title: Effects of OR-1896, a metabolite of levosimendan, on force of contraction and Ca2+ transients under acidotic condition in aequorin-loaded canine ventricular myocardium.
abstract: We performed experiments in canine ventricular trabeculae loaded with aequorin to elucidate the influence of acidosis on the positive inotropic effect (PIE) of OR-1896 (R)- N-[4-(4-methyl-6-oxo-1,4,5,6-tetrahydro-pyridazin-3-yl)-phenyl]-acetamide, an active metabolite of levosimendan. The concentration-response curve (CRC) for OR-1896 was biphasic in acidotic conditions (pH(o) 6.6) that was essentially the same as that in control conditions (pH(o) 7.4). The CRC for PIE of OR-1896 reached a plateau at 10(-6) M (first phase) and it became steeper again at 10(-3) M (second phase). Under acidotic conditions the efficacy of the first phase was 10% of the maximal response to isoproterenol (ISO(max)) and the PIE was associated with an increase in Ca(2+) transients of 2% of ISO(max) (P<0.05). The sensitivity of myofilaments to Ca(2+) ions was increased by OR-1896 in acidotic conditions, whereas the relationship of Ca(2+) transients and contractile force during an increase in elevation of the extracellular Ca(2+) concentration and during application of dihydroouabain was shifted prominently to the right in acidotic conditions. In conclusion, OR-1896 elicited a PIE due to an increase in the sensitivity of myofilaments to Ca(2+) ions even in acidotic conditions. The PIE of OR-1896 in acidotic conditions was much less than that in control conditions because the effect of the compound to induce an increase in intracellular Ca(2+) mobilization was markedly attenuated in acidotic conditions.
authors.affiliation: Department of Pharmacology, Yamagata University School of Medicine, 2-2-2 Iida-nishi, Yamagata 990-9585, Japan.
authors.forename: Reiko
authors.initials: R
authors.lastname: Takahashi
authors.forename: Masao
authors.initials: M
authors.lastname: Endoh
mesh_terms: D000081:Acetamides / Q000378:metabolism / Q000494:pharmacology*; D000138:Acidosis / Q000378:metabolism*; D000331:Aequorin / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D005260:Female; D006352:Heart Ventricles / Q000187:drug effects / Q000378:metabolism; D006835:Hydrazones / Q000378:metabolism / Q000494:pharmacology; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects* / Q000502:physiology; D011724:Pyridazines / Q000378:metabolism / Q000494:pharmacology*; D000077464:Simendan
chemical_list: D000081:Acetamides; D006835:Hydrazones; C414222:N-(4-(4-methyl-6-oxo-1,4,5,6-tetrahydropyridazin-3-yl)phenyl)acetamide; D011724:Pyridazines; D000077464:Simendan; D000331:Aequorin; D002118:Calcium