Fetal lung epithelial cells contain two populations of amiloride-sensitive Na+ channels.

Active Na+ transport by the alveolar epithelium plays a major role in reabsorption of the fetal lung fluid after birth. We characterized the biochemical and physiological characteristics of Na+ conductive pathways in distal fetal lung epithelial (FLE) cells isolated from 20-day-old rat fetuses. We demonstrated that a polyclonal antibody to Na+ channel protein (NaAb) binds to the plasma membranes of FLE cells. In Western blot studies, this NaAb and an anti-idiotypic monoclonal antibody to the amiloride-binding subunit of the Na+ channel protein recognized 150- and 90-kDa polypeptides in plasma membrane vesicles of FLE. 22Na+ flux measurements across plasma membrane vesicles of FLE revealed the existence of electrogenic Na+ transport, which was twice as high as the corresponding adult value. One hundred micromolars of amiloride, benzamil, and 5-(N-ethyl-N-isopropyl)-2'-4'-amiloride inhibited 30, 40, and 70% of the electrogenic Na+ transport across plasma membrane vesicles of FLE cells, respectively. The half-maximum inhibition of electrogenic Na+ transport by these substances occurred between 0.3 and 1 microM. [3H]benzamil equilibrium binding studies in membrane vesicles of FLE cells revealed the existence of two binding sites that had dissociation constant values of 19 and 1,525 nM, respectively. These data indicate the presence of both high- and low-amiloride affinity Na+ conductive pathways (channels) in FLE cells.