L28 Pancreatic biliary intestinal secretion 2011

L28 Pancreatic biliary intestinal secretion 2011 -...

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-1/7- COPYRIGHT Prof. Beyenbach Mammalian Physiology BIOAP 4580 2011 SECRETIONS OF THE DIGESTIVE SYSTEM П Pancreatic, Biliary, and Intestinal Secretion 1. Pancreatic endocrine and exocrine secretions. The pancreas has endocrine and exocrine functions. Endocrine functions include secretion of insulin into the blood stream by β-cells in response to elevated blood glucose concentrations (after a meal). Endocrine glucagon is secreted by α-cells. Glucagon stimulates glycogenolysis and gluconeogenesis in the liver. Endocrine somatostatin is secreted by δ- cells. Somatostatin inhibits the secretion of a number of hormones: growth hormone, insulin, glucagon and vasoactive peptide (VIP). VIP is another pancreatic endocrine secretion. It stimulates fluid secretion by the pancreas and the small intestine, and it relaxes smooth muscle of blood vessels. Exocrine functions are the secretion of electrolytes (Na + , K + , Cl - and HCO 3 - ) and the secretion of digestive enzymes. The microscopic make-up of the exocrine part of the pancreas is essentially like that of the salivary gland (Fig. 1). a) The acinar cells of the pancreas, responding to ACh (acetylcholine) and CCK, secrete an isosmotic fluid of NaCl, a plasma-like solution, under basal conditions (Fig. 2). However, depending on the composition of the diet (high protein or carbohydrate) the same cells secrete a host of enzymes specific to the diet. These enzymes include protease precursors, α-amylase, lipases, ribonucleases, elastases, and collagenases (Table 1). Fig. 1. Structure and function of the exocrine pancreas. Acinar cells secrete digestive enzymes and electrolytes; small ducts secrete NaHCO 3 . b) The duct cells of the gland secrete a HCO 3 - rich solution (Figs. 1, 2). Note the cooperation between Cl - channels and Cl/HCO 3 exchange transport at the apical membrane.
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-2/7- Fig. 2. Secretory electrolyte transport mechanisms in the pancreas: a) secretion of NaCl in the acinar portion of the gland, b) secretion of NaHCO 3 in the ductal portion of the gland. c) Secretion of digestive enzymes. Next to the secretion of an isosmotic fluid of NaCl, acinar cells secrete a variety of enzymes into the acinar lumen. Trypsin, chymotrypsin, carboxypeptidases, and elastases target the digestion of proteins (Table 1). Lipases and phospholipases aim at the digestion of lipids. Phospholipids and amylases respectively pursue the digestion of carbohydrates. Nucleases take delight digesting DNA and RNA (Table 1). Note that precursors of proteases, i.e. inactive enzymes, are secreted into the lumen as trypsinogen, chymotrypsinogen and procarboxypeptidases (Fig.3). They are activated in the lumen of the small intestine where enzyme activity is useful for digesting food (and not for digesting the pancreas or the ducts leading to the intestine). In brief, the enzyme enterokinase bound to the apical membrane of intestinal epithelial cells and sticking out into the lumen, converts trypsinogen to trypsin. Trypsin autocatalytically
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L28 Pancreatic biliary intestinal secretion 2011 -...

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