Lecture 26

Buffering Systems of the Kidney:

• Excretion: Done by the kidneys.  If we get a lot of H ions, we can excrete them in the urine to control the acidity of the kidney.  When we talked about the nephrons we talked about H transports that transported H+ into the lumen/filtrate
  
  • There are 3 transport mechanisms for H+.
    
    • 1.) Na+ in and H+ out Antiport – in the apical membranes of the Proximal Tubule and Thick Ascending Loop of Henle, which pushes H+ ions out to the lumen
      
    • 2.) In Intercalated Type A cells we have an ATP powered H+ pump in apical membrane, which pumps H+ ions out.
      
    • 3.) In the Intercalated Type A cells again, there is a minor ATP powered transport antiport that pumps K+ ions in and H+ ions out.
      
      • There is a great ability to kick out H+ ions into the filtrate for all 3 mechanisms
        
  • The urine has a pH limit of 4.4. It cannot go below this b/c the concentration gradient is too strong for the transports to work against the 4.4 pH
    
  • The plasma/blood is a pH of 7.4.
    
  • pH is a logarithmic function, and a one interval drop in pH is a 10-fold increase in the hydrogen ion concentration. It can only have about 1000 times the hydrogen ion concentration as the blood plasma. 7.4 to 4.4 is the 1000X difference in hydrogen ion concentration.
    
  • In the absence of buffering systems, the 4.4 pH would occur very quickly and the body would go into acidosis very quickly.
    
  • There are buffering systems for the urine in the renal nephron – the proof that these work is that normal urine pH is somewhere in the range of 5 or higher.
    

 
 

We have 3 tubular Urine Buffering Systems that work to maintain that, so we can get rid of all the H ions (for Urine pH):

1. Bicarbonate System
   

• Bicarbonate system (neutralizes H ions, but is how bicarb gets reabsorbed)
  
• In the Early Proximal Tubule when we had Na+ flooding out of this (via the Na/K pumps in the basolateral membrane) Na+ is not being followed by Cl, rather it is being followed by Bicarbonate, but bicarbonate couldn't cross the apical membrane nor could it go paracellular through the Tight Junctions, but this is how it is going to work (need a transformation)
  
• We are going to have Sodium Bicarbonate coming out in the filtrate, and it is being freely filtered, and thus the concentration of bicarbonate in the filtrate is approximately the same as the concentration of bicarbonate in the plasma.  Virtually all bicarbonate is reabsorbed. 
  
• So how this occurs is that is has to be transformed.   We take this bicarbonate and add the H ion (HCO3 +H+), this is going to give us à Carbonic Acid H₂CO₃ (this slowly dissociates to à H2O + CO2 (very slow step). However in the presence in the enzyme Carbonic Anhydrase (CA) this reaction of bicarbonate changing to H2O and CO2 will occur extremely quickly.  CA is exposed at the lumen surface at the apical membrane of the Proximal Tubule and at the apical membrane of the Thick Ascending Loop of Henle. Presenting CA at the surface of a cell is very unusual (we find it at these 2 points in the kidney, but we also find it in the capillaries of the lungs b/c at that point we want everything to turn to CO2 there so it leaves).
  
• The Bicarbonate is very quickly being changed to H2O and CO2. 
  
• If the CO2 stays in the water and leaves with the urine, then buffering happens.
  
• If the CO2 (bicarb has a change when getting across the membrane so it cant do it, but CO2 has no charge so it can enter into the cell and it can keep going through into the interstitium and go into the capillary and be lost in the lungs.
  
• If the CO2 gets lost in the lungs then it has buffered the urine – but most of the time that is not what happens (this system can buffer the urine in that way, but it generally doesn't happen that way) 
  
• We have Carbonic Anhydrase free floating inside the cell, and we also have H2O. 
  
• There is a very small amount of the water is always dissociating to a H ion and a Hydroxy group (OH) (it very quickly changes back) but a small amount of this is being made by the water all over your body. 
  
• If we take this CO2 and we add it to this Hydroxy group (OH), and in the presence of Carbonic Anhydrase, this will become a Bicarbonate. This is in the Proximal Tubule and Thick Ascending Loop of Henle.
  
• This is a reversible reaction and it depends upon the removal of these 2 things: 1.) the H ion and 2.) the bicarbonate. 
  
  • In other words if we can pull these away and get rid of the H ion and the Bicarbonate, we will pushes this in this direction.
    
• We already have the Na pump coming back in going into the lumen. 1Na+ and 3 HCO3 (Bicarbonate) Cotransport out – both Na and HCO3 being kicked back out to the interstitium – this is how bicarbonate is reabsorbed. 
  
• As soon as the bicarbonate is being produced it is being kicked out into the interstitium, and the H ion is being kicked out into the lumen, and this is how bicarbonate is reabsorbed.  It is absorbed into the cell as CO2, and it is changed back into bicarbonate where it gets kicked out into the interstitium. This is how bicarbonate can follow Na in the Early Proximal Tubule
  
  • The bicarbonate system would buffer if CO₂ is lost via respiration or urine. CO₂ is fairly soluble in water.
    

There are several transports for the bicarbonate to get through the basolateral membrane.

• 1.) This 1Na+ and 3 HCO3 Cotransport exists in Proximal Tubule and Thick Ascending Loop of Henle, but it only exists in the basolateral membrane
  
• 2.) In the Thick Ascending Loop of Henle and in the Intercalated Type A cells we have a Bicarbonate out and a Cl in Cotransporter.
  
• 3.) We also had a Potassium and Bicarbonate Cotransport found only in the Thick Ascending Loop of Henle taking out Bicarbonate into the interstitium
  
  • The Bicarbonate system not only does buffering, but it also kicks bicarbonate back out into the interstitium
    
• All 3 exist in the Thick Ascending Loop of Henle.
  
• This is how bicarbonate is reabsorbed
  
• There are transports to carry the bicarbonate out.
  
• The bicarbonate cannot cross using the paracellular route, it cannot cross the apical membrane.
  

 
 

1. The Phosphate system:
   

• P is not found in large amounts in the blood. It is reabsorbed in the Early Proximal Tubule.
  
• The original filtrate does not contain enough phosphate.
  
• The more the acidic it is (the more H ions we add into the solutions) the more the reaction is driven in the other direction = H₂PO₄ ß à HPO₄^- + H⁺
  
• If we draw a pK curve (a pK curve is how much of a specific ion is in which form at which pH's) the pK level is where it is 50:50 (the point of inflection)
  
• Our pK is roughly 6.8. When the filtrate came into the Proximal Tubule it was 7.4, as it made its way down, it was probably about pK of 7.0 when it left the Proximal Tubule. When we kick out our urine it is a pH of between 5 and 6. Not only do we have a low amount of phosphate, but the pK is against us because it will not function as a buffer, it will not pick up the H ions as easily. When we hit the Thin Descending Loop of Henle the water is going to concentrate the P and the H ions dramatically. By the time this all reaches the Distal Tubule, this is all concentrated enough and the pH is low enough to do major buffering. At the end of the Descending Loop of Henle you are getting some sort of buffering out of this.  H2O is not going to leave until you get to the Principal Cells.