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Two mechanisms are responsible for the positive inotropic effect of the cardiotonic drug milrinone, i.e., inhibition of type III cAMP phosphodiesterase (PDE III), and displacement of endogenous adenosine from A1 inhibitory receptor. Since PDE III inhibition may increase the likelihood of cardiac arrhythmias by increasing cAMP content, our attention focused on the synthesis of new compounds with more pronounced characteristics as adenosine antagonists. In this work, four new milrinone analogues were studied, in comparison with the parent drug, for their effects on the contractility of guinea pig isolated atrial preparations, their ability to antagonize endogenous adenosine at the level of A1 receptor, and to inhibit the activity of PDE III partially purified from guinea pig heart. The new compounds present various chemical substitutions with respect to the parent drug: in compounds SF397 (methyl 5-cyano-2-methyl-6-oxo-1,6-dihydropyridine-3-carboxylate) and SF399 (benzyl 5-cyano-2-methyl-6-oxo-1,6-dihydropyridine-3-carboxylate), the 4-pyridil moiety of milrinone was replaced with a methoxycarbonyl and a benzyloxycarbonyl group, respectively; the same structural modifications were also associated with the replacement of the cyano-group in 5-position with an acetyl group in compounds SF416 (methyl 5-acetyl-2-methyl-6-oxo-1,6-dihydropyridine-3-carboxylate) and SF419 (benzyl 5-acetyl-2-methyl-6-oxo-1,6-dihydropyridine-3-carboxylate). All the new compounds had a marked positive inotropic effect, most of them also being more active and more potent than milrinone. When their affinity for A1 receptor was assessed as the displacement of [3H] 8-cyclopentyl-1,3-dipropylxanthine ([3H]DPCPX) from cardiac membranes, SF397 and SF399 showed affinity (Ki of about 600 nM) similar to that of milrinone (Ki 550 nM). By contrast, SF416 and SF419 had very low (Ki of about 10,000 nM) or scarce (Ki of about 2,000 nM) anti-adenosine component, respectively. All the new compounds inhibited PDE III activity, their Ki values proceeding in the following order: milrinone (3.80 μM) 2+ uptake, Ca2+ATPase, Na+/K+ATPase, Na+/Ca2+ exchange carrier) or a receptor (β-adrenoceptor) mainly involved in the control of cardiac contractility. None of the tested compounds inhibited enzyme or transport systems; however, SF397, SF399 and SF416, although to different extents, had a direct β-adrenergic action. Indications about structure-activity relationships are tentatively discussed, in order to obtain useful information for the design of new analogues with better pharmacological profiles. [ABSTRACT FROM AUTHOR]