Ion transport proteins identified in alveolar type I (ATI) and alveolar type II cells (ATII). In ATII cells a variety of ion transporting proteins have been identified (ENaC: epithelial Na+ channel; CNG: cyclic-nucleotide-gated channel; Kv: voltage-gated potassium channels; Na+/K+-ATPase: sodium/potassium ATPase; Kir: inward rectifying K+ channel; KCa: calcium-activated potassium channel; CFTR: cystic fibrosis transmembrane conductance regulator; ClC: voltage-sensitive Cl− channels; GABAA: γ-aminobutyric acid type A Cl− channel; NKCC: sodium/potassium two chloride cotransporter; AE: anion exchanger). For clarity of the scheme, the subtypes of the different K+ channels were omitted. ATI cells are similarly equipped with ion transporting proteins. In addition these cells express aquaporin 5 (AQP5). The molecular identity of the K+ channel described is not known yet.
