Identification of alpha1-adrenoceptor subtypes in the human prostatic urethra

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Abstract
To identify the α1-adrenoceptor subtypes in the human prostatic urethra, we compared the potencies of various α1-adrenoceptor agonists and antagonists in inhibiting [3H]tamsulosin binding to human prostatic urethral membranes with their potencies in inhibiting the binding of (+)-β-([125I]iodo-4-hydroxyphenyl)ethylaminomethyl-tetralone ([125I]HEAT) to cloned human α1a, α1b and α1d subtypes. The α1A-selective antagonists 5-methylurapidil and (+)niguldipine showed higher affinities for both cloned α1a and urethral α1-adrenoceptors than for cloned α1b- and α1d-adrenoceptors. NS-49, (R)-3′-(2-amino-1-hydroxyethyl)-4′-fluoromethanesulfonanilide hydrochloride, recently characterized as an α1A-selective agonist, also showed high affinity for the cloned α1a subtype and urethral α1-adrenoceptors. Prazosin showed lower affinity for α1-adrenoceptors in the human prostatic urethra than for any of the three cloned α1-adrenoceptors. Comparison of the affinities of α1-adrenoceptor agonists and antagonists for human prostatic urethral α1-adrenoceptors to their affinities for the three cloned α1 subtypes indicated a close correlation between the affinities for human urethral α1 and the cloned α1a-adrenoceptors. However, prazosin did not conform to this pattern. These findings suggest that the predominant α1-adrenoceptor in the human urethra is the α1A subtype, and that an α1L subtype which has been characterised by its low affinity for prazosin, may also be present.

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