On closer study it is found that pancreatic secretion consists of two phases: 1. Nervous Phase 2. Chemical Phase.
1. Nervous Phase:
Pancreatic secretion starts 1—2 minutes after taking food. When the vagi are cut, the secretion is abolished. This proves that the initial phase of pancreatic secretion is a reflex response. The stimulus for this reflex arises in the mouth during mastication as well as in the stomach when food enters the latter (gastro-pancreatic reflex). It is to be noted that this reflex is purely unconditioned.
There is no conditioned stimulus here. This stands in great contrast with the nervous phase of gastric secretion. That the vagi are the motor nerves of pancreas is proved by the fact that when they are stimulated, increased pancreatic secretion takes place. This vagal secretion is rich in enzyme but has very little effect on bicarbonate concentration. In addition there is a local cholinergic reflex mechanism independent of vagal innervation.
Thus, the vagal juice has got a greater digestive power. Acetylcholine is the mediator and parasympathomimetic drugs, e.g., pilocarpine are also effective. The response is blocked by atropine. Inhibition of secretion may also be obtained by stimulating adrenergic nerves producing vasoconstriction and constriction of ducts. Thus stimulation of sympathetic nerves may decrease secretion of pancreatic juice by reducing the blood flow through the organ and flow of juice through the duct.
2. Chemical Phase:
There are many controversial opinions regarding the cell responding to a stimulus with a characteristic type of functional activity regardless of the nature of stimulus. It is now thought that the cells of intralobular ducts secrete water and bicarbonate and the enzymes are secreted by acinous cells. It has already been noted that the rate of secretion of pancreatic juice rises sharply when the gastric contents enter duodenum.
This is the onset of chemical phase. Bayliss and Starling isolated a loop of jejunum maintaining its vascular supply intact but destroying all possible nerve connections. When acid was introduced into this loop, pancreas was found to secrete. Since there is no nervous connection between this loop and the pancreas, it is evident that the stimulus must be carried to pancreas through blood stream.
That HCl itself is not the stimulus is proved by the fact that injection of the acid into the portal vein did not produce any effect. But when acid extracts of intestinal mucosa were injected into the portal vein, pancreatic secretion was stimulated. This proves that the acid liberates some substance from the mucous membrane which acts as the real stimulus.
This substance is called secretin originally by Bayliss and Starling. Recently this has been separated into the following different components:
(a) Secretin,
(b) Pancreozymin,
The first two which are related to the pancreas are described below:
a. Nature and Action of Secretin:
It can be extracted from the mucous membrane of the duodenum and upper part of the small intestine with water, 0.4% HCl, soap solution or dilute alkali. From the nature of the solvents it is obvious that during normal process of digestion secretin can be extracted by the HCl of the gastric chyme as well as by the alkali of bile.
Secretin has been isolated by various workers and is believed to be a polypeptide, containing 27 amino acid residues. Its molecular weight is about 5,000. It is rapidly destroyed by pepsin and trypsin in alkaline or neutral medium. It remains stable in acid solution. Recently secretin has been synthesised. When secretin is injected intravenously, the flow of pancreatic juice increases (Fig. 9.40). This juice is watery, rich in bicarbonate but poor in enzyme.
b. Pancreozymin:
It is a polipeptide containing 33 amino acids. This causes release of zymogen granules from the pancreatic acinous cells resulting in release of a pancreatic juice rich in enzymes but poor in bicarbonate.
Influence of Various Foodstuffs on Pancreatic Secretion:
It has been observed that pancreatic secretion remains high for about three hours and then declines. This is due to the fact that stomach is completely emptied within three hours when mixed meal is ingested so contact of gastric chyme is lost with the mucosa of proximal part of the small intestine.
Pancreatic juice is found to vary in quality and quantity with different types of food. Meat stimulates a secretin type of response, i.e., large volume, more alkali and less enzyme. Fat elicits a vagal type of response, i.e., moderate volume, low alkali and rich in enzyme. Bread produces a mixed type of response.