Renal Regulation of Urea Phosphate Calcium and Magnesium

A. Urea

- Fifty percent of the filtered urea is reabsorbed passively in the proximal tubule.

- The distal tubule, cortical collecting ducts, and outer medullary collecting ducts are impermeable to urea; thus, no urea is reabsorbed by these segments.

- ADH increases the urea permeability of the inner medullary collecting ducts. Urea reabsorption from inner medullary collecting ducts contributes to urea recycling in the inner medulla and to the development of the corticopapillary osmotic gradient.

B. Phosphate

- Eighty-five percent of the filtered phosphate is reabsorbed in the proximal tubule by Na+-phosphate cotransport. Because distal segments of the nephron do not reabsorb phosphate, 15% of the filtered load is excreted in urine.

- Parathyroid hormone (PTH) inhibits phosphate reabsorption in the proximal tubule by activating adenylate cyclase, generating cyclic AMP (cAMP), and inhibiting Na+-phosphate cotransport. Therefore, PTH causes phosphaturia and increased urinary cAMP.

- Phosphate is a urinary buffer for H+; excretion of H2P04 is called titratable acid.

- Sixty percent of the plasma Ca2+ is filtered across the glomerular capillaries.

-Together, the proximal tubule and thick ascending limb reabsorb more than 90% of the filtered Ca2+ by passive processes that are coupled to Na+ reabsorption.

- Loop diuretics (e.g., furosemide) cause increased urinary Ca2+ excretion. Because Ca2+ reabsorption is linked to Na+ reabsorption in the loop of Henle, inhibiting Na+ reabsorption with a loop diuretic also inhibits Ca2+ reabsorption. If volume is replaced, loop diuretics can be used in the treatment of hypercalcemia.

-Together, the distal tubule and collecting duct reabsorb 8% of the filtered Ca2+ by an active process.

1. PTH increases Ca2+ reabsorption by activating adenylate cyclase in the distal tubule.

2. Thiazide diuretics increase Ca2+ reabsorption and therefore decrease Ca2+ excretion. For this reason, thiazides are used in the treatment of hypercalciuria.

- is reabsorbed in the proximal tubule, thick ascending limb of the loop of Henle, and distal tubule.

- In the thick ascending limb, Mg2+ and Ca2+ compete for reabsorption; therefore, hypercalcemia causes an increase in Mg2+ excretion (by inhibiting Mg2+ reabsorption). Likewise, hypermagnesemia causes an increase in Ca2+ excretion (by inhibiting Ca2+ reabsorption).

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