Pathophysiology and mechanism of action
Blood pressure regulation is inextricably linked to the circulating blood volume (CBV). Voloreceptors located in the atria and pulmonary vessels respond to stretch and transmit afferent impulses via the medulla oblongata to the hypothalamus. In response, a hormonal reaction is triggered:
- The neurohypophysis increases the secretion of antidiuretic hormone (vasopressin), which causes water retention and vasospasm.
- The release of natriuretic peptides (types A, B, and C) decreases, which also leads to increased reabsorption of sodium and water, and vasospasm.
The antihypertensive effect of diuretics has a dual mechanism. First, they reduce CBV. Second, they cause vasodilation, which is considered the determining factor. The pathogenesis of vascular dilation is as follows:
- Elimination of excess sodium from the body.
- Decrease in sodium concentration directly within the vascular wall (reduction of angiomyocyte "overload").
- Reduction of transmembrane calcium ion transport into the cell.
- Reduction of vascular wall hyperreactivity to vasoconstrictor stimuli.
- Vasodilation $\rightarrow$ drop in total peripheral vascular resistance (TPVR) $\rightarrow$ decrease in BP.
Main drugs and their characteristics
Several main classes of diuretics are used in the pharmacotherapy of hypertension: thiazide, thiazide-like, loop, and aldosterone antagonists.
Thiazide-like diuretics A prominent representative is Indapamidum (indapamide). This is a sulfonamide derivative with a non-thiazide structure. The drug is rapidly and completely absorbed orally, reaching peak concentration in 1–2 hours. The 18-hour half-life ensures a prolonged effect.
- Dosing feature: At the therapeutic dose (2.5 mg/day), indapamide has virtually no diuretic effect. Blood pressure drops solely due to a reduction in TPVR. If the dose is increased, the antihypertensive effect does not become stronger, but pronounced diuresis is provoked.
Thiazide and loop diuretics
- Hydrochlorothiazide: A classical thiazide drug for systematic treatment. Often used as part of combination drugs (e.g., Adelphan, Triresid K).
- Furosemide: A loop diuretic. When administered intravenously, it is capable of rapidly lowering blood pressure. This effect is not related to fluid loss, but to a direct venodilating effect that instantly reduces cardiac preload.
Role in therapy and management of adverse effects
Diuretics are prescribed both as monotherapy (thiazides are drugs of choice in the early stages of essential hypertension) and in combinations. The rationale for combination therapy is simple: most other antihypertensive drugs cause a compensatory retention of water and electrolytes. Diuretics successfully counteract this effect, potentiate the action of concomitant therapy, and normalize the electrolyte balance of the vascular wall.
The problem of electrolyte loss Loop, thiazide, and thiazide-like agents share a common adverse effect — they cause hypokalemia and hypomagnesemia. Orthostatic hypotension is also possible during administration. To correct electrolyte disorders, the following are used:
- Combination with potassium and magnesium supplements.
- Combination with potassium-magnesium-sparing diuretics: triamterene or aldosterone antagonists (spironolactone, also known as veroshpiron or aldactone).
Drugs for special clinical situations
In acute heart failure, specific groups of drugs are used: natriuretic peptide analogues, endothelin antagonists, and vasopressin antagonists.
To relieve hypertensive crises, direct myotropic agents can be used, which directly relax the smooth muscle of blood vessels:
- Bendazol (Dibazol): Produces a moderate and short-lived effect, more often used in combinations.
- Magnesium sulfate: Administered intramuscularly. Major risk: intravenous administration of magnesium sulfate can cause dangerous depression of the respiratory center.
Briefly: answers to topical questions
What metabolic adverse effects are characteristic of prolonged intake of thiazide and thiazide-like diuretics?
Prolonged intake of thiazide and thiazide-like diuretics is characterized by disorders of carbohydrate, lipid, purine, and water-electrolyte metabolism.
- Hypokalemia — decrease in serum potassium levels.
- Hyponatremia — a rare but severe electrolyte disturbance.
- Hyperuricemia — a dose-dependent increase in uric acid concentration, provoking gout attacks.
- Hyperglycemia — increased insulin resistance and risk of developing diabetes mellitus.
- Dyslipidemia — increased levels of atherogenic lipoproteins.
What are the contraindications to the prescription of thiazide diuretics?
The main absolute contraindication to the prescription of thiazide diuretics is renal failure with a creatinine clearance of less than 30 mL/min, as the drugs become ineffective at such values. Loop diuretics are an alternative. In cases of hyperuricemia and gout, thiazide diuretics should be used with caution and serum uric acid levels should be monitored; a transient increase in its level does not require discontinuation of the drug.
With which specific classes of antihypertensive drugs is it most rational to combine diuretics?
It is most rational to combine diuretics with four main classes of antihypertensive drugs.
- ACE inhibitors (ACEIs) — the combination suppresses counter-regulatory mechanisms.
- Angiotensin II receptor blockers (ARBs) — provide pharmacological synergy.
- Calcium channel blockers (CCBs) — enhance the antihypertensive effect.
- Beta-blockers (BBs) — used for a more pronounced reduction in blood pressure.
Such combinations ensure better treatment tolerability and effective suppression of blood pressure-raising mechanisms.
What is the molecular mechanism of action of aldosterone antagonists (spironolactone)?
Spironolactone belongs to potassium-sparing diuretics and is a competitive mineralocorticoid receptor antagonist. It blocks aldosterone and deoxycorticosterone receptors located in the terminal segment of the distal tubules and the cortical part of the collecting ducts. Aldosterone stimulates the synthesis of epithelial sodium channels (ENaC); aldosterone antagonists block the action of this hormone.
Which drugs belong to the group of thiazide-like diuretics besides indapamide?
Besides indapamide, thiazide-like diuretics include chlorthalidone and clopamide. They act in the initial segment of the distal tubules and block the sodium-chloride cotransporter.
To memorize the pathogenesis of vasodilation from diuretics, use the rule "Minus Sodium — Minus Calcium — Minus Tone". Sodium leaves the vessel wall → calcium does not follow it in → the vessel relaxes (TPVR drops).
Class and exam questions
Why doesn't indapamide at a dose of 2.5 mg cause frequent urination?
At this therapeutic dose, the drug works exclusively as a vasodilator, reducing total peripheral vascular resistance. The diuretic effect manifests only with an unjustified increase in the dose, which does not lead to an additional decrease in blood pressure.
How does intravenous furosemide lower blood pressure so rapidly?
The rapid antihypertensive effect is due to a direct venodilating effect. The drug dilates veins, which instantly reduces myocardial preload even before a pronounced diuresis begins.
Why are diuretics combined with other blood pressure medications?
Many antihypertensive agents cause compensatory retention of water and electrolytes in the body. Diuretics eliminate this adverse effect and potentiate the overall antihypertensive action.
What else to study on this topic
- Neurohumoral regulation: the role of the hypothalamus and neurohypophysis in controlling CBV.
- Mechanism of action of aldosterone antagonists (spironolactone).
- Pharmacokinetics of thiazide diuretics with long-term use.
- Specific aspects of magnesium sulfate administration in hypertensive crises and risks of intravenous injection.
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