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.