Antagonism of novel inotropic agents at A1 adenosine receptors and m-cholinoceptors in human myocardium

Contrasting Effects of Inhibition of Phosphodiesterase 3 and 5 on Cardiac Function and Interstitial Fibrosis in Rats With Isoproterenol-Induced Cardiac Dysfunction

Myocardial relaxation and stiffness are influenced by fibrillar collagen content. Cyclic nucleotide signaling regulators have been investigated targeting more effective modulation of collagen deposition during myocardial healing process. To assess the effects of phosphodiesterase type 3 and phosphodiesterase type 5 inhibitors on cardiac function and left ventricular myocardial fibrosis in catecholamine-induced myocardial injury, sildenafil and pimobendan were administered to male Wistar rats 24 hours after isoproterenol injection. Echocardiography and electrocardiogram were performed to assess kinetic and rhythm changes during 45 days of drug administration. At the end of study, type I and type III collagen were measured through immunohistochemistry analysis, and left ventricular pressure was assessed through invasive method. Echocardiography assessment showed increased relative wall thickness at 45 days in pimobendan group with significant diastolic dysfunction and increased collagen I deposition compared with nontreated positive group (3.03 ± 0.31 vs. 2.73 ± 0.28%, P < 0.05). Diastolic pressure correlated positively with type I collagen (r = 0.54, P < 0.05). Type III collagen analysis did not demonstrate difference among the groups. Sildenafil administration attenuated type I collagen deposition (2.15 ± 0.51 vs. positive group, P < 0.05) and suggested to be related to arrhythmic events. Arrhythmic events were not related to the quantity of fibrillar collagen deposition. Although negative modulation of collagen synthesis through cyclic nucleotides signaling have shown promising results, in this study, pimobendan postconditioning resulted in increased collagen type I formation and severe diastolic dysfunction while sildenafil postconditioning reduced collagen type I deposition and attenuated diastolic dysfunction.

A review of the pharmacology and clinical uses of pimobendan

OBJECTIVE

To review the pharmacology, research developments, and clinical uses of pimobendan

DATA SOURCES

Original research articles and clinical studies from 1984 to August 2011.

VETERINARY DATA SYNTHESIS

Pimobendan is approved for use in dogs for the treatment of congestive heart failure (CHF) secondary to chronic valvular heart disease (CVHD) and dilated cardiomyopathy (DCM). Expert-based veterinary guidelines recommend the use of pimobendan in the management of acute, hospital-based therapy for patients with CHF attributable to CVHD.

CONCLUSIONS

The use of pimobendan, an inodilator with phosphodiesterase 3 (PDE3) inhibitory and calcium-sensitizing properties, is regarded as a component of the standard of care in the management of dogs with CHF secondary to both DCM and CVHD. Further studies are warranted to confirm the safety and efficacy of pimobendan for the off-label use of this drug in asymptomatic CVHD, pulmonary arterial hypertension, asymptomatic myocardial diseases, CHF from all other causes and in cats with CHF.

Drugs to facilitate recovery of neuromuscular blockade and muscle strength

Several drugs that quicken recovery from neuromuscular blockade caused by vecuronium in anesthetized patients are reviewed. Ulinastatin, a protease inhibitor, is thought to promote the release of acetylcholine at the neuromuscular junction and increases hepatic blood flow and urine volume. For this reason, ulinastatin quickens recovery from neuromuscular blockade in anesthetized patients receiving vecuronium. Additionally, pretreatment with ulinastatin avoids prolongation of vecuronium-induced neuromuscular blockade in patients with hepatic cirrhosis. Gabexate mesilate is also a protease inhibitor. During a continuous infusion of gabexate mesilate, recovery from neuromuscular blockade was quickened. Amino acid-enriched solution supplies energy to the skeletal muscles and causes an increase in muscle strength. An infusion of amino acid-enriched solution hastens recovery from neuromuscular blockade in anesthetized patients. When amino acids supply energy to the skeletal muscles, they simultaneously produce heat in the skeletal muscles. This thermal generation may be closely related to fast recovery from neuromuscular blockade. Amino acid-enriched solution makes recovery from neuromuscular blockade quick and avoids hypothermia during general anesthesia. Milrinone, a phosphodiesterase III inhibitor, is supposed to increase the release of acetylcholine at the neuromuscular junction and make the neuromuscular junction sensitive to acetylcholine. Therefore, recovery from neuromuscular blockade is hastened. Nicorandil enhances membrane K+ conductance in skeletal muscle and increases contraction of the skeletal muscle. Thus, nicorandil quickens recovery from neuromuscular blockade.

Effect of milrinone on vecuronium-induced neuromuscular block

We examined the effect of milrinone, a phosphodiesterase III inhibitor, on neuromuscular block induced by vecuronium. Thirty adult patients were randomly assigned to one of two equal groups: the milrinone group and the control group. Subjects in the milrinone group received an intravenous loading dose of milrinone 5 microg x kg-1x min-1 for 10 min, followed by an infusion at a rate of 0.5 microg x kg-1x min-1. Subjects in the control group received normal saline at a rate of 0.1 ml x kg-1 x h-1. Thirty minutes after the beginning of the infusion of milrinone, anaesthesia was induced with intravenous thiopental 4 mg x kg-1 and fentanyl 2 microg x kg-1, and was maintained with isoflurane in oxygen and nitrous oxide. Neuromuscular blockade was monitored electromyographically at the adductor pollicis muscle. The times from the administration of vecuronium 0.1 mg.kg-1 to the onset of neuromuscular block and the return of the first, second, third, and fourth response of the train-of-four were compared between the two groups. Times to the recovery of the ratio of the first twitch to the control twitch to 25%, 50% and 75%, and times to the recovery of train-of-four ratio to 25%, 50% and 75% were also compared between the two groups. The onset of neuromuscular block in the milrinone group was significantly slower than in the control group. The times to the returns of the four twitches of the train-of-four, times to recovery of the ratio of the first twitch to the control twitch to 25% and 50%, and the times to the recovery of the train-of-four ratio to 25% and 50% were significantly shorter in the milrinone group than in the control group. We conclude that milrinone delays the onset of neuromuscular blockade but hastens its recovery in anaesthetised patients receiving vecuronium.

Muscarinic receptors in the mammalian heart

In the mammalian heart, cardiac function is under the control of the sympathetic and parasympathetic nervous system. All regions of the mammalian heart are innervated by parasympathetic (vagal) nerves, although the supraventricular tissues are more densely innervated than the ventricles. Vagal activation causes stimulation of cardiac muscarinic acetylcholine receptors (M-ChR) that modulate pacemaker activity via I(f) and I(K.ACh), atrioventricular conduction, and directly (in atrium) or indirectly (in ventricles) force of contraction. However, the functional response elicited by M-ChR-activation depends on species, age, anatomic structure investigated, and M-ChR-agonist concentration used. Among the five M-ChR-subtypes M(2)-ChR is the predominant isoform present in the mammalian heart, while in the coronary circulation M(3)-ChR have been identified. In addition, evidence for a possible existence of an additional, not M(2)-ChR in the heart has been presented. M-ChR are subject to regulation by G-protein-coupled-receptor kinase. Alterations of cardiac M(2)-ChR in age and various kinds of disease are discussed.

Quantitative structure-activity relationships of cardiotonic agents

Quantitative structure-activity relationships (QSARs) of different cardiotonic agents are presented. A critical analysis of all QSARs provides a very vivid picture of the mechanisms of varying cardiotonic agents. The cardiotonics can be broadly put into 2 categories: cardiac glycosides and nonglycoside cardiotonics, which include phosphodiesterase of type III (PDE III) inhibitors, sympathomimetic (adrenergic) stimulants, A1-selective adenosine antagonists, Ca2+ channel activators and vasopressin antagonists. For cardiac glycosides, QSARs reveal that the position of carbonyl oxygen in their lactone moiety and shifting of the lactone ring from its original position or its replacement by another group would be crucial for their activity. The carbonyl group or its isostere like CN is indicated to be the sole binding entity and the hydrogen bonding through this group is considered to be the most likely binding force. For nonglycoside cardiotonics that include PDE III inhibitors and A1-selective antagonists, a five-point model has been established for their activity, the salient features of which are: (1) the presence of a strong dipole, (2) an adjacent acidic proton, (3) a methyl-sized lipophilic space, (4) a relatively flat overall topography and (5) a basic or hydrogen-bond acceptor site opposite to the dipole. For Ca2+ channel activators, the importance of steric, electrostatic, lipophilic and hydrogen-bonding properties of molecules is indicated, while for vasopressin antagonists the lipophilic and electronic properties are suggested to be the most important.

Muscarinic receptors in the failing human heart

In the human heart, as in the heart of several other species, muscarinic receptors are predominantly of the M2-subtype that couple via a pertussis toxin-sensitive Gi-protein to inhibit adenylyl cyclase. However, it is not clear whether an additional muscarinic receptor subtype exists in the human heart. In human right atrium, stimulation of muscarinic M2 receptors causes direct negative inotropic and chronotropic effects; in human ventricular myocardium, however, the negative inotropic effect can be only achieved when basal force of contraction has been pre-stimulated by cyclic AMP-elevating agents such as beta-adrenoceptor agonists, forskolin or phosphodiesterase inhibitors (indirect effect); this has been shown in various in vitro and in vivo studies. Evidence has accumulated that in chronic heart failure vagal activity is decreased. Cardiac muscarinic M2 receptor density and functional responsiveness (inhibition of adenylyl cyclase activity and negative inotropic effects), however, are not considerably changed when compared with non-failing hearts although cardiac Gi-activity is increased.

New inotropic agents: milrinone analogs

1. Two new milrinone analogs, 3-acetyl-6-phenyl-2(1H)-pyridone (SF 348) and 3-acetyl-7-methyl-7,8-dihydro-2,5(1H, 6H) quinolinone (SF 349), increase the contractile activity of spontaneously beating and electrically driven atria isolated from reserpine-treated guinea-pigs. 2. Propranolol 0.1 microM drastically inhibits the contractile effect of SF 348, whereas that of SF 349 is unaffected. Preincubation of the atria with adenosine-deaminase suppresses the cardiac activity of SF 349, but does not affect that of SF 348. 3. SF 349 competitively antagonizes the negative inotropic effect induced by N6-(R-phenylisopropyl)-adenosine (R-PIA) and displaces N6-cyclohexyl[3H]-adenosine (3H-CHA) from its binding sites to A1 receptors in the guinea-pig heart. 4. The positive inotropic effect of SF 348 is largely sustained by activation of beta-adrenoceptors, whereas SF 349 acts by displacing endogenous adenosine from its inhibitory (A1) receptors in the atria.

Synthesis and cardiotonic activity of novel pyrimidine derivatives: crystallographic and quantum chemical studies

The synthesis of ethyl or methyl 4-substituted or unsubstituted 2-(dimethylamino)-5-pyrimidinecarboxylates 10-20, which is mainly carried out by reaction of ethyl or methyl 2-[(dimethylamino)methylene]-3-oxoalkanoates with 1,1-dimethylguanidine, is described. The above esters were hydrolyzed to the relative carboxylic acids 21-30, which were decarboxylated to the corresponding 2,4-disubstituted pyrimidines 31-40. All the new synthesized pyrimidines were evaluated in spontaneously beating and electrically driven atria from reserpine-treated guinea pigs. Their effects were compared to those induced by milrinone in both atria preparations. Compound 28 (4-benzyl-2-(dimethylamino)-5-pyrimidinecarboxylic acid) was the most effective positive inotropic agent, while the corresponding methyl ester 17 reduced both the contractile force and the frequency of guinea pig atria. An antagonism toward the negative influence exerted by endogenous adenosine on the heart seems to be involved in the contractile activity of compound 28. By contrast, compound 17 might be partial agonist at the purinergic inhibitory (A1) receptor. X-ray analysis carried out on 17 and 28 and molecular modeling investigations extended also to related derivatives allowed a possible rationalization between structure and inotropic activity for this series of compounds.

EMD 53998 acts as Ca(2+)-sensitizer and phosphodiesterase III-inhibitor in human myocardium

The effect of EMD 53998 (EMD) (0.1-100 mumol/l), chemically a racemic thiadiazinone derivative, suggested to be a potent Ca(2+)-sensitizer, was studied in human failing and nonfailing left ventricular myocardium. For comparison, the effects of the pyridazinone derivative pimobendan (0.1-300 mumol/l), isoprenaline (Iso) (0.001-3 mumol/l) as well as CaCl2 (1.8-15 mmol/l Ca2+) were investigated. The positive inotropic responses were examined in electrically driven (1 Hz, 37 degrees C) human left ventricular papillary muscle strips from terminally failing hearts (NYHAIV, n = 24) and nonfailing donor hearts (NF, n = 9). The effect of EMD on the Ca(2+)-sensitivity of skinned fiber preparations from the very same human failing hearts were studied as well. EMD and pimobendan increased force of contraction (FOC) in a concentration-dependent manner. As judged from the EC50-values, EMD increased FOC more potently than pimobendan. EMD was significantly more effective than pimobendan to increase FOC in papillary muscle strips from NYHA IV (EMD: +2.5 +/- 0.1 mN; pimobendan: +0.8 +/- 0.2 mN) as well as from nonfailing hearts (EMD: +3.1 +/- 0.5 mN; pimobendan: +1.2 +/- 0.2 mN). Only in terminally failing myocardium, EMD increased FOC as effectively as Iso. After inotropic stimulation with EMD, pimobendan, or Iso, carbachol (1000 mumol/l) reduced FOC in left ventricular papillary muscle strips, indicating a cAMP-dependent mode of action. In skinned fiber experiments, EMD increased Ca(2+)-sensitivity significantly more (p < 0.01) than pimobendan.

IN CONCLUSION

EMD increases FOC in human myocardium via sensitizing of the contractile proteins towards Ca2+ and by inhibition of phosphodiesterase III-isoenzymes. EMD is a potent calcium sensitizing agent in human myocardium. Thiadiazinone derivatives could be one step in the evolution to more potent and selective calcium-sensitizers.