The therapeutic potential of novel cardiotonic agents

Neonatal blood pressure support: the use of inotropes, lusitropes, and other vasopressor agents

A solid understanding of the mechanisms of action of cardiovascular medications used in clinical practice along with efforts to develop comprehensive hemodynamic monitoring systems to improve the ability to accurately identify the underlying pathophysiology of cardiovascular compromise are essential in the management of neonates with shock. This article reviews the mechanisms of action of the most frequently used cardiovascular medications in neonates. Because of paucity of data from controlled clinical trials, evidence-based recommendations for the clinical use of these medications could not be made. Careful titration of the given medication with close monitoring of the cardiovascular response might improve the effectiveness and decrease the risks associated with administration of these medications.

Cardiac Ca2+ signaling and Ca2+ sensitizers

The role of Ca2+ in cardiac excitation-contraction (E-C) coupling has been established by simultaneous measurements of contractility and Ca2+ transients by means of aequorin in intact myocardium and Ca2+ sensitive fluorescent dyes in single myocytes. The E-C coupling process can be classified into 3 processes: upstream (Ca2+ mobilization), central (Ca2+ binding to troponin C) and downstream mechanism (thin filament regulation and crossbridge cycling). These mechanisms are regulated differentially by various inotropic interventions. Positive force-frequency relationship and effects of beta-adrenoceptor stimulation, phosphodiesterase 3 inhibitors and digitalis are essentially exerted via upstream mechanism. Alpha-adrenoceptor stimulation, endothelin-1, angiotensin II, and clinically available Ca2+ sensitizers, such as levosimendan and pimobendan, act by a combination of the upstream and central/downstream mechanism. The Frank-Starling mechanism and effects of Ca2+ sensitizers such as EMD 57033 and Org 30029 are primarily induced via the central/downstream mechanism. Whereas the upstream and central mechanisms are markedly suppressed in failing myocytes and under acidotic conditions, Ca2+ sensitizers such as EMD 57033 and Org 30029 can induce cardiotonic effects under such conditions. Ca2+ sensitizers have high therapeutic potential for the treatment of contractile dysfunction in congestive heart failure and ischemic heart diseases, because they have energetic advantages and less risk of Ca2+ overload and can maintain effectiveness under pathological conditions.

Could Ca2+ sensitizers rescue patients from chronic congestive heart failure?

Attempts to ameliorate cardiac contractile dysfunction by Ca(2+) mobilizers, such as catecholamines, phosphodiesterase (PDE) inhibitors and digitalis, play an important role in pharmacotherapy for congestive heart failure (CHF), but these agents possess disadvantages in causing Ca(2+) overload resulting in arrhythmogenicity and damage to cardiomyocytes. Ca(2+) sensitizers that act directly on contractile proteins are free from the risk of Ca(2+) overload and they could improve haemodynamic parameters with minimum increase in energy expenditure even under pathological conditions, including acidosis and stunned myocardium. Beneficial effects of levosimendan (that acts by combination of Ca(2+) sensitization and PDE inhibition) on CHF due to hypertensive cardiomyopathy in Dahl/Rapp rats as reported in this issue demonstrate the potential of oral levosimendan in long-term treatment of chronic CHF. Since chronic CHF in clinical settings is much more complex, careful analysis of clinical outcomes will be required to establish the therapeutic relevance of Ca(2+) sensitizers in the treatment of chronic CHF.

Acute heart failure: inotropic agents and their clinical uses

Inotropic agents are indispensable for the improvement of cardiac contractile dysfunction in acute or decompensated heart failure. Clinically available agents, including sympathomimetic amines (dopamine, dobutamine, noradrenaline) and selective phosphodiesterase-3 inhibitors (amrinone, milrinone, olprinone and enoximone) act via cAMP/protein kinase A (PKA)-mediated facilitation of intracellular Ca2+ mobilisation. Phosphodiesterase-3 inhibitors also have a vasodilatory action, which plays a role in improving haemodynamic parameters in certain patients, and are termed inodilators. The available inotropic agents suffer from risks of Ca2+ overload leading to arrhythmias, myocardial cell injury and ultimately, cell death. In addition, they are energetically disadvantageous because of an increase in activation energy and cellular metabolism. Furthermore, they lose their effectiveness under pathophysiological conditions, such as acidosis, stunned myocardium and heart failure. Pimobendan and levosimendan (that act by a combination of an increase in Ca2+ sensitivity and phosphodiesterase-3 inhibition) appear to be more beneficial among existing agents. Novel Ca2+ sensitisers that are under basic research warrant clinical trials to replace available inotropic agents.

Signal transduction and Ca2+ signaling in intact myocardium

The experimental procedures to simultaneously detect contractile activity and Ca(2+) transients by means of the Ca(2+) sensitive bioluminescent protein aequorin in multicellular preparations, and the fluorescent dye indo-1 in single myocytes, provide powerful tools to differentiate the regulatory mechanisms of intrinsic and external inotropic interventions in intact cardiac muscle. The regulatory process of cardiac excitation-contraction coupling is classified into three categories; upstream (Ca(2+) mobilization), central (Ca(2+) binding to troponin C), and/or downstream (thin filament regulation of troponin C property or crossbridge cycling and crossbridge cycling activity itself) mechanisms. While a marked increase in contractile activity by the Frank-Starling mechanism is associated with only a small alteration in Ca(2+) transients (downstream mechanism), the force-frequency relationship is primarily due to a frequency-dependent increase of Ca(2+) transients (upstream mechanism) in mammalian ventricular myocardium. The characteristics of regulation induced by beta- and alpha-adrenoceptor stimulation are very different between the two mechanisms: the former is associated with a pronounced facilitation of an upstream mechanism, whereas the latter is primarily due to modulation of central and/or downstream mechanisms. alpha-Adrenoceptor-mediated contractile regulation is mimicked by endothelin ET(A)- and angiotensin II AT(1)-receptor stimulation. Acidosis markedly suppresses the regulation induced by Ca(2+) mobilizers, but certain Ca(2+) sensitizers are able to induce the positive inotropic effect with central and/or downstream mechanisms even under pathophysiological conditions.

Dual regulation of myofilament Ca2+ sensitivity by levosimendan in normal and acidotic conditions in aequorin-loaded canine ventricular myocardium

Experiments were carried out in canine ventricular trabeculae loaded with aequorin to investigate the effects of levosimendan {(R)-([4-(1,4,5,6-tetrahydro-4-methyl-6-oxo-3-pyridazinyl)phenyl]-hydrazono)-propanedinitrile} on contractile force and Ca(2+) transients in normal and acidotic conditions. The concentration-response curve for the positive inotropic effect (PIE) of levosimendan was bell-shaped, that is, it declined markedly at 10(-4) M after achieving the maximum at 10(-5) M in normal (pH(o)=7.4) and acidotic conditions (pH(o)=6.6). The positive inotropic effect (PIE) of levosimendan up to 10(-5) M was associated with an increase in Ca(2+) transients and a shift of the relationship of Ca(2+) transients and force to the left of that of elevation of [Ca(2+)](o). Levosimendan at 10(-4) M elicited a negative inotropic effect (NIE) in association with a further increase in Ca(2+) transients, and during washout Ca(2+) transients increased further, while the force was abolished before both signals recovered to the control. In acidotic conditions, the relationship of Ca(2+) transients and force during the application of levosimendan in normal conditions was essentially unaltered, whereas the PIE was suppressed due to attenuation of the increase in Ca(2+) transients. In summary, in intact canine ventricular myocardium, levosimendan elicits a dual inotropic effect: at lower concentrations, it induces a PIE by a combination of increases in Ca(2+) transients and Ca(2+) sensitivity, while at higher concentrations it elicits an NIE due to a decrease in Ca(2+) sensitivity. Acidosis inhibits the PIE of levosimendan due to suppression of the increase in Ca(2+) transients in response to the compound.