Endocrine Mechanisms 



757 



Chapter 8). Glycogen is reversibly broken down to produce glucose- 1 -phos- 

 phate under the influence of an enzyme phosphorylase, a process which has 

 been referred to as phosphorolysis because of its similarity to hydrolysis. In 

 phosphorolysis H3PO4 participates in the reaction in a manner comparable to 

 that of H2O in hydrolysis. Under the influence of the enzyme phosphoglu- 

 comutase the phosphate group is reversibly shifted within the molecule to 

 the 6-position. The resulting glucose-6-phosphate is reversibly split by the 

 action of phosphatase to yield glucose and inorganic phosphate. Glucose-6- 

 phosphate and fructose-6-phosphate, with which the former comes into equi- 

 librium under the influence of the enzyme isomerase, comprise the primary 

 substrates in the aerobic and anaerobic energy-yielding oxidative metabol- 

 ism of carbohydrates. 



A number of endocrine glands in the mammal are known to influence 

 the intermediary metabolism of carbohydrates and consequently the carbo- 

 hydrate contents of body tissues. Adrenin, from the adrenal medulla, ele- 

 vates blood sugar by accelerating the formation of lactic acid from glycogen 

 in muscle and of glucose from glycogen in the liver. In muscle, after adren- 



MUSCLE 



LIVER. 



Gluco^en 



Glucose 



Lactic Qc'id 



BLOOD 



Fig. 286. Diagram representing the gross metabolic transformations of carbohydrate in 

 muscle and liver occurring in the higher vertebrate. 



alin administration, hexose monophosphate increases and inorganic phos- 

 phate decreases. It would appear that adrenalin increases phosphorylase ac- 

 tivity, since this is the predominant reaction common to both muscle and 

 liver in these changes. Adrenalin accelerates this reaction only in living 

 cells, being ineffective in enzyme solutions. 



Insulin from the islets of Langerhans of the pancreas appears to affect 

 carbohydrate metabolism primarily through acceleration of the change, glu- 

 cose to glycogen, in both muscle and liver. ^°^ Insulin is not essential to this 

 process at high blood sugar levels but is essential within the physiological 

 range of blood sugar concentrations. In fact glycogen may be formed from 

 glucose even in hypoglycemic animals, after insulin treatment. There is 

 also some evidence that insulin has an inhibitory action on adrenalin-accel- 

 erated phosphorolysis in the liver. 



Hormones of the adrenal cortex influence the intermediary metabolism 

 involving carbohydrates in mammals.^^- ^"^ After adrenalectomy rats and 

 mice may be maintained in health by administration of sodium salts and 

 glucose. However, such individuals, when fasted, suffer a rapid drop in 



