Lecture #30: Pharmacology of Diuretic Agents Part I: Principles of Diuretic Action and Pharmacology of Carbonic Anhydrase Inhibitors and Loop Diuretics

What is the primary goal of diuretic therapy?

Decrease extracellular fluid volume by increasing renal sodium and water excretion, thereby restoring and maintaining euvolemia.

What are natriuresis and diuresis?

Natriuresis is increased urinary Na+ excretion; diuresis is increased urine/water output. Diuretics generally increase Na+ excretion with accompanying water loss.

What determines the magnitude of a diuretic's effect?

The effect depends largely on its nephron site of action, duration of action, and dietary salt intake; blocking segments that normally reabsorb larger fractions of filtered Na+ produces greater diuresis.

What compensatory mechanisms are activated by diuretic-induced ECF volume contraction?

Decreased ECF/effective circulating volume activates the SNS, RAAS/aldosterone, and ADH, promoting Na+ and water reabsorption and opposing the diuretic effect.

How does RAAS activation oppose diuretic therapy?

Volume depletion activates RAAS → increased aldosterone → increased distal Na+ reabsorption with accompanying water retention, counteracting natriuresis and diuresis.

How does ADH oppose diuretic-induced volume loss?

Reduced effective circulating volume causes non-osmotic ADH release → AQP2 insertion in collecting ducts → increased free-water reabsorption.

How are most diuretics delivered to their luminal sites of action?

Most are highly protein-bound and therefore not freely filtered; they are secreted from blood into the proximal tubule by organic anion transporters (OATs) and then act from the luminal side.

How can OAT inhibition decrease diuretic efficacy?

OAT inhibition decreases proximal tubular secretion of the diuretic, reducing drug concentration within the nephron and therefore decreasing delivery to its luminal site of action.

What is the key mechanism underlying diuretic-induced K+ wasting?

Upstream Na+ transport blockade increases Na+ delivery to distal principal cells → increased Na+ reabsorption through ENaC creates a lumen-negative potential → increased K+ secretion through ROMK → urinary K+ loss.

How does aldosterone enhance diuretic-induced K+ wasting?

Volume depletion activates RAAS → aldosterone increases Na+/K+-ATPase, ENaC, and ROMK activity/expression → increased Na+ reabsorption and lumen negativity → increased K+ secretion.

Why are renal prostaglandins important for diuretic efficacy?

PGE2 and PGI2 promote renal vasodilation, maintain renal blood flow and GFR, support natriuresis, and oppose Na+ and ADH-mediated water reabsorption.

How do NSAIDs reduce diuretic efficacy?

NSAIDs inhibit renal PGE2/PGI2 synthesis → afferent arteriolar vasoconstriction → decreased renal blood flow and GFR; they also promote Na+/water retention and can impair OAT-mediated diuretic delivery.

Where do carbonic anhydrase inhibitors primarily act?

The proximal tubule is the primary site; carbonic anhydrase inhibition also occurs in the distal nephron/collecting duct.

Which carbonic anhydrase inhibitor is emphasized by name in this lecture?

Acetazolamide is the major starred CA inhibitor; the class also includes methazolamide and the ophthalmic agents dorzolamide and brinzolamide.

What is the mechanism of action of carbonic anhydrase inhibitors?

They reversibly inhibit membrane-bound and cytoplasmic carbonic anhydrase → decrease H+ generation/secretion and HCO3− reabsorption → impair NHE3-mediated Na+ reabsorption → increase Na+, HCO3−, and water excretion.

Why does CA inhibition decrease proximal Na+ reabsorption?

CA inhibition decreases intracellular H+ generation, so less H+ is available for NHE3 exchange with luminal Na+ → decreased Na+ reabsorption and mild natriuresis/diuresis.

Why are carbonic anhydrase inhibitors weak diuretics?

Although they inhibit proximal Na+/HCO3− reabsorption, downstream nephron segments have a large capacity to reabsorb the additional Na+ delivered to them, limiting net Na+ and water loss.

Which substances have increased urinary excretion with CA inhibitors?

CAIs increase urinary Na+, HCO3−, K+, and Ca2+; they have little to no effect on Mg2+ excretion.

How do CA inhibitors affect urine and blood pH?

Increased urinary HCO3− makes the urine more alkaline, while depletion of plasma HCO3− decreases blood pH and produces mild metabolic acidosis.

What acid-base disorder is characteristic of CA inhibitors?

Mild hyperchloremic metabolic acidosis due to urinary HCO3− loss and increased Cl− reabsorption to maintain electroneutrality.

Why is the diuretic effect of CA inhibitors self-limited?

HCO3− loss lowers plasma and filtered HCO3−, reducing the substrate on which the drug's natriuretic effect depends; volume contraction and metabolic acidosis also promote compensatory Na+ reabsorption.

How quickly can the CA inhibitor diuretic effect become self-limited?

The lecture describes the natriuretic/diuretic effect as becoming self-limited after approximately 2–4 days of continuous therapy.

Why isn't the metabolic acidosis produced by CA inhibitors self-limited?

CA inhibition continues to prevent HCO3− reabsorption and depletes the blood's buffering capacity; only the natriuretic/diuretic effect becomes self-limited as filtered HCO3− decreases.

Why can CA inhibitors cause hypokalemia?

Increased distal Na+ delivery plus aldosterone-mediated ENaC activity increases the lumen-negative potential in principal cells, promoting K+ secretion and urinary K+ loss.

What are the major therapeutic uses of acetazolamide/CA inhibitors?

Prevention and treatment of acute mountain sickness, open-angle glaucoma, and treatment of metabolic alkalosis; they have limited usefulness for edema because of weak/self-limited diuresis.

How does acetazolamide treat acute mountain sickness?

CA inhibition causes HCO3− loss and mild metabolic acidosis → stimulates peripheral chemoreceptors and respiratory center → sustained hyperventilation → increased CO2 exhalation and improved oxygenation.

How do CA inhibitors treat open-angle glaucoma?

CA inhibition in the ciliary body decreases bicarbonate production and aqueous humor secretion → decreased intraocular pressure and reduced risk of optic nerve damage.

How can acetazolamide treat metabolic alkalosis?

It increases urinary HCO3− excretion, helping reduce the elevated body bicarbonate responsible for alkalosis.

Why can CA inhibitors cause calcium kidney stones?

CAIs increase urinary Ca2+ and alkalinize the urine; alkaline urine favors precipitation of calcium phosphate crystals.

What are important adverse effects of CA inhibitors?

Mild hyperchloremic metabolic acidosis, mild K+ wasting/hypokalemia, calcium phosphate kidney stones, paresthesias, somnolence, dizziness, ataxia, and headache.

What are major contraindications to CA inhibitors?

Acidosis, severe renal disease, cirrhosis or other severe hepatic disease, and hypersensitivity to the drug or formulation.

Why should CA inhibitors be avoided in severe hepatic disease/cirrhosis?

They can reduce NH4+ production/excretion, an important mechanism of acid and nitrogen handling, potentially contributing to hyperammonemia.

Where do loop diuretics act?

The thick ascending limb (TAL) of the loop of Henle, where they inhibit the Na+/K+/2Cl− cotransporter NKCC2.

Which loop diuretics are emphasized in the lecture?

Furosemide, torsemide, bumetanide, and ethacrynic acid.

What is the mechanism of loop diuretics?

They inhibit NKCC2 in the TAL → prevent Na+, K+, and Cl− reabsorption → profound natriuresis and diuresis.

Why are loop diuretics the most potent diuretics?

Approximately 25% of filtered Na+ is normally reabsorbed in the TAL through NKCC2; blocking this high-capacity transport system produces profound and rapid Na+ and water excretion.

What is the ceiling effect of loop diuretics?

Once NKCC2 is maximally inhibited, increasing drug concentration cannot block more transporter activity; downstream Na+ reabsorption further limits Na+ excretion to roughly the fraction normally handled by the TAL.

Which electrolytes have increased urinary excretion with loop diuretics?

Na+, Cl−, K+, Ca2+, and Mg2+ are excreted in increased amounts.

Why do loop diuretics increase urinary Ca2+ and Mg2+?

NKCC2 inhibition prevents K+ recycling and eliminates the lumen-positive electrical potential in the TAL that normally drives paracellular Ca2+ and Mg2+ reabsorption.

Why do loop diuretics cause hypokalemia?

Increased distal Na+ delivery and RAAS/aldosterone activation increase ENaC-mediated Na+ reabsorption in principal cells, producing a lumen-negative potential that promotes K+ secretion.

What acid-base disturbance is associated with loop diuretics?

Hypokalemic metabolic alkalosis caused by K+ loss, increased aldosterone-mediated H+ secretion, volume contraction, RAAS activation, increased proximal HCO3− reabsorption, and hypochloremia that impairs HCO3− excretion.

How do loop diuretics affect the kidney's ability to concentrate urine?

NKCC2 blockade disrupts the countercurrent multiplier and collapses the medullary osmotic gradient, reducing collecting-duct water reabsorption even when ADH is present; therefore concentrated urine cannot be produced normally.

How do loop diuretics impair urine dilution?

The TAL normally reabsorbs solute without water and dilutes tubular fluid; NKCC2 blockade disables this diluting segment, so the kidney cannot maximally dilute urine during high water intake.

What are the major clinical uses of loop diuretics?

They are used for fluid overload/edema, heart failure, hypercalcemia, and BP lowering particularly in patients with heart failure, kidney disease, or cirrhosis.

Why are loop diuretics useful for hypercalcemia?

They abolish the TAL lumen-positive potential required for paracellular Ca2+ reabsorption, thereby increasing urinary Ca2+ excretion.

Why may torsemide be preferred over furosemide in refractory edema?

Furosemide has variable oral absorption (~50–60%), which can worsen with GI edema in heart failure; torsemide has approximately 100% oral bioavailability, more predictable absorption, and a longer half-life/duration.

What is unique about ethacrynic acid?

It is the loop diuretic in the lecture that lacks a sulfonamide moiety; however, it is not more effective or pharmacokinetically advantageous and has a higher risk of ototoxicity.

What important toxicity is associated with loop diuretics, especially ethacrynic acid?

Ototoxicity; ethacrynic acid has a greater risk, particularly when combined with other ototoxic drugs such as aminoglycosides.

What mechanisms contribute to loop diuretic resistance?

Post-diuretic Na+ reabsorption, distal nephron hypertrophy/upregulation of NCC and ENaC, RAAS/aldosterone and ADH activation, reduced renal perfusion, impaired drug absorption or OAT secretion, high Na+ intake, and interacting drugs such as NSAIDs.

Which patients are particularly susceptible to loop diuretic-refractory edema?

Patients with heart failure, chronic kidney disease, cirrhosis with ascites, nephrotic syndrome, and secondary hyperaldosteronism.

Why can the combination of an NSAID, ACE inhibitor/ARB, and diuretic cause acute kidney injury?

NSAID → afferent constriction by blocking prostaglandins; ACE inhibitor/ARB → efferent dilation by blocking angiotensin II; diuretic → reduced effective circulating volume. Together these markedly reduce glomerular filtration pressure and GFR, increasing AKI risk.