Abstract
α1A-adrenergic receptor (AR) primarily mediates the contraction of the
prostatic and cavernous smooth muscles. Among clinically available α1-AR
antagonists for the medical management of benign prostatic hyperplasia (BPH),
tamsulosin has a modest selectivity for α1A- and α1D- over α1B-ARs.
To compare the effects of various α1-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) α1A-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 α1-AR
antagonists, and the inhibitory activities of prazosin, doxazosin, and terazosin
were not statistically different. All α1-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 α1-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.