PharmPK Discussion - Multiple receptor site binding

• On 20 May 2007 at 14:56:25, elisa maulin (elisamaulin.-a-.yahoo.com) sent the message
  

  
  The following message was posted to: PharmPK
  
  I have a seemingly easy question to which I am having
  difficulty getting an answer.
  
  My question concerns how a drug binds to multiple
  receptor systems.
  
  For example, epinephrine at low doses activates beta2,
  and at high doses it activates alpha1.
  
  The question is, at high doses, is it ONLY activating
  alpha1, or is it having some maximized or saturable
  effect on beta1 such that it can spill over to alpha1?
  
  Preskorn gives an example of venlafaxine, which binds
  to the serotonin and norepinephrine transporters.  He
  makes the point that at one dose it is essentially
  binding only to the serotonin transporter, and at a
  higher dose it begins to bind to the NET.
  
  http://www.preskorn.com/columns/9911.html
  
  So if a drug binds to three systems with Ki 1x, 10x,
  and 100x, does that mean that at the lowest dose it
  only binds to the 1x system, and at a higher dose it
  only binds to the 1x and 10x, and at a higher dose it
  binds to all three?  And, at the highest doses, the 1x
  system's effects are "maximized?"
  
  But the real question is, how can one calculate the
  relative binding given Kis (or some other factor?)
  
  Thanks in advance for your help.
  
  

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