Permissiveness
Although cortisol has no direct vasoactive properties, it augments the vasoconstrictive effects of catecho|amines and angiotensin II. The above graph shows that administration of norepinephrine produced a limited degree of vasoconstriction that was markedly increased following pretreatment with cortisol.
This effect is termed permlssiveness and occurs when one hormone allows another to exert its maximal effect. Cortisol exerts its potentiating effect in part
through upregulation of alpha-1 adrenergic receptors on vascular smooth muscle cells. In adrenal insufficiency, low glucocorticoid levels can contribute to hypotensive crisis by decreasing vascular responsiveness to angiotensin II and norepinephrine.
Additive & Synergistic Effects
The combination of 2 drugs with similar actions can have a synergistic or additive effect.
When the combined effect of 2 drugs is equal to the sum of their individual effects, it is termed an additive response.
When the combined effect exceeds the sum of the individual drug effects, the interaction is called synergistic.
In this case, cortisol's influence on vascular responsiveness is best described as permissive rather than synergistic as cortisol has no intrinsic vasoconstrictive effects.
Antagonism
Competitive = change ED50 = shift right
Non-competitive = change Emax = shift down
Adverse Drug Reactions