Comparison of relaxation responses of cavernous and trigonal smooth muscles from rabbits by alpha1-adrenoceptor antagonists; prazosin, terazosin, doxazosin, and tamsulosin.

Alpha1a-adrenergic receptor (AR) primarily mediates the contraction of the prostatic and cavernous smooth muscles. Among clinically available alpha1-AR antagonists for the medical management of benign prostatic hyperplasia (BPH), tamsulosin has a modest selectivity for alpha1A- and alpha1D- over alpha1B-ARs. To compare the effects of various alpha1-AR antagonists on relaxation responses of cavernous and trigonal smooth muscles, isometric tension studies with relatively selective (tamsulosin) and non-selective (prazosin, doxazosin, and terazosin) alpha1A-AR antagonists, were conducted in the cavernous and trigonal muscle strips of rabbits (n=10 each). Tamsulosin had the strongest inhibitory effect on contraction of trigonal smooth muscle among the various alpha1-AR antagonists, and the inhibitory activities of prazosin, doxazosin, and terazosin were not statistically different. All alpha1-AR antagonists caused concentration-dependent relaxation of the cavernous muscle strips. Tamsulosin was shown to have greater potency than prazosin (more than 100-fold), doxazosin (more than 1000-fold), and terazosin (more than 1000-fold), in relaxation of cavernous smooth muscle. In conclusion, tamsulosin might be the most effective drug among the four commonly used alpha1-AR antagonists for the medical management of BPH. Tamsulosin might be a potential substitute for phentolamine in combination with vasoactive agents as an intracavernous injection therapy for patients with erectile dysfunction.