Mechanism of action at the molecular level
The primary site of action of loop diuretics is the epithelium of the thick ascending limb of the loop of Henle. The drugs selectively block the NKCC2 transport system (sodium-potassium-chloride cotransporter) located on the apical membrane of the cells. Normally, this transporter ensures the coupled influx of one sodium ion ($Na^+$), one potassium ion ($K^+$), and two chloride ions ($2Cl^-$).
Blockade of the NKCC2 protein leads to a chain of sequential changes:
- Inhibition of electrolyte reabsorption: the primary effect is a sharp decrease in the absorption of $Na^+$, $K^+$, and $Cl^-$.
- Osmotic diuresis: non-reabsorbed ions retain water in the tubules along the osmotic gradient, which provokes abundant fluid excretion.
- Excretion of divalent cations: due to impaired potassium transport, the transepithelial potential required for passive paracellular reabsorption of magnesium ($Mg^{2+}$) and calcium ($Ca^{2+}$) is altered. Magnesium loss with prolonged administration causes hypomagnesemia. Hypocalcemia develops rarely, as calcium is compensatorily reabsorbed in the distal tubules.
The effectiveness of this group of drugs is preserved under any fluctuations in the acid-base balance (ABB) of the blood.
Effect on hemodynamics and prostaglandins
The therapeutic effect of loop diuretics is not limited to renal mechanisms:
- Stimulation of prostaglandin synthesis: the drugs activate cyclooxygenase-2 (COX-2) expression, causing an increase in the level of prostaglandin E2, which additionally inhibits sodium reabsorption.
- Vascular effect: furosemide and ethacrynic acid have a vasodilating effect on the pulmonary vessels. This decrease in tone occurs before the onset of the diuretic effect.
In acute pulmonary edema, clinical improvement is achieved due to two factors: early vasodilation (unloading of the pulmonary circulation) and subsequent reduction in circulating blood volume (CBV).
Pharmacokinetic parameters
The drugs are characterized by a high rapid response and a short latent period.
| Drug | Route of administration | Onset of action | Duration | Metabolism and excretion |
|---|---|---|---|---|
| Furosemide | Oral / IV | 30 min / 3–4 min | 3–4 h / 1–2 h | Glucuronidation; excretion via kidneys and GI tract |
| Bumetanide | Oral | 30–40 min | 4–6 h | Metabolism by the cytochrome P450 system |
| Ethacrynic acid | Oral / IV | 1 h / 15 min | 6–8 h / 3–4 h | Excretion via kidneys and GI tract |
Renal excretion is carried out through glomerular filtration and active secretion in the proximal segment of the nephron.
Indications and safety profile
Clinical use:
- Edematous syndrome: pulmonary edema, cerebral edema, chronic heart failure, liver cirrhosis, angioedema.
- Cardiovascular and obstetric conditions: hypertensive crisis, severe arterial hypertension, preeclampsia.
- Other indications: hypercalcemia, forced diuresis in acute poisoning.
Adverse effects:
- Electrolyte and metabolic disorders: hypokalemia (risk of hypokalemic alkalosis), hypomagnesemia, hyperuricemia (decreased uric acid secretion against the background of a drop in CBV), hyperglycemia, hyperlipidemia.
- Specific toxicity: ototoxicity (hearing loss, highest with ethacrynic acid).
- Allergological history: furosemide and bumetanide are sulfonamide derivatives. Patients with sulfonamide allergies are prescribed ethacrynic acid.
Briefly: answers to topical questions
Which drugs belong to the group of loop diuretics?
The group of loop diuretics includes such drugs as furosemide, bumetanide, torasemide, and ethacrynic acid.
The side effects of loop diuretics are easy to remember by the rule "3 HYPO + 3 HYPER + Hearing": HYPOkalemia, HYPOmagnesemia, HYPOvolemia; HYPERuricemia, HYPERglycemia, HYPERlipidemia; plus ototoxicity.
Class and exam questions
Why does hypocalcemia develop extremely rarely during furosemide therapy?
Despite the reduction of $Ca^{2+}$ reabsorption in the loop of Henle due to changes in the transepithelial potential, calcium ions are actively absorbed in the distal tubules of the nephron, which compensates for its losses.
How does furosemide reduce the load in pulmonary edema before the onset of diuresis?
The drug stimulates the synthesis of prostaglandin E2 via COX-2, causing direct dilation of the pulmonary vessels and unloading the pulmonary circulation even before an increase in urination.
Which agent should be chosen for an allergy to sulfonamide drugs?
In this case, ethacrynic acid is prescribed because it does not contain a sulfonamide group, unlike furosemide and bumetanide.
What else to study on this topic
- Protocol for forced diuresis in acute poisoning
- Prevention of hypokalemic alkalosis with potassium-sparing agents
- Comparative characteristics of the ototoxicity of loop diuretics
- The role of prostaglandins E2 in the regulation of tubular reabsorption
Master the entire topic in the «Pharmacology» course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Loop diuretics" and 5,304 more subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 116 video lessons on the subject
Other topics in the section "Diuretics"
Free video lessons: pharmacologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.