PHARMACOLOGY OF EPINEPHRINE 117 



creased irritability manifests itself in the appearance of extra- 

 systoles and is not prevented by vagotomy. 10 It is favored 

 by chloroform narcosis. Under chloroform anesthesia ventric- 

 ular fibrillation is easily induced. Barium salts and strophan- 

 thin also sensitize the heart to these effects of epinephrine. 

 The central vagal stimulation, which, as we have seen, arises 

 reflexly as a result of the hypertension, also acts on the sinus 

 and on the bundle of Tawara to give heart block or other de- 

 rangements of rhythm. 



According to Stella, 586 the reflex bradycardia which follows 

 the injection of epinephrine is due to the effects of the elevated 

 arterial pressure acting on the sensitive regions of the carotid 

 sinus and the territory of distribution of the depressor nerves. 

 The degree of this action is conditioned by a state of increased 

 reflex excitability of the cardiac vagus centers induced by the 

 injected epinephrine. 



Oliver and Schafer 480 first demonstrated the immediate and 

 marked rise of blood pressure produced by injecting an extract 

 of the adrenal medulla. Contraction of the peripheral arteries 

 is so marked as to cause a shrinkage of many organs, as re- 

 corded by a plethysmograph. Several seconds after an intra- 

 venous injection (subcutaneous administration is ineffective) 

 of 0.05 mgm. per kilo of epinephrine into an animal, the arterial 

 blood pressure rapidly rises, reaches a maximum in about 15 

 seconds, continues to rise slightly for the next \ to \\ minutes, 

 and then gradually recedes. In the dog, the pulse rate may 

 be increased to double its value at first but is slowed as the 

 pressure rises and often becomes irregular. The reduced pulse 

 rate continues for sometime after the blood pressure has re- 

 turned to normal. 



The maximum pressor effect elicitable by epinephrine varies 

 in different animals. In the rabbit the normal pressure is 

 rarely doubled. In the cat and dog, on the other hand, pres- 

 sures as high as 250 to 300 mms. are obtainable after vagotomy. 

 The rise in pressure is proportional to the natural logarithm 



