Renal Tubular Transport Flashcards
What are the basic mechanisms for
•Passive or “downhill” transport:
–Simple diffusion: “down” electrochemical gradient via lipid bilayer or aqueous channels
–Facilitated diffusion: “down” electrochemical gradient; specific carriers are required
What are the basic mechanisms for active transcellular solute movement?
Energy dependent uphill process.
primary active transport
secondary active transport.
Pinocytosis
–Primary active transport:
against electrochemical gradient; ATP hydrolysis provides energy
–Secondary active transport:
“downhill” movement of one substance provides energy for “uphill” movement of another substance
•Cotransport, countertransport
What percentage of filtrate does the proximal tubule reabsorb?
What is absorbed here?
•Proximal tubule reabsorbs 60-80% of the filtrate
–Most of filtered H2O, Na+, K+, Cl-, bicarbonate, Ca2+, phosphate
–Normally, all the filtered glucose, amino acids
What is secreted in the proximal tubule?
•Several organic anions and cations (including drugs, drug metabolites, creatinine, urate) are secreted in proximal tubule
What transporter is highlighted for the proximal tubular transport?
Na-K-ATPase
Are urea and Cl- secreted by the proximal tubule?
no
What facilitates the net unidirectional transport of Na+ in the PCT?
•Polarity of epithelial cell membranes facilitates net unidirectional transport
What powers the transport of Na+ in PCT reabsorption?
- Ultimately powered by Na+,K+ ATPase in basolateral membrane
- Na+ reabsorption is usually coupled to transport of or exchange for another solute
Na+ Reabsorption is Linked to what kind of transport?
Transcellular Transport
Paracellular Reabsorption of Cl- and Urea in Early PCT is not an active process. What does it depent on?
•dependent on Na+ and H2O reabsorption
In the early PCT, there are no Cl- transporter. As Na+ and water are reabsorbed, Cl- and urea become more concentrated in luminal fluid. What provides the driving force for paracellular reabsorption?
What else allows transport of Cl-?
–Modest concentration gradient between lumen and peritubular interstitium provides driving force for paracellular reabsorption
–There are specific Cl- channels (typically in the form of anion exchanger) in the later PCT that allows transcellular transport
What does the transcellular transport of Cl- lead to?
•This creates a slightly positive charge in the tubular fluid, which helps drive paracellular reabsorption of Ca, Mg, and K
Where are organic nutrients (glucose, amino acids) reabsorbed?
The PCT
How do the kidneys regulate plasma concentrations of glucose and A.A.’s?
They don’t. Thats the role of the liver and endocrine system
What is the Basic Mechanism of Tubular Reabsorption of Glucose & Amino Acids
•Secondary active transport; only transcellular pathways
•Uptake across luminal membrane:
–Against concentration gradient
–Coupled to Na+ entry down its electrochemical gradient
–Ultimately dependent on Na-K-ATPase
•Exit cells through basolateral membrane by facilitated diffusion
If filtered amount (load) of glucose (= GFR · Pglucose) exceeds a certain rate:
–Capacity of nephrons to reabsorb all the filtered glucose is exceeded (same with amino acid transport)
–Glucose appears in the urine (glucosuria)
–Osmotic diuresis!
•Can you identify a disease in which saturation of the Na-glucose cotransporters occurs?
Diabetes
Tubular handling of organic acids and bases is affected by pH of
of luminal fluid, H+ in the tubular lumen favors reabsorption of organic acids, but traps organic bases in the lumen
Characterize the descending limb of the loop of henle’s permeability.
•Descending limb is highly permeable to H2O, moderately permeable to solutes
Characterize the permeability of the ascending limb of the loop of henle
•Ascending limb is highly permeable to solutes, but impermeable to water
What is the first portion of the early distal tubule?
The next part is highly convoluted, what is it permeable to? Impermeable to?
macula densa
•Next part is highly convoluted and permeable to most ions, but is impermeable to urea and water (diluting segment)
Describe the permeability of the late distal tubule along with it’s secretion/reabsorption
•Impermeable to urea; reabsorbs Na+ and secretes K+, under hormonal influence; secretes H+ against a large concentration gradient; permeability to water is controlled by ADH
What cell types are found in the late distal tubule?
Principle cells
intercalated cells
What is the role of principle cells in the late distal tubule?
–Na+ reabsorption and K+ secretion
•K+ diffuses out of cell and into the tubular fluid
–Site of potassium-sparing diuretics
•They inhibit the stimulatory effect of aldosterone at this site
•Can also directly block sodium channels on the luminal membranes, decreasing the effectiveness of the Na-K pump
What is the role of the late distal tubules intercalated cells?
–Secrete H+ ions via H-ATPase transporter
What is the final site for processing of urine?
Medullary collecting duct
The medullary collecting duct is permeable to urea, and can secrete H+ ions against their concentration gradient. How is the permeability of water controlled here?
ADH
What are two factors promoting fluid movement into peritubular capillaries?
What is the consequence of this?
- High plasma colloid osmotic pressure
- Low hydrostatic pressure in these capillaries
•Consequence: almost as much fluid is reabsorbed as was initially filtered into Bowman’s capsule
Where does aldosterone work? what does it lead to reabsorption of? Secretion of?
Site of action: late distal tubule and collecting duct
Effects: Increase NaCl reabsorption
increase K+ secretion
also H+ secretion
Where does angiotensin II work? what does it lead to reabsorption of? Secretion of?
Site of action: proximal tubule, thick ascending loop of henle/distal tubule, collecting tubule
Effects: Increase NaCl reabsorption
increase H+ secretion
Where does ADH work? what does it lead to reabsorption of? Secretion of?
Site of action: distal tubule/collecting tubule and duct
Effects: Increase H2O reabsorption
Where does ANP work? what does it lead to reabsorption of? Secretion of?
Site of action: distal tubule/collecting tubule and duct
Effects: decrease Na+ reabsorption
Where does parathyroid hormone work? what does it lead to reabsorption of? Secretion of?
Site of action: proximal tubule, thick ascending loop of henle/distal tubule
Effects: decrease PO43- reabsorption
Increase Ca2+ reabsorption
