Sodium and Water Balance I: ADH Regulation and Clinical Correlations

Learning Outcomes

• Describe the sites and magnitude of fluid absorption along the nephron when both a concentrated urine and a dilute urine are being formed.

• Describe the action of antidiuretic hormone (also known as arginine vasopressin, AVP) at the collecting duct.

• Explain how the action of antidiuretic hormone (AVP) on the kidney controls whole-body water balance.

• Describe how disorders in antidiuretic hormone (AVP) secretion affect water balance.

Corticopapillary Hyperosmotic Gradient and Nephron Osmolarity

• Osmolarity increases in the nephron as it descends, which causes water to be reabsorbed passively by osmosis.

• Regional Permeability Differences: Water reabsorption does not happen uniformly throughout the nephron. Permeability differs significantly between the ascending and descending arms of the Loop of Henle (LOH).

• ADH Supervising Role: Water absorption in the late Distal Convoluted Tubule (DCT) and the Collecting Duct (CD) is under the close supervision of Antidiuretic Hormone (ADH).

• The Gradient Mechanism:

  • The interstitium is hypertonic (high solute concentration) relative to the tubule.

  • In the cortex-to-papilla transition, the hyperosmotic interstitium facilitates passive water reabsorption.

  • For the first time in the nephron, $H_2O$ reabsorption is not automatically coupled to the reabsorption of solutes in these specific segments because permeability is ADH-dependent.

Sites and Magnitude of Fluid Absorption

Water reabsorption is categorized into two types: Obligatory (independent of bodily conditions) and Facultative (dependent on hydration status and ADH).

• Proximal Tubule (PCT): Reabsorbs approximately $70\%$ of filtered water. This is obligatory absorption.

• Thin Descending Limb (TDLOH): Reabsorbs approximately $20\%$ of filtered water. This is also obligatory absorption.

• Thin/Thick Ascending Limb (ALOH): Virtually $0\%$ water reabsorption occurs here; these segments are impermeable to water.

• Distal Tubule (DCT) and Collecting Duct (CD): Magnitude of reabsorption varies by hydration status (Facultative).

Comparative Magnitude of Water Excretion

• Normal State:

  • DCT absorption: $3-5\%$

  • CD absorption: $0-5\%$

  • Urine output: Approximately $1.5\,l/day$ ($0.8-1\%$ of filtered load).

• Volume Expanded State (Overhydrated):

  • Body does not need to conserve water; ADH is low.

  • DCT absorption: $4\%$

  • CD absorption: Approximately $0-4\%$

  • Urine output: Up to $12\,l/day$ ($6\%$ of filtered load).

• Dehydrated State:

  • Body maximizes reabsorption under high ADH.

  • DCT absorption: $5\%$

  • CD absorption: Approximately $5\%$

  • Urine output: Minimum of $0.3\,l/day$ ($0.2\%$ of filtered load).

• Note: The body must excrete at least $300\,ml$ of water daily to dissolve solutes for excretion.

Diuresis vs. Anti-Diuresis

Diuresis (Dilute Urine Formation)

• Occurs when there is an excess of water in the body.

• ADH Levels: Low or Zero.

• Mechanism: Water reabsorption in the PCT and thin TDLOH remains normal ($291\,mOsm$ to $1200\,mOsm$ at the bend). However, the DCT and CD become impermeable to $H_2O$.

• Result: Large volume of dilute urine. Volume can reach $12-18\,l/day$ ($10\%$ of GFR). Osmolarity can be as low as $50\,mOsm/Kg/water$.

Anti-Diuresis (Concentrated Urine Formation)

• Occurs during dehydration to conserve water.

• ADH Levels: High.

• Mechanism: ADH promotes Aquaporin-2 (AQP2) membrane fusion in the late DCT and CD, enhancing reabsorption.

• Urea Permeability: High ADH also increases urea permeability in the inner medullary collecting duct by inserting urea transporters. This enhances urea retention and maintains the high inner medullary gradient required for water reabsorption.

• Result: Small volume of concentrated urine. Urine osmolarity can reach $1200\,mOsm/kg/water$. Volume can be as low as $0.3-0.5\,l/day$.

Mechanism of Action of ADH at the Collecting Duct

1. ADH (Arginine Vasopressin) binds to V2 receptors on the basolateral membrane of principal cells.

2. This binding promotes the conversion of $ATP$ to $cAMP$ via the enzyme adenylate cyclase.

3. $cAMP$ activates Protein Kinase A (PKA).

4. Activated PKA promotes the fusion of Aquaporin-2 (AQP2) water channels into the apical (luminal) membrane.

5. This increases the permeability of the membrane to $H_2O$, allowing water to move into the cell and then into the blood.

Osmoregulation: The Osmolar Control Pathway

• Trigger: Increased plasma osmolarity (e.g., due to sweating or breathing during exercise).

• Sensors: Osmoreceptors, which are specialized neuronal cells located in the anterior hypothalamus in an area called the Vascular Organ of Lamina Terminalis (OVLT).

• Mechanism of Sensation:

  • High plasma osmolarity causes water to leave the osmoreceptor cells via AQP4 channels.

  • The osmoreceptor neurons shrink.

  • Shrinkage triggers signaling to the Supraoptic Nuclei (SON).

• Response: Activation of the thirst response and the secretion of ADH from the posterior pituitary.

• Feedback Loop: Once water is gained, osmoreceptors swell and stop firing, inhibiting further ADH secretion.

Osmoregulation: Non-Osmolar Control Pathways

Baroreceptor Pathway

• Trigger: Decreased blood volume/Effective Circulating Volume (ECV).

• Sensors: Baroreceptors located in the Aortic Arch and Carotid Sinus.

• Mechanism: These are specialized stretch receptors. When arterial blood pressure falls, their firing rate decreases.

• Response: Signals are sent to the Medulla Oblongata and then to the Hypothalamus. A low firing rate triggers the thirst response and ADH secretion.

Juxtaglomerular (JG) Apparatus Pathway

• Trigger: Low blood volume detected in the kidney.

• Mechanism:

  1. Granular juxtaglomerular cells detect low volume and release Renin.

  2. Renin converts Angiotensinogen to Angiotensin I.

  3. Angiotensin Converting Enzyme (ACE) converts Angiotensin I to Angiotensin II.

• Response: Angiotensin II triggers the thirst response and ADH secretion.

Integration of ADH Control: Volume Overrides Tonicity

• Set Point: The plasma osmolarity at which ADH begins to increase is approximately $280\,mOsm$ (range of $270-285\,mOsm$ in healthy individuals).

• Sensitivity:

  • Osmolar control is very sensitive, responding to a $2-3\%$ change in osmolarity.

  • Non-osmolar control requires a larger change ($>10\%$ decrease in blood volume) but produces a much larger ADH output (up to $50\,pg/ml$).

The "Volume Overrides Tonicity" Principle

• Volume Depleted (~15%): The set point shifts to the left. ADH is secreted even if plasma osmolarity is below the normal $280\,mOsm$ threshold.

• Volume Expanded (~15%): The set point shifts to the right. ADH secretion is suppressed even if plasma osmolarity is high.

• Clinical Significance (Circulatory Collapse): In scenarios like circulatory collapse, the body prioritizes volume over tonicity. ADH secretion and water conservation are maintained to increase ECF volume, even if it leads to plasma hypotonicity.

ADH Stimulants and Depressants

• ADH Stimulants (Increase ADH Levels):

  • Opioids

  • Anti-depressants (SSRIs, TCAs)

  • MDMA (Ecstasy): A recreational drug that can lead to death via water intoxication.

  • Nicotine

• ADH Depressants (Decrease ADH Levels):

  • Alcohol

Pathophysiology: Hyponatremia and Hypernatremia

Hyponatremia ($<135\,mM$ Na+)

• Cause: Net loss of $Na^+$ or net gain of $H_2O$.

• Major Reasons:

  • Disturbances of Salt Balance: Hypoaldosteronism, Thiazides, vomiting, diarrhea.

  • Disturbances of Water Input: Primary Psychological Polydipsia (excessive volitional water intake).

  • Disturbances of Water Output (SIADH): Syndrome of Inappropriate ADH Secretion.

SIADH vs. Water Intoxication

• SIADH: Excessive ADH leads to high urine osmolarity, hyponatremia, and hypervolemia. Common causes include post-operative stress ($30\%$), head trauma ($20\%$), and ectopic ADH production from tumors ($20\%$).

• Differentiation:

  • Water Intoxication: Rapid volume change, potentially fatal brain dysfunction. Urinary $Na^+$ is very low.

  • SIADH: Urinary $Na^+$ is high because high blood volume suppresses Renin and Angiotensin II, decreasing sodium reabsorption to compensate for volume overload, which ironically makes hyponatremia worse.

Hypernatremia ($>145\,mM$ Na+)

• Major Reasons:

  • Salt Balance: Dietary intake, IV administration, Hyperaldosteronism.

  • Water Output: Decreased intake (unconscious patients), vomiting/diarrhea, or lack of ADH effect (Diabetes Insipidus).

Diabetes Insipidus (DI)

DI is a non-functional ADH system resulting in polyuria, polydipsia, hypernatremia, and hypotension.

• Neurogenic / Central DI: Failure of ADH secretion due to lesions in the hypothalamus or pituitary. Treated with synthetic ADH.

• Nephrogenic DI: Failure of principal cells to respond to ADH (often due to V2 receptor mutations). Treated with a restricted $Na^+$ diet.

Differential Diagnosis via Water Restriction Test

• Normal Patient: Water restriction $\rightarrow$ increased plasma osmolarity $\rightarrow$ ADH release $\rightarrow$ Concentrated urine ($1200\,mOsm$).

• Central DI: Water restriction $\rightarrow$ increased plasma osmolarity $\rightarrow$ no ADH $\rightarrow$ Dilute urine ($<200\,mOsm$). If external ADH is administered, the patient responds by concentrating urine ($\sim 800\,mOsm$).

• Nephrogenic DI: Water restriction $\rightarrow$ increased plasma osmolarity $\rightarrow$ Dilute urine. External ADH administration has no effect because the kidneys are unresponsive.

Questions & Discussion

• The lecture included scheduled breaks for integrated questions to test understanding of ADH control and feedback loops.

• Interactive recap recordings and QR codes were provided to scan and answer questions regarding Sodium and Water Balance Part 1 and Part 2.