Functional role of cholinoceptors and purinoceptors in human isolated atrial and ventricular heart muscle

Acetylcholine Reduces L-Type Calcium Current without Major Changes in Repolarization of Canine and Human Purkinje and Ventricular Tissue

Vagal nerve stimulation (VNS) holds a strong basis as a potentially effective treatment modality for chronic heart failure, which explains why a multicenter VNS study in heart failure with reduced ejection fraction is ongoing. However, more detailed information is required on the effect of acetylcholine (ACh) on repolarization in Purkinje and ventricular cardiac preparations to identify the advantages, risks, and underlying cellular mechanisms of VNS. Here, we studied the effect of ACh on the action potential (AP) of canine Purkinje fibers (PFs) and several human ventricular preparations. In addition, we characterized the effects of ACh on the L-type Ca²⁺ current (I_CaL) and AP of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and performed computer simulations to explain the observed effects. Using microelectrode recordings, we found a small but significant AP prolongation in canine PFs. In the human myocardium, ACh slightly prolonged the AP in the midmyocardium but resulted in minor AP shortening in subepicardial tissue. Perforated patch-clamp experiments on hiPSC-CMs demonstrated that 5 µM ACh caused an ≈15% decrease in I_CaL density without changes in gating properties. Using dynamic clamp, we found that under blocked K⁺ currents, 5 µM ACh resulted in an ≈23% decrease in AP duration at 90% of repolarization in hiPSC-CMs. Computer simulations using the O'Hara-Rudy human ventricular cell model revealed that the overall effect of ACh on AP duration is a tight interplay between the ACh-induced reduction in I_CaL and ACh-induced changes in K⁺ currents. In conclusion, ACh results in minor changes in AP repolarization and duration of canine PFs and human ventricular myocardium due to the concomitant inhibition of inward I_CaL and outward K⁺ currents, which limits changes in net repolarizing current and thus prevents major changes in AP repolarization.

Acetylcholine Reduces IKr and Prolongs Action Potentials in Human Ventricular Cardiomyocytes

Vagal nerve stimulation (VNS) has a meaningful basis as a potentially effective treatment for heart failure with reduced ejection fraction. There is an ongoing VNS randomized study, and four studies are completed. However, relatively little is known about the effect of acetylcholine (ACh) on repolarization in human ventricular cardiomyocytes, as well as the effect of ACh on the rapid component of the delayed rectifier K⁺ current (I_Kr). Here, we investigated the effect of ACh on the action potential parameters in human ventricular preparations and on I_Kr in human induced pluripotent stem-cell-derived cardiomyocytes (hiPSC-CMs). Using standard microelectrode technique, we demonstrated that ACh (5 µM) significantly increased the action potential duration in human left ventricular myocardial slices. ACh (5 µM) also prolonged repolarization in a human Purkinje fiber and a papillary muscle. Optical mapping revealed that ACh increased the action potential duration in human left ventricular myocardial slices and that the effect was dose-dependent. Perforated patch clamp experiments demonstrated action potential prolongation and a significant decrease in I_Kr by ACh (5 µM) in hiPSC-CMs. Computer simulations of the electrical activity of a human ventricular cardiomyocyte showed an increase in action potential duration upon implementation of the experimentally observed ACh-induced changes in the fully activated conductance and steady-state activation of I_Kr. Our findings support the hypothesis that ACh can influence the repolarization in human ventricular cardiomyocytes by at least changes in I_Kr.

Considerations for an In Vitro, Cell-Based Testing Platform for Detection of Adverse Drug-Induced Inotropic Effects in Early Drug Development. Part 1: General Considerations for Development of Novel Testing Platforms

Drug-induced effects on cardiac contractility can be assessed through the measurement of the maximal rate of pressure increase in the left ventricle (LVdP/dt_max) in conscious animals, and such studies are often conducted at the late stage of preclinical drug development. Detection of such effects earlier in drug research using simpler, in vitro test systems would be a valuable addition to our strategies for identifying the best possible drug development candidates. Thus, testing platforms with reasonably high throughput, and affordable costs would be helpful for early screening purposes. There may also be utility for testing platforms that provide mechanistic information about how a given drug affects cardiac contractility. Finally, there could be in vitro testing platforms that could ultimately contribute to the regulatory safety package of a new drug. The characteristics needed for a successful cell or tissue-based testing platform for cardiac contractility will be dictated by its intended use. In this article, general considerations are presented with the intent of guiding the development of new testing platforms that will find utility in drug research and development. In the following article (part 2), specific aspects of using human-induced stem cell-derived cardiomyocytes for this purpose are addressed.

Low Resting Membrane Potential and Low Inward Rectifier Potassium Currents Are Not Inherent Features of hiPSC-Derived Cardiomyocytes

Human induced pluripotent stem cell (hiPSC) cardiomyocytes (CMs) show less negative resting membrane potential (RMP), which is attributed to small inward rectifier currents (IK1). Here, IK1 was measured in hiPSC-CMs (proprietary and commercial cell line) cultured as monolayer (ML) or 3D engineered heart tissue (EHT) and, for direct comparison, in CMs from human right atrial (RA) and left ventricular (LV) tissue. RMP was measured in isolated cells and intact tissues. IK1 density in ML- and EHT-CMs from the proprietary line was similar to LV and RA, respectively. IK1 density in EHT-CMs from the commercial line was 2-fold smaller than in the proprietary line. RMP in EHT of both lines was similar to RA and LV. Repolarization fraction and IK,ACh response discriminated best between RA and LV and indicated predominantly ventricular phenotype in hiPSC-CMs/EHT. The data indicate that IK1 is not necessarily low in hiPSC-CMs, and technical issues may underlie low RMP in hiPSC-CMs.

Concise Review: Criteria for Chamber-Specific Categorization of Human Cardiac Myocytes Derived from Pluripotent Stem Cells

Human pluripotent stem cell‐derived cardiomyocytes (PSC‐CMs) have great potential application in almost all areas of cardiovascular research. A current major goal of the field is to build on the past success of differentiation strategies to produce CMs with the properties of those originating from the different chambers of the adult human heart. With no anatomical origin or developmental pathway to draw on, the question of how to judge the success of such approaches and assess the chamber specificity of PSC‐CMs has become increasingly important; commonly used methods have substantial limitations and are based on limited evidence to form such an assessment. In this article, we discuss the need for chamber‐specific PSC‐CMs in a number of areas as well as current approaches used to assess these cells on their likeness to those from different chambers of the heart. Furthermore, describing in detail the structural and functional features that distinguish the different chamber‐specific human adult cardiac myocytes, we propose an evidence‐based tool to aid investigators in the phenotypic characterization of differentiated PSC‐CMs. Stem Cells2017;35:1881–1897

Application of the RIMARC algorithm to a large data set of action potentials and clinical parameters for risk prediction of atrial fibrillation

Ex vivo recorded action potentials (APs) in human right atrial tissue from patients in sinus rhythm (SR) or atrial fibrillation (AF) display a characteristic spike-and-dome or triangular shape, respectively, but variability is huge within each rhythm group. The aim of our study was to apply the machine-learning algorithm ranking instances by maximizing the area under the ROC curve (RIMARC) to a large data set of 480 APs combined with retrospectively collected general clinical parameters and to test whether the rules learned by the RIMARC algorithm can be used for accurately classifying the preoperative rhythm status. APs were included from 221 SR and 158 AF patients. During a learning phase, the RIMARC algorithm established a ranking order of 62 features by predictive value for SR or AF. The model was then challenged with an additional test set of features from 28 patients in whom rhythm status was blinded. The accuracy of the risk prediction for AF by the model was very good (0.93) when all features were used. Without the seven AP features, accuracy still reached 0.71. In conclusion, we have shown that training the machine-learning algorithm RIMARC with an experimental and clinical data set allows predicting a classification in a test data set with high accuracy. In a clinical setting, this approach may prove useful for finding hypothesis-generating associations between different parameters.

K+ current changes account for the rate dependence of the action potential in the human atrial myocyte

Ongoing investigation of the electrophysiology and pathophysiology of the human atria requires an accurate representation of the membrane dynamics of the human atrial myocyte. However, existing models of the human atrial myocyte action potential do not accurately reproduce experimental observations with respect to the kinetics of key repolarizing currents or rate dependence of the action potential and fail to properly enforce charge conservation, an essential characteristic in any model of the cardiac membrane. In addition, recent advances in experimental methods have resulted in new data regarding the kinetics of repolarizing currents in the human atria. The goal of this study was to develop a new model of the human atrial action potential, based on the Nygren et al. model of the human atrial myocyte and newly available experimental data, that ensures an accurate representation of repolarization processes and reproduction of action potential rate dependence and enforces charge conservation. Specifically, the transient outward K(+) current (I(t)) and ultrarapid rectifier K(+) current (I(Kur)) were newly formulated. The inwardly recitifying K(+) current (I(K1)) was also reanalyzed and implemented appropriately. Simulations of the human atrial myocyte action potential with this new model demonstrated that early repolarization is dependent on the relative conductances of I(t) and I(Kur), whereas densities of both I(Kur) and I(K1) underlie later repolarization. In addition, this model reproduces experimental measurements of rate dependence of I(t), I(Kur), and action potential duration. This new model constitutes an improved representation of excitability and repolarization reserve in the human atrial myocyte and, therefore, provides a useful computational tool for future studies involving the human atrium in both health and disease.

Muscarinic receptors in the mammalian heart

In the mammalian heart, cardiac function is under the control of the sympathetic and parasympathetic nervous system. All regions of the mammalian heart are innervated by parasympathetic (vagal) nerves, although the supraventricular tissues are more densely innervated than the ventricles. Vagal activation causes stimulation of cardiac muscarinic acetylcholine receptors (M-ChR) that modulate pacemaker activity via I(f) and I(K.ACh), atrioventricular conduction, and directly (in atrium) or indirectly (in ventricles) force of contraction. However, the functional response elicited by M-ChR-activation depends on species, age, anatomic structure investigated, and M-ChR-agonist concentration used. Among the five M-ChR-subtypes M(2)-ChR is the predominant isoform present in the mammalian heart, while in the coronary circulation M(3)-ChR have been identified. In addition, evidence for a possible existence of an additional, not M(2)-ChR in the heart has been presented. M-ChR are subject to regulation by G-protein-coupled-receptor kinase. Alterations of cardiac M(2)-ChR in age and various kinds of disease are discussed.

Muscarinic receptors in the failing human heart

In the human heart, as in the heart of several other species, muscarinic receptors are predominantly of the M2-subtype that couple via a pertussis toxin-sensitive Gi-protein to inhibit adenylyl cyclase. However, it is not clear whether an additional muscarinic receptor subtype exists in the human heart. In human right atrium, stimulation of muscarinic M2 receptors causes direct negative inotropic and chronotropic effects; in human ventricular myocardium, however, the negative inotropic effect can be only achieved when basal force of contraction has been pre-stimulated by cyclic AMP-elevating agents such as beta-adrenoceptor agonists, forskolin or phosphodiesterase inhibitors (indirect effect); this has been shown in various in vitro and in vivo studies. Evidence has accumulated that in chronic heart failure vagal activity is decreased. Cardiac muscarinic M2 receptor density and functional responsiveness (inhibition of adenylyl cyclase activity and negative inotropic effects), however, are not considerably changed when compared with non-failing hearts although cardiac Gi-activity is increased.

Effects of bimakalim on human cardiac action potentials: comparison with guinea pig and nicorandil and use-dependent study

Electrophysiologic effects of K(ATP) channel openers (KCOs) are rarely studied for tissue and species specificity, and use-dependent investigations in human tissues are lacking. We therefore investigated in vitro the concentration-dependent effects of the KCO bimakalim [from 10 nM to 10 microM, at 1,000 ms of cycle length (CL) and 37 degrees C] on human (atrium, n = 4, and ventricle, n = 6) and guinea pig (atrium, n = 7, and ventricle, n = 6) transmembrane action potential (AP). The frequency relation (from CL 1,600 to 300 ms, 31 degrees C) of human atrial AP duration 90% (APD90) shortening (10 microM vs. baseline, n = 7) also was determined. A parallel study was performed with the KCO nicorandil (from 10 nM to 1 mM, n = 3) in human atrial APs, at 31 degrees C. Resting membrane potential and maximal upstroke velocity of AP were not modified by bimakalim at maximal concentration, whereas AP amplitude was decreased in both guinea pig preparations (p < 0.05); APD90 was shortened in all tissues (p < 0.01). Median effective concentration (EC50) for APD90 shortening at 37 degrees C was 0.54 and 2.74 microM in atrial and ventricular human tissue, respectively, and 8.55 and 0.89 microM in atrial and ventricular guinea pig tissue, respectively. In human atrial tissue at 31 degrees C, EC50 with bimakalim was 0.39 microM; a much higher value was seen with nicorandil (210 microM). Bimakalim (10 microM)-induced APD90 shortening as a function of stimulation rate was greatest at longest CL. Evidence is provided for (a) species (human vs. guinea pig) and tissue (atrium vs. ventricle) differential AP sensitivity to bimakalim; (b) an approximately 500-fold higher efficacy of bimakalim versus nicorandil to shorten human atrial APD90; and (c) normal use-dependence of human atrial APD90 shortening with bimakalim at 10 microM.

Endogenous muscarinic activity attenuates adrenergic inotropic effects in field stimulated atrial myocardium from children with congenital heart defects

To analyse the possible influence of endogenous muscarinic activity on the inotropic effects of endogenously released noradrenaline in field stimulated myocardial preparations from atria of children with congenital heart defects, we studied the maximal effect of the muscarinic antagonist atropine (1.5 micromol L(-1)). Maximal force of contraction increased by 12.8 +/- 2.0% (SEM), while the maximal rate of development of the force increased by 16.7 +/- 2.7% (SEM). Time to half maximal developed force was 57 +/- 5 s (SEM). Time to peak force, time to relax to the 20% level and relaxation time all decreased significantly after atropine. Compared with endogenous adrenoceptor stimulation alone, the combined effects of partial muscarinic and adrenergic receptor stimulation thus were moderate reductions of the maximal force of contraction and maximal rate of development of the force and increased time to peak force, time to relax to the 20% level and relaxation time. The main effect of the endogenous muscarinic activity probably was to attenuate the effect of the beta-adrenoceptor stimulation. The endogenous muscarinic activity in field stimulated atrial preparations from children is significant, and has to be taken into account in experimental set-ups.

Muscarinic M1 receptor activation reduces maximum upstroke velocity of action potential in mouse right atria

We investigated whether acetylcholine affects cardiac action potentials through the muscarinic M1 in addition to M2 receptors in spontaneously beating mouse isolated right atria. A conventional glass microelectrode technique was used for the purpose. Acetylcholine (3-10 microM) reduced the maximum upstroke velocity of the action potentials (Vmax), followed by an increase. It shortened action potential duration at 90% repolarization, hyperpolarized the resting membrane and decreased the rate of beating. Atropine (3-100 nM) concentration dependently antagonized these effects of acetylcholine. Pirenzepine (10 and 30 nM), a selective muscarinic M1 receptor antagonist, antagonized acetylcholine (5 microM)-induced reduction of Vmax without affecting other effects of acetylcholine. In addition, pirenzepine (30 nM) induced an immediate and linear acceleration of the VmaX reduced by acetylcholine. In contrast, AF-DX 116 (11(¿2-[(diethylamino)-methyl]-1-piperidyl¿acetyl)-5,11-dihydro-6 H-pyridol[2,3-b][1,4]benzodiazepine-6-one base, 30-300 nM), a selective muscarinic M2 receptor antagonist, failed to antagonize acetylcholine-induced reduction of Vmax, but abolished its increase. It antagonized the shortening of action potential duration, membrane hyperpolarization and decreased the beating rate. McN-A-343 (4-(m-chlorophenyl-carbamoyloxy)-2-butynyltrimethylammonium chloride, 100 and 300 microM), a muscarinic M1 receptor agonist, reduced Vmax and prolonged action potential duration, while oxotremorine (100-300 nM), a muscarinic M2 receptor agonist, evoked reverse effects. These results suggest that acetylcholine exerts a mixed effect on Vmax, consisting of a reduction and a facilitation, possibly mediated by concurrent activation of muscarinic M1 and M2 receptors, respectively, in isolated right atria of mice.

Adenosine as an antiarrhythmic agent

Adenosine produces acute inhibition of sinus node and atrioventricular (AV) nodal function. This profound but short lived electrophysiologic effect makes adenosine a suitable agent for treating supraventricular tachycardias (SVT) that incorporate the sinus node or AV node as part of the arrhythmia circuit, or for unmasking atrial tachyarrhythmias or ventricular pre-excitation. Its antiadrenergic properties also make it an effective agent for use with some unique atrial and ventricular tachycardias. Appropriate dosing and rapid bolusing with intravenous administration is required. Recognition of infrequent proarrhythmic risks and potential drug interactions with xanthine derivatives and dipyridamole should maximize its safe and effective use. This review will highlight adenosine's mechanism of action, administration, clinical indications, efficacy, and risks when used in tachyarrhythmic management.

The shape of human atrial action potential accounts for different frequency-related changes in vitro

We aimed at investigating frequency-related changes of human atrial action potential (AP) in vitro to see whether baseline AP shape might account for different responses to increasing stimulation rates. Human right atrial trabeculae (n = 48) obtained from adult (n = 38, mean age 59 +/- 8, range 45-72 years) consecutive patients (approximately equal to 30% of those operated upon by a single surgeon; 1.26 preparations per patient, range 1-2) were superfused in an organ bath with oxygenated (O2 content 16 ml/l) and modified (NaHCO3 25.7 mmol/l) Tyrode's solution at 31 degrees C. Baseline electrophysiology (pacing: 1 ms duration, 2-4 mA current intensity) at cycle length (CL) of 1000 ms was recorded in 90% (43 out of 48) of the preparations. The frequency-related protocol (CL from 1600 to 300 ms) was, however, undertaken in 23 (48%) preparations because 20 (42%) became pacing unresponsive immediately after baseline recordings. No statistical differences were seen when baseline electrophysiological parameters (mean +/- SD) were grouped according to late pacing responsiveness (n = 43 vs. n = 23): respectively, resting membrane potential (RMP) was -74 +/- 6 vs. -75 +/- 4 mV, maximal upstroke velocity (Vmax) 172 +/- 60 vs. 173 +/- 39 V/s, AP amplitude (APA) 89 +/- 11 vs. 91 +/- 8 mV and AP durations were at 30% (APD30%) 10 +/- 13 vs. 13 +/- 18 ms, 50% (APD50%) 45 +/- 79 vs. 62 +/- 91 ms and 90% (APD90%) 383 +/- 103 vs. 407 +/- 108 ms. To classify baseline AP shape, two criteria were adopted: criterion 1 ("objective"), based on APA (cut-off 90 mV) and APD90% (cut-off 500 ms) computed values and criterion 2 ("visual") derived from the literature. These criteria enabled us to differentiate three AP shape types: type 1 (spike and dome), type 3 (no dome) and type 4 (extremely prolonged). At baseline, the two criteria diagnosed different proportions of AP shape types. There were, however, no intra-type statistical differences among electrophysiological parameters. By criterion 1, analysis of variance (ANOVA) showed significant inter-type differences of RMP,Vmax, APA, APD50 and 90% and by criterion 2 of APA, APD30, 50 and 90%, respectively. To facilitate comparisons with previous published data, criterion 2 was selected to analyse frequency-related changes of AP shape types. At low stimulation rate, ANOVA for repeated measures (with Greenhouse-Geisser epsilon correction) showed inter-type differences for APD30, 50 and 90% (P = 0.00005). RMP, Vmax, APA and APD90% were overall frequency-related (P = 0.00005). Inter-type frequency-related differences were however seen only for APD90%. Human atrial AP durations (30, 50 and 90%) enable differentiation among AP shape types (1, 3 and 4). By a standardized use-dependent protocol overall RMP, Vmax, APA and APD90% are frequency-related. AP shape accounts for frequency-related changes of APD90% only. A type 4 AP shape with much prolonged AP duration had a flat frequency dependence. At high stimulation rates, adult type 1 and 3 AP shapes are indistinguishable. Use-dependent and pharmacological investigations in human atrial myocytes need to take AP shape into account.

Characterization of the positive and negative inotropic effects of acetylcholine in the human myocardium

In the human isolated myocardium, acetylcholine (10(-9) to 10(-3) M) elicited a biphasic inotropic effect (a decrease in the lower and an increase in the higher concentration range) in atrial and a positive inotropic effect in ventricular trabeculae. However, under conditions of raised contractility achieved by exposure to noradrenaline (10(-5) M), only negative inotropic effects were observed in both atria and ventricles. Atropine (10(-6) M), but not propranolol (10(-6) M), antagonized both positive and negative inotropic effects of acetylcholine, thus showing that the responses were mediated by muscarinic acetylcholine receptors. The use of subtype selective muscarinic receptor antagonists (10(-7) to 10(-5) M), pirenzepine (M1 > M3 > M2), AF-DX 116 (11-([2-[(diethylamino)-methyl]-1-piperidyl]acetyl)-5,11-dihydro-6H- pyridol[2,3-b][1,4]benzodiazepine-6-one base; M2 > M1 > M3) and HHSiD (p-fluorohexahydro-siladifenidol hydrochloride; M3 > or = M1 >> M2) revealed that the negative inotropic effect of acetylcholine in atrial as well as the positive inotropic effect in ventricular trabeculae were best antagonized by AF-DX 116 and not by pirenzepine, suggesting the involvement of the muscarinic M2 receptor subtype, possibly linked to different second messenger systems. On the other hand, the positive inotropic effect of acetylcholine (10(-6) to 10(-3) M) in the atrial tissue, observed only in preparation with depressed contractility, was not effectively antagonized by either AF-DX 116 or HHSiD, but was significantly reduced by pirenzepine. (ABSTRACT TRUNCATED AT 250 WORDS)

Resting and action potentials of nonischemic and chronically ischemic human ventricular muscle

INTRODUCTION

The effect of chronic ischemia on the electrical properties of human cardiac tissue is not well understood.

METHODS AND RESULTS

Membrane potentials were studied using microelectrode techniques in isolated human ventricular tissues obtained from nonischemic (n = 17) or chronically ischemic (n = 7) myocardium. In normal Tyrode's solution, resting potential (Vr) was lower in ischemic (-70.1 +/- 2.12 mV) than in nonischemic muscles (-77.6 +/- 0.93 mV; mean +/- SEM; P < 0.05). In high [K]o (> 10 mM) media, Vr was of similar magnitude in both types of tissue (in 21.6 mM [K]o, Vr was -53.1 +/- 2.24 mV in nonischemic and -49.6 +/- 2.03 mV in ischemic preparations; n = 7 each; P > 0.05). Lowering [K]o caused persistent hyperpolarization in nonischemic muscles, but caused depolarization in chronically ischemic preparations (in 2.7 mM [K]o, Vr was -84.9 +/- 2.74 mV and -61.7 +/- 7.72 mV, respectively; n = 7; P < 0.05). Pinacidil (100 microM) normalized the response of chronically ischemic preparations to [K]o. Action potentials (APs) from nonischemic tissues varied in shape and could show aberrations. Epinephrine (1.5 microM) and 4-aminopyridine (3 mM) increased the AP duration, while butanedione monoxime (20 mM) and tetrodotoxin (1 microM) shortened it. In chronically ischemic muscles, the AP was characterized by the absence of a plateau and the presence of a slow phase of final repolarization.

CONCLUSION

The differential effect of low [K]o on the resting membrane potential of nonischemic and chronically ischemic tissues suggests a change in the properties or the regulation of background K+ channels during chronic ischemia.

Different pharmacological responses of atrium and ventricle: studies with human cardiac tissue

It has been recently reported that 5-hydroxytryptamine (5-HT) increases force of contraction in atrial tissue but not in ventricular tissue. In the present study with trabeculae obtained from non-diseased human hearts, we investigated whether this difference in the contractile responses is specific for 5-HT or is also observed for other substances: calcitonin gene-related peptide (CGRP), angiotensin II, adenosine, somatostatin and acetylcholine. CGRP (10(-9) to 10(-7) M) and angiotensin II (10(-9) to 10(-5) M) caused concentration-dependent increases in force of contraction in atrial trabeculae (up to 36 +/- 8% and 42 +/- 8% of the response to 10(-5) M noradrenaline, respectively). Similar to 5-HT, no effects were observed with CGRP and angiotensin II in ventricular trabeculae. Adenosine (10(-8) to 10(-5) M) and somatostatin (10(-8) to 10(-6) M) caused concentration-dependent negative inotropic effects on baseline atrial contractility (-54 +/- 17% and -51 +/- 25%, respectively), but no response was found on baseline ventricular contractility. Adenosine, but not somatostatin, reduced force of contraction after pre-stimulation with 10(-5) M noradrenaline in atrial tissue and, to a lesser extent, in ventricular tissue. Acetylcholine exhibited a biphasic concentration-response curve in the atrial tissue, consisting of an initial negative inotropic response (10(-9) to 10(-7) M, from 120 +/- 41 mg at baseline to 48 +/- 16 mg at 10(-7) M), followed by a positive inotropic response (10(-6) to 10(-3) M, from 48 +/- 16 mg at 10(-7) M to 77 +/- 15 mg).(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of P1- and muscarinic-receptor agonists upon cAMP-dependent and independent inotropic responses of guinea-pig cardiac preparations

1. The indirect negative inotropic effects of P1- and muscarinic-receptor agonists were compared by examining how the responses of isolated guinea-pig left atria and papillary muscles to positive inotropes were affected by the presence of the P1-receptor agonist, L-PIA or the muscarinic-receptor agonist, carbachol. 2. The indirect negative inotropic effects of L-PIA and carbachol were similar: both attenuated the responses of left atria more effectively than those of papillary muscles; and both more effectively attenuated responses to the cAMP-dependent positive inotropes, isoprenaline and forskolin. 3. However, there were differences: L-PIA, but not carbachol, was able at high concentrations to inhibit the responses of left atria to the calcium channel opener Bay K 8644; and at concentrations that produced similar direct negative inotropic effects, L-PIA consistently attenuated the positive inotropes more effectively than carbachol. 4. These findings are consistent with L-PIA being able to activate an additional negative inotropic mechanism that carbachol cannot.

Electrophysiologic and inotropic effects of alpha-adrenoceptor stimulation in human isolated atrial heart muscle

The effects of alpha-adrenoceptor stimulation on force of contraction were investigated in human atrial heart muscle and compared with those of beta-adrenoceptor stimulation. The maximal positive inotropic effect produced by stimulation of alpha-adrenoceptors with phenylephrine (in the presence of atenolol 10 mumol/l) was significantly smaller than that seen in response to beta-adrenoceptor stimulation with isoprenaline. The maximal effect of phenylephrine (25% of the maximal effect of isoprenaline) required far higher concentrations (1 mmol/l) than isoprenaline (100 nmol/l); the EC50 values amounted to 33.1 mumol/l and 3.3 nmol/l, respectively. In the presence of the alpha-adrenoceptor blocking agent phentolamine (1 mumol/l), the concentration-response curve of phenylephrine was displaced to higher concentrations of the agonist; under these conditions, the EC50 value amounted to 52.5 mumol/l. The effects of the catecholamines noradrenaline and adrenaline on force of contraction remained unchanged in the presence of phentolamine (1 mumol/l) or prazosin (1 mumol/l). The positive inotropic effect of phenylephrine (1 mmol/l) was associated with a slight decrease in action potential duration; the effects on action potential were completely blocked in the presence of phentolamine (1 mumol/l). These findings support the view that selective stimulation of alpha-adrenoceptors may mediate a small but detectable positive inotropic effect in human atrial tissue under in vitro conditions. The requirement of high concentrations of alpha-adrenoceptor agonists and the lack of effects of the endogenous catecholamines adrenaline and noradrenaline on alpha-adrenoceptors (in concentrations which fully elicit the beta-adrenoceptors-mediated response) do not provide a basis for a functional role of alpha-adrenoceptor-mediated effects under in vivo conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Validation of a human atrial trabecular preparation for evaluation of inotropic substances

Assessment of cardioactive substances is usually performed using animal tissue, with the effects being extrapolated to humans, thereby potentially introducing errors due to species differences. In order to validate the use of human atrial tissue, known positive and negative inotropic agents were tested on trabeculae obtained from patients' atrial appendages at the time of cardiac surgery requiring, cardiopulmonary bypass. Trabeculae were selected according to strict criteria: cross-sectional area less than 1.0 mm2, resting force (RF) less than 0.7 g, and developed force (DF) greater than 0.8 g. Each trabecula received only one drug in a cumulative dose manner. Where necessary, the vehicle used to dissolve or stabilize the drug solution was also tested. In addition, the relative DF of "no-drug," "time-only" controls were measured during the same time period. After adjusting for the effect of time on the preparation, relative DF was increased to 157% by dobutamine (1.5 x 10(-5) M), to 136% by amrinone (5.6 x 10(-4) M), and to 117% by ouabain (2 x 10(-7) M). The relative DF decreased with nifedipine and propranolol, with 50% inhibition for both drugs being 1.5 x 10(-7) M. Although human ventricular muscles might be more appropriate to use in order to determine the effects observed with the whole heart, they are extremely difficult to obtain on a regular basis. The results of this study show that the atrial trabecular preparation offers an acceptable alternative.

86Rubidium efflux and negative inotropy induced by P1- and muscarinic-receptor agonists in guinea-pig left atria. Effects of potassium channel blockers

Guinea-pig isolated left atria, paced at 2 Hz were incubated with 86rubidium (86Rb) for 120 min. They were then washed every 2 min for 2 hr, each sample being retained for scintillation counting. Left atrial isometric tension was recorded simultaneously. A concentration-response curve for the muscarinic agonist carbachol or the P1-receptor agonists adenosine and L-N6-phenyl-isopropyladenosine (L-PIA) was obtained. Antagonists were present from 20 min before agonist exposure. The rate constant (k) for 86Rb efflux was calculated for each 2 min sample and the mean increase for each concentration of agonist determined. In the absence of drugs there was no significant alteration in the rate constant during the 2 hr experimental period. Adenosine, L-PIA and carbachol produced concentration-related increases in rate constant for 86Rb efflux. The adenosine and L-PIA concentration-response curves were virtually superimposed upon the curves for the negative inotropic responses. The 86Rb efflux induced by adenosine was antagonized in an apparently parallel manner by 8-phenyltheophylline (8-PT) indicating involvement of P1-receptors. Alone, the putative potassium channel blockers, 4-aminopyridine (4-AP) and bromobenzoylmethyladamantylamine (BMA) caused, respectively, no change and a reduction in resting 86Rb efflux immediately prior to the agonist exposure. 4-AP reduced the L-PIA- and adenosine-induced increases in 86Rb efflux and, to a lesser extent, the negative inotropic response to adenosine. BMA caused "flattening" of the dose-response curves for 86Rb efflux induced by L-PIA, adenosine and carbachol with a significant reduction in response at the highest concentrations of adenosine and carbachol. The negative inotropic response to adenosine was also reduced. These results suggest that 4-AP and BMA block the P1-receptor-linked potassium channels and that BMA interacts with common K+ channels linked to P1- and muscarinic receptors. The negative inotropic responses of the guinea-pig left atrium to P1- and muscarinic agonists can be attributed, at least in part, to the opening of outward K+ channels.

Antagonism of novel inotropic agents at A1 adenosine receptors and m-cholinoceptors in human myocardium

The effects of the new inotropic agents saterinone, sulmazole, UD-CG 212.Cl and milrinone at A1 adenosine receptors and m-cholinoceptors were evaluated in human myocardium from patients with heart failure. At A1 adenosine receptors, all compounds inhibited 3H-DPCPX-binding to ventricular membrane preparations at micromolar concentrations. As judged from the K1-values, the rank order of potency was saterinone greater than sulmazole greater than UD-CG 212.Cl greater than milrinone. The new inotropic agents also displaced the binding of 3H-QNB at m-cholinoceptors. Except for saterinone, the concentration ranges of mean Ki-values were considerably higher at m-cholinoceptors than at A1 adenosine receptors. The rank order of potency was saterinone greater than sulmazole greater than UD-CG 212.Cl greater than milrinone. Competition of the A1 adenosine receptor agonist R-PIA to 3H-DPCPX-binding showed a biphasic curve with a shallow slope (Hill coefficient nH = 0.63) and revealed two affinity states of the A1 adenosine receptor. In the presence of guanine nucleotides [Gpp(NH)p], the competition curve showed one low affinity class of binding sites and was shifted to the right. In contrast, the competition curves of the new inotropic agents were characterized by a monophasic, steeper slope (mean Hill coefficient nH = 0.98). Guanine nucleotides had no effect. Similar results were obtained with saterinone and carbachol at m-cholinoceptors. Competition with carbachol revealed three affinity states of the m-cholinoceptor, the super-high affinity binding was reversed by Gpp(NH)p. Competition with saterinone revealed one class of binding sites which was not influenced by Gpp(NH)p.(ABSTRACT TRUNCATED AT 250 WORDS)