EXPRESSION OF THE EPITHELIAL NA+ CHANNEL IN THE DEVELOPING RAT LUNG

The adult mature fetal, but not immature fetal, lung is capable of act ively transporting Na+ from the alveolar space. The reason for the imp aired Na+ transport in the immature lung is not known; however, the ap ical membrane Na+ channel is the rate-limiting step for epithelial Na transport. This study determined whether transcripts coding for the a dult rat colonic epithelial Na+ channel (alpharENaC) were present in t he fetal and adult lung and whether they were developmentally regulate d. Similarly sized alpharENaC transcripts were identified in RNA isola ted from fetal and adult whole rat lung, primary cultures of fetal and adult alveolar epithelium, and adult rat whole kidneys, suggesting th at the lung alpharENaC is a similar transcript to that found in the sa lt-deprived rat colonic epithelium. There were low mRNA levels in 17- to 18-day gestational age (GA) fetal lungs and epithelium (term GA = 2 2 days), but these levels increased markedly during the saccular stage of lung development (20 days GA) and remained high in adult lungs. Th e combined administration of thyroid-releasing hormone and dexamethaso ne to pregnant rats between 16 and 18 days GA induced the expression o f lung alpharENaC in their fetuses. We conclude that alpharENaC is exp ressed in mature fetal and adult alveolar epithelium and that it is in fluenced by hormones known to alter maturation of the fetal lung.