Functional interaction of H2-receptors and 5HT4-receptors in atrial tissues isolated from double transgenic mice and from human patients

Clebopride stimulates 5-HT4-serotonin receptors in the human atrium

Clebopride resembles in its structural formula metoclopramide. Clebopride, an approved drug, is used to treat gastrointestinal diseases. Here, we tested the hypothesis that clebopride like metoclopramide acts as a partial agonist at human cardiac 5-HT4-serotonin-receptors. Clebopride enhanced the force of contraction (FOC) in isolated, electrically stimulated (1 Hz) left atrial preparations (LA) from transgenic mice with cardiac specific overexpression of the human 5-HT4-serotonin receptors (5-HT4-TG). Subsequently applied GR125487 (1 µM), a specific 5-HT4-serotonin-receptor antagonist, diminished this positive inotropic effect (PIE) of clebopride in LA from 5-HT4-TG. Clebopride failed to heighten FOC in LA from littermate wild-type mouse hearts (WT). Clebopride augmented the beating rate in isolated right atrial preparations (RA) from 5-HT4-TG but unable to do so in RA from WT. Clebopride alone (up to 10 µM) failed to augment FOC in isolated electrically stimulated (1Hz) human right atrial preparations (HAP) obtained during open heart surgery from adult patients with severe coronary heart disease. Interestingly, in the presence of the phosphodiesterase III inhibitor cilostamide, clebopride heightened FOC in HAP. GR125487 attenuated this PIE in HAP. Furthermore, when 1 µM serotonin had raised FOC in HAP, additionally applied 10 µM clebopride diminished FOC in HAP. We conclude that clebopride can act as an agonist and as an antagonist at 5-HT4-serotonin receptors in the human atrium.

Bromopride stimulates 5-HT4-serotonin receptors in the human atrium

Bromopride, an analogue of metoclopramide, is approved in some countries to treat gastrointestinal diseases. These therapeutic effects of bromopride are explained by antagonism at D2-dopamine receptors in the gut and the brain. We tested here the hypothesis that bromopride acts as an agonist or antagonist at the human cardiac 5-HT4-serotonin receptors. To this end, the force of contraction (FOC) was measured in isolated human atrial preparations (HAP), in isolated left atrial preparations (LA), and in isolated spontaneously beating right atrial (RA) preparations from mice with cardiac specific overexpression of the human 5-HT4-serotonin receptors (5-HT4-TG). Bromopride concentration dependently increased FOC in LA from 5-HT4-TG. The positive inotropic effect (PIE) of bromopride in LA from 5-HT4-TG was abolished by GR125487, a 5-HT4-serotonin receptor antagonist. Only in the presence of the phosphodiesterase III inhibitor cilostamide did bromopride raise FOC under isometric conditions in HAP. The PIE of 10 µM bromopride in HAP was extinguished by 1 µM GR125487. When serotonin had elevated FOC in HAP, additionally applied bromopride reduced FOC. These data suggest that bromopride is a partial agonist at human cardiac 5-HT4-serotonin receptors.

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.

Mosapride stimulates human 5-HT4-serotonin receptors in the heart

Mosapride (4-amino-5-chloro-2-ethoxy-N-[[4-[(4-fluorophenyl) methyl]-2-morpholinyl]-methyl] benzamide) is a potent agonist at gastrointestinal 5-HT4 receptors. Mosapride is an approved drug to treat several gastric diseases. We tested the hypothesis that mosapride also stimulates 5-HT4 receptors in the heart. Mosapride increased the force of contraction and beating rate in isolated atrial preparations from mice with cardiac overexpression of human 5-HT4-serotonin receptors (5-HT4-TG). However, it is inactive in wild-type mouse hearts (WT). Mosapride was less effective and potent than serotonin in raising the force of contraction or the beating rate in 5-HT4-TG. Only in the presence of cilostamide (1 μM), a phosphodiesterase III inhibitor, mosapride, and its primary metabolite time dependently raised the force of contraction under isometric conditions in isolated paced human right atrial preparations (HAP, obtained during open heart surgery). In HAP, mosapride (10 μM) reduced serotonin-induced increases in the force of contraction. Mosapride (10 µM) shifted the concentration-response curves to serotonin in HAP to the right. These data suggest that mosapride is a partial agonist at 5-HT4-serotonin receptors in HAP.

Contractile Effects of Semaglutide in the Human Atrium

Semaglutide is a glucagon-like peptide 1 receptor (GLP-1R) agonist. GLP-1R agonists are used to treat type 2 diabetes and obesity. It is currently unknown whether semaglutide can directly increase force of contraction (FOC) in the human heart. We tested the hypothesis that semaglutide might increase the FOC in the isolated human atrium. To this end, we conducted contraction experiments in isolated human right atrial muscle preparations (HAP). HAP were obtained during open-heart surgery. We detected a concentration- and time-dependent positive inotropic effect (PIE) of semaglutide in HAP. These PIEs were accompanied by increases in the rates of tension development and tension relaxation and a reduction in muscle relaxation time. The PIE of semaglutide in HAP was attenuated by H89, an inhibitor of the cyclic AMP-dependent protein kinase and by ryanodine, an inhibitor of sarcoplasmic Ca2+ release. Semaglutide up to 100 nM failed to exert a PIE in isolated electrically paced (1 Hz) wild-type mouse left atrial preparations studied for comparison. Our data suggest that semaglutide can increase the FOC in the atria of patients at therapeutic drug concentrations.

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.

Serotonin Receptors in Myocardial Infarction: Friend or Foe?

Acute myocardial infarction (AMI) is one of the leading causes of death worldwide and treatment costs pose a major burden on the global health care system. Despite the variety of treatment options, individual recovery can be still poor and the mortality rate, especially in the first few years after the event, remains high. Therefore, intense research is currently focused on identifying novel target molecules to improve the outcome following AMI. One of the potentially interesting targets is the serotonergic system (5-HT system), not at least because of its connection to mental disorders. It is known that patients suffering from AMI have an increased risk of developing depression and vice versa. This implicates that the 5-HT system can be affected in response to AMI and might thus represent a target structure for patients' treatment. This review aims to highlight the importance of the 5-HT system after AMI by describing the role of individual serotonin receptors (5-HTR) in the regulation of physiological and pathophysiological responses. It particularly focuses on the signaling pathways of the serotonin receptors 1, 2, 4, and 7, which are expressed in the cardiovascular system, during disease onset, and the following remodeling process. This overview also emphasizes the importance of the 5-HT system in AMI etiology and highlights 5-HTRs as potential treatment targets.

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.

Clonidine stimulates force of contraction via histamine H2 receptors in the human atrium

Clonidine has various clinical effects mediated by agonism of α1- or α2-adrenoceptors and the blocking of hyperpolarization-activated-nucleotide-gated pacemaker channels (HCN). It is unknown whether clonidine can also stimulate human cardiac histamine H2 receptors (hH2Rs). We used isolated electrically stimulated left and spontaneously beating right atrial preparations from mice overexpressing the hH2R specifically in the heart (H2-TG), and spontaneously beating right atrial preparations of guinea pigs for comparison. Moreover, we studied isolated electrically stimulated muscle strips from the human right atrium. Clonidine (1, 3, and 10 µM) increased force of contraction in isolated left atrial preparations from H2-TG mice. In contrast, clonidine reduced the spontaneous beating rate in right atrial preparations from H2-TG. Clonidine raised the beating rate in guinea pig right atrial preparations. Clonidine failed to increase the force of contraction but reduced beating rate in wild-type litter mate mice (WT). In WT, histamine failed to increase the force of contraction in left atrial preparations and beating rate in right atrial preparations. Clonidine (10 µM) increased the force of contraction in isolated human right atrial preparations. The positive inotropic effect in the human atrium was attenuated by cimetidine (10 µM). Clonidine increased the beating rate of the isolated spontaneously beating guinea pig right atrium and acted as a H2R partial agonist. Furthermore, clonidine showed binding to the guinea pig H2R (100 µM) using HEK cells in a recombinant expression system (pKi < 4.5) but hardly to the human H2R. These data suggest that clonidine can functionally activate cardiac human H2R.

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.

Cardiac Roles of Serotonin (5-HT) and 5-HT-Receptors in Health and Disease

Serotonin acts solely via 5-HT4-receptors to control human cardiac contractile function. The effects of serotonin via 5-HT4-receptors lead to positive inotropic and chronotropic effects, as well as arrhythmias, in the human heart. In addition, 5-HT4-receptors may play a role in sepsis, ischaemia, and reperfusion. These presumptive effects of 5-HT4-receptors are the focus of the present review. We also discuss the formation and inactivation of serotonin in the body, namely, in the heart. We identify cardiovascular diseases where serotonin might play a causative or additional role. We address the mechanisms which 5-HT4-receptors can use for cardiac signal transduction and their possible roles in cardiac diseases. We define areas where further research in this regard should be directed in the future, and identify animal models that might be generated to this end. Finally, we discuss in what regard 5-HT4-receptor agonists or antagonists might be useful drugs that could enter clinical practice. Serotonin has been the target of many studies for decades; thus, we found it timely to summarise our current knowledge here.

Ergotamine Stimulates Human 5-HT4-Serotonin Receptors and Human H2-Histamine Receptors in the Heart

Ergotamine (2'-methyl-5'α-benzyl-12'-hydroxy-3',6',18-trioxoergotaman) is a tryptamine-related alkaloid from the fungus Claviceps purpurea. Ergotamine is used to treat migraine. Ergotamine can bind to and activate several types of 5-HT₁-serotonin receptors. Based on the structural formula of ergotamine, we hypothesized that ergotamine might stimulate 5-HT₄-serotonin receptors or H₂-histamine receptors in the human heart. We observed that ergotamine exerted concentration- and time-dependent positive inotropic effects in isolated left atrial preparations in H₂-TG (mouse which exhibits cardiac-specific overexpression of the human H₂-histamine receptor). Similarly, ergotamine increased force of contraction in left atrial preparations from 5-HT₄-TG (mouse which exhibits cardiac-specific overexpression of the human 5-HT₄-serotonin receptor). An amount of 10 µM ergotamine increased the left ventricular force of contraction in isolated retrogradely perfused spontaneously beating heart preparations of both 5-HT₄-TG and H₂-TG. In the presence of the phosphodiesterase inhibitor cilostamide (1 µM), ergotamine 10 µM exerted positive inotropic effects in isolated electrically stimulated human right atrial preparations, obtained during cardiac surgery, that were attenuated by 10 µM of the H₂-histamine receptor antagonist cimetidine, but not by 10 µM of the 5-HT₄-serotonin receptor antagonist tropisetron. These data suggest that ergotamine is in principle an agonist at human 5-HT₄-serotonin receptors as well at human H₂-histamine receptors. Ergotamine acts as an agonist on H₂-histamine receptors in the human atrium.

Cardiovascular effects of bufotenin on human 5-HT4 serotonin receptors in cardiac preparations of transgenic mice and in human atrial preparations

It is unclear whether bufotenin (= N,N-dimethyl-serotonin = 5-hydroxy-N,N-dimethyl-tryptamine), a hallucinogenic drug, can act on human cardiac serotonin 5-HT₄ receptors. Therefore, the aim of the study was to examine the cardiac effects of bufotenin and for comparison tryptamine in transgenic mice that only express the human 5-HT₄ receptor in cardiomyocytes (5-HT₄-TG), in their wild-type littermates (WT) and in isolated electrically driven (1 Hz) human atrial preparations. In 5-HT₄-TG, we found that both bufotenin and tryptamine enhanced the force of contraction in left atrial preparations (pD2 = 6.77 or 5.5, respectively) and the beating rate in spontaneously beating right atrial preparations (pD2 = 7.04 or 5.86, respectively). Bufotenin (1 µM) increased left ventricular force of contraction and beating rate in Langendorff perfused hearts from 5-HT₄-TG, whereas it was inactive in hearts from WT animals, as was tryptamine. The positive inotropic and chronotropic effects of bufotenin and tryptamine were potentiated by an inhibitor of monoamine oxidases (50 µM pargyline). Furthermore, bufotenin concentration- (0.1-10 µM) and time-dependently elevated force of contraction in isolated electrically stimulated musculi pectinati from the human atrium and these effects were likewise reversed by tropisetron (10 µM). We found that bufotenin (10 µM) increased the phosphorylation state of phospholamban in the isolated perfused hearts, left and right atrial muscle strips of 5-HT₄-TG but not from WT and in isolated human right atrial preparations. In summary, we showed that bufotenin can increase the force of contraction via stimulation of human 5-HT₄ receptors transgenic mouse cardiac preparations but notably also in human atrial preparations.

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.