New inotropic agents: milrinone analogs.

1.1. Two new milrinone analogs, 3-acetyl-6-phenyl-2(IH)-pyridone (SF 348) and 3-acetyl-7-methyl-7,8-dihydro-2,5(1H, 6H) quinolinone (SF 349), increase the contractile activity of spontaneously beating and electrically driven atria isolated from reserpine-treated guinea-pigs.2.2. Propranolol 0.1 μM drastically inhibits the contractile effect of SF 348, whereas that of SF 349 is unaffected. Preincubation of the atria with adenosine-deaminase suppresses the cardiac activity of SF 349, but does not affect that of SF 348.3.3. SF 349 competitively antagonizes the negative inotropic effect induced by N6-(R-phenylisopropyl)-adenosine (R-PIA) and displaces N6-cyclohexyl[3h]-adenosine (3H-CHA) from its binding sites to A1 receptors in the guinea-pig heart.4.4. The positive inotropic effect of SF 348 is largely sustained by activation of β-adrenoceptors, whereas SF 349 acts by displacing endogenous adenosine from its inhibitory (A1) receptors in the atria.