Lecture #31: Pharmacology of Diuretic Agents Part II: Pharmacology of Thiazide Diuretics, Mineralocorticoid Receptor Inhibitors and Other Potassium-Sparing Diuretics and Osmotic Diuretics

Where do thiazide diuretics act?

Thiazide diuretics inhibit the Na+-Cl− cotransporter (NCC) in the distal convoluted tubule, where approximately 5–10% of filtered Na+ is normally reabsorbed.

Which thiazide diuretics are emphasized in the lecture?

Hydrochlorothiazide and chlorthalidone are starred high-yield drugs; other agents include chlorothiazide, indapamide, metolazone, methyclothiazide, and bendroflumethiazide.

What is the mechanism of thiazide diuretics?

NCC inhibition in the DCT decreases NaCl reabsorption → increases Na+, Cl−, and water excretion → moderate natriuresis and diuresis.

Why are thiazides less potent than loop diuretics?

The DCT normally reabsorbs only about 5–10% of filtered Na+, whereas the TAL handles a larger fraction; therefore NCC blockade produces moderate rather than profound diuresis.

How do thiazides acutely lower blood pressure?

They reduce ECF and plasma volume → decrease venous return and cardiac output → lower blood pressure.

How do thiazides lower blood pressure chronically?

Chronic Na+ depletion decreases vascular smooth muscle responsiveness to vasoconstrictors → sustained reduction in peripheral vascular resistance.

Why are thiazides important in primary hypertension?

They are recommended first-line agents because they provide effective and consistent BP reduction with strong cardiovascular outcome data.

How does chlorthalidone differ pharmacokinetically from hydrochlorothiazide?

Chlorthalidone has a very long half-life of about 40–60 hours and duration of action of 24–72 hours, producing smoother 24-hour BP control but a greater risk of hypokalemia than HCTZ.

What are high-yield features of metolazone?

Metolazone remains effective with reduced GFR, has some proximal tubular activity, and has powerful synergy with loop diuretics, but can cause more intense electrolyte disturbances and is not used routinely for uncomplicated hypertension.

What is sequential nephron blockade?

Combining a loop diuretic that blocks NKCC2 in the TAL with a thiazide that blocks NCC in the DCT prevents distal reabsorption of the Na+ load escaping the loop and produces synergistic natriuresis/diuresis.

How do thiazides affect potassium?

They cause K+ wasting and can produce hypokalemia.

Why do thiazides cause K+ wasting?

NCC blockade increases distal Na+ delivery; volume contraction activates RAAS/aldosterone → ENaC and Na+/K+-ATPase activity increase → lumen becomes more negative → K+ secretion increases.

What acid-base disturbance do thiazides cause?

They can cause hypokalemic metabolic alkalosis due to K+ loss, increased H+ secretion, volume contraction, RAAS activation, hypochloremia, and bicarbonate retention.

How do thiazides affect calcium excretion?

They decrease urinary Ca2+ excretion by increasing transcellular Ca2+ reabsorption in the DCT.

How does NCC blockade increase Ca2+ reabsorption?

Lower intracellular Na+ increases basolateral NCX1 activity → lowers intracellular Ca2+ → increases Ca2+ entry through apical TRPV5 → Ca2+ is exported basolaterally through NCX1 and PMCA1b.

Why are thiazides useful in idiopathic hypercalciuria?

They decrease urinary Ca2+ concentration and calcium salt supersaturation, which can reduce recurrent calcium stone formation in selected patients.

How do thiazides affect magnesium?

Chronic therapy can cause Mg2+ wasting and hypomagnesemia by remodeling the DCT and reducing activity/expression of the apical TRPM6 Mg2+ channel.

Why do thiazides cause hyponatremia?

They impair the kidney's ability to dilute urine while preserving its ability to concentrate urine; ADH-mediated water reabsorption and thirst can therefore cause water retention out of proportion to Na+ retention → dilutional hyponatremia.

Do thiazides impair urine concentrating ability?

No. They impair the diluting segment but preserve the medullary gradient and collecting-duct response to ADH, which contributes to their high risk of hyponatremia.

How do thiazides treat nephrogenic diabetes insipidus/vasopressin resistance?

Mild volume contraction activates RAAS and decreases GFR → increases proximal Na+ and water reabsorption → decreases fluid delivery to ADH-sensitive collecting ducts → urine volume can fall by about 30–50%.

What metabolic adverse effects are associated with thiazides?

Hyperglycemia/insulin resistance, hyperuricemia/gout, and mild hyperlipidemia.

How can thiazides cause hyperglycemia?

Hypokalemia impairs ATP-sensitive K+ channel function and insulin secretion from pancreatic beta cells, while thiazides also decrease tissue insulin sensitivity.

Why can thiazides cause hyperuricemia and gout?

Volume contraction increases uric acid reabsorption and decreases urate secretion through proximal tubular urate transport mechanisms such as URAT1.

What are major adverse effects of thiazides?

ECF volume depletion, hypotension, hyponatremia, hypokalemic metabolic alkalosis, hypomagnesemia, uncommon hypercalcemia, hyperuricemia/gout, hyperglycemia, and hyperlipidemia.

How do NSAIDs reduce thiazide effectiveness?

NSAIDs cause afferent arteriolar vasoconstriction, decreasing renal blood flow and drug delivery to the DCT, and increase upstream Na+/water reabsorption, blunting the antihypertensive and diuretic effects.

How do thiazides affect lithium levels?

Increased Na+ loss causes compensatory proximal reabsorption of Na+ and Li+ → decreased lithium clearance → increased plasma lithium and risk of toxicity.

Why do thiazides increase the risk of digoxin toxicity?

Thiazide-induced hypokalemia increases digoxin binding/effects at the Na+/K+-ATPase and promotes intracellular Ca2+ overload, increasing arrhythmia risk.

How does probenecid affect thiazide therapy?

Probenecid inhibits organic anion transporters, reducing thiazide secretion into proximal tubular fluid and therefore decreasing drug delivery and efficacy.

Where do potassium-sparing diuretics act?

They act in aldosterone-sensitive principal cells of the late DCT, connecting tubule, and collecting duct.

Why are potassium-sparing agents weak diuretics?

Only about 2–5% of filtered Na+ is normally reabsorbed in principal cells, so blocking this segment produces only mild natriuresis.

What are the two major classes of potassium-sparing diuretics?

ENaC inhibitors and mineralocorticoid receptor antagonists (MRAs).

Which ENaC inhibitors are emphasized?

Amiloride and triamterene.

What is the mechanism of ENaC inhibitors?

They directly block luminal ENaC → decrease Na+ entry and Na+ reabsorption → reduce the lumen-negative potential → decrease K+ secretion through ROMK.

Why are ENaC inhibitors potassium sparing?

Reduced Na+ reabsorption diminishes the lumen-negative electrochemical gradient that normally drives K+ secretion, so K+ is retained.

What is the major adverse effect of ENaC inhibitors?

Hyperkalemia, especially in patients with renal impairment or when combined with other K+-retaining drugs.

Which drugs increase hyperkalemia risk with ENaC inhibitors?

ACE inhibitors, ARBs, direct renin inhibitors, NSAIDs, K+ supplements, and calcineurin inhibitors.

What is the role of amiloride in lithium-induced nephrogenic diabetes insipidus?

Amiloride blocks ENaC and prevents Li+ entry into principal cells, reducing intracellular Li+ accumulation, AQP2 downregulation, and collecting-duct resistance to vasopressin.

What is the role of ENaC inhibitors in Liddle syndrome?

Liddle syndrome is caused by gain-of-function ENaC mutations; ENaC inhibitors plus Na+ restriction directly counter the excessive Na+ reabsorption, hypertension, and hypokalemia.

Which mineralocorticoid receptor antagonists are emphasized?

Spironolactone and eplerenone; finerenone is a nonsteroidal selective MRA also discussed.

What is the mechanism of MR antagonists?

They competitively block cytosolic aldosterone receptors → prevent aldosterone-dependent gene transcription → decrease ENaC, Na+/K+-ATPase, and ROMK expression/activity → increase Na+/water excretion while retaining K+ and H+.

How does the effect of MR antagonists relate to aldosterone concentration?

The higher the patient's aldosterone level, the greater the potential electrolyte and natriuretic effects of MR antagonism.

What are major clinical uses of spironolactone/eplerenone?

Hypertension, edema, primary hyperaldosteronism, heart failure, cirrhotic ascites, and use with another diuretic to reduce K+ wasting.

Why is spironolactone preferred for ascites due to cirrhosis?

Cirrhosis produces effective arterial hypovolemia and strong RAAS/aldosterone activation, so direct aldosterone antagonism targets a major mechanism of Na+ and water retention; spironolactone is considered the diuretic of choice.

Why are MR antagonists useful in heart failure?

They oppose aldosterone-mediated Na+ retention and K+ loss and also block aldosterone effects associated with inflammation, myocardial remodeling, and fibrosis.

What endocrine adverse effects are specific to spironolactone?

Gynecomastia, impotence, and decreased libido due to androgen receptor antagonism and interference with testosterone synthesis.

Why does eplerenone cause fewer endocrine adverse effects than spironolactone?

Eplerenone is more selective for the mineralocorticoid receptor and therefore has less antiandrogen activity.

What important interaction involves eplerenone and CYP3A4?

Eplerenone undergoes CYP3A4 metabolism; strong CYP3A4 inhibitors or inducers should be avoided, and grapefruit juice increases eplerenone exposure.

What acid-base effect can potassium-sparing diuretics cause?

Retention of K+ and H+ can produce mild metabolic acidosis.

What is mannitol?

Mannitol is an osmotic diuretic that is freely filtered, minimally reabsorbed, pharmacologically inert, and retained in tubular fluid where it raises luminal osmolality.

Where does mannitol have its greatest nephron effect?

The proximal tubule and descending limb of the loop of Henle because these segments are highly water permeable; it also reduces water reabsorption farther downstream.

How does mannitol produce diuresis?

Increased tubular fluid osmolality holds water in the nephron → increased luminal flow → decreased passive Na+ reabsorption and rapid water excretion.

How does mannitol lower intracranial pressure?

It increases plasma osmolality → draws water from brain parenchyma into the extracellular/intravascular compartment → water is then excreted by the kidneys.

What are the major therapeutic uses of mannitol?

Rapid reduction of elevated intracranial pressure/cerebral edema and acute reduction of intraocular pressure, including acute ophthalmic emergencies.

What serious complication can mannitol cause in heart failure?

Acute intravascular volume expansion can markedly increase preload and pulmonary capillary pressure, causing or worsening pulmonary edema.

What major electrolyte abnormalities can occur with mannitol?

Early volume expansion can cause dilutional hyponatremia; excessive water loss during diuresis can cause dehydration, hypernatremia, and hyperkalemia.

Why is mannitol contraindicated in anuria?

If it cannot be excreted, mannitol accumulates extracellularly and causes marked volume expansion, pulmonary edema, and potentially severe cardiovascular/renal complications.

What are major contraindications to mannitol?

Severe dehydration, severe renal disease/anuria, renal dysfunction after mannitol, active cranial bleeding except during craniotomy, progressive heart failure, and severe pulmonary edema/congestion.