Two groups of α-blockers are registered for the treatment of hypertension, though not as first choice. Doxazosin and prazosin are α1-adrenergic receptor antagonists. They block the postsynaptic α1-receptors on smooth muscle cells in the wall of arteries and veins. This results in vasodilation and thus a reduction of arteriolar peripheral resistance and venous capacitance. The action of these drugs does not alter sympathetic tone, cardiac output, or heart frequency. α1-receptor antagonists have a beneficial effect on blood lipid levels: they decrease the LDL cholesterol and increase HDL.
An important adverse effect of α1-receptor blockers is the so-called first dose phenomenon: at the beginning of treatment (after 90 minutes), serious hypotensive
complaints such as dizziness, palpitations, headache, and fatigue can occur. Ketanserin and urapidil are in essence also α1-adrenergic receptor blockers, but with an α1-lytic action. Ketanserin has an unclear mechanism of action; apart from α1 blocking effects, it might also act at serotonergic receptors and on the CNS. Urapidil is a selective α1-blocker and acts centrally (via 5HT1A receptor stimulation).
Bradycardia, arrhythmias, and hypokalaemia are contraindications for the use of ketanserin or urapidil. Adverse effects are headache, dizziness, and fatigue. Ketanserin and urapidil interact with antiarrhythmics. Monitoring the effects with an ECG is required. In combination with diuretics, there exists a risk of hypokalaemia. Hence, combination with a potassium-sparing diuretic is recommended.
The first dose of an α-blocker should always be given at night because of the risk of
Extra info: Severe hypotension is a well-known effect after the first dose of an α-blocker.
Extra info: α-blockers improve relaxation of the smooth muscle cells of the bladder neck/prostate gland by blocking the α receptors there. See the section on prostate hyperplasia in the chapter Nephrology & Urology.