Physiological basis of diuresis and the role of ADH
The regulation of water reabsorption is closely linked to the structure of the collecting ducts and the principal cells of their epithelium. A key role in this process is played by antidiuretic hormone (ADH), which controls the permeability of cell membranes.
- The hormone regulates the number of water channels — aquaporins (type AQP2) — on the apical membrane.
- In the presence of ADH, water channels are inserted into the membrane, ensuring active water reabsorption.
- Hormone deficiency or a genetic defect in aquaporins leads to a condition known as diabetes insipidus, in which the body loses its ability to concentrate urine.
Group I: Agents affecting tubular epithelium
This extensive group combines drugs acting on transport systems in specific segments of the nephron:
- Loop diuretics act in the thick ascending limb of the loop of Henle by blocking $Na^+-K^+$ cotransport. These include furosemide, bumetanide, and ethacrynic acid.
- Thiazide and thiazide-like diuretics affect the early distal convoluted tubule by inhibiting $Na^+-Cl^-$ cotransport. Representatives: hydrochlorothiazide, indapamide, chlortalidone, and others.
- Potassium-sparing agents act in the late distal tubule and collecting ducts.
Potassium-sparing diuretics and other groups
Drugs that conserve potassium ions in the body are classified according to their intracellular mechanism of action:
- Sodium channel blockers (triamterene, amiloride) act directly on epithelial sodium channels (ENaC).
- Aldosterone antagonists (spironolactone, eplerenone) block mineralocorticoid receptors.
In addition to the aforementioned groups, osmotic diuretics and miscellaneous agents are distinguished, among which are carbonic anhydrase inhibitors (acetazolamide) and xanthine derivatives (aminophylline).
Briefly: answers to topical questions
What are the side effects and complications of loop diuretics?
The main side effects and complications of loop diuretic therapy are electrolyte imbalances and hemodynamic disorders.
- Electrolyte loss — the drugs cause hypokalemia and hypomagnesemia.
- Renal hypoperfusion — occurs when furosemide is administered in the setting of hypovolemia.
- Thromboembolic complications — the risk of their development significantly increases when the drug is used under conditions of hypovolemia.
To correct electrolyte imbalances, combination with potassium and magnesium supplements or potassium-magnesium-sparing diuretics is required. If serum potassium drops below 3.0 mmol/L, furosemide should be discontinued.
What are the indications and contraindications for thiazide diuretics?
Thiazide diuretics are used to treat cardiovascular diseases and endocrine disorders, but have limitations in gout. Indications for use:
- Hypertension — they are baseline agents, drugs of choice for monotherapy in early stages.
- Chronic heart failure — used for symptomatic treatment and relief of congestion (edema, dyspnea).
- Diabetes insipidus — used due to their paradoxical antidiuretic effect.
Contraindications:
- Acute gout attack — hydrochlorothiazide may be contraindicated in the acute phase, as it can cause uric acid retention in the body (hyperuricemia).
The loop of Henle corresponds to loop diuretics (furosemide), the distal tubules to thiazides, and the collecting ducts to potassium-sparing diuretics (spironolactone).
Class and exam questions
Where exactly in the nephron do loop diuretics act?
Loop diuretics act in the thick ascending limb of the loop of Henle, where they block the $Na^+-K^+$ cotransporter.
Which drugs belong to sodium channel blockers?
Sodium channel blockers include triamterene and amiloride, which act directly on epithelial channels (ENaC).
What is the function of antidiuretic hormone?
ADH regulates the water permeability of collecting duct cell membranes by controlling the number of water channels — aquaporins (AQP2).
What else to study on this topic
- Genetic polymorphism of aquaporins in diabetes insipidus
- Molecular mechanisms of $Na^+-K^+$ and $Na^+-Cl^-$ cotransporters
- Pharmacological differences between thiazide and thiazide-like diuretics
- The role of mineralocorticoid receptors in the action of aldosterone antagonists
- Specific effects of carbonic anhydrase inhibitors and xanthines
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