Splanchnic circulation - regulation mostly metabolic

 

 

 

 

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Gastrin

[WG21:p489]

Produced by G cells in the lateral walls of the glands in antral portion of stomach

4 forms: G34, G17, G14, and carboxyl terminal tetrapeptide

G17 is the main one. T1/2=2-3min

Actions

Stimulation of

• Gastric acid secretion
• Pepsin secretion
• Mucosa growth in stomach, and small and large intestines
• Gastric motility
• Insulin secretion (only after a protein meal, not a carbohydrate meal)
• Glucagon secretion

Regulation

Stimulated by

• Gastrin-releasing polypeptide
  * Not by Ach
• Presence of penylalanine and tryptophan

Inhibited by

• Acid in antrum
  * By direct action on G cells and by release of somatostatin

 

Cholecystokinin (CCK)

Aka cholecystokinin-pancreozymin (CCK-PZ)

Secreted by endocrine cells (I cells) in the upper intestine

Also found in other places e.g. nerves, brain

T1/2 = 5 min

Action

• Contraction of the gallbladder
• Secretion of pancreatic juice rich in ENZYMES
• Augments action of secretin (which causes secretion rich in HCO3)
• Inhibis gastric emptying
• Trophic effect on pancreas
• Increases secretion of enterokinase
• May enhance motility of small bowel and large bowel
• Contraction of pyloric sphincter
  * Along with secretion
• Stimulate glucagon secretion
  * With gastrin

Regulation

Increased by

• Contact of intestinal mucosa with products of digestion
  * Esp peptides and amino acids, fatty acids (more than 10 carbon)

NB:

• Positive feedback situation
  --> Stimulation release of bile and pancreatic juice
  --> Bile and pancreatic juice break down fat and protein
  --> These products stimulates further CCK

 

Secretin

Secreted by S cells deep in the glands of the mucosa of the upper portion of small intestine

T1/2 = 5 min

Actions

• Increases secretion of bicarbonate by duct cells of the pancreas and biliary tract
  --> Causes watery, alkaline pancreatic juice
  * via cAMP
• Decreases gastric acid secretion
• May cause contraction of pyloric sphincter

 

Gastric inhibitory polypeptide (GIP)

Produced by K cells in mucosa in duodenum and jejunum.

Action

• In large doses, inhibits gastric secretion and motility
  --> But not in physiological level
• May stimulate insulin secretion at physiological level

Regulation

• Stimulated by glucose and fat in duodenum

 

VIP

Markedly stimulates intestinal secretion of electrolyte and water

Relaxation of intestinal smooth muscle

Dilation of peripheral blood vessels

Inhibition of gastric acid secretion

Potentiates action of acetylcholine in salivary gland

 

Motilin

Secreted by enterochromaffin cells and Mo cells in the stomach and bowels

Acton

Contraction of smooth muscle in the stomach and intestine

Major regulator of MMC (migrating motor complexes)

 

Somatostatin

Inhibits secretion of gastrin, VIP, GIP, secretin, and motilin

Secretion stimulated by acid in stomach

 

 

Saliva

Contains

Lingual lipase

Salivary alpha-amylase

Mucin

IgA

Lactoferrin

Proline-rich protein

 

 

Function

Facilitates swallowing

Keep mouth moist

Solvent for taste

Aid speech

Keep mouth and teeth clean

Some antibacterial action

 

Stimulated by parasympathetic nerve

 

Swollow involves afferent impulse in trigeminal, glossopharyngeal, and vagus nerve

Effecterent to pharyngeal musculature and tongue via trigeminal, facial and hypoglossal

 

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3 components of lower oesophageal sphincter

[WG21:p494]

Oesophageal smooth muscle
* i.e. intrinsic sphincter

Fibres of the crural portion of the diaphragm
* i.e. extrinsic sphincter

Oblique or sling fibres of the stomach wall
* Creating a flap valve

 

Tone of LES is under neural control

Release of Ach from vagal endings
--> Intrinsic sphincter to contract

Release of NO and VIP from interneurons innervated by other vagal fibres
--> Relax of LES

Extrinsic sphincter is innervated by the phrenic nerve
--> Coordination with respiration and contractions of chest and abdo muscles

 

 

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[WG21:p501]

Pancreatic juice

 

1500mL of pancreatic juice secreted per day

Contains

• Trypinogen
  * Converted to trypsin by brush border enzyme enteropeptidase (enterokinase)
• Trypsin inhibitor
• HCO3
• Chymotrypsinogen
• Proelastase
• Procarboxypeptidase
• Pancreatic lipase
• Pancreatic alpha-amylase
• Phospholipase A2

 

 

Trypsin converts chymotrypsinogen into chymotrypsin and also activate trypsinogen

Regulation

• Secretin
  --> Increase HCO3 content and volume
  (acting on pancreatic ducts)
• CCK
  --> Increase enzyme content but low in volume
  (acting on acinar cells)
• Ach
  --> Also increase enzyme content
  (acting on acinar cells via phospholipase C)

NB: In small ducts, HCO3 is secreted

In larger ducts, HCO3 is reabsorbed in exchange for Cl-

[WG21:p502]

 

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[WG21:p504]

Blood flow from portal space (with bile duct, portal vein, and hepatic artery)
--> Towards central vein

Functions of liver

1. Formation and secretion of bile
2. Metabolism and vitamin metabolism
3. Inactivation of substances
4. Synthesis of plasma protein
5. Immunity

 

 

 

Bile salt

2 principle bile salts are cholic acid and chenodeoxycholic acid

In colon, bacterial conversion:

Cholic acid --> deoxycholic acid

Chenodeoxycholic acid --> lithocholic acid

Percentage in bile:

• Cholic acid = 50%
• Chenodeoxycholic acid = 30%
• Deoxycholic acid = 15%
• Lithocholic acid = 5%

 

Function

Reduce surface tension

Emulsification of fat (in conjunction with phospholipids, monoglycerides)
--> Forming micelles

 

Micelles are only formed when bile salt reach a certain level

• 95% of bile salts are absorbed from small intestines
• Most are absorbed from terminal ileum
  * By Na-bile salt symporter
  * Powered by basolateral Na-K ATPase

Remaining bile salts enter the colon and are converted to salts of deoxycholic acid and lithocholic acid

Lithocholic acids are relatively insoluble and is mostly excreted in stools
* Only 1% resorbed

 

Total bile pool =3.5g

Rate of synthesis = 0.2-0.4g/day

Bile recycles about twice per meal and 6-8 times per day

 

Regulation

Cholagogues (Gallbladder contraction)

• CCK
  * Triggered by presence of fatty acids and amino acid in duodenum

Choleretics (Increase in secretion)

• Stimulation of vagus nerve
• Secretin
• Bile salt