Electrophysiological effects of FK664, a new cardiotonic agent, on preparations from guinea pig ventricle and from rabbit sino-atrial node.
Effects of the cardiotonic agent FK664, 6-(3, 4-dimethoxy-phenyl)-1-ethyl-4-mesitylimino-3-methyl-3,4-dihydro-2 (1H)-pyrimidone, on isolated guinea pig ventricular muscles and rabbit sinus node pacemaker cells were studied using micro-electrode techniques. In ventricular muscles driven at 0.5-1.0 Hz, FK664 above 3 mumol.litre-1 caused an increase in contractile force and a shortening of time to peak tension. This positive inotropic effect of FK664 was accompanied by a slight elevation of the early plateau phase of the action potential, while other action potential variables were unaffected. The change in contractile force induced by FK664 was abolished in a low Ca2+ medium (0.12 mmol.litre-1) or by treatment with ryanodine (2 mumol.litre-1), whereas it was relatively well preserved in the preparations pretreated with nefedipine (1 mumol.litre-1). The slow action potentials induced by isoprenaline (0.3 mumol.litre-1) in high K+ medium (30 mmol.litre-1) and the slow inward current measured by single sucrose gap voltage clamp at a holding potential of -40 mV were unaffected by FK664. In sinus node pacemaker cells, FK664 (1-10 mumol.litre-1) caused a dose dependent acceleration of phase 4 depolarisation and a shortening of spontaneous firing cycle length. This positive chronotropic effect of FK664 was markedly inhibited in a low Ca2+ medium (0.3 mmol.litre-1). These findings suggest that FK664 has positive inotropic and chronotropic effects on the heart, due to an enhancement of transsarcolemmal calcium influx through the low threshold, dihydropyridine insensitive Ca2+ channel population.
Kodama
I
I
Department of Pharmacology, University of Hawaii, School of Medicine, Honolulu 96822.
Anno
T
T
Sudo
Y
Y
Satake
N
N
Shibata
S
S
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000493:pharmacokinetics; D015220:Calcium Channels / Q000378:metabolism; D002316:Cardiotonic Agents / Q000494:pharmacology*; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects*; D006352:Heart Ventricles / Q000187:drug effects; D009200:Myocardial Contraction / Q000187:drug effects*; D009206:Myocardium / Q000378:metabolism; D009543:Nifedipine / Q000494:pharmacology; D010210:Papillary Muscles / Q000187:drug effects; D011744:Pyrimidinones / Q000494:pharmacology*; D011817:Rabbits; D012433:Ryanodine / Q000494:pharmacology; D012508:Sarcolemma / Q000378:metabolism; D012849:Sinoatrial Node / Q000187:drug effects*; D013779:Tetrodotoxin / Q000494:pharmacology
D015220:Calcium Channels; D002316:Cardiotonic Agents; D011744:Pyrimidinones; D012433:Ryanodine; D013779:Tetrodotoxin; C062761:FK 664; D009543:Nifedipine; D002118:Calcium