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Antidiuretic
Antidiuretic
Antidiuretic
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An antidiuretic is a substance that helps to control fluid balance in an animal's body by reducing urination,[1] opposing diuresis.[2] Its effects are opposite that of a diuretic. The major endogenous antidiuretics are antidiuretic hormone (ADH; also called vasopressin) and oxytocin. Both of those are also used exogenously as medications in people whose bodies need extra help with fluid balance via suppression of diuresis. In addition, there are various other antidiuretic drugs, some molecularly close to ADH or oxytocin and others not. Antidiuretics reduce urine volume, particularly in diabetes insipidus (DI), which is one of their main indications.

The antidiuretic hormone class includes vasopressin (ADH), argipressin, desmopressin, lypressin, ornipressin, oxytocin, and terlipressin. Miscellaneous others include chlorpropamide and carbamazepine.

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Antidiuretic

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An antidiuretic is a substance that reduces to help control in the body by opposing . The primary natural antidiuretic is antidiuretic (ADH), also known as arginine vasopressin (AVP), a nonapeptide synthesized in the and released from the gland that primarily regulates retention by the kidneys to maintain osmotic balance and . ADH acts on the collecting ducts of the , increasing their permeability to through the insertion of channels into the apical membrane of principal cells, thereby reducing urine output and concentrating urine. Its release is primarily stimulated by increased detected by osmoreceptors in the or by decreased sensed by , with sensitivity to osmolality changes as small as 2 mOsm/L. In addition to its antidiuretic effects, ADH exerts vasoconstrictive actions at higher concentrations by binding to V1 receptors on vascular , helping to elevate during . Dysregulation of ADH can lead to conditions such as syndrome of inappropriate antidiuretic secretion (SIADH), causing , or , characterized by excessive due to ADH deficiency or renal resistance. Synthetic antidiuretics, such as , mimic ADH's effects and are used therapeutically.

Definition and Types

Definition

An antidiuretic is any substance or hormone that inhibits , the production of , by promoting the of in the kidneys, resulting in the formation of concentrated and a reduction in overall volume. This action helps conserve and maintain osmotic balance. The primary example of an antidiuretic is (ADH), also known as , a consisting of nine that is synthesized in the supraoptic and paraventricular nuclei of the and subsequently released from the gland into the bloodstream. The term "antidiuretic" was coined in the early to describe the effects of pituitary extracts that opposed , following observations in by researchers such as Arturo Farini and Richard von den Velden, who demonstrated the antidiuretic properties of these extracts in treating . Through its antidiuretic effects, ADH plays a fundamental role in maintaining body fluid homeostasis by counteracting dehydration and regulating plasma osmolality, ensuring that water is retained when bodily needs demand it.

Natural and Synthetic Antidiuretics

Natural antidiuretics primarily encompass endogenous peptide hormones that regulate water balance by promoting renal water reabsorption. The principal natural antidiuretic is arginine vasopressin (AVP), also known as antidiuretic hormone (ADH), a nonapeptide hormone synthesized in the hypothalamus and consisting of nine amino acids with a characteristic disulfide bridge forming a cyclic structure. In humans, AVP binds to vasopressin V2 receptors to exert its antidiuretic effects, distinguishing it from its vasoconstrictive actions mediated by V1 receptors. Oxytocin, another hypothalamic nonapeptide structurally similar to AVP, differing at amino acid positions 3 and 8, exhibits minor antidiuretic activity at high concentrations, primarily through cross-activation of V2 receptors, though its primary roles involve uterine contraction and milk ejection. Synthetic antidiuretics are engineered analogs of natural vasopressin designed to enhance specificity, duration of action, and resistance to enzymatic degradation. Desmopressin (1-deamino-8-D-arginine vasopressin, or DDAVP) is a prominent example, modified from AVP by removal of the N-terminal amino group (deamination) and substitution of L-arginine with D-arginine at position 8, resulting in a longer plasma half-life of 90–190 minutes compared to AVP's shorter duration and increased selectivity for V2 receptors over V1. This structural alteration minimizes vasoconstrictive side effects while preserving antidiuretic potency, making desmopressin suitable for conditions requiring targeted water retention. Terlipressin, or triglycyl-lysine vasopressin, represents another synthetic variant where three glycine residues are added to the N-terminus of lysine vasopressin, conferring prodrug-like properties that slowly release active vasopressin; it demonstrates dose-dependent antidiuretic effects at low doses (0.05–1.0 μg/kg) alongside its primary vasoconstrictive applications. While hormone-based agents dominate antidiuretic pharmacology, non-hormonal examples include certain and antagonists of (ANP), which indirectly enhance water retention by counteracting natriuretic and diuretic pathways. However, these are less commonly emphasized compared to vasopressin derivatives due to their broader physiological impacts and limited clinical specificity for antidiuresis.

Physiological Mechanisms

Role of Antidiuretic Hormone (ADH)

Antidiuretic hormone (ADH), also known as arginine vasopressin (AVP), is synthesized in the magnocellular neurons of the supraoptic and paraventricular nuclei within the hypothalamus. It is initially produced as a larger precursor protein called preprovasopressin, which consists of 164 amino acids and includes a signal peptide, the mature AVP nonapeptide, neurophysin II, and a glycopeptide known as copeptin. During transport along the axonal projections to the posterior pituitary, the precursor undergoes enzymatic processing in the Golgi apparatus and secretory granules, cleaving it into the active 9-amino-acid hormone ADH, along with neurophysin II and copeptin, which are released in equimolar ratios. Once synthesized, ADH is packaged into secretory granules and transported via the hypothalamo-neurohypophyseal tract to the gland, where it is stored until release into the systemic circulation. Release occurs in response to physiological stimuli, such as increased detected by hypothalamic osmoreceptors, leading to that maintains . In circulation, ADH has a short of approximately 10-20 minutes, primarily due to rapid metabolism by vasopressinases in the liver and kidneys, as well as renal excretion. ADH exerts its primary antidiuretic effects by binding to V2 receptors, which are G-protein-coupled receptors predominantly expressed on the basolateral membrane of principal cells in the kidney's collecting ducts, promoting water reabsorption to concentrate urine. It also interacts with V1 receptors, particularly the V1a subtype on vascular cells, inducing at higher concentrations to support regulation. Additionally, through V1b (or V3) receptors in the , ADH stimulates (ACTH) release, contributing to stress responses, though these non-renal actions are secondary to its central role in .

Action in the Kidney

Antidiuretic hormone (ADH), also known as , primarily targets the collecting ducts of the in the , where it enhances water permeability to facilitate . In the absence of ADH, the collecting ducts remain relatively impermeable to , allowing dilute to be excreted. Upon binding to ADH, the epithelial cells lining these ducts undergo a series of intracellular changes that insert water channels into the , enabling the to conserve and produce concentrated . This action is crucial for maintaining by preventing excessive water loss. At the molecular level, ADH exerts its effects by binding to vasopressin V2 receptors (V2R) on the basolateral of principal cells in the collecting ducts. This binding activates the G-protein-coupled receptor, which stimulates adenylate cyclase to increase intracellular (cAMP) levels. Elevated cAMP then activates (PKA), leading to the of regulatory proteins that promote the trafficking and insertion of (AQP2) water channels from intracellular vesicles into the apical facing the tubular lumen. AQP2 channels allow water to move passively down its osmotic gradient from the into the cell, where it can then exit via basolateral aquaporin-3 and -4 channels into the bloodstream. This regulated insertion of AQP2 is a rapid process, occurring within minutes of ADH stimulation, and is reversible upon hormone withdrawal. The water reabsorption process in the collecting ducts relies on an osmotic gradient established earlier in the by the countercurrent multiplier in the loop of Henle. The descending limb of the loop is permeable to water, which exits due to the hypertonic medullary , while the ascending limb actively transports ions out, creating a progressively increasing osmolarity in the medulla up to 1200 mOsm/L. This hypertonicity draws water from the collecting ducts through AQP2 channels under ADH influence, concentrating the urine and minimizing water excretion. Without this gradient, even with increased permeability, effective reabsorption would be impaired. In quantitative terms, ADH action can dramatically alter output; in its absence, the kidneys may produce up to 20 liters of dilute per day, whereas maximal ADH stimulation reduces this to less than 1 liter of highly concentrated , demonstrating the hormone's pivotal role in . This capacity underscores the efficiency of the renal response in adapting to varying hydration states.

Regulation of Antidiuretic Activity

Osmotic Regulation

Osmoreceptors responsible for the osmotic regulation of antidiuretic hormone (ADH), also known as , are located in the anterior , particularly within the organum vasculosum of the (OVLT). These specialized neurons detect subtle changes in , with sensitivity to increases as small as 1-2 mOsm/kg above the normal range of 280-295 mOsm/kg. The OVLT lacks a complete blood-brain barrier, allowing direct exposure to circulating solutes and enabling rapid osmotic sensing. The threshold for ADH is approximately 285 mOsm/kg, beyond which plasma ADH levels rise linearly with increasing osmolality, ensuring precise control of . This linear relationship allows for graded responses: minor elevations in osmolality trigger modest ADH release, while larger increases provoke stronger to restore . Sodium ions primarily drive this osmotic signal due to their dominant contribution to extracellular osmolality, though other solutes like and glucose can also influence activity when their concentrations fluctuate significantly. In the osmotic feedback loop, elevates , activating OVLT osmoreceptors that project to the supraoptic and paraventricular nuclei in the . This stimulation prompts ADH release from the , which acts on the renal collecting ducts to enhance water reabsorption via channels, thereby diluting the plasma and suppressing further ADH secretion. This mechanism maintains within narrow limits, preventing both hyperosmolar and hypoosmolar states.

Non-Osmotic Regulation

Non-osmotic regulation of antidiuretic hormone (ADH), also known as , involves mechanisms that stimulate its release independently of changes, primarily in response to alterations in effective circulating volume or other physiological stressors. These pathways allow the body to prioritize volume conservation during conditions like , overriding the primary osmotic control that maintains plasma . Baroreceptors located in the , , and left atrium play a central role in detecting reductions in or . During , such as in hemorrhage, these exhibit decreased firing rates, which are transmitted via afferent vagal and glossopharyngeal nerves to the and subsequently to the , stimulating ADH release from the . This response promotes renal reabsorption and to restore hemodynamic stability. Additional neural inputs further modulate ADH secretion through non-osmotic means. Angiotensin II, generated by the renin-angiotensin-aldosterone system in response to , directly stimulates hypothalamic neurons to potentiate ADH release. Similarly, stimuli such as , , and various forms of stress activate hypothalamic pathways, including the emetic reflex and central catecholaminergic systems, leading to increased ADH . Non-osmotic stimuli typically require a substantial volume deficit, such as greater than 10% loss of effective circulating volume, to significantly override the dominant osmotic regulation threshold. This ensures that remains the baseline control under normal conditions, with volume-sensitive mechanisms activating only during severe perturbations. Clinically, these non-osmotic pathways contribute to in scenarios involving acute volume depletion or postoperative states, where , , or surgical stress triggers excessive ADH release, impairing free water excretion despite normal or low osmolality.

Pathophysiological Implications

Deficiency States

Deficiency states of antidiuretic activity primarily manifest as , a group of disorders characterized by the inability to concentrate due to insufficient antidiuretic hormone (ADH) action, leading to excessive water loss. These conditions result in exceeding 3 liters per day, often up to 20 liters in severe cases, and are rare, affecting approximately 1 in 25,000 individuals worldwide. The primary forms include , , and gestational diabetes insipidus during pregnancy. Central diabetes insipidus arises from ADH deficiency due to damage or dysfunction of the or gland, such as from head trauma, tumors, infections, or neurosurgical interventions. This leads to reduced or absent ADH , impairing reabsorption in the renal collecting ducts and causing dilute output despite . In severe cases, patients may experience of up to 20 liters per day, accompanied by intense thirst. Nephrogenic diabetes insipidus results from renal resistance to ADH, where the kidneys fail to respond appropriately even in the presence of elevated ADH levels, leading to persistently low below 300 mOsm/kg. Common causes include genetic in the vasopressin V2 receptor gene (AVPR2) on the , accounting for about 90% of congenital cases, or acquired factors such as and chronic hypercalcemia. This form can be complete, with no urine concentration ability, or partial, allowing some response to ADH and milder . Across these deficiency states, core symptoms include with a preference for cold water, recurrent , and if fluid intake is inadequate, potentially progressing to , seizures, or in untreated cases. Gestational diabetes insipidus, a transient form unique to , occurs in 2 to 4 per 100,000 pregnancies due to increased placental vasopressinase activity that degrades ADH, typically resolving postpartum.

Excess States

Excess states of antidiuretic hormone (ADH) activity, also known as antidiuretic excess, occur when there is inappropriate or excessive of ADH, leading to impaired water excretion, water retention, and dilutional . This condition disrupts the normal balance of fluid and electrolytes, resulting in low serum sodium levels despite normal total body sodium, often presenting as euvolemic . The hallmark is continued ADH action that is not suppressed by low , causing the kidneys to reabsorb excessive water independently of osmotic needs. The syndrome of inappropriate ADH secretion (SIADH) represents the prototypical excess state, characterized by unsuppressed ADH release from the or ectopic sources, leading to with inappropriately concentrated urine. Ectopic ADH production commonly arises from malignancies, particularly small cell lung cancer (SCLC), which accounts for up to 70% of tumor-related cases, as tumor cells autonomously synthesize and release ADH. (CNS) disorders, such as , subarachnoid hemorrhage, infections, trauma, or tumors, can also trigger SIADH by disrupting regulatory pathways in the or pituitary. Diagnostic criteria for SIADH include serum sodium below 135 mEq/L, serum osmolality less than 275 mOsm/kg, greater than 100 mOsm/kg, and urine sodium exceeding 40 mEq/L in the setting of euvolemia. Beyond SIADH, other causes contribute to antidiuretic excess, including pharmacological agents that either stimulate ADH release or enhance its renal effects. Selective serotonin reuptake inhibitors (SSRIs) and are notable examples, with SSRIs promoting ADH secretion via serotonin-mediated pathways and carbamazepine increasing renal sensitivity to ADH. can mimic or precipitate SIADH through reduced and altered , often resolving with thyroid hormone replacement. Postoperative states, particularly following major , may induce transient ADH hypersecretion due to , , or stress, leading to acute water retention. Symptoms of antidiuretic excess vary by acuity and severity of hyponatremia, stemming primarily from caused by hypo-osmolar swelling of brain cells. In acute forms (developing over less than 48 hours), severe (serum sodium <125 mEq/L) can manifest as headache, nausea, vomiting, confusion, and potentially life-threatening seizures or coma due to rapid brain edema. Chronic excess states (>48 hours) allow for partial adaptation, presenting with subtler symptoms such as fatigue, gait instability, , and increased risk of falls or from prolonged hypo-osmolality. Pathophysiologically, excessive ADH binds to V2 receptors in the renal collecting ducts, activating channels to enhance water reabsorption, which expands intravascular volume and dilutes serum sodium. This mild volume expansion suppresses the renin-angiotensin-aldosterone system, reducing sodium reabsorption in the and contributing to . Consequently, serum levels are low due to increased renal clearance from the expanded effective circulating volume, further distinguishing excess states from other hyponatremic conditions.

Clinical and Therapeutic Applications

Diagnostic Approaches

Diagnostic approaches to assess antidiuretic function primarily involve laboratory measurements of antidiuretic hormone (ADH) levels, dynamic tests to evaluate renal concentrating ability and hormonal responses, and to identify structural abnormalities in the hypothalamic-pituitary axis. These methods help differentiate between central and nephrogenic forms of antidiuretic deficiency, as well as excess states like the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Plasma ADH, also known as arginine vasopressin (AVP), is measured using () or () techniques, which detect concentrations as low as 0.5-1 pg/mL after sample extraction to account for the hormone's instability in plasma. Normal basal plasma ADH levels typically range from 1 to 5 pg/mL, though values can vary with hydration status and assay sensitivity; elevated levels may indicate SIADH, while low or undetectable levels suggest (DI). The water deprivation test evaluates the kidney's ability to concentrate in response to ADH, confirming antidiuretic deficiency when fails to rise appropriately. In this protocol, is restricted for up to 8-18 hours while monitoring body weight, , and every 1-2 hours; is halted if body weight exceeds 3-5% or plasma sodium reaches 145-150 mmol/L to prevent severe . Healthy individuals achieve above 800 mOsm/kg, whereas values below 300 mOsm/kg after deprivation indicate DI; subsequent administration of (a synthetic ADH analog) distinguishes central DI (urine osmolality increases >50%) from nephrogenic DI (minimal response). Magnetic resonance imaging (MRI) of the and is essential for identifying structural causes of antidiuretic dysfunction, such as lesions disrupting ADH synthesis or release in central DI. High-resolution T1-weighted MRI with contrast reveals the posterior pituitary's normal "bright spot" due to ADH storage vesicles; its absence or ectopic location suggests central DI, while thickened or enhancing or hypothalamic masses indicate infiltrative, neoplastic, or inflammatory etiologies like tumors or .

Pharmacological Interventions

Pharmacological interventions targeting (ADH) activity include synthetic analogues that mimic its renal effects to treat deficiency states and receptor antagonists that block V2 receptors to manage excess states. acetate, a selective V2 receptor and synthetic analogue of ADH, is the cornerstone therapy for , effectively reducing by enhancing water reabsorption in the collecting ducts. Intranasal administration is commonly used, with typical doses ranging from 10 to 40 mcg daily, divided into one to three applications, providing 8 to 20 hours of antidiuresis per dose. Oral tablets offer an alternative for long-term management, starting at 0.05 mg (50 mcg) twice daily and titrated up to 0.4 mg (400 mcg) three times daily based on urinary output and osmolality. Vasopressin (arginine vasopressin), the natural , is employed primarily for its vasoconstrictive properties in acute esophageal variceal but also exerts antidiuretic effects that contribute to potential complications. Administered intravenously at an initial rate of 0.2 to 0.4 units per minute, with escalation to 0.8 units per minute if needed, it controls hemorrhage in up to 80% of cases initially, though its antidiuretic action can lead to unintended water retention. Vasopressin V2 receptor antagonists, or vaptans, inhibit antidiuretic activity to promote free water excretion in conditions like the syndrome of inappropriate antidiuretic hormone secretion (SIADH). , an oral selective V2 antagonist, is approved for euvolemic associated with SIADH, with dosing initiated at 15 mg once daily and adjusted in 15 mg increments to a maximum of 60 mg daily, guided by serum sodium monitoring to achieve gradual correction. Conivaptan, a dual V1a/V2 antagonist for intravenous use, addresses acute in hospitalized patients, featuring a 20 mg infused over 30 minutes followed by 20 to 40 mg per 24 hours for up to 4 days, effectively raising serum sodium by 4 to 6 mEq/L in the first 24 hours. Route of administration varies by clinical context: intranasal is favored for outpatient management of due to its convenience and exceeding 10%, while intravenous routes are reserved for acute scenarios, such as or conivaptan infusions during emergencies to ensure rapid onset and precise control. Side effects of ADH agonists like and include from excessive activation and water retention, occurring in up to 5% of patients and potentially leading to seizures if sodium falls below 125 mEq/L. Vaptans such as and conivaptan, conversely, frequently cause (up to 16% incidence) and dry mouth (up to 13%) due to aquaresis, which increases volume by 3 to 5 liters daily and requires fluid intake guidance to avoid .

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