MadSci Network: Medicine

Re: What is co-transport of sodium in the kidney

Date: Mon Mar 19 10:38:11 2001
Posted By: Joshua Chai, Medical student, Medical Sciences, University of Cambridge
Area of science: Medicine
ID: 983890703.Me

There are transporter "pumps" to transport  specific compounds across the 
plasma membrane in almost every cells in the body. They can either make 
use of the energy derived from ATP hydrolysis or depend on the so-
called "co-transportation" mechanisms to "pump" ions or proteins against 
their electrochemical gradients. In case of the renal tubular epithelial 
cell, the co-transporters use energy derived from ONE ion running DOWN the 
electrochemical gradient to power another UPHILL transport so that the ion 
concerned can be concentrated on one side of the membrane. If the 
transport is in the same direction it is called symporter, otherwise it is 
termed antiporter. Notice that in co-transportation, the electrochemical 
gradient of the species running downhill (usually hydrogen ions or sodium 
ions) has to be eventually maintained by another counter-acting active 
tranport mechanism, usually by means of the sodium-potassium ATPase, so 
often it is also called secondary active transport. Just to give an 
example, in the luminal membrane of the epithelial cells in the renal 
proximal convoluted tubule, there is a symporter for glucose and sodium co-
transportation, i.e. glucose and sodium are being transported into the 
epithelial cell in the same direction. Quite obviously, glucose is 
transported actively against the concentration gradient using the energy 
derived from sodium running down its gradient. The electrochemical 
gradient of sodium, on the other hand, is maintained by the sodium-
potassium ATPase in the baso-lateral membrane. For more details, you can 
consult any of the standard physiology textbook, e.g. Guyton & 
Hall, 'Textbook of Medical Physiology', Saunders, gives a very detailed 

Joshua Chai
Medical Student
University of Cambridge

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