Background: Selective phosphodiesterase (PDE3) inhibitors improve cardiac contractility and may use in congestive heart failure. However, their proarrhythmic potential is the most important side effect. Methods: In this research, we evaluated the potential cardiotonic activity of six new synthesized selective PDE3 inhibitors (6-hydroxy-4-methylquinolin-2(1H)-one derivatives) using the spontaneously beating atria model. In each experiment, atrium of reserpine-treated rat was isolated and the contractile and chronotropic effects of a synthesized compound were assessed. The 3-isobutyl-1-methylxanthine, a non-selective PDE inhibitor, was used for comparison. Results: The results showed that, among new compounds, the best pharmacological profile was obtained with the compound 6-[4-(4-methylpiperazine-1-yl)-4-oxobutoxy]-4-methylquinolin-2(1H)-one, C6, which displayed selectivity for increasing the force of contraction (168 ± 5% change over the control) rather than the frequency rate (138 ± 5% change over the control) at 300 μM. However, C6 at concentrations of 10 and 100 μM produced left and upward shift in the positive inotropic concentrationresponse curve of isoprenaline. The -log EC50 of isoprenaline was 8.843 ± 0.171 in the absence, 9.448 ± 0.138 and 9.456 ± 0.107 in the presence of 10, 100 μM of C6, respectively (P<0.001, n = 6). Also, amrinone, a selective PDE3 inhibitor, shifted the isoprenaline concentration-response curve to the left and upward. The concentration of 10 and 100 μM amrinone decreased -log EC50 of isoprenaline to 9.527 ± 0.287 and 9.423 ± 0.243, respectively (P<0.001, n = 6). Moreover, the positive chronotropic effect of isoprenaline was not affected by amrinone or C6. Conclusion: This study provides functional evidence for the positive inotropic effect of C6. Considering the augmentation of isoprenaline positive inotropic concentration-response with C6 and amrinone, we conclude that C6 produces its effect via potentiation of cAMP-dependent signaling system and possibly by inhibition of PDE3 activity.

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