February 17, 2006

Salivary and Gastric Secretion

Ron Lynch, PhD 626-2472

SALIVARY GLANDS Objectives: Salivary Secretion (1200 ml/day) 1. What are the primary functions of saliva and the basic constituents which provide these functions. 2. How is salivary secretion regulated: What are the primary activators of salivary secretion? What role does blood flow play in elevating secretion of saliva? I, Functional Anatomy. There are many saliva secreting glands within the oral cavity, however, three sets of principal glands secrete greater than 99% of all fluid. The largest are the bilateral pair of Parotid glands. Secretions from the parotids have a high water content (serous secretion) which generally accounts for 60% of all saliva. The other two primary pairs are Submaxillary glands and the Sublingual glands.

II. General Glandular Structure

The primary salivary glands resemble grapes on a stem. The bolbus portion is called the acinus, and fluid moving from the acini of the gland is collected into a series of enlarging ducts. The functional properties and permeability of the cells in these two regions are quite different. The acinus is very permeable to water, and acts in filtering water and ionic components from blood. The acinus is highly vascularized so flow of blood around this region can increase dramatically upon activation. In contrast to acinar cells, ductal cells are very impermeant to water.

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February 17, 2006

Salivary and Gastric Secretion

Ron Lynch, PhD 626-2472

III. Composition of saliva. The components that confer function to saliva include water, ionic and organic factors. The primary ionic components contained in saliva include Na+, K+, Cl- and HCO3- . In addition, small amounts of Ca2+, I-, H2PO4- , F- and SCN- are present. The primary organic components of saliva include the enzymes α-amylase, lysozyme and lingual lipase. Also present at a relatively high concentration are mucopolysaccharides and glycoproteins (mucus). IV. Function of Saliva: Saliva performs two specific functions in the oral cavity: Digestive, and Protective which includes Lubrication. A. Digestive: α-amylase acts to begin starch digestion, and lipase fat digestion. The pH optima of amylase is 7.0 while that of ligual lipase is 4.0. Thus, amylase is active only before its environment becomes acidified. Since the proximal stomach stores food with limited mixing, the interior of a bolus of food can remain non-acidified for a variable period within the stomach. Depending on the bulk of a meal, amylase can assist in initial starch breakdown, although in general it's role normally is not significant relative to that played by the pancreatic and intestinal (brush border) enzymes. B. Protective: F-, SCN-, immunoglobulin A, lysozyme and lactoferrin all act to kill bacteria or retard bacterial growth in the mouth; secreted HCO3 buffers pH. The mucus secretion acts to coat the interior of the mouth to prevent abrasion and to coat the food to aid in swallowing. Saliva also acts as a solvent and aids in taste sensation. V.

Mechanism of Secretion: The acinus is the site where secretion of fluid and organic components takes place in the salivary glands. The acinus produces saliva by filtration of plasma from blood coupled with Ca2+ activated anion (HCO3- / Cl-) and protein secretion. A. Filtration: Filtration through acinar cells is primarily passive. Net fluid movement is accomplished by regulating blood flow to the acinus. Blood flow to the acinus is elevated in association with secretory stimuli, leading to increased blood pressure around the acinus. The elevation in interstitial pressure drives filtration through the acinar cells, thereby elevating net fluid flux into the lumen. The large fluid flux carries ions as well as some plasma protein into the lumen by solvent drag. B. Organic components: Specific proteins are produced and secreted from acinar cells upon activation (e.g., amylase, mucopolysaccharides), while others are simply filtered (IgG). The mucopolysacchadies are stored within granules which are released when intracellular Ca2+ concentration increases.

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February 17, 2006

Salivary and Gastric Secretion

Ron Lynch, PhD 626-2472

Ionic components: Bicarbonate secretion is under control of Ca2+. An anion channel in the apical membrane of acinar cells is more selective for HCO3 than Cl- and is opened by elevated intracellular Ca2+. Thus, when Ca2+ becomes elevated, the Ca2+ activated anion channel resident in the acinar cell apical membrane opens and HCO3 selectively flows with water into the lumen. C. Role for Carbonic Anhydrase (CA) in "activated" bicarbonate secretion: + A recurrent theme in cells which secrete either H or HCO3- is the presence of the enzyme +

carbonic anhydrase. In the presence of CO2 CA forms carbonic acid that dissociates into H +

and HCO 3 − . Generally the H and HCO3 are transported in opposite directions. CA recently has been found in salivary acinar cells, and is important for the increase in cellular HCO3content during active salivation. It appears CA activity is increased by mass action; i.e., during cell depolarization energy utilization is increased which stimulates mitochondrial respiration. The subsequent increase in CO2 drives the CA mediated production of carbonic 2+

acid; the dissociated bicarbonate enters the lumen via an apical Ca +

H

+

+

crosses basolateral membranes via Na /H

activated anion channel.

exchange. Inhibition of CA by drugs

(acetozolamide) decreases cytosolic HCO3 levels and increases Cl- in saliva under activated 2+

conditions, because Cl- can move through the Ca activated anion channel when cytosolic HCO3 levels are not elevated. This accounts for the fact that acetozolamide blocks CA and salivary HCO3 production without a changing fluid or protein flux. VI. Regulation of Secretion:

Since the active components of salivary secretion are

expressed in the acinus, this is where the regulators of salivation act. Sympathetic stimulation through β-adrenergic receptors elevates protein secretion. However, through α-adrenergic receptors on vascular smooth muscle cells, sympathetic stimuli also can elicit vasoconstriction. Since increases in blood flow are required to increase salivary flow rate, sympathetic stimulation by itself leads to secretion of a very viscus saliva. This is observed in coincidence with the "fight or flight" and other stressful sympathetic mechanisms. Parasympathetic stimuli on the other hand, activate HCO3- and protein secretion at the level of the acinar cells, and fluid flux by elevating blood flow via relaxation of vascular smooth muscle. Acetylcholine activates the acinar cell while Substance P appears to be the vasodilatory transmitter thereby acting by elevating blood flow. No hormones have significant influence on salivary secretion rate, or composition. 2/17/2006 8:51 AM

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February 17, 2006

Salivary and Gastric Secretion

Ron Lynch, PhD 626-2472

GASTRIC SECRETIONS: (2000 ml/day) Objectives: 1. Determine the components of gastric secretions, the cells from which the specific components arise, and the function of each component. 2. Identify the basic constituents that provide the digestive and protective functions of gastric secretions. 3. What is the basic cellular mechanisms by which HCl is produced and secreted from the oxyntic cell? a. What are the key transport and enzymatic processes required for activated HCl secretion, and where in the cell are they located? 4. Determine how HCl secretion is regulated at the cellular level: a. What are the primary secretagogues for activating HCl secretion? b. What are the second messenger pathways through which the activators of HCl secretion operate? I.

Functional Anatomy - The secretory glands in the stomach can be delineated into 3 regional divisions. Cardiac glands are mucus secreting glands found primarily in proximal stomach. Gastric (or Oxyntic) glands are located in the central 3/4 of the stomach. These glands penetrate into the submucosa and secrete primarily hydrochloric acid (HCl), and Pepsinogen. There are also mucus secreting cells at the regions of the gland nearest the stomach lining. The third type of gland is smaller in size and located in the terminal Antral region. These Pyloric glands secrete mucus into stomach and gastrin into the blood.

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February 17, 2006

Salivary and Gastric Secretion

Ron Lynch, PhD 626-2472

II. Cellular Properties and Functions of the Oxyntic Glands.

Gastric Lumen Gastric Pits Columnar Epithelium Lamina Propria

Mucosa

Gastric Gland SubMucosa

Lymph Node Lymphatics

Mucularis Serosa

Gastric or Oxyntic glands are protrusions into the lining of the stomach. In the non-activated state, the density of the glands make the surface of the stomach look demarcated by cracks or pits. Lining the opening of the gastric pits near the epithelial surface are mucus and HCO3- secreting cells. The cells within the oxyntic gland and stomach epithelia are connected by tight junctions making them highly impermeant. This feature together with secretion of mucus and HCO3- provide the stomach surface with a protective barrier. Anything that alters the coupling between cells causes defects in the barrier and subsequent ulceration in the prescence of HCl. Similar cells are found lining cardiac and pyloric glands. Further down the gastric pits are multi-potential cells (mucus neck cells) which can migrate to the surface, or differentiate into secretory cells that remain within the gland. There are two types of secretory cells within the gland. These are the oxyntic (or parietal) cells and the peptic (or chief) cells. Parietal cells secrete HCl and the vitamin B12 binding protein intrinsic factor. Intrinsic factor is a glycoprotein which is essential for normal absorption of Vitamin B12 in the intestine. Peptic cells secrete pepsinogen. Pepsin is a proteolytic enzyme secreted in an inactive form (pepsinogen) and converted by stomach acidity or by autocatalysis to pepsin. Pepsin expresses maximal activity at pH < 5.0.

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February 17, 2006

Salivary and Gastric Secretion

Ron Lynch, PhD 626-2472

Basal (non-activated) secretion of HCl is relatively low, but significant. Resting stomach pH can be as low as 2.0. With maximal stimulation, lumenal pH can reach 1.0 or lower. Thus, the gradient between cell (pH 7.0) and lumen can be as high as 106. The HCl secreted by oxyntic cells produces an acid environment in the stomach lumen which helps to kill bacteria and to activate pepsin. However, the primary digestive role for HCl is to solubilize connective tissue and dissociate ionic bonds between molecules.

III. Mechanism of Hydrochloric Acid Secretion: Oxyntic cell. Upon activation of the oxyntic cell, mitochondial metabolism is elevated producing CO2. CO2 produced by cellular metabolism or entering the cell from the blood reacts with H2O under the influence of carbonic anhydrase to form carbonic acid which then dissociates into + + H+ and HCO3- . H+ are actively secreted into the lumen in exchange for K+ by the H -K ATPase while the HCO3- diffuses out of the cell into the blood in exchange for C1- via a HCO3-/Cl- antiporter. The release of HCO3 to the blood raises venous pH. The rise in venous pH that accompanies luminal H+ secretion is called the "alkaline tide".

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February 17, 2006

Salivary and Gastric Secretion

Ron Lynch, PhD 626-2472

+ Electroneutality must be maintained as the cell secretes H to the lumen and HCO3- to the blood. + During activation, Cl-/HCO3- antiport and Na /Cl- cotransport in the basolateral membrane assure + that cell Cl- levels are elevated. C1- is then secreted into the lumen through co-transport with K . + + + + Since the H ATPase exchanges H for K , and equi-molar Cl- is secreted with K , there is no net + K flux under activated (active secretion) conditions rather net HCl secretion.

Regulation of HCl Secretion: HCl from oxyntic cells is increased by Ach (parasympathetic), gastrin (endocrine) and histamine (paracine). Gastrin release from G cells is stimulated by gastrin releasing peptide (neural; submucosal plexus), and inhibited by somatostatin (paracrine).

Mechanisms whereby secretagogues elicit acid secretion from the parietal cells. The actions of blocking agents are shown: atropine (A), cimetidine (CM), lanthanum ion (La2+), and protaglandin E2 (PGE2). (from Johnson LR. editor: Physiology of the gastrointestinal tract, New York, 1981, Raven Press.)

Nerves whose cell bodies originate in the submucosal plexus release Ach that opens voltage 2+ dependent Ca channels after binding to oxyntic cells. Blockers of the voltage dependent channels block the neuronal mediated response. Gastrin is the most potent stimulus for HCl secretion. Gastrin release into the blood from G cells (in pyloric glands) is stimulated by peptides in the lumen and by the release of gastrin releasing peptide (GRP) from nerves whose cell bodies reside in the sub-mucosal plexus. Gastrin from the blood binds to specific receptors 2+ 2+ on the oxyntic cells, and leads to an elevation of cell Ca by releasing Ca from intracellular stores. Histamine also can stimulate HCl secretion, but it is not known if the secretion of histamine itself is regulated. The histamine containing cells (also known as Enterochromaffin cells, ECL) are located in the gastric pits, and appear to secrete continuously. The released histamine diffuses through the extracellular space binding to specific receptors (H2 subtype) on the oxyntic cell elevating cAMP concentration, which weakly activates the oxyntic cell. Histimine likely drives basal HCl secretion. 2/17/2006 8:51 AM

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February 17, 2006

V.

Salivary and Gastric Secretion

Ron Lynch, PhD 626-2472

Major Mechanisms for Stimulation of Gastric Acid Secretion: Secretion from all gastro-intestinal glands can be delineated into 3 phases: Cephalic, Gastric and Intestinal. The cephalic phase is initiated by central anticipatory signals like smell of food or time of day. The Gastric phase begins as food reaches the stomach eliciting distension and elevating the luminal pH by buffering its contents. Strong stimulation of HCl secretion is initiated by stomach distension which elicits local reflexes (release of Ach on oxyntic cells; GRP on G-cells). Also, buffering of lumenal pH inhibits somatostatin release removing a break on gastrin secretion, and nutrients in the stomach lumen (amino acids) directly stimulate G cells to secrete. The Intestinal phase is driven by mild activation of stretch, osmo- and nutrient receptors in the intestinal lumen though again these stimuli are minor relative to the Gastric stimuli. For Cephalic stimuli, reflexes of central origin elicit initial responses coordinated within the neural plexi, but without local stimuli (gastric distension, lumenal amino acids), the secretory response is relatively weak amounting to usually 10% of the maximal HCl output. Conversely, gastric signals are generated within the canal. For example, amino acids act directly on the G-cell and local pH buffering reduces G-cell inhibition, thereby providing strong signals for gastrin release with gastrin being the strongest activator of HCl secretion. Moreover, local distension in the stomach provides rapid and intense input to the neural plexi leading to strong activation of oxyntic cell secretion (release of Ach), as well as coordinated increases in stomach motility. Major Mechanisms for Stimulation of Gastric Acid Secretion

Phase

Stimulus

Pathway

Cephalic

Chewing, swallowing, etc.

Vagus nerve to Plexi 1. Parietal cells 2. G cells Local and vagovagal reflexes to 1. Parietal cells 2. G cells 1. Intestinal endocrine cells

Gastric

Gastric distension

Intestinal

Protein digestion products in duodenum: weak stimulants

InterDigestive

Basal Release

ECL Cells- paracrine

Stimulus to Parietal Cell Acetylcholine Gastrin, Histamine Acetylcholine Gastrin Enterooxyntin

Histamine

Modified from Johnson LR: Gastrointestinal physiology, ed 3, St. Louis, 1985, The CV Mosby Co. from MI Grossman.

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February 17, 2006

VI.

Salivary and Gastric Secretion

Ron Lynch, PhD 626-2472

Inhibition of Gastric Secretion: Signals originating from duodenum. There are several primary enteric mechanisms which can act to inhibit gastric secretion. These mechanisms are most effective when chyme pH < 3. Acid (pH < 5.0), stretch and protein products in the intestine generate short and long enterogastric reflexes which inhibit acid production in the stomach, and also can inhibit gastrin release leading to reduced acid secretion. In addition, pH < 2.0 in the stomach promotes somatostatin release, which decreases gastrin secretion. It is clear that the level of acid in the duodenum and pyloris are the primary signals for feedback regulation of oxyntic secretion. However, several other parameters may also contribute to the feedback. For example, hyperosmotic solutions in the duodenum will lower acid secretion apparently by releasing a hormone from the intestinal wall that feeds back to the oxyntic cells (i.e., the candidate hormone: enterogastrone). The other signals are identical to those previously discussed for the regulation of gastric emptying.

Body

ECL

Antrum

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