Effects of the positive inotropic agent Org 30029 on developed force and aequorin light transients in intact canine ventricular myocardium

Targeting the sarcomere to correct muscle function

Various human diseases can disrupt the balance between muscle contraction and relaxation. Sarcomeric modulators can be used to readjust this balance either indirectly by intervening in signalling pathways or directly through interaction with the muscle proteins that control contraction. Such agents represent a novel approach to treating any condition in which striated muscle function is compromised, including heart failure, cardiomyopathies, skeletal myopathies and a wide range of neuromuscular conditions. Here, we review agents that modulate the mechanical function of the sarcomere, focusing on emerging compounds that target myosin or the troponin complex.

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.

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.

The therapeutic potential of novel cardiotonic agents

During the course of treatment of heart failure patients, cardiotonic agents are inevitable for improvement of myocardial dysfunction. Clinically available agents, such as beta-adrenoceptor agonists and selective phosphodiesterase 3 inhibitors, act mainly via cyclic AMP/protein kinase A-mediated facilitation of Ca(2+) mobilisation (upstream mechanism). These agents are associated with the risk of Ca(2+) overload leading to arrhythmias, myocardial cell injury and premature cell death. In addition, they are energetically disadvantageous because of an increase in activation energy and metabolic effects. Cardiac glycosides act also via an upstream mechanism and readily elicit Ca(2+) overload with a narrow safety margin. No currently available agents act primarily via an increase in the myofilament sensitivity to Ca(2+) ions (central and/or downstream mechanisms). Novel Ca(2+) sensitisers under basic research may deserve clinical trials to examine the therapeutic potential to replace currently employed agents in acute and chronic heart failure patients. Molecular mechanisms of action of Ca(2+) sensitisers are divergent. In addition, they show a wide range of discrete pharmacological profiles due to additional actions associated with individual compounds. Therefore, the outcome of clinical trials has to be explained carefully based on these mechanisms of actions.

Mechanisms of action of novel cardiotonic agents

Regulation of myocardial contractility by cardiotonic agents is achieved by an increase in intracellular Ca2+ mobilization (upstream mechanism), an increase in Ca2+ binding affinity to troponin C (central mechanism), or facilitation of the process subsequent to Ca2+ binding to troponin C (downstream mechanism). cAMP mediates the regulation induced by Ca2+ mobilizers such as beta-adrenoceptor agonists and selective phosphodiesterase III inhibitors acting through the upstream mechanism. These agents act likewise on the central mechanism to decrease Ca2+ sensitivity of troponin C in association with the cAMP-mediated phosphorylation of troponin I. In addition to such a well-known action of cAMP, recent experimental findings have revealed that Ca2+ sensitizers, such as levosimendan, OR-1896, and UD-CG 212 Cl, require the cAMP-mediated signaling for induction of Ca2+ sensitizing effect. These agents shift the [Ca2+] -force relationship to the left, but their positive inotropic effect (PIE) is inhibited by carbachol, which suppresses selectively the cAMP-mediated PIE. These findings imply that cAMP may play a crucial role in increasing the myofilament Ca2+ sensitivity by cross-talk with the action of individual cardiotonic agents. No clinically available cardiotonic agents act primarily via Ca2+ sensitization, but the PIE of pimobendan and levosimendan is partly mediated by an increase in myofilament Ca2+ sensitivity. Evidence is accumulating that cardiotonic agents with Ca2+ sensitizing action are more effective than agents that act purely via the upstream mechanism in clinical settings. Further clinical trials are required to establish the effectiveness of Ca2+ sensitizers in long-term therapy for congestive heart failure patients.

Increase in myofibrillar Ca2+ sensitivity induced by UD-CG 212 Cl, an active metabolite of pimobendan, in canine ventricular myocardium

We performed experiments in canine ventricular trabeculae loaded with aequorin to elucidate the mechanism of positive inotropic effect of UD-CG 212 Cl (4,5-dihydro-6-[2-(4-hydroxyphenyl)-1H-benzimidazole-5-yl]-5-methyl-3(2H)-pyridazinone), an active metabolite of pimobendan. The maximum response to UD-CG 212 Cl achieved at 10(-5) M was 18% of ISOmax and it was associated with an increase in Ca2+ transients of 7% of ISOmax. For a given increase in force, the increase in Ca2+ transients induced by UD-CG 212 Cl was less than that induced by elevation of [Ca2+]o. The positive inotropic effect of UD-CG 212 Cl was not associated with an impairment of relaxation and it was abolished by carbachol. In conclusion, UD-CG 212 Cl has a positive inotropic effect partly due to an increase in myofibrillar Ca2+ sensitivity that is exerted via cross talk with a signal transduction pathway that involves cAMP.

Positive inotropic effects of calcium sensitizers on normal and failing cardiac myocytes

Calcium sensitizers increase myocardial contractile function without affecting Ca2+ homeostasis, which might be beneficial in the treatment of patients with heart failure. However, it remains uncertain whether Ca sensitizers induce quantitatively similar inotropic responses in control and failing hearts. To compare their effects in normal versus failing hearts at the cellular level, shortening mechanics and intracellular calcium ([Ca2+]i) transient were simultaneously measured in the left ventricular myocytes isolated from normal dogs (n = 8) and dogs with rapid pacing-induced heart failure (n = 7). CGP 48506 and EMD 57033 exerted a positive inotropic effect in a dose (0.1-3 microM)-dependent manner in both normal and heart failure myocytes. The percent increase of cell shortening magnitude was comparable between the two groups. CGP 48506 and EMD 57033 did not affect the diastolic cell length and resting [Ca2+]i level. They did not affect the duration of [Ca2+]i transient dynamics. Thus Ca2+ sensitizers exerted comparable positive inotropic effects without affecting the rest cell length and rest [Ca2+]i in normal and heart failure myocytes.

Inotropic effects of OR-1896, an active metabolite of levosimendan, on canine ventricular myocardium

We performed experiments in dog ventricular trabeculae loaded with aequorin to elucidate the mechanism of positive inotropic effect of (R)-N-[4-(4-methyl-6-oxo-1,4,5, 6-tetrahydro-pyridazin-3-yl)-phenyl]-acetamide (OR-1896), an active metabolite of (R)-([4-(1,4,5, 6-tetrahydro-4-methyl-6-oxo-3-pyridazinyl)phenyl]-hydrazono)-pr opaned initrile (levosimendan). Concentration-response curve for OR-1896 was biphasic: positive inotropic effect of OR-1896 reached a plateau at 10(-5) M (1st phase) and the concentration-response curve became steeper at 10(-3) M and higher (2nd phase). Maximum response of the 1st phase was 29% of maximal response to isoproterenol and associated with an increase in Ca(2+) transients of 13% of the maximal response to isoproterenol. For a given increase in force, the increase in Ca(2+) transients by OR-1896 was lower than that induced by elevation of [Ca(2+)](o). The positive inotropic effect of OR-1896 was not associated with impairment of relaxation and it was abolished by carbachol. In conclusion, OR-1896 has a positive inotropic effect partly due to an increase in myofibrillar Ca(2+) sensitivity that is exerted via cross-talk with signal transduction mediated by cAMP.

Effects of OR-1896, an active metabolite of levosimendan, on contractile force and aequorin light transients in intact rabbit ventricular myocardium

We performed experiments in rabbit ventricular papillary muscles loaded with aequorin to elucidate the mechanism of positive inotropic effect (PIE) of OR-1896, an active metabolite of levosimendan. The concentration-response curve (CRC) for OR-1896 was biphasic: PIE of OR-1896 reached a plateau at 10(-5) M (first phase), and the CRC became steeper at 10(-3) M and higher (second phase). Maximal response of the first phase was 11% of the maximal response to isoproterenol (ISOmax) and associated with an increase in Ca2+ transients of 5% of ISOmax. For a given degree of PIE, the increase in Ca2+ transients by OR-1896 was lower than that induced by elevation of [Ca2+]o. The PIE of OR-1896 was not associated with impairment of relaxation, and it was abolished by carbachol. In conclusion, OR-1896 has a PIE partly due to an increase in myofibrillar Ca2+ sensitivity that is exerted through crosstalk with signal transduction mediated by cyclic adenosine monophosphate (cAMP).

(-)-Enantiomer EMD 57439 antagonizes the Ca2+ sensitizing effect of (+)-enantiomer EMD 57033 on diastolic function but not on systolic function in rabbit ventricular cardiomyocytes

EMD 53998 (5-[1-(3,4-dimethoxybenzoyl)-1,2,3,4-tetrahydro-6-quinolyl]-6-meth yl-3,6-dihydro-2H-1,3,4-thiadiazin-2-one), the racemic mixture of (+)-enantiomer EMD 57033 and (-)-enantiomer EMD 57439, is a prototype of Ca2+ sensitizers that act via a central and/or down-stream mechanism in cardiac E-C coupling. In rabbit ventricular cardiomyocytes loaded with indo-1/AM, EMD 53998 and EMD 57033 shifted the relationship between Ca2+ transients and cell shortening (systolic function) to the left to the same extent as compared with that of elevation of [Ca2+]o. EMD 57439 did not elicit a positive inotropic effect (PIE). The PIE of EMD 57033 was associated with a more pronounced decrease in the diastolic cell length than that of EMD 53998, whereas the systolic effects of these compounds were equivalent. These results indicate that weak phosphodiesterase (PDE) III inhibition may exert a differential action on diastolic and systolic function. Thus, EMD 57439 antagonizes the Ca2+-sensitizing effect of EMD 57033 on diastolic function with no effect on systolic function, which may lead to a decrease in diastolic cell length of a lesser extent with the racemate EMD 53998 compared with (+)-enantiomer EMD 57033.

The force-interval relationship in human myocardium

The force-interval relationship is an important modulator of contractility in mammalian myocardium. In a number of mammalian species, increasing the frequency of stimulation results in an increase in force of contraction. Over the last 10 years, the effects of atrial pacing have been closely examined in normal human subjects and in patients with dilated cardiomyopathy, and the effects of the stimulation frequency have been investigated in isolated preparations from nonfailing and failing human hearts. An abnormal force-interval relationship in vivo and in vitro has been a consistent finding in patients with dilated cardiomyopathy, whereby an increase in stimulation frequency fails to increase the contractile response. The force-interval relationship of cardiac muscle has been shown to reflect intracellular calcium cycling and sarcoplasmic reticulum function. Therefore, agents that affect excitation-contraction coupling, in particular intracellular calcium mobilization and sarcoplasmic reticulum function, modulate the response of contraction force to stimulation frequency.

Effects of a novel cardiotonic agent, Org 9731, on force and aequorin light transients in intact ventricular myocardium of the dog: involvement of a cyclic AMP-mediated mechanism and myofibrillar responsiveness to Ca2+ ions

The action of a novel cardiotonic agent, Org 9731 (4-fluoro-N-hydroxy-5, 6-dimethoxy-benzo[b]thiophene-2-carboximidamide methanesulphonate), on intracellular aequorin light transients and isometric contractions was investigated in ventricular trabeculae isolated from dogs. The positive inotropic effect of Org 9731 at 3 microM and higher (up to 0.1 mM) was associated with an increase in the amplitude of the intracellular Ca2+ transient, but the effect of the compound at 0.3 and 1 mM was accompanied by a decrease of the transient. The maximum inotropic response to Org 9731 was approximately 70% of the maximum response to isoproterenol, while the maximum increase in the amplitude of Ca2+ transients produced by Org 9731 was about 30% of the maximum increase induced by isoproterenol. The duration of isometric contractions was prolonged by Org 9731 at 0.3 and 1 mM, with accompanying prolongation of the duration of light transients. The concentration-response curve for the positive inotropic effect of Org 9731 was markedly shifted by carbachol (3 microM), being moved to the right and downward, and the maximum response to Org 9731 was about 10% of that to isoproterenol in the presence of carbachol. Carbachol abolished the increase in the light transient and the accumulation of adenosine 3',5'-cyclic monophosphate (cyclic AMP) induced by Org 9731. These results indicate that Org 9731 increases cardiac contractility, mainly through the accumulation of cyclic AMP up to a concentration of 0.1 mM and also by increasing the responsiveness of myofibrils to Ca2+ ions at 0.3 mM and higher in association with the attenuation of Ca2+ transients. The structure-activity relationship implies that the introduction of a fluorine atom at position 4 of the benzothiophene ring of Org 30,029 attenuated its Ca(2+)-sensitizing action but markedly increased the activity of mechanisms dependent on cyclic AMP.

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.