Role of histamine in controlling gastric acid
secretion:
Histamine in an amino acid derivative , it formed
continually in small amount in gastric
mucosa,either in response to acid in the stomach
or for other reasons , this amount , acting by itself
,causes little acid secretion , however , whenever
Ach , & gastrine stimulate the pariatal cells at the
same time , then even small amount of histamine
greatly enhance scid secretion , when the action
of histamine is blocked by antihistaminic drug
like cimetidine , neither Ach, nor gastrine can
cause significant amount ofd acid secretion , thus
histamine is necessary cofacter for exciting acid
secretion
SO Multiplicative effect of Ach, ,gastrine &histamin in
stimulating acid secretion because no one of the 3
primary stimulater for acid secretion is effective in
causing secretion of more than slight amount of acid
when function alone.
Regulation of pepsinogen secretion :
Occures in response to 2 signals
1- stimulation of peptic cells by Ach released from vagus
nerve or other enteric nerve .
2- stimulation of peptic secretion in response to acid in
stomach.
Gastrin released during acid secretion has an additional
direct effect in stimulating peptic cell, so that the rate of
secretion of pepsinogene is influanced by amount of
acid in stomach .
Feed back inhibition of gastric secretion by
excess acid :
When gastric acidity increase (PH below 3)the
gastrin mechanism for stimulating gastric
acid secretion become blocked, this result
from 2 factors:
1- greatly enhanced acidity depress or block the
secretion of gastrin itself by G cell
2- the acid cause inhibitory nervous reflex , that
inhibit gastric secretion .
This feedback inhibition of gasric glands plays
important role in protecting stomach against
excessive acidity which promote peptic
ulceration , also it is important in maintaining
optimal PH for function of pep[tic enzyme in
the digestive process (PH about 8)
Phases of gastric secretion :
1- cephalic phase :
Occur even before food enters the stomach , it
result from sight , smell, thought ,or taste of
food , neurogenic signals that originate in the
cerebral cortex or in appetite center of
hypothalamus trasmitted directly to the
stomach , this phase account about 20%of
gastric secretion
2- gastric phase :
The local enteric reflex & gastrin mechanism
causes secretion of gastric juice that
continues several hours that the food remains
in the stomach , this phase account 70% of
gastric secretion
3- intestinal phase :
The presence of food in upper S.I
(particularly the duodenum) cause the
stomach to secret small amount of gastric
juice , probably because of release small
amount of gastrin by the duodenal mucosa
in response to distention , or chemical
stimului, as well as several other
hormones.
Inhibition of gastric secretion by intestinaL
facters :
Although chyme stimulate gastric secretion
during intestinal phase , it paradoxically
often inhibit secretion during gastric phase
this result from:
1- presence of food in S.I intiate local
enterogastric reflex transmitted through
enterc system as well as through extrinsic
sympathetic & vagus nerve that inhibit
stomach secretion , this reflex intiated by
distention of S.I. presence of acid , protein
breakdown products
2- presence of acid , fat , protein breack down
products , hyper or hypo-osmottic fluid or any
irritating factors in the upper S.I cause the
release of several intestinal hormones , one of
these is secretin.
The functional importance of inhibition of
gastric secretion by intestinal factors is
probably to slow the release of chyme from
stomach when the S.I.is already filled , in fact
the enterogastric reflex & these inhibitory
hormones usually reduce stomach motility , at
the same time reduces gasric sectretion .,