07.01.4 Selective α1- Adrenoceptor Antagonist

Prazosin is a selective a₁-adrenoceptor blocker. The a₁-adrenoceptor blockade with prazosin causes a decrease in the noradrenaline a₁-adrenoceptor mediated vasoconstriction, which effectively gives a vasodilation, and as a result there is a decrease in blood pressure. Prazosin is occasionally used as 3rd/4th line treatment of hypertension. Thus, if people have very high blood pressure, which is still not controlled with 2 or 3 of the standard drugs for the treatment hypertension, Prazosin may be added to the regimen to increase the likelihood of controlling blood pressure.

Tamsulosin is the first example of an uroselective blocker, that is a selective a₁-adrenoceptor blocker, which additionally shows selectivity for a_1A-adrenoceptors (which are found on the prostate) over a_1B-adrenoceptors, which are found of blood vessels. Thus, tamsulosin has a greater ability to reverse noradrenaline a_1A-adrenoceptor-mediated contraction of the smooth muscle of the prostate than to reverse noradrenaline a_1B-adrenoceptor-mediated contraction of the blood vessels. Tamsulosin is used in the treatment of benign prostatic hyperplasia where the growth of the prostrate obstructs the bladder neck, making urination difficult in older men. Tamsulosin relaxes the prostate muscle to reduce the obstruction. Tamsulosin also reduces the a₁-adrenoceptor contraction in the neck and sphincters of the urinary bladder, which makes urinating easier.