Cross PHYSIOLOGY 451 RENAL PHYSIOLOGY Dr. Michael Fill, Lecturer velcro

Maintaining salt (NaCl) and H 2 0 balance is a key function of the kidney. Na +, Cl - and H 2 0 are all freely filtered. AND….huge amounts of these are filtered. AND….most of what’s filtered is reabsorbed Note: Na, Cl & H 2 0 are not normally secreted Renal Handling of Sodium, Chloride and Water “the core of renal physiology” Some Generalizations: Na + Reabsorption is active, via the transcellular route and is powered by the basolateral Na-K-ATPase. Cl - Reabsorption is passive (paracellular) and active (transcellular). Regardless of route, it is always coupled somehow to Na + reabsorption. Indeed, parallel Cl - reabsorption is implied when describing Na + reabsorption. H 2 0 Reabsorption is by osmosis and secondary to reabsorption of solute, particularly Na + and those dependent on Na + reabsorption.

Overview of Na + Reabsorption along the Nephron 65% of filtered Na + is reabsorbed from the proximal tubule. 25% of filtered Na + is reabsorbed from the thick ascending limb. 5% of filtered Na + is reabsorbed from the distal tubule. 4-5% of filtered Na + is reabsorbed from the collecting duct.

Proximal Tubule: Na + Reabsorption Na + Reabsorption Stepwise: 1.Na-K-ATPase keeps intracellular Na level low. This means there is a gradient across apical membrane. 2.Filtered Na + is transported across apical membrane several ways. 3.Na + entering the cell is then moved across basolateral membrane. Important Points: Reabsorption of other solutes are linked to Na + reabsorption. Without the Na-K-ATPase, the Na + gradient that powers reabsorption of Na & other solutes would not exist.

Cl - reabsorption parallels reabsorption of Na +. Remember this diagram? A Helpful Concept: “Electronuetrality Rule” any volume of solution (no membranes separating stuff here) will have equal numbers of cations and anions. Thus, a solution with 140 mM Na +, will have 140 mM of anions. In the plasma, the most abundant anions are Cl - (~110 mM) & HCO 3 - (~24 mM). Proximal Tubule: Cl - Reabsorption

Two Routes of Cl - Reabsorption: 1.Paracellular 2.Transcellular

Two Routes of Cl - Reabsorption: 1.Paracellular Through “not so tight” tight junctions P assive, down electrochemical gradient Depends indirectly on Na + transport Most Cl- reabsorption via this route 2.Transcellular Proximal Tubule: Cl - Reabsorption

Complicated apical process which depends directly on Na + transport Apical transport is essentially Cl-Na-symport Basolateral transport via the Cl-K-symporter Two Routes of Cl - Reabsorption: 1.Paracellular Through “not so tight” tight junctions P assive, down electrochemical gradient Depends indirectly on Na + transport Most Cl- reabsorption via this route 2.Transcellular Proximal Tubule: Cl - Reabsorption

H 2 0 Reabsorption Overview H 2 0 Reabsorption is by osmosis and secondary to reabsorption of solute. Kidney’s must be able to “separate salt from H 2 0”. Obvious but important. If you drink excess H 2 0 (no salt), then your kidneys must excrete the excess H 2 0. If you eat excess salt (no H20), then your kidneys must excrete the excess salt. Evidence that the kidney’s do this is the body’s capacity to generate dilute or concentrated urine.

Comparison of H 2 0 and Na + Handling Four Significant Points : 1. Equal amounts of H 2 0 & Na + are reabsorbed from proximal tubule. 2. H 2 0 & Na + are both reabsorbed from loop of Henle, but from different parts of the loop. Overall, the loop reabsorbs more Na + than H Na + is reabsorbed from the distal tubule. H 2 0 is not. 4. Both Na + and H 2 0 are reabsorbed from collecting duct. The amounts of each are variable & controlled.

What defines when and where H 2 0 moves along the nephron? Answer: H 2 0 moves only down osmotic gradients ( no H 2 0 pumps here ) H 2 0 moves only if it can (a H 2 0 permeable pathway must exist) The Osmotic Gradients Proximal Tubule: Na + & Na-dependent solute reabsorption creates gradient Loop & Collecting Duct: High salt (NaCl) and urea levels in medulla provide gradient Possible H 2 0 Permeation Pathways H 2 0 may move through lipid bilayers, aquaporins or tight junctions. Basolateral membranes: always highly H 2 0 permeable because they contain a certain type of aquaporin. Apical membrane & tight junction: H 2 0 permeability vary along the nephron. Distal Tubule Zero “ Collecting Duct Low, but regulated“ Ascending Limb Zero “ Descending Limb High “ Proximal Tubule High Apical H 2 0 Permeability

Cortex Medulla High Solute Level What happens when H 2 0 permeability of collecting duct is very low? Dilute tubular fluid moves down collecting duct and remains dilute. H 2 0 Perm Low Tubular Fluid Dilute Here Overview of “Urine Concentration” Control

Cortex Medulla High Solute Level Result dilute urine What happens when H 2 0 permeability of collecting duct is very low? Dilute tubular fluid moves down collecting duct and remains dilute. H 2 0 Perm Low Tubular Fluid Dilute Here

Cortex Medulla High Solute Level H 2 0 Perm High Tubular Fluid Dilute Here What happens if H 2 0 permeability of collecting duct is high? High solute level in medulla means there is a large osmotic gradient that favors H 2 0 movement out. H 2 0 moves out of tubular fluid and the fluid becomes more concentrated. Overview of “Urine Concentration” Control

Preview of “Urine Concentration” Control Cortex Medulla High Solute Level H 2 0 Perm High Tubular Fluid Dilute Here What happens if H 2 0 permeability of collecting duct is high? High solute level in medulla means there is a large osmotic gradient that favors H 2 0 movement out. H 2 0 moves out of tubular fluid and the fluid becomes more concentrated. Result Concentrated Urine (H 2 0 was conserved) Key Points to Remember (so far): 1) H 2 0 is moving down osmotic gradient. 2) It only moves if there is a H 2 0 permeable pathway available. 3) How much H 2 0 moves will depend on…. - Gradient size - Degree of H 2 0 permeability.

Na +, Cl - and H 2 0 Handling Varies in Different Renal Segments Proximal Tubule … There is “iso-osmotic” reabsorption Loop of Henle … There is “separation of salt & H 2 0” Distal Tubule & … Reabsorption is “regulated” ( by hormones ) Collecting Duct

“Isosmotic reabsorption” from PROXIMAL TUBULE Proximal Tubule is in the cortex The interstitium of the cortex is iso-osmotic to plasma This figure was shown earlier. All Glucose Reabsorbed

What’s happening with Na + ? It’s concentration stays constant ! “Isosmotic reabsorption” from PROXIMAL TUBULE

Clearly, Na + is being reabsorbed. Na + reabsorption is what drives reabsorption of HC0 3 - and the nutrients. Na + concentration inside the tubule stays constant because H 2 0 is also reabsorbed along the tubule. (i.e. fluid volume decreases) VOLUME This is called…. “Isosmotic Volume Reabsorption” H 2 0 always follows solute….the main extracellular solute is Na +. When ever a little Na + moves, a little H 2 0 follows it. In other words, Na + & H 2 0 reabsorption keep pace….osmolarity & [Na + ] stay constant as tubular fluid volume decreases. What’s happening with Na + ? It’s concentration stays constant ! “Isosmotic reabsorption” from PROXIMAL TUBULE

Cl - is also being reabsorbed…. So…Why does Cl - level rise? VOLUME Cl - level rises because… Early on: HC0 3 - is the primary anion following the cations. Later : HC0 3 - levels drop & Cl - starts following the cations. Remember, Cl - is reabsorbed passively via the paracellular route. Cl - level levels out later along the proximal tubule. This means Cl - and H 2 0 reabsorption are matching each other later on in proximal tubule. What’s happening with Cl - ? It’s concentration rises ! “Isosmotic reabsorption” from PROXIMAL TUBULE

Important Fact: The Loop ( the loop overall ) always reabsorbs more Na + than H 2 0. This means that the fluid leaving the loop is always more dilute than the fluid that entered it. Same diagram as before: 1. Na + & H 2 0 reabsorption physically separated (in different loop segments) 2. H 2 0 reabsorbed from descending limb. 3. Na + reabsorbed from ascending limb (thick ascending limb) Loop of Henle Separates Salt & H 2 0 1st2nd3rd

1.Na + reabsorption powered by gradient generated by basolateral Na-K-ATPase. 2.There is a unique apical Na + transport. Na-K-2Cl symport 3.Na-K-2Cl symporter is target of a very common loop-diuretic (furosemide = lasix) 4.Na-K-2Cl symporter requires all 3 ions to operate. Apical K channel assures there will be lumenal K available to keep it going. Loop of Henle : Na + Reabsorption ( Thick Ascending Limb )

Like ascending limb of loop, distal tubule is not permeable to H 2 0 but Na + reabsorption occurs Distal Tubule : Na + Reabsorption 1.Na + reabsorption powered by gradient generated by basolateral Na-K-ATPase. 2.Another unique apical Na + transport. Na-Cl symport 3.Na-Cl symporter is the target of another type of diuretic (thiazide) 4.Note presence of apical Ca channels. Parathyroid hormone regulates these. (more later about this)

Two Types of Cells Present Principal cells (70% of total cells present) are specialized to handle Na + & H 2 0. Intercalated cells (2 subtypes called  /  or A/B) are specialized to handle Cl - & pH. 1.Again…Na + reabsorption powered by the basolateral Na-K-ATPase. 2.Another unique apical Na + transport a Na + channel (These are not like the Na channels associated with the AP.) 3.This Na + channel is regulated by the hormone aldosterone. 4.Several diuretics also target this Na + channel or its aldosterone regulation. (more about this later) Collecting Duct : Na + Reabsorption Now a Closer Look

Here is another view of the Principal Cell. 1.The H 2 0 permeability of collecting duct is inherently very low but can be very high if Antidiuretic Hormone (ADH) is present. (ADH = vasopressin) 2. ADH is a peptide hormone released from posterior pituitary. It acts on the apical membranes of principal cells. (again, more about this later) Collecting Duct : Na + & H 2 0 Reabsorption

First…Collecting duct is only site in the nephron where H 2 0 permeability is hormonally regulated. ADH increases H 2 0 permability. (normally its very low) Second…Collecting duct of inner medulla is a little bit different. Even in absence of ADH, it has some small amount of H 2 0 permeability. So, some H 2 0 will always be reabsorbed there. Third…ADH action is not all-or-none. A little ADH increases H 2 0 permeability a little. More ADH increases it more. Fourth…ADH increases apical H 2 0 permeability by stimulating fusion of aquaporin-containing membrane vesicles. In absence of ADH, these are withdrawn by endocytosis. Question #1: What happens to the tubular fluid if no ADH is present? Collecting duct H 2 0 permeability is low. A large volume dilute urine will be excreted. Collecting Duct : Na + & H 2 0 Reabsorption Important Points to Remember:

First…Collecting duct is only site in the nephron where H 2 0 permeability is hormonally regulated. ADH increases H 2 0 permability. (normally its very low) Second…Collecting duct of inner medulla is a little bit different. Even in absence of ADH, it has some small amount of H 2 0 permeability. So, some H 2 0 will always be reabsorbed there. Third…ADH action is not all-or-none. A little ADH increases H 2 0 permeability a little. More ADH increases it more. Fourth…ADH increases apical H 2 0 permeability by stimulating fusion of aquaporin-containing membrane vesicles. In absence of ADH, these are withdrawn by endocytosis. Important Points to Remember: Question #2: What happens to the tubular fluid when ADH is present? Collecting duct H 2 0 permeability is high. A small volume concentrated urine will be excreted. Collecting Duct : Na + & H 2 0 Reabsorption