Doxazosin relaxes ureteral smooth muscle and inhibits epinephrine-induced ureteral contractility in vitro

OBJECTIVES Although recent evidence has supported increased ureteral stone passage with selective alpha(1) adrenergic receptor antagonists, no mechanistic study evaluating ureteral relaxation by alpha, antagonism has been reported to date. We evaluated whether the alpha-bocker doxazosin reduces spontaneous, and inhibits alpha, -agonist-induced ureteral contractility. Additionally, alpha-receptor subtypes in normal and obstructed human ureter were analyzed. METHODS We exposed porcine ureters in organ tissue baths with 0.1, 1.0, or 10 mu M doxazosin and recorded the tension for 90 minutes. After the initial treatment, a concentration-response curve of epinephrine or phenylephrine (1 nM to 10 mu M) was generated. The experiment was repeated with the proximal, mid-, and distal ureter. The relative expression of the alpha 1A, 1B, and 1D receptor subtypes in normal and obstructed human ureters was analyzed using immunoblotting. RESULTS Doxazosin reduced the spontaneous ureteral contractility rates in a concentration-dependent fashion by 23% to 34%. A more pronounced relaxation effect by doxazosin was evident when epinephrine was introduced to the tissues. In 1 and 10 mu M doxazosin-pretreated tissues, epinephrine caused 89% and 100% relaxation, respectively. Phenylephrine-induced contractions were antagonized by doxazosin but not reversed to any relaxant function. No differential expression of alpha(1)-receptor subtypes was identified in the obstructed versus normal ureters. CONCLUSIONS The results of our study have shown that alpha(1)-receptor blockade decreases ureteral contractility and inverses the effect of epinephrine, providing even greater relaxation. We hypothesize that alpha receptor blockade might relax the ureter and induce stone passage by way of epinephrine activation of beta receptors. Additional studies should be performed to validate this hypothesis and to compare various alpha(1)-receptor subtype antagonists.