Cellular Mechanism of Action
Guanethidine acts exclusively at the terminals of sympathetic nerve fibers. Its mechanism of action differs fundamentally from that of reserpine and is based on 'hijacking' neuronal transport systems. The process occurs in several stages:
- Neuronal uptake. Guanethidine actively competes with the physiological neurotransmitter—norepinephrine—for reuptake transport systems (NET). As a result, drug molecules are taken up by the sympathetic fiber.
- Vesicular entry. Once inside the cell, guanethidine enters synaptic vesicles (where the neurotransmitter is stored) and physically displaces norepinephrine.
- Neurotransmitter degradation. Norepinephrine released from vesicles into the cytoplasm does not reach the synaptic cleft; it is rapidly degraded intracellularly by the enzyme monoamine oxidase (MAO).
- Blockade of synthesis and release. Once anchored in the vesicles, guanethidine blocks both the enzymatic synthesis of new norepinephrine from dopamine and directly inhibits exocytosis—the release of the neurotransmitter.
The ultimate result of these cellular processes is profound depletion of norepinephrine stores.
Pharmacodynamics and Hemodynamic Effects
The primary clinical target of guanethidine is the cardiovascular system. As neurotransmitter stores are depleted, a marked decrease in sympathetic tone occurs. Blood pressure falls through two parallel mechanisms:
- Total peripheral resistance (TPR) decreases due to vasodilation.
- Cardiac output decreases.
Pharmacokinetic features: characterized by a prolonged latency period. Time is required for norepinephrine stores to deplete to a level that affects blood pressure. The maximum antihypertensive effect develops only on days 7–8 of continuous therapy. After complete withdrawal of guanethidine, the residual effect persists for up to two weeks until sympathetic nerve terminals regenerate their pool of neurotransmitters.
Adverse Effects and Comparison with Reserpine
The safety profile of guanethidine has distinct features determined by both the specifics of its action and compensatory reactions of the autonomic nervous system.
Differences from reserpine (advantages):
- CNS sparing. Guanethidine is completely unable to cross the blood-brain barrier (BBB). It does not cause drug-induced depression, sedation, or lethargy.
- Adrenal sparing. The drug does not affect the secretion of catecholamines by adrenal medullary cells. This is because the adrenal medulla lacks the membrane reuptake transport systems that are critical for guanethidine to enter the cell.
Specific hazard: Kluchevoi disadvantage of guanethidine distinguishing it from reserpine is the development of severe orthostatic hypotension. When a patient abruptly moves from a horizontal to a vertical position, blood vessels lacking sympathetic control are unable to constrict rapidly, leading to a critical drop in blood pressure.
Symptoms of parasympathetic dominance: As with reserpine, blockade of the sympathetic system leads to dominant parasympathetic tone. Clinically, this manifests as pronounced bradycardia, persistent nasal congestion, and GI stimulation (increased glandular secretion and diarrhea).
Medical Status and Current Use
Due to the high risk of severe orthostatic reactions and the availability of safer antihypertensive drugs, guanethidine is currently practically excluded from routine clinical practice. Its primary niche today is experimental research (e.g., pharmacological modeling of chemical sympathectomy in laboratory animals).