Alpha- and beta-adrenoceptor-blocking agents are divided into primary subgroups on the basis of their receptor selectivity. All of these agents are pharmacologic antagonists or partial agonists and most are reversible and competitive in action. Because α and β blockers differ markedly in their effects and clinical applications, these drugs are considered separately in the following discussion.
Subdivisions of the α blockers are based on selective affinity for α1 versus α2 receptors or a lack thereof. Other features used to classify the α-blocking drugs are their reversibility and duration of action.
Irreversible, long-acting—Phenoxybenzamine is the prototypical long-acting α blocker; it differs from other adrenoceptor blockers in being irreversible in action. It is slightly α1 selective. Reversible, shorter-acting—Phentolamine is a competitive, reversible blocking agent that does not distinguish between α1 and α2 receptors. Alpha1-selective—Prazosin is a highly selective, reversible pharmacologic α1 blocker. Doxazosin, terazosin, and tamsulosin are similar drugs. The advantage of α1 selectivity is discussed in the following text. Alpha2-selective—Yohimbine and rauwolscine are α2-selective competitive pharmacologic antagonists. They are used primarily in research applications.
Alpha-blocking drugs are all active by the oral as well as the parenteral route, although phentolamine is rarely given orally. Phenoxybenzamine has a short elimination half-life but a long duration of action—about 48 h—because it binds covalently to its receptor. Phentolamine has a duration of action of 2–4 h when used orally and 20–40 min when given parenterally. Prazosin and the other α1-selective blockers act for 8–24 h.
Phenoxybenzamine binds covalently to the α receptor, thereby producing an irreversible (insurmountable) blockade. The other α-blocking agents are competitive antagonists, and their effects can be surmounted by increased concentrations of agonist. This difference may be important in the treatment of pheochromocytoma because a massive release of catecholamines from the tumor may overcome a reversible blockade.
High-Yield Terms to Learn
|Competitive blocker ||A surmountable antagonist (eg, phentolamine); one that can be overcome by increasing the dose of agonist |
|Epinephrine reversal ||Conversion of the pressor response to epinephrine (typical of large doses) to a blood pressure–lowering effect; caused by α blockers, which unmask the β2 vasodilating effects of epinephrine |
|Intrinsic sympathomimetic activity (ISA) ||Partial agonist action by adrenoceptor blockers; typical of several β blockers (eg, pindolol, acebutolol) |
|Irreversible blocker ||A nonsurmountable inhibitor, usually because of covalent bond formation (eg, phenoxybenzamine) |
|Membrane-stabilizing activity (MSA) ||Local anesthetic action; typical of several β blockers (eg, propranolol) |
|Orthostatic hypotension ||Hypotension that is most marked in the upright position; caused by venous pooling (typical of α blockade) or inadequate blood volume (caused by blood loss or excessive diuresis) |