314 DIGESTION 



tones, and some amino acids. The enzyme is secreted by the zymogenic 

 cells in an inactive form, pepsinogen. Pepsinogen is converted to pepsin 

 by hydrogen ions and pepsin (see p. 273). Rennin is present in par- 

 ticularly high concentrations in the stomachs of young mammals. This 

 enzyme is involved in the curdling of milk. Rennin catalyzes the con- 

 version of casein to soluble paracasein. Paracasein combines with cal- 

 cium ion to form insoluble calcium paracaseinate. Rennin obtained from 

 calf stomach is used commercially to curdle milk in cheese making. 

 Pepsin can also curdle milk, but accomplishes this result by a process 

 different from that of rennin. 



The flow of gastric juice is regulated by several factors. Nervous im- 

 pulses due to various stimuli, such as food in the mouth, or even the 

 odor or sight of food, cause secretion. Fear or worry have been shown 

 to suppress secretion. Mechanical pressure inside the stomach has a 

 slight effect, but the presence of certain foods {e.g., meat extract, peptone) 

 in the stomach causes a tremendous increase in gastric flow. This effect 

 of food seems to be independent of the nervous system, and some evidence 

 has been obtained to show that a substance may be present in gastric 

 mucosa which reacts with a food component to form a hormone, gastrin 

 (p. 308). This substance is liberated into the blood and causes gastric 

 secretion. The injection of histamine, a compound present in gastric 

 mucosa as well as in other body tissues, causes the secretion of a gastric 

 juice which is high in hydrochloric acid but low in pepsin, in contrast 

 to the normal composition. The composition of chyme leaving the 

 stomach for the duodenum has an effect on gastric digestion. Thus when 

 fat or acid are placed into the duodenum, gastric secretion and motility 

 are inhibited. A material extracted by l\'y from intestinal mucosa, when 

 injected into the blood stream, produces the same type of depression 

 of gastric activity. It is a hormone called enter ogastr one (p. 309) . Uro- 

 gastrone, isolated from urine, has a similar effect. 



Various chemicals have a profound effect on the secretion and composi- 

 tion of gastric juice. Ethyl alcohol leads to a secretion high in hydro- 

 chloric acid and mucin and low in pepsin; liver, meat, and vegetable 

 extracts are powerful stimulants of normal gastric juice secretion, while 

 acid depresses secretion. The concentration of hydrochloric acid is 

 chronically lowered or raised in certain pathological conditions. The 

 hydrochloric acid is completely absent (achlorhydria) , e.g., in pernicious 

 anemia, and is produced in excessive amounts (hyperacidity) in most 

 cases of duodenal ulcers. 



Bland diets, for example, milk, are used in treating ulcer patients to 

 prevent excessive gastric secretion. The high buffering capacity of such 

 diets helps to neutralize the free acid of the gastric secretion. 



