CARBOHYDRATE METABOLISM 201 



ROLE OF THE MEDULLA 



Although the adrenal cortex is responsible for the hypogly- 

 cemia which accompanies adrenal insufficiency, the medulla 

 may play a part in counteracting the hypoglycemic reaction 

 which follows the injection of certain drugs, as for example, 

 insulin. Thus cats in which the adrenal medullae are excised 

 or in which the right gland is removed while the left is 

 denervated are said to be hypersensitive to insulin. 88 This 

 hypersensitivity is not due to deficiency of the hepatic store of 

 glycogen which may be normal. Since insulin (c/. Chapter 

 VII) stimulates the secretion of epinephrine, one might antici- 

 pate a more pronounced hyperglycemia to follow the injection 

 of insulin in animals in which the medulla has been inactivated. 

 This stimulation of epinephrine secretion by insulin was dem- 

 onstrated in the anastomosis experiments of Houssay, Lewis, 

 and Molinelli. 312 In rats and in rabbits, on the other hand, in 

 which an incomplete adrenalectomy has been performed, the 

 increased sensitivity to insulin which follows the excision of 

 the glands disappears with time according to some authors. 588 

 It is possible that in these animals the discharge of epinephrine 

 which follows insulin injection is of minor importance (as com- 

 pared to the cat or dog) in compensating for the hypoglycemic 

 action of insulin. Sundberg, 606 however, observed a greater 

 sensitivity in rabbits in which cortical tissue only was present 

 and in which there was no evidence of cortical insufficiency. 

 Takats and Cathbert 619 have also found that denervation of 

 the adrenals raises the sugar tolerance of normal dogs, in- 

 creases their responsiveness to insulin, and decreases their 

 responsiveness to the hyperglycemic action of epinephrine. 



The theory, that the hyperglycemia following epinephrine 

 injections is caused by an inhibition of insulin secretion from 

 the pancreatic islet cells, is refuted by the occurrence of this 

 hyperglycemia in depancreatized animals. The hypoglycemia 

 which follows the primary hyperglycemia induced by epineph- 

 rine has been attributed to stimulation of the vagus which 

 causes an increased secretion of insulin. 



