§ 5.212 CARBOHYDRATE METABOLISM 195 



carnivores may become diabetic if treated with growth hormone, 

 but that the rat can continue growing under the same treatment, 

 with Uttle disturbance of its sugar balance, except in extreme cases. 

 Glucagon secretion can also be stimulated directly by a low content 

 of sugar in the blood (cf. § 5.521 ; Saka, 1952). 



The action of glucagon is usually estimated from its effect upon 

 the glucose level in the blood; but it is claimed that the hydro- 

 cortisone acts at two stages in the process of gluconeogenesis ; by 

 stimulating the transformation of fats and proteins to pyruvic acid, 

 and also by increasing the blood-glucose at the expense of glucose- 

 6-phosphate in the tissues and liver. The transformation of 

 glycogen stores to free glucose via phosphorylated glucose is 

 facilitated by thyroxine (§ 5.111); but the latter hormone alone is 

 not able to release glucose into the blood stream (Fig. 5-8). 



As has been mentioned, hydrocortisone secretion is stimulated 

 by the endocrinokinetic hormone ACTH, and glucagon by STH. 



Adrenaline can also have a diabetogenic effect in liberating 

 sugar from liver glycogen into the blood. This is presumably an 

 indirect effect, since adrenaline causes a slow release of ACTH 

 (§ 4.231). The latter must be the main cause of the hyperglycaemia 

 associated with stress and excitement, and (like the comparable 

 sinus gland reaction in Callinectes) is also under direct nervous 

 stimulation. 



5.212 Decrease in blood-sugars by antidiabetogenic hormones 



Arthropoda. a hormone of this type has not so far been 

 reported for the Crustacea. 



Insecta. It has long been postulated that an antidiabetogenic 

 hormone is released just prior to moulting, and it is now believed 

 to be the same as the moult-promoting hormone, ecdysone, 

 from the prothoracic glands, or their equivalent in the ring 

 gland of Calliphora (Dennell, 1949). The secretion of the mouhing 

 hormone is under endocrinokinetic control from the neuro- 

 secretory cells of the intercerebrum. 



Vertebrata. The main antidiabetogenic hormone of vertebrates 

 is insulin, secreted from the ^ cells of the pancreatic islets of 

 langerhans (§ 2.222). This hormone lowers the level of sugars in 

 the blood by facilitating the supply of glucose to the tissues ; but 



