CARL F. CORI 



development of regulatory systems and the organism has gained 

 in performance, but at the same time the different parts of the 

 organism have become highly interdependent. 



Hormone Effects 



As a general outline, the following factors are known which 

 enter into a regulatory system such as that of the blood sugar 

 level: (7) specialized tissues which respond to changes in the 

 blood sugar concentrations, e.g., the sympathetic centers in the 

 hypothalamic region and the islet tissue of the pancreas; (2) 

 effectors, which are partly nervous, partly humoral; and {3) 

 targets, which are the enzymatic reactions specifically influenced 

 by the effectors. It is on the last point that a few additional 

 comments may be made. Although phosphorylase has been 

 implicated in the metabolic action of epinephrine, hexokinase 

 in that of insulin, and oxidative phosphorylation in that of 

 thyroxin, none of these systems is as yet sufficiently well under- 

 stood to make one confident that the results obtained so far ex- 

 plain the actions of these hormones in the intact animal. 



Epinephrine and Glucagon 



The characteristic metabolic action of epinephrine in the 

 intact animal consists in a rapid breakdown of glycogen in liver 

 and muscle, even when at the time of injection the reaction is 

 proceeding in the opposite direction. Thus, epinephrine not 

 only causes increased phosphorylase activity but also has an 

 effect on the direction in which phosphorylase activity is pro- 

 ceeding. 



Phosphorylase occurs in liver and muscle in an active and 

 inactive form, and there are enzymes present which can convert 

 one form into the other with great rapidity. The active form in 

 muscle, phosphorylase a, is a dimer of the inactive form, phos- 

 phorylase b (13). Although phosphorylase b can be activated 



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