TRPV4 Contributes to Resting Membrane Potential in Retinal Müller Cells: Implications in Cell Volume Regulation.

Neural activity alters osmotic gradients favoring cell swelling in retinal Müller cells. This swelling is followed by a regulatory volume decrease (RVD), partially mediated by an efflux of KCl and water. The transient receptor potential channel 4 (TRPV4), a nonselective calcium channel, has been proposed as a candidate for mediating intracellular Ca elevation induced by swelling. We previously demonstrated in a human Müller cell line (MIO-M1) that RVD strongly depends on ion channel activation and, consequently, on membrane potential (V ). The aim of this study was to investigate if Ca influx via TRPV4 contributes to RVD by modifying intracellular Ca concentration and/or modulating V in MIO-M1 cells. Cell volume, intracellular Ca levels, and V changes were evaluated using fluorescent probes. Results showed that MIO-M1 cells express functional TRPV4 which determines the resting V associated with K channels. Swelling-induced increases in Ca levels was due to both Ca release from intracellular stores and Ca influx by a pathway alternative to TRPV4. TRPV4 blockage affected swelling-induced biphasic response (depolarization-repolarization), suggesting its participation in modulating V changes during RVD. Agonist stimulation of Ca influx via TRPV4 activated K channels hyperpolarizing V and accelerating RVD. We propose that TRPV4 forms a signaling complex with Ca and/or voltage-dependent K channels to define resting V and V changes during RVD. TRPV4 involvement in RVD depends on the type of stimuli and/or degree of channel activation, leading to a maximum RVD response when Ca influx overcomes a threshold and activates further signaling pathways in cell volume regulation. J. Cell. Biochem. 118: 2302-2313, 2017. © 2017 Wiley Periodicals, Inc.