Katz: K and Ca regulation Flashcards
What percent of K is reabsorbed in the proximal tubule via tight junctions?
55-65%
What part of the nephron absorbs more K than the proximal tubule?
TAL
80-90% can be reabsorbed at the principle cell
What happens to principle cells during a high K diet?
Dietary K moves into all cells (esp principle) via Na/K ATPase pumps. Principle cells then secrete the K into the nephron lumen to achieve K homeostasis.
K excretion rate»_space; K filtered load during a high K diet
How does K secretion occur in principle cells?
- Aldosterone sensitive ROMK are OPEN unless aldosterone is absent (low K situation)
High K diet>
increased K and increased aldosterone>
stimulated ROMK expression
- Flow inducible BK channels are inserted into principle cell lumen when there is a positive K balance.
Hi K>
BK expression
What are the dangers of hyperkalemia?
Cells can’t repolarize–> arrythmias (hypokalemia can do this too)
What happens if someone has a LOW K diet?
- Principle BK cells not expressed
- ROMK channels decreased
- Alpha/type A intercalated cell in DCT and CCD actively reabsorb K so that the K excretion rate is about 1% of the K filtered load
Why is hypokalemia often accompanied by a loss of H (alkalosis)?
Hypokalemia (low K diet, vomiting, diarrhea)> reabsorb K and secrete H into urine> Make bicarb> secreted into plasma> alkalosis
Describe the mechanism of K reabsorption in an alpha intercalated cell.
- K/H antiporter brings K into the cell and pushes H into the lumen
- K diffuses down gradient out channel on basolateral membrane and into the peritubular capillary.
What happens when Ca free plasma levels decrease?
Decrease Ca>
CaSR (Ca sensing receptor)>
Increase in PTH>
Returns Ca upward
What percent of Ca is bound to albumin?
40% the other 60% is free
How does an increase in PTH lead to increased Ca release?
PTH stimulates osteoclast bone resorption
Increased PTH> inhibits osteoblasts (stop making bone)> stimulates osteoclasts> secrete H> dissolves calcium phosphate> Ca and phosphate are reabsorbed by osteoclast> secreted
What affect does PTH have on vit D?
Stimulates ACTIVATION of renal vit D>
increased intestinal Ca absorption
How much of ingested Ca do you normally absorb?
20%
How does PTH stimulate renal Ca reabsorption?
- PTH stimulates PTH receptor
- Increased luminal Ca channel expression
- Ca moves down gradient and increases conc of Ca in cell>
- Na/Ca antiporter moves Ca out of cell
- Ca is reabsorbed
SAVES Ca
If you eat 20 mm of Ca how much is absorbed and how much is excreted?
4 mm absorbed
16 excreted in feces
What happens to the 4 mm absorbed Ca in the kidenys?
4 is excreted by kidneys, and almost 400 mm of Ca were filtered
How many mm of Ca undergo bone resorption and bone formation
15 mm of Ca for both
How many mm of Ca does bone contain?
20,000 mm of ca as hydroxyapatites
Adult skeleton comprises 14% of total body weight and 1/2 is watettr.
How many mm of Ca are in the entire extracellular space?
30 mm
What causes kidney stone? How do you decrease your chance of getting a kidney stone?
Increased Ca phosphate/Ca oxalate>
precipitation rxn in urine
tx: drink water!
How can a diseased kidney lead to renal osteodystrophy?
diseased kidney> renal production of active vit D decreases> intestinal Ca absorption decreases> Ca free plasma level decreases> increase in PTH> increase bone reabsorption> weak bones
Why do many pts with advanced chronic kidney disease take calcitriol to tx elevated levels of parathyroid hormone?
Trying to save Ca but not absorb H
