AUTONOMIC NEUROEFFECTOR TRANSMISSION 



235 



the neurotransmitters in the blood. Such attempts 

 have been made and there is some evidence for the 

 opinion that the neurotransmitter of the adrenergic 

 system normally occurs in small quantities in the 

 blood. However, since the methods of demonstrating 

 norepinephrine in blood require fairly large quanti- 

 ties of blood and are rather laijorious (70, 91), they 

 have not been widely used. Some indirect informa- 

 tion has been obtained by studying the excretion in 

 urine (67, 130). Even if norepinephrine occurs in 

 peripheral blood it remains to be shown that it is 

 derived from postganglionic nerves and not from the 



suprarenal medulla or from chromafhn cells. Proof of 

 its overflow and passage into the blood has been given, 

 however, by studies on the excretion in urine in 

 adrenalectomized patients. In these patients the only 

 important sources of norepinephrine can be the post- 

 ganglionic nerves. For the same rea.sons as outlined 

 above the excretion of acetylcholine, after treatment 

 of the organism with physostigmine, will not allow 

 any conclusions as to the release at the postganglionic 

 nerve endings since acetylcholine is also released at 

 manv other sites. 



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