Effects of psilocin and psilocybin on human 5-HT4 serotonin receptors in atrial preparations of transgenic mice and humans.

Several fungi belonging to the genus Psilocybe, also called "magic mushrooms", contain the hallucinogenic drugs psilocybin and psilocin. They are chemically related to serotonin (5-HT). In addition to being abused as drugs, they are now also being discussed or used as a treatment option for depression. Here, we hypothesized that psilocybin and psilocin may act also on cardiac serotonin receptors and studied them in vitro in atrial preparations of our transgenic mouse model with cardiac myocytes-specific overexpression of the human 5-HT4 receptor (5-HT4-TG) as well as in human atrial preparations. Both psilocybin and psilocin enhanced the force of contraction in isolated left atrial preparations from 5-HT4-TG, increased the beating rate in isolated spontaneously beating right atrial preparations from 5-HT4-TG and augmented the force of contraction in the human atrial preparations. The inotropic and chronotropic effects of psilocybin and psilocin at 10 µM were smaller than that of 1 µM 5-HT on the left and right atria from 5-HT4-TG, respectively. Psilocybin and psilocin were inactive in WT. In the human atrial preparations, inhibition of the phosphodiesterase III by cilostamide was necessary to unmask the positive inotropic effects of psilocybin or psilocin. The effects of 10 µM psilocybin and psilocin were abrogated by 10 µM tropisetron or by 1 µM GR125487, a more selective 5-HT4 receptor antagonist. In summary, we demonstrated that psilocin and psilocybin act as agonists on cardiac 5-HT4 receptors.

The Contractile Machines of the Heart.

This chapter will describe basic structural and functional features of the contractile apparatus of muscle cells of the heart, namely, cardiomyocytes and smooth muscle cells. Cardiomyocytes form the contractile myocardium of the heart, while smooth muscle cells form the contractile coronary vessels. Both muscle types have distinct properties and will be considered with respect to their cellular appearance (brick-like cross-striated versus spindle-like smooth), arrangement of contractile proteins (sarcomeric versus non-sarcomeric organization), calcium activation mechanisms (thin-filament versus thick-filament regulation), contractile features (fast and phasic versus slow and tonic), energy metabolism (high oxygen versus low oxygen demand), molecular motors (type II myosin isoenzymes with high adenosine diphosphate [ADP]-release rate versus myosin isoenzymes with low ADP-release rates), chemomechanical energy conversion (high adenosine triphosphate [ATP] consumption and short duty ratio versus low ATP consumption and high duty ratio of myosin II cross-bridges [XBs]), and excitation-contraction coupling (calcium-induced calcium release versus pharmacomechanical coupling). Part of the work has been published (Neuroscience - From Molecules to Behavior", Chap. 22, Galizia and Lledo eds 2013, Springer-Verlag; with kind permission from Springer Science + Business Media).

Pacemaking in the lymphatic system.

Lymphatic collecting vessels exhibit spontaneous phasic contractions that are critical for lymph propulsion and tissue fluid homeostasis. This rhythmic activity is driven by action potentials conducted across the lymphatic muscle cell (LMC) layer to produce entrained contractions. The contraction frequency of a lymphatic collecting vessel displays exquisite mechanosensitivity, with a dynamic range from <1 to >20 contractions per minute. A myogenic pacemaker mechanism intrinsic to the LMCs was initially postulated to account for pressure-dependent chronotropy. Further interrogation into the cellular constituents of the lymphatic vessel wall identified non-muscle cell populations that shared some characteristics with interstitial cells of Cajal, which have pacemaker functions in the gastrointestinal and lower urinary tracts, thus raising the possibility of a non-muscle cell pacemaker. However, recent genetic knockout studies in mice support LMCs and a myogenic origin of the pacemaker activity. LMCs exhibit stochastic, but pressure-sensitive, sarcoplasmic reticulum calcium release (puffs and waves) from IP3R1 receptors, which couple to the calcium-activated chloride channel Anoctamin 1, causing depolarisation. The resulting electrical activity integrates across the highly coupled lymphatic muscle electrical syncytia through connexin 45 to modulate diastolic depolarisation. However, multiple other cation channels may also contribute to the ionic pacemaking cycle. Upon reaching threshold, a voltage-gated calcium channel-dependent action potential fires, resulting in a nearly synchronous calcium global calcium flash within the LMC layer to drive an entrained contraction. This review summarizes the key ion channels potentially responsible for the pressure-dependent chronotropy of lymphatic collecting vessels and various mechanisms of IP3R1 regulation that could contribute to frequency tuning.

Unveiling a Rare Side Effect: A Report of a Unique Case of Second-Degree Type 2 Sinoatrial Node Exit Block Induced by Adenosine Infusion.

Adenosine is a widely used pharmacologic agent in the field of cardiology, predominantly for the termination of supraventricular tachycardias and diagnostic purposes. Most of the side effects are short-lasting due to its very short half-life. Fatal complications of adenosine are rare but can include ventricular fibrillation, ventricular tachycardia, and asystole. Proper medical supervision and monitoring are crucial to minimize risks. We report a unique case of a second-degree type 2 sinoatrial node exit block following intravenous adenosine administration in a 25-year-old male presenting with palpitations.

Lysergic acid diethylamide stimulates cardiac human H2 histamine and cardiac human 5-HT4-serotonin receptors.

Lysergic acid diethylamide (LSD) is an artificial hallucinogenic drug. Thus, we hypothesized that LSD might act 5-HT4 serotonin receptors and/or H2 histamine receptors. We studied isolated electrically stimulated left atrial preparations, spontaneously beating right atrial preparations, and spontaneously beating Langendorff-perfused hearts from transgenic mice with cardiomyocyte-specific overexpression of the human 5-HT4 receptor (5-HT4-TG) or of the H2-histamine receptor (H2-TG). For comparison, we used wild type littermate mice (WT). Finally, we measured isometric force of contraction in isolated electrically stimulated muscle strips from the human right atrium obtained from patients during bypass surgery. LSD (up to 10 µM) concentration dependently increased force of contraction and beating rate in left or right atrial preparations from 5-HT4-TG (n = 6, p < 0.05) in 5-HT4-TG atrial preparations. The inotropic and chronotropic effects of LSD were antagonized by 10 µM tropisetron in 5-HT4-TG. In contrast, LSD (10 µM) increased force of contraction and beating rate in left or right atrial preparations, from H2-TG. After pre-stimulation with cilostamide (1 µM), LSD (10 µM) increased force of contraction in human atrial preparations (n = 6, p < 0.05). The contractile effects of LSD in human atrial preparations could be antagonized by 10 µM cimetidine and 1 µM GR 125487. LSD leads to H2-histamine receptor and 5-HT4-receptor mediated cardiac effects in humans.

Methamphetamine increases force of contraction in isolated human atrial preparations through the release of noradrenaline.

We measured the cardiac contractile effects of the sympathomimetic amphetamine-like drug methamphetamine alone and in the presence of cocaine or propranolol in human atrial preparations. For a more comprehensive analysis, we also examined the effects of methamphetamine in preparations from the left and right atria of mice and, for comparison, analyzed the cardiac effects of amphetamine itself. In human atrial preparations, methamphetamine and amphetamine increased the contractile force, the relaxation rate, and the rate of tension development, and shortened the time to maximum tension and the time to relaxation. Likewise, in mice preparations, methamphetamine and amphetamine increased the contractile force in the left atrium and increased the beating rate in the right atrium. The effect in human atrial preparations started at 1 µM, therefore methamphetamine was less effective and potent than isoproterenol in increasing contractile force. These positive inotropic effects of methamphetamine were greatly attenuated by 10 µM cocaine and abolished by 10 µM propranolol. The inotropic effects of methamphetamine in human atrial preparations were associated with, and are believed to be mediated at least in part by, an increase in the phosphorylation state of the inhibitory subunit of troponin. In conclusion, the sympathomimetic central stimulant drug methamphetamine (as well as amphetamine) increased contractile force and protein phosphorylation, presumably through a release of noradrenaline in isolated human atrial preparations. Thus, methamphetamine acts as an indirect sympathomimetic in the human atrium.

Negative Chronotropic Cardiovascular Changes in Lumbar Spine Surgery: A Potential Spinal-Cardiac Reflex?

Cardiovascular changes following lumbar spine surgery in a prone position are exceedingly rare. Over the past 20 years, a total of six cases have been published where patients experienced varying degrees of bradycardia, hypotension, and asystole, which could be attributed to intraoperative dural manipulation. As such, there is emerging evidence for a potential neural-mediated spinal-cardiac reflex. The authors report their experience of negative chronotropy during an elective lumbar spine surgery that coincided with dural manipulation and review the available literature. A 34-year-old male presented with a long-standing history of lower back pain recently deteriorating to bilaterally radiating leg pain, with restricted left leg raise, and numbness at the left L5 dermatomal territory. The patient was an athletic police officer with no comorbidities or past medical history. Magnetic resonance imaging lumbosacral spine revealed spinal stenosis most pronounced at L4/L5 and disc bulges at L3/L4 and L5/S1. The patient opted for lumbar decompression surgery. After an unremarkable comprehensive preoperative workup, including cardiac evaluation (electrocardiogram, echocardiogram), the patient was induced general anesthesia in a prone position. A lumbar incision was made from L2 to S1. When the left L4 nerve root was retracted while removing the prolapsed disc at L4/L5, the anesthetist cautioned the surgeon of bradycardia (34 beats per minute [bpm]), and the surgery was immediately stopped. The heart rate improved to 60 bpm within 30 seconds. When the root was later retracted again, a second episode of bradycardia occurred for 4 minutes with heart rate declining to 48 bpm. The surgery was stopped, and after 4 minutes, the anesthetist administered 600 µg of atropine. The heart rate then rose to 73 bpm within 1 minute. Other potential causes for bradycardia were excluded. The total blood loss was estimated to be 100 mL. He remains well at his 6-month follow-up and has returned to work as normal. Akin to previously published cases, each episode of bradycardia coincided with dural manipulation, which may indicate a possible reflex between the spinal dura mater and the cardiovascular system. Such a rare adverse event may occur even in seemingly healthy, young individuals, and anesthetists should caution the operating surgeon of bradycardias to exclude operative manipulation of the dura as the cause. While this phenomenon is only reported in a handful of lumbar spine surgery cases, it provides evidence for a potential spinal-cardiac physiological reflex in the lumbar spine that may be neural mediated and should be investigated further.

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.

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.

Increased nitric oxide availability worsens the cardiac performance during early re-perfusion period in adult rats.

OBJECTIVES: Re-perfusion is the standard therapy for acute myocardial infarction, despite the associated pathologies that may contribute to irreversible myocardial injury. The present study aims to clarify the alterations in cardiac activities in response to experimental cardiac ischemic arrest followed by re-perfusion in isolated hearts perfused with nitric oxide (NO) donor, l-arginine, or NO inhibitor, Nω-Nitro-l-arginine methyl ester hydrochloride (l-NAME), to shed light on the possible role of NO in the re-perfusion process. METHODS: Hearts isolated from adult Wistar rats were studied on Langendorff preparation under basal conditions and during 30 min re-perfusion following 30 min of total global ischemia. Rats were randomly divided into three groups; control and l-arginine or l-NAME infused heart groups. Cardiac tissue content of malondialdhyde, catalase and nitrite was also measured. RESULTS: Compared to the control group, both l-arginine and l-NAME infused hearts showed increased basal chronotropy and myocardial flow rate. Following ischemia and during the whole period of re-perfusion, the three groups demonstrated significant deterioration in the inotropic activity and compromised myocardial flow rate. l-arginine infused hearts revealed depressed inotropy and chronotropy, weak systolic and diastolic functions with compromised myocardial flow at early 5 min of re-perfusion, yet with significantly higher myocardial flow rate by the end of re-perfusion. CONCLUSIONS: Reducing NO availability by l-NAME revealed mild impact on the ischemia re-perfusion induced contractile dysfunction, whereas excess NO worsens cardiac performance at the early re-perfusion period.

Levosimendan increases the phosphorylation state of phospholamban in the isolated human atrium.

Levosimendan (up to 10 µM) given alone failed to increase force of contraction in isolated electrically stimulated (1 Hz) left atrial (LA) preparations from wild-type mice. Only in the additional presence of 0.1 µM rolipram, an inhibitor of the activity of phosphodiesterase IV, levosimendan increased force of contraction in LA and increased the phosphorylation state of phospholamban at amino acid serine 16. Levosimendan alone increased the beating rate in isolated spontaneously beating right atrial preparations from mice and this effect was potentiated by rolipram. The positive inotropic and the positive chronotropic effects of levosimendan in mouse atrial preparations were attenuated by 10 µM propranolol. Finally, we studied the contractile effects of levosimendan in isolated electrically stimulated (1 Hz) right atrial preparations from the human atrium (HAP), obtained during cardiac surgery. We detected concentration-dependent positive inotropic effects of levosimendan alone that reached plateau at 1 µM levosimendan in HAP (n = 11). Levosimendan shortened time of tension relaxation in HAP. Cilostamide (1 µM), an inhibitor of phosphodiesterase III, or propranolol (10 µM) blocked the positive inotropic effect of levosimendan in HAP. Levosimendan (1 µM) alone increased in HAP the phosphorylation state of phospholamban. In conclusion, we present evidence that levosimendan acts via phosphodiesterase III inhibition in the human atrium leading to phospholamban phosphorylation and thus explaining the positive inotropic effects of levosimendan in HAP.

Cardiac effects of ephedrine, norephedrine, mescaline, and 3,4-methylenedioxymethamphetamine (MDMA) in mouse and human atrial preparations.

The use of recreational drugs like ephedrine, norephedrine, 3,4-methylenedioxymethamphetamine (MDMA), and mescaline can lead to intoxication and, at worst, to death. One reason for a fatal course of intoxication with these drugs might lie in cardiac arrhythmias. To the best of our knowledge, their inotropic effects have not yet been studied in isolated human cardiac preparations. Therefore, we measured inotropic effects of the hallucinogenic drugs ephedrine, norephedrine, mescaline, and MDMA in isolated mouse left atrial (mLA) and right atrial (mRA) preparations as well as in human right atrial (hRA) preparations obtained during cardiac surgery. Under these experimental conditions, ephedrine, norephedrine, and MDMA increased force of contraction (mLA, hRA) and beating rate (mRA) in a time- and concentration-dependent way, starting at 1-3 µM but these drugs were less effective than isoprenaline. Mescaline alone or in the presence of phosphodiesterase inhibitors did not increase force in mLA or hRA. The positive inotropic effects of ephedrine, norephedrine, or MDMA were accompanied by increases in the rate of tension and relaxation and by shortening of time of relaxation and, moreover, by an augmented phosphorylation state of the inhibitory subunit of troponin in hRA. All effects were greatly attenuated by cocaine (10 µM) or propranolol (10 µM) treatment. In summary, the hallucinogenic drugs ephedrine, norephedrine, and MDMA, but not mescaline, increased force of contraction and increased protein phosphorylation presumably, in part, by a release of noradrenaline in isolated human atrial preparations and thus can be regarded as indirect sympathomimetic drugs in the human atrium.

Chronotropic index during 6-minute walk and acute respiratory events in COPDGene.

BACKGROUND: Lower heart rate (HR) increases during exercise and slower HR recovery (HRR) after exercise are markers of worse autonomic function that may be associated with risk of acute respiratory events (ARE). METHODS: Data from 6-min walk testing (6MWT) in COPDGene were used to calculate the chronotropic index (CI) [(HR immediately post 6MWT - resting HR)/((220 - age) - resting HR)] and HRR at 1 min after 6MWT completion. We used zero-inflated negative binomial regression to test associations of CI and HRR with rates of any ARE (requiring steroids and/or antibiotics) and severe ARE (requiring emergency department visit or hospitalization), among all participants and in spirometry subgroups (normal, chronic obstructive pulmonary disease [COPD], and preserved ratio with impaired spirometry). RESULTS: Among 4,484 participants, mean follow-up time was 4.1 years, and 1,966 had COPD. Among all participants, CI-6MWT was not associated with rate of any ARE [adjusted incidence rate ratio (aIRR) 0.98 (0.95-1.01)], but higher CI-6MWT was associated with lower rate of severe ARE [0.95 (0.92-0.99)]. Higher HRR was associated with a lower rate of both any ARE [0.97 (0.95-0.99)] and severe ARE [0.95 (0.92-0.98)]. Results were similar in the COPD spirometry subgroup. CONCLUSION: Heart rate measures derived from 6MWT tests may have utility in predicting risk of acute respiratory events and COPD exacerbations.

Effect of Clonidine Hydrochloride on Isolated Newborn Rat Heart.

The concentration dependenies of the chronotropic response and changes in blood supply to the isolated heart of 7-day-old newborn rats induced by application of α2-adrenergic receptor agonist clonidine hydrochloride in concentrations of 10-9-10-6 M were revealed. The minimum concentration of α2-adrenergic receptor agonist caused tachycardia, while higher concentrations led to bradycardia. The maximum effect manifesting in a decrease in coronary flow was recorded at the minimum concentration of the agonist, while the highest concentration had no effect on the coronary flow. When comparing these results with those obtained in control adult rats, we found that the most pronounced differences in the chronotropic effects were observed after addition of the minimum concentration of the α2-adrenergic receptor agonist: bradycardia in adult rats and tachycardia in newborns. The maximum differences in coronary flow parameters were observed after addition of α2-adrenergic receptor agonist in the maximum concentration that induced a two-phase response in adult rats and had no effect on the blood supply in newborns.

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

In the past, we generated transgenic mice that overexpress the human histamine 2 (H2)-receptor (H2-TG) or that overexpress the human serotonin 4 (5-HT4)-receptor (5-HT4-TG) in the heart. Here, we crossbred these lines of mice to generate double transgenic mice that overexpress both receptors (DT). This was done to study a conceivable interaction between these receptors in the mouse heart as a model for the human heart. When in left atria, initially, force of contraction was elevated maximally with 1 µM serotonin, and subsequently, histamine was cumulatively applied; a biphasic effect of histamine was noted: the force of contraction initially decreased, maximally at 10 nM histamine, and thereafter, the force of contraction increased again at 1 µM histamine. Notably, functional interaction between 5-HT and histamine was also identified in isolated electrically stimulated trabeculae carneae from human right atrium (obtained during cardiac surgery). These functional and biochemical data together are consistent with a joint overexpression of inotropically active H2-receptors and 5-HT4-receptors in the same mouse heart. We also describe an antagonistic interaction on the force of contraction of both receptors in the mouse atrium (DT) and in the human atrial muscle strips. We speculate that via this interaction, histamine might act as a "brake" on the cardiac actions of 5-HT via inhibitory GTP-binding proteins acting on the activity of adenylyl cyclase.

The proarrhythmic effects of hypothermia in atria isolated from 5-HT4-receptor-overexpressing mice.

We investigated whether hypothermia would be arrhythmogenic in mice that overexpress the human 5-HT4 receptor only in their cardiac myocytes (5-HT4-TG). Contractile studies were performed in isolated, electrically driven (1 Hz) left and spontaneously beating right atrial preparations of 5-HT4-TG and littermate wild-type control mice (WT). Hypothermia (23 °C) decreased the force of contraction in the mouse right and left atrial preparations. Moreover, the concentration-dependent positive inotropic effects of 5-HT were blunted but still shifted to lower 5-HT concentrations in the left 5-HT4-TG atria in hypothermia compared to normothermia (37 °C). Furthermore, hypothermia increased the incidence of right atrial arrhythmias in 5-HT4-TG more than in WT mice. In contrast, at 37 °C, lowering the potassium concentration from 5.2 to 2.0 mM also induced arrhythmias in the right atrium, but with a similar incidence in WT and 5-HT4-TG mice. In contrast, 10 µM d,l-sotalol and 300 µM erythromycin did not induce arrhythmias. Hypothermia was accompanied by the increased expression of heat shock protein 70 (HSP70) in WT but not in 5-HT4-TG mice. We concluded that without the stimulation of 5-HT4-receptors by exogenous agonists, a simple temperature reduction can increase arrhythmias in 5-HT4-TG mice. It is tempting to speculate that in human patients, 5-HT4 receptors might contribute to potentially deadly hypothermia-induced arrhythmias.

Histamine can be Formed and Degraded in the Human and Mouse Heart.

Histamine is metabolized by several enzymes in vitro and in vivo. The relevance of this metabolism in the mammalian heart in vivo is unclear. However, histamine can exert positive inotropic effects (PIE) and positive chronotropic effects (PCE) in humans via H2-histamine receptors. In transgenic mice (H2-TG) that overexpress the human H2 receptor in cardiomyocytes but not in wild-type littermate mice (WT), histamine induced PIE and PCE in isolated left or right atrial preparations. These H2-TG were used to investigate the putative relevance of histamine degrading enzymes in the mammalian heart. Histidine, the precursor of histamine, increased force of contraction (FOC) in human atrial preparations. Moreover, histamine increased the phosphorylation state of phospholamban in human atrium. Here, we could detect histidine decarboxylase (HDC) and histamine itself in cardiomyocytes of mouse hearts. Moreover, our data indicate that histamine is subject to degradation in the mammalian heart. Inhibition of the histamine metabolizing enzymes diamine oxidase (DAO) and monoamine oxidase (MAO) shifted the concentration response curves for the PIE in H2-TG atria to the left. Moreover, activity of histamine metabolizing enzymes was present in mouse cardiac samples as well as in human atrial samples. Thus, drugs used for other indication (e.g. antidepressants) can alter histamine levels in the heart. Our results deepen our understanding of the physiological role of histamine in the mouse and human heart. Our findings might be clinically relevant because we show enzyme targets for drugs to modify the beating rate and force of the human heart.

Cardiovascular effects of metoclopramide and domperidone on human 5-HT4-serotonin-receptors in transgenic mice and in human atrial preparations.

It is unclear whether metoclopramide and domperidone act on human cardiac serotonin 5-HT4-receptors. Therefore, we studied transgenic mice that only express the human 5-HT4 receptor in cardiomyocytes in the atrium and in the ventricle (5-HT4-TG), their wild type-littermates (WT) and isolated human atrial preparations. We found that only metoclopramide but not domperidone enhanced the force of contraction in left atrial preparations (pEC50 = 6.0 ± 0.1; n = 7) from 5-HT4-TG, isolated spontaneously beating right atrial preparations (pEC50 = 6.1 ± 0.1; n = 7) from 5-HT4-TG, Langendorff perfused hearts from 5-HT4-TG, living 5-HT4-TG and human right atrial muscle preparations obtained during bypass surgery of patients suffering from coronary heart disease. The maximum inotropic effect of metoclopramide was smaller (81 ± 2%) than that of 5-HT on the left atria from 5-HT4-TG. The maximum increase in the beating rate due to metoclopramide was 93 ± 2% of effect of 5-HT on right atrial preparations from 5-HT4-TG. Metoclopramide and domperidone were inactive in WT. We found that metoclopramide but not domperidone increased the phosphorylation state of phospholamban in the isolated perfused hearts or muscle strips of 5-HT4-TG, but not in WT. Metoclopramide, but not domperidone, shifted the positive inotropic or chronotropic effects of 5-HT in isolated left atrial and right atrial preparations from 5-HT4-TG dextrally, resp., to higher concentrations: the pEC50 of 5-HT for increase in force was in the absence of metoclopramide 8.6 ± 0.1 (n = 5) versus 8.0 ± 0.3 in the presence of 1 µM metoclopramide (n = 5; P < 0.05); and the beating rate was 7.8 ± 0.2 (n = 7) in the absence of metoclopramide versus 7.2 ± 0.1 in the presence of 1 µM metoclopramide (n = 6; P < 0.05). These results suggested that metoclopramide had an antagonistic effect on human cardiac 5-HT4 receptors. In summary, we showed that metoclopramide, but not domperidone, was a partial agonist at human cardiac 5-HT4-receptors.

Chronotropic Index and Acute Exacerbations of Chronic Obstructive Pulmonary Disease: A Secondary Analysis of BLOCK COPD.

Rationale: The chronotropic index quantifies the proportion of the expected heart rate increase that is attained during exercise. The relationship between the chronotropic index and acute exacerbations of chronic obstructive pulmonary disease (AECOPDs) has not been evaluated. Objectives: To determine whether a higher chronotropic index during a 6-minute walk (CI-6MW) is associated with lower risk of AECOPD and whether the CI-6MW is a marker of susceptibility to adverse effects of metoprolol in chronic obstructive pulmonary disease (COPD). Methods: We analyzed data from the BLOCK COPD (Beta-Blockers for the Prevention of AECOPDs) trial. We used Cox proportional hazards models to investigate the relationship between the CI-6MW and the time to AECOPDs. We also tested for interactions between study group assignment (metoprolol vs. placebo) and the CI-6MW on the time to AECOPDs. Results: Four hundred seventy-seven participants with exacerbation-prone COPD (mean forced expiratory volume in 1 second, 41% of predicted) were included in this analysis. A higher CI-6MW was independently associated with a decreased risk of AECOPDs of any severity (adjusted hazard ratio per 0.1 increase in CI-6MW of 0.88; 95% confidence interval, 0.80-0.96) but was not independently associated with AECOPDs requiring hospitalization (adjusted hazard ratio, 0.94; 95% confidence interval, 0.81-1.05). There was a significant interaction by treatment assignment, and in a stratified analysis, the protective effects of a higher CI-6MW on AECOPDs were negated by metoprolol use. Conclusions: A higher CI-6MW is associated with a decreased risk of AECOPDs and may be an indicator of susceptibility to the adverse effects of metoprolol.

Phosphodiesterases 2, 3 and 4 can decrease cardiac effects of H2-histamine-receptor activation in isolated atria of transgenic mice.

Histamine exerts cAMP-dependent positive inotropic effects (PIE) and positive chronotropic effects (PCE) on isolated left and right atria, respectively, of transgenic mice which overexpress the human H2-receptor in the heart (=H2-TG). To determine whether these effects are antagonized by phosphodiesterases (PDEs), contractile studies were done in isolated left and right atrial preparations of H2-TG. The contractile effects of histamine were tested in the additional presence of the PDE-inhibitorserythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride (EHNA, 1 µM, PDE2-inhibitor) or cilostamide (1 µM, PDE3-inhibitor), rolipram (10 µM, a PDE4-inhibitor), and their combinations. Cilostamide (1 µM) and EHNA (1 µM), rolipram (1 µM), and EHNA (1 µM) and the combination of rolipram (0.1 µM) and cilostamide (1 µM) each increased the potency of histamine to elevate the force of contraction (FOC) in H2-TG. Cilostamide (1 µM) and rolipram (10 µM) alone increased and EHNA (1 µM) decreased alone, and their combination increased the potency of histamine to increase the FOC in H2-TG indicating that PDE3 and PDE4 regulate the inotropic effects of histamine in H2-TG. The PDE inhibitors (EHNA, cilostamide, rolipram) alone did not alter the potency of histamine to increase the heart beat in H2-TG whereas a combination of rolipram, cilostamide, and EHNA, or of rolipram and EHNA increased the potency of histamine to act on the beating rate. In summary, the data suggest that the PCE of histamine in H2-TG atrium involves PDE 2 and 4 activities, whereas the PIE of histamine are diminished by activity of PDE 3 and 4.