Pancreas function

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EXOCRINE PANCREAS FUNCTIONS AND DISORDERS

• M.Prasad Naidu
• MSc Medical Biochemistry,
• Ph.D.Research Scholar

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• The pancreas secretes 15003000 mL of isosmotic
  alkaline (pH gt8) fluid per day containing about
  20 enzymes.
• The pancreatic secretions provide the enzymes
  needed to effect the major digestive activity of
  the gastrointestinal tract and provide an optimal
  pH for the function of these enzymes.

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• Regulation of Pancreatic Secretion
• The exocrine pancreas is influenced by intimately
  interacting hormonal and neural systems.
• Gastric acid is the stimulus for the release of
  secretin from the duodenum, which stimulates the
  secretion of water and electrolytes from
  pancreatic ductal cells.
• Release of cholecystokinin (CCK) from the
  duodenum and proximal jejunum is largely
  triggered by long-chain fatty acids, certain
  essential amino acids (tryptophan, phenylalanine,
  valine, methionine), and gastric acid itself.
• CCK evokes an enzyme-rich secretion from acinar
  cells in the pancreas.

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• Water and Electrolyte Secretion
• Bicarbonate is the ion of primary physiologic
  importance within pancreatic secretion.
• The ductal cells secrete bicarbonate
  predominantly derived from plasma (93) more than
  from intracellular metabolism (7).
• Bicarbonate enters through the sodium bicarbonate
  cotransporter with depolarization caused by
  chloride efflux through the cystic fibrosis
  transmembrane conductance regulator (CFTR).
• Secretin and VIP, both of which increase
  intracellular cyclic AMP, act on the ductal cells
  opening the CFTR in promoting secretion.
• CCK, acting as a neuromodulator, markedly
  potentiates the stimulatory effects of secretin.
• Acetylcholine also plays an important role in
  ductal cell secretion. Bicarbonate helps
  neutralize gastric acid and creates the
  appropriate pH for the activity of pancreatic
  enzymes and bile salts.

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• Enzyme Secretion
• The acinar cell is highly compartmentalized and
  is concerned with the secretion of pancreatic
  enzymes.
• Proteins synthesized by the rough endoplasmic
  reticulum are processed in the Golgi and then
  targeted to the appropriate site.
• Amylolytic enzymes such as amylase, hydrolyze
  starch to oligosaccharides and to the
  disaccharide maltose.
• The lipolytic enzymes include lipase,
  phospholipase A2, and cholesterol esterase. Bile
  salts inhibit lipase in isolation, but colipase,
  another constituent of pancreatic secretion,
  binds to lipase and prevents this inhibition.
  Bile salts activate phospholipase A and
  cholesterol esterase.

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• Proteolytic enzymes include endopeptidases
  (trypsin, chymotrypsin), which act on internal
  peptide bonds of proteins and polypeptides
  exopeptidases (carboxypeptidases,
  aminopeptidases), which act on the free carboxyl-
  and amino-terminal ends of peptides,
  respectively and elastase.
• The proteolytic enzymes are secreted as inactive
  precursors and packaged as zymogens.
• Ribonucleases (deoxyribonucleases, ribonuclease)
  are also secreted.
• Enterokinase, an enzyme found in the duodenal
  mucosa, cleaves the lysine-isoleucine bond of
  trypsinogen to form trypsin.
• Trypsin then activates the other proteolytic
  zymogens and phospholipase A2 in a cascade
  phenomenon.
• All pancreatic enzymes have pH optima in the
  alkaline range.

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• The nervous system initiates pancreatic enzyme
  secretion.
• The stimulatory neurotransmitters are
  acetylcholine and gastrin-releasing peptides.
• These neurotransmitters activate
  calcium-dependent second messenger systems,
  resulting in the release of zymogen granules.

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• Autoprotection of the Pancreas
• Autodigestion of the pancreas is prevented by the
  packaging of pancreatic proteases in precursor
  form and by the synthesis of protease inhibitor
  i.e., pancreatic secretory trypsin inhibitor
  (PSTI) (SPINK1) , which can bind and inactivate
  about 20 of trypsin activity.
• Mesotrypsin, chymotrypsin c, and enzyme y can
  also lyse and inactivate trypsin.
• These protease inhibitors are found in the acinar
  cell, the pancreatic secretions, In addition, low
  calcium concentration within the cytosol of
  acinar cells in the normal pancreas promotes the
  destruction of spontaneously activated trypsin.
• Loss of any of these protective mechanisms leads
  to zymogen activation, autodigestion, and acute
  pancreatitis.

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• Exocrine-Endocrine Relationships
• Insulin appears to be needed locally for secretin
  and CCK to promote exocrine secretion thus, it
  acts in a permissive role for these two hormones.
• Enteropancreatic Axis and Feedback Inhibition
• The available evidence supports the concept that
  the duodenum contains a peptide called
  CCK-releasing factor that is involved in
  stimulating CCK release.
• It appears that serine proteases inhibit
  pancreatic secretion by inactivating a
  CCK-releasing peptide in the lumen of the small
  intestine.
• Acidification of the duodenum releases secretin,
  which stimulates vagal and other neural pathways
  to activate pancreatic duct cells, which secrete
  bicarbonate.

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Acute Pancreatitis

• Pancreatic inflammatory disease may be classified
  as (1) acute pancreatitis or (2) chronic
  pancreatitis.
• The pathologic spectrum of acute pancreatitis
  varies from interstitial pancreatitis, which is
  usually a mild and self-limited disorder, to
  necrotizing pancreatitis, in which the extent of
  pancreatic necrosis may correlate with the
  severity of the attack and its systemic
  manifestations.

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• Causes of Acute Pancreatitis
• Gallstones (including microlithiasis)
• Alcohol (acute and chronic alcoholism)
  Hypertriglyceridemia
• Endoscopic retrograde cholangiopancreatography
  (ERCP),
• blunt abdominal trauma
• Postoperative
• Drugs (azathioprine, 6-mercaptopurine,
  sulfonamides, estrogens, tetracycline, valproic
  acid, anti-HIV medications)
• Sphincter of Oddi dysfunction

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• Hypertriglyceridemia is the cause of acute
  pancreatitis in 1.33.8 of cases.
• Any factor (e.g., drugs or alcohol) that causes
  an abrupt increase in serum triglycerides to
  levels gt11 mmol/L (1000 mg/dL) can precipitate a
  bout of acute pancreatitis.
• Finally, patients with a deficiency of
  apolipoprotein CII have an increased incidence of
  pancreatitis apolipoprotein CII activates
  lipoprotein lipase, which is important in
  clearing chylomicrons from the bloodstream.
• Approximately 25 of cases of acute pancreatitis
  are drug related.

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• Several recent studies have suggested that
  pancreatitis is a disease that evolves in three
  phases.
• The initial phase is characterized by
  intrapancreatic digestive enzyme activation and
  acinar cell injury.
• Trypsin activation appears to be mediated by
  lysosomal hydrolases such as cathepsin B that
  become colocalized with digestive enzymes in
  intracellular organelles it is currently
  believed that acinar cell injury is the
  consequence of trypsin activation.
• The second phase of pancreatitis involves the
  activation, chemoattraction, and sequestration of
  leukocytes and macrophages in the pancreas,
  resulting in an enhanced intrapancreatic
  inflammatory reaction.

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• The third phase of pancreatitis is due to the
  effects of activated proteolytic enzymes and
  cytokines, released by the inflamed pancreas, on
  distant organs.
• Activated proteolytic enzymes, especially
  trypsin, not only digest pancreatic and
  peripancreatic tissues but also activate other
  enzymes such as elastase and phospholipase A2.
• The active enzymes and cytokines then digest
  cellular membranes and cause proteolysis, edema,
  interstitial hemorrhage, vascular damage,
  coagulation necrosis, fat necrosis, and
  parenchymal cell necrosis.
• Cellular injury and death result in the
  liberation of bradykinin peptides, vasoactive
  substances, and histamine that can produce
  vasodilation, increased vascular permeability,
  and edema with profound effects on many organs,
  most notably the lung.
• The systemic inflammatory response syndrome
  (SIRS) and acute respiratory distress syndrome
  (ARDS) as well as multiorgan failure may occur as
  a result of this cascade of local as well as
  distant effects.

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• There appear to be a number of genetic factors
  that can increase the susceptibility and/or
  modify the severity of pancreatic injury in acute
  pancreatitis.
• Four susceptibility genes have been identified
• (1) cationic trypsinogen mutations ,
• (2) pancreatic secretory trypsin inhibitor ,
• (3) CFTR, and
• (4) monocyte chemotactic protein (MCP-1).
• Experimental and clinical data indicate that
  MCP-1 may be an important inflammatory mediator
  in the early pathologic process of acute
  pancreatitis, a determinant of the severity of
  the inflammatory response, and a promoter of
  organ failure.

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• Pancreatic Secretory Trypsin Inhibitor (Psti)
  Gene Mutations
• PSTI, or SPINK1, is a 56-amino-acid peptide that
  specifically inhibits trypsin by physically
  blocking its active site. SPINK1 acts as the
  first line of defense against prematurely
  activated trypsinogen in the acinar cell.
• Recently, it has been shown that the frequency of
  SPINK1 mutations in patients with
  idiopathicchronic pancreatitis is markedly
  increased, suggesting that these mutations may be
  associated with pancreatitis.

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• Laboratory Data
• The diagnosis of acute pancreatitis is usually
  established by the detection of an increased
  level of serum amylase and lipase.
• Values threefold or more above normal virtually
  clinch the diagnosis if gut perforation,
  ischemia, and infarction are excluded.
• Serum lipase activity increases in parallel with
  amylase activity.
• A threefold elevated serum lipase value is
  usually diagnostic of acute pancreatitis these
  tests are especially helpful in patients with
  nonpancreatic causes of hyperamylasemia.

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• Hypocalcemia occurs in 25 of patients.
  Leukocytosis occurs frequently.
• Hyperglycemia
• Hyperbilirubinemia serum bilirubin gt68 mol/L
  (gt4.0 mg/dL) occurs in 10 of patients.
• Hypertriglyceridemia occurs in 510 of patients,
  
• Finally, the electrocardiogram is occasionally
  abnormal in acute pancreatitis with ST-segment
  and T-wave abnormalities simulating myocardial
  ischemia.

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• Chronic Pancreatitis and Pancreatic Exocrine
  Insufficiency
• Chronic pancreatitis is a disease process
  characterized by irreversible damage to the
  pancreas as distinct from the reversible changes
  noted in acute pancreatitis.
• The condition is best defined by the presence of
  histologic abnormalities, including chronic
  inflammation, fibrosis, and progressive
  destruction of both exocrine and eventually
  endocrine tissue.

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• Causes
• Alcoholic
• Tobacco smoking
• Hypercalcemia
• Hyperlipidemia
• Chronic renal failure
• CFTR mutations
• SPINK1 mutations
• Isolated autoimmune chronic pancreatitis
• Postnecrotic (severe acute pancreatitis)
• Recurrent acute pancreatitis

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• alcoholism is the most common cause of clinically
  apparent chronic pancreatitis, while cystic
  fibrosis is the most frequent cause in children.
• Recent investigations have indicated that up to
  15 of patients with idiopathic pancreatitis may
  have pancreatitis due to genetic defects .
• mutations of CFTR.
• This gene functions as a cyclic AMPregulated
  chloride channel.
• In patients with cystic fibrosis, the high
  concentration of macromolecules can block the
  pancreatic ducts.

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• Patients with chronic pancreatitis seek medical
  attention predominantly because of two symptoms
  abdominal pain or maldigestion and weight loss.
• In contrast to acute pancreatitis, the serum
  amylase and lipase levels are usually not
  strikingly elevated in chronic pancreatitis.
• The diagnostic test with the best sensitivity and
  specificity is the hormone stimulation test
  utilizing secretin. It becomes abnormal when 60
  of the pancreatic exocrine function has been
  lost. This usually correlates well with the onset
  of chronic abdominal pain.

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Assessment of pancreatic function

• Measurement of pancreatic enzymes
• Amylase or alpha 1,4 glucosidase is the major
  enzyme which digests starch.
• The serum amylase contains the P ( pancreatic )
  
• S ( salivary) isoenzymes.
• These two can be distinguised by the inhibition
  test.
• Normal amylase level in serum is 50 120 Units.

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• The level rises within 5 hr of the onset of acute
  pancreatitis.
• The level reaches a peak(4-6 fold) within 12
  hours.
• Within 2-4 days of the attack, the level returns
  to normal.
• As the serum amylase level starts falling,
  urinary amylase level rises.
• Amylase level in blood is mildly increased in
  cases of cholecystitis, peptic ulcer, diseases of
  mesentry obstruction of intestine.
• Chronic pancreatitis no change or only mild
  elevation is noticed .

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• CLEARANCE RATIO
• If the sample is collected too early, the serum
  amylase levels may not show the expected rise.
• If the sample is collected too late, again serum
  amylase may be low due to necrosis of the
  pancreatic tissue.
• Calculation of clearance ratio will avoid these
  defects.
• CR Urine amylase level x Scr x
  100
• Serum amylase level Ucr
  
• ( Scr Ucr is serum urinary creatinine level )
• In patients with acute pancreatitis, the ratio
  varies from 7-15.
• The normal ratio is 1-4.4

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• serum lipase
• Major lipolytic enzyme which hydrolyzes glycerol
  esters of long chain fatty acids.
• Normal level in serum 30-235 U/L
• The level in blood is highly elevated(2-50 fold)
  in acute pancreatitis.This persists for 7-14
  days.
• Thus lipase remains elevated longer than amylase.
• Lipase is not elevated in salivary diseases.
• Lipase estimation has advantage over amylase.

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• Macroamylasemia
• In macroamylasemia, amylase circulates in the
  blood in a polymer form too large to be easily
  excreted by the kidney. Patients with this
  condition demonstrate an elevated serum amylase
  value, a low urinary amylase value, and a Cam/Ccr
  ratio of lt1. The presence of macroamylase can be
  documented by chromatography of the serum. The
  prevalence of macroamylasemia is 1.5 of the
  nonalcoholic general adult hospital population.
  Usually macroamylasemia is an incidental finding
  and is not related to disease of the pancreas or
  other organs.
• Macrolipasemia has now been documented in a few
  patients with cirrhosis or non-Hodgkin's
  lymphoma. In these patients, the pancreas
  appeared normal on ultrasound and CT examination.
  Lipase was shown to be complexed with
  immunoglobulin A. Thus, the possibility of both
  macroamylasemia and macrolipasemia should be
  considered in patients with elevated blood levels
  of these enzymes.

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• The secretin test, used to detect diffuse
  pancreatic disease, is based on the physiologic
  principle that the pancreatic secretory response
  is directly related to the functional mass of
  pancreatic tissue.
• In the standard assay, secretin is given IV in a
  dose of 0.2 g/kg of synthetic human secretin as a
  bolus. Normal values for the standard secretin
  test are (1) volume output gt2 mL/kg per hour, (2)
  bicarbonate (HCO3) concentration gt80 mmol/L, and
  (3) HCO3 output gt10 mmol/L in 1 hour.
• The most reproducible measurement, giving the
  highest level of discrimination between normal
  subjects and patients with chronic pancreatic
  exocrine insufficiency, appears to be the maximal
  bicarbonate concentration.

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• There may be a dissociation between the results
  of the secretin test and other tests of
  absorptive function. For example, patients with
  chronic pancreatitis often have abnormally low
  outputs of HCO3 after secretin but have normal
  fecal fat excretion. Thus the secretin test
  measures the secretory capacity of ductular
  epithelium, while fecal fat excretion indirectly
  reflects intraluminal lipolytic activity.
• It must be noted that, an abnormal secretin test
  result suggests only that chronic pancreatic
  damage is present.

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• The amount of human elastase (normal
  level175-1500µg/g of stool reflects the
  pancreatic output of this proteolyticenzyme.
  Decreased elastase activity in stool(lt100 µg/g of
  stool) is an excellent test to detect severe
  pancreatic exocrine insufficiency in patients
  with chronic pancreatitis and cystic fibrosis
  provided that the stool specimen is solid.
• The fecal elastase-1 and small bowel biopsy are
  useful in the evaluation of patients with
  suspected pancreatic steatorrhea. The fecal
  elastase level will be abnormal and small bowel
  histology will be normal in such patients.

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• Fat balance studies
• The astimation of fat in stool is done
• When feces contains split fatty acids, it points
  to a normal pancreatic function,but defective
  absorption.
• If fat excreated is neutral fat, is due to
  defective digestion, is more in favour of
  pancreatic disease
• Steatorrhea does not occur until intraluminal
  levels of lipase are markedly reduced,
  underscoring the fact that only small amounts of
  enzymes are necessary for intraluminal digestive
  activities.
• Normal fat level in stool is
• Children upto 6yrs of age -lt2g/d
• Thereafter 2-6g/d
• Steatorrhoea-gt7g/d

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• Estimation of sweat electrolytes
• In pancreatic fibrocystic disease,sodium
  chloride are increased in sweat.
• The disease is characterized by thick viscous
  secretions of exocrine glands.
• Sweat chloride levels of gt60mmol/L,on two
  separate occations, is diagnostic of cystic
  fibrosis.

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