Effect of viscous load on the ciliary beat frequency (CBF). The CBF dropped ~35 percent within the low viscosity range of 2 to 37 cP (2–15 percent dextran solutions), but no further decrease was observed at higher viscosities (37–200 cP; corresponding to 15–30 percent dextran solutions). We related this CBF autoregulatory mechanism to the activity of TRPV4 channels. Under low viscosity the channel shows little or no activity. Increasing viscosity activates TRPV4, triggering the autoregulatory response to maintain a correct CBF. Preventing TRPV4-mediated Ca²⁺ entry (Ca²⁺-free or Gd³⁺-containing dextran solutions) or inhibiting PLA₂ (upstream of TRPV4 activation) resulted in marked reduction of CBF (modified from reference [8]).