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chapter 22
Metabolic Homeostasis
TABLE 22-1
Constituents o f Endocrine Pancreas
Cell Type
Hormone
Structure
Structurally Analogous Hormones
a
Glucagon
29 amino acids
Secretin
Vasoactive intestinal peptide
Gastric inhibitory polypeptide
Glicentin
ß
Insulin
Two chains:
A
= 2 1
amino acids
B=30 amino acids
IGF-I (somatomedin C)
IGF-II
Relaxin
Nerve growth factor
8
Somatostatin
Two forms:
S-14=14 amino acids
S-28
=2
8
amino acids
None known
F
Pancreatic polypeptide
36 amino acids
None known
homeostasis. The protein constituents of plasma do not
serve as significant sources of carbon and nitrogen for
cells.
Albumin
One of the smallest and the most abundant plasma pro-
teins,
albumin
plays a significant role in osmotic regula-
tion and transport of free fatty acids. Albumin is synthe-
sized in the liver at a rate of approximately 14 g/d, or
1 0
%
of the total protein synthesis of the body. Deviations from
the normal concentration of albumin in plasma can indi-
cate the state of hepatic function. Albumin is also present
in interstitial fluid.
Lipoproteins
The plasma lipoproteins play a central role in metabolic
homeostasis. As discussed in Chapter 20, a
lipoprotein
is a
complex composed of triacylglycerol, protein, cholesterol
(free or esterified), and phospholipids. The phospholipid
and protein form an external coat to render the particles
soluble in plasma for transport around the body. The roles
of lipoproteins in metabolic homeostasis are presented
in Chapter 20.
22.3 Endocrine Pancreas
and Pancreatic Hormones
The endocrine pancreas is pivotal in metabolic homeosta-
sis and an integral component of metabolic regulation (see
also Chapter 30). It is composed of 1-2 million islets of
Langerhans scattered throughout the organ and forming
2-3% of its total weight. The islets contain at least
four cell types—
a, p, 8,
and F—that secrete glucagon,
insulin, somatostatin, and pancreatic polypeptide, respec-
tively (Table 22-1). The a and
P
cells make up 20% and
75% of the total, respectively. Most islets have a charac-
teristic distribution of cells;
a
and
S
cells are located at
the periphery and
p
cells the center, but close intercellular
communication exists between different cell types. The
islet appears to function as an integrated unit rather than
as four independent types of cell. Islets in the posterior
portion of the head of the pancreas may contain up to 70%
of F cells; those in the other regions of the pancreas may
have fewer or none.
Insulin
Structure and Synthesis
The structures of insulin and proinsulin are given in
Figure 22-6. Proinsulin is a single polypeptide chain of
8 6
amino acids that permits correct alignment of three
pairs of disulfide bonds. Insulin is composed of an A chain
of 21 amino acids and a B chain of 30 amino acids, the
chains being held together by two disulfide bonds. A third
disulfide bond is present within the A chain. Human in-
sulin differs from porcine insulin by a single amino acid
and from beef insulin by three amino acids. These sub-
stitutions do not significantly affect activity; hence the
widespread use of bovine and porcine insulin in clinical
therapy. Human insulin and its analogues have been syn-
thesized by recombinant technology for clinical use. In
one analogue, two amino acid residues, namely prolyl and
lysyl residues at positions 28 and 29 on the B chain, re-
spectively, are switched. This analogue is called
lispro
