indication

For the treatment of high blood pressure and management of edema.

pharmacology

Thiazides such as hydrochlorothiazide promote water loss from the body (diuretics). They inhibit Na⁺/Cl^- reabsorption from the distal convoluted tubules in the kidneys. Thiazides also cause loss of potassium and an increase in serum uric acid. Thiazides are often used to treat hypertension, but their hypotensive effects are not necessarily due to their diuretic activity. Thiazides have been shown to prevent hypertension-related morbidity and mortality although the mechanism is not fully understood. Thiazides cause vasodilation by activating calcium-activated potassium channels (large conductance) in vascular smooth muscles and inhibiting various carbonic anhydrases in vascular tissue.

mechanism of action

Hydrochlorothiazide, a thiazide diuretic, inhibits water reabsorption in the nephron by inhibiting the sodium-chloride symporter (SLC12A3) in the distal convoluted tubule, which is responsible for 5% of total sodium reabsorption. Normally, the sodium-chloride symporter transports sodium and chloride from the lumen into the epithelial cell lining the distal convoluted tubule. The energy for this is provided by a sodium gradient established by sodium-potassium ATPases on the basolateral membrane. Once sodium has entered the cell, it is transported out into the basolateral interstitium via the sodium-potassium ATPase, causing an increase in the osmolarity of the interstitium, thereby establishing an osmotic gradient for water reabsorption. By blocking the sodium-chloride symporter, hydrochlorothiazide effectively reduces the osmotic gradient and water reabsorption throughout the nephron.Hydrochlorothiazide, a thiazide diuretic, inhibits water reabsorption in the nephron by inhibiting the sodium-chloride symporter (SLC12A3) in the distal convoluted tubule, which is responsible for 5% of total sodium reabsorption. Normally, the sodium-chloride symporter transports sodium and chloride from the lumen into the epithelial cell lining the distal convoluted tubule. The energy for this is provided by a sodium gradient established by sodium-potassium ATPases on the basolateral membrane. Once sodium has entered the cell, it is transported out into the basolateral interstitium via the sodium-potassium ATPase, causing an increase in the osmolarity of the interstitium, thereby establishing an osmotic gradient for water reabsorption. By blocking the sodium-chloride symporter, hydrochlorothiazide effectively reduces the osmotic gradient and water reabsorption throughout the nephron.

toxicity

The most common signs and symptoms observed are those caused by electrolyte depletion (hypokalemia, hypochloremia, hyponatremia) and dehydration resulting from excessive diuresis. If digitalis has also been administered, hypokalemia may accentuate cardiac arrhythmias. The oral LD50 of hydrochlorothiazide is greater than 10 g/kg in the mouse and rat.

biotransformation

Hydrochlorothiazide is not metabolized.

absorption

50-60%

half life

5.6 and 14.8 hours

route of elimination

Hydrochlorothiazide is not metabolized but is eliminated rapidly by the kidney. Hydrochlorothiazide crosses the placental but not the blood-brain barrier and is excreted in breast milk.

drug interactions

Deslanoside: Possible electrolyte variations and arrhythmias

Diazoxide: Significant hyperglycemic effect

Digitoxin: Possible electrolyte variations and arrhythmias

Digoxin: Possible electrolyte variations and arrhythmias

Dofetilide: Increased risk of cardiotoxicity and arrhythmias

Lithium: The thiazide diuretic, hydrochlorothiazide, may increase serum levels of lithium.

Tenoxicam: Tenoxicam may antagonize the blood pressure lowering effect of Hydrochlorothiazide. Monitor for changes in the therapeutic effect of Hydrochlorothiazide if Tenoxicam is initiated, discontinued or dose changed.

Trandolapril: The thiazide diuretic, Hydrochlorothiazide, may increase the hypotensive effect of Trandolapril. Hydrochlorothiazide may also increase the nephrotoxicity of Trandolapril. Monitor for postural hypotension at initiation of concomitant therapy and renal dysfunction during chronic therapy.

Treprostinil: Additive hypotensive effect. Monitor antihypertensive therapy during concomitant use.