124 KINETIC HORMONES — II 



not to be dependent upon any nerve reflex between the gut sur- 

 face and either the cells secreting the factor or the pancreas. 

 Direct introduction of acid into the circulation is not effective in 

 causing the pancreatic secretion ; whereas introduction of extracts 

 of the duodenal mucosa brings on the flow of secretion within 

 about one minute. 



Small quantities of secretin can also be extracted from the gastric 

 mucosa; but the main source is from the duodenum, and con- 

 sequently its secretion is only brought about by placing acid in 

 this part ot the gut. Acid in the stomach or the lower part of the 

 intestine has no effect in stimulating pancreatic secretion. The 

 natural acid is hydrochloric from the stomach; but any buffer 

 that keeps the duodenal contents between pH 4 and 5 stimulates 

 the production of secretin (Grossman, 1950). Peptones and 

 amino acids are much less effective (Fig. 4-4). 



The pancreatic glands which are stimulated by secretin produce 

 an alkaline solution containing NaHCOg; as this secretion brings 

 the duodenal contents to neutrality, the stimulation of secretin 

 stops and the flow of alkali from the pancreas also stops, the end 

 point of this hormone-controlled titration having been reached. 

 Secretin also increases the flow of bile from the liver ; but enzyme 

 secretion from the pancreas is stimulated by the vagus nerve or by 

 pancreozymin (Table 14). 



Cold-blooded Vertebrata. Elasmobranchs, salmon, frogs, 

 salamanders and some reptiles, such as tortoises, have all been 

 found to yield extracts, which act like secretin in mammals; but 

 it is not certain that all the extracts contain the same chemical as 

 the mammalian hormone: they may owe their activity to the 

 presence of histamine, which is a potent secretagogue for the 

 pancreatic glands. There is no evidence for the hormone having 

 any natural function in these animals. 



Ayes. Claims made for the presence of secretin in birds appear to 

 be founded on some confusion between this hormone and gastrin. 

 They are said, however, to respond to injection of secretin. 



Digestive enzyme secretion by the pancreas 



Mammalia. Pancreozymin, like secretin, is secreted from the 

 mammalian duodenal mucosa and acts upon the pancreas (Fig. 



