Does sildenafil citrate affect myometrial contractile response to nifedipine in vitro?

Objective: We sought to test the hypothesis that sildenafil citrate (SC) at low concentrations potentiates the tocolytic effects of nifedipine in vitro. Study Design: Myometrial biopsies were obtained from 22 term nonlaboring women undergoing scheduled cesarean delivery. Tissue strips were suspended in organ chambers for isometric tension recording, and incubated for 30 minutes with either SC at 231 ng/mL or solvent. The effects of cumulative doses (10 ^-10 to 10⁵ mol/L) of nifedipine on spontaneous and oxytocin-induced uterine contractility were then determined. Areas under the contraction curve were compared using 1-way analysis of variance with Tukey post hoc test (significance: P < .05). Results: Nifedipine significantly inhibited spontaneous and oxytocin-induced myometrial contractility. Preincubation with SC increased response to nifedipine and significantly potentiated its inhibitory effect at 10⁸ mol/L, without affecting oxytocin-induced contractile response. Conclusion: At concentrations within a therapeutic window, SC increases myometrial sensitivity to nifedipine.