Maxi K+ channel mediates regulatory volume decrease response in a human bronchial epithelial cell line

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Abstract
The cell regulatory volume decrease (RVD) response triggered by hypotonic solutions is mainly achieved by the coordinated activity of Cl− and K+ channels. We now describe the molecular nature of the K+ channels involved in the RVD response of the human bronchial epithelial (HBE) cell line 16HBE14o−. These cells, under isotonic conditions, present a K+current consistent with the activity of maxi K+ channels, confirmed by RT-PCR and Western blot. Single-channel and whole cell maxi K+ currents were readily and reversibly activated following the exposure of HBE cells to a 28% hypotonic solution. Both maxi K+ current activation and RVD response showed calcium dependency, inhibition by TEA, Ba2+, iberiotoxin, and the cationic channel blocker Gd3+ but were insensitive to clofilium, clotrimazole, and apamin. The presence of the recently cloned swelling-activated, Gd3+-sensitive cation channels (TRPV4, also known as OTRPC4, TRP12, or VR-OAC) was detected by RT-PCR in HBE cells. This channel, TRPV4, which senses changes in volume, might provide the pathway for Ca2+ influx under hypotonic solutions and, consequently, for the activation of maxi K+channels.