Gut hormone release

 

The gastroenteropancreatic endocrine cells of the gut can be divided into open or closed type. The open type cell is thought to be a taste cell for the gut which responds to stimulation by food substances and other hormones by releasing its contained granules and hormones. The mechanism of release of this granules have been discussed previously. What needs to be added now is the fact that these secretions occur in quanta and not in continuum just as the mechanism in the neuron synaptic vesicle release.

 

When Bayliss and Starling discovered the response of secretin producing cell to acidification of mucosa. It was easy to postulate a mechanism of release which involves a neural facility. Before Bayliss and Starling, the mechanism of gut hormone secretion was explained by the palvovian nerve response.

 

It was then believed that a neuronal antenna was placed on top of the mucosa which when stimulated causes a second order neuron to fire at the target cell. Bayliss and Starling modified this view and completely renewed the aspect of nervous stimulation of gut endocrine cell.

 

However, a revival of Pavlovian nervism occurred in the middle part of this century mainly championed by Grossman. He suggested that a neural mechanism is involved in luminal stimulation through an antenna that is connected to a second order neuron which is in turn connected to a closed type endocrine cell that becomes stimulated as a result of luminal stimulus of the neural antenna on the mucosa of the gut.

 

A recent explanation of the mechanism of enteroendocrine hormone release in the gut was proposed by Fujita and Kobayashi that

1)         In view of the fact that no demonstrative neural structure is found histologically at the mucosal interface of most endocrine cells, the existence of such antenna like nerve ending is only speculative.

2)         In view of the stimulus secretion coupling method described for the adrenal medulary cells which is non-neural suggested by Douglas and Robin (1961), there is no reason not to believe that such a phenomena may not occur in the gut endocrine cells since both family of cells belong to the paraneuron group.

3)         Observation of increased blood gastrin level in a post-vagotomy patent after luminal stimulation by meat extract suggests a non-neural mechanism.

4)         Atropine does not impair gastrin release produced by luminal stimuli

5)         Human G cells are not vagally innervated; only parietal cells receive vagal innervation. It has however been shown that G cells are approached by neural elements 300A0 away suggesting some degree of neuronal interreaction.

 

On the basis of the above, Fujita and his associates put forward the hypothesis on the gut luminal stimulation which led to basal secretion through a mechanism of microvilli tuft luminal stimulation of open type gut endocrine cells which causes the excitation of granules which are then causes the excitation of granules which are then released in quanta through basal emiocytosis.

 

 

 

Endocrine secretion

Paraneurons

APUD

  

 

 

 


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