Inotropic and Chronotropic Profile of MCI-154: Comparison with Isoproterenol and Imazodan in Guinea Pig Cardiac Preparations

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Abstract
Although recent studies indicate that MCI-154 exerts novel positive inotropic actions in heart muscle, the chronotropic properties of this new drug remain undefined. The present study compared the inotropic/chronotropic profile of MCI-154 with those of a nonselective .beta.1/.beta.2-agonist (isoproterenol, Iso) and a type III phosphodiesterase inhibitor (imazodan, CI-914) in cardiac preparations isolated from guinea pigs. The inotropic efficacy of MCI-154 was approximately equal to that of Iso and CI-194 in electrically paced (1 Hz) atrial muscle, with each agent increasing isometric contractile tension .apprx. 170% above basal (predrug) values. The inotropic EC50 for Iso (2.9 .+-. 0.7 .times. 10-9 M) was several orders of magnitude less than that for MCI-154 (1.4 .+-. 0.4 .times. 10-4 M) and CI-914 (1.1 .+-. 0.2 .times. 10-4 M). The inotropic potency of MCI-154 was equivalent in atrial and left ventricular nyocardium. Both Iso and CI-194 substantially increased spontaneous beating frequency of sinoatrial preparations, and the inotropic/chronotropic potency ratio for each was unity. In contrast, MCI-154 exerted a slight negative chronotropic action on basal sinoatrial rate. Moreover, the negative chronotropic influence of MCI-154 was increased several-fold in the presence of Iso-stimulated maximal increases in heart rate (HR), and this inhibitory chronotropic action of MCI-154 was not prevented by muscarinic receptor blockade with atropine. These findings indicate that MCI-154 has a unique inotropic/chronotropic profile in cardiac tissues of guinea pigs in that this drug (a) efficaciously increased myocardial contractility, (b) had minimal effect on basal sinoatrial automaticity and yet (c) markedly inhibited sympathetically mediated sinus tachycardia.

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