Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 26Overview of Gastrointestinal Function & Regulation 447

VIP

VIP contains 28 amino acid residues (Table 26–6). It is found
in nerves in the gastrointestinal tract and thus is not itself a hor-
mone, despite its similarities to secretin. Prepro-VIP contains
both VIP and a closely related polypeptide (PHM-27 in hu-
mans, PHI-27 in other species). VIP is also found in blood, in
which it has a half-life of about 2 minutes. In the intestine, it
markedly stimulates intestinal secretion of electrolytes and
hence of water. Its other actions include relaxation of intestinal
smooth muscle, including sphincters; dilation of peripheral
blood vessels; and inhibition of gastric acid secretion. It is also
found in the brain and many autonomic nerves (see Chapter
7), where it often occurs in the same neurons as acetylcholine.
It potentiates the action of acetylcholine in salivary glands.
However, VIP and acetylcholine do not coexist in neurons that
innervate other parts of the gastrointestinal tract. VIP-secret-
ing tumors (VIPomas) have been described in patients with se-
vere diarrhea.

MOTILIN

Motilin is a polypeptide containing 22 amino acid residues that
is secreted by enterochromaffin cells and Mo cells in the stom-

ach, small intestine, and colon. It acts on G protein-coupled re-

ceptors on enteric neurons in the duodenum and colon and on

injection produces contraction of smooth muscle in the stom-

ach and intestines. Its circulating level increases at intervals of

approximately 100 min in the interdigestive state, and it is a

major regulator of the migrating motor complexes (MMCs)

(Figure 26–24) that control gastrointestinal motility between

meals. Conversely, when a meal is ingested, secretion of motil-

in is suppressed until digestion and absorption are complete.

The antibiotic erythromycin binds to motilin receptors, and

derivatives of this compound may be of value in treating pa-

tients in whom gastrointestinal motility is decreased.

SOMATOSTATIN

Somatostatin, the growth-hormone-inhibiting hormone origi-

nally isolated from the hypothalamus, is secreted as a paracrine

by D cells in the pancreatic islets (see Chapter 21) and by similar

D cells in the gastrointestinal mucosa. It exists in tissues in two

forms, somatostatin 14 and somatostatin 28, and both are se-

creted. Somatostatin inhibits the secretion of gastrin, VIP, GIP,

secretin, and motilin. Its secretion is stimulated by acid in the

lumen, and it probably acts in a paracrine fashion to mediate the

inhibition of gastrin secretion produced by acid. It also inhibits

FIGURE 26–24 Migrating motor complexes (MMCs). Note that the complexes move down the gastrointestinal tract at a regular rate dur-
ing fasting, that they are completely inhibited by a meal, and that they resume 90–120 minutes after the meal. (Reproduced with permission from
Chang EB, Sitrin MD, Black DD: Gastrointestinal, Hepatobiliary, and Nutritional Physiology. Lippincott-Raven, 1996.)

Phase I -

Phase II -

Phase III - Regular spike potentials

and contractions

No spike potentials,

no contractions

Irregular spike potentials

and contractions

Phases of

MMC III

II

I

MEAL

Stomach

Propagation

rate

(5 cm/m)

Distal

ileum

~90 min

Resumption

of MMCs