Renal handling of Ca, PO4, Mg

Brenner and Rector, The Kidney. 

Calcium/Phosphate regulation short PowerPoint

Calcium

99% in bones and teeth

1% in plasma and cells

plasma Ca

• total = 8.5 to 10.5 mg/dL
• free (ionized) 45% of total
• bound (reversibly) to protein 40% of total
• complexed to anions 5% of total
• corrected for low protein

 

 correction is wrong

Ca homeostasis

• PTH
• releases Ca from bone
• PTH released when plasma Ca falls
• PTH activates vitamin D pre-hormone to increase Ca reabsorption from gut
•  
•  
• Ca dysfunction
•  surfactant
• clotting
• neuromuscular
•  
• Sources of calcium
• diet
• supplements
• drugs

Hypercalcemia; e.g., cancer, excess Vit. D

• diuresis
• kidney stones
• chronic renal failure

Renal handling of Calcium

• proximal tubule - passive diffusion in paracellular channels driven by positive potential in lumen
• thick ascending loop of Henle - ditto due to Na-K-2Cl transporter.
• Ca sensing receptor in proximal tubule, loop, distal tubule, CD
• increased Ca intake
• suppresses PTH
• Ca binds to CaSR = inhibits luminal K channel and basolateral NaK ATPase
• stops Ca reabsorption
• causes polyuria
• decreases NaCl reabsorption
• reduces medullary osmotic gradient
• impairs ADH induced water permeability
• polyuria also occurs in renal failure due to impaired gradient
• distal convoluted tubule
• Ca enters tubule cells via TRPV5 Ca channel
• Ca binds to protein (to avoid apoptosis) and is transported to basolateral membrane
• thiazide diuretics cause hypocalciuria (beneficial side effect - reduces kidney stones)
• reduced ECF volume causes increased Na reabsorption in proximal tubule and increased Ca reabsorption

Phosphorus

• energy storage
• urine buffer
• bone component
• signal transduction
• total body = 700 g
• 85% in bones and teeth
• normal serum PO4 = 3-4.5 mg/dL
• Ca x PO4 >70 mg/dL = risk of precipitate (calciphylaxis) - blocks blood flow
• Regulation of PO4
• PTH, vit D, FGF-23
• renal
• proximal tubule - PO4 coupled to Na reabsorption (cotransport)
• PTH causes endocytosis of Na-PO4 cotransporter
• defect in receptor, Gs protein, or adenylyl cyclase = pseudohypoparathyroidism
• hyperphosphatemia
•  

Magnesium

• total body = 24 g
• 50-60% in bone
• 40-50% in ECF
• 1% in ICF
• renal handling
• absorbed passively - follows Na

Hypercalcemia

• stones, bones, groans, thrones, psychiatric overtones

• treatment

• fluids
• lasix

http://bio1152.nicerweb.com/Locked/media/ch45/homeostasis-calcium.html