Biochemical mechanisms for the inotropic effect of the cardiotonic drug milrinone

The interface between monitoring and physiology at the bedside

Hemodynamic instability as a clinical state represents either a perfusion failure with clinical manifestations of circulatory shock or heart failure or 1 or more out-of-threshold hemodynamic monitoring values, which may not necessarily be pathologic. Different types of causes of circulatory shock require different types of treatment modalities, making these distinctions important. Diagnostic approaches or therapies based on data derived from hemodynamic monitoring assume that specific patterns of derangements reflect specific disease processes, which respond to appropriate interventions. Hemodynamic monitoring at the bedside improves patient outcomes when used to make treatment decisions at the right time for patients experiencing hemodynamic instability.

Pharmaceutical management of decompensated heart failure syndrome in children: current state of the art and a new approach

Prompt initiation of appropriate and intensive treatment in children with decompensated heart failure is crucial to avoid irreversible end-organ dysfunction. Initial management of these children includes transfer to the pediatric cardiac intensive care unit, basic hemodynamic monitoring, and establishment of intravenous access. Inotropic support should be instituted peripherally before obtaining central venous and arterial access. The team should be prepared for emergent intubation and initiation of mechanical circulatory support. Two experienced physicians should work together to obtain vascular access and manage sedation, airway control, and cardiovascular support. Acute heart failure syndrome in children may be related to cardiomyopathy, myocarditis, congenital heart disease, and acute rejection post heart transplantation. Each of these causes requires a different approach. Fulminant myocarditis may lead to severe morbidity and requires intensive support, although its outcome is considered to be good. Acute heart failure related to newly diagnosed dilated cardiomyopathy may represent end-stage heart failure; therefore, long-term mechanical circulatory support and heart transplantation may be considered to avoid other end-organ dysfunction. Hypertrophic cardiomyopathy may lead to acute decompensation due to 1) left ventricular outflow obstruction, 2) restrictive physiology leading to pulmonary hypertension, or 3) myocardial ischemia associated with coronary artery bridging. Decompensated heart failure associated with congenital heart disease usually represents end-stage heart failure and requires thorough evaluation for heart transplantation. Children with single-ventricle physiology who develop decompensated heart failure after a Fontan procedure are not candidates for mechanical circulatory support and therefore may not survive to heart transplantation. Acute heart failure due to posttransplantation acute rejection requires aggressive antirejection treatment, which places these patients at significant risk for overwhelming opportunistic infections. In our opinion, mechanical circulatory support should be initiated early in children who present with end-stage heart failure associated with hemodynamic instability to avoid end-organ damage.

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.

New milrinone analogues: in vitro study of structure-activity relationships for positive inotropic effect, antagonism towards endogenous adenosine, and inhibition of cardiac type III phosphodiesterase

Two mechanisms are responsible for the positive inotropic effect of the cardiotonic drug milrinone, i.e., inhibition of type III cAMP phosphodiesterase (PDE III), and displacement of endogenous adenosine from A(1) inhibitory receptor. Since PDE III inhibition may increase the likelihood of cardiac arrhythmias by increasing cAMP content, our attention focused on the synthesis of new compounds with more pronounced characteristics as adenosine antagonists. In this work, four new milrinone analogues were studied, in comparison with the parent drug, for their effects on the contractility of guinea pig isolated atrial preparations, their ability to antagonize endogenous adenosine at the level of A(1) receptor, and to inhibit the activity of PDE III partially purified from guinea pig heart. The new compounds present various chemical substitutions with respect to the parent drug: in compounds SF397 (methyl 5-cyano-2-methyl-6-oxo-1,6-dihydropyridine-3-carboxylate) and SF399 (benzyl 5-cyano-2-methyl-6-oxo-1,6-dihydropyridine-3-carboxylate), the 4-pyridil moiety of milrinone was replaced with a methoxycarbonyl and a benzyloxycarbonyl group, respectively; the same structural modifications were also associated with the replacement of the cyano-group in 5-position with an acetyl group in compounds SF416 (methyl 5-acetyl-2-methyl-6-oxo-1,6-dihydropyridine-3-carboxylate) and SF419 (benzyl 5-acetyl-2-methyl-6-oxo-1,6-dihydropyridine-3-carboxylate). All the new compounds had a marked positive inotropic effect, most of them also being more active and more potent than milrinone. When their affinity for A(1) receptor was assessed as the displacement of [(3)H] 8-cyclopentyl-1,3-dipropylxanthine ([(3)H]DPCPX) from cardiac membranes, SF397 and SF399 showed affinity (K(i) of about 600 nM) similar to that of milrinone (K(i) 550 nM). By contrast, SF416 and SF419 had very low (K(i) of about 10000 nM) or scarce (K(i) of about 2000 nM) anti-adenosine component, respectively. All the new compounds inhibited PDE III activity, their K(i) values proceeding in the following order: milrinone (3.80 microM) <SF397 (7.00 microM) <SF399 (8.80 microM) <SF416 (35.00 microM) SF419< (155.00 microM). To better characterize the mechanisms responsible for the positive inotropic response of the new compounds, we also investigated the effects of new analogues on some systems (ATP-dependent Ca(2+) uptake, Ca(2+)ATPase, Na(+)/K(+)ATPase, Na(+)/Ca(2+) exchange carrier) or a receptor (beta-adrenoceptor) mainly involved in the control of cardiac contractility. None of the tested compounds inhibited enzyme or transport systems; however, SF397, SF399 and SF416, although to different extents, had a direct beta-adrenergic action. Indications about structure-activity relationships are tentatively discussed, in order to obtain useful information for the design of new analogues with better pharmacological profiles.

Mechanisms of positive inotropic action of flosequinan, hydralazine, and milrinone on mammalian myocardium

Flosequinan is an arterial and venous dilator that also has a positive inotropic effect at relatively higher doses. The purpose of this study was to determine the mechanism of this positive inotropic effect in ferret papillary muscles loaded with the Ca2+ indicator, aequorin. Over the range of doses from 10(-6) to 10(-3) M, flosequinan produced a 61 +/- 9% increase in peak tension that was accompanied by a corresponding increase in [Ca2+]i. This positive inotropic effect was not selectively blocked by addition to the perfusate of procaine 0.6 microM, tetrodotoxin 10(-6) M or by verapamil, 5 x 10(-8) M. In contrast, the positive inotropic effect of flosequinan, but not milrinone or hydralazine, was potentiated by prior addition of ouabain 3 nM to enhance intracellular Ca2+ via reduction of the Na+/Ca2+ exchange. Moreover, antagonists of Na+/Ca2+ exchange, including cadmium 10 microM, amiloride 600 microM and choline substitution for 1/3 Na+ in the perfusate, blocked the response to flosequinan but not hydralazine or milrinone. These results indicate that flosequinan produces a positive inotropic effect by reduction of Na+/Ca2+ exchange in mammalian myocardium. Moreover, flosequinan has the potential to interact synergistically with other positive inotropic agents such as digoxin that affect Na+/Ca2+ exchange by direct or indirect actions.

An analysis of the mechanism of the inotropic action of some milrinone analogues in guinea-pig isolated atria

1. It has been reported previously that the milrinone analogues, ethyl 5-cyano-1,6-dihydro-2-methyl-6-oxo-3 pyridine carboxylate (I) and ethyl 5-cyano-1,6-dihydro-2-ethyl-6-oxo-3 pyridine carboxylate (II) exert a positive inotropic effect (EC50 = 15.6 +/- 0.2 microM and 40.3 +/- 0.1 microM) both on spontaneously beating and on electrically driven atria from reserpine-treated guinea-pigs. In the present study the mechanism of the inotropic action of these two agents was investigated. 2. In electrically driven left atrium from reserpine-treated guinea-pigs the EC50 values for inotropic activity for compounds (I) and (II) corresponded to that of milrinone (EC50 = 25 +/- 0.1 microM) but compound (I) induced a greater maximum effect. This corresponded to a percentage increase in developed tension over control of 63 +/- 0.3 whereas the maximum inotropic effect of milrinone was 48 +/- 0.3 and that of compound (II) was 47 +/- 0.2. 3. The inotropic activity of compounds (I) and (II) (10-100 microM) was resistant to propranolol (0.1 microM), thus excluding the involvement of beta-adrenoceptors. 4. Since the inotropism induced by compounds (I) and (II) was not reduced by carbachol (1 nM-0.5 microM), an action involving changes in adenosine 3':5'-cyclic monophosphate (cyclic AMP) can be excluded. 5. The inotropic action of compounds (I) and (II) was blocked selectively by 8-phenyltheophyline (10 microM) or adenosine deaminase (2 u ml-1). 6. Both (I) and (II) inhibited, in an apparently competitive manner, the negative inotropic effect induced by N6-(L-phenylisopropyl) adenosine (L-PIA), a stable adenosine agonist. The pA2 values for (I) and (II) were 4.79 and 4.36, respectively.7. In rat brain compounds (I) and (II) inhibited the specific binding of N6-cyclohexyl[3H]-adenosine- ([3H]-CHA) with an IC50 of 0.18 + 0.01 mM and 0.25 + 0.02 mm, respectively, which were similar to their IC50 values for blocking the PIA-induced negative inotropic effect and which are also in the range of concentrations that are effective in inducing positive inotropism in guinea-pig atria.8. The results from the present study suggest that antagonism of endogenous purines causes positive inotropism without affecting intracellular cyclic AMP levels.