Sodium & Potassium Balance – Detailed Study Notes

A. Actions of ADH

Renal (Collecting-Duct) Actions

• Controls water permeability of the apical membrane in principal cells of the collecting duct.
• Binds to V2 receptors on the basolateral membrane → activates cAMP/PKA pathway → phosphorylation of vesicle-associated proteins.
• Causes rapid insertion of pre-formed water channels Aquaporin-2 (AQP2) into the apical membrane.
  • In the absence of ADH AQP2 is removed by endocytosis → water impermeability → dilute urine.
• Stimulates UT-A1 urea transporters → ↑ urea recycling, strengthening medullary osmotic gradient.
• (Minor) Enhances ENaC-mediated Na⁺ reabsorption, contributing to water reabsorption by raising medullary osmolality.
• Net effect: ↑ water reabsorption, ↑ urine osmolality, ↓ plasma osmolality, restoration of ECFV.

Cardiovascular Actions

• Potent vasoconstrictor (V1A receptors on vascular smooth muscle) → ↑ TPR → ↑ arterial pressure.
• Activates vagal afferents → reflex bradycardia (slowing of heart rate).

Endocrine/Other Actions

• Stimulates ACTH release via V1B (V3) receptors in anterior pituitary → links water conservation to stress axis.
• Acts synergistically with Ang II in hypovolaemic states.

B. Aquaporins

Classification

• Orthodox isoforms (strict water channels): AQP1, 2, 4, 5, 6, 8.
• "Cocktail" (aquaglyceroporins – water + small solutes): AQP3, 7, 9, 10.

Renal Distribution & Significance

• $AQP1$: Proximal tubule + thin descending limb; constitutive, ensures massive obligatory water reabsorption.
• $AQP2$: Apical membrane of collecting-duct principal cells; ADH-regulated.
• $AQP3$ and $AQP4$: Basolateral membrane of principal cells; provide exit pathway for absorbed water.
• Pathology links: Mutations in AQP2 or V2 receptor → Nephrogenic Diabetes Insipidus. Aquaporin dysregulation implicated in polycystic kidney disease, CHF, liver cirrhosis.

C. Protection of Renal Medullary Cells from Hypertonicity

• Challenge: Interstitial osmolality in the inner medulla can reach $>1200\;\text{mOsm·kg}^{-1}$ → potential protein denaturation & cell shrinkage.
• Thick Ascending Limb (TAL) Machinery: $NKCC2$ cotransporter + $ROMK$ channel generate NaCl gradient.
• Hypertonicity-induced transcription factor: TonEBP/NFAT5 up-regulates genes buffering osmotic stress.

Key Adaptive Mechanisms (TonEBP Targets)

1. Compatible Organic Osmolyte Accumulation
   • Transporters: SMIT (inositol), BGT1 (betaine), TauT (taurine).
   • Enzymes: Aldose Reductase (AR) → sorbitol synthesis; Neuropathy Target Esterase (NTE) → glycerophosphorylcholine.
   • Purpose: Increase intracellular osmolarity without perturbing protein function.
2. Heat-Shock Proteins (e.g., HSP70) → molecular chaperones limit protein unfolding.
3. Modulation of membrane transport: reduced ROMK activity prevents K⁺ loss; alterations in NaCl transport fine-tune gradient.

Clinical relevance: Maladaptation may contribute to medullary hypoxic injury, analgesic nephropathy.

D. Sodium Balance

Intake & Output

• Average diet: $100\text{–}400\;\text{mmol Na}^+!/\text{day}$ (≈ 6–24 g salt).
• Output at rest, cool climate:
  • Sweat: negligible.
  • Faeces: negligible.
  • Urine: matches intake (regulation range $\approx100\text{–}400\;\text{mmol·day}^{-1}$).
• Kidneys must vary excretion to avoid dramatic changes in ECFV & BP; failure would cause oedema or circulatory collapse.

Detection of Na⁺ Load Changes

1. ↑ Na⁺ intake → ↑ plasma osmolality →
   • (1) Thirst → ↑ water intake.
   • (2) ADH release → water retention.
     → Restores osmolality but expands ECFV.
2. Effective Circulating Volume (ECV) – portion of ECF that actually perfuses tissues; sensed rather than total ECFV.
   • Dependent on vascular volume, pressures & cardiac output.
   • In disease (CHF, cirrhosis) ECV ↓ despite expanded ECFV → triggers Na⁺ retention.

Sensors of ECV

• Low-pressure receptors: atria, pulmonary vasculature, large veins.
• High-pressure baroreceptors: carotid sinus, aortic arch, juxtaglomerular (JG) apparatus.
• CNS & hepatic sensors (secondary).

Neural & Hormonal Pathways

• Afferent: vagal/glossopharyngeal nerves → medulla → hypothalamus.
• Efferent: alters GFR & tubular reabsorption via
  • Sympathetic nerve activity (α-adrenergic vasoconstriction & direct tubular effects).
  • Hormones: RAS, Aldosterone, Noradrenaline, ADH, ANP, Prostaglandins.
  • Starling forces in peritubular capillaries (hydrostatic & colloid osmotic pressure).

Hormonal Modulators of Na⁺ Excretion

1. Renin-Angiotensin System (RAS)

• ↓ ECV → JG renin release → ↑ Ang II.
• Ang II lowers Na⁺ excretion via:
  a. Stimulating aldosterone (zona glomerulosa).
  b. Direct proximal-tubule Na⁺ reabsorption (Na⁺/H⁺ exchanger, Na⁺-K⁺-ATPase).
  c. Renal vasoconstriction → ↑ filtration fraction (FF) → ↑ peritubular $\pi<em>{c}$ & ↓ $P</em>{c}$ → promotes proximal Na⁺ uptake.
  d. ↓ medullary blood flow → less osmotic washout → ↑ passive Na⁺ reabsorption in thin ascending limb.

2. Aldosterone

• Steroid; genomic + non-genomic actions (latency 60–90 min).
• Acts on principal cells (collecting duct & DCT):
  • ↑ apical ENaC & ROMK synthesis/activation.
  • ↑ basolateral Na⁺-K⁺-ATPase.
  • Net: ↑ Na⁺ reabsorption; coupled K⁺ & H⁺ secretion (risk of hypokalaemic alkalosis).
• Controls ~2 % of filtered Na⁺ (but crucial fine-tuning).
• Stimuli: (1) Ang II (most important), (2) ↑ plasma K⁺ (high sensitivity), (3) ↓ plasma Na⁺ (weak), (4) ACTH (minor for aldosterone, major for cortisol).

3. Noradrenaline (SNS)

• α₁ vasoconstriction → ↓ GFR, ↑ FF.
• Direct tubular stimulation of Na⁺ uptake (proximal)
• Activates renin via β₁ receptors on JG cells.

4. Atrial Natriuretic Peptide (ANP)

• 28-aa peptide; stored in atrial granules.
• Release stimulus: atrial distension; granule exocytosis.
• Synthesised as pro-ANP → activated by corin (membrane serine protease); ANP & corin also expressed in kidney (paracrine effect).
• Receptor: NPR-A (guanylyl cyclase linked) → ↑ cGMP.

Actions:

1. Natriuresis & Diuresis
   • ↑ GFR (afferent dilation, efferent constriction, mesangial relaxation).
   • ↑ glomerular permeability (↑ Kf).
   • ↑ medullary blood flow → washout of gradient.
   • Direct tubular inhibition of Na⁺ transporters (Na⁺-K⁺-ATPase, NHE3, NKCC2, ENaC).
   • Maximal effect in medullary collecting duct.
2. Vasodilation → ↓ BP.
3. ↓ responsiveness to vasoconstrictors.
4. Lowers plasma renin, aldosterone & ADH → counter-regulatory to RAS/ADH.

5. Prostaglandins (mainly PGE₂, PGI₂)

• Synthesised in kidney; high when Ang II & ADH are high (e.g., volume depletion).
• Vasodilatory, inhibit Na⁺ reabsorption; provide local brake on excess Na⁺ retention.
• Clinical note: NSAID inhibition during volume contraction → marked Na⁺ & water retention.

E. Potassium Balance

Physiological Importance

• Major intracellular cation: $\text{[K⁺]}_i \approx 150\;\text{mmol·L}^{-1}$; extracellular $\approx 4\;\text{mmol·L}^{-1}$.
• Determines resting membrane potential of excitable cells via Nernst relationship:
  $E<em>m = -61 \log\frac{[K^+]</em>i}{[K^+]_e} \; (\text{mV})$.
• Needed for protein synthesis, cell growth, enzyme activity.
• Plasma ranges: normokalaemia $3.5\text{–}5.5\;\text{mmol·L}^{-1}$; >5.5 hyperkalaemia, <3.5 hypokalaemia.

Intake & Output

• Diet: $50\text{–}100\;\text{mmol·day}^{-1}$ (e.g., single rockmelon ≈ 50 mmol).
• Not tightly regulated at GI level in humans.
• Output: Urine 45–90 mmol·day⁻¹ (adjustable); Sweat <5; Faeces 5–10.

Transcellular K⁺ Shifts (Buffering Post-prandial Load)

Promote K⁺ uptake into cells (preventing hyperkalaemia)

• Insulin – stimulates Na⁺-K⁺-ATPase.
• β₂-Adrenoceptor activation (Adrenaline) – ↑ Na⁺-K⁺-ATPase.
• Aldosterone – genomic & non-genomic cellular uptake.

Promote K⁺ efflux (risk of hyperkalaemia)

• Metabolic acidosis (H⁺/K⁺ exchange).
• ↑ Plasma osmolality (water efflux drags K⁺).
• Cell lysis (trauma, rhabdomyolysis, tumour lysis).
• Strenuous exercise (K⁺ release from working muscle).

Renal Handling

• Filtered freely.
• Proximal tubule + TAL: reabsorb ≥80 % (paracellular & NKCC2-mediated pathways).
• Distal nephron (CNT & Collecting Duct) determines final excretion (1 % to >200 % of filtered load).
  • α-Intercalated cells: reabsorb K⁺ via H⁺/K⁺-ATPase (relatively fixed).
  • Principal cells: secrete K⁺ via ROMK & BK channels (variable, regulated).

Determinants of K⁺ Secretion (Principal Cells)

1. Intracellular [K⁺] (driving force).
2. Luminal [K⁺] (concentration gradient; influenced by tubular flow).
3. Transepithelial Potential Difference (V_te) – lumen negative favours K⁺ exit; generated by ENaC-mediated Na⁺ uptake.
4. Luminal K⁺ Permeability – number & open probability of ROMK/BK channels.

Factors Increasing K⁺ Secretion / Excretion

• Aldosterone – enhances (1), (3), (4). Hyperaldosteronism → hypokalaemia.
• High K⁺ intake – raises plasma & intracellular K⁺; stimulates aldosterone; ↑ V_te.
• ↑ Distal Flow Rate (e.g., osmotic diuresis, loop/thiazide diuretics) – lowers luminal [K⁺] & washes away secreted K⁺.
• ↑ Distal Na⁺ Delivery – more ENaC activity → more lumen negativity.
• Impermeant anions in tubular fluid (HCO₃⁻, SO₄²⁻, NO₃⁻) – lumen negative.
• Alkalosis – cellular K⁺ uptake exchange & increased HCO₃⁻ in lumen.

Acid–Base Effects on K⁺ Renal Handling

• Acute Alkalosis: ↑ K⁺ secretion (↑ intracellular K⁺, lumen negativity, ↑ ROMK activity; HCO₃⁻ impermeant).
• Acute Acidosis: ↓ secretion (competition of H⁺ for Na⁺-K⁺-ATPase, depolarised membrane).
• Chronic Acidosis: total K⁺ excretion ↑ because proximal Na⁺Cl⁻ & water reabsorption is inhibited → ↑ distal flow & Na⁺ delivery.

Pathophysiological Correlates

• Hypokalaemia: diuretics, hyperaldosteronism, vomiting/alkalosis, insulin overdose.
• Hyperkalaemia: renal failure, hypoaldosteronism (Addison’s, ACE-I, ARB, K⁺-sparing diuretics), acidosis, tissue breakdown.
• Therapeutic note: High K⁺ diet lowers BP & stroke risk; Mechanisms include vasodilation, natriuresis, modulation of baroreflex ("health benefits" – McDonough & Youn, 2017).

Integrative & Clinical Connections

• Water, Na⁺ and K⁺ balances are inter-linked via hormonal cross-talk (ADH ↔ RAS ↔ ANP ↔ Aldosterone).
• Disorders of one system often manifest in the others: e.g., SIADH → hyponatraemia; hyperaldosteronism → metabolic alkalosis & hypokalaemia; volume depletion + NSAIDs → acute Na⁺ retention & renal failure.
• Understanding transporter physiology underpins pharmacology: loop diuretics (block NKCC2 → Na⁺/K⁺ loss), thiazides (block NCC), K⁺-sparing diuretics (ENaC blockers, aldosterone antagonists).
• Ethical & practical consideration: public-health strategies (salt reduction, K⁺-rich diets) influence BP & cardiovascular morbidity.