In this article we will discuss about Gastrointestinal Hormone:- 1. Introduction to Gastrointestinal Hormone 2. Biomedical Importance of Gastrointestinal Hormone 3. Features.
Introduction to Gastrointestinal Hormone:
1. Many hormones are liberated by the gastrointestinal tract—more than any other single organ.
2. The gastrointestinal hormones assist in digestive processes of the foodstuffs moving the digested products into the extracellular space through the intestinal mucosal cells, moving those products to distant cells through the circulation, and expelling waste products.
Biomedical Importance of Gastrointestinal Hormone:
1. Excessive production of several of the gastrointestinal hormones cause disease syndrome. The physicians cannot diagnose accurately unless they are aware of these syndromes.
2. These hormones have also close link to neuropeptides.
Features of Gastrointestinal Hormones:
A. Diversity of actions:
1. The classic definition of hormone is satisfied by many of the gastrointestinal peptides. Examples include gastrin, secretin, GIP, CCK, motilin, PP and enteroglucagon.
2. Other gastrointestinal peptides have paracrine actions or to act in a neurocrine fashion. Examples include VIP, somatostatin, substance P, BLI, Enkephalin, and neurotensin. Although these substances are found in high concentration in neurons, they are either not found in the circulation under normal conditions or have such short plasma half-lives that they would not be effective.
B. Location of Gastrointestinal Peptide-producing cells:
1. Many of the gastrointestinal peptides are found in the nerves in gastrointestinal tissues and most of them are also present in the central nervous system.
2. Synthesis of the peptides by central nervous system tissue is difficult to prove, but new techniques of molecular biology should establish whether genes coding for these substances are active.
3. The function of these peptides in the central and peripheral nervous system is under investigation.
4. The distribution of the gastrointestinal hormones is mentioned in Table 16.2.
C. Precursors and Multiple forms:
1. Secretin only exists in a single form.
2. The presence of multiple forms of gastrointestinal peptides in gastrointestinal tissues and in the circulation impedes the definition of the number and nature of these molecules.
D. Overlapping structure and function of gastrointestinal peptides:
1. In accordance with the sequence and functional similarity many of these hormones can be placed in one of the two families. These are the gastrin family and secretin family.
2. The gastrin family consists of gastrin and CCK.
3. The secretin family includes secretin, glucagon, GIP, VIP, and glicentin (which has glucagon-like immuno-reactivity but is a distinct peptide).
4. The neurocrine peptides neurotensin, bombesin-like peptides, substance P, and somatostatin bear no structural similarity to any other gastrointestinal peptide. They have very short plasma half-lives and may play no physiologic role in plasma.
E. Mechanism of Action:
1. Pancreatic acinar cells have got six different classes of receptors.
These are for:
(a) Muscarinic cholinergic agents,
(b) The gastrin CCK family,
(c) Bombesin and related peptides,
(d) The physalaemin-substance P family,
(e) Secretin and VIP.
(f) Cholera toxin.
2. The peptide-receptor complexes activate two distinct intracellular mechanisms. One involves the mobilization of intracellular calcium stores, and the other involves the activation of adenylate cyclase and the generation of cAMP.
3. The mechanisms do not cross-over, i.e., gastrin does not change cAMP levels, nor does secretin affect the intracellular Ca++ level. These two systems converge at some point.
4. The peptides causing Ca++ mobilization in the pancreatic acinar cells also affect the metabolism of phosphatidylinositol and enhance its conversion to diacylglycerol and various inositol phosphates. These effects are associated with depolarization of the acinar cell which may be involved in amylase secretion.
5. The mechanism of the actions of cAMP, Ca++, and phospholipids on amylase secretion is similar in many respects.