Natriuretic peptide C receptor in the developing sheep lung: role in perinatal transition

Background: At birth, the release of surfactant from alveolar type II cells (ATIIs) is stimulated by increased activity of the beta-adrenergic/adenylyl cyclase/cyclic 3'-5' adenosine monophosphate-signaling cascade. Atrial natriuretic peptide (ANP) stimulates surfactant secretion through natriuretic peptide receptor A (NPR-A). ANP inhibits adenylyl cyclase activity through its binding to NPR-C. We wished to further understand the role of the NPR-C in perinatal transition. Methods: We studied ATII expression of NPR-C in fetal and newborn sheep using immunohistochemistry, and surfactant secretion in isolated ATIIs by measuring ³[H] choline release into the media. Results: ANP induced surfactant secretion, and, at higher doses, it inhibits the stimulatory effect of the secretagogue terbutaline. ATII NPR-C expression decreased significantly after birth. Premature delivery also markedly decreased ANP and NPR-C in ATIIs. Co-incubation of terbutaline (10^-4 M) with ANP (10^-6 M) significantly decreased ³[H] choline release from isolated newborn ATII cells when compared with terbutaline alone; this inhibitory effect was mimicked by the specific NPR-C agonist, C-ANP (10^-10 M). Conclusion: ANP may act as an important epithelial-derived inhibitor of surfactant release in the fetal lung, and downregulation of ANP and NPR-C following birth may sensitize ATII cells to the effects of circulating catecholamines, thus facilitating surfactant secretion.