76: Atrial natriuretic peptide (ANP) production and receptors in the human uterus and effect on myometrial relaxation

Abstract

Natriuretic peptides induce uterine relaxation. We identify human myometrial ANP production, ANP receptor expression, and physiology in human tissue and primary cultured cells. In an IRB-approved study, gravid human myometrium was obtained from cesarean section patients (21 to 41 weeks gestation). Isometric tension studies were performed on 2 ×10 mm strips of uterine tissue. ANP dose-curve (beginning at 3 pM) was determined. Western blot analysis was performed on uterine tissue samples for ANP receptors (A, B, and C subtypes). Radioimmunoassay (RIA)measured ANP, cAMP, and cGMP in myometrial tissue, primary myometrial cultured cells, and cell culture media in control cells and oxytocin (30 nM) or nifedipine (10μM) treated cells. In primary cultured myometrial cells, micromolar ANP had little effect on intracellular calcium in the presence or absence of oxytocin. In dramatic contrast, very low dosages (picomolar levels) of ANP have statistically significant effect on human myometrial tissue contraction rate and amplitude. In fresh human myometrial tissue, we demonstrated the presence of three ANP receptor subtypes (A, B, and C). Normalized to total protein loaded and smooth muscle specific alpha-actin, the relative ANP receptor subtype expression was measured in term, preterm, hypertensive, preeclamptic, and normotensive patients in parallel with RIA determination of ANP, cAMP, and cGMP levels. A complex pattern of ANP receptor and ANP concentration was observed. Primary cultured myometrial tissue lack a response to nanomolar ANP. In contrast, fresh myometrial tissue contraction is modulated by picomolar ANP. All ANP receptor subtypes are present in fresh myometrial tissue but may decrease in culture (future experiments). ANP and all three ANP receptor subtypes are present in human myometrial tissue and may serve an important role in treating preterm labor.