114 MEDULLA 



does not prevent the action of epinephrine, but actually (after 

 the fibers are degenerated) renders the tissue more easily ex- 

 citable. 



Although most of its actions are sympathomimetic, epineph- 

 rine exerts certain effects which indicate a parasympathetic- 

 like action. Thus in the frog and turtle, epinephrine constricts 

 the pulmonary blood vessels which are supposed to be under 

 parasympathetic control. 405 The effects of epinephrine which 

 follow its injection with neurine, acetylcholine, and related 

 drugs or in the presence of a low calcium concentration, are 

 also explained by assuming some parasympathomimetic action 

 of epinephrine. 270 



The action of epinephrine is characterized by the absence of 

 any measurable latent period; by its dependence upon the 

 epinephrine concentration, which if maintained constant, may 

 elicit an unabated reaction for a long period of time; and by 

 the rapid subsidence of its effects when the drug is removed 

 from its site of action. 410 ■ 537 



In general, the effects of epinephrine are dependent upon a 

 number of conditions. The state of tone of the organ may 

 determine the extent of the action. Thus a normally dilated 

 bronchiole is scarcely affected by epinephrine while the con- 

 stricted bronchiole is readily dilated. As Auer and Meltzer 451 

 first showed, interruption of the sympathetic pathways in- 

 creases the effect exerted by epinephrine. Thus the denerv- 

 ated iris is 10 to 40 times as sensitive to the drug as the intact 

 organ. The ionic concentration of the inorganic constituents 

 of the medium also affects the action of epinephrine. The 

 calcium ion and the potassium ion act synergistically with 

 epinephrine; other ions (hydrogen, hydroxyl, sodium, mag- 

 nesium, etc.) also affect the action of epinephrine. 639 



The alkaloids of ergot (ergotamine and ergotoxine) have the 

 peculiar property of inhibiting the effects of epinephrine even 

 in doses which in themselves induce no great functional 

 changes. Although this is true of most actions of epinephrine, 



