Cell Biology and Physiology of the Kidney¶
Chapter 320 | Part 9: Disorders of the Kidney and Urinary Tract · Part 9 – Renal & Urinary Tract Disorders · Chapter 320
Key Clinical Points¶
- The kidney is highly differentiated, containing nearly 30 different cell types forming filtering capillaries and segmented nephrons.
- Glomerular count varies by birth weight: ~900,000 in normal-birth-weight adults; as few as 225,000 in low-birth-weight adults.
- Autoregulation of GFR is mediated by three factors: myogenic reflex (afferent), tubuloglomerular feedback (TGF) via the macula densa, and Angiotensin II (efferent).
- Proximal tubule reabsorbs ~60% of filtered Na+ and water, ~90% of bicarbonate, and nearly all glucose and amino acids.
- Loop of Henle creates a hypertonic medullary interstitium via countercurrent multiplication; the thick ascending limb is impermeable to water.
- Podocyte integrity (slit-pore membrane) depends on proteins including nephrin, podocin, and TRPC6; mutations cause heavy proteinuria.
- SGLT2 inhibitors target Na+/glucose cotransporters in the proximal tubule for managing Type 2 Diabetes.
- Loop diuretics target NKCC2 in the thick ascending limb; Thiazides target Na+/Cl− cotransporters in the distal convoluted tubule.
- Carbonic anhydrase inhibitors (e.g., acetazolamide) block proximal bicarbonate reabsorption and are used to alkalinize urine.
- Potassium-sparing diuretics include Amiloride (blocks apical Na+ channels) and Spironolactone/Eplerenone (block mineralocorticoid receptors).
DEFINITION & OVERVIEW¶
• Complexity: One of the most highly differentiated organs; nearly 30 different cell types form filtering capillaries and segmented nephrons. • Nephron Population: ◦ Normal-birth-weight adults: ~900,000 glomeruli. ◦ Low-birth-weight adults: as few as 225,000 (associated with higher risk of complications). • Nephron Types: ◦ Cortical: Majority; located in mid-to-outer cortex; short loops of Henle; shared peritubular capillaries. ◦ Juxtamedullary: Fewer; at border of cortex/medulla; long loops of Henle; individual vasa recta. • Functional Roles: ◦ Cortical: Perform most glomerular filtration (larger afferent arterioles). ◦ Juxtamedullary: Create osmotic gradient for urine concentration via long loops.
Renal Masses and Imaging¶
• Clinical Presentation: Palpable masses may indicate cystic disease (PCKD), congenital ureteral pelvic obstruction, or renal cell carcinoma. • Imaging Modalities: ◦ Ultrasound: Effective for determining size/symmetry; detecting cysts/masses; less effective for stones. ◦ CT (Noncontrast): Standard test for nephrolithiasis (noted risk of cumulative radiation). ◦ MRI: Useful for evaluating and following renal masses, including renal cell carcinoma. ◦ Radioisotope scan: Determines percentage of function in each kidney.
EMBRYOLOGIC DEVELOPMENT¶
• Nephrogenesis: Driven by genes (e.g., Pax2, Lhx1, Notch2, Hnf1b, Tcf21, Pdgfrb) and morphogenic cues; involves ureteric bud induction and metanephric blastema. ◦ S-shaped nephrons form from induced mesenchyme → differentiate into segments. ◦ VEGF-A & Angiopoietin-1: Guide capillary formation and podocyte differentiation. • Podocytes: ◦ Structure: Form slit-pore membrane with neighbors to filter plasma water/solute. ◦ Components: Nephrin, annexin-4, CD2AP, FAT, ZO-1, P-cadherin, podocin, TRPC6, PLCE1, and Neph 1-3 proteins. ◦ Clinical Note: Mutations in these proteins result in heavy proteinuria. ◦ Maintenance: Podocytes are replaced by migrating parietal epithelia; failure leads to proteinuria.
Nephron Segments¶
• Autoregulation of GFR: Not linear with pressure due to three factors: ◦ Myogenic Reflex: Afferent arteriole → responds to pressure changes (constricts if high, dilates if low). ◦ Tubuloglomerular Feedback (TGF): Macula densa (thick ascending limb) senses NaCl/flow. → High flow/NaCl → ATP release → Adenosine → Afferent constriction. → Low flow/NaCl → Reduced adenosine → Afferent dilation. ◦ Angiotensin II: Released in response to low blood flow → Efferent vasoconstriction → Increases glomerular hydrostatic pressure. • TGF Modulators: ◦ Enhancers: Angiotensin II, Reactive Oxygen Species. ◦ Blunters: Nitric Oxide (NO).
EPITHELIAL SOLUTE TRANSPORT¶
• Polarized Epithelia: ◦ Structure: Tight junctions separate apical (lumen) and basolateral (interstitium) membranes. ◦ Function: Allows asymmetric distribution of membrane proteins for directional transport. • Transport Types: ◦ Cellular: Movement across both membranes. ◦ Paracellular: Movement between cells through tight junctions. • Epithelial Classification: ◦ Leaky (Low-resistance): e.g., Proximal tubule; suited for bulk fluid reabsorption. ◦ Tight (High-resistance): e.g., Collecting duct; allows refined control of transport.
Membrane Transport Mechanisms¶
• Active Transport: Requires ATP; includes Na+/K+-ATPase, H+-ATPases, and Ca2+-ATPases. ◦ Function: Creates concentration gradients (e.g., Na+ gradient) to drive other processes. • Passive Transport: Movement via channels driven by concentration/electrical gradients. • Facilitated Diffusion: Mediated by carriers/uniporters (e.g., GLUT2 for glucose). • Secondary Active Transport: Cotransporters (symporters) or antiporters; can be electroneutral or electrogenic. • Regulation: ◦ Sensing: Via apical microvilli, primary cilia, or mechanosensing channels. ◦ Signaling: Mediated by intracellular Ca2+ or purinergic receptor-mediated pathways.
SEGMENTAL NEPHRON FUNCTIONS¶
• Proximal Tubule: ◦ Reabsorption: ~60% Na/water; ~90% bicarbonate; nearly all glucose and amino acids. ◦ Mechanism: High surface area (brush border) + leaky junctions → bulk flow driven by peritubular capillary pressure. ◦ Glucose: Mediated by SGLT2 (apical) and GLUT2 (basolateral). → SGLT2 inhibitors block reabsorption to lower blood glucose. ◦ Bicarbonate: Requires Carbonic Anhydrase; → Acetazolamide blocks this, used to alkalinize urine. ◦ Acid Secretion: Ammonia (NH3) production from glutamine; Titratable acid (H2PO4-). ◦ Chloride: Reabsorbed via apical formate exchange and basolateral K+/Cl- cotransporter. ◦ Minerals: 60-70% Ca, ~85% Phosphate reabsorbed. → Vitamin D activation (1α-hydroxylase) & FGF23/PTH regulation of phosphate.
Loop of Henle¶
• Function: Creates hypertonic medullary interstitium via countercurrent multiplication. ◦ Thick Ascending Limb: Impermeable to water; active transport of Na, K, and Cl (NKCC2).
Distal Convoluted Tubule¶
• Function: Refined control of ion transport. ◦ Thiazide-sensitive transporters: Target for thiazide diuretics.
Cortical and Medullary Collecting Ducts¶
• Principal Cells: Regulate Na+ reabsorption, K+ secretion, and water permeability (Vasopressin/Aldosterone). ◦ Aldosterone: Increases Na+ reabsorption and K+ secretion. ◦ Vasopressin: Increases water permeability via aquaporins. ◦ ANP: Inhibits sodium reabsorption.
INHERITED DISORDERS OF RENAL TUBULAR TRANSPORT¶
• Table 320-1 Summary: → Proximal Tubule Disorders: ◦ Proximal renal tubular acidosis (SLC4A4) ◦ Fanconi-Bickel syndrome (SLC2A2) ◦ Isolated renal glycosuria (SLC5A2) ◦ Cystinuria (Type I: SLC3A1; Non-type I: SLC7A9) ◦ Lysinuric protein intolerance (SLC7A7) ◦ Dicarboxylic aminoaciduria (SLC1A1) ◦ Hartnup disorder (SLC6A19) ◦ Hypophosphatemic nephrolithiasis/osteoporosis 1 (SLC34A1) ◦ Hereditary hypophosphatemic rickets with hypercalcemia (SLC34A3) ◦ Renal hypuricemia (Type 1: SLC22A12; Type 2: SLC2A9) ◦ Dent's disease & X-linked variants (CLCN5) → Loop of Henle Disorders: ◦ NKCC2 mutations (SLC12A1), ROMK (KCNJ1), ClC-Kb (CLCNKB), Barttin (BSND), Calcium-sensing receptor (CASR), Claudin-16 (CLDN16), Claudin-19 (CLDN19), ATP1G1. → Distal Tubule & Collecting Duct Disorders: ◦ Gitelman syndrome (SLC12A3) ◦ Primary hypomagnesemia with secondary hypocalcemia (TRPM6) ◦ Pseudoaldosteronism (Liddle's syndrome) (SCNN1B, SCNN1G) ◦ Recessive pseudohypoaldosteronism type 1 (SCNN1A, SCNN1B, SCNN1G) ◦ Gordon’s hyperkalemia-hypertension syndrome (WNK1, WNK4) ◦ X-linked nephrogenic diabetes insipidus (AVPR2) ◦ EAST/SeSAME syndrome (KCNJ10) ◦ Nephrogenic diabetes insipidus (autosomal) (AQP2) ◦ Distal renal tubular acidosis (SLC4A1, ATP6V1B1, ATP6V0A4).
PHARMACOLOGIC MANIPULATION OF TUBULAR TRANSPORT¶
• Loop Diuretics: ◦ Target: NKCC2 in the thick ascending limb. ◦ Effect: Block Na, K, and Cl reabsorption. • Thiazides: ◦ Target: Na/Cl cotransporter in the distal convoluted tubule. ◦ Effect: Promote sodium excretion. • Carbonic Anhydrase Inhibitors (e.g., Acetazolamide): ◦ Target: Proximal tubule. ◦ Effect: Block bicarbonate reabsorption; used to alkalinize urine. • Potassium-Sparing Diuretics: ◦ Amiloride: Blocks apical Na+ channels in the distal nephron. ◦ Spironolactone/Eplerenone: Antagonize mineralocorticoid receptors (Aldosterone) to block basolateral Na+/K+-ATPase.
Clinical Targets¶
• SGLT2 Inhibitors: Block Na-glucose cotransporters in proximal tubules → lower blood glucose; used in DM and CKD.
KEY PEARLS & HIGH-YIELD POINTS¶
• Differentiation: The kidney is highly specialized with ~30 cell types. • Autoregulation: Myogenic (Afferent), TGF (Macula Densa/Afferent), and Ang II (Efferent) ensure stable GFR. • Proximal Tubule: Primary site for bulk reabsorption; essential for bicarbonate, glucose, and amino acids. • SGLT2: Key target in the proximal tubule for managing hyperglycemia. • Loop vs. Thiazide: Loop diuretics act on NKCC2 (thick ascending limb); Thiazides act on Na/Cl (distal convoluted tubule). • Potassium Sparing: Amiloride and Spironolactone/Eplerenone are used to block sodium reabsorption without significant potassium loss. • Sodium Balance: Sodium intake → Net Na+ balance → (Increase → Edema; Decrease → Volume depletion).
Reference Tables¶
TABLE 320-1 Inherited Disorders Affecting Renal Tubular Ion and Solute Transport¶
Harrison's 22e, p.2368
| DISEASE OR SYNDROME | PROTEIN (GENE) | OMIMa |
|---|---|---|
| Disorders Involving the Proximal Tubule | ||
| Proximal renal tubular acidosis | Sodium bicarbonate cotransporter (SLC4A4, 4q21) | 604278 |
| Fanconi-Bickel syndrome | Glucose transporter, GLUT2 (SLC2A2, 3q26.2) | 227810 |
| Isolated renal glycosuria | Sodium glucose cotransporter (SLC5A2, 16p11.2) | 233100 |
| Cystinuria | ||
| Type I | Cystine, dibasic and neutral amino acid transporter (SLC3A1, 2p16.3) | 220100 |
| Non-type I | Amino acid transporter, light subunit (SLC7A9, 19q13.1) | 600918 |
| Lysinuric protein intolerance | Amino acid transporter (SLC7A7, 4q11.2) | 222700 |
| Dicarboxylic aminoaciduria | Glutamate transporter (SLC1A1, 9q24.2) | 222730 |
| Hartnup disorder | Neutral amino acid transporter (SLC6A19, 5p15.33) | 34500 |
| Hypophosphatemic nephrolithiasis/osteoporosis 1 | Sodium phosphate cotransporter (SLC34A1, 5q35.3) | 612286 |
| Hereditary hypophosphatemic rickets with hypercalcemia |
Sodium phosphate cotransporter (SLC34A3, 9q34) | 241530 |
| Renal hypouricemia | ||
| Type 1 | Urate-anion exchanger (SLC22A12, 11q13) | 220150 |
| Type 2 | Urate transporter, GLUT9 (SLC2A9, 4p16.1) | 612076 |
| Dent’s disease | Chloride channel, ClC-5 (CLCN5, Xp11.22) | 300009 |
| X-linked recessive nephrolithiasis with renal failure | Chloride channel, ClC-5 (CLCN5, Xp11.22) | 310468 |
| X-linked recessive hypophosphatemic rickets | Chloride channel, ClC-5 (CLCN5, Xp11.22) | 307800 |
| Disorders Involving the Loop of Henle | ||
| Sodium, potassium chloride cotransporter (SLC12A1, 15q21.1) Potassium channel, ROMK (KCNJ1, 11q24) Chloride channel, ClC-Kb (CLCNKB, 1p36) Chloride channel accessory subunit, Barttin (BSND, 1p31) |
||
| Calcium-sensing receptor (CASR, 3q13.33) | ||
| Calcium-sensing receptor (CASR, 3q13.33) | ||
| Claudin-16 (CLDN16, 3q27) | ||
| Claudin-19 (CLDN19, 1p34.2) | ||
| Sodium potassium ATPase, γ-subunit (ATP1G1, 11q23) 1 |
||
| Disorders Involving the Distal Tubule and Collecting Duct | ||
| Gitelman syndrome | Sodium chloride cotransporter (SLC12A3, 16q13) | 263800 |
| Primary hypomagnesemia with secondary hypocalcemia |
Melastatin-related transient receptor potential cation channel 6 (TRPM6, 9q22) | 602014 |
| Pseudoaldosteronism (Liddle’s syndrome) | Epithelial sodium channel β and γ subunits (SCNN1B, SCNN1G, 16p12.1) | 177200 |
| Recessive pseudohypoaldosteronism type 1 | Epithelial sodium channel, α, β, and γ subunits (SCNN1A, 12p13; SCNN1B, SCNN1G, 16pp12.1) | 264350 |
| Pseudohypoaldosteronism type 2 (Gordon’s hyperkalemia-hypertension syndrome) |
Kinases WNK-1, WNK-4 (WNK1, 12p13; WNK4, 17q21.31) | 145260 |
| X-linked nephrogenic diabetes insipidus | Vasopressin V2 receptor (AVPR2, Xq28) | 304800 |
| EAST/SeSAME syndrome | Potassium channel Kir4.1 (KCNJ10, 1q23.2) | 612780 |
| Nephrogenic diabetes insipidus (autosomal) | Water channel, aquaporin-2 (AQP2, 12q13) | 125800 |
| Distal renal tubular acidosis | ||
| autosomal dominant | Anion exchanger-1 (SLC4A1, 17q21.31) | 179800 |
| autosomal recessive | Anion exchanger-1 (SLC4A1, 17q21.31) | 602722 |
| with neural deafness | Proton ATPase, β1 subunit (ATP6V1B1, 2p13.3) | 192132 |
| with normal hearing | Proton ATPase, 116-kD subunit (ATP6V0A4, 7q34) | 602722 |