Physio Renal Transport I (38)

• Na⁺
• Cl^-
• HCO₃^-
• Water
• the kidneys filter an enormous amount of water each day: ~180 L daily
• more than 99% of that filtered volume is reabsorbed as the fluid makes its way through the renal tubule
• water, Na⁺, Cl^-, HCO₃^- & 100% of filtered glucose & AAs are reabsorbed
• 
• unless any of these substances are in excess, none should be present in the urine

1. to remove waste products

2. to maintain solute & water homeostasis - the kidneys have to be able to balance excretion w/ intake

- by filtering this large volume & reabsorbing MOST of it, a minuscule change in the fraction of substance reabsorbed can drastically alter the excretion of that solute or water

• K⁺
• the kidney has the ability to selectively secrete K⁺, however MOST of it is still reabsorbed

• a breakdown product of proteins that is also an important osmolyte in the interstitial fluid of the kidney
• is important for generating a high urine osmolality

• the Apical surface of tubular epithelial cells corresponds to the INSIDE of the renal tubule, aka where the tubular fluid flows to eventually become the urine

• the Basolateral surface of tubular epithelial cells corresponds to the interstitial fluid of the medulla, where peritubular capillaries lie to pick up water & solutes reabsorbed during nephron processing

• reabsorption occurs from the tubular lumen → through the tubular epithelial cells → into the interstitium → then finally back into the blood

• 1. Passive/Facilitated Transporters: move solutes passively down their concentration gradient
• - eg. Glucose Transporter in the basolateral membrane of renal epithelial cells

• 2. Cotransporters (Symporters): move solutes in the same direction, 1 up & 1 down its concentration gradient
• - eg. Na⁺/Glucose cotransporter in the apical membrane; couples the downhill movement of Na+ into epithelial cells w/ the uphill movement of glucose (again) into epithelial cells

• 3. Antiporters: move solutes in opposite directions, again 1 up & 1 down its concentration gradient
• - eg. Na⁺/H⁺ antiporter in the apical membrane; couples the downhill movement of Na⁺ into epithelial cells w/ the uphill movement of H⁺ out into the proximal tubule lumen

• b/c of the Na⁺/K⁺ ATPase
• a transporter in the basolateral membrane of all cells in the body (not just the proximal tubule) that actively transports Na⁺ OUT of the cell in exchange for K⁺ into the cell
• the driving force for this process is ATP hydrolysis - it’s what supplies the gradient for all those previously mentioned coupled transporters

• invagination of a region of the plasma membrane to capture material in the extracellular space & internalized it within an endosome
• that material can then be trafficked to lysosomes for degradation & reutilization
• this is important in the kidney on the apical epithelial cell membrane for the uptake of PEPTIDES & PROTEINS that have somehow gotten through the filtration barrier & are now in the tubular lumen

• the movement of solutes & water THROUGH epithelial cells & then into the interstitial fluid
• 

the movement of solutes & water BETWEEN cells through tight junctions where epithelial cells adhere to each other (again from the lumen to the interstitial space)

• when solutes are reabsorbed by moving with water taken from the renal tubule into the interstitium via a paracellular route
• happens in certain places but DOESN’T happen in others

• 67% (about 2/3rds)
• such reabsorption occurs in an iso-osmotic manner b/c reabsorption of water follows uptake of solutes closely (solutes are absorbed → they create an osmotic gradient → this new gradient facilitates water reabsorption)

• they are the SAME
• even though all these solutes are being reabsorbed in the proximal tubule, there’s no change in osmolarity b/c water is following the ions out
• tubular osmolarity stays the same throughout the proximal tubule

• the Na⁺/K⁺ ATPase in epithelial cells’ basolateral membrane
• it maintains a low cytoplasmic Na⁺ concentration (~10 mM) in the cytoplasm of tubular cells relative to the extracellular concentration which is typically ~140 mM (value for plasma)

• the most important transporter for Na⁺ reuptake in the 1st half of the proximal tubule is the apical Na⁺/H⁺ Antiporter

• it moves Na⁺ into tubular epithelial cells (↓ its concentration gradient) coupled to H⁺ efflux

• Na⁺ is then basolaterally pumped OUT of the cell into the interstitium by the Na⁺/K⁺ ATPase

HCO₃^-

• to maintain pH homeostasis - it facilitates bicarbonate reabsorption

• effluxed protons combine w/ bicarbonate in the tubular lumen to form carbonic acid

• that carbonic acid is then converted into water & CO₂ by carbonic anhydrase- by generating CO₂, a ‘gaseous form of acid’ can now diffuse into the tubular epithelial cell across the apical membrane

• • in the cell a cytoplasmic carbonic anhydrase converts it & water back into H₂CO₃
• • H₂CO₃ then dissociates into protons & bicarbonate

• recycled H⁺ has been used to convert luminal bicarbonate 1st into carbonic acid then CO₂

• that CO₂ diffuses across the apical membrane where it’s reformed into H₂CO₃ INSIDE the cell

• H₂CO₃ can then dissociate into HCO₃^- & H⁺ (which continue to promote the cycle)

• the protons here are CYCLING, there’s no net acid secretion in the proximal tubule - simply using acid as a way to reabsorb bicarbonate

• it’s transported into the interstitium/back into the blood via a basolateral Cl^-/ HCO₃^- exchanger
• (similar to BAN3 that carries out Cl^-/ HCO₃^- exchange in RBCs)
• another mechanism is via the Na⁺/HCO₃^- cotransporter
• 

• diuretics
• if you block bicarbonate reabsorption, more Na⁺ tends to stay in the renal tubule (to maintain electroneutrality)
• the retention of solutes in the lumen KEEPS water in the tubule lumen → more fluid stays in the urine → hence it’s a diuretic

• Na⁺ uptake is coupled to & drives the reabsorption of NUTRIENTS: glucose, sugars, AAs

• • happens via a series of Na⁺ couple cotransporters in the apical membrane
• - the downhill movement of Na⁺ into the cell is driving the uphill movement of sugars/AAs into the cell (against its concentration gradient)

• then these nutrients exit the cell via passive transporters on the basolateral side of the epithelial cell

• caused by a genetic defect in an apical Na⁺ coupled amino acid transporter involved in cysteine transport

• in this disease cysteine & other AAs stay in the tubular lumen & can form SALTS w/ Ca²⁺

• those Ca²⁺ salts aggregate, precipitate, & lead to the formation of kidney stones

• in this case there is a direct relationship between the absence of a Na⁺ coupled transporter & the appearance of kidney stones in a patient

• the concentration of substances like sugars, lactate, AAs, & bicarbonate are decreasing

• the concentration of Na⁺ remains constant b/c its uptake is happening iso-osmotically: as Na⁺ is being reabsorbed, H₂O follows along behind it

• b/c there isn’t much Cl^- reuptake in the 1st half of the proximal tubule, it’s concentration actually increases somewhat: this is important in the later generation of a transepithelial potential

• here there is also an apical Na⁺/H⁺ Antiporter

• however now secreted protons aren’t combining w/ bicarbonate but instead w/ other anions like formate

• so effluxed H⁺ combines w/ small charged anions (formate) to form formic acid, which then diffuses back into the cell

• formate is the the unprotonated charged form

• formic acid is the protonated uncharged form

• • formic acid is actually small enough to DIFFUSE across the apical epithelial cell membrane into the proximal tubular epithelial cells
• 

• once formic acid has diffused into the renal epithelial cell, it can dissociate into it’s charged form formate (HCOO⁻) & a proton (H⁺)

• this formate can then be used by an Antiporter in the apical side of epithelial cells that couples the efflux of formate back into the tubular fluid w/ the reabsorption of Cl^-

• there’s a lot of Cl^- in the tubular fluid that needs to be reabsorbed & this represents a TRANScellular mechanism of Cl^- reuptake

• once inside the cell, Cl^- can be transported out the basolateral side via a K⁺/Cl^- cotransporter

• as a result of Na⁺ & H₂O being reabsorbed in the 1st part of the proximal tubule, Cl^- concentration in the tubular lumen increases

• that provides a driving force for Cl^- to move from the lumen into the interstitium paracellulary

• • this movement of a negative species creates a transepithelial potential between the lumen & interstitial space
• - this isn’t the same thing as a transmembrane potential; this gradient is much smaller

• however it still provides a significant driving force now for the uptake of additional CATIONS paracellularly

• the uptake of Cl^- ions leaves the tubular fluid more positively charged

• that transepithelial potential helps drives the uptake of Na⁺, K⁺, Ca²⁺, Mg²⁺, any cationic species can sense this potential & MOVE toward the interstitium

• *the Cl^- diffusion potential (via a transepithelial gradient) is what promotes paracellular movement of cations from the tubular lumen into the interstitium

1. Thick Ascending limb of the loop of Henle

2. Collecting Duct

in these 2 places the molecular bases of the transepithelial potential is less clear (unlike in the proximal tubule, where it’s clearly due to increased Cl^- concentration & then subsequent movement)

• 25% of Na⁺ & Cl^-
• 23% of water
• this reabsorption is happening sequentially (instead of together like in the proximal tubule)

• thin Descending limb: highly permeable to water but impermeable to ions

• thin Ascending limb: impermeable to water but permeable to Na⁺ & Cl^-

• thick Ascending limb: again ion but no water reabsorption

• as the tubular fluid makes it’s way down the thin descending limb, it’s surrounded by an interstitium that has a HIGH osmolarity (starts at 300 mOsm, can get as high as 1200 mOsm at the bottom of the medulla)

• there’s a HUGE osmotic gradient between the lumen & the interstitium

• to equilibrate this gradient, either salt or water could move; b/c the descending thin segment is IMPERMEABLE to salt, water moves OUT of the tube into the interstitium

• therefore at the tip of the Loop of Henle, the osmolarity in the tubular lumen is equal to the interstitial osmolarity (~1200 mOsm)

• impermeable to water, but permeable to salt (solutes)

• • as the fluid makes its way back up the loop of Henle, to equilibrate the osmolarity salt moves out of the tube into the interstitium
• - higher up in the medulla the interstitium has a lower & lower osmolarity (decreases from 1200 mOsm back to ~300 mOsm)

• the Na⁺/K⁺/2Cl^- Cotransporter in the Apical membrane (facing the tubular lumen)
• * is the most important transporter in the thick ascending limb
• it couples the downhill movement of Na⁺ (downhill b/c of the basolateral Na⁺/K⁺ ATPase) to the uphill movement of K⁺ & Cl^- all into the epithelial cells
• 
• (there’s also a Na⁺/H⁺ antiporter)

• a 1200 AA protein containing 12 transmembrane segments & 2 large cytoplasmic domains that’s driven by the (ATPase facilitated) Na⁺ gradient
• it allows for K⁺ & Cl^- reabsorption
• 
• 

• Furosemide, a diuretic
• by blocking this transporter, more ions (solute) get left in the tubular lumen which means more water is retained there as well → more fluid ends up in the urine

• Bartter’s syndrome, which is characterized by hypokalemia
• the reason low K⁺ levels occur is b/c this transporter provides an important route of K⁺ reabsorption

YES

• same as in the proximal tubule, the sign of the transepithelial potential is luminal positive relative to the interstitium; this helps DRIVE cation reuptake from the tubular lumen

• * important difference between the thin/thick ascending limb & the proximal tubule is that they’re IMPERMEABLE to water; there can be NO solvent drag here

• what IS happening at the epithelial tight junctions is like electrophoresis → ions pass through w/o being carried by water

• 7% of the filtered Na⁺ & Cl^-
• a VARIABLE amount of water depending on the amount of ADH (Antidiuretric Hormone)

• High ADH→ high water permeability → extensive reabsorption of water → concentrated urine
• Low ADH→ low water permeability → extensive excretion of water → dilute urine

• • Na⁺/Cl^- Cotransporter in the apical membrane
• allows downhill movement of Na⁺ into the cell (driven by the Na⁺/K⁺ ATPase facilitated gradient) & secretion of Cl^- into the tubular fluid

• * the early distal tubule is IMPERMEABLE to water - there is continuous salt reabsorption but no water is taken out of the tubular fluid at this point

• the fluid in the tubular lumen therefore becomes hypOosmolar (has a lower osmolarity than plasma, ~50 mOsm)

• Thiazide, a diuretic
• by blocking this transporter, more salt stays in the tubular lumen which means more water is retained there as well → more fluid ends up in the urine

• 1. Principal Cells: on top
• 2. Intercalated Cells: on the bottom

• 1. Na⁺ Channels
• - allow Na⁺ to move down its concentration gradient into epithelial cells

• 2. K⁺ Channels
• - critical for maintaining homeostasis: are the primary route by which the kidney can secrete K⁺

• 3. Water Channels, aka Aquaporins
• - are present in vesicles beneath the apical membrane & moved from these vesicles to the apical membrane in response to ADH

• occurs when a patient either fails to make or respond to ADH, meaning they’re unable to regulate water reabsorption
• they can’t increase the water permeability of the CD so a very dilute urine is being generated constitutively

• up to 10%
• that means if the principal cells of the late distal tubule & CD are NOT reabsorbing water, 10% of that filtered water will be lost in the urine
• b/c 180 L of fluid is filtered a day, an extra 18 L will be lost via the urine a day
• a patient w/ uncontrolled diabetes insipidus is urinating 18 L a day (10x a typical volume) - the only way they’re avoiding death by dehydration is by consuming massive amounts of water

• Amiloride
• keeps Na⁺ in the renal tubule → water stays there → diuretic

• Liddle’s Syndrome (*gain of function mutation)
• the mutated channels excessively reabsorb Na⁺, which causes ↑ BP
• is characterized by HYPERactivity of these Na+ channels
• symptom of Liddle’s syndrome = HTN, a direct consequence of excessive Na⁺ reabsorption in the principal cells

• α- & β-intercalated cells
• these cells are specialized for Acid Secretion
• they have in their apical membrane an ATP-drive ion pump, the Vacuolar ATPase (V-ATPase)

• transports protons from the epithelial cytoplasm into the tubular lumen
• is driven by ATP hydrolysis
• is critical in maintaining pH
• 

• Renal Tubule Acidosis
• patients are unable to secrete a sufficient amount of acid into the urine & as a result the plasma becomes overly acidic

• they secrete base (bicarbonate) across the basolateral membrane
• cytoplasmic carbonic anhydrase combines CO₂ & H₂O to form H₂CO₃, which dissociates into H⁺ & HCO₃^-
• bicarbonate is secreted across the basolateral membrane & protons are secreted across the apical membrane, maintaining the cytoplasms neutral state

• it is a luminal NEGATIVE potential
• as a result paracellular Cl^- reabsorption occurs

• 
• PT: proximal tubule
• TAL: thick ascending limb
• DT: distal tubule
• CD: collecting duct

• increased plasma osmolarity (more so than decrease BP or volume)
• as a result it only affects WATER reabsorption, not salt reabsorption
• does so by controlling aquaporins in principle cells of CD

• designed to prevent large changes in Na⁺ excretion when GFR changes

• when there’s uptake of water/solutes into epithelial cells from the tubular lumen & then they diffuse out into the interstitium, most of those substances are then going back into the plasma

• movement of water (& solutes) into blood is favored by P_i & π_c but opposed by P_c & π_i (hydrostatic/oncotic forces)

• hydrostatic pressure in the interstitium (P_i) favors fluid movement into the capillary
• hydrostatic pressure in the peritubular capillary (P_c) retards fluid movement into the capillary

• oncotic pressure in the interstitium (π_i) retards fluid movement into the capillary
• oncotic pressure in the peritubular capillary lumen (π_c) favors fluid movement into the capillary

• the concentration of proteins in the plasma leaving nephrons via the efferent arterioles will increase (b/c more of the fluid has been filtered)

• as a result of that increased protein concentration, there will be a higher oncotic pressure in the capillary lumen (↑π_c)

• that ensures that more reabsorbed water REENTERS the peritubular capillary (will be pulled in to dilute that ‘high’ solute concentration)

• • this phenomenon helps balance out what could be a large fluid loss as a result of increased GFR
• • (extrinsic factors can override this: ADH, ANG II, Aldosterone, NE)