Pancreatic Secretion

Pancreatic exocrine secretion consists of an aqueous or bicarbonate component and an enzymatic component. The aqueous component consists primarily of water and sodium bicarbonate and is produced by the cells lining the pancreatic ducts. The aqueous component neutralizes duodenal contents, preventing injury to the duodenal mucosa and bringing the contents within the pH range necessary for optimal enzymatic digestion of nutrients. The enzymatic or protein component is a low-volume secretion from the pancreatic acinar cells that contains enzymes for the digestion of all major foodstuffs. Unlike the enzymes secreted into saliva and gastric juice, the pancreatic enzymes are essential for normal digestion and absorption.

Functional Anatomy of the Pancreas

The structure of the exocrine pancreas resembles a cluster of grapes and its functional units are similar to the salivons of the salivary glands. Pyramidal acinar cells are oriented with the apices toward a lumen to form an acinus (Fig. 18). Groups of acini form lobules separated from each other by areolar tissue. The lumen of each spherical acinus is drained by a ductule whose epithelium extends into the acinus in the form of centroacinar cells. Within each lobule, ductules join to form intralobular ducts. These in turn drain into extra-lobular ducts, which join to form the major pancreatic collecting duct draining the gland.

The acinar cells comprise approximately 80% of the pancreas by volume and secrete a small volume of juice containing the pancreatic enzymes. Ductule cells, which comprise about 4% of the gland, together with the centroacinar cells, secrete the aqueous component, a large volume secretion of water and NaHCO3. The endocrine cells of the pancreas account for only 2% of its mass and are found in the islets of Langerhans distributed throughout the pancreatic parenchyma. The islets contain both the insulin-secreting fi cells and the

FIGURE 18 Schematic diagram illustrating the histology of a functional unit of the pancreas.

glucagon-secreting a cells. The islets also contain large amounts of somatostatin, which may act as a paracrine to inhibit the release of both insulin and glucagon. The pancreas also produces pancreatic polypeptide, a candidate hormone that inhibits pancreatic exocrine secretion (see Chapter 32).

Efferent nerves from both the sympathetic and parasympathetic systems influence pancreatic secretion. Sympathetic innervation is provided by postganglionic fibers from the celiac and superior mesenteric plexuses. Sympathetic fibers enter the pancreas along with the arteries to the organ. Parasympathetic preganglionic fibers to the pancreas are contained in branches of the vagus nerves. These fibers course down the surface of the stomach, entering the pancreas from the antral-duodenal region. Within the pancreas, the vagal fibers terminate either at acini and islets or on other intrinsic cholinergic nerves of the pancreas. Parasympathetic nerves stimulate pancreatic exocrine secretion, whereas sympathetic nerves are largely inhibitory.

Aqueous Component of Pancreatic Secretion

The 100-g human pancreas secretes approximately 1 L of fluid per day, which is sufficient to neutralize most of the acid entering the duodenum. At all rates of secretion, pancreatic juice is essentially isotonic with plasma. At low rates of secretion, pancreatic juice is primarily a solution of Na+ and Cl", whereas at high rates, Na + and HCO" predominate. The concentration of Na+ in pancreatic juice approximately equals its concentration in plasma, and at all rates of secretion K+ is also found in concentrations equal to its plasma levels (Fig. 19).

As in gastric juice, the ionic composition of pancreatic juice varies with the rate of secretion (Fig. 19). Just as the concentrations of the cations Na+ and H+ were

Exchange Between And Ions

FIGURE 19 Relationship between the principal ions in pancreatic juice and secretory rate. Concentrations of the ions in plasma are shown for comparison.

FIGURE 18 Schematic diagram illustrating the histology of a functional unit of the pancreas.

FIGURE 19 Relationship between the principal ions in pancreatic juice and secretory rate. Concentrations of the ions in plasma are shown for comparison.

34. Secretion reciprocally related in gastric juice, the concentrations of Cl" and HCO" vary inversely in pancreatic juice. As one might expect, theories analogous to those proposed to explain the ionic composition of gastric juice also explain the variation in pancreatic juice composition. It is believed that one cell type, perhaps the acinar cell, secretes a small volume of juice that primarily contains Na+ and Cl". This fluid is the basal secretion and is equal to only about 2% of the maximal secretory rate. In response to stimulation, other cell types—the ductule cells and the centroacinar cells— secrete large volumes of fluid containing primarily Na+ and HCO". As the rate of secretion increases, the small amount of Cl" is diluted by the HCO", and pancreatic juice becomes a solution of Na+ and HCO" with small amounts of Cl". This concept is analogous to the two-component hypothesis used to explain the composition of gastric juice. Evidence also exists to indicate that as HCO3" moves through the duct system of the pancreas, it moves down its concentration gradient, leaving the ducts in exchange for Cl". Thus, the higher the secretory rate, the less time for exchange to occur, and the more likely that pancreatic juice will contain primarily HCO". At low secretory rates, this exchange moves toward completeness and Cl" becomes the predominant anion. In reality, the final composition of pancreatic juice is probably due to both of these processes.

The aqueous component is secreted by the ductule and centroacinar cells and may contain HCO3" at a concentration equal to several times its concentration in plasma. The lumen of the pancreatic ducts is 5-9 mV negative with respect to the blood. Therefore, HCO3" is secreted against both its chemical and electrical gradients. This has been interpreted as evidence that HCO3" is actively transported across the apical surface of the ductule cell. Although the exact mechanism has not been elucidated, evidence has accumulated for the HCO3" secretory mechanism illustrated in Fig. 20. In this model the (Na + ,K+)-ATPase of the basolateral membrane creates an electrochemical gradient for Na+ to move into the cell in exchange for H+, which leaves the cell against its concentration gradient. There is some evidence that a H+-ATPase may actually pump H+ out of the cell. In either case, CO2 diffuses readily into the alkalinized cell, combining with water to form HCO". This latter step is catalyzed by carbonic anhydrase. The continued movement of H+ across the basolateral membrane leads to a buildup of HCCV and the movement of HCCV across the apical membrane in exchange for Cl". The Cl"-HCO3" exchange mechanism is found in many secretory cells of the gastrointestinal tract. As H+ leaves the cell, it combines with HCO3" in the plasma to produce more CO2, which is free to diffuse into the cell. Because of this secretory process, venous

Blood

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