Impact of NO on ET-1- and AM-induced inotropic responses: potentiation by combined administration

Physiological regulation of cardiac contractility by endogenous reactive oxygen species

Increased production of reactive oxygen species (ROS) has been linked to the pathogenesis of congestive heart failure. However, emerging evidence suggests the involvement of ROS in the regulation of various physiological cellular processes in the myocardium. In this review, we summarize the latest findings regarding the role of ROS in the acute regulation of cardiac contractility. We discuss ROS-dependent modulation of the inotropic responses to G protein-coupled receptor agonists (e.g. β-adrenergic receptor agonists and endothelin-1), the potential cellular sources of ROS (e.g. NAD(P)H oxidases and mitochondria) and the proposed end-targets and signalling pathways by which ROS affect contractility. Accumulating new data supports the fundamental role of endogenously generated ROS to regulate cardiac function under physiological conditions.

Atrial myocardium is the predominant inotropic target of adrenomedullin in the human heart

Adrenomedullin (ADM) is an endogenous peptide with favorable hemodynamic effects in vivo. In this study, we characterized the direct functional effects of ADM in isolated preparations from human atria and ventricles. In electrically stimulated human nonfailing right atrial trabeculae, ADM (0.0001–1 μmol/l) increased force of contraction in a concentration-dependent manner, with a maximal increase by 35 ± 8% (at 1 μmol/l; P < 0.05). The positive inotropic effect was accompanied by a disproportionate increase in calcium transients assessed by aequorin light emission [by 76 ± 20%; force/light ratio (ΔF/ΔL) 0.58 ± 0.15]. In contrast, elevation of extracellular calcium (from 2.5 to 3.2 mmol/l) proportionally increased force and aequorin light emission (ΔF/ΔL 1.0 ± 0.1; P < 0.05 vs. ADM). Consistent with a cAMP-dependent mechanism, ADM (1 μmol/l) increased atrial cAMP levels by 90 ± 12%, and its inotropic effects could be blocked by the protein kinase A (PKA) inhibitor H-89. ADM also exerted positive inotropic effects in failing atrial myocardium and in nonfailing and failing ventricular myocardium. The inotropic response was significantly weaker in ventricular vs. atrial myocardium and in failing vs. nonfailing myocardium. In conclusion, ADM exerts Ca2+-dependent positive inotropic effects in human atrial and less-pronounced effects in ventricular myocardium. The inotropic effects are related to increased cAMP levels and stimulation of PKA. In heart failure, the responsiveness to ADM is reduced in atria and ventricles.

Synergistic activation of salmon cardiac function by endothelin and beta-adrenergic stimulation

The aim was to find out the effects of endothelin-1 (ET-1) in salmon (Salmo salar) cardiac contractile and endocrine function and its possible interaction with beta-adrenergic regulation. We found that ET-1 has a positive inotropic effect in salmon heart. ET-1 (30 nM) increased the contraction amplitude 17+/-4.7% compared with the basal level. beta-Adrenergic activation (isoprenaline, 100 nM) increased contraction amplitude 30+/-13.1%, but it did not affect the contractile response to ET-1. ET-1 (10 nM) stimulated the secretion of salmon cardiac natriuretic peptide (sCP) from isolated salmon ventricle (3.3+/-0.14-fold compared with control) but did not have any effect on ventricular sCP mRNA. Isoprenaline alone (0.1-1,000 nM) did not stimulate sCP release, but ET-1 (10 nM) together with isoprenaline (0.1 nM) caused a significantly greater increase of sCP release than ET-1 alone (5.4+/-0.07 vs. 3.3+/-0.14 times increase compared with control). The effects on the contractile and secretory function could be inhibited by a selective ETA-receptor antagonist BQ-610 (1 microM), whereas ETB-receptor blockage (by 100 nM BQ-788) enhanced the secretory response. Thus ET-1 is a phylogenetically conserved regulator of cardiac function, which has synergistic action with beta-adrenergic stimulation. The modulatory effects of ET-1 may therefore be especially important in situations with high beta-adrenergic tone.

Adrenomedullin: regulator of systemic and cardiac homeostasis in acute myocardial infarction

During and following acute myocardial infarction, a variety of endogenous mediators are elevated, one of which is adrenomedullin (AM). AM is a multifunctional peptide that has been identified as having a putative beneficial role following an ischemic insult at both systemic and local levels. Classically described as a potent vasodilator, natriuretic, and diuretic agent, experimental infarct models also demonstrate AM to exhibit antiproliferative and antiapoptotic functions in the myocardium, counterregulating the effects of mediators such as angiotensin-II and endothelin-1. Less well documented are the angiogenic and inflammatory modulating potentials of AM, which may also contribute toward reducing adverse ventricular remodeling. The review examines clinical and experimental studies, looking at the effects of AM and cellular mechanisms that could be involved in mediating cardioprotective effects and ultimately optimizing left ventricular remodeling. Finally, the possibility of enhancing endogenous actions of AM by pharmacological intervention is considered.

Direct coronary vasodilator action of adrenomedullin is mediated by nitric oxide

Increased circulating levels of adrenomedullin (ADM) cause peripheral vasodilatation and hypotension, accompanied by cardiac actions including tachycardia and increases in cardiac contractility, cardiac output, coronary conductance (CC) and coronary blood flow (CBF). It is unclear to what extent these cardiac effects are direct actions of ADM or secondary to the hypotension and altered cardiac loading. The direct cardiac actions of ADM were examined in conscious sheep previously implanted with aortic and coronary flow probes, and an indwelling left coronary artery cannula. Responses to infusion of ADM (0.5 microg kg(-1) h(-1) for 1 h) into the left coronary artery or jugular vein were compared (n=6). The effect of blockade of nitric oxide (NO) synthase with intracoronary (i.c.) N(omega)-nitro-l-arginine (l-NNA; 1.5 mg kg(-1) h(-1), infused for 2 h before and during ADM infusion, was assessed to determine whether the responses to ADM were mediated by NO (n=5). I.c. ADM caused large and sustained increases in CC (0.35+/-0.07-0.55+/-0.13 ml min(-1) mmHg-1, P<0.05) and CBF (28+/-6-42+/-9 ml min(-1), P<0.05), but had no effect on arterial pressure or indices of cardiac contractility (first differential of the upstroke of systole and peak aortic flow rate). Intravenous infusion of ADM had no effects. I.c. l-NNA, at a dose that abolished the coronary vasodilator action of acetylcholine, blocked ADM-induced coronary vasodilatation. In conclusion, ADM had a direct coronary vasodilator action that was mediated by release of endogenous NO and resulted in increased CBF. There was no evidence for a direct inotropic action of ADM.

Dual role of endothelin-1 via ETA and ETB receptors in regulation of cardiac contractile function in mice

An increase in coronary perfusion pressure leads to increased cardiac contractility, a phenomenon known as the Gregg effect. Exogenous endothelin (ET)-1 exerts a positive inotropic effect; however, the role of endogenous ET-1 in the contractile response to elevated load is unknown. We characterized here the role of ETA and ETB receptors in regulation of contractility in isolated, perfused mouse hearts subjected to increased coronary flow. Elevation of coronary flow from 2 to 5 ml/min resulted in 80 +/- 10% increase in contractile force (P < 0.001). BQ-788 (ETB receptor antagonist) augmented the load-induced contractile response by 35% (P < 0.05), whereas bosentan (ETA/B receptor antagonist) and BQ-123 (ETA receptor antagonist) attenuated it by 34% and 56%, respectively (P < 0.05). CV-11974 (ANG II type 1 receptor antagonist) did not modify the increase in contractility. These results show that endogenous ET-1 is a key mediator of the Gregg effect in mouse hearts. Moreover, ET-1 has a dual role in the regulation of cardiac contractility: ETA receptor-mediated increase in contractile force is suppressed by ETB receptors.

Endothelin-1 contributes to the Frank-Starling response in hypertrophic rat hearts

Endothelin-1 is involved in mechanical load-induced cardiac growth processes; it also has effects on contractility. The interaction of endothelin-1 and the Frank-Starling response is unknown. The present study aimed to characterize the role of endothelin-1 in the regulation of the Frank-Starling response, one of the major mechanisms regulating cardiac contractile force, in both normal and hypertrophied hearts. Nontransgenic rat hearts and hypertrophic hearts of hypertensive double transgenic rats harboring human angiotensinogen and renin genes were studied in a Langendorff isolated heart setup with a liquid-filled balloon inside the left ventricle used to measure contractile parameters. The rats were studied at compensated phase, before showing any signs of heart failure. Compensated hypertrophy in double transgenic rat hearts resulted in improved contractility at a given level of preload when compared with nontransgenic rat hearts. Hearts of both rat lines showed preserved Frank-Starling responses, that is, increased contractile function in response to increased end-diastolic pressure. The mixed endothelin A/B receptor antagonist bosentan attenuated the Frank-Starling response by 53% (P<0.01) in the double transgenic hearts but not in nontransgenic hearts. The diastolic parameters remained unaffected. The left ventricles of the double transgenic rat hearts showed an 82% higher level of endothelin type A receptor mRNA and a 25% higher level of immunoreactive endothelin-1 compared with nontransgenic rat hearts. The type 1 angiotensin II receptor antagonist CV-11974 had no significant effect on contractile function in response to load in either strain. These results show that endogenous endothelin-1 contributes to the Frank-Starling response in hypertrophied rat hearts by affecting systolic performance.

Apelin, the novel endogenous ligand of the orphan receptor APJ, regulates cardiac contractility

The orphan receptor APJ and its recently identified endogenous ligand, apelin, exhibit high levels of mRNA expression in the heart. However, the functional importance of apelin in the cardiovascular system is not known. In isolated perfused rat hearts, infusion of apelin (0.01 to 10 nmol/L) induced a dose-dependent positive inotropic effect (EC50: 33.1+/-1.5 pmol/L). Moreover, preload-induced increase in dP/dt(max) was significantly augmented (P<0.05) in the presence of apelin. Inhibition of phospholipase C (PLC) with U-73122 and suppression of protein kinase C (PKC) with staurosporine and GF-109203X markedly attenuated the apelin-induced inotropic effect (P<0.001). In addition, zoniporide, a selective inhibitor of Na+-H+ exchange (NHE) isoform-1, and KB-R7943, a potent inhibitor of the reverse mode Na+-Ca2+ exchange (NCX), significantly suppressed the response to apelin (P<0.001). Perforated patch-clamp recordings showed that apelin did not modulate L-type Ca2+ current or voltage-activated K+ currents in isolated adult rat ventricular myocytes. Apelin mRNA was markedly downregulated in cultured neonatal rat ventricular myocytes subjected to mechanical stretch and in vivo in two models of chronic ventricular pressure overload. The present study provides the first evidence for the physiological significance of apelin in the heart. Our results show that apelin is one of the most potent endogenous positive inotropic substances yet identified and that the inotropic response to apelin may involve activation of PLC, PKC, and sarcolemmal NHE and NCX.

Overexpression of sarcolemmal calcium pump attenuates induction of cardiac gene expression in response to ET-1

The function of the plasma membrane calmodulin-dependent calcium ATPase (PMCA) in myocardium is unknown. PMCA is localized in caveolae, 50- to 100-nm membrane invaginations, which also contain receptors for endothelin-1 (ET-1) and various other ligands. PMCA has been suggested to play a role in regulation of caveolar signal transduction. We studied the effects of the hypertrophic agonist ET-1 and increased coronary perfusion pressure on cardiac synthesis of B-type natriuretic peptide (BNP) in transgenic rats overexpressing the human PMCA 4CI in isolated perfused heart preparation. ET-1 infusion for 2 h increased BNP mRNA levels twofold in left ventricles (LV) of nontransgenic rats, whereas no increase was noted in PMCA rat hearts. Similar responses were seen in adrenomedullin and c-fos mRNA levels, and in immunoreactive BNP secretion. Increased mechanical load produced by elevated perfusion pressure induced similar 1.5- to 1.6-fold increases in LV BNP mRNA in both nontransgenic and PMCA rat hearts. These results show that cardiac overexpression of PMCA attenuates ET-1-stimulated early induction of cardiac gene expression, suggesting that PMCA may modulate myocardial growth responses.