Relaxant action of sildenafil citrate (Viagra) on human myometrium of pregnancy.

Objective: The purpose of this study was to investigate the effect of the cyclic guanosine monophosphate phosphodiesterase 5-specific inhibitor, sildenafil citrate, on the contractions of isolated pregnant human myometrium.

Study Design: Myometrial samples were obtained from women who underwent elective cesarean delivery. Myometrial contractions that were recorded in response to sildenafil in the absence and presence of the potassium channel blocker, tetraethylammonium or the guanylate cyclase inhibitor, methylene blue (10 micromol/L) were studied. One-way analysis of variance with post-hoc analysis was used to test differences among groups.

Results: Sildenafil caused relaxation of myometrium in a concentration-dependent manner. The log(10) EC(50) value for this relaxation in the presence of 20 mmol/L tetraethylammonium was significantly different (P<.01) than values that were obtained with sildenafil alone or sildenafil in the presence of either methylene blue or 5 and 10 mmol/L tetraethylammonium.

Conclusion: Myometrial relaxation that is evoked by the direct application of sildenafil occurs independently of cyclic guanosine monophosphate. Potassium channels appear to be the likely candidates in mediating this response.