DOPAMINE ACTS AT THE SAME RECEPTORS AS NORADRENALINE IN THE RAT ISOLATED VAS DEFERENS

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Abstract
1 The proposal that dopamine activates a different population of receptors from those activated by noradrenaline and phenylephrine to cause contraction of the rat vas deferens has been investigated using a preparation of the epididymal half of this tissue. 2 In preparations preincubated in cocaine, oestradiol and propranolol, to block sites of amine loss and β-adrenoceptors, noradrenaline was the most, and dopamine the least, potent of the three agonists. Phentolamine competitively inhibited each of the agonists to a similar extent. Prazosin also inhibited the actions of the three agonists to a similar extent. These results indicate that the three agonists activate a single population of α1-adrenoceptors to cause contraction in this preparation. 3 In experiments using the prostatic half of the rat vas deferens, in the presence of cocaine, oestradiol, propranolol and prazosin, noradrenaline was approximately 40 times more potent than dopamine in causing inhibition of twitches induced by electrical field stimulation. Yohimbine competitively antagonized the effects of the two agonists to a similar extent indicating that both act at the same population of α2-adrenoceptors. 4 Taken together, these findings do not lend support to proposals that there are populations of specific dopamine receptors located pre- and postjunctionally in the rat vas deferens.