The inhibitory effect of the flavonoid dioclein was assessed on purified vascular cyclic nucleotide phosphodiesterase isoforms (EC 3.1.4.17, PDE1-5) in comparison with 8-methoxymethyl-isobutylmethylxanthine (8-MM-IBMX) and vinpocetine which are currently used as PDE1 inhibitors. The mechanism underlying the vasorelaxant effect of dioclein was investigated in human saphenous vein. Dioclein inhibited PDE1 more selectively than vinpocetine and 8-MM-IBMX, with IC(50) values of 2.47+/-0.26 and 1.44+/-0.35 microM, respectively in basal- and calmodulin-activated states. Dioclein behaved as a competitive inhibitor for cGMP hydrolysis by PDE1 in basal- and calmodulin-activated states (K(i)=0.62+/-0.14 and 0.55+/-0.07 microM, respectively), indicating this inhibitory effect to be independent of calmodulin interactions. In addition, dioclein induced a concentration-dependent relaxation of human saphenous vein which was independent on the presence of functional endothelium (EC(50) values of 7.3+/-3.1 and 11+/-2.7 microM, respectively with and without endothelium). 8-MM-IBMX relaxed human saphenous vein with an EC(50)=31+/-16 microM, whereas vinpocetine did not cause any vasorelaxation at concentrations up to 100 microM. Rp-8-pCPT-cGMPS, which inhibits cGMP-dependent protein kinase (PKG), blocked the vasodilator effect of dioclein, whereas H-89, which is a cAMP-dependent protein kinase (PKA) inhibitor, had a minor inhibitory effect. Our data show that dioclein is a potent calmodulin-independent selective inhibitor of PDE1 and that inhibition of PDE1 is involved in the PKG-mediated vasorelaxant effect of dioclein in human saphenous vein. Furthermore, dioclein may represent a new archetype to develop more specific PDE1 inhibitors.