Renal Mineral handling

Question 1

How is calcium handled in the kidneys?

Answer 1

In the glomerulus calcium that is not protein bound is freely filtered and there is reabsorption along the nephron.
Of filtered calcium 70% is reabsorbed in the proximal tubule and a further 20% in the thick ascending limb of the loop of Henle. This reabsorption is mainly passive and paracellular and driven by sodium reabsorption.

Question 2

How does sodium reabsorption effect calcium?

Answer 2

Sodium reabsorption causes water reabsorption which raises tubular calcium concentration causing calcium to diffuse out of the tubules. The positive
lumen potential also encourages calcium to leave the tubule.

Question 3

What causes secondary hypoparathyroidism?

Answer 3

Renal disease where Ca cannot be excreted well and a diet too high in Ca.

Question 4

What can cause nutritional secondary hyperparathyroidism?

Answer 4

• Excess P intake (or diets high in cation
‘binders’ such as cereals, wheat bran) May weaken bones osteomalacia.
. Oxalates (e.g. some grasses) may also bind to
calcium).

Question 5

All phosphate that is not protein bound is

freely filtered where?

Answer 5

In the glomerulus. There is reabsorption along the nephron.

Question 6

Of filtered phosphate how much is reabsorbed in the proximal tubules by a transcellular process that relies on sodium reabsorption?

Answer 6

80%. Apical phosphate entry is by co-transport with sodium via the sodium phosphate co-transporter (NPT2). This transporter is downregulated by PTH.

Question 7

80-90% of the 15% organic phosphate in soft tissue are

bound to Mg, Na or Ca. What can this lead to?

Answer 7

(forms complex) (lead to bile and kidney stones)

Question 8

What can cause hyperphosphataemia?

Answer 8

*Phosphate supplements or enemas (small dogs and cats).

• Cell lysis may release P
  — (Lymphomas, rhabdomyolysis, soft tissue trauma, haemolysis)
• Hyperthryroidism.
• AKI or CKD resulting in reduce kidney excretion.

Question 9

How much magnesium is found in bone, soft tissue and ECF?

Answer 9

54% is in bone. 45% in soft tissues.

and just 1% in the extracellular fluid.

Question 10

Magnesium that is not protein bound is freely filtered in the glomerulus and there is reabsorption along the nephron. Only 30% is reabsorbed in the
proximal tubule. The majority is reabsorbed where?

Answer 10

The thick ascending limb by passive paracellular movement driven by the transepithelial potential. A further 5% is reabsorbed in the distal tubules.

Question 11

What do PTH and calcitonin do to paracellular magnesium reabsorption?

Answer 11

Increase it.

Question 12

Both loop diuretics and thiazides do what to magnesium excretion?

Answer 12

Increase it.

Question 13

The most prevelant cation is?

Answer 13

Calcium.

Question 14

Dietary intake of Ca and P can be high so there is a relience on kidneys to excrete the excess. During renal failure this cannot be done effectively and Ca and P will rise in blood. What hormones regulate Ca/P?
.

Answer 14

PTH and Vitamin D regulate Ca/P. Kidney important in these hormone axes.

Question 15

What three effects does PTH have on the kidney?

Answer 15

• It increases vitamin D synthesis; (ii)
• It reduces proximal tubular phosphate reabsorption which increases phosphate excretion
• it increases distal tubular calcium reabsorption by activating calcium channels.