Functional and radioligand binding characterization of the a1L-adrenoceptor subtype of the human vas deferens
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Summary: Alpha1-adrenoceptor antagonists can cause ejaculatory dysfunction as an adverse effect. Contractions of the human vas deferens are mediated via α1A-adrenoceptors, and this study investigated whether the low affinity state of this receptor (α1L-adrenoceptor) is involved in mediating contractions of this tissue. The potency of agonists and the affinity of receptor subtype selective antagonists were determined in functional experiments and in [3H]tamsulosin binding experiments to identify the α1-adrenoceptor subtype population present in the human vas deferens. The α1A-adrenoceptor selective agonist A61603 was a full agonist and was 250-fold more potent than noradrenaline. Prazosin antagonized contractile responses to phenylephrine with a low affinity (pKd = 8.6). Only high concentrations of RS17053 antagonized responses to phenylephrine and yielded a relatively low affinity estimate of 7.0. BMY7378 (α1D-adrenoceptor selective) gave a low affinity estimate (pKd = 6.7), whilst tamsulosin (α1A- and α1D-adrenoceptor selective) had a high affinity (pKd = 9.9). [3H]Tamsulosin bound to human vas deferens membranes with a high affinity (pKd = 10.0). Prazosin, RS17053 and BMY7378 competed with [3H]tamsulosin with low affinities for a single population of binding sites (pKd values of 8.5, 7.2 and 6.3, respectively). These functional and radioligand binding data indicate that the human vas deferens possesses a homogeneous population of α1-adrenoceptors which have the pharmacological properties of the putative α1L-adrenoceptor, the same functional receptor previously identified in the human prostate.
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