Liquid movement across the surface epithelium of large airways

The cystic fibrosis transmembrane conductance regulator CFTR gene is found on chromosome 7 [ Kerem, B., Rommens, J.M., Buchanan, J.A., Markiewicz, D., Cox, T.K., Chakravarti, A., Buchwald, M., Tsui, L.C., 1989. Identification of the cystic fibrosis gene: genetic analysis. Science 245, 1073–1080; Riordan, J.R., Rommens, J.M., Kerem, B., Alon, N., Rozmahel, R., Grzelczak, Z., Zielenski, J., Lok, S., Plavsic, N., Chou, J.L., et al., 1989. Identification of the cystic fibrosis gene: cloning and characterization of complementary DNA. Science 245, 1066–1073] and encodes for a 1480 amino acid protein which is present in the plasma membrane of epithelial cells [Anderson, M.P., Sheppard, D.N., Berger, H.A., Welsh, M.J., 1992. Chloride channels in the apical membrane of normal and cystic fibrosis airway and intestinal epithelia. Am. J. Physiol. 263, L1–L14]. This protein appears to have many functions, but a unifying theme is that it acts as a protein kinase C- and cyclic AMP-regulated Cl− channel [Winpenny, J.P., McAlroy, H.L., Gray, M.A., Argent, B.E., 1995. Protein kinase C regulates the magnitude and stability of CFTR currents in pancreatic duct cells. Am. J. Physiol. 268, C823–C828; Jia, Y., Mathews, C.J., Hanrahan, J.W., 1997. Phosphorylation by protein kinase C is required for acute activation of cystic fibrosis transmembrane conductance regulator by protein kinase A. J. Biol. Chem. 272, 4978–4984]. In the superficial epithelium of the conducting airways, CFTR is involved in Cl− secretion [Boucher, R.C., 2003. Regulation of airway surface liquid volume by human airway epithelia. Pflugers Arch. 445, 495–498] and also acts as a regulator of the epithelial Na+ channel (ENaC) and hence Na+ absorption [Boucher, R.C., Stutts, M.J., Knowles, M.R., Cantley, L., Gatzy, J.T., 1986. Na+ transport in cystic fibrosis respiratory epithelia. Abnormal basal rate and response to adenylate cyclase activation. J. Clin. Invest. 78, 1245–1252; Stutts, M.J., Canessa, C.M., Olsen, J.C., Hamrick, M., Cohn, J.A., Rossier, B.C., Boucher, R.C., 1995. CFTR as a cAMP-dependent regulator of sodium channels. Science 269, 847–850]. In this chapter, we will discuss the regulation of these two ion channels, and how they can influence liquid movement across the superficial airway epithelium.