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Voltage- and time-dependent properties of the recombinant rat vanilloid receptor (rVR1)

Martin J. Gunthorpe, Mark H. Harries, Rab K. Prinjha, John B. Davis and Andrew Randall

1. Whole-cell voltage-clamp techniques were used to investigate the capsaicin-, voltage- and time-dependent properties of the rat vanilloid receptor (rVR1) stably expressed in human embryonic kidney (HEK) 293 cells.

2. At a holding potential of -70 mV, application of capsaicin (0·03-30 µM) to HEK 293 cells expressing the rVR1 receptor led to the appearance of inward currents (EC₅₀, 497 nM; Hill coefficient, n_H, 2·85) which were reversibly antagonized by 10 µM capsazepine.

3. Current-voltage relationships, determined using depolarizing or hyperpolarizing voltage ramps, had reversal potentials close to 0 mV, exhibited substantial outward rectification and possessed a region of negative slope conductance at holding potentials negative to around -70 mV. Further experiments indicated that the outward rectification and the region of negative slope conductance did not result from external block of the channel by either Ba²⁺, Ca²⁺ or Mg²⁺.

4. During our characterization of rVR1, it became apparent that the rectification behaviour of this receptor was not entirely instantaneous as might be expected for a ligand-gated ion channel, but rather displayed clear time-dependent components. We characterized the kinetics of these novel gating properties in a series of additional voltage-step experiments.

5. The time-dependent changes in rVR1-mediated conductance due to membrane depolarization or repolarization occurred with bi-exponential kinetics. On depolarization to +70 mV the time-dependent increase in outward current developed with mean time constants of 6·7 ± 0·7 and 51·8 ± 18·4 ms, with the faster time constant playing a dominant role (64·4 ± 3·8 %). Similar kinetics also described the decay of 'tail currents' observed on repolarization. Furthermore, these time-dependent changes appeared to be unaffected by the removal of extracellular divalent cations and were not significantly voltage dependent.

6. Our data reveal that rVR1 exhibits substantial time- and voltage-dependent gating properties that may have significance for the physiology of sensory transduction of nociceptive signals.

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