Pancreatic and Salivary Glands

The pancreas and major salivary glands are compound exocrine glands

The exocrine pancreas and major salivary glands are compound exocrine glands—specialized secretory organs that contain a branching ductular system through which they release their secretory products. The principal function of these exocrine glands is to aid in the digestion of food. The saliva produced by the salivary glands lubricates ingested food and initiates the digestion of starch. Pancreatic juice, rich in and digestive enzymes, neutralizes the acidic gastric contents that enter the small intestine and also completes the intraluminal digestion of ingested carbohydrate, protein, and fat. Each of these exocrine glands is under the control of neural and humoral signals that generate a sequential and coordinated secretory response to an ingested meal. We discuss the endocrine pancreas in Chapter 51 .

Morphologically, the pancreas and salivary glands are divided into small but visible lobules, each of which represents a subdivision of the parenchyma and is drained by a single intralobular duct ( Fig. 43-1 A ). Groups of lobules separated by connective tissue septa are drained by larger interlobular ducts. These interlobular ducts empty into a main duct that connects the entire gland to the lumen of the gastrointestinal tract.

Within the lobules reside the microscopic structural and functional secretory units of the gland. Each secretory unit is composed of an acinus and a small intercalated duct. The acinus represents a cluster of 15 to 100 acinar cells that synthesize and secrete proteins into the lumen of the epithelial structure. In the pancreas, acinar cells secrete ~20 different digestive zymogens (inactive enzyme precursors) and enzymes. In the salivary glands, the principal acinar cell protein products are α-amylase, mucins, and proline-rich proteins. Acinar cells from both the pancreas and salivary glands also secrete an isotonic, plasma-like fluid that accompanies the secretory proteins. In all, the final acinar secretion is a protein-rich product known as the primary secretion.

Each acinar lumen is connected to the proximal end of an intercalated duct. Distally, the intercalated ducts fuse with other small ducts to form progressively larger ducts that ultimately coalesce to form the intralobular duct that drains the lobule. Although the ducts provide a conduit for the transport of secretory proteins, the epithelial cells lining the ducts also play an important role in modifying the fluid and electrolyte composition of the primary secretion. Thus, the final exocrine gland secretion represents the combined product of two distinct epithelial-cell populations, the acinar cell and the duct cell.

In addition to acini and ducts, exocrine glands contain a rich supply of nerves and blood vessels. Postganglionic parasympathetic and sympathetic fibers contribute to the autonomic regulation of secretion through the release of cholinergic, adrenergic, and peptide neurotransmitters that often bind to receptors on the acinar and duct cells. These efferent fibers also regulate blood flow. Both central and reflex pathways contribute to the neural regulation of exocrine secretion. The autonomic nerves also carry afferent pain fibers that are activated by glandular inflammation and trauma. The vasculature not only provides oxygen and nutrients for the gland but also carries the hormones that help to regulate secretion.

Acinar cells are specialized protein-synthesizing cells

Acinar cells—such as those in the pancreas (see Fig. 43-1 B ) and salivary glands (see Fig. 43-10 , below)—are polarized epithelial cells that are specialized for the production and export of large quantities of protein. Thus, the acinar cell is equipped with extensive rough endoplasmic reticulum. However, the most characteristic feature of the acinar cell is the abundance of electron-dense secretory granules at the apical pole of the cell. These granules are storage pools of secretory proteins, and they are poised to release their contents after stimulation of the cell by neurohumoral agents. The secretory granules of pancreatic acinar cells contain the mixture of zymogens and enzymes required for digestion. The secretory granules of salivary acinar cells contain either α-amylase (in the parotid gland) or mucins (in the sublingual glands). Secretory granules in the pancreas appear uniform, whereas those in the salivary glands often exhibit focal nodules of condensation within the granules known as spherules.

The pancreatic acinar cell has served as an important model for elucidating protein synthesis and export via the secretory pathway ( Fig. 43-2 ). Synthesis of secretory proteins (see pp. 34–35 ) begins with the cellular uptake of amino acids and their incorporation into nascent proteins in the rough endoplasmic reticulum (ER). Vesicular transport mechanisms then shuttle the newly synthesized proteins to the Golgi complex.

Within the Golgi complex, secretory proteins are segregated away from lysosomal enzymes. Most lysosomal enzymes require the mannose-6-phosphate receptor for sorting to the lysosome (see p. 40 ). However, the signals required to direct digestive enzymes into the secretory pathway remain unclear.

Secretory proteins exit the Golgi complex in condensing vacuoles or immature secretory granules. These large membrane-bound structures are acidic and maintain the lowest pH within the secretory pathway.

Maturation of the condensing vacuole to a secretory or zymogen granule is marked by condensation of the proteins within the vacuole and pinching off of membrane vesicles. The diameter of a secretory (zymogen) granule is about two thirds that of a condensing vacuole, and its content is more electron dense. Secretory proteins are stored in zymogen granules that are located in the apical region of the acinar cell. The bottom portion of Figure 43-2 shows the results of a pulse-chase experiment that follows the cellular itinerary of radiolabeled amino acids as they move sequentially through the four major compartments of the secretory pathway.

Exocytosis, the process by which secretory granules release their contents, is a complex series of events that involves the movement of the granules to the apical membrane, fusion of these granules with the membrane, and release of their contents into the acinar lumen. Secretion is triggered by stimulation of cell-surface receptors by either hormones or neurotransmitters (neurohumoral stimulation). At the onset of secretion, the surface area of the apical plasma membrane transiently increases as much as 30-fold. Thereafter, activation of an apical endocytic pathway leads to retrieval of the secretory granule membrane for recycling and a decrease in the area of the apical plasma membrane back to its resting value. Thus, during the steady state of secretion, secretory granule membrane is simultaneously delivered to and retrieved from the apical membrane.

Before the exocytotic event, vacuolar-type H pumps (see pp. 118–119 ) in the vesicle membrane use the energy of ATP hydrolysis to transport H ⁺ from the cytosol to the lumen of the vesicle. This transport process sets up both a chemical gradient for H ⁺ (inside acid) and an electrical gradient (inside positive) across the vesicle membrane. Cl ⁻ channels in the vesicle membrane can then allow Cl ⁻ to flow into the vesicle lumen, in parallel with the H ⁺ , so that the overall process is HCl movement from cytosol to vesicle lumen. Water follows by osmosis. The above “secretion” of minute amounts of Cl ⁻ into the secretory vesicle may contribute to the hydration of proteins within the granule before vesicle fusion. Furthermore, exocytosis of the vesicle contents may lead to a transient acidification of the acinar lumen and may modulate adjacent epithelial cells.

The cytoskeleton of the acinar cell plays an important role in the regulation of exocytosis. A component of the actin network appears to be required for delivery of the secretory granules to the apical region of the cell. A second actin network, located immediately below the apical membrane, acts as a barrier that blocks fusion of the granules with the apical plasma membrane. On stimulation, this second network reorganizes and then releases the blockade to permit the secretory granules to approach the apical plasma membrane. Fusion of the granules with the plasma membrane probably requires the interaction of SNAREs on secretory granules and the apical plasma membrane, as well as various other factors (see p. 37 ). After fusion, the granule contents enter the acinar lumen and move down the ducts into the gastrointestinal tract.

Duct cells are epithelial cells specialized for fluid and electrolyte transport

Pancreatic and salivary duct cells are polarized epithelial cells specialized for the transport of electrolytes across distinct apical and basolateral membrane domains. Duct epithelial cells contain specific membrane transporters and an abundance of mitochondria to provide energy for active transport, and they exhibit varying degrees of basolateral membrane infolding that increases the membrane surface area of pancreatic duct cells (see Fig. 43-1 C ) and salivary duct cells (see Fig. 43-10 C ). Although some duct cells contain prominent cytoplasmic vesicles, the synthetic machinery (i.e., ER and Golgi complex) of the duct cell is, in general, much less developed than that of the acinar cell.

Duct cells exhibit a considerable degree of morphological heterogeneity along the length of the ductal tree. At the junction between acinar and duct cells, and protruding into the pancreatic lumen, are small cuboidal epithelial cells known as centroacinar cells. These cells express very high levels of carbonic anhydrase (see p. 630 ) and presumably play a role in secretion. The epithelial cells of the most proximal (intercalated) duct are squamous or low cuboidal, have an abundance of mitochondria, and tend to lack cytoplasmic vesicles. These features suggest that the primary function of these cells is fluid and electrolyte transport. Progressing distally, the cells become more cuboidal columnar and contain more cytoplasmic vesicles and granules. These features suggest that these cells are capable of both transport of fluid and electrolytes and secretion of proteins. Functional studies indicate that the types of solute transport proteins within duct cells differ depending on the cell's location in the ductal tree.

Ion transport in duct cells is regulated by neurohumoral stimuli that act through specific receptors located on the basolateral membrane. As is the case for cells elsewhere in the body, duct cells can increase transcellular electrolyte movement either by activating individual transport proteins or by increasing the number of transport proteins in the plasma membrane.

Goblet cells contribute to mucin production in exocrine glands

In addition to acinar and duct cells, exocrine glands contain varying numbers of goblet cells. These cells secrete high-molecular-weight glycoproteins known as mucins (see p. 874 ). When hydrated, mucins form mucus. Mucus has several important functions, including lubrication, hydration, and mechanical protection of surface epithelial cells. Mucins also play an important immunological role by binding to pathogens and interacting with immune-competent cells. These properties may help to prevent infections. In the pancreas, mucin-secreting goblet cells are primarily found among the epithelial cells that line the large distal ducts. They can account for as many as 25% of the epithelial cells in the distal main pancreatic duct of some species. In the salivary gland, goblet cells are also seen in the large distal ducts, although in less abundance than in the pancreas. However, in many salivary glands, mucin is also secreted by acinar cells.

The acinar cell secretes digestive proteins in response to stimulation

To study secretion at the cellular level, investigators use enzymatically separated single acini (15 to 100 cells) or mechanically dissected single lobules (250 to 1000 cells). The measure of secretion is the release of digestive proteins into the incubation medium. The amount released over a fixed time interval is expressed as a percentage of the total content at the outset of the experiment. Because amylase is released in a fully active form, it is common to use the appearance of amylase activity as a marker for secretion by acinar cells.

When the acinar cells are in an unstimulated state, they secrete low levels of digestive proteins via a constitutive secretory pathway. Acinar cells stimulated by neurohumoral agents secrete proteins via a regulated pathway. Regulated secretion by isolated acinar cells in vitro is detected within 5 minutes of stimulation and is energy dependent. During a 30- to 60-minute stimulation period, acinar cells typically secrete 5 to 10 times more amylase than with constitutive release. However, during this period of regulated secretion, the cells typically secrete only 10% to 20% of the digestive proteins stored in their granules. Moreover, acinar cells are able to increase their rate of protein synthesis to replenish their stores.

The acinar cell exhibits two distinct patterns of regulated secretion: monophasic and biphasic ( Fig. 43-3 A ). Increasing levels of an agonist that generates a monophasic dose-response relationship (e.g., gastrin-releasing peptide [GRP]) causes secretion to reach a maximal level that does not fall with higher concentrations of the agent. In contrast, increasing levels of a secretagogue that elicits a biphasic dose-response relationship (e.g., cholecystokinin and carbachol) causes secretion to reach a maximal level that subsequently diminishes. As discussed below, this biphasic response may reflect the presence of functionally separate high-affinity and low-affinity receptors, and is related to the pathogenesis of acute pancreatitis. N43-1

Acetylcholine and cholecystokinin mediate the regulated secretion of proteins by pancreatic acinar cells

Although at least a dozen different receptors are present on the plasma membrane of the pancreatic acinar cell, the most important in regulating protein secretion is the M ₃ muscarinic acetylcholine (ACh) receptor (see p. 341 ), located on the basolateral membrane and also found in many glandular tissues.

Two closely related receptors for cholecystokinin (CCK) are distinguished by their structure, affinity for ligands, and tissue distribution (see p. 867 ). Although both CCK receptors may be activated by CCK or gastrin, the CCK ₁ receptor (previously known as CCK _A , encoded by the CCK1R gene) has a much higher affinity for CCK than for gastrin, whereas the CCK ₂ receptor (previously known as CCK _B , encoded by CCK2R gene) has approximately equal affinities for CCK and gastrin. In some species, both forms of the CCK receptor are present on the acinar cell. Although CCK ₁ receptors are present in human acinar cells, their function is unknown. N43-2

M ₃ and CCK receptors have many similarities: both are basolateral, both are linked to the Gα _q heterotrimeric G protein, both use the phospholipase C (PLC)/Ca ²⁺ signal-transduction pathway (see pp. 58–60 ), and both lead to increased enzyme secretion from the acinar cell.

Numerous other receptors—including those for gastrin-releasing peptide (GRP; see p. 868 ), calcitonin gene–related peptide (CGRP; see p. 1067 ), insulin (see pp. 1035–1050 ), secretin (see pp. 886–887 ), somatostatin (see pp. 993–994 ), and vasoactive intestinal peptide (VIP; see Table 41-1 )—are also found on the pancreatic acinar cell. Although these other receptors may also play a role in regulating secretion, protein synthesis, growth, and transformation, their precise physiological functions remain to be clearly defined.

Activation of receptors that stimulate different signal-transduction pathways may lead to an enhanced secretory response. For example, as shown in Figure 43-3 B , simultaneous stimulation of the high-affinity CCK receptor (which acts via [Ca ²⁺ ] _i ) and the VIP receptor (which acts via cAMP) generates an additive effect on secretion. Alternatively, acinar cells that have previously been stimulated may become temporarily refractory to subsequent stimulation. This phenomenon is known as desensitization.

Ca ²⁺ is the major second messenger for the secretion of proteins by pancreatic acinar cells

Ca ²⁺

Much of the pioneering work on the role of intracellular Ca ²⁺ in cell signaling has been performed on the pancreatic acinar cell ( Fig. 43-4 A ). Generation of a cytosolic Ca ²⁺ signal is a complex summation of cellular events (see p. 60 ). Even when the acinar cell is in the resting state, the cytosolic free Ca ²⁺ level ([Ca ²⁺ ] _i ) oscillates slowly. Maximal stimu­latory (i.e., physiological) concentrations of CCK or ACh increase the frequency of the oscillations (see Fig. 43-4 B ) but have less effect on the amplitude. This increase in the frequency of [Ca ²⁺ ] _i oscillations is required for protein secretion by acinar cells. In contrast, a supramaximal (i.e., hyperstimulatory) concentration of CCK or ACh generates a sudden [Ca ²⁺ ] _i spike that is 2 to 10 times greater than that seen with physiological stimulation and eliminates further [Ca ²⁺ ] _i oscillations. This [Ca ²⁺ ] _i spike and the subsequent absence of oscillations are associated with an inhibition of secretion that appears to be mediated by disruption of the cytoskeletal components that are required for secretion.

In addition to proteins, the pancreatic acinar cell secretes a plasma-like fluid

Besides secreting proteins, acinar cells in the pancreas secrete an isotonic, plasma-like fluid ( Fig. 43-5 ). This NaCl-rich fluid hydrates the dense, protein-rich material that the acinar cells secrete. The fundamental transport event is the secretion of Cl ⁻ across the apical membrane. N43-4 For transcellular (plasma-to-lumen) movement of Cl ⁻ to occur, Cl ⁻ must move into the cell across the basolateral membrane. As in many other Cl ⁻ -secreting epithelial cells (see p. 139 ), in the acinar cell basolateral Cl ⁻ uptake occurs via an Na/K/Cl cotransporter. The Na-K pump generates the Na ⁺ gradient that energizes the Na/K/Cl cotransporter. The K ⁺ entering through the Na-K pump and via the Na/K/Cl cotransporter exits through K ⁺ channels that are also located on the basolateral membrane. Thus, a pump, a cotransporter, and a channel are necessary to sustain the basolateral uptake of Cl ⁻ into the acinar cell.

The rise in [Cl ⁻ ] _i produced by basolateral Cl ⁻ uptake drives the secretion of Cl ⁻ down its electrochemical gradient through channels in the apical membrane. As the transepithelial voltage becomes more lumen negative, Na ⁺ moves through the cation-selective paracellular pathway (i.e., tight junctions) to join the Cl ⁻ secreted into the lumen. Water also moves through this paracellular pathway, as well as via aquaporin water channels on the apical and basolateral membranes. Therefore, the net effect of these acinar cell transport processes is the production of an isotonic, NaCl-rich fluid that accounts for ~25% of total pancreatic fluid secretion.

Like the secretion of protein by acinar cells, secretion of fluid and electrolytes is stimulated by secretagogues that raise [Ca ²⁺ ] _i . In the pancreas, activation of muscarinic receptors by cholinergic neural pathways and activation of CCK receptors by humoral pathways increase the membrane conductance of the acinar cell. A similar effect is seen with GRP. Apical membrane Cl ⁻ channels and basolateral membrane K ⁺ channels appear to be the effector targets of the activated Ca ²⁺ signaling pathway. Phosphorylation of these channels by Ca ²⁺ -dependent kinases is one likely mechanism that underlies the increase in open channel probability that accompanies stimulation.

The pancreatic duct cell secretes isotonic NaHCO ₃

The principal physiological function of the pancreatic duct cell is to secrete an -rich fluid that alkalinizes and hydrates the protein-rich primary secretions of the acinar cell. The apical step of transepithelial secretion ( Fig. 43-6 ) is mediated in part by a Cl-HCO ₃ exchanger, a member of the SLC26 family (see Table 5-4 ) that secretes intracellular into the duct lumen. Luminal Cl ⁻ must be available for this exchange process to occur. Although some luminal Cl ⁻ is present in the primary secretions of the acinar cell, anion channels on the apical membrane of the duct cell provide additional Cl ⁻ to the lumen in a process called Cl ⁻ recycling. The most important of these anion channels is the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-activated Cl ⁻ channel that is present on the apical membrane of pancreatic duct cells (see p. 120 , Box 43-1 ). Cl ⁻ recycling is facilitated by the coactivation of CFTR and SLC26 exchangers through direct protein-protein interactions. In some species, such as the rat and mouse, pancreatic duct cells also contain a Ca ²⁺ -activated Cl ⁻ channel on the apical membrane; this channel also provides Cl ⁻ to the lumen for recycling. Apical Cl ⁻ channels, including CFTR, may also directly serve as conduits for movement from the duct cell to the lumen.

The intracellular that exits the duct cell across the apical membrane arises from two pathways. N43-5 The first is direct uptake of via an electrogenic Na/HCO ₃ cotransporter (NBCe1-B or SLC4A4; see p. 122 ), which presumably operates with an stoichiometry of 1 : 2. The second mechanism is the generation of intracellular from CO ₂ and OH ⁻ , catalyzed by carbonic anhydrase (see p. 630 ).The OH ⁻ in this reaction, along with H ⁺ , is derived from H ₂ O. Thus, the H ⁺ that accumulates in the cell must be extruded across the basolateral membrane. One mechanism of H ⁺ extrusion is Na-H exchange. The second mechanism for H ⁺ extrusion across the basolateral membrane, at least in some species, is an ATP-dependent H pump. Pancreatic duct cells contain acidic intracellular vesicles (presumably containing vacuolar-type H pumps) that are mobilized to the basolateral membrane of the cell after stimulation by secretin, a powerful secretagogue (see below). Indeed, H pumps are most active under conditions of neurohumoral stimulation. Thus, three basolateral transporters directly or indirectly provide the intracellular that pancreatic duct cells need for secretion: (1) the electrogenic Na/HCO ₃ cotransporter, (2) the Na-H exchanger, and (3) the H pump. The physiological importance of these three acid-base transporters in humans has yet to be fully established. The pancreatic duct cell accounts for ~75% of total pancreatic fluid secretion.

N43-5

Secretion by the Pancreatic Duct

The current model for secretion by the pancreatic duct is very similar to that outlined in Figure 43-6 . However, we can now add some important details about the apical step of secretion. The Cl-HCO ₃ exchanger at the apical membrane is a member of the SLC26 family ( )—previously known as the SAT family—specifically, SLC26A6 (also known as CFEX). We now appreciate that SLC26A6, which is capable of exchanging several different anions (e.g., Cl ⁻ , , oxalate), is electrogenic ( ). When mediating Cl-HCO ₃ exchange, it appears that SLC26A6 exchanges two ions for every Cl ⁻ ion. This stoichiometry would strongly favor the efflux of across the apical membrane of the pancreatic duct cell.

As noted in the text, the Cl ⁻ that enters the cell via SLC26A exits the cell via apical Cl ⁻ channels, principally CFTR. Interestingly, it appears that an interaction between the SLC26A6 protein and CFTR greatly increases the open probability of CFTR ( ).

Another member of the SLC26 family—SLC26A3—also is present in the apical membrane of pancreatic duct cells. SLC26A3 is also electrogenic but has a stoichiometry opposite to that of SLCA6, two Cl ⁻ ions for every . This transporter would extrude Cl ⁻ (and take up ) from the duct cell across the apical membrane. Its physiological function might be to reabsorb at times when the duct is not secreting or to contribute to the recycling of Cl ⁻ when the duct is secreting .

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