Transepithelial Measurements of Bicarbonate Secretion in Calu-3 Cells

The importance of impaired HCO ₃ secretion in the pathophysiology of the pancreas of cystic fibrosis (CF) patients has been well documented and known for many years ( 1 ). Studies in 1990s from the Welsh ( 2 ) and Boucher ( 3 ) laboratories suggested that HCO ₃ secretion may also be impaired in the airways of CF patients. Perhaps because Cl ^- secretion has been assumed to be of higher importance and thus received the greater attention, the transcellular mechanisms of HCO ₃ secretion remain poorly understood and underinvestigated. Studies from our own laboratory ( 4 ) and the laboratories of Wine and Widdicombe ( 5 ) have now established that the human airway serous cell line, Calu-3 cells, secrete HCO ₃ and not Cl ^- , in response to a cAMP-mediated secretory agonist. Serous cells are the most abundant cell type of the submucosal glands ( 6 ) and are the predominate site of CFTR expression in the airways ( 7 , 8 ). Thus, a better understanding of the transport mechanisms of serous cells is of critical importance in establishing how mutations in CFTR lead to submucosal gland and airways pathophysiology. The focus of this chapter is to provide a description of the methods used to study Calu-3 cell monolayers grown on permeable supports by the short-circuit current (I _SC ) technique. In addition, methods for ion flux studies and ion substitution studies of short-circuited monolayers are described along with background information necessary to interpret the experimental results specifically as they relate to the net secretion of HCO ₃ . To introduce the subject of HCO ₃ secretion, we provide a brief description of the mechanisms of transepithelial HCO ₃ secretion as they are presently understood.

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