Christina,
    Receptor location is of critical importance in virtually all receptor pathways.  Throughout all living organisms there myriad examples of receptor pathways in which the same basic receptor is being used with very modest changes for 2 completely independent functions.  The actions of antihistamines are a perfect example of this, benadryl is a great antihistamine targeting the H1 histamine receptor and relieving alergic reactions, but it also crosses the blood brain barier and hits that same receptor and causes drowsiness.  Claritin also tagets H1 but has greatly reduced brain penentration and as such it doesn't generally cause drowsiness.  

Peripherally, the H1 receptor is involved in response to antigens and vasodialation, in the brain it is involved processes related to sleeping and waking states.  Same molecule, same receptor, completely different function based on location.  This doesn't even touch on the many G-protein receptor kinase pathways that are believed to perform specific cellular functions based mainly on localization within single cell, not unique active site stucture, or ligand.

As a fellow scientist, I find your dismissive comments to other knowlegable scientists such as Randy or Peter, as though you are clearly better informed, to be disturbing.  Regardless of your level of knowledge berating people as ignorant is not an appropriate form of communication.   

Disclaimer: I am a chemist not a biologist

Mark

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