Functional and radioligand binding characterization of the α1L-adrenoceptor subtype of the human vas deferens

Alpha₁-adrenoceptor antagonists can cause ejaculatory dysfunction as an adverse effect. Contractions of the human vas deferens are mediated via α_1A-adrenoceptors, and this study investigated whether the low affinity state of this receptor (α_1L-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 α_1A-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 (pK_d = 8.6). Only high concentrations of RS17053 antagonized responses to phenylephrine and yielded a relatively low affinity estimate of 7.0. BMY7378 (α_1D-adrenoceptor selective) gave a low affinity estimate (pK_d = 6.7), whilst tamsulosin (α_1A- and α_1D-adrenoceptor selective) had a high affinity (pK_d = 9.9). [³H]Tamsulosin bound to human vas deferens membranes with a high affinity (pK_d = 10.0). Prazosin, RS17053 and BMY7378 competed with [³H]tamsulosin with low affinities for a single population of binding sites (pK_d 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 α_1L-adrenoceptor, the same functional receptor previously identified in the human prostate.