308 



HORMONES 



made between them and "secretogogues." The latter are chemical sub- 

 stances in the food, or derived from food during digestion, which directly 

 or indirectly stimulate the secretion of digestive juices. They are not 

 classified as hormones because they are not produced by the body. 



Gastrin. This hormone is produced by the mucosa of the lower, or 

 pyloric, end of the stomach and to a lesser extent by the mucosa of the 

 duodenum (that portion of the small intestine immediately beyond the 

 stomach). It is secreted into the blood stream (though not directly) 

 as a result of stimulation by secretogogues and has the effect of increasing 

 the flow of gastric juice. The juice so formed is high in hydrochloric acid 

 but low in pepsin. 



Gastrin can be extracted from suitable mucosa and has been purified 

 considerably, although not completely. It is destroyed by proteolytic 

 enzymes or by boiling in one-tenth normal sodium hydroxide solution 

 and is precipitated by trichloracetic acid. It is probably a peptide or 

 low molecular weight protein. Histamine in very small doses has the 

 same effect as gastrin, and it may be that histamine actually is the hor- 

 mone. This has not been proved, however, mainly because the effective 

 level of histamine is too low to be detected in the blood stream by the 

 analytical methods at present available. An observable gastric response 

 is produced in human beings by the injection of only 0.004 /xg. of hista- 

 mine per kilogram body weight per minute. 



Secretin. This substance stimulates the secretion of water by the pan- 

 creas and of bile by the liver. The .increased flow of pancreatic juice 

 is relatively poor in enzyme content.^ Secretin exists in an inactive 

 form (prosecretin) in the mucosa of the upper small intestine, or duode- 

 num, and is released by the action of dilute hydrochloric acid from the 

 stomach (pH ca. 4.6). 



Two crystalline secretin preparations have been isolated, both in the 

 form of salts with picrolonic acid. One appears to be a peptide, while 

 the other is a compound of low molecular weight. The peptide, however, 

 can be extensively hydrolyzed by aminopolypeptidase without loss of 

 secretin activity. It is suggested that the two products may be related 

 in much the same way as thyroxine and thyroglobulin. The exact 

 formula of secretin is not known. 



C holecystokinin. The name of this hormone (literally ''gall bladder 

 mover") indicates its physiological function which is to stimulate con- 

 traction and emptying of the gall bladder. Like secretin, it is produced 

 in the mucosa of the duodenum. It is secreted indirectly into the blood 

 stream whenever fat, fatty acids, peptone, or dilute hydrochloric acid 

 enter the intestine. It has been only partially purified, and its chemical 



^ Another gastrointestinal hormone, the existence of which is very probable but not 

 conchisively proved, stimulates the secretion of enzijmes by the pancreas. Purifipd 

 prei)a rations of this substance, pancreozymin, have no effect on the volume of pan- 

 creatic secretion. 



