Polycystic kidney disease is characterised by the massive enlargement of fluid-filled epithelial cysts that disrupt kidney function. Hanaoka et al. (1996) demonstrated that the cystic fibrosis transmembrane conductance regulator (CFTR) Cl^– channel plays a key role in fluid accumulation within the lumen of some, but not all, cysts from patients with polycystic kidney disease. This suggests that other types of Cl^– channels might play an important role in cyst development and enlargement.

Using MDCK type I cells that express CFTR, we previously demonstrated that CFTR-driven fluid secretion increases the size of MDCK cysts (Li & Sheppard, 2002). In this study, we investigated the role of Ca²⁺-activated and volume-sensitive Cl^– channels in cyst formation and growth. We began by assessing the function of Ca²⁺-activated and volume-sensitive Cl^– channels in MDCK cells using the iodide efflux assay (for Methods, see Lansdell et al. 1998). To stimulate Ca²⁺-activated and volume-sensitive Cl^– channels, we used ionomycin (1 µM) and a 50 % hypotonic solution, respectively; to activate CFTR Cl^– channels, we used forskolin (10 µM). In type I MDCK cells, forskolin, ionomycin and hypotonicity all stimulated an efflux of iodide and the magnitude of iodide efflux decreased in the rank order: hypotonicity (82 ± 9 nmol min^-1) >> ionomycin (18 ± 2 nmol min^-1) = forskolin (14 ± 3 nmol min^-1; means ± S.E.M.; n = 6 for all values). In contrast, in type II MDCK cells that lack CFTR, only hypotonicity stimulated an efflux of iodide. However, the magnitude of the response was drastically smaller than that stimulated by hypotonicity in type I MDCK cells (10 ± 1 nmol min^-1; n = 6).

For cyst growth studies, we used cysts grown either in collagen gels or culture dishes for 6 days. Type I MDCK cysts formed using both substrates. However, type II MDCK cysts only formed in culture dishes. In the absence of drugs, a few large type I cysts with well-defined walls formed in culture dishes. However, many small type II cysts with poorly defined walls formed in culture dishes in the absence of drugs. In collagen gels, forskolin (10 µM) and ionomycin (1 µM) stimulated the formation and growth of type I MDCK cysts, but were without effect on type II MDCK cysts. The number and size of cysts grown in the presence of ionomycin (1 µM) were significantly smaller than those grown in the presence of forskolin (10 µM; P < 0.01; Student’s unpaired t test). A 50 % hypotonic solution stimulated the growth of both type I and type II MDCK cysts. Interestingly, cysts grown in the presence of a 50 % hypotonic solution developed into a solid mass of cells after 2-4 days. We interpret these data to suggest that the CFTR Cl^– channel plays a dominant role in MDCK cyst formation and growth.

This work was supported by the NKRF.