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

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.

Role of Ca(2+)/calmodulin-dependent protein kinase II in the regulation of the cardiac L-type Ca(2+) current during endothelin-1 stimulation

Endothelin-1 (ET-1) shows a positive inotropic effect on cardiac muscle. Although the L-type Ca(2+) current (I(Ca)) is one of the important determinants of cardiac excitation-contraction coupling, the effect of ET-1 on the I(Ca) is not always clear. The controversial results appear to be due to different patch-clamp methods. The present study measured the effect of ET-1 on the I(Ca) of rat ventricular myocytes using the perforated patch-clamp technique. The holding potential was set to -40 mV, and depolarization was applied every 10 s. ET-1 (10 nM) increased the I(Ca) in a monophasic manner. The current reached a steady state 15 min after the application of ET-1, when the measurement was done. Endothelin receptor subtype expression was also investigated using Western immunoblotting. ET(A)-receptor protein was expressed, but ET(B)-receptor protein was not expressed, in the cell membranes of rat ventricular myocytes. The effect of ET-1 on the I(Ca) was inhibited by a selective ET(A)-receptor antagonist, BQ-123, but not by a selective ET(B)-receptor antagonist, BQ-788. The effect was inhibited by protein kinase C (PKC) inhibitor chelerythrine and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93, but not by its inactive analog KN-92. The effect of ET-1 was also blocked by another CaMKII inhibitor, autocamtide-2-related inhibitory peptide. These results suggest that ET-1 increases the I(Ca) via the ET(A)-receptor-PKC-CaMKII pathway.

The cardiovascular physiology and pharmacology of endothelin-1

One year after the discovery in 1980 that the endothelium was obligatory for acetylcholine to relax isolated arteries, it was clearly shown that the endothelium could also promote contraction. In 1988, Dr Yanagisawa's group identified endothelin-1 (ET-1) as the first endothelium-derived contracting factor. The circulating levels of this short (21 amino acids) peptide were quickly determined in humans and it was reported that in most cardiovascular diseases, circulating levels of ET-1 were increased and ET-1 was then recognized as a likely mediator of pathological vasoconstriction in human. The discovery of two receptor subtypes in 1990, ET(A) and ET(B), permitted optimization of bosentan, which entered clinical development in 1993, and was offered to patients with pulmonary arterial hypertension in 2001. In this report, we discuss the physiological and pathophysiological role of endothelium-derived ET-1, the pharmacology of its two receptors, focusing on the regulation of the vascular tone and as much as possible in humans. The coronary bed will be used as a running example, but references to the pulmonary, cerebral, and renal circulation will also be made. Many of the cardiovascular complications associated with aging and cardiovascular risk factors are initially attributable, at least in part, to endothelial dysfunction, particularly dysregulation of the vascular function associated with an imbalance in the close interdependence of NO and ET-1, in which the implication of the ET(B) receptor may be central.

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.

CaMKII-dependent reactivation of SR Ca(2+) uptake and contractile recovery during intracellular acidosis

In hearts, intracellular acidosis disturbs contractile performance by decreasing myofibrillar Ca(2+) response, but contraction recovers at prolonged acidosis. We examined the mechanism and physiological implication of the contractile recovery during acidosis in rat ventricular myocytes. During the initial 4 min of acidosis, the twitch cell shortening decreased from 2.3 +/- 0.3% of diastolic length to 0.2 +/- 0.1% (means +/- SE, P < 0.05, n = 14), but in nine of these cells, contractile function spontaneously recovered to 1.5 +/- 0.3% at 10 min (P < 0.05 vs. that at 4 min). During the depression phase, both the diastolic intracellular Ca(2+) concentration ([Ca(2+)](i)) and Ca(2+) transient (CaT) amplitude increased, and the twitch [Ca(2+)](i) decline prolonged significantly (P < 0.05). In the cells that recovered, a further increase in CaT amplitude and a reacceleration of twitch [Ca(2+)](i) decline were observed. The increase in diastolic [Ca(2+)](i) was less extensive than the increase in the cells that did not recover (n = 5). Blockade of sarcoplasmic reticulum (SR) function by ryanodine (10 microM) and thapsigargin (1 microM) or a selective inhibitor of Ca(2+)-calmodulin kinase II, 2-[N- (2-hydroxyethyl)-N-(4-methoxybenzenesulfonyl)] amino-N-(4-chlorocinnamyl)-N-methyl benzylamine (1 microM) completely abolished the reacceleration of twitch [Ca(2+)](i) decline and almost eliminated the contractile recovery. We concluded that during prolonged acidosis, Ca(2+)-calmodulin kinase II-dependent reactivation of SR Ca(2+) uptake could increase SR Ca(2+) content and CaT amplitude. This recovery can compensate for the decreased myofibrillar Ca(2+) response, but may also cause Ca(2+) overload after returning to physiological pH(i).

Role of endothelin-1 receptors in healthy anaesthetized rabbits

1. Many diseases are associated with elevated endothelin (ET)-1 plasma concentrations. In order to understand the consequence of this elevation, in the present study the effects of exogenous ET-1 on the entire organsim were investigated, in particular with respect to the role of ETA and ETB receptors in the cardiovascular system. In open-chest rabbits, left ventricular (LV) pressure (LVPmax, LVPed), dP/dtmax and dP/dtmin were recorded in ejecting and isovolumically beating hearts to determine cardiac function. In addition, heart rate (HR), aortic pressure (AoP) and aortic flow (AoF) were measured. Total peripheral resistance (TPR) was calculated from mean AoP and AoF. 2. In the first series of experiments (n = 11), ET-1 (0.5 nmol/kg; bolus) produced a non-significant reduction in HR. Systolic function, in terms of AoF, LVPmax and dP/dtmax, was improved; for example, LVPmax was increased significantly (69 +/- 10 vs 106 +/- 20 mmHg for control and ET-1, respectively; P < 0.05). Similarly, early relaxation (dP/dtmin) was improved. In parallel, TPR rose significantly (0.25+/-0.07 vs 0.35+/-0.1 mmHg/min per mL for control and ET-1, respectively; P < 0.05). Isovolumic measurements showed corresponding responses. 3. In the second series of experiments (n = 7), animals were pretreated with an ETA receptor antagonist (330 nmol/min per kg FR 139317). After ETA receptor blockade, the administration of ET-1 had no significant effect on cardiac function or vasomotion. 4. In the third series of experiments (n = 6), animals were pretreated with an ETB receptor antagonist (10 nmol/min per kg BQ 788). In this series of experiments, the effects of ET-1 on cardiac function and vasomotion were the same as in the first series of experiments, except for the effect on HR, which decreased by 35% after ET-1. 5. In our experimental model, exogenous ET-1 exerted a clear-cut positive inotropic effect, together with the anticipated peripheral vasoconstriction via ETA receptors.

Adrenomedullin and myocardial contractility in the rat

The effects of adrenomedullin in the regulation of myocardial contractility were investigated in the rat. In papillary muscles (n=6), adrenomedullin (0.1 to 10 nM) failed to show contractile effects. NO (nitric oxide) synthase inhibition with N(G)-nitro-L-arginine (L-NOARG) did not unmask any inotropic effect of adrenomedullin. The positive inotropic effect of isoprenaline (0. 01 nM to 10 microM) was identical after adrenomedullin, after L-NOARG, and after L-NOARG plus adrenomedullin (n=6 each). In field-stimulated rat ventricular myocytes, adrenomedullin (1, 10, and 100 nM; n=4 each) had impact neither on cell shortening nor on Ca(2+) transients. In isolated constant-flow perfused hearts (7.3+/-0.3 ml/min), adrenomedullin (1 nM, n=9; 10 nM, n=7) induced significant coronary vasodilation (-28%, -50%). In conclusion, adrenomedullin is a potent coronary vasodilator, but has no significant effects on myocardial contractility in the rat.

Investigating the dual nature of endothelin-1: ischemia or direct arrhythmogenic effect?

Endothelin-1 (ET-1) is a potent vasoconstrictor peptide, which may also elicit severe ventricular arrhythmias. The aims of our study were to compare the effects of total left anterior descending coronary artery (LAD) occlusion to intracoronary (ic.) ET-1 administration and to investigate the pathomechanism of ET-1 induced arrhythmias in 3 groups of anesthetized, open-chest mongrel dogs. In group A (n=10) a total LAD occlusion was carried out for 30 min, followed by a 60 min reperfusion period. In groups B and C ET-1 was administered into LAD for 30 min at a rate of 30 pmol/min (n=6) and 60 pmol/min (n=8). Epi- and endocardial monophasic action potential (MAP) recordings were performed to detect electrophysiologic changes and ischemia Blood samples for lactate measurements were collected from the coronary sinus (CS) and from the femoral artery. Infrared imaging was applied to follow epimyocardial heat emission changes. At the end of the ET-1 infusion period coronary blood flow (CBF) was reduced significantly in groups B and C (deltaCBF30MIN B: 21+/-2%, p<0.05; C: 35+/-2%, p<0.05), paralleled by a significant epimyocardial temperature decrease in group C (deltaT30MIN: -0.65+/-0.29 degrees C, p<0.05). Two dogs died of ventricular fibrillation (VF) in the reperfusion period in group A. Ventricular premature contractions and non-sustained ventricular tachycardic episodes appeared in group B, whereas six dogs died of VF in group C. Significant CS lactate level elevation indicating ischemia was observed only in group A from the 30th min occlusion throughout the reperfusion period (control vs. 30 min: 1.3+/-0.29 vs. 2.2+/-0.37 mmol/l, p<0.05). Epi- and endocardial MAP durations (MAPD90) and left ventricular epicardial (LV(EPI)) upstroke velocity decreased significantly in group A in the occlusion period. ET-1 infusion significantly increased LV(EPI) MAPD90 in group B and both MAPD90-s in group C. In conclusion, ischemic MAP and CS lactate changes were observed only in group A. Although ET-1 reduced CBF significantly in groups B and C, neither MAP nor lactate indicated ischemic alterations. ET-1 induced major ventricular arrhythmias appeared before signs of myocardial ischemia developed, though reduced CBF presumably contributed to sustaining the arrhythmias.

Effects of an endothelin receptor antagonist TAK-044 on myocardial energy metabolism in ischemia/reperfused rat hearts

The purpose of this study was to investigate the effects of an endothelin-receptor antagonist TAK-044 on functional defects and metabolic derangement in myocardial ischemia/reperfusion injury. We sequentially measured high-energy phosphate metabolites and intracellular pH by phosphorus magnetic resonance spectroscopy during 35-min global ischemia followed by 60-min reperfusion in Langendorff-perfused rat hearts. TAK-044 (initial loading by 3 mg/kg followed by perfusion with 100 nM solution) was administered in two different ways: before ischemia or immediately after reperfusion. In addition, we investigated the effects of TAK-044 on functional defects and metabolic alterations induced by hydrogen peroxide (200 microM, 30 min). The recoveries of left ventricular developed pressure after reperfusion in TAK-044 groups (51 +/-12% in TAK-I, 61 +/- 12% in TAK-R) were better than in control (10 +/- 5% in control; p < 0.01). Increases in left ventricular end-diastolic pressure (LVEDP) in TAK-044 groups (22 +/- 5 mm Hg in TAK-I, 24 +/- 5 mm Hg in TAK-R) were less than in control (38 +/- 3 mm Hg; p < 0.01). Adenosine triphosphate (ATP) (33 +/- 5% in TAK-I, 28 +/- 4% in TAK-R) in TAK-044 groups were higher than in control (13 +/- 3%; p < 0.01). The creatine phosphokinase (CPK) release during reperfusion in TAK-044 groups (3.3 +/- 1.5 IU/g wet wt/60 min in TAK-I, 3.5 +/- 2.5 IU/g wet wt/60 min in TAK-R) were lower than in control (13.8 +/- 3.9 IU/g wet wt/60 min; p < 0.05). In contrast, TAK-044 did not attenuate the myocardial injury induced by hydrogen peroxide. TAK-044, even if administered simultaneous with coronary reperfusion, attenuated myocardial ischemia/ reperfusion injury. The energy-preservative effect of TAK-044 could be associated with the good functional recovery in ischemia/reperfused rat hearts.