Pharmacological characterization of effects of UK-1745, a novel cardiotonic agent with beta-adrenoceptor-blocking action, in aequorin-loaded canine right ventricular muscle

UK-1745, a derivative of furoindolinone, is a novel cardiotonic agent that was designed to have both beta-adrenoceptor-blocking and cardiotonic activity. The aim of this study was to clarify the mode of action of UK-1745 in the canine and rabbit myocardium. UK-1745 elicited a weak but definite concentration-dependent positive inotropic effect in association with a decrease in the total duration of contraction: in particular, a decrease in the relaxation time in isolated canine right ventricular trabeculae. The maximum positive inotropic effect of UK-1745 was achieved at 3x10(-5)m and amounted to approximately 15% of the maximum response to isoproterenol. The EC50 for the positive inotropic effect of UK-1745 was 3.3x10(-6)m. Carbachol, a muscarinic receptor agonist, at 3x10(-6)m completely inhibited the positive inotropic effect of UK-1745. UK-1745 shifted the concentration-response curve for isoproterenol to the right with pA2 value of 5.70. By contrast, UK-1745 at 3x10(-7)to 3x10(-5)m shifted the concentration-response curve for forskolin to the left. In aequorin-loaded ventricular trabeculae, UK-1745 induced a positive inotropic effect that was accompanied by an increase in Ca2+ transients. It did not affect the relationship between the amplitude of Ca2+ transients and peak force as compared with that associated with elevation of the extracellular concentration of Ca2+ ions ([Ca2+]o). The level of cyclic AMP in tissue was not significantly increased at 3x10(-5)m UK-1745. The present results indicate that UK-1745 exerts a positive inotropic effect mainly via a cyclic AMP-dependent mechanism but, in addition, it has beta-adrenoceptor-blocking activity over the same range of concentrations. A drug with such a pharmacological profile might have the potential advantage of avoiding Ca2+ overload and superfluous oxygen consumption, which may contribute to the unfavorable effects of novel cardiotonic agents that act purely by inhibition of phosphodiesterase III.