Modulation of TRPV4 Channel Activity by ATP in Rat and Human Urinary Bladder
Seneca D. Hutson, BSc.1, Katarina Zvarova, MD, PhD2, Peter Zvara, MD, PhD2.
1University of Vermont, Burlington, VT, USA, 2University of Southern Denmark, Odense, Denmark.
BACKGROUND: Transient receptor potential V4 (TRPV4) channels involved in bladder sensory signaling represent a promising target for the treatment of bladder overactivity. Stretch-induced ATP release from the urothelium stimulates sensory nerve endings. Existing experimental evidence suggests that TRPV4 channels are likely involved in this process through activation of ATP release, although the exact mechanism of this effect needs further elucidation. In this study we investigated functional interaction between selective TRPV4 modulating drugs and ATP in rat and human bladder.
METHODS: Bladder tissue was functionally assessed using in vitro myography. Experiments used full thickness strips from healthy rat bladder and partial thickness strips containing detrusor muscle and urothelium from the bladder of adult female patients undergoing radical cystectomy. Contractile response of the bladder tissue to TRPV4-modulating drugs (agonist - GSK1016790A, antagonist - HC067047) and their interaction with P2X2/3 receptor agonist adenosine triphosphate (ATP) were evaluated in Ca-containing solution. Ca2+-free solution was then used to demonstrate the reliance of these events on calcium.
RESULTS: TRPV4 agonist (GSK1016790A) evoked dose-dependent increases in the baseline tone and amplitude of phasic contractions of muscle strips from both rat (n=6) and human (n=3) bladders. The response first developed at GSK concentration of 1nM and reached a peak at 100nM. This effect was significantly reduced by pretreatment with 1ÁM TRPV4 antagonist (HC067047). ATP (1mM) administered before GSK completely blocked the GSK effect. Adding 1 - 100Ám of ATP at the peak of GSK-evoked response increased the amplitude of phasic activity. A 1mM dose of ATP, at the peak of GSK-evoked response, reversed the effect of GSK, returning the tone to baseline and completely abolishing phasic activity. In Ca2+-free solution, both tonic and phasic responses to GSK were reduced by >90%. These effects were observed in both rat and human tissue.
CONCLUSIONS: Our results show that TRPV4 increases tone and amplitude of phasic activity in bladder strips in both human and rat bladder, supporting its likely role in bladder overactivity. Lower concentrations of ATP potentiate TRPV4-induced phasic activity and higher concentrations of ATP completely inhibit all TRPV4 bladder effects. These observations point to the possibility that ATP may modulate TRPV4 channel activity during bladder filling via negative feedback, thus contributing to the control of bladder sensory signaling.
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