Solute recovery

• The proximal tubule contains microvilli and the tight junction between the cells are much more porous than normal, which allows for some ions to get past.
• The kidney recover ions, water and small organic molecules such as amino acids and glucose through a process known as solute recovery.
• The primary active transporters include Na/K ATPase and H ATPase, which are the most common ones in the plasma membrane.
• The solute carrier family (SLC) proteins, which number around 300 types, are mostly co-transporters powered by established concentration gradient of Na
• Aquaporins, ion channels, and protein endocytosis receptors are also involved in solute recovery.
• Solute recovery in the proximal tubule requires energy because the filtrate and the plasma concentrations are at equilibrium.
• The primary active transporters are located in the basal side of the cell and use energy to form an imbalance, high K and low Na inside the cells, which can be used by co transporters on the apical side to drive resorption
• In the loop of Henle there is regulated potassium leakage into the lumen through the Renal Outer Medullary potassium channel (ROMK)
• SLC22 family includes:

  • organic cation transporters(OCTs)
  • organic anion transporters (OATs)
  • organic cation/carnation transporters (OCNTs)
• organic cation transporters(OCTs)

  • allows passive movement in either direction (so things go down-gradient), causing passive drift of cations into the basal membrane cells which are then actively are transported into the lumen
  • thing carried out include: endogenous amines (e.g., dopamine), drugs like cimetidine and morphine and antihistamines
• organic anion transporters (OATs)

  • uses Na/K ATPase to cause imbalance to drive influx of ⍺-ketoglucorate, which is used as an anti porter to drive anion transport into the basal membrane cell
  • the anions passively drift into the lumen, issues in this drift can cause issues as the anions can build up in the cells to a toxic level
  • transportes: methotrexate, furosemide, penicillin, etc.
• SLC5A1 transports glucose in 1:1 ratio and SLC5A2 transports in a 2:1 ratio
• Sodium resorption in the proximal tubule occurs through an anti-porter which transports a proton into the lumen in exchange
• Sodium resorption in the distal tubule occurs through a co-transporter which transports a chloride into the cell as well
• potassium recovery in the loop of Henle occurs through a co-transporter which transports a sodium and two chloride into the cell as well
• amino acids recovery in the proximal tubule occurs through a co-transporter which transports two chloride ions and a sodium into the cell as well
• glucose recovery in the proximal tubule and a little in the loop of Henle occurs through a co-transporter which transports a sodium into the cell as well
• Phosphate recovery in the proximal tubule occurs through a co-transporter which transports sodium into the cell as well
• the proton that is transported into the lumen through the sodium proton anti-porter, binds to bicarbonate in the lumen to form carbonic acid which is broken down into water and CO2
• CO2 formed in the lumen of the nephron passively flows back into the cell and react to form bicarbonate, which is then co-transported back into the plasma
• if there are protons remaining when the bicarbonate has been taken up, it means that the body is in acidosis
• a proton can react with phosphate and ammonia in the lumen, which results in the net excretion of protons
• There are 2 types of intercalated cells:

  • type A
  • type B
• Type A intercalated cells actively excrete protons out of the body and transports bicarbonate back into the plasma, and can be used to correct acidosis
• Type B intercalated cells resorb protons back into the body, and excrete bicarbonate out of the body, and can be used to correct alkalosis
• water is passively recovered through aquaporin and the concentration gradient created through Na^+ recovery systems
• Calcium recovery is mainly paracellular (between cells), passive and driven by osmosis once the urine has become more concentrated
• Protein uptake occurs in the proximal tubule cells and is driven receptor-mediated endocytosis for proteins using general receptors such as megalin
• ammonia/ammonium comes from the catabolism of glutamine which forms ammonia (passively go into the lumen) and ammonium (actively transported into the lumen with a sodium, proton co-transporter)