Mechanism of action and pharmacodynamics
The exact molecular mechanism by which hydralazine (Hydralazinum) affects smooth muscle cells remains unclear. Nevertheless, the pharmacodynamics and hemodynamic effects of the drug have been studied in considerable detail.
The main site of action of this myotropic agent is resistance vessels, which include arterioles and small arteries. By acting on them, the drug causes pronounced vasodilation (widening of the lumen). The physiological consequence of such vascular wall relaxation is a significant decrease in total peripheral vascular resistance (TPVR). It is this drop in TPVR that causes the final therapeutic effect: a pronounced decrease in blood pressure.
Pharmacokinetics and genetic polymorphism
The pharmacokinetic profile of hydralazine is of particular interest due to its pronounced genetic polymorphism. The drug can be administered both intravenously and orally (per os). When taken orally, absorption from the gastrointestinal tract is very active, with about 90% of the active substance being absorbed.
However, high absorption does not guarantee a high concentration in the systemic circulation. The drug's bioavailability turns out to be low and highly variable. The reason lies in the potent "first-pass" effect through the liver, where the drug undergoes intensive metabolism with the formation of active metabolites. The key biotransformation process here is acetylation.
The activity of the metabolizing enzyme is genetically determined, which divides patients into two main groups:
- "Slow acetylators": metabolism proceeds more slowly, so the systemic bioavailability of the drug reaches approximately 30%.
- "Fast acetylators": the enzyme works extremely actively, destroying most of the drug before it enters the systemic circulation. In such patients, bioavailability drops to 10%.
Regarding temporal characteristics, when taken orally, the maximum hypotensive effect develops in 1–2 hours. The total duration of the drug's action ranges from 6 to 8 hours.
Side effects and safety profile
Despite its antihypertensive efficacy, the use of hydralazine is limited by its safety profile. Adverse effects affect several body systems at once:
- Cardiovascular system: reflex tachycardia occurs in response to vasodilation and a drop in blood pressure. In addition, sodium and water retention in the body is characteristic, which clinically manifests as the development of edema.
- Gastrointestinal tract: patients frequently encounter dyspeptic disorders, including nausea, vomiting, diarrhea, and anorexia (loss of appetite).
- Central nervous system: the most frequent adverse event from the CNS is headache.
Of particular concern is the specific toxicity of the drug. An hydralazine overdose can lead to the development of a severe complication: lupus-like syndrome. In its clinical manifestations, this condition mimics systemic lupus erythematosus.
Dihydralazine
Dihydralazine (Dihydralazinum) is a related compound. In its chemical structure and pharmacological characteristics, it is almost completely similar to hydralazine.
In modern medical practice, dihydralazine is rarely used as a standalone agent. As a rule, it is part of combination drugs. A classic example of such an agent is "Adelphan-Esidrex K", where dihydralazine is used together with other components.
Briefly: answers to topical questions
What is the molecular mechanism of vascular smooth muscle relaxation under the action of hydralazine?
The exact molecular mechanism of vascular smooth muscle relaxation under the action of hydralazine is currently unclear. It is known that the drug's target is predominantly resistance vessels (arterioles and small arteries). Acting on these structures leads to a decrease in total peripheral vascular resistance (TPVR) and blood pressure.
What are the main indications for the use of hydralazine in clinical practice?
The main indications for the use of hydralazine are chronic congestive heart failure and urgent conditions accompanied by high blood pressure.
- Chronic congestive heart failure — the drug is prescribed as an alternative to ACE inhibitors when they cannot be used (for example, in severe renal failure or hypotension).
- Eclampsia — used as a direct vasodilator for the most rapid possible reduction of blood pressure.
Hydralazine has no independent significance as a first-line drug.
What active components are part of the combination drug "Adelphan-Esidrex K" in addition to dihydralazine?
In addition to dihydralazine, the fixed-dose combination antihypertensive drug "Adelphan-Esidrex K" includes three more active components.
- Reserpine — a sympatholytic (at a dose of 0.0001 g).
- Hydrochlorothiazide — a diuretic (at a dose of 0.01 g).
- Potassium chloride — an electrolyte correction component (at a dose of 0.6 g), added to prevent hypokalemia caused by the thiazide.
This combination provides synergy of action and convenience for the patient.
To remember the specific side effect of hydralazine, use the association: "HYDRA bites like a WOLF" (in Russian, hydra/wolf sound-alike) — HYDRAlazine in overdose causes a LUPUS-like (wolf-like) syndrome.
Class and exam questions
Why does the bioavailability of hydralazine vary so much among different patients?
This is due to the genetic polymorphism of liver enzymes responsible for acetylation. In "fast acetylators", the drug is broken down more actively, and its bioavailability drops to 10%, whereas in "slow" ones it is about 30%.
Which vessels does hydralazine predominantly act on?
The drug selectively dilates resistance vessels: arterioles and small arteries. This leads to a decrease in total peripheral vascular resistance (TPVR) and a drop in blood pressure.
What is the special feature of dihydralazine use?
It is structurally and pharmacologically similar to hydralazine, but in clinical practice, it is usually used not as monotherapy, but as part of combination drugs (for example, "Adelphan-Esidrex K").
What else to study on this topic
- Molecular hypotheses of the mechanism of action of hydralazine on smooth muscle.
- Pathogenesis of the development of lupus-like syndrome in overdose.
- Mechanisms of reflex tachycardia in response to vasodilation.
- Clinical significance of sodium and water retention during the intake of myotropic drugs.
- Principles of combination therapy using dihydralazine.
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