α_1L-Adrenoceptor mediation of smooth muscle contraction in rabbit bladder neck: a model for lower urinary tract tissues of man

Abstract

1. The α₁-adrenoceptor population mediating contractile responses to noradrenaline (NA) in smooth muscles of the bladder neck from rabbit (RBN) has been characterized by use of quantitative receptor pharmacology.

2. Experiments with several `key' α₁-adrenoceptor antagonists of varying subtype selectivities (RS-17053, BMY 7378, indoramin, 5-methylurapidil, prazosin, REC 15/2739, SNAP 5089, terazosin, WB 4101, tamsulosin, (+)-cyclazosin and RS-100329) were conducted. Schild regression analyses yielded affinity (mean pK_b) estimates of 7.1, 6.2, 8.6, 8.6, 8.4, 9.3, 7.0, 7.4, 8.9, 10.0, 7.1 and 9.3, respectively, although deviations from unit Schild regression slope question the robustness of data for RS-17053 and SNAP 5089.

3. The nature of antagonism by these agents and the profile of affinity determinations generated together suggest that a single α₁-adrenoceptor subtype mediates contractile responses of RBN to NA. Additional studies with phenylephrine indicated also an agonist-independence of this profile. Pharmacologically, this profile was reminiscent of that described as `α<sub>1L</sub>'-adrenoceptor, which has been shown to mediate contractions of several tissues including lower urinary tract (LUT) tissues of man. Furthermore, a similarity was noticed between the `α_1L'-adrenoceptor described here in RBN and the rabbit and human cloned α_1a-adrenoceptor (based on data from both whole cell radioligand binding at 37°C and [³H]-inositol phosphates accumulation assays), characterizations of which have been published elsewhere.

4. In conclusion, the RBN appears to provide a predictive pharmacological assay for the study of NA-induced smooth muscle contraction in LUT tissues of man.

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