Resorption of alveolar fluid at birth: Cellular mechanisms

Na enters the cell through the apical surface of both ATI and ATII cells via amiloride-sensitive epithelial Na channels (ENaC), both highly selective channels (HSC) and nonselective channels (NSC), and via cyclic nucleotide gated channels (seen only in ATI cells). Electroneutrality is conserved with chloride movement through cystic fibrosis transmembrane conductance regulator (CFTR) or through chloride channels (CLC) in ATI and ATII cells, and/or paracellularly through tight junctions. The increase in cell Na stimulates Na-K-ATPase activity on the basolateral aspect of the cell membrane, which drives out three Na ions in exchange for two K ions, a process that can be blocked by the cardiac glycoside ouabain. If the net ion movement is from the apical surface to the interstitium, an osmotic gradient would be created, which would in turn direct water transport in the same direction, either through aquaporins or by diffusion.