Postsynaptic alpha adrenergic receptor subtypes mediating contraction of human urinary bladder base and prostatic urethra were investigated in vitro. Alpha adrenergic receptor agonists, noradrenaline and phenylephrine, induced contraction dose-dependently in these tissues. Alpha adrenergic receptor antagonists, prazosin and yohimbine, competitively antagonized the contraction induced by these agonists in the bladder base as well as prostatic urethra. The mean pA2 values for the antagonists in the bladder base and prostatic urethra were as follows: 8.89 and 8.96 for prazosin and 6.30 and 6.45 for yohimbine, respectively. Comparison of pA2 values of prazosin and yohimbine in both the tissues with those values found in the literature for these compounds in a variety of tissues containing alpha-1 or alpha-2 adrenergic receptor indicates that the postsynaptic alpha adrenergic receptor subtype in the human urinary bladder base and prostatic urethra is the alpha-1 type, not the alpha-2 type.

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