# Clinical Cases: Tubular Function - Reabsorption and Secretion ## Case 1: Diabetic Glucosuria and SGLT2 Inhibitor Therapy ### Patient Presentation A 58-year-old female with type 2 diabetes mellitus presents for diabetes management. Her hemoglobin A1c is 8.4% despite maximum dose metformin and a sulfonylurea. She also has stage 3 CKD with eGFR 45 mL/min/1.73m2 and albuminuria (UACR 180 mg/g). ### History of Present Illness - Type 2 diabetes for 12 years - Hypertension controlled on lisinopril - BMI 32 kg/m2 - Fasting glucose levels 160-200 mg/dL - No symptoms of hypoglycemia ### Physical Examination - Blood pressure: 138/84 mmHg - Weight: 89 kg - No peripheral edema - Monofilament sensation reduced in both feet - Urine dipstick: 2+ glucose ### Workup **Laboratory Studies:** - HbA1c: 8.4% - Fasting glucose: 182 mg/dL - Creatinine: 1.4 mg/dL (eGFR 45) - UACR: 180 mg/g - Urinalysis: 2+ glucose, trace protein ### Diagnosis **Type 2 Diabetes with Diabetic Kidney Disease - Candidate for SGLT2 Inhibitor Therapy** ### Discussion This case illustrates glucose handling in the proximal tubule: - **Normal Glucose Reabsorption**: The lecture describes that SGLT2 (90%) and SGLT1 (10%) in the proximal tubule normally reabsorb all filtered glucose. The transport maximum (Tm) is approximately 375 mg/min. - **Renal Threshold**: When plasma glucose exceeds ~180-200 mg/dL, the transport maximum is exceeded and glucose appears in urine. Her fasting glucose of 182 mg/dL is at the threshold. - **SGLT2 Inhibitor Mechanism**: Dapagliflozin or empagliflozin block SGLT2, deliberately causing glucosuria to lower blood glucose. This also delivers more sodium to the macula densa, activating tubuloglomerular feedback and reducing intraglomerular pressure. ### Treatment - Add empagliflozin 10 mg daily - Continue lisinopril for renal protection - Counsel about increased urinary frequency and genital yeast infection risk - Monitor creatinine (expect small initial rise of 3-5 mL/min which is hemodynamic and protective) ### Clinical Pearl SGLT2 inhibitors work by blocking proximal tubule glucose reabsorption, causing intentional glucosuria. The renal protective effects extend beyond glucose control to include reduced intraglomerular pressure through tubuloglomerular feedback. --- ## Case 2: Gitelman Syndrome ### Patient Presentation A 19-year-old female presents with fatigue, muscle cramps, and episodes of muscle weakness. She has had these symptoms intermittently since childhood but they have worsened recently during summer heat. ### History of Present Illness - Recurrent muscle cramps since age 10 - Salt craving - No vomiting or diarrhea - No diuretic or laxative use - Excessive thirst but normal urine output - Mother had similar symptoms ### Physical Examination - Blood pressure: 102/68 mmHg (low-normal) - Heart rate: 76 bpm - No edema - Mild tetany with Chvostek sign positive - Normal neurologic examination otherwise ### Workup **Laboratory Studies:** - Potassium: 2.8 mEq/L (low) - Magnesium: 1.4 mg/dL (low) - Bicarbonate: 30 mEq/L (high - metabolic alkalosis) - Calcium: 9.2 mg/dL (normal) - Urine calcium: Low (hypocalciuria - 24-hour urine calcium 40 mg/day) - Urine potassium: 45 mEq/day (inappropriately high) - Renin and aldosterone: Both elevated **Genetic Testing:** - SLC12A3 gene mutation identified (NCC transporter) ### Diagnosis **Gitelman Syndrome** ### Discussion This case demonstrates distal convoluted tubule function: - **NCC Transporter**: The lecture describes the thiazide-sensitive sodium-chloride cotransporter (NCC) in the DCT. Gitelman syndrome results from loss-of-function mutations in NCC, mimicking chronic thiazide use. - **Hypocalciuria Mechanism**: When NCC is blocked, less sodium enters DCT cells, lowering intracellular sodium. This enhances basolateral Na+/Ca2+ exchange (NCX), increasing calcium reabsorption - opposite to loop diuretics which cause hypercalciuria. - **Hypomagnesemia**: The DCT is the major site of regulated magnesium reabsorption; dysfunction causes magnesium wasting. - **Hypokalemia and Alkalosis**: Volume depletion from sodium wasting activates RAAS, increasing aldosterone-mediated potassium secretion and hydrogen ion secretion. ### Treatment - Oral potassium supplementation (60-80 mEq/day) - Oral magnesium supplementation (essential - K won't normalize without Mg) - Liberal salt intake - Potassium-sparing diuretics (amiloride) may help - Avoid situations causing additional potassium loss ### Clinical Pearl Gitelman syndrome mimics thiazide diuretic use while Bartter syndrome mimics loop diuretic use. The key differentiating feature is urine calcium: Gitelman causes hypocalciuria (thiazide effect) while Bartter causes hypercalciuria (loop diuretic effect). --- ## Case 3: Fanconi Syndrome from Multiple Myeloma ### Patient Presentation A 67-year-old male presents with bone pain, fatigue, and recurrent fractures. He is found to have anemia and hypercalcemia on routine labs. ### History of Present Illness - Progressive lower back pain for 6 months - Fatigue and weakness - 15-pound unintentional weight loss - Increased thirst and urination - Two pathologic rib fractures from minimal trauma ### Physical Examination - Blood pressure: 128/78 mmHg - Pallor - Point tenderness over lumbar spine and ribs - No hepatosplenomegaly - No lymphadenopathy ### Workup **Laboratory Studies:** - Hemoglobin: 9.2 g/dL - Creatinine: 2.1 mg/dL - Calcium: 11.8 mg/dL (elevated) - Albumin: 3.2 g/dL - Total protein: 10.5 g/dL (elevated - protein gap) - Glucose (serum): 95 mg/dL (normal) - Uric acid: 2.1 mg/dL (low) - Phosphorus: 2.2 mg/dL (low) - Bicarbonate: 18 mEq/L (low) **Urinalysis:** - Glucose: 2+ (despite normal serum glucose) - Protein: 2+ - pH: 5.8 **Additional Studies:** - SPEP: M-spike present (IgG kappa) - 24-hour urine: Glucosuria, aminoaciduria, phosphaturia - Skeletal survey: Lytic lesions in spine, ribs, skull ### Diagnosis **Multiple Myeloma with Light Chain-Induced Fanconi Syndrome** ### Discussion This case illustrates proximal tubule dysfunction: - **Fanconi Syndrome**: The lecture describes generalized proximal tubule dysfunction affecting multiple transport systems. Light chain deposition in proximal tubule cells impairs: - Glucose reabsorption via SGLT2 (glucosuria at normal serum glucose) - Amino acid reabsorption (aminoaciduria) - Phosphate reabsorption (phosphaturia, hypophosphatemia) - Uric acid reabsorption (hypouricemia) - Bicarbonate reabsorption via NHE3 (Type 2 proximal RTA) - **Proximal Tubule Functions**: The PCT normally reabsorbs 65-70% of filtered sodium, water, glucose, amino acids, phosphate, and bicarbonate. Global dysfunction causes loss of all these substances. ### Treatment - Hematology consultation for myeloma treatment - Chemotherapy with bortezomib-based regimen - Phosphate supplementation for hypophosphatemia - Bicarbonate supplementation for metabolic acidosis - Bisphosphonates (with caution given renal function) - Hydration to prevent cast nephropathy ### Clinical Pearl Fanconi syndrome causes glucosuria despite normal blood glucose - the proximal tubule simply cannot reabsorb the filtered glucose. This distinguishes it from diabetic glucosuria where glucose appears because of hyperglycemia exceeding the normal transport maximum. --- ## Image Reference For visual reference of tubular transport concepts, see: - Wikimedia Commons: [Proximal convoluted tubule](https://commons.wikimedia.org/wiki/Category:Proximal_convoluted_tubule) - Histology showing brush border - Radiopaedia: [Renal tubular acidosis](https://radiopaedia.org/articles/renal-tubular-acidosis) - Clinical information - StatPearls: [Nephron Histology](https://www.ncbi.nlm.nih.gov/books/NBK554411/) - Tubular segment identification --- ## Learning Points 1. **Glucose Transport Maximum**: The renal threshold for glucose is ~180-200 mg/dL. Above this, SGLT2 and SGLT1 are saturated and glucosuria occurs. 2. **SGLT2 vs SGLT1**: SGLT2 handles 90% of glucose reabsorption with low affinity/high capacity; SGLT1 handles 10% with high affinity as a "safety net." 3. **NCC and Calcium**: Thiazide diuretics and Gitelman syndrome (both affecting NCC) cause hypocalciuria, useful for treating hypercalciuric nephrolithiasis. 4. **Fanconi Syndrome Components**: Look for the combination of glucosuria with normal glucose, aminoaciduria, phosphaturia, uricosuria, and type 2 RTA. 5. **Na+/K+-ATPase**: This basolateral pump establishes the sodium gradient that drives all secondary active transport in the nephron.