Effects of treatment with a 5-HT4 receptor antagonist in heart failure

Role of Vasoactive Hormone-Induced Signal Transduction in Cardiac Hypertrophy and Heart Failure

Heart failure is the common concluding pathway for a majority of cardiovascular diseases and is associated with cardiac dysfunction. Since heart failure is invariably preceded by adaptive or maladaptive cardiac hypertrophy, several biochemical mechanisms have been proposed to explain the development of cardiac hypertrophy and progression to heart failure. One of these includes the activation of different neuroendocrine systems for elevating the circulating levels of different vasoactive hormones such as catecholamines, angiotensin II, vasopressin, serotonin and endothelins. All these hormones are released in the circulation and stimulate different signal transduction systems by acting on their respective receptors on the cell membrane to promote protein synthesis in cardiomyocytes and induce cardiac hypertrophy. The elevated levels of these vasoactive hormones induce hemodynamic overload, increase ventricular wall tension, increase protein synthesis and the occurrence of cardiac remodeling. In addition, there occurs an increase in proinflammatory cytokines and collagen synthesis for the induction of myocardial fibrosis and the transition of adaptive to maladaptive hypertrophy. The prolonged exposure of the hypertrophied heart to these vasoactive hormones has been reported to result in the oxidation of catecholamines and serotonin via monoamine oxidase as well as the activation of NADPH oxidase via angiotensin II and endothelins to promote oxidative stress. The development of oxidative stress produces subcellular defects, Ca2+-handling abnormalities, mitochondrial Ca2+-overload and cardiac dysfunction by activating different proteases and depressing cardiac gene expression, in addition to destabilizing the extracellular matrix upon activating some metalloproteinases. These observations support the view that elevated levels of various vasoactive hormones, by producing hemodynamic overload and activating their respective receptor-mediated signal transduction mechanisms, induce cardiac hypertrophy. Furthermore, the occurrence of oxidative stress due to the prolonged exposure of the hypertrophied heart to these hormones plays a critical role in the progression of heart failure.

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.

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.

Future scope and challenges for congestive heart failure: Moving towards development of pharmacotherapy

Heart failure is invariably associated with cardiac hypertrophy and impaired cardiac performance. Although several drugs have been developed to delay the progression of heart failure, none of the existing interventions have shown beneficial effects in reducing morbidity and mortality. In order to determine specific targets for future drug development, we have discussed different mechanisms involving both cardiomyocytes and non-myocyte (extracellular matrix) alterations for the transition of cardiac hypertrophy to heart failure as well as for the progression of heart failure. We have emphasized the role of oxidative stress, inflammatory cytokines, metabolic alterations and Ca2+-handling defects in adverse cardiac remodeling and heart dysfunction in hypertrophied myocardium. Alterations in the regulatory process due to several protein kinases as well as participation of mitochondrial Ca2+-overload, activation of proteases and phospholipases and changes in gene expression for subcellular remodeling have also been described for the occurrence of cardiac dysfunction. Association of cardiac arrhythmia with heart failure has been explained as a consequence of catecholamine oxidation products. Since these multifactorial defects in extracellular matrix and cardiomyocytes are evident in the failing heart, it is a challenge for experimental cardiologists to develop appropriate combination drug therapy for improving cardiac function in heart failure.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Serotonin: a platelet hormone modulating cardiovascular disease

Cardiovascular diseases and depression are significant health burdens and increasing evidence suggests a causal relationship between them. The incidence of depression among patients suffering from cardiovascular disease is markedly elevated, and depression itself is an established cardiovascular risk factor. Serotonin 5-hydroxytryptamin (5-HT), a biogenic amine acting as a neurotransmitter and a peripheral hormone, is involved in the pathogenesis of both, cardiovascular disease and depression. Novel cardiovascular functions of 5-HT have recently been described and will be summarized in this review. 5-HT has a broad spectrum of functions in the cardiovascular system, yet the clinical or experimental data are partly conflicting. There is further research needed to characterize the clinical effects of 5-HT in particular tissues to enable targeted pharmacological therapies.

Noninvasive stratification of postinfarction rats based on the degree of cardiac dysfunction using magnetic resonance imaging and echocardiography

The myocardial infarction (MI) rat model plays a crucial role in modern cardiovascular research, but the inherent heterogeneity of this model represents a challenge. We sought to identify subgroups among the post-MI rats and establish simple noninvasive stratification protocols for such subgroups. Six weeks after induction of MI, 49 rats underwent noninvasive examinations using magnetic resonance imaging (MRI) and echocardiography. Twelve sham-operated rats served as controls. Increased end-diastolic left ventricular (LV) pressure and lung weight served as indicators for congestive heart failure (CHF). A clustering algorithm using 13 noninvasive and invasive parameters was used to identify distinct groups among the animals. The cluster analysis revealed four distinct post-MI phenotypes; two without congestion but with different degree of LV dilatation, and two with different degree of congestion and right ventricular (RV) affection. Among the MRI parameters, RV mass emerged as robust noninvasive marker of CHF with 100% specificity/sensitivity. Moreover, LV infarct size and RV ejection fraction further predicted subgroup among the non-CHF and CHF rats with excellent specificity/sensitivity. Of the echocardiography parameters, left atrial diameter predicted CHF. Moreover, LV end-diastolic diameter predicted the subgroups among the non-CHF rats. We propose two simple noninvasive schemes to stratify post-MI rats, based on the degree of heart failure; one for MRI and one for echocardiography.NEW & NOTEWORTHY In vivo phenotyping of rats is essential for robust and reliable data. Here, we present two simple noninvasive schemes for the stratification of postinfarction rats based on the degree of heart failure: one using magnetic resonance imaging and one based on echocardiography.

Review of novel therapeutic targets for improving heart failure treatment based on experimental and clinical studies

Heart failure (HF) is a major public health priority due to its epidemiological transition and the world's aging population. HF is typified by continuous loss of contractile function with reduced, normal, or preserved ejection fraction, elevated vascular resistance, fluid and autonomic imbalance, and ventricular dilatation. Despite considerable advances in the treatment of HF over the past few decades, mortality remains substantial. Pharmacological treatments including β-blockers, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, and aldosterone antagonists have been proven to prolong the survival of patients with HF. However, there are still instances where patients remain symptomatic, despite optimal use of existing therapeutic agents. This understanding that patients with chronic HF progress into advanced stages despite receiving optimal treatment has increased the quest for alternatives, exploring the roles of additional pathways that contribute to the development and progression of HF. Several pharmacological targets associated with pathogenesis of HF have been identified and novel therapies have emerged. In this work, we review recent evidence from proposed mechanisms to the outcomes of experimental and clinical studies of the novel pharmacological agents that have emerged for the treatment of HF.

Heart failure: novel therapeutic approaches

Heart failure (HF) is a complex clinical syndrome that can result from any structural or functional cardiac disorders that impairs the ability of the ventricle to fill with or eject blood. Despite effective medical interventions, mortality and morbidity remain substantial. There have been significant advances in the therapy of HF in recent decades, such as the introduction of beta-blockers and antagonists of the renin-angiotensin system but still there is a major unmet need for better therapies for HF. In the present era, pathophysiology of HF has been explored. Various novel pathways, molecular sites have been identified, which contribute to the progression of the disease. By targeting these sites, newer pharmacological agents have been developed, which can play a promising role in the treatment of HF. This article focuses on recent advancements in pharmacotherapy of HF, which include agents targeting myocardial contractility, cytokines and inflammation, fibrosis and remodeling, myocardial metabolism, oxidative stress, and other newly defined pathways.

Reversal of subcellular remodelling by losartan in heart failure due to myocardial infarction

This study tested the reversal of subcellular remodelling in heart failure due to myocardial infarction (MI) upon treatment with losartan, an angiotensin II receptor antagonist. Twelve weeks after inducing MI, rats were treated with or without losartan (20 mg/kg; daily) for 8 weeks and assessed for cardiac function, cardiac remodelling, subcellular alterations and plasma catecholamines. Cardiac hypertrophy and lung congestion in 20 weeks MI-induced heart failure were associated with increases in plasma catecholamine levels. Haemodynamic examination revealed depressed cardiac function, whereas echocardiographic analysis showed impaired cardiac performance and marked increases in left ventricle wall thickness and chamber dilatation at 20 weeks of inducing MI. These changes in cardiac function, cardiac remodelling and plasma dopamine levels in heart failure were partially or fully reversed by losartan. Sarcoplasmic reticular (SR) Ca²⁺-pump activity and protein expression, protein and gene expression for phospholamban, as well as myofibrillar (MF) Ca²⁺-stimulated ATPase activity and α-myosin heavy chain mRNA levels were depressed, whereas β-myosin heavy chain expression was increased in failing hearts; these alterations were partially reversed by losartan. Although SR Ca²⁺-release activity and mRNA levels for SR Ca²⁺-pump were decreased in failing heart, these changes were not reversed upon losartan treatment; no changes in mRNA levels for SR Ca²⁺-release channels were observed in untreated or treated heart failure. These results suggest that the partial improvement of cardiac performance in heart failure due to MI by losartan treatment is associated with partial reversal of cardiac remodelling as well as partial recovery of SR and MF functions.

Differential regulation of C-type natriuretic peptide-induced cGMP and functional responses by PDE2 and PDE3 in failing myocardium

Recently, we showed C-type natriuretic peptide (CNP)-induced negative inotropic (NIR) and positive lusitropic response (LR) in failing rat heart. We wanted to study whether, and if so, how phosphodiesterases (PDEs) regulate CNP-induced cyclic 3',5'-guanosine monophosphate (cGMP) elevation and functional responses. Inotropic and lusitropic responses were measured in left ventricular muscle strips and cyclic nucleotide levels, PDE activity and phospholamban (PLB) and troponin I (TnI) phosphorylation were measured in ventricular cardiomyocytes from Wistar rats with heart failure 6 weeks after myocardial infarction. CNP-mediated increase in global cGMP was mainly regulated by PDE2, as reflected by a marked amplification of the cGMP increase during PDE2 inhibition and by a high PDE2 activity in cardiomyocytes. PDE3 inhibition, on the other hand, caused no significant cGMP increase by CNP. The functional consequences did not correspond to the changes of cGMP. PDE3 inhibition increased the potency of the CNP-induced NIR and LR, while PDE2 inhibition desensitized the CNP-induced NIR, but not LR. A role for PDE2 on the maximal LR and PDE5 on the maximal NIR to CNP was revealed in the presence of PDE3 inhibition. CNP increased PLB phosphorylation about 25- to 30-fold and tended to increase TnI phosphorylation about twofold. As a whole, CNP-induced functional responses were only modestly regulated by PDEs compared to the cAMP-mediated functional responses to β1-adrenoceptor stimulation, which are highly regulated by PDEs. There is a mismatch between the CNP-induced cGMP increase and functional responses. Global cGMP levels are mainly regulated by PDE2 after CNP stimulation, whereas the functional responses are modestly regulated by both PDE2 and PDE3, indicating cGMP compartmentation by PDEs affecting CNP-induced responses in failing hearts.

Cardiac PDEs and crosstalk between cAMP and cGMP signalling pathways in the regulation of contractility

Elucidation of cAMP and cGMP signalling in the heart remains a hot topic, and new regulatory mechanisms continue to appear. Studying the influence of phosphodiesterases on 5-HT4 receptor signalling in porcine atrium, a paper from this issue of the journal expands findings of a crosstalk between cardiac cGMP and cAMP signalling recently discovered in failing rat ventricle to a different species and cardiac region. The overall data suggest that cGMP, produced following stimulation of the NPR-B receptor for C-type natriuretic peptide (CNP), inhibits cAMP degradation by phosphodiesterase 3 and thereby enhances cAMP-mediated signalling from β-adrenoceptors and 5-HT4 receptors to inotropic effects. In porcine atrium, this effect can be seen both as an increase in inotropic effect and as a reduced fade of the inotropic effect with time. Thus, accumulating evidence brings together several active fields of research, including cardiac phosphodiesterases, compartmentation of cyclic nucleotide signalling and the field of natriuretic peptides. If present in human hearts, this effect of CNP may have clinical implications.

Synthesis and pharmacological properties of a new hydrophilic and orally bioavailable 5-HT4 antagonist

5-HT4 receptor antagonists have been suggested to have clinical potential in treatment of atrial fibrillation, diarrhea-prone irritable bowel syndrome and urinary incontinence. Recently, the use of 5-HT4 antagonists has been suggested to have a therapeutic benefit in heart failure. Affinity for the hERG potassium ion channel and increased risk for prolonged QT intervals and arrhythmias has been observed for several 5-HT4 ligands. Serotonin may also have beneficial effects in the central nervous system (CNS) through stimulation of the 5-HT4 receptor, and reduced distribution of 5-HT4 antagonists to the CNS may therefore be an advantage. Replacing the amide and N-butyl side chain of the 5-HT4 receptor antagonist SB207266 with an ester and a benzyl dimethyl acetic acid group led to compound 9; a hydrophilic 5-HT4 antagonist with excellent receptor binding and low affinity for the hERG potassium ion channel. To increase oral bioavailability of carboxylic acid 9, two different prodrug approaches were applied. The tert-butyl prodrug 11 did not improve bioavailability, and LC-MS analysis revealed unmetabolized prodrug in the systemic circulation. The medoxomil ester prodrug 10 showed complete conversion and sufficient bioavailability of 9 to advance into further preclinical testing for treatment of heart failure.

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.

Agents increasing cyclic GMP amplify 5-HT4-elicited positive inotropic response in failing rat cardiac ventricle

Activation of 5-HT₄ receptors in failing ventricles elicits a cAMP-dependent positive inotropic response which is mainly limited by the cGMP-inhibitable phosphodiesterase (PDE) 3. However, PDE4 plays an additional role which is demasked by PDE3 inhibition. The objective of this study was to evaluate the effect of cGMP generated by particulate and soluble guanylyl cyclase (GC) on the 5-HT₄-mediated inotropic response. Extensive myocardial infarctions were induced by coronary artery ligation in Wistar rats, exhibiting heart failure 6 weeks after surgery. Contractility was measured in left ventricular preparations. Cyclic GMP was measured by EIA. In ventricular preparations, ANP or BNP displayed no impact on 5-HT₄-mediated inotropic response. However, CNP increased the 5-HT₄-mediated inotropic response as well as the β₁-adrenoceptor (β₁-AR)-mediated response to a similar extent as PDE3 inhibition by cilostamide. Pretreatment with cilostamide eliminated the effect of CNP. Inhibition of nitric oxide (NO) synthase and soluble GC by l-NAME and ODQ, respectively, attenuated the 5-HT₄-mediated inotropic response, whereas the NO donor Sin-1 increased this response. The effects were absent during PDE3 inhibition, suggesting cGMP-dependent inhibition of PDE3. However, in contrast to the effects on the 5-HT₄ response, Sin-1 inhibited whereas l-NAME and ODQ enhanced the β₁-AR-mediated inotropic response. cGMP generated both by particulate (NPR-B) and soluble GC increases the 5-HT₄-mediated inotropic response in failing hearts, probably through inhibition of PDE3. β₁-AR and 5-HT₄ receptor signalling are subject to opposite regulatory control by cGMP generated by soluble GC in failing hearts. Thus, cGMP from different sources is functionally compartmented, giving differential regulation of different G_s-coupled receptors.

Synthesis and pharmacological properties of novel hydrophilic 5-HT4 receptor antagonists

Serotonin (5-hydroxytryptamine, 5-HT) is an important signalling molecule in the human body. The 5-HT(4) serotonin receptor, coupled to the G protein G(s), plays important physiological and pathophysiological roles in the heart, urinary bladder, gastrointestinal tract and the adrenal gland. Both 5-HT(4) antagonists and agonists have been developed in the aim to treat diseases in these organs. 5-HT(4) agonists might have beneficial effects in the central nervous system (CNS) and therefore, 5-HT(4) antagonists might cause CNS side effects. In this study, we have developed new amphoteric 5-HT(4) antagonists. A series of cyclic indole amide derivatives possessing an oxazine ring and a piperidine alkane carboxylic acid side chain and the corresponding prodrug esters were synthesized and their binding to 5-HT(4) receptors and antagonist properties were evaluated. In addition, an indole ester without the oxazine ring and the corresponding indole amide derivatives were also tested. Octanol-water distribution (LogD(Oct7.4)) was tested for some of the synthesized ligands. The main structure-affinity characteristics of the 5-HT(4) compounds tested were that the prodrug esters show higher affinity than their corresponding free acids, indole esters show higher affinity than the corresponding amides and ligands containing the oxazine ring in the indole skeleton show higher affinity than indole derivatives not containing the ring. One representative prodrug ester and its corresponding free acid were tested for binding on a panel of receptors and showed preserved selectivity for the 5-HT(4) receptor. These new molecules may be useful to target peripheral 5-HT(4) receptors.

The expanded biology of serotonin

Serotonin is perhaps best known as a neurotransmitter that modulates neural activity and a wide range of neuropsychological processes, and drugs that target serotonin receptors are used widely in psychiatry and neurology. However, most serotonin is found outside the central nervous system, and virtually all of the 15 serotonin receptors are expressed outside as well as within the brain. Serotonin regulates numerous biological processes including cardiovascular function, bowel motility, ejaculatory latency, and bladder control. Additionally, new work suggests that serotonin may regulate some processes, including platelet aggregation, by receptor-independent, transglutaminase-dependent covalent linkage to cellular proteins. We review this new "expanded serotonin biology" and discuss how drugs targeting specific serotonin receptors are beginning to help treat a wide range of diseases.

Effect of piboserod, a 5-HT4 serotonin receptor antagonist, on left ventricular function in patients with symptomatic heart failure

AIMS

Myocardial 5-HT(4) serotonin (5-HT) receptors are increased and activated in heart failure (HF). Blockade of 5-HT(4) receptors reduced left ventricular (LV) remodelling in HF rats. We evaluated the effect of piboserod, a potent, selective, 5-HT(4) serotonin receptor antagonist, on LV function in patients with HF.

METHODS AND RESULTS

This was a prospective, double-blind, parallel group trial in patients with NYHA class II-IV HF and LV ejection fraction (EF) < or =0.35. Patients receiving standard HF treatment were randomized to placebo (n = 70) or piboserod 80 mg (n = 67) for 24 weeks including 4 weeks up titration. The primary endpoint was LVEF measured by cardiac magnetic resonance imaging (MRI). Secondary endpoints were LV volumes, N-terminal pro-brain natriuretic peptide, norepinephrine, quality of life, and 6 min walk test. Piboserod significantly increased LVEF by 1.7% vs. placebo (CI 0.3, 3.2, P = 0.020), primarily through reduced end-systolic volume from 165 to 158 mL (P = 0.060). There was a trend for greater increase in LVEF (2.7%, CI -1.1, 6.6, P = 0.15) in a small subset of patients not on chronic beta-blocker therapy. There was no significant effect on neurohormones, quality of life, or exercise tolerance. Patients on piboserod reported more adverse events, but numbers were too small to identify specific safety issues.

CONCLUSION

Although patients with chronic HF had a small but significant improvement in LVEF when treated with piboserod for 24 weeks, the result was not reflected in significant changes in other efficacy parameters, and its clinical relevance remains uncertain.

5-HT4-elicited positive inotropic response is mediated by cAMP and regulated by PDE3 in failing rat and human cardiac ventricles

BACKGROUND AND PURPOSE

The left ventricle in failing hearts becomes sensitive to 5-HT parallelled by appearance of functional G(s)-coupled 5-HT(4) receptors. Here, we have explored the regulatory functions of phosphodiesterases in the 5-HT(4) receptor-mediated functional effects in ventricular muscle from failing rat and human heart.

EXPERIMENTAL APPROACH

Extensive myocardial infarctions were induced by coronary artery ligation in Wistar rats. Contractility was measured in left ventricular papillary muscles of rat, 6 weeks after surgery and in left ventricular trabeculae from explanted human hearts. cAMP was quantified by RIA.

KEY RESULTS

In papillary muscles from postinfarction rat hearts, 5-HT(4) stimulation exerted positive inotropic and lusitropic effects and increased cAMP. The inotropic effect was increased by non-selective PDE inhibition (IBMX, 10 microM) and selective inhibition of PDE3 (cilostamide, 1 microM), but not of PDE2 (EHNA, 10 microM) or PDE4 (rolipram, 10 microM). Combined PDE3 and PDE4 inhibition enhanced inotropic responses beyond the effect of PDE3 inhibition alone, increased the sensitivity to 5-HT, and also revealed an inotropic response in control (sham-operated) rat ventricle. Lusitropic effects were increased only during combined PDE inhibition. In failing human ventricle, the 5-HT(4) receptor-mediated positive inotropic response was regulated by PDEs in a manner similar to that in postinfarction rat hearts.

CONCLUSIONS AND IMPLICATIONS

5-HT(4) receptor-mediated positive inotropic responses in failing rat ventricle were cAMP-dependent. PDE3 was the main PDE regulating this response and involvement of PDE4 was disclosed by concomitant inhibition of PDE3 in both postinfarction rat and failing human hearts. 5-HT, PDE3 and PDE4 may have pathophysiological functions in heart failure.

Effects of serotonin in failing cardiac ventricle: signalling mechanisms and potential therapeutic implications

Previously, cardioexcitation by serotonin (5-hydroxytryptamine, 5-HT) was believed to be confined to atria in mammals including man, and mediated through 5-HT(4) receptors in pig and man, but 5-HT(2A) receptors in rat. Recent studies, reviewed here, demonstrate that functional 5-HT(4) receptors can be revealed in porcine and human ventricular myocardium during phosphodiesterase inhibition, and that 5-HT(4) receptor mRNA is increased in human heart failure. In rats, functional 5-HT(4) and 5-HT(2A) receptors appear in the cardiac ventricle during heart failure and mediate inotropic responses through different mechanisms. 5-HT(2A) receptor signalling resembles that from alpha(1)-adrenoceptors and causes inotropic effects through increased myosin light chain phosphorylation, resulting in Ca(2+) sensitisation. 5-HT(4) receptor signalling resembles that from beta-adrenoceptors and causes inotropic effects through a pathway involving cAMP and PKA-mediated phosphorylation of proteins involved in Ca(2+) handling, resulting in enhanced contractility through increased Ca(2+) availability. Cyclic AMP generated through 5-HT(4) receptor stimulation seems more efficiently coupled to increased contractility than cAMP generated through beta-adrenoceptor stimulation. Increasing contractility through cAMP is considered less energy efficient than Ca(2+) sensitisation and this may be one reason why beta-adrenoceptor antagonism is beneficial in heart failure patients. Treatment of heart failure rats with the 5-HT(4) antagonist SB207266 (piboserod) resulted in potentially beneficial effects, although small. Further studies are needed to clarify if such treatment will be useful for patients with heart failure.

Serotonin increases L-type Ca2+ current and SR Ca2+ content through 5-HT4 receptors in failing rat ventricular cardiomyocytes

Rats with congestive heart failure (CHF) develop ventricular inotropic responsiveness to serotonin (5-HT), mediated through 5-HT2A and 5-HT4 receptors. Human ventricle is similarly responsive to 5-HT through 5-HT4 receptors. We studied isolated ventricular cardiomyocytes to clarify the effects of 5-HT on intracellular Ca2+ handling. Left-ventricular cardiomyocytes were isolated from male Wistar rats 6 wk after induction of postinfarction CHF. Contractile function and Ca2+ transients were measured in field-stimulated cardiomyocytes, and L-type Ca2+ current ( ICa,L) and sarcoplasmic reticulum (SR) Ca2+ content were measured in voltage-clamped cells. Protein phosphorylation was measured by Western blotting or phosphoprotein gel staining. 5-HT4- and 5-HT2A-receptor stimulation induced a positive inotropic response of 33 and 18% (both P < 0.05) and also increased the Ca2+ transient (44 and 6%, respectively; both P < 0.05). ICa,L and SR Ca2+ content increased only after 5-HT4-receptor stimulation (57 and 65%; both P < 0.05). Phospholamban serine16 (PLB-Ser16) and troponin I phosphorylation increased by 26 and 13% after 5-HT4-receptor stimulation ( P < 0.05). 5-HT2A-receptor stimulation increased the action potential duration and did not significantly change the phosphorylation of PLB-Ser16 or troponin I, but it increased myosin light chain 2 (MLC2) phosphorylation. In conclusion, the positive inotropic response to 5-HT4 stimulation results from increased ICa,L and increased phosphorylation of PLB-Ser16, which increases the SR Ca2+ content. 5-HT4 stimulation is thus, like β-adrenoceptor stimulation, possibly energetically unfavorable in CHF. 5-HT2A-receptor stimulation, previously studied in acute CHF, induces a positive inotropic response also in chronic CHF, probably mediated by MLC2 phosphorylation.

Cardiovascular responses produced by 5-hydroxytriptamine:a pharmacological update on the receptors/mechanisms involved and therapeutic implications

The complexity of cardiovascular responses produced by 5-hydroxytryptamine (5-HT, serotonin), including bradycardia or tachycardia, hypotension or hypertension, and vasodilatation or vasoconstriction, has been explained by the capability of this monoamine to interact with different receptors in the central nervous system (CNS), on the autonomic ganglia and postganglionic nerve endings, on vascular smooth muscle and endothelium, and on the cardiac tissue. Depending, among other factors, on the species, the vascular bed under study, and the experimental conditions, these responses are mainly mediated by 5-HT(1), 5-HT(2), 5-HT(3), 5-HT(4), 5-ht(5A/5B), and 5-HT(7) receptors as well as by a tyramine-like action or unidentified mechanisms. It is noteworthy that 5-HT(6) receptors do not seem to be involved in the cardiovascular responses to 5-HT. Regarding heart rate, intravenous (i.v.) administration of 5-HT usually lowers this variable by eliciting a von Bezold-Jarisch-like reflex via 5-HT(3) receptors located on sensory vagal nerve endings in the heart. Other bradycardic mechanisms include cardiac sympatho-inhibition by prejunctional 5-HT(1B/1D) receptors and, in the case of the rat, an additional 5-ht(5A/5B) receptor component. Moreover, i.v. 5-HT can increase heart rate in different species (after vagotomy) by a variety of mechanisms/receptors including activation of: (1) myocardial 5-HT(2A) (rat), 5-HT(3) (dog), 5-HT(4) (pig, human), and 5-HT(7) (cat) receptors; (2) adrenomedullary 5-HT(2) (dog) and prejunctional sympatho-excitatory 5-HT(3) (rabbit) receptors associated with a release of catecholamines; (3) a tyramine-like action mechanism (guinea pig); and (4) unidentified mechanisms (certain lamellibranch and gastropod species). Furthermore, central administration of 5-HT can cause, in general, bradycardia and/or tachycardia mediated by activation of, respectively, 5-HT(1A) and 5-HT(2) receptors. On the other hand, the blood pressure response to i.v. administration of 5-HT is usually triphasic and consists of an initial short-lasting vasodepressor response due to a reflex bradycardia (mediated by 5-HT(3) receptors located on vagal afferents, via the von Bezold-Jarisch-like reflex), a middle vasopressor phase, and a late, longer-lasting, vasodepressor response. The vasopressor response is a consequence of vasoconstriction mainly mediated by 5-HT(2A) receptors; however, vasoconstriction in the canine saphenous vein and external carotid bed as well as in the porcine cephalic arteries and arteriovenous anastomoses is due to activation of 5-HT(1B) receptors. The late vasodepressor response may involve three different mechanisms: (1) direct vasorelaxation by activation of 5-HT(7) receptors located on vascular smooth muscle; (2) inhibition of the vasopressor sympathetic outflow by sympatho-inhibitory 5-HT(1A/1B/1D) receptors; and (3) release of endothelium-derived relaxing factor (nitric oxide) by 5-HT(2B) and/or 5-HT(1B/1D) receptors. Furthermore, central administration of 5-HT can cause both hypotension (mainly mediated by 5-HT(1A) receptors) and hypertension (mainly mediated by 5-HT(2) receptors). The increasing availability of new compounds with high affinity and selectivity for the different 5-HT receptor subtypes makes it possible to develop drugs with potential therapeutic usefulness in the treatment of some cardiovascular illnesses including hypertension, migraine, some peripheral vascular diseases, and heart failure.