CARL F. CORI 



content of muscle is increased by epinephrine, which should 

 favor glycogen synthesis rather than breakdown. No indication 

 has been found for an increase in inorganic phosphate which 

 would favor breakdown. One could refer here again to the 

 compartments of the cell, but this is not a satisfactory explana- 

 tion since it is not amenable, at the present time, to experimental 

 investigation. 



Glucagon, a protein recently isolated from the pancreas in 

 crystalline form, increases the phosphorylase activity of the liver 

 in the same way as epinephrine (19). In contrast to epi- 

 nephrine, glucagon does not cause an increase in blood lactic acid 

 in the intact animal, and it also has no action on the phosphory- 

 lase a content of the isolated diaphragm. It is possible that 

 glucagon cannot penetrate into the muscle cell. What the com- 

 mon denominator, if any, might be between epinephrine and 

 glucagon in their action on the liver is unknown; possibly 

 glucagon causes the liberation of a sympathomimetic amine in 

 the liver. In conclusion it might be pointed out that epi- 

 nephrine and glucagon probably act on the enzymes which main- 

 tain a balance between active and inactive phosphorylase rather 

 than on phosphorylase itself, but this awaits verification in a 

 cell-free enzyme system. 



Muscular activity also influences the relative amounts of 

 active and inactive phosphorylase in muscle. Continuously 

 active muscles, such as heart and diaphragm, contain a higher 

 percentage of phosphorylase a than resting skeletal muscle. 

 Stimulation of isolated frog gastrocnemius at a rate of 60 single 

 shocks per minute for 1 to 2 minutes, i.e., conditions which per- 

 mit work at a steady state without development of fatigue, 

 resulted in a marked increase in the percentage of phosphorylase 

 a. On the other hand, repeated tetanic stimulation during 1 

 to 2 minutes until the muscle showed fatigue, resulted in an 

 almost complete disappearance of phosphorylase a. These ex- 

 periments, which have not as yet been reported, illustrate again 

 the dynamic balance which exists in muscle between inactive 

 and active phosphorylase. 



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