Culture-induced alterations in alveolar type II cell Na+ conductance

Changes in Na+ transport in rat alveolar type II (ATII) cells during culture were quantified and related to alterations in spatial distribution of proteins antigenically related to amiloride-sensitive Na+ channels. Adult rat ATII cells were cultured for periods ranging from 24 to 96 h. When patch clamped in the whole cell mode, both freshly isolated and cultured ATII cells exhibited outwardly rectified Na+ currents. At 0 and 24 h in culture, these currents were equally inhibited by amiloride, benzamil, and 5-(N-ethyl-N-isopropyl)-2',4'-amiloride (inhibitory constant approximately 1 microM). These conductive pathways were equally permeable to Na+ and K+. Immunocytochemical localization at 0 or 24 h in culture revealed the presence of plasma membrane antigenic sites; after 48 h, the appearance of intracellular antigenic sites increased significantly. A single band of molecular mass 135 kDa in membrane proteins of freshly isolated ATII cells was recognized in Western blots; at 48 h in culture, two lower bands with molecular masses of 75 and 65 kDa were detected in either membrane or cytoplasmic proteins. Photolabeling with 2'-methoxy-5'-nitrobenzamil showed that the 135-, 75-, and 65-kDa bands contained amiloride-binding sites. These results suggest the presence of low amiloride affinity conductive pathways in freshly isolated and cultured ATII cells. Culturing ATII cells resulted in internalization and possible breakdown of these pathways and decreased Na+ transport.