Lecture 6 -- Digestive Processes in the Stomach

What is the enzyme that digests starch in the stomach?

Amylase

At what pH is amylase active?

Greater than 6.

What enzyme digests protein in the stomach?

Pepsin

At what pH is pepsin active?

Low pH

What is the basic unit of starch?

Maltose (two glucose molecules linked by a 1-4 α-glycosidic bond)

What are the two main components of starch?

Amylose (Double helix) and amylopectin (Branched chain)

What kind of bonds can amylase digest?

α-glycosidic bonds.

Why can't mammals digest cellulose?

Mammalian enzymes cannot break down β-glycosidic bonds found in cellulose.

What is unique about the saliva of carnivores regarding starch digestion?

Carnivores have low carbohydrates in their diet
→ They do not produce salivary amylase
→ Starch digestion starts at small intestine

How do omnivores adapt to carbohydrate digestion?

Omnivores e.g. pigs have high level of starch in their diets
→ Fundus is derived from the fundic region → Only mucus is produced = No acid secretion → Allow salivary amylase to function for a longer time

How do horses adapt to carbohydrate digestion?

Working horses can have increased level of starch in their diet
→ Because they have salivary amylase + Their fundus are derived from oesophageal region → Only mucous secretion → No acid secretion → More time for salivary amylase to break down starch

How do ruminants adapt to carbohydrate digestion?

Ruminants still have low level of starch in their diet
→ Fresh grass start off as hydrolysed carbohydrates that are linked with alpha glycoside bond → Convert into structural carbohydrates to allow grass to grow faster
BUT still absence of salivary amylase

What substances primarily make up gastric juice?

Hydrochloric acid (HCl) and pepsinogen.

Why is pepsinogen secreted in its inactive form?

To prevent self-digestion of the stomach wall by pepsin

What is the primary function of hydrochloric acid in the stomach?

To activate pepsinogen to pepsin and provide an acidic environment for protein digestion
P.S. pepsin is autocatalysis → Can activate more pepsinogen

How is HCl secreted in the stomach?

Step 1: CO2 in capillary diffuse into the parietal cells → Combines with H2O to form H2CO3 → Dissociates to release H+ ions + HCO3- ions
Step 2a: HCO3- ions reabsorbed back to capillaries with an exchange of Cl- → Cl- ions are secreted into the lumen of the stomach
Step 2b: H+ secrete to lumen with an exchange of Na+ with the use of ATP
Step 3: H+ combine with Cl- to form HCl

Why urine sample cannot be taken right after eating?

Urine pH slightly increase because the secretion of HCO3- during the production of HCl → Inaccurate sampling

What are the functions of HCl?

1. Convert pepsinogen to pepsin

2. Kill bacteria → Prevent fermentation

   p.s. in horses + pigs: large part of stomach does not produce acid → fermentation of starch does occur

3. Provide acidic environment for pepsin to digest protein

4. Degrades large chunk of connective and muscle tissue into smaller digested particle

How long does HCl secretion occur after a meal in dogs?

Maximum secretion occurs 2-3 hours after a meal.

How long does HCl secretion occur after a meal in pigs?

Continous

What stimulate the secretion of HCl and pepsin?

→ Gastrin bind to its receptors on ECL cells → Secrete histamine → Histamine binds to its receptors on parietal cells (mainly) instead of chief cells → Stimulates HCl secretion

→ ACh and Histaminę has their own receptor on parietal cells/ goblet cells/ chief cells → Increase secretion of mucus/ pepsinogen/ HCl

These two processes can be regulated within ENS/ parasynpathetic nervous system via vagus nerve

What inhibit the secretion of HCl, pepsinogen in stomach?

1. When pH <2.0, gastrin stop release = HCl stop release to prevent ulceration

2. Before food enters stomach, H+ is low but not buffer → Gastrin inhibit BUT once food reaches into food, H+ decrease → Release of gastrin
   P.S. Protein buffering system:
   → Protein are made up of amino acids, which contain amino group (NH2) → Accept H+ and become NH3+ → Mop up some H+ in stomach → Stimulate gastrin release

What is the cephalic phase of gastric secretion?

Before food has entered stomach → Sight/ smell/ taste/ presence of food in mouth → Stimulates 1.HCl and pepsinogen secretion from parietal cells and chief cells through the vagus nerve 2. Gastrin release → Increase HCl secretion from parietal cells

What triggers the gastric phase of secretion?

After food has entered stomach → Stomach expansion + peptides in the lumen after food has entered → Stimulate 1.HCl and pepsinogen secretion from parietal cells and chief cells through the vagus nerve 2. Gastrin release → Increase HCl secretion from parietal cells

What triggers the intestinal phase of secretion?

Food component + Acidity of chyme

What hormones are involved in the intestinal phase of secretion?

Secretin and cholecystokinin (CCK)

How do duodenal signals affect gastric secretion?

They inhibit stomach motility and gastric juice secretion

What are the pathophysiology of gastric/ duodenal ulceration?

Increase H+ production (Duodenal ulceration)/ Decrease protective function (Gastric ulceration)

→ HCl + Pepsin damage epithelial cells and underlying tissue → ulceration

→ Damage cell produce histamine → Stimulates acid secretion

→ Villi damage by acid → Increase secretion + decrease absorption → Cause diarrhoea

What are the two types of ulcers discussed?

Gastric ulcers and duodenal ulcers.

What are the causes of gastric or duodenal ulceration?

1. Helicobacter Pylori in humans

2. NSAIDs → Prostaglandin stimulate production of mucous + HCO3- → NSAIDs Inhibit prostaglandin secretion

3. Mast cell tumours/ mast cell leukaemia → Produce excess histamine → Increase HCl production

4. Grastrin producing tumors → Produce gastrin → Increase HCl production

What are the treatment of gastric/ duodenal ulceration

1. PPI

2. Antihistamine

3. Antacid

4. Mucosal binding agents

How does antihistamine works?

Block the action of histamine on H2 receptors in the stomach → Reduce HCl production

How does PPi works?

The transporter protein (proton pump) is responsible for secreting the H+ ions out the parietal cell to the lumen of the gut -> Proton pump inhibitor block this process

How does antacid works?

Antacids neutralize stomach acid