The inotropic effect of the active metabolite of levosimendan, OR-1896, is mediated through inhibition of PDE3 in rat ventricular myocardium.
OBJECTIVE
We recently published that the positive inotropic response (PIR) to levosimendan can be fully accounted for by phosphodiesterase (PDE) inhibition in both failing human heart and normal rat heart. To determine if the PIR of the active metabolite OR-1896, an important mediator of the long-term clinical effects of levosimendan, also results from PDE3 inhibition, we compared the effects of OR-1896, a representative Ca2+ sensitizer EMD57033 (EMD), levosimendan and other PDE inhibitors.


METHODS
Contractile force was measured in rat ventricular strips. PDE assay was conducted on rat ventricular homogenate. cAMP was measured using RII_epac FRET-based sensors.


RESULTS
OR-1896 evoked a maximum PIR of 33 ± 10% above basal at 1 μM. This response was amplified in the presence of the PDE4 inhibitor rolipram (89 ± 14%) and absent in the presence of the PDE3 inhibitors cilostamide (0.5 ± 5.3%) or milrinone (3.2 ± 4.4%). The PIR was accompanied by a lusitropic response, and both were reversed by muscarinic receptor stimulation with carbachol and absent in the presence of β-AR blockade with timolol. OR-1896 inhibited PDE activity and increased cAMP levels at concentrations giving PIRs. OR-1896 did not sensitize the concentration-response relationship to extracellular Ca2+. Levosimendan, OR-1896 and EMD all increased the sensitivity to β-AR stimulation. The combination of either EMD and levosimendan or EMD and OR-1896 further sensitized the response, indicating at least two different mechanisms responsible for the sensitization. Only EMD sensitized the α1-AR response.


CONCLUSIONS
The observed PIR to OR-1896 in rat ventricular strips is mediated through PDE3 inhibition, enhancing cAMP-mediated effects. These results further reinforce our previous finding that Ca2+ sensitization does not play a significant role in the inotropic (and lusitropic) effect of levosimendan, nor of its main metabolite OR-1896.
Department of Pharmacology, Faculty of Medicine, University of Oslo and Oslo University Hospital, Oslo, Norway; K.G. Jebsen Cardiac Research Centre, Faculty of Medicine, University of Oslo, Oslo, Norway; Center for Heart Failure Research, Faculty of Medicine, University of Oslo, Oslo, Norway.
Øivind
Ø
Ørstavik
Department of Pharmacology, Faculty of Medicine, University of Oslo and Oslo University Hospital, Oslo, Norway; K.G. Jebsen Cardiac Research Centre, Faculty of Medicine, University of Oslo, Oslo, Norway; Center for Heart Failure Research, Faculty of Medicine, University of Oslo, Oslo, Norway.
Ornella
O
Manfra
Department of Pharmacology, Faculty of Medicine, University of Oslo and Oslo University Hospital, Oslo, Norway; K.G. Jebsen Cardiac Research Centre, Faculty of Medicine, University of Oslo, Oslo, Norway; Center for Heart Failure Research, Faculty of Medicine, University of Oslo, Oslo, Norway.
Kjetil Wessel
KW
Andressen
Center for Heart Failure Research, Faculty of Medicine, University of Oslo, Oslo, Norway; Department of Cardiology, Oslo University Hospital, Oslo, Norway.
Geir Øystein
GØ
Andersen
Department of Pharmacology, Faculty of Medicine, University of Oslo and Oslo University Hospital, Oslo, Norway; K.G. Jebsen Cardiac Research Centre, Faculty of Medicine, University of Oslo, Oslo, Norway; Center for Heart Failure Research, Faculty of Medicine, University of Oslo, Oslo, Norway.
Tor
T
Skomedal
Department of Pharmacology, Faculty of Medicine, University of Oslo and Oslo University Hospital, Oslo, Norway; K.G. Jebsen Cardiac Research Centre, Faculty of Medicine, University of Oslo, Oslo, Norway; Center for Heart Failure Research, Faculty of Medicine, University of Oslo, Oslo, Norway.
Jan-Bjørn
JB
Osnes
Department of Pharmacology, Faculty of Medicine, University of Oslo and Oslo University Hospital, Oslo, Norway; K.G. Jebsen Cardiac Research Centre, Faculty of Medicine, University of Oslo, Oslo, Norway; Center for Heart Failure Research, Faculty of Medicine, University of Oslo, Oslo, Norway.
Finn Olav
FO
Levy
Department of Pharmacology, Faculty of Medicine, University of Oslo and Oslo University Hospital, Oslo, Norway; K.G. Jebsen Cardiac Research Centre, Faculty of Medicine, University of Oslo, Oslo, Norway; Center for Heart Failure Research, Faculty of Medicine, University of Oslo, Oslo, Norway.
Kurt Allen
KA
Krobert
D000081:Acetamides / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000502:physiology; D002316:Cardiotonic Agents / Q000494:pharmacology*; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000378:metabolism; D054684:Cyclic Nucleotide Phosphodiesterases, Type 3 / Q000378:metabolism; D008297:Male; D009200:Myocardial Contraction; D009206:Myocardium / Q000201:enzymology*; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000502:physiology; D058987:Phosphodiesterase 3 Inhibitors / Q000494:pharmacology*; D011724:Pyridazines / Q000494:pharmacology*; D017208:Rats, Wistar
D000081:Acetamides; D002316:Cardiotonic Agents; C414222:N-(4-(4-methyl-6-oxo-1,4,5,6-tetrahydropyridazin-3-yl)phenyl)acetamide; D058987:Phosphodiesterase 3 Inhibitors; D011724:Pyridazines; D000242:Cyclic AMP; D054684:Cyclic Nucleotide Phosphodiesterases, Type 3; D002118:Calcium