Comparison of the densities of 5-HT4 receptors, beta 1- and beta 2-adrenoceptors in human atrium: functional implications

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.

Regulation of basal and norepinephrine-induced cAMP and ICa in hiPSC-cardiomyocytes: Effects of culture conditions and comparison to adult human atrial cardiomyocytes

BACKGROUND

There is ongoing interest in generating cardiomyocytes derived from human induced pluripotent stem cells (hiPSC) to study human cardiac physiology and pathophysiology. Recently we found that norepinephrine-stimulated calcium currents (I_Ca) in hiPSC-cardiomyocytes were smaller in conventional monolayers (ML) than in engineered heart tissue (EHT). In order to elucidate culture specific regulation of β₁-adrenoceptor (β₁-AR) responses we investigated whether action of phosphodiesterases (PDEs) may limit norepinephrine effects on I_Ca and on cytosolic cAMP in hiPSC-cardiomyocytes. Results were compared to adult human atrial cardiomyocytes.

METHODS

Adult human atrial cardiomyocytes were isolated from tissue samples obtained during open heart surgery. All patients were in sinus rhythm. HiPSC-cardiomyocytes were dissociated from ML and EHT. Förster-resonance energy transfer (FRET) was used to monitor cytosolic cAMP (Epac1-camps sensor, transfected by adenovirus). I_Ca was recorded by whole-cell patch clamp technique. Cilostamide (300 nM) and rolipram (10 μM) were used to inhibit PDE3 and PDE4, respectively. β₁-AR were stimulated with the physiological agonist norepinephrine (100 μM).

RESULTS

In adult human atrial cardiomyocytes, norepinephrine increased cytosolic cAMP FRET ratio by +13.7 ± 1.5% (n = 10/9, mean ± SEM, number of cells/number patients) and I_Ca by +10.4 ± 1.5 pA/pF (n = 15/10). This effect was not further increased in the concomitant presence of rolipram, cilostamide and norepinephrine, indicating saturation by norepinephrine alone. In ML hiPSC-cardiomyocytes, norepinephrine exerted smaller increases in cytosolic cAMP and I_Ca (FRET +9.6 ± 0.5% n = 52/21, number of cells/number of ML or EHT, and I_Ca + 1.4 ± 0.2 pA/pF n = 34/7, p < 0.05 each) and both were augmented in the presence of the PDE4 inhibitor rolipram (FRET +16.7 ± 0.8% n = 94/26 and I_Ca + 5.6 ± 1.4 pA/pF n = 11/5, p < 0.05 each). Cilostamide increased the response to norepinephrine on FRET (+12.7 ± 0.5% n = 91/19, p < 0.05), but not on I_Ca. In EHT hiPSC-cardiomyocytes, norepinephrine responses on both, FRET and I_Ca, were larger than in ML (FRET +12.1 ± 0.3% n = 87/32 and I_Ca + 3.3 ± 0.2 pA/pF n = 13/5, p < 0.05 each). Rolipram augmented the norepinephrine effect on I_Ca (+6.2 ± 1.6 pA/pF; p < 0.05 vs. norepinephrine alone, n = 10/4), but not on FRET.

CONCLUSION

Our results show culture-dependent differences in hiPSC-cardiomyocytes. In conventional ML but not in EHT, maximum norepinephrine effects on cytosolic cAMP depend on PDE3 and PDE4, suggesting immaturity when compared to the situation in adult human atrial cardiomyocytes. The smaller I_Ca responses to norepinephrine in ML and EHT vs. adult human atrial cardiomyocytes depend at least partially on a non-physiological large impact of PDE4 in hiPSC-cardiomyocytes.

Contribution of miRNAs to ion-channel remodelling in atrial fibrillation

Atrial fibrillation (AF) is the most commonly encountered clinical arrhythmia associated with pronounced mortality and morbidity, which are related to palpitations, fainting, congestive heart failure, and stroke. Prolonged episodes of AF promote AF persistence mainly due to electrical remodelling that alters ion-channel expression and/or function. MicroRNAs (miRNAs), a new class of non-coding mRNAs of around 22 nucleotides in length, have recently emerged as one of the key players in the gene-expression regulatory networks. The potential roles of miRNAs in controlling AF have recently been investigated. Several recent studies have provided promising results for a better understanding of the molecular mechanisms of AF. In this review, we summarize the mechanism of miRNAs as regulators of ion-channel gene expression and their role in causing AF through electrical remodelling.

Arrhythmias, elicited by catecholamines and serotonin, vanish in human chronic atrial fibrillation

Atrial fibrillation (AF) is the most common heart rhythm disorder. Transient postoperative AF can be elicited by high sympathetic nervous system activity. Catecholamines and serotonin cause arrhythmias in atrial trabeculae from patients with sinus rhythm (SR), but whether these arrhythmias occur in patients with chronic AF is unknown. We compared the incidence of arrhythmic contractions caused by norepinephrine, epinephrine, serotonin, and forskolin in atrial trabeculae from patients with SR and patients with AF. In the patients with AF, arrhythmias were markedly reduced for the agonists and abolished for forskolin, whereas maximum inotropic responses were markedly blunted only for serotonin. Serotonin and forskolin produced spontaneous diastolic Ca(2+) releases in atrial myocytes from the patients with SR that were abolished or reduced in myocytes from the patients with AF. For matching L-type Ca(2+)-current (ICa,L) responses, serotonin required and produced ∼ 100-fold less cAMP/PKA at the Ca(2+) channel domain compared with the catecholamines and forskolin. Norepinephrine-evoked ICa,L responses were decreased by inhibition of Ca(2+)/calmodulin-dependent kinase II (CaMKII) in myocytes from patients with SR, but not in those from patients with AF. Agonist-evoked phosphorylation by CaMKII at phospholamban (Thr-17), but not of ryanodine2 (Ser-2814), was reduced in trabeculae from patients with AF. The decreased CaMKII activity may contribute to the blunting of agonist-evoked arrhythmias in the atrial myocardium of patients with AF.

Human 5-HT₄receptor stimulation in atria of transgenic mice

In human atrium, serotonin (5-HT) exerts pleiotropic effects, which are thought to be mediated via 5-HT4 receptors. Here, we used transgenic mice (TG) that overexpress the human 5-HT4(a) receptor under control of the heart-specific α-myosin heavy chain promoter in the atria (and ventricles). Contractile studies were performed in isolated electrically driven left atrial preparations and spontaneously beating right atrial preparation of TG and littermate control mice (wild type (WT)). 5-HT increased force of contraction and phospholamban phosphorylation on serine 16 only in left atrial preparations from TG but not from WT. In contrast, β-adrenoceptor stimulation of left atrial preparations by isoprenaline increased force of contraction with similar pEC50 values and to a similar maximum extent in both TG and WT. The contractile effects of 5-HT in left atrial preparations from TG could be blocked by the 5-HT4 receptor-specific antagonists GR125487 or GR113808. In right atrial preparations from WT and TG, the β-adrenoceptor agonist isoprenaline exerted a positive chronotropic effect with similar pEC50 values and similar maximum effects. Only in right atrial preparations from TG but not WT, 5-HT exerted a positive chronotropic effect that could be attenuated by 5-HT4 receptor-specific antagonists. Finally, in left atrial preparations of TG, a higher incidence of arrhythmias was noted compared to WT. The present data indicate that the human 5-HT4 receptors expressed in mouse atria are functional. This is the first transgenic model to study this human receptor in the atrium ex vivo or in vivo.

The cardiac ventricular 5-HT4 receptor is functional in late foetal development and is reactivated in heart failure

A positive inotropic responsiveness to serotonin, mediated by 5-HT₄ and 5-HT_2A receptors, appears in the ventricle of rats with post-infarction congestive heart failure (HF) and pressure overload-induced hypertrophy. A hallmark of HF is a transition towards a foetal genotype which correlates with loss of cardiac functions. Thus, we wanted to investigate whether the foetal and neonatal cardiac ventricle displays serotonin responsiveness. Wistar rat hearts were collected day 3 and 1 before expected birth (days -3 and -1), as well as day 1, 3, 5 and 113 (age matched with Sham and HF) after birth. Hearts from post-infarction HF and sham-operated animals (Sham) were also collected. Heart tissue was examined for mRNA expression of 5-HT₄, 5-HT_2A and 5-HT_2B serotonin receptors, 5-HT transporter, atrial natriuretic peptide (ANP) and myosin heavy chain (MHC)-α and MHC-β (real-time quantitative RT-PCR) as well as 5-HT-receptor-mediated increase in contractile function exvivo (electrical field stimulation of ventricular strips from foetal and neonatal rats and left ventricular papillary muscle from adult rats in organ bath). Both 5-HT₄ mRNA expression and functional responses were highest at day -3 and decreased gradually to day 5, with a further decrease to adult levels. In HF, receptor mRNA levels and functional responses reappeared, but to lower levels than in the foetal ventricle. The 5-HT_2A and 5-HT_2B receptor mRNA levels increased to a maximum immediately after birth, but of these, only the 5-HT_2A receptor mediated a positive inotropic response. We suggest that the 5-HT₄ receptor is a representative of a foetal cardiac gene program, functional in late foetal development and reactivated in heart failure.

Ontogenic changes of the control by phosphodiesterase-3 and -4 of 5-HT responses in porcine heart and relevance to human atrial 5-HT(4) receptors

BACKGROUND AND PURPOSE

Atrial inotropic responses to 5-HT mediated through 5-HT(4) receptors fade, presumably through phosphodiesterase (PDE) activity. We investigated the influence of a selective inhibitor of PDE3 (cilostamide) or of PDE4 (rolipram) on the fade of 5-HT responses in atrial muscle.

EXPERIMENTAL APPROACH

5-HT responses were compared, ex vivo, on sinoatrial beating rate of newborn piglets, porcine atrial and ventricular force, and human atrial force. cAMP levels were assessed in piglet atrium.

KEY RESULTS

5-HT-evoked sinoatrial tachycardia did not fade and was not potentiated by cilostamide (300 nmol.L(-1)) or rolipram (1 micromol.L(-1)). Inotropic responses to 5-HT faded in atria from piglets, adolescent pigs and humans. Cilostamide reduced atrial fade of 5-HT responses in adolescent pigs and humans but not in newborn piglets. Cilostamide disclosed 5-HT ventricular responses in newborn, but not adolescent pigs. Rolipram reduced fade of atrial 5-HT responses in newborn and adolescent pigs but not in humans. Concurrent cilostamide + rolipram abolished fade of 5-HT responses in porcine left atria and facilitated ventricular 5-HT responses, but did not reduce residual fade in human atrium in the presence of cilostamide. 5-HT-evoked increases in cAMP faded; fade was abolished by concurrent cilostamide + rolipram.

CONCLUSIONS AND IMPLICATIONS

PDE3-induced control of porcine 5-HT responses differed in atrium and ventricle and changed with age. PDE3 and PDE4 jointly prevented fade of inotropic and cAMP responses to 5-HT in porcine atrium. Unlike porcine atria, only PDE3 induced fade of 5-HT responses in human atria.

Selective desensitization of the 5-HT4 receptor-mediated response in pig atrium but not in stomach

BACKGROUND AND PURPOSE

The time dependency of the effect of 5-HT(4) receptor agonists depends on many specific regulatory mechanisms, which vary between tissues. This has important implications with regard to the effects of endogenous 5-HT, as well as to the clinical use of 5-HT(4) receptor agonists, and might contribute to tissue selectivity of agonists.

EXPERIMENTAL APPROACH

The progression and desensitization of 5-HT(4) receptor-mediated responses were evaluated in an organ bath set-up using two, clinically relevant, porcine in vitro models: gastric cholinergic neurotransmission and atrial contractility.

KEY RESULTS

Exposure of gastric tissue to 5-HT or to the selective 5-HT(4) receptor agonists prucalopride and M0003 results in a sustained non-transient effect during exposure; after washout, the response to a subsequent challenge with 5-HT shows no clear desensitization. Incubation of left atrial tissue with 5-HT resulted in a transient response, leading after washout to a marked desensitization of the subsequent response to 5-HT. The selective 5-HT(4) receptor agonists prucalopride and M0003 induce only very weak atrial responses whereas they are very effective in desensitizing the atrial response to 5-HT. The observations also suggest that the properties of prucalopride and M0003 to bind to and/or activate the 5-HT(4) receptor differ from those of 5-HT. This difference might have contributed to the observed desensitization.

CONCLUSIONS AND IMPLICATIONS

The high potency of prucalopride and M0003 in desensitizing the response to 5-HT together with their low efficacy in the atrium emphasizes the cardiac safety of this class of 5-HT(4) receptor agonists.

Phosphorylation of phospholamban and troponin I through 5-HT4 receptors in the isolated human atrium

We studied the mRNA expression and function of 5-hydroxytryptamine (5-HT) receptors as well as their signal transduction in right atrial tissue from patients undergoing cardiac surgery and right ventricular tissue from human donor hearts. In isolated, electrically driven strips from human right atrium, 5-HT exerted concentration-dependent positive inotropic effects (EC(50) value = 0.10 +/- 0.01 microM) and hastened relaxation (positive lusitropic effect). The 5-HT(4) receptor antagonists SB203186 or GR125487 antagonised these effects. 5-HT (2 microM) increased the content of cyclic adenosine monophosphate (cAMP) from 6.86 +/- 1.36 to 19.1 +/- 2.45 pmol/mg protein (n = 6, p < 0.05) but did not alter the tissue content of inositol-1,4,5-trisphosphate (IP(3)). With reverse transcription polymerase chain reaction, mRNAs coding for the 5-HT(4) receptor splice variants 5-HT(4(a)), 5-HT(4(b)) and 5-HT(4(c)) were detected in human right atrium and right ventricle. 5-HT(2A) mRNA only was measurable in human atrium. Expression level of total 5-HT(4) receptor mRNA in the right ventricle amounted to 41% (n = 5-8) of that in the right atrium. 5-HT (2 microM) increased the atrial phosphorylation states of phospholamban to 168% at serine-16 and to 150% at threonine-17 (n = 4; p < 0.05) and of the inhibitory subunit of troponin to 150% (n = 6; p < 0.05). In conclusion, the positive inotropic and lusitropic effects of 5-HT in electrically driven human right atria are mediated via 5-HT(4) receptors. These effects are accompanied by and probably due to an increase in cAMP content and the subsequent elevation of the phosphorylation state of Ca(2+) regulatory proteins.

5-HT4 receptor agonists: similar but not the same

5-Hydroxytryptamine(4) (5-HT(4)) receptors are an interesting target for the management of patients in need of gastrointestinal (GI) promotility treatment. They have proven therapeutic potential to treat patients with GI motility disorders. Lack of selectivity for the 5-HT(4) receptor has limited the clinical success of the agonists used until now. For instance, next to their affinity for 5-HT(4) receptors, both cisapride and tegaserod have appreciable affinity for other receptors, channels or transporters [e.g. cisapride: human ether-a-go-go-related gene (hERG) is K(+) channel and tegaserod: 5-HT(1) and 5-HT(2) receptors]. Adverse cardiovascular events observed with these compounds are not 5-HT(4) receptor-related. Recent efforts have led to the discovery of a series of selective 5-HT(4) receptor ligands, with prucalopride being the most advanced in clinical development. The selectivity of these new compounds clearly differentiates them from the older generation compounds by minimizing the potential of target-unrelated side effects. The availability of selective agonists enables the focus to shift to the exploration of 5-HT(4) receptor-related differences between agonists. Based on drug- and tissue-related properties (e.g. differences in receptor binding, receptor density, effectors, coupling efficiency), 5-HT(4) receptor agonists are able to express tissue selectivity, i.e. behave as a partial agonist in some and as a full agonist in other tissues. Furthermore, the concept of ligand-directed signalling offers great opportunities for future drug development by enlarging the scientific basis for the generation of agonist-specific effects in different cell types, tissues or organs. Selective 5-HT(4) receptor agonists might thus prove to be innovative drugs with an attractive safety profile for better treatment of patients suffering from hypomotility disorders.

Quantitative mRNA analysis of serotonin 5-HT4 receptor isoforms, calcium handling proteins and ion channels in human atrial fibrillation

Serotonin 5-HT(4) receptors are present in human atrial myocytes and have been proposed to contribute to the generation of atrial fibrillation (AF). Here, we quantified 5-HT(4) receptors as well as other key genes involved in cardiac rhythm and contraction in right atrial appendages of patients with chronic AF (CAF) and acute AF (AAF). Right atrial appendages were obtained from eleven patients in sinus rhythm (SR), five with AAF and six with CAF (>12 months). TaqMan real time quantitative RT-PCR was performed on total RNA. Results were normalised to the average of three housekeeping genes, cyclophilin, GADPH and RL-19. The rank order of expression of h5-HT(4) receptors variants was (b)>(a)>(g)>(c) in the group of patients in SR. In AAF, we found a strong decrease in h5-HT(4(b)), h5-HT(4(c),) and h5-HT(4(g)) transcripts. In CAF patients, the mRNA expression level of the h5-HT(4(b)) isoform significantly increased two fold versus SR. A similar increase was reported for beta(1)-adrenergic receptor, connexin 43 and the L-type Ca(2+) channel CaCNA1C subunit. Interestingly, CAF was associated with a strong increase in the expression of Na(+)/Ca(2+) exchanger and the voltage-dependent Na(+) channel SCN5A subunit. Our results indicate that h5-HT(4(b)) is the dominant cardiac isoform of human 5-HT(4) receptors and its expression is increased in CAF. These data support the involvement of 5-HT(4) receptors in atrial arrhythmia.

Fading of 5-HT4 receptor-mediated inotropic responses to 5-hydroxytryptamine is caused by phosphodiesterase activity in porcine atrium

Inotropic responses to 5-hydroxytryptamine (5-HT) in human and porcine atrium can fade, suggesting 5-HT(4) receptor desensitization. De Maeyer et al., however, show in this issue that inhibition of phosphodiesterases with isobutyl-methyl-xanthine prevents fading of 5-HT(4) receptor-mediated responses to 5-HT and the partial agonist prucalopride in porcine atrium.

Porcine left atrial and sinoatrial 5-HT(4) receptor-induced responses: fading of the response and influence of development

1.--In this study, we aimed to characterize in vitro the effects of the benzofuran 5-HT(4) receptor agonists prucalopride, R149402 and R199715 and the indolic agents tegaserod and 5-HT in the atria of young pigs (10-11 weeks) and newborn piglets. 2.--In the paced left atrium of young pigs, only 5-HT results in positive inotropic responses when administered cumulatively (maximal effect relative to isoprenaline=53%, pEC(50)=6.8); however, all agonists showed lusitropic effects. Noncumulative administration results in greater positive inotropic responses for 5-HT and induces moderate positive inotropic responses for the other agonists; these responses fade. 3.--Phosphodiesterase (PDE) enzyme inhibition with 3-isobutyl-1-methylxanthine (IBMX; 20 microM) enhances the responses to cumulatively administered 5-HT (maximal effect=89%, pEC(50)=7.7) and reveals clear positive inotropic effects for prucalopride, tegaserod, R149402 and R199715; fading is abolished. The maximal effect of the benzofurans is less pronounced than that of the indoles. 4.--In the spontaneously beating right atrium of young pigs, all agonists show chronotropic activity when administered cumulatively in the absence of IBMX, without fade. Benzofurans behaved as partial agonists compared to 5-HT (maximal effect=54%, pEC(50)=6.5). 5.--In newborns, the inotropic activity of the agonists in the IBMX-treated left atrium was less pronounced than in the young pig; the same applied for the chronotropic response in the right atrium, except for 5-HT. 6.--In conclusion, the atrial responses to 5-HT(4) receptor activation increase in the first months of life; the inotropic response is regulated by PDEs. Prucalopride, R149402 and R199715 are partial agonists compared to 5-HT.

Pharmacodynamic effects of a novel prokinetic 5-HT receptor agonist, ATI-7505, in humans

ATI-7505, an investigational 5-HT(4) receptor agonist, was designed to have similar activity as cisapride without the cardiac adverse effects, i.e. without QT prolongation. In addition, ATI-7505 is not metabolized by CYP450. The aim of the study was to assess the effect of ATI-7505 on gastrointestinal (GI) and colonic transit in healthy humans. A randomized, parallel-group, double-blind, placebo-controlled study evaluated effects of 9-day treatment with ATI-7505 (3, 10 or 20 mg t.i.d.) on scintigraphic GI and colonic transit in healthy volunteers (12 per group). Primary endpoints were gastric-emptying (GE) T(1/2), colonic geometric centre (GC) at 24 h and ascending colon (AC) emptying T(1/2). Daily stool diaries were kept. Analysis of covariance assessed overall treatment group differences, followed by post hoc unadjusted pairwise comparisons. There were borderline overall treatment effects (decrease) on GE T(1/2) (P = 0.154); the 20 mg t.i.d. of ATI-7505-accelerated GE vs placebo (P = 0.038). ATI-7505 increased colonic transit (GC24, P = 0.031) with fastest transit at 10 mg t.i.d. vs placebo (P = 0.065). ATI-7505 accelerated AC emptying T(1/2) (overall P = 0.075) with 10 mg dose vs placebo (P = 0.042). There was looser stool (Bristol stool form scale, overall P = 0.056) with the 10 and 20 mg t.i.d. doses. No safety issues were identified. ATI-7505 accelerates overall colonic transit and tends to accelerate GE and AC emptying and loosen stool consistency.

Investigation of serotonin type 4 receptor expression in human and non-human primate gastrointestinal samples

BACKGROUND

The serotonin type 4 (5-HT4) receptor has been associated with functions of the gastrointestinal tract such as modulation of the peristaltic reflex, smooth muscle tone, intestinal secretion and visceral sensitivity. The activation of peripheral 5-HT4 receptors with agonists such as tegaserod has been shown to accelerate gastric emptying and improve symptoms of constipation in animals and humans. However, detailed data on the expression profile and on the localization of this receptor subtype are lacking so far.

OBJECTIVE

To study the pattern and expression levels of 5-HT4 receptor messenger RNA expression in the gut.

METHOD

Normal tissue samples were collected from the whole gastrointestinal tract of patients undergoing abdominal surgery and, in addition, of monkeys. We performed a comprehensive analysis of 5-HT4 receptor expression by quantitative reverse transcription-polymerase chain reaction, using human and non-human primate tissues from the oesophagus to the rectum. In addition, the brain and heart of non-human primates were analysed.

RESULTS

Significantly higher levels of 5-HT4 receptor mRNA were measured in the human stomach, duodenum, jejunum, ileum and caecum and also in the corresponding non-human primate gut segments, ranging from 2- to 12-fold compared with the liver. No differences were found between females and males of both human and non-human primates.

CONCLUSIONS

These results show 5-HT4 receptor mRNA expression throughout the gastrointestinal tract in humans and primates, and also support the preclinical and clinical findings of 5-HT4 receptors ligands exhibiting multiple effects throughout the gastrointestinal tract.

5-Hydroxytryptamine receptors in the human cardiovascular system

The human cardiovascular system is exposed to plasma 5-hydroxytryptamine (5-HT, serotonin), usually released from platelets. 5-HT can produce harmful acute and chronic effects. The acute cardiac effects of 5-HT consist of tachycardia (preceded on occasion by a brief reflex bradycardia), increased atrial contractility and production of atrial arrhythmias. Acute inotropic, lusitropic and arrhythmic effects of 5-HT on human ventricle become conspicuous after inhibition of phosphodiesterase (PDE) activity. Human cardiostimulation is mediated through 5-HT4 receptors. Atrial and ventricular PDE3 activity exerts a protective role against potentially harmful cardiostimulation. Chronic exposure to high levels of 5-HT (from metastatic carcinoid tumours), the anorectic drug fenfluramine and its metabolites, as well as the ecstasy drug 3,4-methylenedioxymethamphetamine (MDMA) and its metabolite 3,4-methylenedioxyamphetamine (MDA) are associated with proliferative disease and thickening of cardiac valves, mediated through 5-HT2B receptors. 5-HT2B receptors have an obligatory physiological role in murine cardiac embryology but whether this happens in humans requires research. Congenital heart block (CHB) is, on occasion, associated with autoantibodies against 5-HT4 receptors. Acute vascular constriction by 5-HT is usually shared by 5-HT1B and 5-HT2A receptors, except in intracranial arteries which constrict only through 5-HT1B receptors. Both 5-HT1B and 5-HT2A receptors can mediate coronary artery spasm but only 5-HT1B receptors appear involved in coronary spasm of patients treated with triptans or with Prinzmetal angina. 5-HT2A receptors constrict the portal venous system including oesophageal collaterals in cirrhosis. Chronic exposure to 5-HT can contribute to pulmonary hypertension through activation of constrictor 5-HT1B receptors and proliferative 5-HT2B receptors, and possibly through direct intracellular effects.

Functional studies of the 5'-untranslated region of human 5-HT4 receptor mRNA

The serotonin 5-HT4 receptor (where 5-HT stands for 5-hydroxy-tryptamine) is a member of the seven transmembrane-spanning G-protein-coupled family of receptors and mediates many cellular functions both in the central nervous system and at the periphery. In the present study, we isolated and characterized the 5'-flanking region of the h5-HT4 (human 5-HT4) receptor. We demonstrate the existence of a novel exon that corresponds to the 5'-untranslated region of the h5-HT4 receptor gene. RNase protection analysis and reverse transcriptase-PCR experiments performed on human atrial RNA demonstrated that the major transcription start site of the h5-HT4 receptor gene is located at -3185 bp relative to the first ATG codon. In addition, a 1.2 kb promoter fragment which drives the transcription of the 5-HT4 receptor was characterized. The promoter region lacks TATA and CAAT canonical motifs in the appropriate location, but contains putative binding sites for several transcription factors. Transient transfection assays revealed that the (-3299/-3050) gene fragment possesses the ability to promote the expression of the luciferase reporter gene in human cell lines. In contrast, the promoter was silent in monkey COS-7 cells, indicating the requirement of specific factors to initiate transcription in human cells. In addition to the promoter element, enhancer activity was found in a region (-220/-61) located in the long 5'-untranslated region. Mutational analysis, gel shift and transfection assays identified an Nkx2.5 (NK2-transcription-factor-related 5)-like binding site as a regulatory sequence of this enhancer. Our results suggest a complex regulation of the h5-HT4 receptor gene expression involving distinct promoters and non-coding exons.

Functional serotonin 5-HT4 receptors in porcine and human ventricular myocardium with increased 5-HT4 mRNA in heart failure

Serotonin (5-hydroxytryptamine, 5-HT) increases contractile force and elicits arrhythmias through 5-HT(4) receptors in porcine and human atrium, but its ventricular effects are unknown. We now report functional 5-HT(4) receptors in porcine and human ventricle. 5-HT(4) mRNA levels were determined in porcine and human ventricles and contractility studied in ventricular trabeculae. Cyclic AMP-dependent protein kinase (PKA) activity was measured in porcine ventricle. Porcine and human ventricles expressed 5-HT(4) receptor mRNA. Ventricular 5-HT(4(b)) mRNA was increased by four times in 20 failing human hearts compared with five donor hearts. 5-HT increased contractile force maximally by 16% (EC(50)=890 nM) and PKA activity by 20% of the effects of (-)-isoproterenol (200 microM) in ventricular trabeculae from new-born piglets in the presence of the phosphodiesterase-inhibitor 3-isobutyl-1-methylxanthine. In ventricular trabeculae from adult pigs (3-isobutyl-1-methylxanthine present) 5-HT increased force by 32% (EC(50)=60 nM) and PKA activity by 39% of (-)-isoproterenol. In right and left ventricular trabeculae from failing hearts, exposed to modified Krebs solution, 5-HT produced variable increases in contractile force in right ventricular trabeculae from 4 out of 6 hearts and in left ventricular trabeculae from 3 out of 3 hearts- range 1-39% of (-)-isoproterenol, average 8%. In 11 left ventricular trabeculae from the failing hearts of four beta-blocker-treated patients, pre-exposed to a relaxant solution with 0.5 mM Ca(2+) and 1.2 mM Mg(2+) followed by a switch to 2.5 mM Ca(2+) and 1 mM Mg(2+), 5-HT (1-100 microM, 3-isobutyl-1-methylxanthine present) consistently increased contractile force and hastened relaxation by 46% and 25% of (-)-isoproterenol respectively. 5-HT caused arrhythmias in three trabeculae from 3 out of 11 patients. In the absence of phosphodiesterase inhibitor, 5-HT increased force in two trabeculae, but not in another six trabeculae from 4 patients. All 5-HT responses were blocked by 5-HT(4) receptor antagonists. We conclude that phosphodiesterase inhibition uncovers functional ventricular 5-HT(4) receptors, coupled to a PKA pathway, through which 5-HT enhances contractility, hastens relaxation and can potentially cause arrhythmias.

A single mutation in the 5-HT4 receptor (5-HT4-R D100(3.32)A) generates a Gs-coupled receptor activated exclusively by synthetic ligands (RASSL)

To better understand G-protein-coupled receptor (GPCRs) signaling, cellular and animal physiology, as well as gene therapy, a new tool has recently been proposed. It consists of GPCR mutants that are insensitive to endogenous ligands but sensitive to synthetic ligands. These GPCRs are called receptor activated solely by synthetic ligands (RASSL). Only two examples of such engineered receptors have been described so far: one G(i)-coupled (opioid receptors) and one G(s)-coupled (beta(2)-adrenergic receptors). Here, we describe the first RASSL related to serotonin receptors (D100(3.32)A G(s)-coupled 5-HT(4) receptor or 5-HT(4)-RASSL). 5-HT(4)-RASSL is generated by a single mutation, is totally insensitive to serotonin (5-HT), and still responds to synthetic ligands. These ligands have affinities in the range of nanomolar concentrations for the mutant receptor and exhibit full efficacy. More interestingly, two synthetic ligands behave as antagonists on the wild type but as agonists on the 5-HT(4)-RASSL.

Inhibitory activity of antibodies against the human atrial 5-HT(4)receptor

Antibodies directed against the second extracellular loop of G protein-coupled receptors have been shown to exert "agonist-like" activities. In order to test the hypothesis that this is a general phenomenon, antibodies were raised in rabbits against a synthetic peptide corresponding to the second extracellular loop of the newly sequenced human cardiac 5-HT(4)receptor. The antibodies were affinity-purified and shown to recognize the 5-HT(4)receptor in immunoblots of Chinese hamster ovary (CHO) cells expressing the receptor. The antibodies had no intrinsic effect but could depress the activation of L -type calcium channel induced by serotonin in human atrial cells. This antagonist-like effect was exerted both by intact IgG and by Fab fragments. These results are physiologically important since it has been shown that the 5-HT(4)receptor could be a target for autoantibodies in mothers at risk of giving birth to children with neonatal atrio-ventricular block.

Pharmacological characterization of the human 5-HT(4(d)) receptor splice variant stably expressed in Chinese hamster ovary cells

The recently identified C-terminal splice variant of the human 5-HT(4) receptor, the h5-HT(4(d)) receptor, was stably expressed in a CHO cell line at 493+/-25 fmol mg(-1) protein. We analysed its pharmacological properties by measuring binding affinities and 5-HT(4) ligand-induced cyclic AMP production. The pharmacological binding profile determined in competition studies with the specific antagonist [(3)H]-GR113808 revealed a rank order of affinity of 5-HT(4) ligands for the h5-HT(4(d)) receptor that was consistent with those previously reported for other 5-HT(4) receptor isoforms. In adenylyl cyclase functional assays, the h5-HT(4(d)) receptor displayed equipotent coupling for all 5-HT(4) agonists tested (EC(50) in the range of 1 - 6 nM). EC(50) values were lower than those previously obtained with the 5-HT(4(e)) receptor stably expressed in CHO cells indicating that the 5-HT(4(d)) receptor was more efficiently coupled to its effector than the 5-HT(4(e)) receptor isoform. Moreover, in terms of agonist efficacy (E(max)), the benzamide derivative, renzapride displayed full agonist properties at the h5-HT(4(d)) receptor (same E(max) as 5-HT) whereas it was previously shown to be a partial agonist at the h5-HT(4(e)) receptor. A constitutive activity of the h5-HT(4(d)) receptor was observed in CHO cells in the absence of any 5-HT(4) ligand. Surprisingly, two 5-HT(4) ligands, SB204070 and RS39604 which are described as highly potent antagonists in various biological models, revealed partial agonist properties at the h5-HT(4(d)) receptor. We conclude that C-terminal tails of 5-HT(4) receptor isoforms may directly influence their functional properties.

Isolation of the serotoninergic 5-HT4(e) receptor from human heart and comparative analysis of its pharmacological profile in C6-glial and CHO cell lines

RT - PCR technique was used to clone the human 5-HT(4(e)) receptor (h5-HT(4(e))) from heart atrium. We showed that this h5-HT(4(e)) receptor splice variant is restricted to brain and heart atrium. Recombinant h5-HT(4(e)) receptor was stably expressed in CHO and C6-glial cell lines at 347 and 88 fmol mg(-1) protein, respectively. Expression of h5-HT(4(e)) receptors at the cell membrane was confirmed by immunoblotting. The receptor binding profile, determined by competition with [(3)H]-GR113808 of a number of 5-HT(4) ligands, was consistent with that previously reported for other 5-HT(4) receptor isoforms. Surprisingly, we found that the rank order of potencies (EC(50)) of 5-HT(4) agonists obtained from adenylyl cyclase functional assays was inversely correlated to their rank order of affinities (K(i)) obtained from binding assays. Furthermore, EC(50) values for 5-HT, renzapride and cisapride were 2 fold lower in C6-glial cells than in CHO cells. ML10302 and renzapride behaved like partial agonists on the h5-HT(4(e)) receptor. These results are in agreement with the reported low efficacy of the these two compounds on L-type Ca(2+) currents and myocyte contractility in human atrium. A constitutive activity of the h5-HT(4(e)) receptor was observed in CHO cells in the absence of any 5-HT(4) ligand and two 5-HT(4) antagonists, GR113808 and ML10375, behaved as inverse agonists. These data show that the h5-HT(4(e)) receptor has a pharmacological profile which is close to the native h5-HT(4) receptor in human atrium with a functional potency which is dependent on the cellular context in which the receptor is expressed.

Review article: cardiac adverse effects of gastrointestinal prokinetics

Gastrointestinal prokinetics, such as metoclopramide, cisapride and levosulpiride, are widely used for the management of functional gut disorders. Recently, several studies have shown that cisapride (a partial 5-HT4 receptor agonist) can induce dose-dependent cardiac adverse effects, including lengthening of the electrocardiographic QT interval, syncopal episodes and ventricular dysrhythmias. Until recently, it was not clear whether these effects were dependent on 5-HT4 receptor activation or related to peculiar characteristics in the molecular structure of single agents within the benzamide class. Experimental evidence now favours the second hypothesis: cisapride possesses Class III antiarrhythmic properties and prolongs the action potential duration through blockade of distinct voltage-dependent K+ channels, thus delaying cardiac repolarization and prolonging the QT interval. Patients at risk of cardiac adverse effects are children, subjects with idiopathic, congenital or acquired long QT syndrome and, in particular, those receiving concomitant medication with Class III antiarrhythmic agents, some H1-receptor antagonists (e.g. terfenadine), or drugs such as azole antifungals (e.g. ketoconazole, itraconazole, miconazole and fluconazole) and macrolide antibacterials (e.g. erythromycin, clarithrod-mycin and troleandomycin), which can inhibit cisapride metabolism by interfering with the CYP3A4 isoenzyme.

Functional gut disorders: from motility to sensitivity disorders. A review of current and investigational drugs for their management

Functional gut disorders include several clinical entities defined on the basis of symptom patterns (e.g., functional dyspepsia, irritable bowel syndrome, functional abdominal pain, functional abdominal bloating), for which there is no established pathophysiological mechanism. Because there is no well-defined pathophysiological target, treatment should be aimed at symptom improvement. Prokinetics and antispasmodics have been widely used in the treatment of functional gut disorders on the assumption that disordered motility is the underlying cause of symptoms, and symptom improvement is indeed achievable with these compounds in some, but not all, patients with features of hypo- or hypermotility, respectively. In the first part of this review, we cover the basic pharmacology and discuss the rationale for the clinical use of prokinetics and antispasmodics. On the other hand, in the past few years, the explosive growth in the research focusing on visceral sensitivity and visceral reflexes has suggested that at least some patients with functional gut disorders have altered visceral perception. Thus, the second part of the review covers these developments and focuses on studies addressing the issue of drugs modulating visceral sensitivity.

Molecular and functional characterization of a 5-HT4 receptor cloned from human atrium

5-Hydroxytryptamine (5-HT) has been shown to exert positive inotropic, chronotropic, and lusitropic effects and to stimulate the L-type calcium channel current (I(Ca)) in human atrial tissue through activation of the pharmacologically defined 5-HT4 receptor subtype. However, the molecular nature of the receptor(s) involved in these effects is still unknown. In the present study, we report the molecular nature of a 5-HT4 receptor cloned from human atrium, h5-HT4A. Sequence analysis reveals that h5-HT4A displays a 93% protein identity with the short form of the 5-HT4 receptor recently isolated from rat brain. h5-HT4A mRNA is expressed in human atrium but not ventricle, and is also found in brain and GI tract. h5-HT4A transiently expressed in COS-7 cells displays a classical 5-HT4 pharmacological profile. However, affinities of the h5-HT4A receptor for agonists such as ML10302, BIMU1, renzapride or zacopride were 4-10-fold lower than the ones found in brain. Moreover, the stimulatory patterns of cAMP formation by h5-HT4A in response to the 5-HT4 agonists ML10302 and renzapride were very similar to the patterns of stimulation of I(Ca) obtained in response to these compounds in human atrial myocytes. We conclude that h5-HT4A likely mediates the effects of 5-HT in human atrium and may differ from 5-HT4 receptor isoforms present in the brain and GI tract.