Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 26Overview of Gastrointestinal Function & Regulation 445

The carboxyl terminal tetrapeptide (CCK 4) also exists in

tissues. The carboxyl terminal is amidated, and the tyrosine

that is the seventh amino acid residue from the carboxyl

terminal is sulfated. Unlike gastrin, the nonsulfated form of

CCK has not been found in tissues. However, derivatization of

other amino acid residues in CCK can occur. The half-life of

circulating CCK is about 5 minutes, but little is known about

its metabolism.

In addition to its secretion by I cells in the upper intestine,

CCK is found in nerves in the distal ileum and colon. It is also

found in neurons in the brain, especially the cerebral cortex,

and in nerves in many parts of the body (see Chapter 7). In

the brain, it may be involved in the regulation of food intake,

and it appears to be related to the production of anxiety and

analgesia. The CCK secreted in the duodenum and jejunum is

probably mostly CCK 8 and CCK 12, although CCK 58 is also

present in the intestine and circulating blood in some species.

The enteric and pancreatic nerves contain primarily CCK 4.

CCK 58 and CCK 8 are found in the brain.

In addition to its primary actions, CCK augments the

action of secretin in producing secretion of an alkaline pan-

creatic juice. It also inhibits gastric emptying, exerts a trophic

effect on the pancreas, increases the synthesis of enterokinase,

and may enhance the motility of the small intestine and colon.

There is some evidence that, along with secretin, it augments

the contraction of the pyloric sphincter, thus preventing the

reflux of duodenal contents into the stomach. Gastrin and

CCK stimulate glucagon secretion, and since the secretion of

FIGURE 26–22 Sites of production of the five gastrointestinal hormones along the length of the gastrointestinal tract. The width of
the bars reflects the relative abundance at each location.

Fundus

Antrum

Duodenum

Jejunum

Ileum

Colon

Gastrin CCK Secretin GIP Motilin

TABLE 26–7 Stimuli that affect gastrin secretion.

Stimuli that increase gastrin secretion

Luminal

Peptides and amino acids

Distention

Neural

Increased vagal discharge via GRP

Bloodborne

Calcium

Epinephrine

Stimuli that inhibit gastrin secretion

Luminal

Acid

Somatostatin

Bloodborne

Secretin, GIP, VIP, glucagon, calcitonin