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Developmental Expression of Endothelial Nitric Oxide Synthase (eNOS) in the Rat Liver

Transition from fetal to postnatal life requires significant changes in cardiac, pulmonary, and hepatic blood flow. As such, there must be changes in vascular control in these vascular systems. Vascular resistance, a major contributor to blood flow, is mediated in the ductus arteriosus and pulmonary vasculature by endothelial nitric oxide synthase (eNOS). This study was conducted to determine the ontogeny of hepatic eNOS expression and activity. Additionally, the expression and activity of inducible nitric oxide synthase (iNOS) was measured to determine whether perinatal hypoxia resulted in detectable levels. NOS mRNA and proteins were determined by reverse transcription PCR assay and semiquantitative Western blot analysis, respectively. NOS activity was measured by the formation of [14C]-citrulline from [14C]-arginine. Localization of eNOS within the liver was determined by immunohistochemistry. eNOS mRNA was detectable at low levels at 18-d gestation and increased after birth, reaching a maximum level (4.5-fold increase) at 20 d of life. Parallel patterns for eNOS protein and activity were seen, with 6.9-fold and 16.1-fold increases, respectively. In the prenatal rat, eNOS was localized to areas of extramedullary hematopoiesis, with little signal in the sinusoids. Postnatally, there was a decrease in staining in the hematopoietic cells and a gradual increase in the staining of the endothelium of the sinusoids and central veins. iNOS mRNA and protein could not be detected at any age. eNOS expression and activity are developmentally regulated, increasing after birth coincident with an initial relative hypoxia and an increase in shear forces upon closure of the ductus venosus.