Test 2 PP 6

Vasopressin 101 Notes

Vasopressin: names, main function, and origin

• AKAs: ADH (antidiuretic hormone), AVP (arginine vasopressin), also called ADH in some contexts.

• Main function: osmoregulation.

• Produced by magnocellular neurons in the hypothalamus:

  • Supraoptic nucleus (SO or SON) is the main area.

  • Paraventricular nucleus (PVN).

• AVP is a peptide hormone:

  • Cannot penetrate cell membranes.

  • Receptors are on the cell membrane (membrane-bound receptors).

• After synthesis, AVP is transported down long magnocellular axons toward the posterior pituitary and terminates at axon terminals in the neurohypophysis (posterior pituitary).

• AVP is stored in large secretory vesicles within axon terminals and released when needed; AVP is made ahead of time to allow immediate release during demand (e.g., hyperkalemia). New AVP is synthesized to replace released stores; typical storage is ample to handle pathological conditions.

Synthesis, transport, and storage in the posterior pituitary

• AVP is produced in the hypothalamus and transported down magnocellular neurons to the posterior pituitary (neurohypophysis).

• Stored in large secretory vesicles in axon terminals of magnocellular neurons.

• Release is rapid when needed; stores are usually sufficient to handle pathological situations.

• After release, reduced AVP is replenished with new synthesis to replace what's released.

Circumventricular organs (CVOs) and osmoreception

• OVLT: organum vasculosum of the lamina terminalis. Detects plasma osmolality (mainly salt).

• SFO: subfornical organ. Helps detect blood sodium.

• Both are members of the circumventricular organs located in the wall of the anterior third ventricle.

• They have fenestrated capillaries, so there is no blood–brain barrier here.

• They can also detect cerebrospinal fluid composition, but are not great at doing so.

• OVLT and SFO participate in signaling that regulates thirst and AVP release.

• There are seven CVOs in the family; they share traits such as fenestrated capillaries and proximity to CSF/Blood interfaces. They are not part of the posterior pituitary.

Triggers for the release of vasopressin

1) Hyperosmolality (mainly due to hypernatremia or severe hyperglycemia):

• OVLT and SFO detect osmolality changes and signal magnocellular neurons.

• This activates the thirst center and AVP release, leading to drinking water and water reabsorption.
  2) Hypotension (hypotension/hypovolemia):

• Low blood pressure/volume triggers R2A (renin–angiotensin II) pathway → AT2 activation → directly stimulates magnocellular neurons.

• Also indirectly stimulates OVLT and SFO, which then directly stimulate magnocellular neurons.
  3) Nausea: a non-osmolyte mechanism that is powerful but not fully understood historically; nausea can trigger AVP release, increasing free water absorption and potentially causing hyponatremia.

How AVP works in the kidney (V2-mediated water reabsorption)

• AVP binds to V2 receptors on principal cells of the collecting ducts.

• This activates a cyclic AMP (cAMP) second messenger system:

  • The rise in $ext{cAMP}$ leads to docking and insertion of aquaporin-2 channels into the apical (luminal) membrane of principal cells.

  • Aquaporin-2 (AQP2) channels allow water reabsorption from the filtrate into the cell, then into the interstitium, and finally into the blood via basolateral pathways.

• Result: free water reabsorption increases, raising blood volume and blood pressure.

• When AVP dissociates, AQP2-containing vesicles are recycled for future use.

• Path of water movement with AVP action:

  • Water moves from filtrate into the cytosol via AQP2 channels on the apical membrane.

  • Water moves from the cytosol into the interstitium via AQP3 and AQP4 channels.

  • Water then enters capillaries via oncotic forces, increasing blood volume and pressure.

• Note: Increased blood volume and pressure are consequences of AVP-mediated water reabsorption.

What happens to the free water reabsorbed?

• The reabsorbed free water dilutes hypernatremic (hyperosmolar) blood, reducing osmolality.

• It also increases intravascular volume, contributing to higher blood pressure.

• The overall outcome of AVP-driven water reabsorption is a more dilute plasma with higher circulating volume.

Hypernatremia: what it is and how to fix it

• Hypernatremia means salty blood; often accompanies hyperosmolality (high plasma osmolality).

• Sodium is the main osmolyte in plasma and interstitial fluid.

• Diagnosis by plasma (serum) sodium levels:

  • [Na^+]_{ ext{plasma}} > 145 ext{ meq/L}

• Common causes of hypernatremia: 1) Not drinking water (thirst mechanism disruption, e.g., lesions in the thirst center) or being too sick to drink water (vomiting/diarrhea). 2) Excessive water loss without equivalent salt loss (vomiting, diarrhea, diuretics, excessive sweating).

  • Sweating is a hypotonic fluid loss, leading to a higher salt:water ratio.
    3) Blocking vasopressin function (diabetes insipidus): manufacturing, transport to posterior pituitary, release, or V2 receptor binding issues; failure to insert AQP2 channels.

• In many cases, the hypernatremia is corrected by thirst-driven water intake and AVP-mediated water reabsorption, but significant cases require renal and fluid management.

Hypernatremia and renal physiology: salt handling and natriuresis

• When salt load is high, the kidney reduces salt reabsorption to help correct hypernatremia.

• Atrial natriuretic peptide (ANP) reduces ENaC channel production by inhibiting aldosterone release from the adrenal glomerulosa:

  • ANP inhibits aldosterone synthesis, resulting in decreased ENaC channel insertion in the collecting duct.

  • Fewer ENaC channels lead to reduced salt reabsorption (natriuresis).

• The primary correction for hypernatremia involves the combination of water intake (driven by AVP and thirst) and reduced salt reabsorption by the kidney.

Persistent hypernatremia: three broad categories of causes

1) Inadequate water intake

• Tumor or other disruption in the hypothalamic thirst center (MPA) may blunt the thirst mechanism.

• Illness that prevents drinking water (vomiting, severe illness) can prevent intake.
  2) Excessive water loss without equivalent salt loss

• Vomiting, diarrhea, diuretics, excessive sweating (hyperhidrosis).

• Sweat is hypotonic; these losses create a higher salt:water ratio.

• Some diuretics can cause hypernatremia via excessive water loss or altered electrolyte handling.
  3) Blocked vasopressin function

• Problems with AVP production, transport, storage, release, or V2 receptor signaling (diabetes insipidus).

• Failure to manufacture or insert aquaporins-2 (AQP2) channels.

Osmotic diuresis in diabetes mellitus and its relation to hypernatremia

• Osmotic diuresis occurs when glucose is not fully reabsorbed from the filtrate (glycosuria):

  • Glucose acts as a filtrate osmolyte that retains water in the filtrate, increasing water excretion.

  • This leads to higher water loss and can precipitate hypernatremia if water intake does not keep pace.

• In diabetes mellitus with poor control (including DKA), ketone bodies also act as osmolytes in the filtrate and contribute to osmotic diuresis.

• Even AVP-stimulated AQP2 channels cannot reabsorb water against the osmotic pull of sugar and ketone osmolytes, worsening free-water loss and hypernatremia.

How does vasopressin help defend against severe hypotension?

• Baroreceptors in carotid sinus, aortic arch, and right atrium detect severe drops in pressure/volume (e.g., > 15 ext{%}).

• They send signals to the nucleus tractus solitarius (NTS) via cranial nerves IX and X.

• NTS signals magnocellular neurons to release large amounts of AVP; AVP is a powerful vasoconstrictor.

• AVP increases blood pressure via vasoconstriction and by supporting blood volume.

AVP vasoconstriction and hypertension mechanisms (V1-mediated)

• AVP binds to V1 receptors on the tunica media of arterioles, triggering a signaling cascade:

  • Release of calcium from the endoplasmic reticulum into the cytosol.

  • Calcium activates actin–myosin to contract the tunica media, narrowing the arteriole lumen.

  • Result: immediate increase in blood pressure due to peripheral vasoconstriction.

• Primary targets: arterioles in skin and muscles are heavily affected, contributing to increased vascular resistance and pressure.

• Additional effect: AVP can also increase myocardial contractility by acting on cardiomyocytes, raising the force of contraction and helping raise blood pressure; AVP does not act on the SA or AV nodes like some other vasoconstrictors (e.g., angiotensin II).

Fun facts: alcohol and smoking effects on AVP and blood pressure

• Alcohol (ethanol) depresses magnocellular neuron activity and AVP release, reducing water reabsorption in the kidney and increasing urine output (polyuria). This can lead to mild hypernatremia and hypovolemia/hypotension.

  • ADH does promote some sodium reabsorption, but alcohol overall tends to cause diuresis.

• Cigarette smoking can increase blood pressure via multiple routes: 1) Triggers AVP release from magnocellular neurons independent of hypernatremia, leading to water reabsorption and potential volume overload and hypertension. 2) Triggers norepinephrine release from the adrenal medulla (chromaffin cells), increasing heart rate and stimulating cardiomyocytes and the SA/AV nodes, contributing to higher BP and afterload.

  • Together, these effects can raise systemic blood pressure.

Diabetes Insipidus (DI)

• A condition related to AVP dysregulation (production, transport, release, or receptor function).

• Mentioned as a topic in Endocrine Week 10, Monday, Sum24, indicating DI as a key related topic to vasopressin physiology.