The cell volume regulatory response following hypotonic shocks is often achieved by the coordinated activation of K ؉ and Cl ؊ channels. In this study, we investigate the identity of the K ؉ and Cl ؊ channels that mediate the regulatory volume decrease (RVD) in ciliated epithelial cells from murine trachea. RVD was inhibited by tamoxifen and 1,9-dideoxyforskolin, two agents that block swelling-activated Cl ؊ channels. These data suggest that swelling-activated Cl ؊ channels play an important role in cell volume regulation in murine tracheal epithelial cells. Ba 2؉ and apamin, inhibitors of K ؉ channels, were without effect on RVD, while tetraethylammoniun had little effect on RVD. In contrast, clofilium, an inhibitor of the KvLQT/IsK potassium channel complex potently inhibited RVD, suggesting a role for the KvLQT/IsK channel complex in cell volume regulation by tracheal epithelial cells. To investigate further the role of KvLQT/IsK channels in RVD, we used IsK knock-out mice. When exposed to hypotonic solutions, tracheal cells from IsK(؉/؉) mice underwent RVD, whereas cells from IsK(؊/؊) failed to recover their normal size. These data suggest that the IsK potassium subunit plays an important role in RVD in murine tracheal epithelial cells.When exposed to hypotonic solutions most cells swell rapidly before recovering their original volume, a response known as regulatory volume decrease (RVD). 1 RVD involves the activation of ionic pathways, mainly ion channels, which permit the passive loss of electrolytes and osmotically obliged water (1). Although the ability of maintaining a constant volume in the face of osmotic stress is important for all cells in the body, the process assumes particular significance in epithelial cells. In the case of the airways, the luminal face of the epithelial cells is covered by a liquid (airway surface liquid, ASL) that modifies its osmolality under different situations, becoming hyperosmolar (e.g. during cold or dry ventilation) or hypoosmolar (e.g. breathing fog) (2-4). These changes in the osmolality of the ASL affect other airway functions, including protein and hydroelectrolytic secretion, and release of inflammatory mediators (5, 6). Despite the significance of ASL to airway physiology and pathophysiology the molecular basis for the ion pathways involved in cell volume regulation is largely unknown. Although the molecular identity of the swelling-activated Cl Ϫ channels has been the subject of several studies in recent years (7-9), little is known about the identity of the swelling-activated K ϩ channels involved in RVD. In this study we have characterized the ionic conductances underlying the RVD response in murine tracheal cells. We have found that the potassium channel complex KvLQT/IsK (10) plays an important role in cell volume regulation by murine tracheal cells. EXPERIMENTAL PROCEDURESAnimals-For this study, we used mice from two different outbred genetic backgrounds. One group yielded control animals, while the other gave wild-type isk(ϩ/ϩ) and null isk(Ϫ/Ϫ) mice (11)....

The regulation of Maxi Cl− channels by 17β‐oestradiol and non‐steroidal triphenylethylene antioestrogens represents a rapid, non‐classical effect of these compounds. In the present study we have investigated the signalling pathways used for the regulation of Maxi Cl− channel activity by oestrogens and antioestrogens in C1300 neuroblastoma cells. Whole‐cell Maxi Cl− currents were readily and reversibly activated by tamoxifen, toremifene and the membrane‐impermeant ethyl‐bromide tamoxifen, only when applied to the extracellular medium. Pre‐treatment of C1300 cells with oestrogen or cAMP prevented the antioestrogen‐induced activation of Maxi Cl− channels. The inhibitory effect of 17β‐oestradiol and cAMP was abolished by the kinase inhibitor staurosporine. Current activation was unaffected by the removal of intracellular Ca2+ and Mg2+, but was completely abolished in the presence of okadaic acid. These results are consistent with the participation of an okadaic acid‐sensitive serine/threonine protein phosphatase in the activation of Maxi Cl− channels. However, neither oestrogen or antioestrogen treatment modified the total activity of the two major serine/threonine phosphatases, PP1 and PP2A, in C1300 cells. Although the role of these Maxi Cl− channels remains unknown, our findings suggest strongly that their modulation by oestrogens and antioestrogens is linked to intracellular signalling pathways.