Selective inhibition by E4080, a novel bradycardic agent, of positive chronotropic responses to norepinephrine in isolated dog hearts

Chronotropic and inotropic effects of terikalant on isolated, blood-perfused atrial and ventricular preparations of dogs

We investigated the effects of terikalant, which blocks inward rectifier K+ current, on the sinus rate, atrial and ventricular contractile force in the isolated, blood-perfused right atrial and left ventricular preparations of dogs, and the effects of terikalant on the negative cardiac responses to acetylcholine, adenosine or pinacidil (an ATP-sensitive K+ channel opener) and on the positive cardiac responses to norepinephrine. Terikalant (1-100 nmol) decreased sinus rate and briefly and slightly increased atrial contractile force in isolated atria. However, terikalant did not increase ventricular contractile force in isolated ventricles. Neither propranolol nor atropine inhibited the positive inotropic and negative chronotropic responses to terikalant, respectively. Terikalant (10 or 30 nmol) did not significantly affect the negative cardiac responses to acetylcholine, adenosine nor pinacidil and the positive responses to norepinephrine. These results suggest that terikalant decreases sinus rate with a small changes in myocardial contractile force and does not affect the cardiac responses to muscarinic and adenosine receptor agonists, ATP-sensitive K+ channel openers nor beta-adrenoceptor agonists in the dog heart.

Inhibition by bertosamil of cardiac responses to pinacidil or Bay k 8644 in isolated dog atria and ventricles

We investigated the effects of a novel bradycardiac agent, bertosamil (3-isobutyl-7-isopropyl-9,9-pentamethylene-3,7-diazabicyclo[3.3.1] nonane sesquihydrogenfumarate), on the sinus rate and atrial contractile force and the left ventricular contractile force in isolated, blood-perfused dog hearts and the blocking effects of bertosamil on the chronotropic and inotropic responses to pinacidil and Bay k 8644 (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)- piridine-5-carboxylate). Bertosamil (0.1-100 nmol) caused transient positive, followed by continuous negative, chronotropic responses and positive inotropic responses in atria, and increased the left ventricular contractile force. Neither propranolol nor atropine affected the cardiac responses to bertosamil. Bertosamil (3-100 nmol) dose dependently attenuated the negative chronotropic and inotropic responses to pinacidil but not to acetylcholine. Bertosamil at a high dose attenuated the positive cardiac responses to Bay k 8644, norepinephrine and isoproterenol. These results suggest that bertosamil inhibits negative cardiac responses mediated by an ATP-sensitive K+ channel but not an acetylcholine muscarinic receptor and, at a high dose, attenuates the L-type Ca2+ channel-mediated positive cardiac responses in isolated dog hearts.