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
