Antagonists and Agonists

[Qrios]

I know that a hormone that binds too a receptor either can be a agonist (activator) or antagonist.
I also know that for estrogen there is two receptors (alpha and beta) and tamoxifen acts as an agonist in one and antagonist in the other.

Now to the questions:
To my knowledge there is more than one androgen receptor but androgens have similar binding affinities for them.

A hormone that is an agonist, does it always activates the receptor?

A androgen that binds to the ar is it an agonist for all androgen receptors?



Also please tell me if anything i wrote here is wrong.

 

[Seth Roberts]

agaonists et al

There is only one receptor for androgens -- the androgen receptor. The anabolic androgenic steroids (testosterone, oxymetholone, oxandrolone, methandrostenolone etc...) are agonists of the androgen receptor to varying degrees. Some of them also have agonistic activity at the progesterone receptor. A few of them are antagonists at the progesterone receptor. It gets quite complicated, but I think I have answered your question.
Seth

 

[Qrios]

"are agonists of the androgen receptor to varying degrees."

It is this I don't really get.
A androgen can have more or less binding affinity but after it has bound, either it activates it or it does not...or?

 

[buffdoc]

OK, maybe this will help. Hormone and other receptor sites are proteins with distinct three dimensional shapes. The biological activity that is transduced, if you will, via the receptor activation, is carried out according to changes in this three dimensional structure. When a particular molecule attaches to a receptor site, the degree of induced conformational change determines the level of agonism.
Agonism is a term which describes activation of a receptor and consequent bio-activity. Hence the term, agonist.
Antagonism is the opposite, hence the term, antagonist.
It is more complicated, however, as sosme substance may be mixed agonist-antagonists.
There are also often subunits of receptors (ie, different sections of the three dimensional protein), which you alluded to in terms of alpha and beta subunits, etc.
But to answer your question: yes, as Seth Roberts pointed out, the agonism can take place to varying degrees, based, in part at least, on the conformational changes in the receptor protins that I'm describing. If this were not true, then all androgens would have the same effect on a mg for mg basis, if absorption were equal. Binding affinities are NOT equal, this is why there are different potencies (eg, testosterone vs DHT). They work their magic to different degrees upon the receptor proteins.

 

[Qrios]

Thanks for the answers!!
You should know it's appreciated!