Endothelin 1 enhances myofilament Ca2+ responsiveness in aequorin-loaded ferret myocardium

Recent insights in the paracrine modulation of cardiomyocyte contractility by cardiac endothelial cells

The cardiac endothelium is formed by a continuous monolayer of cells that line the cavity of the heart (endocardial endothelial cells (EECs)) and the luminal surface of the myocardial blood vessels (intramyocardial capillary endothelial cells (IMCEs)). EECs and IMCEs can exercise substantial control over the contractility of cardiomyocytes by releasing various factors such as nitric oxide (NO) via a constitutive endothelial NO-synthase (eNOS), endothelin-1, prostaglandins, angiotensin II, peptide growth factors, and neuregulin-1. The purpose of the present paper is actually to shortly review recent new information concerning cardiomyocytes as effectors of endothelium paracrine signaling, focusing particularly on contractile function. The modes of action and the regulatory paracrine role of the main mediators delivered by cardiac endothelial cells upon cardiac contractility identified in cardiomyocytes are complex and not fully described. Thus, careful evaluation of new therapeutic approaches is required targeting important physiological signaling pathways, some of which have been until recently considered as deleterious, like reactive oxygen species. Future works in the field of cardiac endothelial cells and cardiac function will help to better understand the implication of these mediators in cardiac physiopathology.

The role of the paracrine/autocrine mediator endothelin-1 in regulation of cardiac contractility and growth

Endothelin-1 (ET-1) is a critical autocrine and paracrine regulator of cardiac physiology and pathology. Produced locally within the myocardium in response to diverse mechanical and neurohormonal stimuli, ET-1 acutely modulates cardiac contractility. During pathological cardiovascular conditions such as ischaemia, left ventricular hypertrophy and heart failure, myocyte expression and activity of the entire ET-1 system is enhanced, allowing the peptide to both initiate and maintain maladaptive cellular responses. Both the acute and chronic effects of ET-1 are dependent on the activation of intracellular signalling pathways, regulated by the inositol-trisphosphate and diacylglycerol produced upon activation of the ET(A) receptor. Subsequent stimulation of protein kinases C and D, calmodulin-dependent kinase II, calcineurin and MAPKs modifies the systolic calcium transient, myofibril function and the activity of transcription factors that coordinate cellular remodelling. The precise nature of the cellular response to ET-1 is governed by the timing, localization and context of such signals, allowing the peptide to regulate both cardiomyocyte physiology and instigate disease.

LINKED ARTICLES

This article is part of a themed section on Endothelin. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.168.issue-1.

Endocardial endothelium is a key determinant of force-frequency relationship in rat ventricular myocardium

We tested the hypothesis that removing endocardial endothelium (EE) negatively impacts the force-frequency relationship (FFR) of ventricular myocardium and dissected the signaling that underlies this phenomenon. EE of rat trabeculae was selectively damaged by brief (<1 s) exposure to 0.1% Triton X-100. Force, intracellular Ca(2+) transient (iCa(2+)), and activity of protein kinase A (PKA) and protein kinase C (PKC) were determined. In control muscles, force and iCa(2+) increased as the stimulation frequency increased in steps of 0.5 Hz up to 3.0 Hz. However, EE-denuded (EED) muscles exhibited a markedly blunted FFR. Neither isoproterenol (ISO; 0.1-5 nmol/l) nor endothelin-1 (ET-1; 10-100 nmol/l) alone restored the slope of FFR in EED muscles. Intriguingly, however, a positive FFR was restored in EED preparations by combining low concentrations of ISO (0.1 nmol/l) and ET-1 (20 nmol/l). In intact muscles, PKA and PKC activity increased proportionally with the increase in frequency. This effect was completely lost in EED muscles. Again, combining ISO and ET-1 fully restored the frequency-dependent rise in PKA and PKC activity in EED muscles. In conclusion, selective damage of EE leads to significantly blunted FFR. A combination of low concentrations of ISO and ET-1 successfully restores FFR in EED muscles. The interdependence of ISO and ET-1 in this process indicates cross-talk between the β1-PKA and ET-1-PKC pathways for a normal (positive) FFR. The results also imply that dysfunction of EE and/or EE-myocyte coupling may contribute to flat (or even negative) FFR in heart failure.

Modulation of myocardial contraction by endocardial and coronary vascular endothelium

Endothelial cells in the heart, both endocardial endothelium and coronary vascular endothelium, influence myocardial contraction in isolated tissue and pump function in intact hearts by releasing diffusible agents that affect subjacent myocardium. Endocardial endothelium releases both nitric oxide (NO) and an unidentified "contraction-prolonging substance" ("endocardin") that respectively decrease and increase the duration of twitch contraction, probably by altering myofibrillar calcium sensitivity. These agents modulate the duration of ejection and the timing of relaxation, but without significantly altering early systolic behavior. Coronary vascular endothelium also releases NO, with similar effects on contraction, and in addition probably releases several other agents. Current work is aimed at identifying all of the agents involved in these novel endothelial influences and studying their potential physiologic and pathophysiologic roles in cardiac contractile and other functions.

Endothelin receptor overexpression alters diastolic function in cultured rat ventricular myocytes

The endothelin (ET) signaling pathway controls many physiological processes in myocardium and often becomes upregulated in heart diseases. The aim of the present study was to investigate the effects of ET receptor upregulation on the contractile function of adult ventricular myocytes. Primary cultured adult rat ventricular myocytes were used as a model system of ET receptor overexpression in the heart. Endothelin receptor type A (ET_A) or type B (ET_B) was overexpressed by Adenoviral infection, and the twitch responses of infected ventricular myocytes were measured after ET-1 stimulation. Overexpression of ET_A exaggerated positive inotropic effect (PIE) and diastolic shortening of ET-1, and induced a new twitch response including twitch broadening. On the contrary, overexpression of ET_B increased PIE of ET-1, but did not affect other two twitch responses. Control myocytes expressing endogenous receptors showed a parallel increase in twitch amplitude and systolic Ca²⁺ in response to ET-1. However, intracellular Ca²⁺ did not change in proportion to the changes in contractility in myocytes overexpressing ET_A. Overexpression of ET_A enhanced both systolic and diastolic contractility without parallel changes in Ca²⁺. Differential regulation of this nature indicates that upregulation of ET_A may contribute to diastolic myocardial dysfunction by selectively targeting myofi lament proteins that regulate resting cell length, twitch duration and responsiveness to prevailing Ca²⁺.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Contractile regulation by overexpressed ETArequires intact T tubules in adult rat ventricular myocytes

Endothelin (ET)-1 regulates the contractility and growth of the heart by binding G protein-coupled receptors of the ET type A receptor (ETA)/ET type B (ETB) receptor family. ETA, the predominant ET-1 receptor subtype in myocardium, is thought to localize preferentially within cardiac T tubules, but the consequences of mislocalization are not fully understood. Here we examined the effects of the overexpression of ETAin conjunction with T-tubule loss in cultured adult rat ventricular myocytes. In adult myocytes cultured for 3 to 4 days, the normally robust positive inotropic effect (PIE) of ET-1 was lost in parallel with T-tubule degeneration and a decline in ETAprotein levels. In these T tubule-compromised myocytes, an overexpression of ETAusing an adenoviral vector did not rescue the responsiveness to ET-1, despite the robust expression in the surface sarcolemma. The inclusion of the actin polymerization inhibitor cytochalasin D (CD) during culture prevented gross morphological changes including a loss of T tubules and a rounding of intercalated discs, but CD alone did not rescue the responsiveness to ET-1 or prevent ETAdownregulation. The rescue of a normal PIE in 3- to 4-day cultured myocytes required both an increased expression of ETAand intact T tubules (preserved with CD). Therefore, the activation of ETAlocalized in T tubules was associated with a strong PIE, whereas the activation of ETAin surface sarcolemma was not. The results provide insight into the pathological cardiac conditions in which ETAis upregulated and T-tubule morphology is altered.

Mechanisms of endothelin-1-induced decrease in contractility in adult mouse ventricular myocytes

BACKGROUND AND PURPOSE

The potent vasoconstrictor polypeptide endothelin-1 (ET-1) plays an important pathophysiological role in progression of cardiovascular diseases and elicits prominent effects on myocardial contractility. Although ET-1 produces a positive inotropy in cardiac muscle of most mammalian species, it induces a sustained negative inotropy in mice. This study was performed to gain an insight into the cellular mechanisms underlying the negative inotropy in adult mouse ventricular myocytes.

EXPERIMENTAL APPROACH

Cell shortening and Ca(2+) transients were simultaneously recorded from isolated mouse ventricular myocytes loaded with the Ca(2+)-sensitive fluorescent dye indo-1.

KEY RESULTS

ET-1 decreased cell shortening in a concentration-dependent manner (pD(2) value of 10.1). The ET-1-induced decrease in cell shortening was associated with a decrease in Ca(2+) transients. In addition, the Ca(2+) transient/cell-shortening relationship was shifted to the right by ET-1, indicating decreased myofilament Ca(2+) sensitivity. The instantaneous relationship of the rising phase of the Ca(2+) transient and cell shortening was shifted to the right by ET-1. Decreased Ca(2+) transients and cell shortening induced by ET-1 were markedly attenuated by the specific Na(+)/Ca(2+) exchange inhibitor SEA0400.

CONCLUSIONS AND IMPLICATIONS

ET-1-induced negative inotropy in mouse ventricular myocytes was mediated by decreased Ca(2+) transients and myofilament Ca(2+) sensitivity. These data are entirely consistent with the involvement of increased Ca(2+) extrusion via the Na(+)/Ca(2+) exchanger in the ET-1-mediated decrease in Ca(2+) transients. Decreased Ca(2+) sensitivity may be due to retardation of cell shortening in response to a rise in Ca(2+) transients.

Cardiovascular endothelins: essential regulators of cardiovascular homeostasis

The endothelin (ET) system consists of 3 ET isopeptides, several isoforms of activating peptidases, and 2 G-protein-coupled receptors, ETA and ETB, that are linked to multiple signaling pathways. In the cardiovascular system, the components of the ET family are expressed in several tissues, notably the vascular endothelium, smooth muscle cells, and cardiomyocytes. There is general agreement that ETs play important physiological roles in the regulation of normal cardiovascular function, and excessive generation of ET isopeptides has been linked to major cardiovascular pathologies, including hypertension and heart failure. However, several recent clinical trials with ET receptor antagonists were disappointing. In the present review, the authors take the stance that ETs are mainly and foremost essential regulators of cardiovascular function, hence that antagonizing normal ET actions, even in patients, will potentially do more harm than good. To support this notion, we describe the predominant roles of ETs in blood vessels, which are (indirect) vasodilatation and ET clearance from plasma and interstitial spaces, against the background of the subcellular mechanisms mediating these effects. Furthermore, important roles of ETs in regulating and adapting heart functions to different needs are addressed, including recent progress in understanding the effects of ETs on diastolic function, adaptations to changes in preload, and the interactions between endocardial-derived ET-1 and myocardial pump function. Finally, the potential dangers (and gains) resulting from the suppression of excessive generation or activity of ETs occurring in some cardiovascular pathological states, such as hypertension, myocardial ischemia, and heart failure, are discussed.

Angiotensin II and cardiac excitation-contraction coupling: questions and controversies

Angiotensin II (AngII) is a circulating peptide that produces a positive inotropic effect in the heart in several species, including humans. The subcellular mechanisms involved in producing this effect have been the focus of numerous studies; however, the results of these studies have generated considerable controversy. Although part of the controversy might arise from species and developmental differences, conflicting results have also been reported in the same species. To further complicate the understanding of the cardiac actions of AngII, the binding of the peptide to its transmembrane G-protein-coupled receptors has been shown to activate signalling cascades that involve numerous second messengers. Among these, inositol 1,4,5-triphosphate (IP3) and protein kinase C (PKC) have been shown to have the potential to modulate either one or both of the two basic mechanisms known to increase contractility: (i) an increase in the intracellular Ca2+ concentration ([Ca2+]i); or (ii) an increase in myofilament responsiveness to Ca2+. The aim of this review is to examine the effect of AngII on the fundamental components of cardiac excitation-contraction coupling: calcium currents, Na+/Ca2+ exchange, sarcoplasmic reticulum (SR)-CaZ+ release, calcium transients and contractile proteins. An answer to the following question is sought: Is the positive inotropic effect of AngII due to an increase in [Ca2+]i, to an increase in myofilament responsiveness to Ca2+, or to both?

Molecular mechanisms in endothelial regulation of cardiac function

Endothelium is now recognized as a massive, regionally specific, multifunctional organ. Given its strategic anatomic location between the circulating blood components and the vascular smooth muscle or the cardiac muscle, it is a biologically significant interface whose dysfunction can be a critical factor in various pathological conditions. Two types of endothelial cells are recognized in the heart, the endocardial endothelial (EE) cells and the microvascular endothelial cells (MVE). Both produce common autacoids and share similar roles in signal transduction induced by neurotransmitters, hormones or mechanical stimuli. They are however two distinct cell populations with dissimilar embryological origin, cytoskeletal organization, receptor mediated functions and electrophysiological properties. Both the MVE and EE are modulators of cardiac performance. Myocardial contraction may be modulated by cardioactive agents such as nitric oxide, prostanoids, endothelin, natriuretic peptides, angiotensin II, kinins, reactive oxygen species and adenyl purines released from the cardiac endothelium. Two mechanisms have been proposed for the signal transduction from EE to the underlying myocytes: stimulus-secretion-contraction coupling and blood-heart barrier. Nitric oxide, bradykinin and myofilament desensitizing agent are probably important in short-term regulation of myocardial functions. Endothelin and Angiotensin II are probably involved in long-term regulation. Besides its sensory function and paracrine modulation of myocardial performance, EE as a blood-heart barrier could be of significance for the ionic homeostasis of the cardiac interstitium. In cardiac diseases, the damage to EE or MVE leading to failure of the endothelial cells to perform its regulatory and modulator functions may have serious consequences. A better understanding of the endothelial signaling pathways in cardiac physiology and pathophysiology may lead to the development of novel therapeutic strategies.

Role of troponin I phosphorylation in protein kinase C-mediated enhanced contractile performance of rat myocytes

Our goal was to define the role of phosphorylated cardiac troponin-I in the adult myocyte contractile performance response to activated protein kinase C. In agreement with earlier work, endothelin enhanced both adult rat myocyte contractile performance and cardiac troponin-I phosphorylation. Protein kinase C participated in both responses. The role of cardiac troponin-I phosphorylation in the contractile function response to protein kinase C was further investigated using gene transfer into myocytes of troponin-I isoforms/mutants lacking one or more phosphorylation sites previously identified in purified cardiac troponin-I. Sarcomeric replacement with slow skeletal troponin-I-abrogated protein kinase C-mediated troponin-I phosphorylation. In functional studies, endothelin slowed relaxation in myocytes expressing slow skeletal troponin-I, while the relaxation rate increased in myocytes expressing cardiac troponin-I. Based on these results, acceleration of myocyte relaxation during protein kinase C activation largely depended on cardiac troponin-I phosphorylation. Experiments with troponin-I isoform chimeras provided evidence that phosphorylation sites in the amino portion of cardiac troponin I-mediated the protein kinase C acceleration of relaxation. The cardiac troponin-I Thr-144 phosphorylation site identified in earlier biochemical studies was not significantly phosphorylated during the acute contractile response. Thus, amino-terminal protein kinase C-dependent phosphorylation sites in cardiac troponin-I are likely responsible for the accelerated relaxation observed in adult myocytes.

ET-1 increases distensibility of acutely loaded myocardium: a novel ETA and Na+/H+ exchanger-mediated effect

This study investigated, in rabbit papillary muscles (n = 61) and human auricular strips (n = 7), effects of endothelin-1 (ET-1; 0.1-10 nM) on diastolic myocardial properties. ET-1 (1 nM) was also given in the presence of selective ET(A) or ET(B) antagonism, nonselective ET(A)/ET(B) antagonism, and Na(+)/H(+) exchanger inhibition. Effects of 6.3 mM Ca(2+) were also studied. ET-1 dose dependently increased inotropism. In contrast to baseline, in the presence of ET-1, resting tension (RT) decreased, after an isometric twitch, 3.4 +/- 1.4, 6.9 +/- 1.5, and 12.5 +/- 3.1% with 0.1, 1, and 10 nM, respectively, reflecting an increase in myocardial distensibility. ET-1 effects were abolished with selective ET(A) as well as with nonselective ET(A)/ET(B) antagonism, whereas they were still present with ET(B) antagonism. Na(+)/H(+) exchanger inhibition abolished ET-1 effects on distensibility, whereas it only partially inhibited positive inotropic effect. Ca(2+) increased inotropism to a similar extent to ET-1 (1 nM) but did not affect distensibility. ET-1 therefore increased diastolic distensibility of acutely loaded human and nonhuman myocardium. This effect is mediated by ET(A) receptors, requires Na(+)/H(+) exchanger activation, and cannot be elicited by Ca(2+).

Cardiac endothelial-myocardial signaling: its role in cardiac growth, contractile performance, and rhythmicity

Experimental work during the past 15 years has demonstrated that endothelial cells in the heart play an obligatory role in regulating and maintaining cardiac function, in particular, at the endocardium and in the myocardial capillaries where endothelial cells directly interact with adjacent cardiomyocytes. The emerging field of targeted gene manipulation has led to the contention that cardiac endothelial-cardiomyocytal interaction is a prerequisite for normal cardiac development and growth. Some of the molecular mechanisms and cellular signals governing this interaction, such as neuregulin, vascular endothelial growth factor, and angiopoietin, continue to maintain phenotype and survival of cardiomyocytes in the adult heart. Cardiac endothelial cells, like vascular endothelial cells, also express and release a variety of auto- and paracrine agents, such as nitric oxide, endothelin, prostaglandin I(2), and angiotensin II, which directly influence cardiac metabolism, growth, contractile performance, and rhythmicity of the adult heart. The synthesis, secretion, and, most importantly, the activities of these endothelium-derived substances in the heart are closely linked, interrelated, and interactive. It may therefore be simplistic to try and define their properties independently from one another. Moreover, in relation specifically to the endocardial endothelium, an active transendothelial physicochemical gradient for various ions, or blood-heart barrier, has been demonstrated. Linkage of this blood-heart barrier to the various other endothelium-mediated signaling pathways or to the putative vascular endothelium-derived hyperpolarizing factors remains to be determined. At the early stages of cardiac failure, all major cardiovascular risk factors may cause cardiac endothelial activation as an adaptive response often followed by cardiac endothelial dysfunction. Because of the interdependency of all endothelial signaling pathways, activation or disturbance of any will necessarily affect the others leading to a disturbance of their normal balance, leading to further progression of cardiac failure.

Myocardial contractile responsiveness to endothelin-1 in the post-infarction rat model of heart failure: effects of chronic quinapril

Cardiac endothelin-1 (ET-1) levels and ET receptor expression are increased in congestive heart failure (CHF). In order to determine whether this results in increased responsiveness of ET-A or ET-B receptors to ET-1, we evaluated the contractile effects of ET-1 in isolated papillary muscles isolated from hearts of control rats and from rats 4 weeks post myocardial infarction (MI) having received no therapy or chronic quinapril therapy. The ET-1 dose-response was biphasic in normal muscles. The use of the selective ET-A receptor antagonist BQ123 and the selective ET-B receptor antagonist BQ788 revealed that the initial decrease in tension was the result of ET-B receptor stimulation. Blockade of nitric oxide (NO) production with L-NAME abolished the initial decrease in tension. MI resulted in CHF that was partially reversed by quinapril. In MI, the positive inotropic effects of ET-1 were enhanced due to the loss of the initial ET-B receptor mediated decrease in tension, as well as an increase in the positive inotropic effects of ET-A receptors. This was associated with an increase in ET-A and ET-B receptor mRNA and a decrease in cardiac ecNOS protein. Four weeks of therapy with quinapril attenuated the positive inotropic effects of ET-1 and prevented the increase in ET-A receptor mRNA. Although quinapril did not restore the effects of ET-B receptor stimulation or prevent the increase in ET-B mRNA, it did restore cardiac ecNOS protein expression. Thus, the inotropic response to ET-1 is biphasic due to an overall positive inotropic effect of ET-A receptor stimulation and an ET-B receptor mediated decrease in contractility at low ET-1 concentrations which appears to be mediated by cardiac ecNOS (NO). In post-MI CHF, responsiveness to ET-A receptors increases and the ET-B mediated negative inotropic response is lost despite an increase in both receptor subtypes. Quinapril therapy attenuates these effects and normalises cardiac ecNOS protein.

Inotropic response of rabbit ventricular myocytes to endothelin-1: difference from isolated papillary muscles

Endothelin-1 (ET-1) increased cell shortening and Ca2+ transients over the concentration of 3 x 10(-11) M to 10(-9) M with EC50 of 8.3 x 10(-11) M in rabbit single ventricular myocytes. Thus ET-1 was approximately 60 times more potent in single myocytes than in papillary muscles (EC50 = 5.1 x 10(-9) M) of the same species. In single myocytes, ET-1 at 10(-8) M elicited an inhibitory response that counteracted the facilitatory response: the concentration-response curve (CRC) for ET-1 was bell shaped. The ET(A)-receptor antagonist BQ-485 shifted CRC for ET-1 to the right in parallel; however, the facilitatory response to 10(-8) M ET-1 was markedly enhanced by BQ-485 and also by the ET(B) antagonist BQ-788. The ET(A)/ET(B) antagonist TAK-044 abolished the ET-1-induced response. These findings indicate that the response to ET-1 of single myocytes is different from that of papillary muscles in concentration dependence, characteristics of the response, and susceptibility to ET-receptor antagonists. Anomalous pharmacological characteristics of ET-1-induced response in rabbit papillary muscles may be due to integrated regulatory mechanisms that may involve also various types of noncardiac cell in ventricular myocardium.

Biphasic inotropic response to endothelin-1 in the presence of various concentrations of norepinephrine in dog ventricular myocardium

The present study was undertaken to investigate the interaction between endothelin-1 (ET-1) and norepinephrine (NE) on contractile regulation in dog ventricular myocardium. ET-1 alone did not elicit any inotropic response in isolated dog ventricular trabeculae (37 degrees C, 0.5 Hz). In the presence of NE at a high concentration (10(-7) M), ET-1 (10(-8) M) elicited a long-lasting negative inotropic effect, while in the presence of NE at a moderate concentration (3 x 10(-8) M) it produced a biphasic inotropic effect: a sustained positive inotropic effect subsequent to a short-lasting negative inotropic effect. In the presence of a lower concentration (10(-9) M) that affected scarcely the basal force of contraction, ET-1 produced a pronounced positive inotropic effect in association with negative lusitropic and negative clinotropic effects in a concentration-dependent manner subsequent to a small transient negative inotropic effect. The presented results indicate that not only the extent, but also the quality of the inotropic response to ET-1 is determined by the level of NE in the biophase. The crosstalk of ET-1 with NE may play a crucial role in pathophysiological regulation of cardiac contractility in intact dog ventricular myocardium.

Calcium handling and role of endothelin-1 in monocrotaline right ventricular hypertrophy of the rat

We investigated the role of endothelin-1 (ET-1) in right ventricular function and intracellular Ca(2+)(Ca(2+)(i)) handling of isolated perfused rat hearts with right ventricular hypertrophy induced by monocrotaline (50 mg/kg). Nine weeks after monocrotaline (n=9) or saline (control n=9) treatment, hearts were perfused isovolumically at 37 degrees C and right ventricular function (fluid-filled balloon), right ventricular intracellular Ca(2+) transients (aequorin bioluminescence method) and the effects of ET-1 were determined. Monocrotaline-treated rats developed considerable right ventricular hypertrophy (right ventricular weight:body weight ratio: 1.07+/-0.13 v. 0.60+/-0.03 in controls P<0.05) and these hearts generated higher right ventricular systolic and diastolic pressure, but similar systolic and diastolic wall stress, indicating a compensated functional state. Hypertrophied hearts demonstrated a prolonged duration of isovolumic contraction (time to 90% decline from peak: 105+/-1 v 89+/-4 ms at 3 m M extracellular Ca(2+) P<0.05), but neither the time to peak pressure (71+/-3 ms) nor time to peak light (25+/-3 ms) were different from controls. The increased duration of contraction correlated with a similar prolongation of the Ca(2+)transient (time to 90% decline from peak: 72+/-4 v 50+/-3 ms P<0.05), indicating a reduced rate of Ca(2+)sequestration in hypertrophic right ventricles. Peak systolic intracellular Ca(2+)was similar in control and hypertrophied hearts (1.04+/-0.02 and 0.99+/-0.02 microM, P>0.05, n=6). ET-1 (1-300 p M) affected neither the time course of right ventricular contraction nor that of the Ca(2+)transient or peak systolic Ca(2+)concentrations. These data are the first measurements of right ventricular Ca(2+)transients in beating normal and hypertrophic hearts. We conclude that ET-1 plays no role in compensated hypertrophy because it affected neither right ventricular function nor intracellular Ca(2+)handling in this model.

Pharmacological analysis by HOE642 and KB-R9032 of the role of Na(+)/H(+) exchange in the endothelin-1-induced Ca(2+) signalling in rabbit ventricular myocytes

The role of Na(+)/H(+) exchange in endothelin-1 (ET-1)-induced increases in Ca(2+) transients and cell shortening was studied in rabbit ventricular myocytes loaded with indo-1/AM. Selective inhibitors of Na(+)/H(+) exchange HOE642 (4-isopropyl-3-methyl-sulphonylbenzoyl guanidine methanesulphonate) and KB-R9032 (N-(4-isopropyl-2,2-dimethyl-3-oxo-3, 4-dihydro-2H-benzo-[1,4]oxazine-6-carbonyl) guanidine methanesulphonate) were used as pharmacological tools for the analysis. ET-1 at 0.1 nM induced an increase in Ca(2+) transients by 45.6%, while it increased cell shortening by 109.6%. For a given increase in cell shortening, the ET-1-induced increase in Ca(2+) transients was much smaller than that induced by isoprenaline (ISO, 10 nM). Pretreatment with HOE642 and KB-R9032 (1 microM) inhibited the increase in cell shortening induced by 0.1 nM ET-1 by 51 and 65. 4%, respectively, without a significant alteration of ET-1-induced increase in Ca(2+) transients. HOE642 and KB-R9032 did not affect baseline levels of cell shortening and peak Ca(2+) transients, and the effects of ISO (10 nM). These results indicate that activation of Na(+)/H(+) exchange by ET-1 may play an important role in the positive inotropic effect and the ET-1-induced increase in myofilament Ca(2+) sensitivity in rabbit ventricular myocytes.

Characterization of endothelin receptors in the anesthetized ferret: a novel model for investigating the functional ET(B) receptor subtypes

The pharmacology of endothelin (ET)-1, big ET-1, ET-3, and S6c were characterized in the anesthetized ferret to assess whether this species would provide a new and suitable nonrodent model to be used in characterization of endothelin antagonists. Unlike other species such as dog, rabbit, and rat, the ferret exhibited a dose-dependent pressor response to both ET-1 and big ET-1 with no preceding vasodilatory response. The median effective concentration (ED50) values were 0.047+/-0.009 and 0.469+/-0.003 nmol/kg for ET-1 and big ET-1, respectively. ET-3 and S6c, however, were found to elicit a transient vasodilatory response preceding the pressor response, with ED50 values of 0.23+/-0.09 and 0.18+/-0.03 nmol/kg, respectively. The rank potency of the agonists for the pressor response was found to be ET-1 > S6c > big ET-1 > ET-3. The ET(A)-specific antagonist BQ-123 was shown to block only partially the ET-1 and big ET-1 pressor response with median antagonistic dose (AD50) of 0.24+/-0.11 and 0.015+/-0.005 mg/kg, i.v., respectively, and blockade of the ET(A) receptor did not uncover an ET(B)-induced vasodilation. The dual ET(A/B) antagonist L-754,142 completely antagonized the ET-1 and big ET-1 pressor responses with AD50 values of 0.195+/-0.063 and 0.019+/-0.006 mg/kg, respectively. The ET(B) antagonist BQ-788 blocked the depressor response of S6c entirely but was unable to antagonize the pressor response completely. BQ-123 was shown to antagonize the S6c pressor response partially, suggesting a possible interaction between the ET(A) and ET(B) receptors in the ferret. The unexpected absence of an ET-1-mediated depressor response but the presence of ET-3 and S6c vasodilation in this species supports the theory that there may be subtypes of the ET(B) receptor. These studies demonstrate that the anesthetized ferret provides a suitable model for assessing the physiological potencies of the endothelins and may provide a tool for further understanding of the diversity of the ET(B) receptor.

Muscarinic regulation of Ca2+ signaling in mammalian atrial and ventricular myocardium

The differential regulation of the contractility of mammalian atrial and ventricular myocardium upon activation of muscarinic receptors can be ascribed, for the most part, to alterations in intracellular Ca2+ transients. However, alterations in myofibrillar sensitivity to Ca2+ ions also contribute to such regulation. In atrial muscle, the following actions are all associated with the corresponding alterations in the amplitude of Ca2+ transients in the same direction as those in the strength of the contractile force: (1) the direct inhibitory action on the basal force of contraction; (2) the increase (recovery) in force that is induced during the prolonged stimulation of muscarinic receptors; and (3) the rebound increase in force induced by washout of muscarinic receptor agonists. In addition, for a given decrease in force induced by muscarinic receptor stimulation in atrial muscle, the amplitude of Ca2+ transients is decreased to a smaller extent than the decrease in amplitude induced by reduction of extracellular Ca2+ concentration ([Ca2+]o), an indication that muscarinic receptor stimulation might increase myofibrillar sensitivity to Ca2+ ions simultaneously with the reduction in the amplitude of Ca2+ transients during induction of the direct inhibitory action. In mammalian ventricular myocardium, the direct inhibitory action of muscarinic receptor stimulation exhibits a wide range of species-dependent variation. A pronounced direct inhibitory action is induced in ferret papillary muscle, which is also associated with a definite increase in myofibrillar sensitivity to Ca2+ ions. By contrast, in the ventricular myocardium of other species including the rabbit and the dog, muscarinic receptor stimulation scarcely affects the baseline Ca2+ transients and the force, but it results in a pronounced decrease in Ca2+ transients and force when applied in the presence of beta-adrenoceptor stimulation, a phenomenon known as 'accentuated antagonism' or the 'indirect inhibitory action' of muscarinic receptor stimulation in mammalian ventricular myocardium. During induction of the indirect inhibitory action in mammalian ventricular myocardium, muscarinic receptor stimulation reverses all the effects induced by beta-adrenoceptor stimulation, including the increase in Ca2+ transients, the positive inotropic and lusitropic effects, and the decrease in myofibrillar sensitivity to Ca2+ ions. The relationship between the amplitude of Ca2+ transients and force is unaffected during induction of the indirect inhibitory action in rabbit and dog ventricular myocardium. The direct and indirect inhibitory actions of muscarinic receptor stimulation on Ca2+ transients have clearly different dependences on frequency: the former is more pronounced at a higher rate of stimulation, while the latter is more pronounced at a lower rate. The more complex interaction of muscarinic receptor and beta-adrenoceptor stimulation in mammalian atrial muscle and ferret ventricular muscle might be explained by the contribution of both the direct and the indirect regulatory mechanisms to the interaction.

Role of Na+/Ca2+ exchange in endothelin-1-induced increases in Ca2+ transient and contractility in rabbit ventricular myocytes: pharmacological analysis with KB-R7943

1. The effects of endothelin-1 (ET-1) on intracellular Ca2+ ion level and cell contraction were simultaneously investigated in rabbit ventricular cardiac myocytes loaded with indo-1/A1. The role of Na+/Ca2+ exchange in ET-1-induced positive inotropic effect (PIE) was examined by using KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulphonate), a selective inhibitor of reverse mode Na+/Ca2+ exchange. 2. ET-1 at 0.3 pM - 1 nM increased cell contraction and Ca2+ transient (CaT) with EC50 values of 2.9 pM and 1.2 pM, respectively, and the increase in amplitude of CaT was much smaller relative to the PIE: ET-1 at 1 nM increased peak cell shortening by 237%, while it increased peak CaT by 167%. For a given level of PIE, ET-1-induced increase in CaT was much smaller than that induced by elevation of [Ca2+]o and by isoprenaline. Therefore, ET-1 shifted the relationship between peak CaT and cell shortening to the left relative to the relationship for increase in [Ca2+]o, an indication that ET-1 increased myofibrillar Ca2+ sensitivity. 3. KB-R7943 at 0.1 microM and higher inhibited contraction and CaT induced by 0.1 nM ET-1 and at 0.3 microM it abolished the increase in CaT while inhibiting the PIE by 48.1%. Over concentration range of 0.1-0.3 microM, KB-R7943 neither inhibited baseline contraction and CaT nor the isoprenaline-induced response, although at 1 microM and higher it had a significant inhibitory action on these responses. 4. These results indicate that in rabbit ventricular myocytes both increases in CaT and myofibrillar Ca2+ sensitivity contribute to the ET-induced PIE, and the activation of reverse mode Na+/Ca2+ exchange may play a crucial role in increase in CaT induced by ET-1 in rabbit ventricular cardiac myocytes.

Histamine H1-receptor-mediated increase in the Ca2+ transient without a change in the Ca2+ current in electrically stimulated guinea-pig atrial myocytes

The effects of histamine on the intracellular Ca2+ concentration ([Ca2+]i), action potential and membrane currents were assessed in single atrial myocytes prepared from guinea-pigs. Histamine caused a concentration-dependent increase in the [Ca2+]i transient in indol/AM loaded myocytes when stimulated electrically at 0.5 Hz. However, the maximum increase in [Ca2+]i transient produced by histamine was less than 50% of that elicited by isoprenaline. The histamine-induced increase in [Ca2+]i transient was significantly inhibited by chlorpheniramine, but not by cimetidine. Pretreatment with nifedipine nearly completely suppressed the histamine-induced increase in [Ca2+]i transient. Cyclopiazonic acid did not affect the histamine response. In the whole-cell current-clamp mode of the patch-clamp method, both histamine and isoprenaline prolonged action potential duration (APD) in atrial myocytes. In the presence of Co2+ or nifedipine, the isoprenaline-induced APD prolongation was abolished and an APD shortening effect was manifested, while histamine still increased APD. The APD prolongation elicited by histamine was reversed by chlorpheniramine. In the voltage-clamp mode, the histamine-sensitive membrane current was inwardly rectifying and reversed close to the calculated value of the K+ equilibrium potential. Histamine had no apparent effect on L-type Ca2+ current, in contrast to the pronounced effect of isoprenaline. These results indicate that in guinea-pig atrial myocytes stimulation of H1-receptors with histamine does not directly activate Ca2+ channels but causes an elevation of [Ca2+]i transient by increasing Ca2+ influx through the channels during the prolonged repolarization of action potentials resulting from inhibition of the outward K+ current.

Heterogeneity and underlying mechanism for inotropic action of endothelin-1 in rat ventricular myocytes

1. To clarify the mechanisms underlying the positive inotropic action of endothelin-1 (ET-1), we investigated the effect of ET-1 on twitch cell shortening and the Ca2+ transient in rat isolated ventricular myocytes loaded with a fluorescent Ca2+ indicator indo-1. 2. There was a cell-to-cell heterogeneity in response to ET-1. ET-1 (100 nM) increased twitch cell shortening in only 6 of 14 cells (44%) and the increase in twitch cell shortening was always accompanied by an increase in the amplitude of the Ca2+ transient. 3. The ET(A)- and ET(B)-receptors antagonist TAK-044 (100 nM) almost reversed both the ET-1-induced increases in twitch cell shortening and in the Ca2+ transient. In the ET-1 non-responding cells, the amplitude of the Ca2+ transient never increased. 4. Intracellular pH slightly increased (approximately 0.08 unit) after 30 min perfusion of ET-1 in rat ventricular myocytes. However, ET-1 did not change the myofilament responsiveness to Ca2+, which was assessed by (1) the relationship between the Ca2+ transient amplitude and twitch cell shortening, and by (2) the Ca2+ transient-cell shortening phase plane diagram during negative staircase. 5. We concluded that there was a cell-to-cell heterogeneity in the positive inotropic effect of ET-1, and that the ET-receptor-mediated positive inotropic effect was mainly due to an increase in the Ca2+ transient amplitude rather than to an increase in myofilament responsiveness to Ca2+.

Mechanism of action of endothelin in rat cardiac muscle: cross-bridge kinetics and myosin light chain phosphorylation

1. The molecular mechanism of inotropic action of endothelin was investigated in rat ventricular muscle by studying its effects on characteristics of isometric twitch, barium-induced steady contracture and the level of incorporation of 32Pi into myosin light chain 2. 2. Exposure of rat papillary muscle to endothelin caused an increase in isometric twitch force but did not alter the twitch-time parameters. 3. Endothelin did not significantly change the maximum contracture tension but did cause an increase in contracture tension at submaximal levels of activation, without changes in the tension-to-stiffness ratio and kinetics of attached cross-bridges. Kinetics of attached cross-bridges were deduced during steady contracture from complex-stiffness values, and in particular from the frequency at which muscle stiffness assumes a minimum value, fmin. Endothelin did not alter fmin. 4. Endothelin caused an increase in the level of incorporation of 32Pi into myosin light chain 2 without a concurrent change in the level of incorporation of 32Pi into troponin I. 5. We conclude that the inotropic action of endothelin is not due to an increase in the kinetics of attached cross-bridges, nor due to a change in the force per unit cross-bridge, but may result from an increased divalent cation sensitivity caused by elevated myosin light chain 2 phosphorylation, resembling post-tetanic potentiation in fast skeletal muscle fibres.

Origin of junctional rhythm during radiofrequency ablation of atrioventricular nodal reentrant tachycardia in patients without structural heart disease

Junctional rhythm is commonly observed during radiofrequency catheter ablation of the fast or slow pathways of atrioventricular nodal reentrant tachycardia (AVNRT). However, the origin of these beats remains unclear. We analyzed the retrograde atrial activation sequence of 16 patients (mean +/- SD: 41.2 +/- 18.9 years old) undergoing catheter ablation for typical AVNRT with detailed catheter mapping of the triangle of Koch. The earliest atrial activations were concordant during tachycardia and junctional rhythm in only 5 of 16 patients. The findings suggest that junctional rhythm is unlikely to represent direct stimulation of the atrioventricular (AV) node via a discrete slow pathway but rather results from enhanced automaticity from > or =1 sites in the AV nodal transitional zone. The ensuing atrial activation pattern results from anisotropic spread from these sites. In addition, these data imply that the original concept of the AV node comprising 2 anatomically defined pathways may not be valid, and that a functionally defined pathway model may be a more accurate representation.

Positive inotropic effect of angiotensin II. Increases in intracellular Ca2+ or changes in myofilament Ca2+ responsiveness?

Although it is well known that Angiotensin II (Ang II) has a direct positive inotropic effect in several species, the mechanisms of this action are still poorly understood. The aim of this review is to analyze the possible subcellular mechanisms underlying Ang II-induced positive inotropic action. The binding of Ang II to its receptor triggers a complex signal transduction cascade that stimulates the intracellular formation of two second messengers, inositol 1,4,5-triphosphate (IP3), and 1,2, diacylglycerol (DAG). IP3 triggers the release of Ca2+ from intracellular stores in several cell types and has been shown to increase myofilament Ca2+ sensitivity. DAG activates protein kinase C (PKC), an enzyme that catalyzes the phosphorylation of different cellular proteins, including several proteins of the myofibrils. Distinct ionic transporters, like the Na+/H+ antiporter and the Na(+)-independent Cl-/HCO3- exchanger, implicated in the regulation of intracellular pH, and the Na+/Ca2+ exchanger which contribute to the intracellular Ca2+ homeostasis, have been shown to be activated by a PKC-dependent mechanism. Thus, either one of the Ang II-induced second messengers, that is, IP3 and DAG, has the potential to affect myocardial contractility by modifying either intracellular Ca2+, myofilament Ca2+ responsiveness, or both. As described herein, the available data do not allow a definitive single model to explain the mechanism of the Ang II-induced positive inotropic effect. Moreover, it is possible that the final action of Ang II on myocardial inotropism is the end product of a complex interaction of several of the mechanisms triggered by the hormone.

Reversal of endothelin-induced vasoconstriction by endothelium-dependent and -independent vasodilators in isolated hearts and vascular rings

Endothelin (ET-1) is a potent endogenous vasoconstrictor. Several factors increase ET-1 release in vitro and ET-1 levels increase in vivo in situations that damage blood vessels. The aim of this study was to test the activity of several differently acting vasodilator drugs on reversing or attenuating the vasoconstrictor effects of exogenously administered ET-1 in isolated guinea-pig hearts, in isolated rings with intact endothelium from canine middle cerebral and basilar arteries, and from guinea-pig aortas. Vasodilator drugs tested up to maximal concentrations were adenosine (ADE), nitroprusside (NP), acetylcholine (ACH), nifedipine (NIF), and butanedione monoxime (BDM), an excitation-uncoupling agent. Variables measured in isolated hearts included coronary flow, percentage oxygen extraction (% O2E), left ventricular pressure (LVP), and myocardial oxygen consumption. It was found that ADE, NP, ACH, and BDM each attenuated the 60% decrease in coronary flow and 20% increase in % O2E elicited by 0.5 nM ET-1 in isolated hearts, but only BDM restored coronary flow, whereas BDM and ADE both restored % O2E. In isolated rings constricted with 20 nM ET-1, BDM restored tone equivalent to that by papaverine, whereas NP and NIF only attenuated the vasoconstriction elicited by ET-1. Ring experiments also demonstrated that the vasodilatory effect of BDM was independent of nitric oxide-dependent pathways and that BDM attenuated vasoconstriction resulting from increased bath KCl. The study suggests that drugs affecting intracellular Ca2+ with a mechanism of action downstream from cell-membrane receptors or intracellular messengers may be more effective for reversing the constrictor effect of ET-1. NP, however, would be a better clinical choice for reversing ET-1-induced vasoconstriction.

Different regulation of myofilament Ca2+ sensitivity in beta-escin-skinned cardiac and vascular smooth muscles

We compared the effects of guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS, an activator of G-protein), phorbol 12,13-dibutylate (PDB, an activator of protein kinase C) and pimobendan (an inotropic agent with Ca2+-sensitizing action) on the Ca2+ sensitivity of the contractile proteins in beta-escin-skinned muscle preparations obtained from rabbit left ventricles and mesenteric arteries. After the skinning procedure, when GTPgammaS (100 microM) or PDB (1 microM) was added to the Ca2+ solutions, pCa50 were significantly increased in preparations obtained from vascular smooth muscle, but not from cardiac muscle, indicating that G-protein- and protein kinase C-mediated direct Ca2+ sensitization may occur only in smooth muscle, but not in cardiac muscle. In contrast, pimobendan (50 microM) increased the Ca2+ responsiveness only in cardiac muscle. Therefore, we conclude that, in addition to the common regulatory factors affecting Ca2+ sensitivity such as intracellular pH and phosphorylation by protein kinase A, there are other means of regulation of Ca2+ sensitivity working differently in cardiac and in vascular smooth muscles.

Species-dependent differences in inotropic effects and phosphoinositide hydrolysis induced by endothelin-3 in mammalian ventricular myocardium

1. Species-dependent variations in the positive inotropic effect (PIE) of endothelin-3 (ET-3), and the relationships between the PIE and specific binding sites for [125I]-ET-3 and the PIE and the acceleration of phosphoinositide hydrolysis by ET-3, were studied in ventricular muscles from the rat, guinea-pig, rabbit, ferret and dog. 2. ET-3 in the presence of (+/-)-bupranolol (0.3 microM) and prazosin (0.3 microM) elicited a concentration-dependent PIE in the ventricular muscle from the rat, guinea-pig, rabbit and ferret. The potency of ET-3 and its efficacy in inducing a PIE were highest in the rabbit, intermediate in the rat and guinea-pig and lowest in the ferret. ET-3 did not have any inotropic effect on ventricular muscle from the dog. 3. Specific high-affinity binding of [125I]-ET-3 was observed with membrane fractions derived from the ventricular muscle of the five species. The maximal specific binding (Bmax) of ET-3 was highest in the rat and guinea-pig, intermediate in the rabbit and ferret and lowest in the dog. The values of KD in the rabbit and dog (33 and 52 pM) were lower than those in the rat, guinea-pig and ferret (141-221 pM). 4. In slices of ventricular muscle from all five species, ET-3 increased the accumulation of [3H]-inositol monophosphate (IP1) in a concentration-dependent manner. The extent of accumulation of IP1 was highest in the rat, intermediate in the guinea-pig and rabbit and lowest in the ferret and dog. 5. The results demonstrate the wide range of variations in the PIE of ET-3 on mammalian ventricular muscles. The variations in the coupling processes subsequent to the acceleration of the hydrolysis of PI, triggered by the binding of ET-3 to its receptor, might be important in these species-dependent differences in the PIE of ET-3.

Endothelin and angiotensin II stimulation of Na+-H+ exchange is impaired in cardiac hypertrophy

We compared the effects of endothelin-1 (ET-1) on intracellular pH, intracellular [Ca2+]i, and cell contraction in hypertrophied adult ventricular myocytes from ascending aortic banded rats and age-matched controls. Intracellular pH (pH(i)) was measured in individual myocytes with SNARF-1, and [Ca2+]i was measured with indo-1, simultaneous with cell motion. Experiments were performed at 36 degrees C in myocytes paced at 0.5 Hz in Hepes-buffered solution (pH(o) 7.40) containing 1.2 mM CaCl2. At baseline, calibrated pH(i), diastolic and systolic [Ca2+]i values, and the amplitude of cell contraction were similar in hypertrophied and control myocytes. Exposure of the control myocytes to 10 nM ET-1 caused an increase in the amplitude of cell contraction to 163+/-22% of baseline (P < 0.05), associated with intracellular alkalinization (pH(i) + 0.08+/-0.02 U, P < 0.05) and a slight increase in peak systolic [Ca2+]i (104+/-11% of baseline, P < 0.05). In contrast, in the hypertrophied myocytes, exposure to ET-1 did not increase the amplitude of cell contraction or cause intracellular alkalinization (-0.01+/-0.02 U, NS). Similar effects were observed in the hypertrophied and control myocytes in response to exposure to 10 nM angiotensin II. ET-1 also increased the rate of recovery from intracellular acidosis induced by the washout of NH4Cl in the control cells, but did not do so in the hypertrophied cells. In the presence of 10 microM 5-(N-ethyl-N-isopropyl)-amiloride, which inhibits Na+-H+ exchange, ET-1 did not cause a positive inotropic effect or intracellular alkalinization in control cells. The activation of protein kinase C by exposure to phorbol ester caused intracellular alkalinization and it increased the rate of recovery from intracellular acidification induced by an NH4Cl pulse in control cells but not in hypertrophied cells. ET-1, as well as angiotensin II, and phorbol ester, fail to stimulate forward Na+-H+ exchange in adult hypertrophied myocytes. These data suggest a defect in the coupling of protein kinase C signaling with Na+-H+ exchange in adult hypertrophied myocytes.

Ca(2+)-dependent and Ca(2+)-independent regulation of contractility in isolated human myocardium

Changes in contractile force of the myocardium may depend on changes in the intracellular Ca2+ concentration, changes in the responsiveness of the myofibrils for Ca2+, or a combination of both. We investigated in isolated muscle strip preparations from human nonfailing and endstage failing hearts the influence of physical (changes in preload, stimulation rate, or rhythm), and pharmacological interventions (alpha- or beta-adrenoceptor-stimulation, endothelin) on developed force of contraction and the corresponding intracellular Ca2+ transients.

METHODS

Isometric contraction, electrical stimulation, 37 degrees C. Simultaneous registration of force of contraction and intracellular Ca2+ transients (aequorin method).

RESULTS

Increases in preload, alpha- and endothelin-receptor stimulation resulted in increases in force of contraction without increasing aequorin light emission. Increasing stimulation rate or increasing rest intervals resulted in parallel increases (nonfailing myocardium) or decreases (failing myocardium) of force of contraction and aequorin light emission. beta-Adrenoceptor-stimulation exerted inotropic and lusitropic effects in human failing myocardium associated with a large, overproportional increase in aequorin light emission.

CONCLUSION

The human heart regulates intrinsic contractility via several subcellular mechanisms. Increases in preload (Frank-Starling-mechanism) and alpha- or endothelin-receptor-stimulation enhance myocardial contractility by increasing the Ca2+ responsiveness of the myofilaments; rate- and rhythm-dependent modulation of the contractile state directly depend on changes in the intracellular Ca(2+)-transients; beta-adrenoceptor stimulation results in an overproportional large increase in intracellular Ca2+ transients, probably due to additional cAMP-dependent Ca(2+)-desensitizing effects on the level of the myofibrils.

Endothelial cell regulation of contractility of the heart

Endocardial and coronary vascular endothelial cells release substances that modify the contraction of cardiac myocytes. The major and possibly the sole up-regulating substance is endothelin. Several down-regulating substances are secreted, but none has yet been specifically identified. The relative amounts of up- and down-regulating substances are related to tissue oxygen tension. As pO2 rises, the concentration of up- and down-regulating substances, respectively, increases and decreases. Endothelin increases isometric force and decreases actomyosin ATPase activity thus increasing the economy of conversion of chemical to hydrodynamic energy. Beta-adrenergic agonists increase ATPase activity through an endothelial cell-dependent mechanism, leading to decreased economy. Therefore, two endothelial cell-dependent systems exist for regulating contractile efficiency: One involving endothelin appears to optimize the contraction for efficiency; the other, the beta-adrenergic-mediated system, optimizes for power.

Effects of endothelin-1 on [Ca2+]i-shortening trajectory and Ca2+ sensitivity in rabbit single ventricular cardiomyocytes loaded with indo-1/AM: comparison with the effects of phenylephrine and angiotensin II

In most mammalian species, activation of myocardial endothelin as well as alpha1-adrenergic and angiotensin receptors leads to an increase in contractile function and myocardial cell hypertrophy, in association with acceleration of PI hydrolysis and with resultant production of IP3 and diacylglycerol. Therefore, these receptors may share a common intracellular signal transduction process in cardiac regulation. Although the pathophysiological relevance of endothelin- and angiotensin-mediated signal transduction has been postulated to play a key role in the progress of congestive heart failure, the details of the regulation are still controversial. We carried out experiments to further study the regulation induced by activation of these receptors. In spite of a wide range of species-dependent variation among mammals in the induction of the cardiotonic effect via these receptors, there is an excellent correlation between the extent of acceleration of PI hydrolysis and the positive inotropic effect (associated with a negative lusitropic effect) of the respective receptor agonists under most experimental conditions in rabbit ventricular myocardium. In isolated rabbit ventricular cardiomyocytes loaded with indo-1/AM, activation of these receptors elicited a very similar changes in the relationship between [Ca2+]i and cell shortening: the [Ca2+]i-shortening trajectory was shifted mainly upwards and the relationship of peak shortening vs peak [Ca2+]i was shifted to the left, an indication that the PIE of these agonists is consistently associated with an increase in [Ca2+]i and in the sensitivity of myofilaments to Ca2+ ions under the same experimental condition. Pieces of evidence in biochemical and pharmacological analyses imply that the products of PI hydrolysis, namely diacylglycerol and subsequent activation of protein kinase C, might play a crucial role in the regulation of cardiac function that is induced upon activation of endothelin, angiotensin and alpha-adrenergic receptors in the rabbit ventricular myocardium.

Paracrine coronary endothelial modulation of diastolic left ventricular function in man: implications for diastolic heart failure

Coordinated release of relaxing and contracting factors from the endothelium modulates arterial distensibility. Recently, a similar release of the same and other factors from the coronary endothelium was shown to modulate myocardial performance in humans. This paracrine modulation of left ventricular (LV) performance by substances released from the coronary endothelium mainly affects diastolic LV function. This was evident from the reduction in end-systolic LV pressure, the earlier onset of LV relaxation and the increased LV diastolic distensibility observed in normal subjects during bi-coronary infusion of substance P. In experimental preparations, substance P elicited similar effects on diastolic LV function, which were attributed to a paracrine myocardial action of nitric oxide (NO) because they were absent after addition of hemoglobin. In normal subjects, the myocardial effects of NO were investigated during bi-coronary infusion of the NO-donor sodium nitroprusside and resembled the effects observed during bi-coronary infusion of substance P. This paracrine control of diastolic LV function by the coronary endothelium is influenced by substrate availability and by many neurohumoral substances, whose plasma levels are raised in heart failure. In transplant recipients, bi-coronary co-infusion of substance P and of L-arginine, the substrate for NO production, potentiated the fall in LV filling pressures. Pretreatment with intravenous dobutamine augmented the drop in LV end-systolic pressures observed during bi-coronary infusion of substance P. In isolated papillary muscles, a higher baseline myocardial c-GMP level, as induced by atrial natriuretic peptide, potentiates the negative inotropic and relaxation hastening effects of NO. In isolated ejecting guinea-pig hearts, an endothelin receptor antagonist improved diastolic LV function and this improvement implies paracrine myocardial action on diastolic LV function not only of NO but also of endothelin. Coronary endothelial control of myocardial function affects LV performance both acutely and chronically. An acute increase in heart rate augments release of NO because of coronary reactive hyperemia, lowers LV filling pressures thereby promoting subendocardial perfusion, and hastens LV relaxation thereby prolonging the diastolic time interval for coronary perfusion. Chronic changes in coronary endothelial function could also influence diastolic LV performance. Enhanced coronary endothelial NO release, as occurs during chronic exercise or pacing, could explain increased LV diastolic distensibility observed in athlete's heart and in tachycardia cardiomyopathy. Reduced endothelial NO release, as occurs with aging or after transplantation, could contribute to reduced LV diastolic distensibility in the elderly or in allograft recipients.

The cardiac endothelium: functional morphology, development, and physiology

Cardiac endothelial cells, regardless of whether they are from endocardial or from coronary (micro)vascular origin, directly modulate performance of the subjacent cardiomyocytes, resulting in control of the onset of ventricular relaxation and rapid filling of the heart. This review summarizes major features of the morphology, embryology, and comparative physiology of cardiac endothelial cells as well as the experimental observations on how cardiac endothelial cells affect the mechanical performance of the heart. As for the underlying mechanisms of the interaction between cardiac endothelial cells and cardiomyocytes, two working hypotheses have been postulated over the past years; (1) interaction mediated through a trans-endothelial physicochemical gradient for various ions (active blood-heart barrier), and (2) interaction mediated through the release by the cardiac endothelial cells of various cardioactive substances, eg, nitric oxide, endothelin, and prostacyclin. These two mechanisms may act in concert or in parallel.

The cardiac endothelium: cardioactive mediators

Endothelial cells within the heart release a number of substances that modulate myocardial contractile function. These agents include nitric oxide, endothelin, prostanoids, adenylpurines, and other substances that have so far been characterized only in bioassay studies. A notable feature of many of these agents is that they influence contractile behavior predominantly by modifying cardiac myofilament properties rather than altering cytosolic Ca2+ transients. A consequence of this subcellular action is often a disproportionate effect on myocardial relaxation and diastolic tone. The paracrine modulation of cardiac myocyte function by endothelial cell factors is likely to be an important mechanism contributing to the overall regulation of cardiac contractile function, both physiologically and in pathological states.

Effects of a new endothelin receptor antagonist, TAK-044, on myocardial stunning in dogs

The effects of a new endothelin receptor antagonist, TAK-044, (cyclo[D-alpha-aspartyl-3-[(4-phenylpiperazin-1-yl)carbonyl]L-alan yl-L-alpha aspartyl-D-2-(2-thienyl)glycyl-L-leucyl-D-tryptophyl]disodium salt, on ischemic and post-ischemic myocardial dysfunction (stunned myocardium) were studied in anesthetized open-chest dogs. A short (15 min) occlusion of the left anterior descending coronary artery followed by 5-h reperfusion significantly reduced myocardial segment shortening during and after the ischemic period in the ischemic region. Regional myocardial blood flow was also decreased significantly 10 min after the occlusion, whereas it returned almost completely to its pre-ischemic value 5 h after reperfusion TAK-044 (3 mg/kg,i.v.) administered 10 min before occlusion significantly improved the reduced myocardial segment shortening in the ischemic region during and after occlusion. Cardiovascular hemodynamics and regional myocardial blood flow in a TAK-044-treated group were identical to those in the control group. These results indicate that endogenous endothelin contributes to the cause of ischemic and post-ischemic myocardial dysfunction without changing either hemodynamics or regional myocardial blood flow.

Localization of the origin of the atrioventricular junctional rhythm induced during selective ablation of slow-pathway conduction in patients with atrioventricular node reentrant tachycardia

During radiofrequency catheter ablation of slow atrioventricular node pathway conduction in patients with atrioventricular node reentrant tachycardia, an atrioventricular junction rhythm is frequently observed. The origin and relation to ablation success of this junctional rhythm was examined in this study. By using standard intracardiac electrophysiology techniques, we studied the radiofrequency energy-induced atrioventricular junctional rhythm in 43 consecutive patients with atrioventricular node reentrant tachycardia undergoing selective ablation of slow-pathway conduction. The frequency of atrioventricular junctional activity was correlated with successful and unsuccessful attempts at ablation of slow-pathway conduction. Also, we compared the sequence of retrograde atrial activation of radiofrequency energy-induced atrioventricular junctional beats in a subgroup of 22 patients with the retrograde activation sequence observed during pacing from the right ventricular apex and the site of successful ablation of slow-pathway conduction. A total of 201 radiofrequency-energy applications was delivered in 43 patients with > or = 5 atrioventricular junctional beat(s) induced during 110 (55%) of 201 ablation attempts. Atrioventricular junctional activity was noted during 98% of successful ablations but only 43% of the unsuccessful attempts (sensitivity, 98%; specificity, 57%; negative predictive value, 99%). The mean time to appearance of atrioventricular junctional beats was 8.8 +/- 4.1 sec (mean +/- SD) after the onset of radiofrequency-energy application. In 22 (100%) of 22 patients in whom detailed atrial mapping was performed, the retrograde atrial activation sequence of the radiofrequency-induced atrioventricular junctional beats was earliest in the anterior atrial septum, identical to that seen during pacing from the right ventricular apex. Earliest retrograde atrial activation was at the posterior septum in all patients during pacing from the successful ablation site, a markedly different activation pattern compared with that seen during either radiofrequency ablation or ventricular pacing. Whereas the occurrence of atrioventricular junctional activity during radiofrequency ablation does not necessarily herald a successful ablation of slow atrioventricular node pathway conduction, its absence strongly suggests that the energy is being applied in an unsuccessful fashion. Furthermore, it appears that radiofrequency energy-induced atrioventricular junctional beats originate not from the endocardium in contact with the ablating catheter tip but instead appear to exit remotely from the anterior atrial septal region. This finding supports the existence of specialized tissues in the atrioventricular junction that preferentially transmit the effects of radiofrequency energy to an anterior exit site, possibly identical to the atrial exit site of the retrograde fast atrioventricular node conduction pathway.

Altered responsiveness to endothelin-1 of myocardium from pacing-induced heart failure model in the dog

In congestive heart failure, in addition to a compensatory increase in neurohumoral activation, there is an increase in the endothelial-derived vasoconstrictive and positive inotropic substance, endothelin. Whether downregulation of the cardiac inotropic effects of this endothelial-derived substance occurs, as has been shown to occur with neurohumoral beta- and alpha-adrenergic agonists, remains unknown. In this study we investigated the effects of endothelin-1 [dose-response curve (10(-11) to 10(-7) M)] on the contractile characteristics of isolated papillary muscles from normal dogs and from dogs with heart failure induced by pacing overdrive, with or without removing endocardial endothelium from the papillary muscles. Endothelin-1 caused a similar absolute increase in myocardial contractile indices in all four groups, except for shortening, which increased more in muscles with heart failure without endocardial endothelium. However, in muscles with an intact endocardial endothelium, the relative increase was greater in muscles from pacing overdrive dogs (failing) as compared with normal dogs (tension increase of 110% vs. 53%, p < 0.01 and shortening increase of 127% vs. 24%, p < 0.01). Also, failing muscles with intact endocardial endothelium began responding to endothelin-1 at lower endothelin-1 concentrations (10(-10) vs. 10(-9) M) than normal muscles with intact endocardial endothelium. Endocardial endothelial removal increased the contractile effects of endothelin-1, whether this was done in normal or failing myocardium. This study thus indicates that, in contrast to other positive inotropic substances, in this model of heart failure there is an increase in sensitivity and relative response to endothelin-1. It also indicates that although endocardial endothelial removal increases the relative effects of endothelin-1 in both normal and failing myocardium, the increased responsiveness of failing myocardium is not endocardial endothelial dependent.

Cardiac alpha(1)-adrenoceptors that regulate contractile function: subtypes and subcellular signal transduction mechanisms

Activation of alpha(1)-adrenoceptors as well as endothelin (ET) and angiotensin II (Ang II) receptors in cardiac muscle is coupled to acceleration of the hydrolysis of phosphoinositide (PI), with resultant production of inositol 1,4,5-trisphosphate (IP(3)) and diacylglycerol. There is an excellent correlation between the extent of acceleration of the PI hydrolysis and the positive inotropic effect (PIE) under most experimental conditions after the administration of a-adrenoceptor agonists, ET and Ang II in the rabbit ventricular muscle. The PIE of the alpha-adrenoceptor agonists, ET and Ang II is associated with a negative lusitropic effect and an increase in the sensitivity of myofilaments to Ca(2)+ ions. The PIE can be selectively inhibited by inhibitors of protein kinase C (PKC) such as staurosporine, NA 0345 and H-7, with little effect on the PI hydrolysis and the PIE of isoproterenol and Bay k 8644. Surprisingly, an activator of PKC, phorbol 12,13-dibutyrate (PDBu), selectively and more completely inhibited the PIE and acceleration of PI hydrolysis induced by the alpha-adrenoceptor agonists as well as by ET and Ang II in the rabbit. These receptor agonists consistently cause intracellular alkalinization by activation of Na+-H+ exchange, while the effects on membrane ion channel activities are divergent. For example, alpha-adrenoceptor agonists cause monophasic prolongation of the action potential, the time course of which coincides well with that of the PIE, while ET and Ang II produce a biphasic change in action potential duration, i.e., the long-lasting prolongation preceded by a transient abbreviation. Alpha-adrenoceptor agonists scarcely affect I(ca), whereas ET elicits a biphasic alteration of the current. In addition, the potassium current, I(K1), is markedly suppressed by alpha-adrenoceptor agonists, but this effect is not revealed with Ang II under the same experimental condition. These results indicate that the effects of alpha(1)-adrenoceptor stimulation are partially shared by those of FT and Ang II receptor activation in the heart. Approximately 60% of the total population of alpha(1)-adrenoceptors in the rabbit ventricle are composed of alpha(1A) subtype, which is susceptible to chlorethylclonidine (CEC) and is predominantly responsible for the alpha(1)-mediated PIE and PI hydrolysis. The remaining fraction that belongs to alpha(1A) subtype is further subclassified into the WB 4101-sensitive (partly coupled to PI hydrolysis) and the niguldipine-sensitive (PI hydrolysis-unrelated) subtypes.

Influence of endothelin 1 on human atrial myocardium--myocardial function and subcellular pathways

The influence of endothelin 1 on isometrically contracting human atrial muscle strip preparations was investigated under physiological conditions (37 degrees C, 1 Hz, Ca2+ 2.5 mM). Endothelin dose-dependently increased isometric tension from 3 x 10(-10) M to 1 x 10(-7) M. At 1 x 10(-7) M the inotropic effect of endothelin was maximum with isometric tension being increased by 32 +/- 6% (n = 11, p < 0.05). At 1 x 10(-7) M endothelin the positive inotropic effect was preceded by a transient negative inotropic effect with a decline in tension by -5 +/- 1% (n = 11, p < 0.05). Endothelin prolonged time from peak tension to 50% relaxation (RT50) by 29 +/- 5%. With BQ123 a competitive antagonist of the ETA receptor positive inotropic effect and the prolongation of relaxation was significantly reduced and initial negative a inotropic effect was abolished, indicating a ETA receptor mediated effect. Preincubation with phorbolmyristateacetate (10(-5) M) to downregulate proteinkinase C (PKC) eliminated the positive inotropic effect of endothelin. Similarly, N-5,5-dimethylamiloride (10(-5) M) which inhibits Na+/H(+)-exchanger activity, abolished the positive inotropic effect of ET. However, with either PMA or DMA the initial transient negative inotropic effect was still present (-13 +/- 7%, n = 9, p < 0.05 and -3 +/- 1%, n = 6, p < 0.05). Furthermore, both substances did not abolish the prolongation of twitch time parameters observed under endothelin. After preincubation with PMA, endothelin prolonged RT50 by 18 +/- 6% and with DMA by 11 +/- 2%. Using the photoprotein aequorin as an indicator for intracellular calcium concentrations showed that the positive inotropic effect was mainly mediated by an increase of systolic intracellular calcium concentrations. Thus, the present data indicate that the positive inotropic effect of endothelin in human atrial myocardium results from activation of PKC with a subsequent activation of the Na+/H(+)-exchanger. However, the initial negative inotropic effects as well as the prolongation of relaxation seem to result from a different intracellular mechanism of endothelin.

Effects of endothelin-1 on the contractility of cardiomyocytes from the spontaneously hypertensive rat

1. Disturbances in cardiovascular responsiveness to endogenous endothelin-1 (ET-1) may play a significant role in the pathogenesis of essential hypertension. In this study the inotropic responses of cardiomyocytes derived from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rat (SHR) strains to ET-1 (10(-11)-10(-8) mol/L) were characterized. Isotonic contraction cycles of ventricular cardiomyocytes isolated from age-matched (11 week) WKY and SHR rats were recorded using a rapid digital imaging technique and evaluated by computation of a range of normalized parameters. 2. The maximum effect of ET-1, eliciting a 60-70% increase in myocyte shortening after 3 min, was observed at 10(-9) mol/L in both strains, and was associated with elevations in the rate of shortening and lengthening, abbreviated latency, contractile cycle prolongation and delayed time to peak shortening. 3. No evidence for a significant strain dependent difference in the relative responsiveness to ET-1 was detected. This finding indicates that altered sensitivity to ET-1 is unlikely to be a major factor underlying the development of hypertension in this model. 4. The distinct nature of the alterations in contractile parameters produced by ET-1 compared with angiotensin II (AII) suggests that the prevailing cellular mechanisms of action of these peptides are different and that ET-1 is not a paracrine or autocrine inotropic intermediate for AII.

Endothelin-1 does not phosphorylate phospholamban and troponin I in intact beating rat hearts

To determine a role of phosphorylation of specific cardiac regulatory proteins in the positive inotropic effect of endothelin-1, we examined phosphorylation of sarcoplasmic reticulum and myofibrillar proteins in perfused beating rat hearts treated with endothelin-1. In parallel experiments, the effects of isoprenaline and phorbol-12,13-dibutyrate (PDB) on protein phosphorylation were also tested. In 32Pi-labeled hearts, perfusion with isoprenaline (100 nM) caused 4.4- and 10.4-fold increases in the degree of phosphorylation of phospholamban in sarcoplasmic reticulum and of troponin I in myofibrils, respectively. In contrast, neither endothelin-1 (100 nM) nor PDB (1 microM) significantly changed the phosphorylation state of these proteins. These findings provide evidence that phosphorylation of major cardiac regulatory proteins is not responsible for the positive inotropic action of endothelin-1.

The effects of various drugs on the myocardial inotropic response

1. The signal transduction process mediated by cyclic AMP that leads to the characteristic positive inotropic effect (PIE) in association with a positive lusitropic effect (acceleration of rate of twitch relaxation) has been well established. Relationships between accumulation of cyclic AMP, changes in intracellular Ca2+ transients and the PIE differ, however, depending on the mechanism of particular drugs that affect different steps in the metabolism of cyclic AMP. Selective partial agonists of beta 1-adrenoceptors and inhibitors of phosphodiesterase (PDE) III cause the accumulation of less cyclic AMP for a given PIE than does isoproterenol. In addition, in aequorin-microinjected canine ventricular muscle, selective inhibitors of PDE III, OPC 18790 and Org 9731, produced smaller decreases in the responsiveness of myofilaments to Ca2+ ions than isoproterenol, while a partial agonist of beta 1-adrenoceptors, denopamine, elicits a decrease in Ca2+ responsiveness of the same extent as does isoproterenol. 2. Activation of myocardial alpha 1-adrenoceptors, as well as stimulation of receptors for endothelin and angiotensin II, which accelerates hydrolysis of phosphoinositide (PI) to result in production of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) are associated with very similar inotropic regulation: (1) the dependence on the species of animals of induction of the PIE; (2) an excellent correlation between the extent of acceleration of hydrolysis of PI and the PIE; (3) isometric contraction curves associated with a negative lusitropic effect; (4) the PIE associated with increases in myofibrillar responsiveness to Ca2+ ions; and (5) the selective inhibition of the PIE by an activator of protein kinase C (PKC), phorbol 12,13-dibutyrate (PDBu), with little effect on the PIE of isoproterenol and Bay k 8644. 3. A novel class of cardiotonic agents, namely, Ca2+ sensitizers such as EMD 53998 and Org 30029, act on the Ca(2+)-binding site of troponin C, increasing the affinity of these sites for Ca2+ ions, or at the actin-myosin interface to facilitate the cycling of cross-bridges. These agents produce a PIE with little change or decrease in Ca2+ transients and may bring about a significant breakthrough in the development of drugs for reversal of myocardial failure in the treatment of congestive heart failure.

Analysis of junctional ectopy during radiofrequency ablation of the slow pathway in patients with atrioventricular nodal reentrant tachycardia

BACKGROUND

Junctional ectopy may occur during radiofrequency (RF) catheter ablation of the slow pathway in patients with atrioventricular nodal reentrant tachycardia (AVNRT). The purpose of the present study was to characterize this junctional ectopy quantitatively.

METHODS AND RESULTS

The subjects of this study were 52 consecutive patients with AVNRT who underwent slow pathway ablation and 5 additional patients included retrospectively because they had developed high-degree atrioventricular (AV) block during the procedure. A combined anatomic and electrogram mapping approach was used for slow pathway ablation, and AVNRT was successfully eliminated in all patients. In the group of 52 consecutive patients, the incidence of junctional ectopy was significantly higher during 52 effective applications of RF energy than during 366 ineffective applications (100% versus 65%, P < .001). Compared with ineffective RF energy applications, successful RF energy applications had a significantly longer duration of individual bursts of junctional ectopy (7.1 +/- 7.1 versus 5.0 +/- 7.0 seconds [+/- SD], P < .05), a greater total number of junctional beats during the applications (24 +/- 16 versus 15 +/- 8, P < .01), and a greater total span of time during which junctional ectopy occurred (19 +/- 15 versus 11 +/- 12 seconds, P < .01). Four of the 52 patients plus an additional 5 patients developed transient AV block lasting 34 +/- 37 seconds. In 1 of the 9 patients who had transient AV block, third-degree AV nodal block requiring a permanent pacemaker recurred 2 weeks later. In each of the 9 patients who developed AV block, there was ventriculoatrial (VA) block in association with junctional ectopy during the RF energy application immediately preceding the AV block. Among 48 patients who did not develop AV block, 17 patients had at least one episode of VA block during junctional ectopy. The positive predictive value of VA block during junctional ectopy for the development of AV block was 19% in the consecutive series of 52 patients. Among 31 patients who always had 1:1 VA conduction in association with junctional ectopy, 12 had poor VA conduction in the baseline state, with a VA block cycle length of at least 500 milliseconds during ventricular pacing.

CONCLUSIONS

In patients with AVNRT undergoing slow pathway ablation, junctional ectopy during the application of RF energy is a sensitive but nonspecific marker of successful ablation. The bursts of junctional ectopy are significantly longer at effective target sites than at ineffective sites. VA conduction should be expected during the junctional ectopy that accompanies slow pathway ablation, even when there is poor VA conduction during baseline ventricular pacing. VA block during junctional ectopy is a harbinger of AV block in patients undergoing RF ablation of the slow pathway. If energy applications are discontinued as soon as VA block occurs, the risk of AV block may be markedly reduced.

Endothelin and vasoactive peptides: new cardiovascular mediators

A newly discovered family of vasoactive peptides, endothelins, have strong inotropic and chronotropic effects on the myocardium. Recent studies suggest a possible role for endothelin in pathophysiological states. The ability of these peptides to stimulate mitogenesis in smooth muscle and fibroblasts, coupled with their contribution to the regulation of gene expression and secretion of other neurohumoral mediators, raises the possibility that endothelins may contribute to the development of heart disease. Elevated levels of circulating endothelin have been found in the setting of elevated peak systolic pressure, pulmonary hypertension myocardial infarction, and congestive heart failure. This paper discusses the potential role of endothelin as a regional mediator of pathophysiological states.