Lecture 7: Tubular Fxn 2 Flashcards
Where does bulk reabsorption of Na + anions and water happen?
Proximal tubule
2/3!!
Where does fine regulation of NaCl, potassium and water homeostasis occur?
DCT
CNT/CD
What gets filtered in the thin descending limb?
Water
What gets filtered in the thin AL?
NaCl
What gets filtered in the thick AL?
25% Na + anions
Where does K secretion occur?
CNT/CD principal cell
What gets filtered in the DCT?
4-8% Na + anions
What gets filtered in CNT/CD?
1-3% Na + anions
What transporter defines the distal convoluted tubule?
Na-Cl cotransporter = NCC
Where does expression of DCT’s NCC begin?
Downstream of macula densa
Where is the NCC located on the DCT?
Apical side
What is the function of the NCC?
Reabsorbs 4-8% of filtered Na+
What is Gitelman’s syndrome?
Loss of function mutation of NCC
Impaired NCC = salt loss to urine
What drug targets NCC?
Thiazides diuretics
How is NCC regulated?
NCC is stimulated by low NaCL intake, AngII or sympathetic tone (beta receptors)
Inhibited by high NaCl intake, high potassium diet/ high [K] in plasma
Regulated by WNK kinases
What do CLC-KB (Barton) channels do?
Mediate chloride ion exit through basolateral side of cell into blood
What is the driving force for the NCC?
Na/K ATPase
How do WNK kinases regulated NCC?
Can inhibit NCC so that no sodium or chloride enter/exit cell - no potassium would be brought in
What is the significance of a lack of aquaporins in the DCT?
No water reabsorption - appropriate since DCT is part of diluting segment of nephron
What kind of cells line the DCT2/first part of IMCD and the second part of IMCD?
DCT2/first part IMCD = Principal cells (PC) and intercalated cells (IC)
Second part of IMCD = PC
What what do DCT2, CNT, and CCD function in?
Na, K, acid-base regulation
What does outer medullary CD (OMCD) participate in?
Acid-base in intercalated cells
What does IMCD participate in?
ANP regulates Na
ADH regulates urea
What do principal cells function in from CNT to IMCD?
ADH-regulated water permeability and reabsorption
What defines principal cells of DCT2/CNT/CCD?
Epithelial Na Channel (ENaC) @ apical side of cell
What drug targets ENaC?
Amiloride/triamterene will directly block ENaC
How do ENac and the Na-K ATPase work together in principal cells?
Generate lumen negative TEP
What does the lumen negative TEP accomplish?
K+ secretion (via ROMK)
H+ secretion (in IC)
Paracellular Cl- reabsorption (via Claudin-4, CL4)
What does aldosterone do in principal cells of DCT2/CNT/CCD?
Aldosterone regulates Na and K transport
Aldosterone stimulates:
Na-K ATPase
ENaC
ROMK
What effect does Aldosterone stimulation have in PC of DCT2/CNT/CCD?
Increases (by enhancing lumen potential)
Na and Cl reabsorption
K secretion
H secretion (via H ATPase in IC)
What are examples of aldosterone antagonists?
Spironolactone
Eplerenone
What happens if there is volume depletion, or high levels of K in plasma?
Volume depletion = stimulate AngII =
1) directly stimulate ENac and 2) increase Aldo to bind at Mineralocorticoid receptor and inhibit Nedd4
Both 1) and 2) enhance Na reabsorption
If high K in plasma:
Stimulate Aldo to bind MR and inhibit Nedd4
What does Nedd4 do in principal cells of DCT2/CNT/CCD?
Nedd4 regulates ENaC by ubiquitylating and internalizing the receptor to downregulate it
Inhibited by Aldo
What is Liddle’s syndrome?
Gain of fxn mutation in ENaC = can’t get ubiquitinated by Nedd4 to internalize it
Results in hyperreabsorption of sodium = mono genetic form of HTN
What are AS channels in principal cells of IMCD?
Amiloride-sensitive (AS) sodium channels @ apical end of IMCD cells
What inhibits AS channels?
Atrial natriuretic peptide (ANP)
What effect does ANP have?
Inhibits AS
Increases GFR
Inhibits renin release
Increases natriuresis
How does ANP work?
ANP released from heart/atria in response to high volume/distension
Stimulates GPCR = cGMP from GTP inhibits AS
What are effects of ENaC inhibition?
Increased Na excretion
Increased Cl excretion (since it drives reabsorption normally)
Retains H+ = acidosis
Retains K+ = hyperkalemia
What do Type A interacalated cells do?
Acid secretion
Bicarbonate reabsorption
Where are Type A intercalated cells expressed?
DCT2 to initial IMCD
Strongly in OMCD
More cells pop up with acid load
What are steps in acid secretion and bicarbonate reabsorption in type A intercalated cells?
Formation of H+ (from water) and bicarbonate (from CO2)
Bicarbonate returns to blood via basolateral Cl-HCO3 exchanger called AE1
H+ gets secreted into tubular fluid by H-ATPase and H-K ATPase
Ammonia gets secreted by RhC glycoprotein to form ammonium when combined with secreted H+ - this traps ammonia in urine!
What is the primary mechanism to match acid excretion to acid loads?
Hydrogen ion excretion as ammonium
What do type B intercalated cells do?
Bicarbonate secretion and acid retention
Where are type B intercalated cells found?
CNT to CCD
Concentrated in CCD
More cells pop up with metabolic alkalosis
What are steps in bicarbonate secretion and acid retention in type B cells?
H+ and bicarbonate formed within cell
H+ returns to blood via basolateral H-ATPase
luminal bicarbonate secretion occurs by Cl-HCO3 exchanger called Pendrin
Describe the “plasticity” of Type A and Type B intercalated cells
Have ability to convert to type A or B depending on what the body needs
What are non A non B intercalated cells involved with?
Where are mainly located?
Aldosterone/AngII stimulated chloride reabsorption
Mainly in CNT
How do non A non B intercalated cells mediate Cl reabsorption in an acid/base neutral way?
Apical Pendrin (Cl/HCO3 exchanger) and H-ATPase
Is Pendrin sodium independent or dependent?
Sodium independent
What are things that bring potassium into cell?
Hormones: insulin, beta2 adrenergic agonists
Alkalosis
What transporter is responsible for the high ratio of intracellular to extracellular potassium?
Na-K ATPase
What is a consequence of chronic kidney disease and potassium handling?
K+ retention leads to colonic K+ secretion if kidneys start failing
How does dietary K+ intake influence potassium reabsorption and secretion?
No effect on proximal reabsorption/ loop of Henle reabsorption - implies not regulated there
Dietary K+ regulates transport of potassium DOWNSTREAM of macula densa
How do ENaC and Na-K ATPase work together for potassium secretion by principal cells?
ENaC and Na-K ATPase generate strong lumen negative TEP, which facilitates K+ secretion into urine
By what mechanisms is K+ secreted into urine?
ENaC brings sodium into cell, Na/K ATPase exchanges Na for K into cells, which exit via:
Primary:
ROMK
Secondary: BK channel (also in IC cells)
How is K+ secretion increased?
High plasma potassium, Aldosterone
High luminal Na delivery
How is K+ secretion inhibited?
Low plasma concentration of potassium or aldosterone (since they inhibit Nedd4)
Aldosterone antagonists
ENaC inhibitors
low luminal Na delivery
Increase and inhibition of NCC have what effect on potassium secretion by principal cells?
Inhibition of NCC = increase Na delivery for downstream ENaC/ROMK K+ secretion
Stimulation of NCC = decrease Na+ delivery downstream
What are examples of factors that increase Na delivery downstream by inhibiting NCC?
Thiazides diuretics and Gitelman’s syndrome inhibit NCC => lower BP, hypokalemia
High K+ diet or plasma [K+] inhibit NCC
What factors decrease Na+ delivery downstream by stimulating NCC?
Low K+ diet or plasma [K+]
Familial hyperkalemic HTN mutations in WNK4/WNK1 kinases
Describe the Aldosterone paradox -
Why not kaliuretic (potassium excretion in urine) when triggered by low NaCl diet or volume loss?
Co-increase in Ang II enhances NCC activity and limits sodium delivery for K secretion
This means less driving force for K secretion despite having Aldosterone onboard
How is NCC a determinant of potassium secretion?
Regulates sodium delivery to lumen of cells downstream in DCT2, CNT/CCD
Describe potassium reabsorption in type A intercalated cells
Luminal H-K ATPase @ apical side of cell reabsorbs K+ from urine
What keeps H-K ATPase active in type A intercalated cells for potassium reabsorption?
Constitutive activity
Stimulation by low [K+] in plasma/diet
What suppresses ADH?
Small reduction in plasma osmolality
Large increases in plasma volume/pressure
EtOH
What is diabetes insipidus?
Impaired ADH responsiveness
What factors increase ADH or make it active?
Small increase in plasma osmolality
Large reduction in plasma volume/pressure
High circulating AngII levels
Where does ADH enhance water permeability?
Principal cells of CNT to IMCD
Describe steps of ADH enhancing water permeability in principal cells
ADH binds basolateral V2 receptor (Gs coupled GPCR = activates PKA)
PKA phosphorylates aquaporin-2 and inserts into apical membrane
ADH also increases aquaporin-2 synthesis
Constitutive basolateral aquaporin-3
What are aquaretics?
V2 receptor antagonists - primarily excrete water to dilute urine
Describe urea recycling in nephron
Urea comes in from GFR (about half reabsorbed in PT)
Urea also comes in from vasa recta via UT-A2 transporter
Urea that gets pumped back into medullary interstitium can then recycle back to enter via UT-A2 again
What is required for urea recycling?
ADH
Why is ADH-induced urea transport important?
Urea helps maximize concentration of urine
With ADH present, when urea/water travel down CNT/CD, water will get pumped out and urea gets concentrated
What are the key structures for generating and maintaining a cortico-medullary osmotic gradient?
Medullary loop of Henle
Vasa recta
Both form a countercurrent multiplier system
What structure maintains the medullary gradient?
Vasa recta act as countercurrent exchangers
Describe vasa recta’s permeability to water and solutes
Permeable to NaCl, urea and water:
Water diffuses out of descending vessels and bypasses the medulla
Solutes move out of ascending vessels and recirculate in medulla
How can the gradient in the vasa recta get washed out?
Large blood flow
Osmotic diuretics - retain water in tubules and in descending vasa recta (traps water and drags it down medulla which messes up solute concentration building down there)
What does intact urine concentration require?
ADH
V2 receptor
Aquaporin-2
Describe diabetes insipidus
(1) central -
Due to impaired generation of ADH; respond to exogenous ADH
(2) nephrogenic - Sometimes inherited (mutations in V2 receptor or AQP-2); more frequently acquired (ex: lithium impairs urine concentrating ability)
What is desmopressin?
Exogenous ADH
Correlate each with distal nephron segment (DCT, CNT/CD) and cell type:
A) sodium reabsorption by a channel
B) sodium reabsorption by a channel regulated by ANP
C) NaCl reabsorption by a cotransporter
D) K reabsorption, naming the apical transporter
A) Principle cells in DCT2/CNT > CCD
Epithelial Na Channel (ENaC)
B) Principle cells of IMCD = Amiloride-sensitive (AS) channel
C) DCT = Na-Cl cotransporter (NCC)
D) OMCD»_space; DCT2 to IMCD = H-K ATPase in type A intercalated cells
Correlate each of the following with DCT, CNT/CD and cell type:
H) bicarbonate secretion, naming apical transporter and the clinical condition in which this transport is important
I) water reabsorption in the presence of ADH, naming the channel
J) Urea reabsorption, naming the transporter and the hormone that increases its activity
H) CNT/CCD (Type B cells and non A non B cells) = Pendrin - metabolic alkalosis
I) CNT/CD (principal cells) = Aquaporin2
J) IMCD cells in CD = UT transporter; stimulated by ADH
Identify the transporters that are
(A) inhibited by thiazide diuretics
(B) inhibited by triamterene and amiloride
A) NCC
B) ENaC
For aldosterone:
(A) list two factors that stimulate secretion
(B) Name aldosterone receptor
(C) explain how it affects Na and K transport in CNT/CCD - identifying transporters and enzymes that are stimulated by aldosterone
(D) name a drug that antagonizes action of aldosterone
(A) AngII (also from volume depletion) or high plasma [K+]
(B) Mineralocorticoid receptor (MR)
(D) spironolactone, eplerenone
(C) Aldo stimulates MR and inhibits Nedd4. Aldo stimulates Na-K ATPase, ENaC and ROMK, resulting in increases of: Na reabsorption, K secretion, Cl reabsorption, H secretion (in intercalated cells)
Name two solutes secreted by type A intercalated cells (ICs) and one solute reabsorbed by them
Name primary mechanism that matches acid excretion to acid load
Secreted = H+, NH3 Reabsorbed = bicarbonate
Primary mechanism that matches excretion to load = hydrogen ion excretion as ammonium
Name two apical transporters in non-A non-B cells
Identify the main function of these cells
Apical transporters =
H-ATPase
Pendrin (HCO3 secretion, Cl reabsorption)
Function: CL- reabsorption in acid base neutral way
Given a typical dietary intake of 100 mEq of potassium/day, predict urinary excretion
Name a hormone that regulates potassium excretion and explain relationship between CKD, hyperkalemia and colonic potassium secretion
approx. 92 mEq excreted
CKD = potassium retention leads to colonic secretion in order to maintain balance
Aldosterone regulates potassium secretion
Describe the overall potassium transport in the distal nephron, naming the major cell types and tubular segments that are most responsible
Secretion in DCT, CNT and CCD = principal cells and ROMK/BK channels
*BK channels also in IC cells
Reabsorption in OMCD = type A IC cells via H-K ATPase
Predict and briefly explain the effect of amiloride and thiazide diuretics on potassium secretion
Thiazides diuretics =
Inhibition of NCC = less Na delivery = less K secretion = hypokalemia
Amiloride/triamterene = block ENaC = less K secretion via ROMK = hypokalemia
