Functional coupling of TRPV4 cationic channel and large conductance, calcium-dependent potassium channel in human bronchial epithelial cell lines

Calcium-dependent potassium channels are implicated in electrolyte transport, cell volume regulation and mechanical responses in epithelia, although the pathways for calcium entry and their coupling to the activation of potassium channels are not fully understood. We now show molecular evidence for the presence of TRPV4, a calcium permeable channel sensitive to osmotic and mechanical stress, and its functional coupling to the large conductance calcium-dependent potassium channel (BKCa) in a human bronchial epithelial cell line (HBE). Reverse transcriptase polymerase chain reaction, intracellular calcium imaging and whole-cell patch-clamp experiments using HBE cells demonstrated the presence of TRPV4 messenger and Ca2+ entry, and outwardly rectifying cationic currents elicited by the TRPV4 specific activator 4 alpha-phorbol 12,13-didecanoate (4 alpha PDD). Cell-attached and whole-cell patch-clamp of HBE cells exposed to 4 alpha PDD, and hypotonic and high-viscosity solutions (related to mechanical stress) revealed the activation of BKCa channels subsequent to extracellular Ca2+ influx via TRPV4, an effect lost upon antisense-mediated knock-down of TRPV4. Further analysis of BKCa modulation after TRPV4 activation showed that the Ca2+ signal can be generated away from the BKCa location at the plasma membrane, and it is not mediated by intracellular Ca2+ release via ryanodine receptors. Finally, we have shown that, unlike the reported disengagement of TRPV4 and BKCa in response to hypotonic solutions, cystic fibrosis bronchial epithelial cells (CFBE) preserve the functional coupling of TRPV4 and BKCa in response to high-viscous solutions.