Gastroenterology

Ammonia and Urea Cycle

July 20, 2017 Gastroenterology, Medicinal Chemistry, Nephrology, Physiology and Pathophysiology No comments , , , , ,

Ammonia (NH3) is a small metabolite that results predominantly from protein and amino acid degradation. It is highly membrane-permeant and readily crosses epithelial barriers in its nonionized form.

Ammonia does not have a physiologic function. However, it is important clinically because it is highly toxic to the nervous system. Because ammonia is being formed constantly from the deamination of amino acids derived from proteins, it is important that mechanisms exist to provide for the timely and efficient disposal fo this molecule. The liver is critical for ammonia catabolism because it is the only tissue in which all elements of the urea cycle, also known as the Krebs-Henseleit cycle, are expressed, providing for the conversion of ammonia to urea. Ammonia is also consumed in the synthesis of nonessential amino acids, and in various facets of intermediary metabolism.

Ammonia Formation and Disposition

Ammonia in the circulation originates in a number of different sites. A diagram showing the major contributors to ammonia levels is shown in 14-1. Note that the liver is efficient in taking up ammonia from the portal blood in health, leaving only approximately 15% to spill over into the systemic circulation.

Intestinal Production

The major contributor to plasma ammonia is the intestine, supplying about 50% of the plasma load. Intestinal ammonia is derived via two major mechanisms. First, ammonia is liberated from urea in the intestinal lumen by enzymes known as ureases. Ureases are not expressed by mammalian cells, but are products of many bacteria, and convert urea to ammonia and carbon dioxide. Indeed, this provides the basis for a common diagnostic test, since H. pylori, which colonizes the gastric lumen and has been identified as a cause of peptic ulcer disease, has a potent urease. Therefore, if patients are given a dose of urea labeled with carbon-13, rapid production of labeled carbon dioxide in the breath is suggestive of infection with this microorganism.

Second, after proteins are digested by either host or bacterial proteases, further breakdown of amino acids generates free ammonia. Ammonia in its unionized form crosses the intestinal epithelium freely, and enters the portal circulation to travel to the liver; however, depending on the pH of the colonic contents, a portion of the ammonia will be protonated to ammonium ion. Because the colonic pH is usually slightly acidic, secondary to the production of short-chain fatty acids, the ammonium is thereby trapped in the lumen and can be eliminated in the stool.

Extraintestinal Production

The second largest contributor to plasma ammonia levels is the kidney. Ammonia is also produced in the liver itself during the deamination of amino acids. Minor additional components of plasma ammonia derive from adenylic acid metabolism in muscle cells, as well as glutamine released from senescent red blood cells.

Urea Cycle

The most important site for ammonia catabolism is the liver, where the elements of the urea cycle are expressed in hepatocytes. Ammonia derived from the sources described earlier is converted in the mitochondria to carbamoyl phosphate, which in turn reacts with ornithine to generate citrulline. Citrulline, in turn, reacts in the cytosol with aspartate, produced by the deamination of glutarate, to yield sequentially arginine succinate then arginine itself. The enzyme arginase then dehydrates arginine to yield urea and ornithine, which returns to the mitochondria and can reenter the cycle to generate additional urea. The net reaction is the combination of two molecules of ammonia with one of carbon dioxide, yielding urea and water.

Urea Disposition

A “mass balance” for the disposition of ammonia and urea is presented in Figure 14-2. As a small molecule, urea can cross cell membranes readily. Likewise, it is filtered at the glomerulus and enters the urine. While urea can be passively reabsorbed across the renal tubule as the urine is concentrated, its permeability is less than that of water such that only approximately half of the filtered load can be reabsorbed. Because of this, the kidney serves as the site where the majority of the urea produced by the liver is excreted. However, some circulating urea may passively back diffuse into the gut, where it is acted on by bacterial ureases to again yield ammonia and (CO2?). Some of the ammonia generated is excreted in the form of ammonium ion; the remainder is again reabsorbed to the handled by the liver once more.

Pancreatic Secretion and Its Regulation

April 17, 2016 Gastroenterology, Physiology and Pathophysiology No comments , , , , , , , ,

Overall View of Pancreatic Secretion Regulation

Pancreatic secretory activity related to meal ingestion occurs in cephalic (20-25%), gastric (10%), and intestinal phases (~60-70%). Pancreatic secretion is activated by a combination of neural and hormonal effectors. During the cephalic and gastric phases, secretions are low in volume with high concentrations of digestive enzymes, reflecting stimulation primarily of acinar cells. This stimulation arises from cholinergic vagal input during the cephalic phase, and vago-vagal reflexes activated by gastric distension during the gastric phase. During the intestinal phase, on the other hand, ductular secretion is strongly activated, resulting in the production of high volumes of pancreatic juice with decreased concentrations of protein, although the total quantity of enzymes secreted during this phase is actually also markedly increased. Ductular secretion during this phase is driven primarily by the endocrine action of secretin on receptors localized to the basolateral pole of duct epithelial cells. The inputs to the acinar cells during the intestinal phase include CCK and 5-HT from the intestine as well as neurotransmitters including ACh and GRP. The large magnitude of the intestinal phase is also attributable to amplification by so-called enteropancreatic reflexes transmitted via the enteric nervous system.

  • Cholinergic vagal input – cephalic phase – acinar secretion
  • Vago-vagal reflexes – gastric phase – acinar secretion
  • Secretin – intestinal phase – ductular (cells) secretion
  • CCK (via vago-vagal reflexes and non-ACh neurotransmitters) by CCK-RP and monitor peptide – intestinal phase – acinar cells
  • 5-HT (via vago-vagal reflexes) – intestinal phase – acinar cells
  • ACh – cephalic, gastirc, and intestinal phases – acinar cells
  • GRP – cephalic, gastirc, and intestinal phases – acinar cells
  • Enteropancreatic reflexes transmitted via the enteric nervous system – intestinal phase – ?

Mechanisms of Regulation of Pancreatic Secretion (Primarily the Intestinal Phase)

CCK

CCK can be considered a master regulator of the duodenal cluster unit, of which the pancreas is an important component. CCK is a potent stimulus of acinar secretion, acting predominantly via CCK1 receptor-dependent stimulation of vagal afferents close to its site of release in the duodenum, thereby evoking vago-vagal reflexes that stimulate acinar cell secretion via cholinergic and non-cholinergic neurotransmitters (GRP, VIP). Threre are also CCK1 receptors on the basolateral pole of acinar cells, but it now seems likely that these are only activated if circulating concentrations of CCK rise to supraphysiologic levels.

In addition to its effects on the pancreas, CCK coordinates the activity of other GI seggments and draining organ, including by contract the gallbladder, relaxing the sphincter of Oddi, and slowing gastric motility to retard gastric emptying and thereby control the rate of delivery of partially digested nutrients to more distal segments of the gut. Finally, CCK can modulate the activity of other neurohumorla regulators in a synergistic fashion. For example, CCK itself is a weak agonist of pancreatic ductular secretion of bicarbonate, but it markedly potentiates the effect of secretin on this transport mechanism.

CCK is synthesized and stored by endocrine cells located predominantly in the duodenum, labeled in some sources as "I" cells. Control of CCK release from these cells is carefully regulated to match the body's needs for CCK bioactivity. In part, this is accomplished by the activity of two luminally active CCK releasing factors, which are small peptides. One of these peptides is derived from cells in the duodenum, called CCK-releasing peptide (CCK-RP). It is likely release into the lumen in response to specific nutrients, including fatty acids and hydrophobic amino acids. The other luminal peptide that controls CCK secretion is monitor peptide, which is a product of pancreatic acinar cells. Release of monitor peptide can be neurally mediated, including by the release of ACh and GRP in the vicinity of pancreatic acinar cells during the cephalic phase, and mediated by subsequent vago-vagal reflexes during the gastric and intestinal phases of the response to a meal. Likewise, once CCK release has been stimulated by CCK-RP, it too can cause monitor peptide release via the mechanisms of vago-vagal reflexes.

When meal proteins and oligopeptides are present in the lumen in large  quantities, they compete for the action of trypsin and other proteolytic enzymes, meaning that CCK-RP and monitor peptide are degraded only slowly. Thus, CCK release is sustained, causing further secretion of proteases and other components of the pancreatic juice. On the other hand, once the meal has been fully digested and absorbed, CCK-RP and monitor peptide will be degraded by the pancreatic proteases. This then lead to the termination of CCK release, and thus a marked reduction in the secretion of pancreatic enzymes.

  • CCK-RP by duodenum cells
  • Monitor peptide by acinar cells

5-HT

5-HT, released from intestinal enterochromaffin cells in response to nutrients, activates a vagovagal reflex that mirrors and augments that of CCK itself. It has been calculated that CCK and 5-HT are each responsible for about 50% of pancreatic enzyme secretion during the intestinal phase.

Secretin

The other major regulator of pancreatic secretion is secretin, which is released from S cells in the duodenal mucosa. When the meal enters the small intestine from the stomach, the volume of pancreatic secretions increases rapidly, shifting from a low-volume, protein-rich fluid to a high volume secretion in which enzymes are present at lower concentrations (although in greater absolute amounts, reflecting the effect of CCK and neural effectors on acinar cell secretion). As the secretory rate rises, the pH and bicarbonate concentration in the pancreatic juice rises, with a reciprocal fall in the concentration of chloride ions. These latters effects on the composition of the pancreatic juice are mediated predominately by the endocrine mediator, secretin.

The S cells in the duodenal mucosa can be considered to act functionally as pH meters, sensing the acidity of the luminal contents. As the pH falls, due to the entry of gastric acid, secretin is released from the S cells and travels through the bloodstream to bind to receptors on pancreatic duct cells, as well as on epithelial cells lining the bile ducts and the duodenum itself. These cells, in turn, are stimulated to secrete bicarbonate into the duodenal lumen, thus causing a rise in pH that will eventually shut off secretin release. The pancreas is quantitatively the most important in the bicarbonate secretory response, although the ability of duodenal epithelial cells to secrete bicarbonate may be critically important to protect them from gastric acid, especially in the first part of the duodenum, which is proximal to the site of entry of the pancreatic juice and bile. In fact, patients suffering from duodenal ulcers have abnormally low levels of duodenal bicarbonate secretion both at rest and in response to luminal acidification.

The threshold for secretin release appears to be a luminal pH of less than 4.5. The mechanism by which the S cells sense the change in luminal acidity, and whether secretin release requires a peptide releasing factor and/or the function of mucosal sensory nerve endings is currently unclear. However, while other meal components, such as fatty acids, have been shown in experimental studies to evoke secretin release, the response to acid appears to be the most important physiologically. Subjects who are unable to secrete gastric acid (achlorhydric) secondary to disease or the administration of proton pump inhibitors, or in whom gastric contents have been neutralized by the oral administration of bicarbonate, fail to release secretin postprandially no matter what type of meal is given.

Gastric Secretion and Its Regulation

April 2, 2016 Gastroenterology, Physiology and Pathophysiology No comments , , , , , , , , , ,

Regulation Mechanism of Gastrirc Secretion

Short and Long Reflexes

Neural input provides an important mechanism for regulation of gastric secretion. Reflexes contribute to both the stimulation and inhibition (e.g., CGRP) of secretion. For example, distension of the stomach wall, sensed by stretch receptors, activates reflexes that stimulate acid secretion at the level of the parietal cell. These reflexes may be so-called short reflexes, which involve neural transmission contained entirely within the enteric nervous system. In addition, long reflexes also contribute to the control of secretion. These involve the activation of primary afferents that travel through the vagus nerve, which in turn are interpreted in the dorsal vagal complex and trigger vagal outflow via efferent nerves that travel back to the stomach and activate parietal cells or other components of the secretory machinery. These long reflexes are aslo called vagovagal reflexes. The relative contribution of short and long reflexes to the control of secretion is unknown. However, it is clear that selective gastric vagotomy eliminates some, although not all, of the gastric secretory response to distension as well as a portion of related gastric motor responses.

Acetylcholine is an important mediator of both short and long reflexes in the stomach. It participates in the stimulation of parietal, cheif, and ECL cells; the supression of D cells; and the synapses between nerves within the enteric nervous system. In addition, a second imporant gastric neurotransmitter is gastrin releasing peptide, or GRP. This neuropeptide is the mammalian homologue of one known as bombesin originally isolated from frog skin. GRP is released by enteric nerves in the vicinity of gastrin-containing G cells in the gastric antrum.

  • Short reflexes, ENS (ACh)
  • Long reflexes/vagovagal reflexes, ANS (ACh)
  • GRP
  • CGRP

Humoral Control

The gastric secretory response is also regulated by soluble factors that originate from endocrine and other regulatory cell types, such as ECL cells. The primary endocrine regulator of gastric secretion is gastrin, which actually consists of a family of peptides. Gastrin travels through the bloodstream from its site of release in the antral mucosa to stimulate parietal and ECL cells via their CCK2 receptors.

PS: CCK2 receptors are expressed on the parietal and ECL cells.

Gastric secretion is also modified by paracrine mediators. Histamine is released from ECL cells under the combined influence of gastrin and ACh, and diffuses to neighboring parietal cells to activate acid secretion via histamine H2 receptors. At one time histamine was thought to be the final common mediator of acid secretion, based in part on the clinical observation that histamine H2 receptor antagonists can profoundly inhibit acid secretion. However, it is now known that parietal cells express receptors for not only histamine, but also ACh (muscarinic m3) and gastrin (CCK2). Because histamine H2 receptors are linked predominantly to signaling pathways that involve cAMP, while ACh and gastrin signal through calcium, when the parietal cell is acted upon simultaneously by all three stimuli, a potentiated, or greater than additive, effect on acid secretion results. The physiological implication of this potentiation, or synergism, is that a greater level of acid secretion can be produced with relatively small increases in each of the three stimuli. The pharmacological significance is that simply interfering with the action of any one of them can significantly inhibit acid secretion.

Acid secretion is also subject to negative regulation by specific mediators. Specifically, somatostatin is released from D cells in response to an axon reflex that release CGRP (calcitonin gene-related peptide) in the antral mucosa when luminal pH falls below 3, and inhibits the release of gastrin from G cells. Elsewhere in the stomach, somatostatin can also exert inhibitory influences on ECL, parietal, and chief cells. The SSTR2 somatostatin receptor is responsible for the inhibitory effects of the peptide in the stomach. In fact, there is data to support the idea that gastirc secretion under resting conditions is tonically suppressed by somatostatin. When stimulated responses occur, they are due not only to the active stimulatory mechanisms disscussed above, but also specific suppression of the inhibitory effects of somatostatin, involving the actions of both ACh (via m2 and m4 receptors) and histamine (via H3 receptors) on D cells.

  • Gastrin
  • Histamine
  • Somatostatin (negative regulation)

Luminal Regulators

Specific luminal constituents also modulate gastric secretion indirectly. The example of pH is described earlier, but acid output, at least, is also increased by components of the meal. Short peptides and amino acids, derived from dietary protein secondary to the action of pepsin released from chief cells, are capable of activating gastrin release from G cells. Aromatic amino acids are the most potent, and "receptors" for these are assumed to reside on the apical membrane of the open G-type endocrine cells, although their structure remains to be defined.

Gastric acid secretion is also activated by alcoholic beverages, coffee, and dietary calcium. The effects of alcoholic beverages may not be due to ethanol itself, but rather the amino acids present in the veverage, particularly in beer and wine. Likewise, the effect of coffee does not appear to be attributable to caffeine, since decaffeinated coffee also increases secretion.

  • Food (bufferring/increasing pH)
  • Food (short peptides and amino acids, alcoholic beverages, coffee, dietary calcium)

Summar of Regulators of Gastric Secretion

Product Source Function
Histamine ECL cells amplify acid secretion
    suppress somatostatin secretion
Gastrin G cells amplify acid secretion
    amplify histamine secretion
GRP Nerves stimuate the secretion of gastrin
CGRP Nerves stimulate the secreton of somatostatin
Ach Nerves amplify acid secretion
    amplify pepsinogen secretion
    amplify histamine secretion
    suppress somatostatin secretion
Somatostatin D cells suppress gastrin secretion
    suppress histamine secretion
    suppress acid secretion
    suppress pepsinogen secretion

Regulation of Gastric Secretion

Regulation of Scretion in the Interdigestive Phase

Between meals, the stomach secretes acid and other secretory products at a low level, perhaps to aid in maintaining the sterility of the stomach. However, because no food is present, and thus no buffering capacity of the gastric content, the low volume of secretions produced nevertheless have a low pH – usually around 3.0. Basal acid output in the healthy human is in the range of 0-11 mEq/h, which can be contrasted with the maximal rates that can be produced by ingestion of a meal, or intravenous administration of gastrin, of 10-63 mEq/h. The basal secretion rate is believed to reflect the combined influences of histamine and ACh, released from ECL cells and nerve endings, respectively, tempered by the influence of somatostatin from fundic D cells. Gastrin secretion during the interdigestive period, on the other hand, is minimal. This is because gastrin release is suppressed by a luminal pH of 3 or below, via the release of somatostatin from antral D cells.

Regulation of Postprandial Secretion

In conjunction with a meal, gastric acid secretion can be considered to occur in three phases – cephalic, gastric, and intestinal. The major portion of secretion occurs during the gastric phase, when the meal is actually present in the stomach. Secretion of other gastric products usually parallels that of the acid.

Cephalic Phase

Even before the meal is ingested, the stomach is readied to receive it by the so-called cephalic phase of secretion. In fact, during the cephalic phase, the functions of several gastrointestinal systems in addition to the stomach begin to be regulated, including the pancreas and gallbladder. Higher brain centers respond to the sight, smell, taste and even thought of food, and relay information to the dorsal vagal complex. In turn, vagal outflow initiates both secretory and motor behavior in the stomach and more distal segments. Gastric secretion occuring during the cephalic phase readies the stomach to receive the meal. Vagal outflow activates enteric nerves that in turn release GRP and ACh. Release of GRP in the vicinity of antral G cells releases gastrin that travels through the bloodstream to activate parietal and chief cells in the endocrine fashion. ACh also suppresss ongoing somatostatin release.

Gastric Phase

The gastric phase of secretion is quantitatively the most important. In addition to vagal influences continuing from the cephalic phase, secretion is now amplified further by mechanical and chemical stimuli that arise from the presence of the meal in the lumen. These include the luminal signals discussed earlier, and signals arising from stretch receptors embedded in the wall of the stomach. Thus, as the stomach distends to accommodate the volume of the meal, these receptors initiate both short and long reflexes to further enhance secretory responses either directly, via the release of ACh in the vicinity of parietal cells, or indirectly, via the release of ACh that activate ECL cells, or GRP that activates G cells to release gastrin. These vago-vagal reflexes also transmit information downstream to ready more distal segments of the intestine to receive the meal. The gastric phase of secretion is also accompanied by a marked increase in gastric blood flow, which supplies the metabolic requirements of the actively secreting cell types.

Due to the combined influence of neurocrine and endocrine signals, further amplified by histamine release from ECL cells, secretory cells of the stomach are highly active during the gastric phase. Moreover, pepsinogen released by chief cells is rapidly cleaved to pepsin in an autocatalytic reaction that occurs optimally at pH 2, and this pepsin then acts on ingested protein to release short peptides and amino acids that further enhance gastrin release. Moreover, many dietary substances, including proteins, are highly effective buffers. Thus, while acid secretory rates remain high, the effective pH in the bulk of the lumen may rise to pH 5. This ensures that the rate of acid secretion during the gastric phase is not attenuated by an inhibition of gastrin release that would otherwise be mediated by somatostatin (when pH <=3).

Intestinal Phase

As the meal moves out of the stomach into the duodenum, the buffering capacity of the lumen is reduced and the pH begins to fall. At a threshold of around pH 3, CGRP triggers somatostatin release from D cells in the gastric antrum, which acts on G cells to suppress gastrin release. Somatostatin released from D cells in the oxyntic mucosa, or from nerve endings, likely also acts directly to inhibit secretory function. This acid-sensing response is a neural pathway that involves the activation of chemoreceptors sensitive to pH, which in turn leads to the release of CGRP via an axon reflex. Other signals also limit the extent of gastric secretion when the meal has moved into the small intestine. For example, the presence of fat in the small intestine is associated with a reduction in gastric secretion. This feedback response is believed to involve several endocrine and paracrine factors, including GIP and CCK, the latter of which binds to CCK1 receptors on D cells.

(The End)