Effects of beta₂ adrenergic agents on mediator release

The activities of beta₂-type drugs appear to be multiple. First is their ability to stimulate adenyl cyclase thereby increasing intracellular levels of cAMP and inhibiting the release of histamine following exposure to antigen. Kinetic studies reveal the increase in intracellular cAMP to be rapid but short-lived and may reflect the requirement for beta agonists to be introduced at virtually the same time antigen is presented to the shock organ. A further constraint upon their effectiveness might be suppression of the cAMP response following prior medication with these agents. Secondly, these agents cause bronchiolar smooth muscle relaxation directly and may in that regard oppose the effects of acetylcholine, histamine, prostaglandins, slow reacting substance of anaphylaxis (SRS-A) or other endogenous mediators of bronchospasm. Finally, there is some suggestion that drugs of this class may have some antihistaminic activity, implying competitive inhibition of histamine at the smooth muscle receptor site.