize the pharmacokinetic parameters of its different formulations. We also describe toxicological risks during clinical therapy such as hypersensitivity, neurotoxicity, gastrointestinal, cardiovascular, haematological, skin and renal toxicity and toxicity to the respiratory system. Since the effect of the drug-form PTX is enhanced by various Taxus spp. extracts, we summarize published clinical intoxications and all fatal poisonings for the Taxus baccata plant. This showed that, despite their significant use in anticancer treatment, attention should also be focused on the risk of fatal intoxication due to ingestion of extracts from these plants, which are commonly found in our surroundings.

Myocardial toxicity induced by silica nanoparticles in a transcriptome profile

The deleterious effects of silica nanoparticles (SiNPs) on human health and the ecological system have gradually gained attention owing to their heavy annual output and extensive global flux. The updated epidemiological or experimental investigations have demonstrated the potential myocardial toxicity triggered by SiNPs, but the underlying mechanisms and long-lasting cardiac effects are still poorly understood. Here, a rat model of sub-chronic respiratory exposure to SiNPs was conducted, and the histopathological analysis and ultrastructural investigation of heart tissues were carried out. More importantly, a comprehensive analysis of whole-genome transcription was utilized in rat heart to uncover key biological and cellular mechanisms triggered by SiNPs. The widening of myocardial space and partial fiber rupture were clearly manifested in rat heart after prolonged SiNPs exposure, particularly accompanied by mitochondrial swelling and cristae rupture. With the aid of Affymetrix GeneChips, 3153 differentially expressed genes (DEGs) were identified after SiNPs exposure, including 1916 down- and 1237 up-regulated genes. GO and KEGG analysis illustrated many important biological processes and pathways perturbed by SiNPs, mainly specializing in cellular stress, energy metabolism, actin filament dynamics and immune response. Signal-net analysis revealed that Prkaca (PKA) plays a core role in the cardiac toxification process of prolonged exposure of SiNPs to rats. Furthermore, qRT-PCR verified that PKA-mediated calcium signaling is probably responsible for SiNPs-induced cardiac injury. Conclusively, our study revealed that SiNPs caused myocardial injury, and particularly, provided transcriptomic insight into the role of PKA-calcium signaling triggered by SiNPs, which would facilitate SiNPs-based nanosafety assessment and biomedicine development.

The Roles of Cardiovascular H2-Histamine Receptors Under Normal and Pathophysiological Conditions

This review addresses pharmacological, structural and functional relationships among H₂-histamine receptors and H₁-histamine receptors in the mammalian heart. The role of both receptors in the regulation of force and rhythm, including their electrophysiological effects on the mammalian heart, will then be discussed in context. The potential clinical role of cardiac H₂-histamine-receptors in cardiac diseases will be examined. The use of H₂-histamine receptor agonists to acutely increase the force of contraction will be discussed. Special attention will be paid to the potential role of cardiac H₂-histamine receptors in the genesis of cardiac arrhythmias. Moreover, novel findings on the putative role of H₂-histamine receptor antagonists in treating chronic heart failure in animal models and patients will be reviewed. Some limitations in our biochemical understanding of the cardiac role of H₂-histamine receptors will be discussed. Recommendations for further basic and translational research on cardiac H₂-histamine receptors will be offered. We will speculate whether new knowledge might lead to novel roles of H₂-histamine receptors in cardiac disease and whether cardiomyocyte specific H₂-histamine receptor agonists and antagonists should be developed.

Cardiac Effects of Novel Histamine H2 Receptor Agonists

In an integrative approach, we studied cardiac effects of recently published novel H2 receptor agonists in the heart of mice that overexpress the human H2 receptor (H2-TG mice) and littermate wild type (WT) control mice and in isolated electrically driven muscle preparations from patients undergoing cardiac surgery. Under our experimental conditions, the H2 receptor agonists UR-Po563, UR-MB-158, and UR-MB-159 increased force of contraction in left atrium from H2-TG mice with pEC50 values of 8.27, 9.38, and 8.28, respectively, but not in WT mice. Likewise, UR-Po563, UR-MB-158, and UR-MB-159 increased the beating rate in right atrium from H2-TG mice with pEC50 values of 9.01, 9.24, and 7.91, respectively, but not from WT mice. These effects could be antagonized by famotidine, a H2 receptor antagonist. UR-Po563 (1 µM) increased force of contraction in Langendorff-perfused hearts from H2-TG but not WT mice. Similarly, UR-Po563, UR-MB-158, or UR-MB-159 increased the left ventricular ejection fraction in echocardiography of H2-TG mice. Finally, UR-Po563 increased force of contraction in isolated human right atrial muscle strips. The contractile effects of UR-Po563 in H2-TG mice were accompanied by an increase in the phosphorylation state of phospholamban. In summary, we report here three recently developed agonists functionally stimulating human cardiac H2 receptors in vitro and in vivo. We speculate that these compounds might be of some merit to treat neurologic disorders if their cardiac effects are blocked by concomitantly applied receptor antagonists that cannot pass through the blood-brain barrier or might be useful to treat congestive heart failure in patients. SIGNIFICANCE STATEMENT: Recently, a new generation of histamine H2 receptor (H2R) agonists has been developed as possible treatment option for Alzheimer's disease. Here, possible cardiac (side) effects of these novel H2R agonists have been evaluated.