lecture 17 Flashcards
what are the hormones found in the GI tract?
- secretin
- gastro inhibitory peptide (GIP)
- glucagon
where are these hormones located?
- epithelial endocrine cells
what are the neurotransmitters in the GI tract?
- VIP (vasoactive intestinal polypeptide)
- PHI (peptide histidine isoleucine)
- PACAP
what is VIP?
- vasoactive inhibitory peptide
- peptide hormone that’s vasoactive in the intestine
- 28 amino acids long
what is VIP involved in?
- digestion
- vasodilation
- blood flow
- causes decrease in blood pressure-relaxes smooth muscle
what is PHI structure?
- 27 amino acids long
- histidine at position 1
- isoleucine at position 27
which neurotransmitters are coreleased and cotransported?
- VIP and PHI
- 13 amino acids similarity (similar biological properties)
what amino acid do all have in common?
- phenylalanine at positon 6
- important in binding to the receptor
what amino acid do all lack?
- cysteine
- suggests flexible structure
what is required for functioning of the VIP family?
- whole sequence
- suggests no active site
what is the criteria for chemical neurotransmission?
- found in neurones, biosynthesis mechanisms present
- concentrated in nerve terminals, released by depolarising nerve stimulation
- application of exogenous material mimics response caused by nerve stimulation
- antagonists have same effect on both exogenous and endogenous effects
- mechanisms of breakdown, removal or re-uptake need to be present
what does bolus formation stimulus cause?
- distension of muscle layers
- activates afferent neurones
where do the afferent neurones extend?
- into myenteric plexus
- in orad to caudad direction
what do the afferent neurones synapse onto?
- interneurones
- efferent neurones
what is the function of interneurones in bolus movement?
- passed down myenteric plexus
- release output into myenteric plexus (VIP)
- causes ascending relaxation
what is the function of motor neurones in bolus movement?
- extend into the circular muscle
- release neurotransmitter
- causes descending contraction
what is the process of bolus movement?
- stimulus = bolus distension
- activates afferent neurones in myenteric plexus which synapse
- causes efferent neurones to go into circular muscle and release VIP
- descending contraction
- bolus moves forward (waves of activity)
- neurones fire onto different efferent neurones/interneurones
- wave of activity moves along circular muscle causing relaxation in waves
- bolus movement
what happens once the bolus moves?
- stimulates the next segment of neurones
- wave moving forward
- relaxation infront, contraction behind
where is VIP found?
- in efferent motor neurones in cell body
- in nerve terminals which extend into circular muscle layers
what are VIP blood levels?
- low
- no large fluctuations
- VIP represents overflow of neurotransmitter
what did Fahrenkrug 1978 experiment set out to look at?
- inhibitory responses in gut motility
- looks at receptive relaxation stimulated by mechanical stimulation of pharynx and oesophagus when swallow
- long reflex (involves vagovagal reflex)
what is the vasovagal reflex?
- GI reflects circuit where afferent and efferent fibres of vagus nerve coordinate responses to gut stimuli
- via dorsal vagal complex in the brain
what treatment doesnt block the response?
- standard cholinergic muscarinic blockers (eg. atropine/adrenergic receptor blockers)
what does the lack of response to this treatment suggest?
- non adrenergic non cholinergic response
what inhibits the inhibitory response?
- hexamethonium
what does this inhibition suggest?
- role of interneurone as blocks cholinergic receptors
what occurs in the Fahrenkrug experiment?
- directly stimulate vagus nerve in neck to mimic effects of vagus nerve
- or mechanically stimulate the pharynx
what occurs with electrical/vagal stimulation?
- increase in VIP in blood leaving stomach
- increased volume of stomach (relaxation)
- mimics receptive relaxation
- long reflex
how is the mechanical stimulation achieved?
- balloon placed in pharynx
- filled with saline
- as balloon expands, stimulates a swallowing response
- oesophageal distension
what are the results from the mechanical stimulation?
- increased VIP in blood leaving stomach
- increased relaxation
what does this experiment show?
- link between VIP and stomach relaxation
what did Grider 1985 test for?
- to study contributions of VIP and ATP to relaxation of smooth muscle
why are VIP and ATP the two main candidate neurotransmitters?
- both found in neurones throughout gut
- both have direct relaxant effects on GI smooth muscle
what occurred pre-method in Grider 1985?
- strips taken from stomach of guinea pig
- treated with atropine to block muscarinic cholinergic responses
- treated with guanethidine to block adrenergic effects
what is the method for Grider?
- either side of stomach strip attached by thread
- one end attached to tissue bath
- other attached to tension transducer
- filled bath with Krebs ringers solution to keep tissue alive
what happens when exogenous VIP was added?
- shows dose/conc dependent increases in relaxation
what happens when exogenous VIP antibody was added?
- reduced response
- antibody binds VIP so less VIP binds to its receptor
- shows VIP has receptor on muscle cells to cause relaxation
what happens when exogenous ATP Is added?
- increased relaxation with increasing ATP
what happens when exogenous BzATP (ATP antagonist) is added?
- reduces the effects of ATP on muscle relaxation
how is the effect of endogenous substrates tested?
- by field stimulation
- electrodes placed either side of tissue in tissue bath
- current passed along stimulating nervous system
- releases neurotransmitters
what does increased cycle frequency of stimulation mean?
- tissue relaxes
- endogenous neurotransmitters are released
what occurs when VIP antibody is added?
- blocks part of the response
- big decrease in relaxation
what does this blockage by VIP antiserum suggest?
- when tissue extends endogenously, VIP is released not ATP
what happened when BzATP was added?
- no effect
what do Grider and Makhlouf test?
- peristaltic type reflexes
- investigates two parts of peristaltic reflex at once
what is the Grider and Makhlouf experiment method?
- take tube of colon tissue
- place glass rod through the colon
- attach one end to the tissue bath
- add tension transducer at orad -> caudad end
- pulley system added in middle to allow tissue stretch
what occurs when orad stretch is stimulated?
- the more stretch , the greater the descending relaxation
what can this Grider and Makhlouf response be blocked by?
- tetrodotoxin (inhibits Na+ channels)
what occurs when stretch its stimulated at the caudad end?
- no increase in VIP release
what does this tell us?
- stretching also causes damage
- stops neurotransmitter release
what did Grider and RIvier test?
- whether stretch and relaxation are linked
what were the results from Grider and Riviers experiment?
- VIP antiserum and BzATP cause reduced relaxation
- for control, greater the stretch = greater the relaxation
what does the VIP antiserum response suggest?
- VIP has to be released before the relaxation
- VIP antibody only works when been released from the tissue
- blocks the response
what does the response suggest about VIP 10-28?
- blocks receptor but doesnt activate
- more inhibition response
how are the effects of PHI tested by Grider and Rivier?
- antibody highly specific for VIP added so mops up the VIP
- leaves the effects of PHI
why is using an antagonist more effective than using an antibody?
- antagonist blocks the effects of anything using the receptor
- effects both VIP and PHI
what is the main receptor for VIP/PHI/PACAP?
- VPAC2 (receptor expressed on smooth muscle)
what experiment was carried out by Grider and Rivier involving VIP and PHI?
- added different exogenous compounds and looked at effects of inhibitors
- measured relaxation
what occurs with presence of exogenous VIP/PHI and antibody?
- blocks VIP response
- little effect of PHI
what other substance do neurones contain?
- nitric oxide synthase (NOS)
what is NO used for?
- relaxant agent on smooth muscle
how is NO produced?
- arginine converted to NO + citrolline by NOS
what occurs with NO and tissue relaxation?
- if stimulate relaxation, NO is released
- oxyhemoglobin added to tissue bath, blocks NANC responses and binds NO
how does exogenous NO cause relaxation of GI smooth muscle?
- increases tissue cGMP levels
- increases intracellular Ca2+
- K+ channel opens
- rapid hyperpolarisation
- relaxation
what did Grider test in 1992?
- stimulating muscle strips (1) by field stimulation to measure released neurotransmitters
- testing isolated cells (2) with different substrates
what occurred in experiment 1 including muscle strips?
- field stimulation = increase all
- +L-NNA = decreased relaxation. decreased VIP release, inhibited NO production
- +L-arginine = increase in all
what occurs in experiment two with isolated muscle cells?
- non had effect on VIP release
- +exogenous VIP = increase relaxation and NO production
- L-NNA = decreased relaxation, inhibited NO production
- +L-arginine = increased both
