SILDENAFIL INHIBITS PHOSPHODIESTERASE TYPE-5 IN HUMAN CLITORAL CORPUSCAVERNOSUM SMOOTH-MUSCLE

Phosphodiesterases play an important physiological role by regulating the intracellular levels of cyclic nucleotides. In this study, we inve stigated the kinetic parameters of inhibition of phosphodiesterase (PD E) type 5 (EC 3.1.4.35, 3',5'-cyclic GMP phosphodiesterase) by a novel , high-affinity, selective PDE type 5 inhibitor, sildenafil, in intact cells and in soluble extracts of human clitoral corpus cavernosum smo oth muscle cells. Sildenafil inhibited cGMP hydrolysis in the crude ex tract (K-i = 7.2 +/- 2.7) and in partially purified preparations (K-i = 9 nM) in a competitive manner, as determined by Dixon plots. Sildena fil was a more effective PDE type 5 inhibitor than zaprinast (K-i = 40 0.0 +/- 76.4 nM, crude extracts; 250 nM, partially purified). Stimulat ion of intracellular cGMP synthesis by the nitric oxide donor sodium n itroprusside resulted in a 3.3- and 2.9-fold increase in cGMP concentr ation in the presence of sildenafil or zaprinast, respectively, compar ed to sodium nitroprusside treatment alone in intact cells at physiolo gical temperatures. These observations suggest that human clitoral cor pus cavernosum smooth muscle tone may be regulated by the synthesis an d release of nitric oxide and that this pathway is dependent on PDE ty pe 5 activity. (C) 1998 Academic Press.