Comparative alpha-1 adrenoceptor subtype selectivity and functional uroselectivity of alpha-1 adrenoceptor antagonists

Abstract

We investigated the relevance of selectivity for a given alpha-1-adrenoceptor subtype for in vivo uroselectivity of several alpha-1-adrenoceptor antagonists (alfuzosin, doxazosin, prazosin, tamsulosin, terazosin and 5-Me-urapidil). Comparison of the affinities of these alpha-1-adrenoceptor antagonists at the cloned alpha-1a, alpha-1b and alpha-1d-adrenoceptor subtypes revealed that tamsulosin and 5-Me-urapidil showed selectivity for the alpha-1a subtype. No significant correlations were found between the affinities for alpha-1b or alpha-1d-adrenoceptors and the pK(B) values obtained against phenylephrine-induced contraction of the rabbit prostate in vitro. In contrast, the antagonist potencies in rabbit prostate were correlated (r = 0.89, P < .05) with the pKi values for the alpha-1a-adrenoceptor subtype. However, the pK(B) values were consistently smaller (by 0.6 to 1.9 log unit) than the pKi values for the alpha-1a-adrenoceptor subtype, a result that suggests that the alpha-1-adrenoceptor mediating urethral contractions does not have all the characteristics of the alpha-1a-adrenoceptor. The simultaneous measurement of urethral and arterial pressures in the same conscious male rat allows evaluation of the functional uroselectivity of these antagonists based on their respective effects on both pressures. Dose ranges were selected according to effects on urethral pressure and most antagonists were found effective within the 3 to 100 microg/kg i.v. range. Alfuzosin markedly decreased urethral pressure and either did not decrease blood pressure (10-30 microg/kg) or slightly decreased it at the highest dose tested (100 microg/kg). Doxazosin did not produce sustained reductions in urethral pressure until a dose of 30 microg/kg. Blood pressure was not reduced until 100 microg/kg. Prazosin reduced urethral pressure and blood pressure within the same dose-range whereas terazosin did not decrease urethral pressure at doses that significantly decreased blood pressure (30 and 100 microg/kg). 5-Me-urapidil, an alpha-1a-selective compound did not significantly modify urethral and blood pressure whereas tamsulosin, another alpha-1a-selective compound reduced urethral pressure and blood pressure within the same dose range. In conclusion, in the conscious male rat the functional uroselectivity is not correlated with a selective affinity for the alpha-1a-adrenoceptor subtype.