Phys Chapter 27: Urine Formation by the Kidneys 2: Tubular Reabsorption and Secretion

Glomerular filtrate enters renal tubules, then flowsàproximal tubuleàloop of Henleàdistal tubuleàcollecting tubuleàcollecting ductàexcreted as urine

The rate at which things are filtered:

- Filtration = glomerular filtration rate x plasma concentration

Glomerular filtration and tubular reabsorption are usually more than urinary excretion

- So a small change in filtration or reabsorption, can cause a large change in urinary excretion

Glomerular filtration is pretty unselective, and almost all solutes in the plasma get filtered, except plasma proteins and things bound to them

Tubular reabsorption is very selective

- Some things, like glucose, are completely reabsorbed, while others are poorly reabsorbed, like urea

- So the kidneys can regulate secretion of each solute independent from others, in order to precisely control the body fluid composition

For a substance to be reabsorbed, it must first be transported across the tubular epithelial membranes into the renal interstitial fluid, and then through the peritubular capillary membrane back into the blood

- Reabsorption across the tubular epithelium into the interstitial fluid includes active or passive transport

- After absorption into the interstitial fluid, water and solutes are transported through the peritubular capillary walls into the blood by bulk flow (aka ultrafiltration), mediated by hydrostatic and colloid osmotic forces

- At the peritubular capillaries, there is a net reabsorptive force that moves the fluid and solutes from the interstitium into the blood

Active transport can move a solute against an electrochemical gradient, and requires energy

- Primary active transport – transport that is coupled directly to an energy source, like ATP

o Ex: sodium-potassium ATPase pump that works at most parts of the renal tubule

- Secondary active transport – transport that is coupled indirectly by an energy source, like an ion gradient

o Ex: reabsorption of glucose by the renal tubule

- Solutes can be reabsorbed by active and/or passive mechanisms

- Water is always reabsorbed by a passive osmosis, where water diffuses from an area of low solute concentration (high water concentration) to an area of high solute concentration (low water concentration)

Solutes can be transported through epithelial cells (transcellular route), or between cells (paracellular)

- Renal tubular cells are held together by tight junctions, like other epithelial cells

- Intercellular spaces lie behind the tight junctions, and separate the epithelial cells of the tubule

- Solutes can be reabsorbed or secreted across the cells through the transcellular pathway, or between the cells by moving across the tight junctions and intercellular spaces by way of the paracellular pathway

o Sodium moves by both routes, but most goes through the transcellular pathway

o In some parts of the kidney, especially the proximal tubule, water is also reabsorbed across the paracellular pathway

§ Substances dissolved in the water, especially potassium, chloride, and magnesium, are carried with the reabsorbed fluid between the cells

Primary active transport is important, because it can move solutes against an electrochemical gradient

- The energy for this active transport comes from hydrolysis of ATP by ATPase

- The ATPase can be membrane bound, and also part of the carrier that moves the solute across the cell membrane

- The primary active transporters of the kidneys are:

o Sodium-potassium ATPase, Hydrogen ATPase, Hydrogen-potassium ATPase, and Calcium ATPase

- Example of primary active transport: reabsorption of sodium across the proximal tubular membrane

o Sodium-potassium ATPase is on the basolateral side of the tubular epithelial cell

§ It hydrolyzes ATP to get energy it uses to transport sodium out of the cell into the interstitium

o At the same time potassium is transported from the interstitium to the inside of the cell

§ This keeps low sodium and high potassium in the cell, giving it a net negative charge

§ This active pumping of sodium out of the cell across the basolateral membrane, allows passive diffusion of sodium across the luminal membrane of the cell, from the lumen into the cell

· Two reasons why:

o There is a concentration gradient favoring sodium diffusion into the cell, because intracellular sodium is low, and tubular fluid sodium is high

o The negative intracellular potential attracts the positive sodium ions form the tubular lumen into the cell

- Active reabsorption of sodium by sodium-potassium ATPase happens in most parts of the tubule

- In the proximal tubule, there’s a brush border on the lumen side of the membrane, that increases the surface area

- The lumen surface also has carrier proteins that bind sodium, and release it inside the cell

o This is facilitated diffusion of sodium through the membrane into the cell

o These sodium carrier proteins are also important for secondary active transport of substances

- So net reabsorption of sodium ions from the tubule lumen back into the blood has three steps:

o Sodium diffuses across the lumen membrane (aka apical membrane) into the cell down an electrochemical gradient established by the sodium-potassium ATPase pump on the basolateral side of the membrane

o Sodium is transported across the basolateral membrane against an electrochemical gradient, by the sodium-potassium ATPase pump

o Sodium, water, and other stuff are reabsorbed from the interstitial fluid into the peritbular capillaries by ultrafiltration

Secondary active reabsorption – two or more substances interact with a carrier protein, and are transported together across the membrane

- As one of the substances, like sodium, diffuses down its electrochemical gradient, the energy released is used to drive another substance, like glucose, against its electrochemical gradient

- So secondary active transport doesn’t need energy directly from ATP or other phosphates

o The energy for it comes from diffusion of another substance down its gradient

- In the proximal tubule, carrier proteins in the brush border combine with a sodium, and an amino acid or glucose, at the same time

o This is so efficient, it removes basically all the glucose and a.a’s from the tubule lumen

- Once they get in the cell, glucose & a.a.’s leave across the basolateral membranes by diffusion, which is driven by ↑ glucose and a.a. concentrations in the cell from transport proteins

- Sodium glucose co-transporters (SGLT2 and SGLT1) are located on the brush border of proximal tubular cells, and carry glucose into the cell cytoplasm against a concentration gradient

o About 90% of filtered glucose is reabsorbed by SGLT2 in the early part of the proximal tubule (S1 segment), and the rest by SGLT1 in the later part of the proximal tubule

- On the basolateral membrane, glucose diffuses out of the cell into the interstitial space, through glucose transporter GLUT2 in the early proximal tubule (S1), and GLUT1 in the later part (S3)

- Transport of glucose against a chemical gradient doesn’t use ATP, but reabsorption of glucose depends on energy expended by the primary active Na+-K+ ATPase pump in the basolateral membrane

o Sodium-potassium ATPase allows for an electrochemical gradient for facilitated diffusion of sodium across the lumen membrane to be maintained

o This downhill diffusion of sodium into the cell provides the energy for simultaneous uphill transport of glucose across the lumen membrane

o So reabsorption of glucose is called secondary active transport, cause glucose itself is reabsorbed uphill against a chemical gradient, but it is secondary to primary active transport of sodium

- A substance is said to undergo “active” transport when any part of its reabsorption involves any primary or secondary active transport

- For glucose, secondary active transport happens at the lumen membrane, passive facilitated diffusion happens at the basolateral membrane, and passive bulk flow happens at the peritubular capillaries

Secondary active secretion-some substances are secreted into the tubules by secondary active transport

- This often involves counter-transport of the substance with sodium ions

o Energy from moving the sodium downhill, will be used to move the substance uphill

- Ex of counter transport: active secretion of protons coupled to sodium reabsorption at the lumen membrane

o Sodium-hydrogen counter transport moves sodium into the cell, and protons out

o Done by the Na+-H+ exchange protein in the brush border of the lumen membrane

o As sodium is carried into the cell, protons are forced out in the opposite direction, into the tubule lumen

Some parts of the tubule, especially the proximal tubule, reabsorb large molecules by pinocytosis

- A large molecule, like a protein, attaches to the brush border of the lumen membrane, and the membrane there then invaginates into the cell and pinches off to form a vesicle

- Once in the cell, the protein is digested into a.a’s, which are reabsorbed through the basolateral membrane into the interstitial fluid

- Pinocytosis requires energy, so it is considered a form of active transport

For most things that are actively reabsorbed or secreted, there is a limit to the rate at which the solute can be transported, called the transport maximum

- This limit is due to saturation of the specific transport systems for that solute

- Happens when the amount of solute delivered to the tubule (tubular load) exceeds the capacity of the carrier proteins and enzymes involved in transporting the solute

- Ex: glucose transport system in the proximal tubule

o Normally glucose isn’t in the urine because almost all of the filtered glucose is reabsorbed in the proximal tubule

o When filtered load exceeds the ability of the tubules to reabsorb glucose, glucose is lost in the urine

o Large increases in GFR and/or plasma glucose will increase filtered load enough that not all of it will be reabsorbed

o The threshold for glucose is the point where glucose first starts showing up in the urine

§ The threshold happens before the transport max of glucose is reached

o One reason for the difference between a threshold and transport max, is that not all nephrons have the same transport max for glucose, so some start excreting glucose before other reach their transport max

o The overall transport max for the kidneys is reached when all nephrons have reached their max capacity to reabsorb glucose

o The plasma glucose of a healthy person almost never is high enough to cause glucose to be excreted in the urine, even after a meal

o Diabetes can get plasma glucose levels high enough for glucose to show up in the urine

Some substances that are passively reabsorbed don’t have a transport max, because their rate of transport is determined by other things

- Things that determine their transport instead:

o The electrochemical gradient for diffusion of the substance across the membrane

o The permeability of the membrane for the substance

o The time that the fluid containing the substance remains within the tubule

§ This itself depends on the tubular flow rate

- This type of transport is called gradient-time transport

- Some actively transported things have characteristics of gradient-time transport

o Ex: sodium reabsorption in the proximal tubule

§ The main reason that sodium transport in the proximal tubule doesn’t have a transport max, is because other factors also limit the reabsorption rate besides max rate of active transport

§ In the proximal tubules, the max transport capacity of the sodium-potassium ATPase pump is usually way more than the actual rate of sodium reabsorption

· One reason for this is that a lot of the sodium transported out of the cell leaks back into the tubule lumen through the epithelial tight junctions

· The rate this backleak happens, depends on permeability of the tight junctions, and interstitial forces that determine rate of bulk flow

· So sodium transport in the proximal tubules follows gradient-time transport instead of max transport

· So the greater the concentration of sodium in the proximal tubules, the greater its reabsorption rate

· Also, the slower the flow rate of tubular fluid, the greater the % of sodium that can be reabsorbed from the proximal tubules

§ In the more distal parts of the nephron, the tight junctions of the epithelial cells are tighter, and transport much less sodium

· So here sodium reabsorption shows max transport

· Hormones like aldosterone can increase this max transport

When a solute moves out of the tubule by active transport, and into the renal interstitium, it creates a concentration difference that causes osmosis of water in the same direction the solutes went in, so from the tubule lumen to the renal interstitium

- Some parts of the renal tubule, especially the proximal tubule, are very permeable to water, and water reabsorption happens so fast that there is only a small concentration gradient for solutes across the tubular membrane

- A lot of osmotic flow of water in the proximal tubules happens through tight junctions of the epithelial cells, and through the cells themselves

o The tight junctions in the proximal tubules aren’t that tight, and allow water and ions to diffuse through them

- As water moves across tight junctions by osmosis, it can carry solutes with it, called solvent drag

- Since reabsorption of water, organic solutes, and ions, is coupled with sodium reabsorption, changes in sodium reabsorption significantly influence the reabsorption of water and solutes

- Starting at the loop of Henle and up to the collecting tubule, the tight junctions are way less permeable, and the epithelial cells have a decreased surface area

o Water can’t move through these tight junctions well, so almost no water is reabsorbed

o Antidiuretic hormone (ADH) greatly increases water permeability in the distal and collecting tubules, but not the loop of Henle, which always has low water permeability

- So water movement across the tubule epithelium can only happen if the membrane is permeable to water, regardless of the osmotic gradient

Reabsorption of chloride:

- When positively charged sodium is reabsorbed through the tubular epithelium cell, negative ions like chloride are transported along with sodium

o Sodium leaving the lumen leaves the lumen negatively charged compared to the interstitium, which causes chloride to diffuse passively through the paracellular pathway