Definition (by Wonder Me!):
Physiologic antagonist is an antagonist so-called because it doesn't exert its antagonistic effects directly on the receptor that produces the effects. Physiologic antagonists bind to a receptor and makes that receptor creates an effect that is opposite to the other receptor.


Clinical implications: Antidotes!


Clinical examples:
- Epinephrine for antihistamine effects:

"There are several substances that have antihistaminergic action despite not being ligands for the histamine receptor. For instance, epinephrine raises arterial pressure through vasoconstriction mediated by A1-adrenergic receptor activation, in contrast to histamine, which lowers arterial pressure. Thus, despite not being true antihistamines because they do not bind to and block the histamine receptor, epinephrine and other such substances are physiological antagonists to histamine." (1)

 - Glucagon for betablockers overdose:

"Glucagon has become an accepted antidote to beta-blocker poisoning because it stimulates cAMP synthesis independent of the beta-adrenergic receptor.^[41] Glucagon has shown positive inotropic and chronotropic effects despite beta-receptor blockade in numerous animal models and in humans." (2)

 - Adrenaline & insulin on blood sugar (3)
- Carbachol & papaverine on smooth muscles (3)
- Noradrenaline & histamines on allergic conditions (3)


Citation:
(1) https://en.wikipedia.org/wiki/Physiological_agonism_and_antagonism
(2) http://www.medscape.com/viewarticle/430202_6
(3) https://books.google.com/books?id=X3cCZQCrrjcC&pg=PA43&lpg=PA43&dq=%22Physiologic+antagonist%22+examples&source=bl&ots=SxQy6ELxCI&sig=fuqUEwMnO93e_fwrCwTgoRUwPOo&hl=en&sa=X&ved=0ahUKEwj4hc-y0fHLAhXqyIMKHeV0BiEQ6AEIOjAF#v=onepage&q=%22Physiologic%20antagonist%22%20examples&f=false