GI Secretion

(Transcribed from Dr. Parmar’s lecture, 18-19 May 2000 by Brian Buschman)

 

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Please DO NOT use these notes as your sole source of lecture material.  Dr. Parmar draws many diagrams that are great for teaching the material which, due to the time it takes me to make computer graphics, have not included in here.

 

 

Regulation of the GI tract

The GI tract is regulates by both hormones and nervous functions.  The hormones can be delivered by one of three methods of actions:

1)      Endocrine which are released into the blood such as secretin.

2)      Paracrine which are released to act on nearby tissues or even in an autocrine method.

3)      Neurocrine are released by neurons such as NE and ACh.

 

The different hormones have different types of receptors on the cell membrane.

• b-receptors are adrenergic (stimulated by catacholamines) and function via a camp second messenger system to cause the release of vesicles.
• a-receptors are also adrenergic and function by forming IP₃ which triggers the release of Ca²⁺ that in turn releases the products.
• ACh receptors are cholinergic (stimulated by ACh) and function via IP₃ like the a-receptors.
• Substance P receptors function like a-receptors as well.
• Neurocrine receptors function by stimulating cell depolarization by increasing cell Na⁺ permeability.  This will lead to the release of Ca²⁺. This release will cause the repolarization via K⁺ channels.

 

Second Messenger Systems (Adenylate Cyclase)

The hormone receptor complex on a membrane bound G-protein complex will lead to activation of the G-protein by replacing GDP with GTP.  It will then activate enzymes such as adenylate cyclase.  In this case hormones bind to the receptor site which activates the regulatory site.  The regulatory site is activated by binding a GTP and releasing it’s GDP.  This will activate the catalytic site which will convert ATP to cAMP.  When it is done it hydrolyzes the GTP to GDT and quite working.

 

Smooth Muscle Contraction

Many of us have forgotten what Dr. Kalli taught us about smooth muscle so Dr. Parmar recapped it.  Ca²⁺ acts on calmodulin to activate myosin light chain kinase.  Myosin light chain kinase will phosphorylate myosin to myosin phosphate.  This causes contraction using the energy from the ATP->ADP reaction.  Relaxation comes when myosin light chain kinase removes the P­_i.

 

IP₃ Pathway

The hormone-receptor complex activates lipase-C to cleave phosphinotyl inosital 1,4-biphosphate to IP₃ and DAG.  The IP₃ acts to release Ca²⁺ and the DAG activated protein kinase C.

 

Stomach Secretion

When you smell or eat food the vagus will cholinergically stimulate the parietal cells to secrete HCl.  It will also stimulate G-cells via gastric releasing peptide (GRP) to release gastrin.  Gastrin will be transported via the blood stream to parietal cells where it will cause HCl secretion.  (Remember that gastrin also stimulates ileal activity by the Gastroileal reflex.)

 

This happens during the relaxation phase when no peristalsis exists.  It allows food to move in without causing gastric reflux.

 

Carbonic Anhydrase

Parietal cells produce HCl with the enzyme carbonic anhydrase (CA). CO₂ diffuses into the parietal cells from the blood stream and are converted by CA into H₂CO₃.

 

Sorry, but for the sake of working on Dr. D’s neuro scare I need to leave this one for later.  Really this stiff is just what Dr. Kalli taught last semester with logic applied.

 

 

 

 

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