The ADH axis compared — diabetes insipidus, SIADH and the drugs that act on V2
Verified · Oct 2026ADH saves water through the V2 receptor. Too little ADH, or a kidney that ignores it, wastes water and raises sodium; too much ADH holds water and lowers it. The drugs either add V2 action, block it, or bypass it.
One receptor, three ways to go wrong
ADH binds the V2 receptor on collecting-duct principal cells; through Gs, cAMP and protein kinase A, aquaporin-2 is inserted into the apical membrane and water is reabsorbed. In central DI ADH secretion fails; in nephrogenic DI ADH is normal but the V2 receptor or its signaling is defective. Either way, large volumes of dilute urine concentrate body fluids, causing hypernatremia. In SIADH ADH stays high despite low plasma osmolality, so water is retained; natriuresis keeps the patient euvolemic, without edema.
The drugs follow the receptor. Desmopressin is a selective V2 agonist that replaces ADH in central DI. Tolvaptan blocks V2, so aquaporin-2 is not inserted and free water is excreted; it treats euvolemic or hypervolemic hyponatremia such as SIADH. Conivaptan is an IV antagonist of both V1A and V2 receptors whose effect in hyponatremia comes through V2 aquaresis. Demeclocycline, a tetracycline whose labeled uses are infections, can cause a nephrogenic, dose-dependent, reversible DI and is effective in SIADH, but it is nephrotoxic and kept for when other therapies fail. Nephrogenic DI itself is treated by stopping the cause — lithium is the common one — and with a thiazide; amiloride may help lithium-induced polyuria.
Correcting sodium safely
In chronic hyponatremia (more than 48 hours) brain cells adapt by expelling osmoles, so raising sodium too fast can cause osmotic demyelination syndrome, with manifestations delayed by a few days. That is why the tolvaptan (SAMSCA) label requires starting and restarting it only in a hospital where serum sodium can be monitored closely, warns that too rapid correction (for example, more than 12 mEq/L in 24 hours) can cause osmotic demyelination, and notes greater risk in SIADH or with a very low baseline sodium.
The same receptor works in both directions. If sodium rises too fast on tolvaptan, the drug is interrupted and hypotonic fluid considered; tolvaptan should not be combined with a V2 agonist such as desmopressin, because it may interfere with desmopressin’s V2-agonist activity. Conversely, desmopressin has been given with 3% saline in severe hyponatremia to prevent overly rapid correction.
Comparison
Three disorders of ADH
| Disorder | What goes wrong | Serum sodium | Urine | Drug treatment |
|---|---|---|---|---|
| Central DI | ADH secretion fails (after brain trauma or surgery, for example) | High (hypernatremia) | Large volume, dilute; osmolality rises after desmopressin | Desmopressin |
| Nephrogenic DI | ADH normal; V2 receptor or signaling defect (hereditary; lithium, hypercalcemia; demeclocycline) | High (hypernatremia) | Large volume, dilute; little or no rise after desmopressin | Stop the cause (e.g. lithium); thiazide; amiloride may help lithium-induced polyuria |
| SIADH | ADH high despite low plasma osmolality (CNS disorders, small cell lung cancer, drugs such as carbamazepine and SSRIs) | Low (hyponatremia), euvolemic | Inappropriately concentrated (osmolality > 100 mOsm/kg; sodium > 40 mEq/L) | Tolvaptan (start in hospital); demeclocycline only when other therapies fail |
Drugs that act on the axis
| Drug | Action | Effect on water | Use on the ADH axis | Key risk |
|---|---|---|---|---|
| Desmopressin | Selective V2 agonist (little pressor activity) | Less, more concentrated urine | Central DI; telling central from nephrogenic DI | Water intoxication and hyponatremia |
| Vasopressin | V1 and V2 agonist | Antidiuresis, plus V1 vasoconstriction | Vasodilatory shock | Ischemia; diabetes insipidus after it is stopped |
| Tolvaptan | Selective V2 antagonist | Free-water excretion (aquaresis); serum sodium rises | Euvolemic or hypervolemic hyponatremia, including SIADH | Too rapid correction → osmotic demyelination; liver injury (Jynarque box) |
| Thiazide diuretics | NCC blockade in the distal tubule | Mild volume depletion → more proximal reabsorption → less urine | Nephrogenic DI | Hyponatremia |
| Amiloride | ENaC blockade (lithium enters principal cells through ENaC) | — | May help lithium-induced polyuria | Hyperkalemia |
— = not stated on the source pages.
High-yield
Practice questions
Q1A 59-year-old man has chronic SIADH from a brain tumor and remains hyponatremic despite other treatment. Tolvaptan is avoided because of his liver disease, and his physician starts demeclocycline.
How does demeclocycline raise his sodium?
- A.It blocks both V1A and V2 receptors
- B.It stimulates the V2 receptor
- C.It produces a nephrogenic, dose-dependent resistance to ADH — a reversible DI-like state
- D.It blocks NCC in the distal tubule
- E.It blocks ENaC in the collecting duct
Show answer
C. It produces a nephrogenic, dose-dependent resistance to ADH — a reversible DI-like state — Demeclocycline, a tetracycline, can cause a diabetes insipidus syndrome that is nephrogenic, dose-dependent and reversible on stopping; that is why it is effective in SIADH, although it is nephrotoxic and used only when other therapies fail. Dual V1A/V2 blockade describes conivaptan, and V2 stimulation is desmopressin, which would worsen hyponatremia. Blocking NCC (a thiazide) is a common cause of hyponatremia, and ENaC blockade is amiloride’s action. Tolvaptan is avoided in underlying liver disease.
Q2A 44-year-old woman taking carbamazepine for epilepsy has a serum sodium of 124 mEq/L, a serum osmolality of 262 mOsm/kg, a urine osmolality of 410 mOsm/kg and a urine sodium of 55 mEq/L. She has no edema and no signs of volume depletion, takes no diuretic, and her thyroid and adrenal tests are normal.
Which mechanism best explains her hyponatremia?
- A.Inappropriately high ADH action retaining free water (SIADH)
- B.Renal resistance to ADH
- C.Thiazide-induced sodium loss
- D.Sodium and water loss from volume depletion
Show answer
A. Inappropriately high ADH action retaining free water (SIADH) — She meets the SIADH pattern — low serum osmolality, urine osmolality above 100 mOsm/kg and urine sodium above 40 mEq/L, no volume depletion, with hypothyroidism and adrenal insufficiency excluded — and carbamazepine is one of the drugs that cause it. Renal resistance to ADH is nephrogenic DI, which causes dilute urine and hypernatremia. She takes no thiazide, and hypovolemic hyponatremia requires volume depletion.
Q3A 63-year-old man stopped lithium a year ago but still has polyuria from nephrogenic DI. Desmopressin had little effect. A thiazide is started, and his urine volume falls.
Why does a diuretic reduce his urine output?
- A.The thiazide restores V2 receptor signaling
- B.The thiazide increases ADH secretion
- C.The thiazide blocks ENaC, so lithium cannot enter the cells
- D.Mild volume depletion increases proximal sodium and water reabsorption, so less urine is made
Show answer
D. Mild volume depletion increases proximal sodium and water reabsorption, so less urine is made — Thiazides block NCC; the increased distal sodium excretion and mild volume depletion trigger compensatory proximal reabsorption of sodium and water, which impairs the kidney’s maximum diluting capacity and decreases urine production. They do not act on the V2 receptor, and in nephrogenic DI ADH secretion is already normal — the defect is in the kidney’s response. Blocking ENaC is amiloride’s action; lithium enters principal cells through ENaC, and amiloride may help lithium-induced polyuria.
Sources
- Physiology, Vasopressin — StatPearls (NCBI)
- Desmopressin — StatPearls (NCBI)
- Arginine Vasopressin Disorder (Diabetes Insipidus) — StatPearls (NCBI)
- DDAVP Tablets (desmopressin) — FDA label — FDA / DailyMed
- Vasostrict (vasopressin) — FDA label — FDA / DailyMed
- Hyponatremia — StatPearls (NCBI)
- Syndrome of Inappropriate Antidiuretic Hormone Secretion — StatPearls (NCBI)
- Jynarque (tolvaptan) — FDA label — FDA / DailyMed
- Samsca (tolvaptan) — FDA label — FDA / DailyMed
- Vaprisol (conivaptan) — FDA label — FDA / DailyMed
- Demeclocycline HCl tablets — FDA label — FDA / DailyMed
- Thiazide Diuretics — StatPearls (NCBI)
- Therapeutic Uses of Diuretic Agents — StatPearls (NCBI)
- Amiloride — StatPearls (NCBI)
- Hydrochlorothiazide — StatPearls (NCBI)
- Thalitone (chlorthalidone) — FDA label — FDA / DailyMed
- DDAVP Injection (desmopressin) — FDA label — FDA / DailyMed
- Amiloride HCl tablets — FDA label — FDA / DailyMed
- Inspra (eplerenone) — FDA label — FDA / DailyMed
Each class page compared lists its full sources; this page reuses facts verified there.
By LearnDrugs
Educational summary for medical students. Always verify against current prescribing information and guidelines. Not medical advice.