Myometrial constitutive nitric oxide synthase expression is increased during human pregnancy

Nitric oxide (MO), derived from L-arginine by the action of nitric oxide sy nthase (NOS), is a mediator of many diverse biological activities, includin g vasodilation, neurotransmission and inhibition of platelet adhesion. A ro le for NO in the maintenance of rat and rabbit pregnancy is supported by a variety of studies. A recent study in women demonstrated that myometrial in ducible NOS (iNOS) expression was greater in the early third trimester than either the late third trimester or in the non-pregnant condition, suggesti ng that increased iNOS expression is involved in the maintenance of human p regnancy. Constitutive NOS (cNOS) expression was not determined. The aim of this study was to compare constitutive NOS (both eNOS and bNOS) expression in the human non-pregnant uterus, preterm pregnant uterus (25-34 weeks ges tation) and term pregnant uterus (>37 weeks gestation) using immunohistoche mistry and Western blotting, Preterm pregnant samples were taken from women with a variety of pathologies necessitating early delivery. We found that eNOS and bNOS protein concentrations were greater in the preterm pregnant m yometrium than nonpregnant myometrium. eNOS, but not bNOS, protein concentr ation was lower in myometrial samples obtained at term compared with those obtained preterm. We conclude that the constitutive isoforms of NOS are als o up-regulated in human pregnancy and may play a role in the maintenance of myometrial quiescence.