Control of Gastric Secretions

8/2/2019 Control of Gastric Secretions

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Control of Gastric secretions

Dr Raghuveer Choudhary

Associate ProfessorDept. of Physiology


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Gastric secretion

Gastric juice- hydrochloric acid

- conversion of pepsinogen to pepsine- bacteriostatic effect

- pepsin- protein digestion- replaceable by pancreatic enzymes

- mucus- protective coating, lubricant- part of gastric mucosal barrier

- intrinsic factor- binds B

12vitamin, absorption in the ileum

- the only indispensable substance in gastric juice


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Functions of HCl:

Turn pepsinogen into pepsin & provide a

medium of low pH favoring pepsinaction;

Kill many swallowed virulent organisms;

Promote the secretion of pacreas & bile;

Promote the absorption of calcium &ferrus

Pepsinogen

Secreted bychief cells & activate by HClFunction: digest proteins into proteoses,

peptones,

polypeptides, with little free amino acids


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Parietal cell - secretingTubulovesicles fuse with

canaliculus, increased surface area

and numbers of H+K+ATP ase

increases acid secretion into lumen

of gut.

H+

Acid secretion is against a 3

million fold concentration

gradient

H+ inside = 4x 10-8

MH+ outside = 0.1 M

NEEDS ENERGY


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MECHANISM OF GASTRIC ACIDSECRETION

The oxyntic cells produce andsecrete acid,described as follows

H+ is produced through the action of carbonic anhydrase,which produces carbonic acid from CO2 and H2O.

The H+/ K+-ATPase is used to pump H+ from the cytoplasminto the stomach lumen in exchange for K+. K+ used in thisexchange process is available from food or saliva, but it is alsosecreted via a luminal membrane K+ channel.


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Cl must be secreted to yield HCl. Cl uptakeinto oxyntic cells from the extracellular fluidoccurs via the Cl/HCO3exchange at thebasolateral cell membrane.

HCO3 exits the cell in such a large quantitythat the gastric venous blood becomes

alkaline; this is known as thepostprandialalkaline tide.


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Cl is secreted into the lumen via a Cl

channel in the luminal membrane, whichresults in the generation of a large lumen-negative transepithelial potential differenceacrossthe stomach mucosa.

H+ is transported against a largeelectrochemical gradient, which is reduced bya lumen-negative

voltage.


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Regulation of Gastric Function

Gastric secretion and motility is divided into threestages:

1) Cephalic

2) Gastric

3) Intestinal phases


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- is stimulated by food in the stomach

- accounts for two-thirds of gastric

secretion.

2) The Gastric Phase


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a) the enterogastric reflex.b) local hormones

Secretin

Cholecystokinin

gastric inhibitory peptide

3) The Intestinal Phase

After entering small intestines,

chyme inhibit gastric secretion and mobilityvia:


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1. The taste or smell of food, tactile

sensations of food in the mouth, oreven thoughts of food stimulate themedulla oblongata (green arrow).

2. Parasympathetic action potentialsare carried by the vagus nerves tothe stomach (pink arrow).

3. Preganglionic parasympatheticvagus nerve fibers stimulatepostganglionic neurons in the entericplexus of the stomach.

4. Postganglionic neurons stimulatesecretion by parietal and chief cellsand stimulate gastrin secretion byendocrine cells.

5. Gastrin is carried through thecirculation back to the stomach(purple arrow), where it stimulatessecretion by parietal and chief cells.

Cephalic Phase

GastrinCirculation

Secretionsstimulated

Taste or smell of foodTactile sensation in mouth

Medulla oblongata

Vagus nerves

Stomach

1

2 3

5

4


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The taste or smell of food, tactile sensations of food in the mouth, oreven thoughts of food stimulate the medulla oblongata (green arrow).

Taste or smell of food

Tactile sensation in mouth

Medulla oblongata

Cephalic Phase


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Parasympathetic action potentials are carried by the vagusnerves to the stomach (pink arrow).

Vagus nerves

Cephalic Phase


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Preganglionic parasympathetic vagus nerve fibers stimulatepostganglionic neurons in the enteric plexus of the stomach.

Vagus nerves

Cephalic Phase


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Cephalic Phase

Postganglionic neurons stimulate secretion by parietal and chiefcells and stimulate gastrin secretion by endocrine cells.

Gastrin

Stomach


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Cephalic Phase

Gastrin is carried through the circulation back to the stomach (purple

arrow), where it stimulates secretion by parietal and chief cells.

Gastrin

Circulation



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Stimulation of acid secretion

cephalic phase

blocked byvagotomy

sham feeding

hypoglycemia

role of GRP(bombesin)


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Stomach

Local reflexesstimulated bystomachdistention

Distention


Vagus nervesMedullaoblongata

1.Distention of the stomach activates aparasympathetic reflex. Action potentialsare carried by the vagus nerves to themedulla oblongata (green arrow).

2.The medulla oblongata stimulates stomachsecretions (pink arrow).

3.Distention of the stomach also activateslocal reflexes that increase stomachsecretions (purple arrow).

Gastric Phase1

2

3


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Local reflexes

stimulated bystomach distention

Distention

Vagus nervesMedullaoblongata

Distention of the stomach activates a parasympathetic reflex.

Action potentials are carried by the vagus nerves to the medulla

oblongata (green arrow).

Gastric Phase

Stomach


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Gastric Phase

Secretions

stimulated

The medulla oblongata stimulates stomach secretions (pink arrow).

Vagus nerves

Stomach

Decreasedgastricsecretions


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Gastric Phase

Local reflexesstimulated bystomach distention

Distention of the stomach also activates local reflexes that

increase stomach secretions (purple arrow).

Stomach

Distention


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Gastric phase

60 % ofacid response

distension

peptides

calciumalcoholcaffeine


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Experiment of Sham feeding byPavlov


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1.Chyme in the duodenum witha pH less than 2 or containingfat digestion products (lipids)inhibits gastric secretions bythree mechanisms.

2.Sensory vagal action potentials

to the medulla oblongata(green arrow) inhibit motoraction potentials from themedulla oblongata (pink arrow).

3.Local reflexes inhibit gastricsecretion (orange arrows).

4.Secretin, gastric inhibitorypolypeptide, and cholecystokininproduced by the duodenum(brown arrows) inhibit gastricsecretions in the stomach.

Intestinal Phase

Secretin, gastric inhibitorypeptide, cholecystokinin

Circulation

pH


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Intestinal Phase

Sensory vagal action potentials to the medulla oblongata (greenarrow) inhibit motor action potentials from the medulla oblongata

(pink arrow).

pH


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Intestinal Phase

Local reflexes inhibit gastric secretion (orange arrows).

pH


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Secretin, gastric inhibitory polypeptide, and cholecystokininproduced by the duodenum (brown arrows) inhibit gastric

secretions in the stomach.

Secretin, gastric inhibitory

peptide, cholecystokinin

pH


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Stimulation of gastric acid secretion


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l f l


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Regulation of gastrin release

Gastrin CellSomatostatin

Cell

Acetylcholine

-


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Control of Gastric Secretions

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