Lecture 4- Renal Transport II Flashcards
What is the typical intake of K+ per day ?
100 mEq
varies a lot on diet
How much K+ is excreted by the kidney?
92% of ingested K+
Having perturbation of extracellular K+ will lead to…
nerve misfiring and or cardiac arrythmias
insulin stimulates
K+ uptake
-slower effect, whereas immediate response occurs from Na+/K+ ATPase
How much filtered K+ is reabsored in proximal tubule?
67%
paracellular; solvent drag
How much K+ is reabsorbed in thick ascending limb?
20%
Na/K/2Cl cotransporter) not regulated
How much K+ is reabsorbed in the distal tubule and CD?
12% (apical H+/K+ ATPase) regulated
K+ secretion via principal cells via
apical K+ channels (in DT and CD)
How is high cytoplasmic K+ maintained?
by Na/K ATPase
What are the main secretion regulation factors?
[K+]plasma, aldosterone, K channel activity, Na channel activity, Na delivery to Principal cells
If you increase [K+]plasma
↑ aldosterone secretion > ↑ Na+/K+ ATPase > ↑ K+ channel activity (& Na+ channel activity > ↑ K+ secretion
Furosemide
↓ activity of Na/K/2Cl cotransporter in thick ascending limb of Loop of Henle > ↓ K+ reabsorption, ↑ K+ excretion
aka “K+ wasting” diuretic
Bartter’s syndrome
Na/K/2Cl transporter is defective – characterized by hypokalemia
Thiazide
↓ activity of Na/Cl cotransporter in distal tubule > ↑ Na+ delivery to Principal cells in DT/CD > ↑ K+ secretion
aka “K+ wasting” diuretic
Giltman’s syndrome
characterized by hypotension (due to increase excretion of NaCl) and hypokalemia
Amiloride
↓ activity of Na+ channels in Principal cells of DT/CD > Na+ stuck in tubule opposes K+ efflux across apical surface into tubule > ↓ K+ secretion
“K+ sparing”
How much of body Ca++ is intracellular?
1%
mainly in ER and in mitochondria; sequestered in SR of muscles
How much of total Ca++ is extracellular?
0.1%
What does cytoplasmic Ca++ do?
functions in signaling, muscle contraction, NT release
How much Ca++ is filtered a day?
540 mEq Ca++/day
40% of plasma Ca++ is bound to protein & not filtered
Ca++ Reabsorption by Proximal Tubule
- 20% by apical Ca++ channels
- Ca++ leaves cell via Ca-ATPase and Na+/Ca++ antiporter in basolateral membrane
- 80% occurs by paracellular route (solvent drag, lumenal positive transepithelial potential)
Ca++ Reabsorption by Thick Ascending Limb of Loop of Henle and DT
-20% reabsorbed by thick ascending limb
mechanism similar to that in proximal tubule (no solvent drag)
-10% reabsorbed by distal tubule and CD
only transcellular reabsorption (uses ATP)
Where is the majority of Ca++ transport occur?
in the proximal tubule via paracellular route
↓ plasma Ca++ effect on calcitiol secretion
↑ calcitriol secretion > ↑ Ca++ reabsorption in gut and distal tubule
↓ plasma Ca++ effect on PTH secretion
↑ PTH secretion > ↑ bone resorption & Ca++ reabsorption by loop of Henle and distal tubule
↑ plasma Ca++ effect on calcitonin secretion
↑ calcitonin secretion > ↑ bone formation
PO43- Function and Distribution
buffer; in formation of bone
86% is in the bone, 14% intracellular
PO43- Reabsorption in the Proximal Tubule
- entry via Na+/PO43- cotransporter
- Crosses basolateral membrane via PO43-/anion antiporter
- Hormone changes respond to changes in proximal tubule reabsorption
↓ plasma PO43- effect on calcitriol
↑ calcitriol secretion > ↑ PO43- reabsorption in kidney and gut
↑ calcitonin effect on PO43-
↑ PO43- incorporation into bone, ↓ reabsorption in kidney
Organic anions secreted by renal tubule
endogenous- prostaglandins, uric acid
anionin drugs- penillin, salicylate, ibuprofin, adefovir (anti-HIV)
all of then are competing for the same transport system
Organic cations secreted by renal tubule
endogenous- epinephrine, norepinephrine
cationic drugs- morphine, amiloride, verapmil (Ca channel blocker), Vinblastine (anti-cancer drug)
Organic Anion Secretion in Proximal Tubule
- Anions taken up from blood by anion/α-ketogluterate antiporter (α-kg recycled via Na+ coupled transporter)
- Anions leave via Cl-/anion exchanger
- Cations enter from blood via passive transporters
- Cations enter renal fluid via cation/H+ antiporter and MDR-related transporter
- Competition between cationic drugs (i.e. morphine) can result in drug toxicity
Competition between PAH and Penicillin
Compete for secretion by organic anion transporter led to increased half-life of penicillin in circulation
MDR-related transporters
- Belongs to ABC family of transporters
- 2 ATP binding domains, 2 membrane domains w/ 6 transmembrane helices each
- Broad specificity for solutes transported
Receptor-mediated endocytosis of peptides and small proteins in Proximal Tubule
Peptides/small proteins not removed during filtration get taking up by endocytosis using megalin and cubulin receptors
Endocytosed materials brought to lysosome for degradation to AAs
Fanconi’s syndrome
Defect in receptors or v-ATPase in PT; protein in urine (proteinuria)