Lecture 1- Control of the gut- hormones Flashcards
Hormones
oPeptidesreleasedfromendocrinecells o Intoportalcirculation
o Passthroughliver
o Entersystemiccirculation
o Endupprettyclosetowheretheywere
paracrine hormones
o Relating to or denoting a hormone which has effect only in the vicinity of the gland secreting it.
o Peptides released by endocrine cells
o Act in local environment
o Diffuse short distances
neurocrine hormones
o Peptides released by neurones in the GI tract
o Released after action potential
how many categories of GI hormones
2
name the 2 broad categories of GI hormones
Gastrin family
Secretin family
Name the hormones of the gastrin family
gastrin
cholecytokinin (CCK)
gastrin is released from
G cells
where is gastrin released into
the antrum of the stomach
effect of gastrin
- stimulates parietal cells to produce HCL
- Increasing gastric acid secretion
- increase motility
Cholecystokinin (CCK) released from
I cells in the duodenum and jejunum
effect of CCK
- increase pancreatic/ gallbladder secretion (enzymes, bile, HCO2)
- by increasing gall bladder contractions
- relaxes sphincter of Oddi
CCK release stimulated by
fat and proteins
Name the hormones of the secretin family
secretin
gastric inhibitory polypeptide (GIP)
secretin is released by
S cells into the duodenum
what stimulates S cells to release secretin
H+ and fatty acids
effect of secretin
increase HCO3 from pancreas/gall bladder
decrease gastric acid secretion
gastric inhibitory polypeptide (GIP) cells found
in the duodenum and jejunum
gastric inhibitory polypeptide (GIP) release stimulated by
sugars, amino acids and fatty acids
GIP effect
- increase insulin
- decrease gastric acid secretion
- decreases intestinal motility
motilin released from
ECL cell
motilin effect
- increase small bowel motility
- increase gastric emptying
somatostatin released from D cells
decrease secretion and action of many hormones
what do parietal cells release
HCL
what triggers parietal cells to release HCL
Parietal cells bear receptors for three stimulators of acid secretion, reflecting a triumverate of neural, paracrine and endocrine control:
- Acetylcholine (muscarinic type receptor)
- Gastrin
- Histamine (H2 type receptor)
histamine released from
ECL cells
Histamine stimulates the parietal cells to secrete HCl.
- gastrin-ECL cell pathway
acetylcholine produced by which type of neurone
parasympathetic
which hormones reduced HCL PRODUCTION
secretin, gastric inhibitory peptide, glucagon and somatostatin.
change in morphology of parietal cells during HCL secretion
When acid secretion is stimulated there is a dramatic change in the morphology of the membranes of the parietal cell. Cytoplasmic tubulovesicular membranes which are abundant in the resting cell virtually disappear in concert with a large increase in the cannalicular membrane. It appears that the proton pump as well as potassium and chloride conductance channels initially reside on intracellular membranes and are transported to and fused into the cannalicular membrane just prior to acid secretion.
mechanism of acid secretion
The key player in acid secretion is a H+/K+ ATPase or “proton pump” located in the cannalicular membrane. This ATPase is magnesium-dependent, and not inhibitable by ouabain. The current model for explaining acid secretion is as follows:
Hydrogen ions are generated within the parietal cell from dissociation of water. The hydroxyl ions formed in this process rapidly combine with carbon dioxide to form bicarbonate ion, a reaction cataylzed by carbonic anhydrase.
Bicarbonate is transported out of the basolateral membrane in exchange for chloride. The outflow of bicarbonate into blood results in a slight elevation of blood pH known as the “alkaline tide”. This process serves to maintain intracellular pH in the parietal cell.
Chloride and potassium ions are transported into the lumen of the cannaliculus by conductance channels, and such is necessary for secretion of acid.
Hydrogen ion is pumped out of the cell, into the lumen, in exchange for potassium through the action of the proton pump; potassium is thus effectively recycled.
Accumulation of osmotically-active hydrogen ion in the cannaliculus generates an osmotic gradient across the membrane that results in outward diffusion of water - the resulting gastric juice is 155 mM HCl and 15 mM KCl with a small amount of NaCl.
