Caffeine has four identifiable actions in vitro, but the relationship
between these actions and the drugs effects in vivo are not well
established. In ascending order of dose response (i.e. from most
sensitive to least sensitive) these are:
1. Adenosine receptor blockade, as you have already mentioned
2. Phosphodiesterase inhibition - this enzyme is responsible for the
breakdown of cAMP and therefore this action of the methylxanthines leads
to increased cAMP 2nd messenger functions.
3. Action at Ca++ channels to increase entry of Ca++ into cells and to
decrease sarcolemma sequestration of Ca++. This may be related to the
weak positive inotropic effect of the se drug at high dose rates
(together with increased cAMP dep protein kinase activity due to number
2?)
4. Binding to GABA receptors at the benzodiazepine site (query clinical
relevance because of high KD).

Ted Whittem (whittem@massey.ac.nz)

===========================================================================

>Caffeine has four identifiable actions in vitro, but the relationship
>between these actions and the drugs effects in vivo are not well
>established. In ascending order of dose response (i.e. from most
>sensitive to least sensitive) these are:
>1. Adenosine receptor blockade, as you have already mentioned

This is the important one.  Tolerance to caffeine is associated
with increased adenosine receptor activity and a shifting of
A1 receptors to the high affinity state (and a decrease of beta-adrenergic 
activity).

>2. Phosphodiesterase inhibition - this enzyme is responsible for the
>breakdown of cAMP and therefore this action of the methylxanthines leads
>to increased cAMP 2nd messenger functions.

From what I understand, caffeine doesn't reach high enough serum
levels to significantly inhibit phosphodiesterase in people.

>3. Action at Ca++ channels to increase entry of Ca++ into cells and to
>decrease sarcolemma sequestration of Ca++. This may be related to the
>weak positive inotropic effect of the se drug at high dose rates
>(together with increased cAMP dep protein kinase activity due to number
>2?)
>4. Binding to GABA receptors at the benzodiazepine site (query clinical
>relevance because of high KD).

Don't think this is clinically relevant.

>Ted Whittem

     --M@