Negative inotropic effect of endothelin-1 in the mouse right ventricle

Developmental Changes in the Excitation-Contraction Mechanisms of the Ventricular Myocardium and Their Sympathetic Regulation in Small Experimental Animals

The developmental changes in the excitation-contraction mechanisms of the ventricular myocardium of small animals (guinea pig, rat, mouse) and their sympathetic regulation will be summarized. The action potential duration monotonically decreases during pre- and postnatal development in the rat and mouse, while in the guinea pig it decreases during the fetal stage but turns into an increase just before birth. Such changes can be attributed to changes in the repolarizing potassium currents. The T-tubule and the sarcoplasmic reticulum are scarcely present in the fetal cardiomyocyte, but increase during postnatal development. This causes a developmental shift in the Ca2+ handling from a sarcolemma-dependent mechanism to a sarcoplasmic reticulum-dependent mechanism. The sensitivity for beta-adrenoceptor-mediated positive inotropy decreases during early postnatal development, which parallels the increase in sympathetic nerve innervation. The alpha-adrenoceptor-mediated inotropy in the mouse changes from positive in the neonate to negative in the adult. This can be explained by the change in the excitation-contraction mechanism mentioned above. The shortening of the action potential duration enhances trans-sarcolemmal Ca2+ extrusion by the Na+-Ca2+ exchanger. The sarcoplasmic reticulum-dependent mechanism of contraction in the adult allows Na+-Ca2+ exchanger activity to cause negative inotropy, a mechanism not observed in neonatal myocardium. Such developmental studies would provide clues towards a more comprehensive understanding of cardiac function.

The Role of G Protein-Coupled Receptors in the Right Ventricle in Pulmonary Hypertension

Pressure overload of the right ventricle (RV) in pulmonary arterial hypertension (PAH) leads to RV remodeling and failure, an important determinant of outcome in patients with PAH. Several G protein-coupled receptors (GPCRs) are differentially regulated in the RV myocardium, contributing to the pathogenesis of RV adverse remodeling and dysfunction. Many pharmacological agents that target GPCRs have been demonstrated to result in beneficial effects on left ventricular (LV) failure, such as beta-adrenergic receptor and angiotensin receptor antagonists. However, the role of such drugs on RV remodeling and performance is not known at this time. Moreover, many of these same receptors are also expressed in the pulmonary vasculature, which could result in complex effects in PAH. This manuscript reviews the role of GPCRs in the RV remodeling and dysfunction and discusses activating and blocking GPCR signaling to potentially attenuate remodeling while promoting improvements of RV function in PAH.

Muscle on a chip: in vitro contractility assays for smooth and striated muscle

INTRODUCTION

To evaluate the viability of a muscle tissue, it is essential to measure the tissue's contractile performance as well as to control its structure. Accurate contractility data can aid in development of more effective and safer drugs. This can be accomplished with a robust in vitro contractility assay applicable to various types of muscle tissue.

METHODS

The devices developed in this work were based on the muscular thin film (MTF) technology, in which an elastic film is manufactured with a 2D engineered muscle tissue on one side. The tissue template is made by patterning extracellular matrix with microcontact printing. When muscle cells are seeded on the film, they self-organize with respect to the geometric cues in the matrix to form a tissue.

RESULTS

Several assays based on the "MTF on a chip" technology are demonstrated. One such assay incorporates the contractility assay with striated muscle into a fluidic channel. Another assay platform incorporates the MTFs in a multi-well plate, which is compatible with automated data collection and analysis. Finally, we demonstrate the possibility of analyzing contractility of both striated and smooth muscle simultaneously on the same chip.

DISCUSSION

In this work, we assembled an ensemble of contractility assays for striated and smooth muscle based on muscular thin films. Our results suggest an improvement over current methods and an alternative to isolated tissue preparations. Our technology is amenable to both primary harvests cells and cell lines, as well as both human and animal tissues.

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.

Endothelin-1 and angiotensin II modulate rate and contraction amplitude in a subpopulation of mouse embryonic stem cell-derived cardiomyocyte-containing bodies

Embryonic stem cell-derived cardiomyocytes (ESC-CMs) have applications in understanding cardiac disease pathophysiology, pharmacology, and toxicology. Comprehensive characterization of their basic physiological and pharmacological properties is critical in determining the suitability of ESC-CMs as models of cardiac activity. In this study we use video microscopy and quantitative PCR to investigate the responses of mouse ESC-CMs to adrenoceptor, muscarinic, angiotensin II (Ang II), and endothelin-1 (ET-1) receptor activation. Isoprenaline (10 nM-10 μM) increased beating rate and contraction amplitude in all beating bodies (BBs), whereas carbachol (up to 1 μM) and the I(f) channel blocker ZD-7288 (10 μM) decreased contraction frequency. ET-1 (0.01-100 nM) reduced contraction amplitude in all BBs and increased contraction frequency in 50% of BBs; these effects were blocked by the ET(A) receptor antagonist BQ123 (250 nM). Ang II (0.01 nM-1 μM) increased both contraction amplitude (all BBs) and frequency (in 50% of BBs), effects blocked, respectively, by losartan (100 nM) and PD123,319 (200 nM). These results indicate the presence of functional ET(A) and both AT₁ and AT₂ receptors in murine ESC-CMs, but their expression and or activity appears to be evident only in a limited set of BBs.

Overexpression of diacylglycerol kinase zeta inhibits endothelin-1-induced decreases in Ca2+ transients and cell shortening in mouse ventricular myocytes

Endothelin-1 (ET-1) is released in various cardiovascular disorders including congestive heart failure, and may modulate significantly the disease process by its potent action on vascular and cardiac muscle cell function and gene regulation. In adult mouse ventricular cardiomyocytes loaded with indo-1, ET-1 induced a sustained negative inotropic effect (NIE) in association with decreases in Ca(2+) transients. The ET-1-induced effects on Ca(2+) transients and cell shortening were abolished in diacylglycerol (DAG) kinase zeta-overexpressing mouse ventricular myocytes. A nonselective protein kinase C (PKC) inhibitor, GF109203X, inhibited the ET-1-induced decreases in Ca(2+) transients and cell shortening in concentration-dependent manners, whereas a selective Ca(2+)-dependent PKC inhibitor, Gö6976, did not affect the ET-1-induced effects. A phospholipase Cbeta inhibitor, U73122, and an inhibitor of phospholipase D, C(2)-ceramide, partially, but significantly, attenuated the ET-1-induced effects. Derivatives of the respective inhibitors with no specific effects, U73343 and dihydro-C(2)-ceramide, did not affect the ET-1-induced effects. Taken together, these results indicate that activation of a Ca(2+)-independent PKC isozyme by 1,2-DAG, which is generated by phospholipase Cbeta and phospholipase D activation and inactivated by phosphorylation via DAG kinase, is responsible for the ET-1-induced decreases in Ca(2+) transients and cell shortening in mouse ventricular cardiomyocytes.

Intracellular mechanisms and receptor types for endothelin-1-induced positive and negative inotropy in mouse ventricular myocardium

We examined the intracellular mechanisms for endothelin-1-induced positive and negative inotropic components that coexist in the mouse ventricular myocardium using isolated ventricular tissue and myocytes from 4-week-old mice. In the presence of SEA0400, a specific inhibitor of the Na+-Ca2+ exchanger, endothelin-1 produced positive inotropy. Endothelin-1, when applied to cardiomyocytes in the presence of SEA0400, did not change the peak amplitude of the Ca2+ transient but increased intracellular pH and Ca2+ sensitivity of contractile proteins. On the other hand, in the presence of dimethylamiloride (DMA), a specific inhibitor of the Na+-H+ exchanger, endothelin-1 produced negative inotropy. In cardiomyocytes, in the presence of DMA, endothelin-1 produced a decrease in peak amplitude of the Ca2+ transient. In the presence of both DMA and SEA0400, endothelin-1 produced neither positive nor negative inotropy. Positive inotropy was blocked by BQ-123 and negative inotropy by BQ-788. These results suggested that endothelin-1-induced positive inotropy is mediated by ET(A) receptors, activation of the Na+-H+ exchanger and an increase in intracellular pH and Ca2+ sensitivity and that the negative inotropy is mediated by ET(B) receptors, activation of the Na+-Ca2+ exchanger and decrease in Ca2+ transient amplitude.

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.

Inotropic effects of ETB receptor stimulation and their modulation by endocardial endothelium, NO, and prostaglandins

Endothelin (ET)-1 acts on ETA and ETB receptors. The latter include ETB1 (endothelial) and ETB2 (muscular) subtypes, which mediate opposite effects on vascular tone. This study investigated, in rabbit papillary muscles ( n = 84), the myocardial effects of ETB stimulation. ET-1 (10−9 M) was given in the absence or presence of BQ-123 (ETA antagonist). The effects of IRL-1620 (ETB1 agonist, 10−10–10−6 M) or sarafotoxin S6c (ETB agonist, 10−10–10−6 M) were evaluated in muscles with intact or damaged endocardial endothelium (EE); intact EE, in the presence of NG-nitro-l-arginine (l-NNA); and intact EE, in the presence of indomethacin (Indo). Sarafotoxin S6c effects were also studied in the presence of BQ-788 (ETB2 antagonist). ET-1 alone increased 64 ± 18% active tension (AT) but decreased it by 4 ± 2% in the presence of BQ-123. In muscles with intact EE, sarafotoxin S6c alone did not significantly alter myocardial performance. Sarafotoxin S6c (10−6 M) increased, however, AT by 120 ± 27% when EE was damaged and by 39 ± 8% or 23 ± 6% in the presence of l-NNA or Indo, respectively. In the presence of BQ-788, sarafotoxin S6c decreased AT (21 ± 3% at 10−6 M) in muscles with intact EE, an effect that was abolished when EE was damaged. IRL-1620 also decreased AT (22 ± 3% at 10−6 M) in muscles with intact EE, an effect that was abolished when EE was damaged or in the presence of l-NNA or Indo. In conclusion, the ETB-mediated negative inotropic effect is presumably due to ETB1 stimulation, requires an intact EE, and is mediated by NO and prostaglandins, whereas the ETB-mediated positive inotropic effect, observed when EE was damaged or NO and prostaglandins synthesis inhibited, is presumably due to ETB2 stimulation.

Positive inotropic effect of endothelin-1 in the neonatal mouse right ventricle

In neonatal mouse right ventricles, endothelin-1 (ET-1, 1-300 nM) induced a dose-dependent increase in twitch contractions and the dose-response curve was shifted to the right by BQ-123 (10 microM), an endothelin ET(A) receptor antagonist. The ET-1 (100 nM)-induced positive inotropy was accompanied by an increase in [Ca(2+)](i) transients without any change in the [Ca(2+)](i)-force relationship. Ryanodine (1 microM) partially decreased the [Ca(2+)](i) transients and contractile force, but did not affect the ET-1 (100 nM)-induced positive inotropy. Reduction of [Na(+)](o) elicited an increase in contractile force, and this effect was significantly inhibited by KB-R7943 (30 microM), an inhibitor of the Na(+)-Ca(2+) exchanger. KB-R7943 (30 microM) almost completely suppressed the positive inotropic effect of ET-1. Activation of protein kinase C (PKC) by phorbol 12,13-dibutylate (100 nM) decreased the contractile force, an effect which was suppressed by bisindolylmaleimide I (3 microM). On the other hand, the ET-1-induced positive inotropic effect was unaffected by bisindolylmaleimide I (3 microM). These results suggest that the positive inotropic effect of ET-1 in neonatal mouse right ventricles is caused by the increase in [Ca(2+)](i) transients through activation of the endothelin ET(A) receptor and the increase in Ca(2+) influx via the Na(+)-Ca(2+) exchanger during an action potential. Furthermore, the ET-1-induced positive inotropy is independent of the effects of PKC, which makes it distinct from the ET-1-mediated pathways reported for cardiac tissues in other species.

Pharmacological evidence for involvement of phospholipase D, protein kinase C, and sodium-calcium exchanger in alpha-adrenoceptor-mediated negative inotropy in adult mouse ventricle

The intracellular signalling pathway for alpha-adrenoceptor-mediated negative inotropy was studied pharmacologically in isolated adult mouse ventricle. The negative inotropy was inhibited by GF-109203X, a nonselective protein kinase C inhibitor. Phorbol 12-myristate 13-acetate also produced sustained negative inotropy, which was inhibited by KB-R7943, a Na(+)/Ca(2+) exchanger inhibitor. The alpha-adrenoceptor-mediated negative inotropy was augmented by RHC-80267, a diacylglycerol lipase inhibitor, but was inhibited either by C(2)-ceramide, a phospholipase D inhibitor, and high concentration of propranolol (50 micro M), which inhibits phosphatidate phosphohydrolase. The inotropy was not affected by U-73122, a phospholipase C inhibitor. Lavendustin-A, a tyrosine kinase inhibitor, also inhibited the negative inotropy. These findings suggest that alpha-adrenoceptor-mediated negative inotropy in adult mouse ventricle is mediated by activation of tyrosine kinase, the phospholipase D-phosphatidate phosphohydrolase pathway, and protein kinase C.