TRPV4 and KCa: Modelling the Perfect Couple?

Abstract

The hypothalamus is responsible for maintaining body fluid osmolarity within a narrow range (∼290-300mOsm) [1]; in particular the paraventricular nucleus (PVN) is thought to have a key role in osmoregulation. Previously we have shown changes in action current frequency upon hypotonic challenge within the PVN using patch-clamp electrophysiology [2], with evidence suggesting these changes are due to the activation of the transient receptor potential vanilloid 4 (TRPV4) and calcium-activated potassium (KCa) channels.Our data supports a functional coupling between the TRPV4 and KCa channels, leading us to the hypothesis that upon hypotonic challenge TRPV4 activates KCa through influx of Ca2+, leading to an efflux of K+, as shown in other cells [3] leading to hyperpolarisation of the cell. This hyperpolarisation itself is suggested to create a positive feedback loop increasing the driving force for Ca2+ entry [4]. To investigate this hypothesis we have developed our existing NEURON model (University of Yale) [5], modelling the action of TRPV4 and KCa.In the model decreased osmolarity caused action potential (AP) frequency reduction, with a 93±3% decrease of AP frequency at 290mOsm and a half maximum of 304±0.4mOsm, dependant on starting parameters. Block of TRPV4 or KCa channels prevented this effect with AP frequency remaining at 100±0% regardless of osmolarity. To test positive feedback we simulated TRPV4 activity and measured simulated TRPV4 Ca2+ current with and without the presence of KCa activity. Breaking the positive feedback loop by block of KCa, as predicted, significantly reduced TRPV4 Ca2+ current by 640±0.1pA/s.cm2 (n=7; p<0.005).This model, together with our previous data provides further evidence for a functional coupling between TRPV4 and KCa channels within neurones in the PVN, supporting our hypothesis of a positive feedback system.