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

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

BACKGROUND AND PURPOSE

Positive inotropic responses (PIR) to 5-hydroxytryptamine (5-HT) are induced in the left ventricle (LV) in rats with congestive heart failure (CHF); this is associated with upregulation of the G(s)-coupled 5-HT(4) receptor. We investigated whether chronic 5-HT(4) receptor blockade improved cardiac function in CHF rats.

EXPERIMENTAL APPROACH

Rats were given either the 5-HT(4) antagonist SB207266 (0.5 mg kg(-1) 24h(-1); MI(int)) or placebo (MI(pl)) through mini-osmotic pumps for 6 weeks subsequent to induction of post-infarction CHF. In vivo cardiac function and ex vivo responses to isoprenaline or 5-HT were evaluated using echocardiography and isolated LV papillary muscles, respectively. mRNA levels were investigated using real-time quantitative RT-PCR.

KEY RESULTS

LV diastolic function improved, with 4.6% lower LV diastolic diameter and 24.2% lower mitral flow deceleration in MI(int) compared to MI(pl). SB207266 reduced LV systolic diameter by 6.1%, heart weight by 10.2% and lung weight by 13.1%. The changes in posterior wall thickening and shortening velocity, cardiac output, LV systolic pressure and (dP/dt)(max), parameters of LV systolic function, did not reach statistical significance. The PIR to isoprenaline (10 microM) increased by 36% and the response to 5-HT (10 microM) decreased by 57% in MI(int) compared to MI(pl). mRNA levels for ANP, 5-HT(4(b)) and 5-HT(2A) receptors, MHCbeta, and the MHCbeta/MHCalpha -ratio were not significantly changed in MI(int) compared to MI(pl).

CONCLUSIONS AND IMPLICATIONS

Treatment with SB207266 to some extent improved in vivo cardiac function and ex vivo myocardial function, suggesting a possible beneficial effect of treatment with a 5-HT(4) receptor antagonist in CHF.

Compartmentalized cAMP signalling is important in the regulation of Ca(2+) cycling in the heart

Co-ordinated myocyte handling of calcium is essential for efficient excitation-contraction coupling in the heart. The calcium cycling activity can be modulated by adrenergic stimulation and subsequent phosphorylation. Important functional consequences of phosphorylation include a greater influx of calcium through the voltage-dependent L-type Ca(2+) channel and a greater release of calcium from SR (sarcoplasmic reticulum) through the ryanodine R2 receptor. Furthermore, a more efficient reuptake through SERCA2 (sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase 2) is a result of phosphorylation of its regulatory protein phospholamban. Compartmentalized signalling is important in this signalling cascade, and A-kinase-anchoring proteins play a central role by providing a high level of specificity.

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.

Increasing synaptic noradrenaline, serotonin and histamine enhances in vivo binding of phosphodiesterase-4 inhibitor (R)-[11C]rolipram in rat brain, lung and heart

Phosphodiesterase-4 (PDE4) is one of the main enzymes that specifically terminate the action of cAMP, thereby contributing to intracellular signaling following stimulation of various G protein-coupled receptors. PDE4 expression and activity are modulated by agents affecting cAMP levels. The selective PDE4 inhibitor (R)-rolipram labeled with C-11 was tested in vivo in rats to analyze changes in PDE4 levels following drug treatments that increase synaptic noradrenaline (NA), serotonin (5HT), histamine (HA) and dopamine (DA) levels. We hypothesized that increasing synaptic neurotransmitter levels and subsequent cAMP-mediated signaling would significantly enhance (R)-[(11)C]rolipram retention and specific binding to PDE4 in vivo. Pre-treatments were performed 3 h prior to tracer injection, and rats were sacrificed 45 min later. Biodistribution studies revealed a dose-dependent increase in (R)-[(11)C]rolipram uptake following administration of the monoamine oxidase (MAO) inhibitor tranylcypromine, NA and 5HT reuptake inhibitors (desipramine [DMI], maprotiline; and fluoxetine, sertraline, respectively), and the HA H(3) receptor antagonist (thioperamide), but not with DA transporter blockers GBR 12909, cocaine or DA D(1) agonist SKF81297. Significant increases in rat brain and heart reflect changes in PDE4 specific binding (total-non-specific binding [coinjection with saturating dose of (R)-rolipram]). These results demonstrate that acute treatments elevating synaptic NA, 5HT and HA, but not DA levels, significantly enhance (R)-[(11)C]rolipram binding. Use of (R)-[(11)C]rolipram and positron emission tomography as an index of PDE4 activity could provide insight into understanding disease states with altered NA, 5HT and HA concentrations.

Prucalopride is a partial agonist through human and porcine atrial 5-HT4 receptors: comparison with recombinant human 5-HT4 splice variants

Prucalopride is a gastrointestinal prokinetic drug that acts through 5-HT4 receptors, but its potential effects on cardiac atrial function are unknown. We investigated the effects of prucalopride on human right atrium, piglet left atrium, and piglet sinoatrial node. The effects of prucalopride on 5-HT4 receptor splice variants a, b, g and i, known to be expressed in human atrium, were studied for comparison. Prucalopride was an inotropic partial agonist, compared with 5-HT, on paced human atrial trabeculae (-logEC50M=7.4) and porcine left atria (-logEC50M=7.2), with intrinsic activity of 0.77 and 0.63 respectively. Prucalopride (1 microM) surmountably antagonized the positive inotropic effects of 5-HT on human (pK(P)=7.2) and porcine (pK(P)=7.1) atrium. Prucalopride was also a chronotropic partial agonist (-logEC50M=7.4, intrinsic activity=0.72 with respect to 5-HT) on spontaneously beating piglet atria. The cardiostimulant effects of prucalopride were prevented by GR113808 (1 microM), consistent with mediation through 5-HT4 receptors. Prucalopride bound to recombinant 5-HT4(a), 5-HT4(b), 5-HT4(g), and 5-HT4(i) receptors, labeled by [3H]GR113808, with pKi values of 7.6, 7.5, 7.4, and 7.8 respectively. Prucalopride stimulated adenylyl cyclase as a partial agonist on 5-HT4(a), 5-HT4(b), and 5-HT4(i) receptors with intrinsic activities of 0.82, 0.86, and 0.78 and -logEC50 values of 7.2, 7.3, and 7.2 respectively. At the 5-HT4(g) receptor prucalopride acted as a full agonist (-logEC50M=8.0) compared with 5-HT in the cell line tested, which was probably due to high receptor expression levels. We conclude that prucalopride is a cardiostimulatory partial agonist through human and porcine 5-HT4 receptors. Since prucalopride acts similarly through 5-HT4(a), 5-HT4(b), 5-HT4(g), and 5-HT4(i) receptors, any of these variants could be involved in the mediation of cardiostimulation.