Pharmacological characterization of the vanilloid receptor in the rat isolated vas deferens

The present study set out to further characterize the vanilloid receptor in the rat isolated vas deferens. In this preparation, both capsaicin and resiniferatoxin (RTX) evoked a concentration-dependent inhibition in the amplitude of electrically-evoked contractions with pEC50 values of 7.62 +/- 0.03 and 12.2 +/- 0.21 respectively. Responses to capsaicin were fast in onset and faded rapidly over a 30-min exposure period, whereas those to RTX were slow in onset and well maintained, an observation believed to reflect pharmacokinetic differences in the rate of penetration to the vanilloid receptor. Responses to both agonists showed mutual cross-desensitization and were antagonized by both the vanilloid-receptor antagonist capsazepine and the ion-channel blocker ruthenium red. The capsaicin analogue, olvanil failed to either mimic or antagonize capsaicin-evoked responses in the rat isolated vas deferens, an effect at variance with previous observations in other tissues. The reason for these differences is unclear, but the possibility of multiple classes of receptor cannot at this stage be ruled out.