What we gastroenterologists must (should) know about pancreatic physiology Joachim Mössner University of Leipzig Prague, April 16, 2010

Physiology of Pancreatic Enzyme Secretion Pancreas synthesizes and secretes digestive enzymes and secretes HCO3 Both are necessary for normal digestion Pancreas stimulated acutely by feeding: –Neuronal Mechanisms: Vagus mediates small cephalic phase –Importance of vagal afferents –Vagal-vagal reflexes –Acinar cells have M1 and M3 receptors –Some direct innervation of pancreas from gut JM 2010

The pancreas is made up of three functional components: Endocrine – Islets 2% Exocrine – Acinar 80%Digestive Enzyme Exocrine - Ducts 8%Bicarbonate Rich Fluid Innervation –Vagal: Acetylcholine main transmitter Acini Ducts Islets –Sympathetic: Norepinephrine main transmitter Islets Blood Vessels JM 2010

Pancreas

Paracrine stimulation within the mucosa Endocrine stimulation

Cholecystokinin, CCK CCK or CCK-PZ; Greek chole, "bile"; cysto, "sac"; kinin, "move"; move the gallbladdder Peptide hormone of the gastrointestinal system & brain Stimulation of digestion of fat & proteins CCK, previously called pancreozymin, synthesized by I-cells in the mucosa of small intestine Secreted from the first segment of the small intestine Role in inducing drug tolerance to opioids? JM 2010

Cholecystokinin, CCK Stimulation of release from I-cells by oligopeptides, certain amino acids (phenylalanine), fatty acids Stimulation of release by releasing peptides present in pancreatic secretions and duodenal mucosa (monitor peptide, …)? Negative feedback inhibition: Destruction of CCK releasing peptides by trypsin JM 2010

Cholecystokinin, CCK Inhibition of gastric emptying & gastric acid secretion Stimulation of pancreatic acinar cells to secrete pancreatic digestive enzymes, hence the old name pancreozymin Stimulation of human acinar cells directly via CCK-A receptors? Stimulation indirectly via CCK-B receptors of nerves: release of acetylcholine JM 2010

Cholecystokinin, CCK Enzymes catalyze digestion of fat, protein, & carbohydrates As levels of substances that stimulate release of CCK drop, concentration of the hormone drops as well Release of CCK inhibited by somatostatin, PYY, … CCK causes increased production of hepatic bile CCK stimulates contraction of gall bladder & relaxation of the sphincter of Oddi Bile salts form amphipathic micells that emulsify fats Triglyceride digestion needs lipase, bile salts, colipase: 2 fatty acids, 1 monoglyceride JM 2010

Endocrine Mechanisms CCK: Paracrine stimulation of vagal afferents Endocrine effect through blood Whether human acinar cells have CCK receptors is controversial JM 2010

Secretin –Major stimulant of HCO3 secretion Insulin –Released from islets. Acts on exocrine cells by portal blood system Nutrients –Supply energy, building blocks and amino acids –Act as a anabolic signal What ends Secretion? –Exit of food from upper small intestine –Feedback inhibition by surplus trypsin in intestinal lumen –Ileal brake JM 2010

REGULATION OF PANCREATIC SECRETION

Stimulus-secretion Coupling of Pancreatic Enzyme Secretion JM 2010

INTRACELLULAR TRANSPORT OF PANCREATIC SECRETORY PROTEINS JM 2010

Secretin First hormone identified by William Bayliss & Ernest Starling in 1902 Produced in S cells of the duodenum in the crypts of Lieberkühn Secretin encoded by the SCT gene Protein with 27 amino acids Release by gastric acid entering the duodenum JM 2010

Secretin Stimulation of watery bicarbonate secretion from, Brunner glands of the duodenum, pancreatic duct cells & acinar cells (?) Inhibition of gastric acid secretion by inhibition of gastrin release Stimulation of adenylate cyclase activity: cAMP second messenger Control of water homeostasis throughout the body Regulation of pH of duodenal contents via control of gastric acid secretion & buffering with bicarbonate Role in osmoregulation in the hypothalamus, pituitary, & kidneys JM 2010

Mechanism of Pancreatic Bicarbonate Secretion JM 2010

S6 Ribosomal protein p70 s6k Insulin receptor CCK receptor Akt/PKB PI3-K mTOR Complex 1 mTOR Complex 1 mRNA eIF4E eIF4G m7GTP 4E-BP1 eIF4E 4E-BP1 eIF4A eEF2K (off) eEF2 (Active)(Inactive) eEF2 Phosphatase Amino acids ACh receptor Regulation of Protein Synthesis in the Pancreas Kinase 80 S initiation complex 60 S AUG Stop AAAAA 40 S

Concentration of Ions in Pancreatic Juice as a Function of Flow JM 2010

Function of the Exocrine Pancreas Acinar Cell Trypsin(ogen) Trypsininhibitor Duct Cell CFTR JM 2010

Trypsin Inhibitor JM 2010

Trypsinogen Activation Peptide Ala-Pro-Phe-Asp-Asp-Asp-Asp-Lys- N- D19AD22GK23RA16V TAP N--C * R122H N29I ************ * **** JM 2010

Activation of Pancreatic Proenzymes in the Intestine involves Enterokinase and activated Trypsin JM 2010

Pancreatic Bicarbonate output increases in response to low Duodenal pH JM 2010

eIF4E S6K1 mRNA 4E-BP1 Amino Acids CCK Ca 2+ NFATs Transcriptional Control of Gene Expression Translational Control of Protein Synthesis ? eIF4E MAPKs calcineurin mTOR Pancreatic Growth (Mitogenesis & Hypertrophy) Regulation of Growth of the Pancreas

Long term effects of hormones and nutrients –Adaptive Growth as in pregnancy, lactation and high protein diet –Regeneration Limited after partial resection or fibrosis Nearly complete after mild acute pancreatitis Newer Material –Role of Vagal afferents in responding to nutrients and CCK –Feedback inhibition on CCK release and pancreatic secretion by active trypsin in lumen –Mechanism of high HCO3 secretion –Importance of nutrients particularly amino acids for the pancreas –Question of enteral vs parenteral feeding JM 2010

Somatostatin Somatostatin (growth hormone-inhibiting hormone, GHIH, somatotropin release-inhibiting factor, SRIF) Peptide hormone –Regulation of the endocrine system –Affects on neurotransmisson & cell proliferation –Interaction with G-protein-coupled somatostatin receptors Two active forms produced by alternative cleavage of a single preproprotein: one of 14 amino acids, the other of 28 amino acids JM 2010

Somatostatin: Stomach JM 2010

Somatostatin Inhibition of release of numerous secondary hormones –gastrin, CCK, secretin, motilin, VIP, GIP, enteroglucagon Lowers rate of gastric emptying Reduces smooth muscle contractions & blood flow within the intestine Suppresses release of pancreatic hormones Inhibits insulin release when somatostatin is released from granules in alpha-1 cells of pancreatic islets of Langerhans Inhibits release of glucagon Suppresses exocrine pancreatic secretion JM 2010

Pancreatic Polypeptide Secreted by PP cells predominantly in the head of the pancreas 36 amino acids Function: selfregulation of pancreas secretion activities (endocrine and exocrine) Effects on hepatic glycogen levels & gastrointestinal secretions Secretion in humans increased after a protein meal, fasting, exercise & acute hypoglycemia, decreased by somatostatin & intravenous glucose JM 2010

Peptide Tyrosin Tyrosin, PYY Released by the ileum Inhibition of pancreatic enzyme secretion JM 2010

Duodenal Lipase and Steatorrhea Duodenal Lipase and Steatorrhea DiMagno et al: N Engl J Med 1973;288:813 JM 2010

Proteases destroy CCK releasing peptides Plasma CCK Enzyme secretion Pancreatic duct pressure Pain Negative Feedback Inhibition of Pancreatic Enzyme Secretion JM 2010

Palliation of Pain in Chronic Pancreatitis: Use of Enzymes Metaanalysis 6 randomized, double blind, placebo controlled studies Statistical analysis demonstrates no benefit for pancreatic enzymes Mossner: Surg Clin North Am 1999; 79: JM 2010

Genes and Pancreatitis Hereditary chronic pancreatitis –Mutations: cationic trypsinogen Cystic fibrosis –CFTR-mutations Idiopathic chronic pancreatitis –Mutations of CFTR: special form of cystic fibrosis –Mutations or trypsin inhibitor SPINK –Chymotrypsin C mutations Tropical pancreatitis –SPINK-mutations, CTRC-mutations Alcohol induced chronic pancreatitis –Polygenetic disease? Role of protective mutations? JM 2010

Risk Factor: Chymotrypsin C p.R254W & p.K247_R254del overrepresented in pancreatitis 30 / % with idiopathic or hereditary pancreatitis vs 21 / 2,8040.7% controls Replication study in alcohol-related diseases: 10 / % in chronic pancreatitis vs 3 / % in liver disease Indian subjects with tropical chronic pancreatitis: 10 / % vs 1 / 841.2% control OR = 13.6; CI = ; P = –Rosendahl, Witt, …. Mössner, Teich, Sahin-Toth: Nature Genetics 2008; 40: JM 2010

Risk Factor: Chymotrypsin C JM 2010

Summary & Conclusion CCK: Stimulator of digestive enzyme secretion –Directly via CCK-A receptors on acinar cells (?) –Indirectly via CCK-B receptors: acetylcholine release CCK: Stimulator of gallbladder contraction Secretin: Stimulator of bicarbonate secretion Activation of trypsinogen by enterokinase Activation of proenzymes by trypsin Termination of enzyme secretion: –Negative feedback (destruction of CCK releasing peptides by trypsin ?) –Ileal brake (PYY, somatostatin) JM 2010

I would like to thank my former mentor John A. Williams, MD, PhD Professor of Medicine and Physiology Ann Arbor, Michigan for providing me several of the slides and teaching me in pancreatic physiology during my stay in San Francisco

JM Years University of Leipzig dies academicus, December 2, 2009 Pancreas Group Leipzig Hans Bödeker Sebastian Gaiser Albrecht Hoffmeister Volker Keim Jonas Rosendahl Lena Selig Niels Teich