§ 5.22 PROTEIN METABOLISM 199 



There is no known hormone that stimulates the secretion of 

 insuHn, unless secretion is found to be related to the increased 

 growth of the gland, which is said to occur as a result of injecting 

 extracts of the posterior pituitary (Staszyc, 1956). This is as yet 

 unconfirmed. 



The claim that STH increases insulin secretion is based on an 

 indirect effect, since it only occurs in those species where STH 

 stimulates glucagon to raise the level of blood-sugars, and this in 

 turn stimulates the insulin secretion (§§ 4.222 and 5.521). 



5.22 PROTEIN METABOLISM 



Hormones play a part in protein metabolism in Arthropoda and 

 Vertebrata, but the results so far reported are not always clear. 

 Noble (1955) points out that, in vertebrates, most attempts to 

 assess nitrogen, which is a characteristic element in proteins, have 

 been based on measurements of the over-all balance in the body, 

 whereas a more realistic picture might be obtained by following 

 reactions in different organs of the body separately. Increased 

 protein catabolism in one organ may be accompanied by an increase 

 in nitrogen excretion (§ 5.222), or it may be mainly responsible for 

 supplying materials for protein anabolism in some other organ. 



Arthropoda. The same difficulties of interpretation are prob- 

 ably true for Arthropoda. Perhaps the best that can be suggested 

 at present is that an eyestalk hormone in Crustacea tends to re- 

 strain protein breakdown and nitrogen excretion and that the 

 Y-organ hormone may inhibit them (Table 24). In Insecta a 

 brain hormone appears to be associated with an actual increase in 

 new protein formation. No hormones are yet known to stimulate 

 catabolism in either Crustacea or Insecta. 



5.221 Restraint of protein catabolism 



Crustacea. It has been claimed that an eyestalk hormone 

 restrains protein catabolism and that removal of the sinus gland, 

 which is the main storage organ of the hormone, results in protein 

 breakdown and loss of nitrogen ; but the evidence is ambiguous, 

 partly because the results seem to vary with stages in the moult 

 and intermoult cycle. 



Positive evidence shows that sinus gland removal causes a loss 



