Ganong's Review of Medical Physiology, 23rd Edition

320
SECTION IV
Endocrine & Reproductive Physiology

ulation of protein synthesis from amino acids entering the cells
and inhibition of protein degradation foster growth.
The anabolic effect of insulin is aided by the protein-
sparing action of adequate intracellular glucose supplies. Fail-
ure to grow is a symptom of diabetes in children, and insulin
stimulates the growth of immature hypophysectomized rats to
almost the same degree as growth hormone.

MECHANISM OF ACTION

INSULIN RECEPTORS

Insulin receptors are found on many different cells in the

body, including cells in which insulin does not increase glu-

cose uptake.

TABLE 21–5
Glucose transporters in mammals.

Function K

m

(mM)

a

Major Sites of Expression

Secondary active transport

(Na

1

-glucose cotransport)

SGLT 1 Absorption of glucose 0.1–1.0 Small intestine, renal tubules

SGLT 2 Absorption of glucose 1.6 Renal tubules

Facilitated diffusion

GLUT 1 Basal glucose uptake 1–2 Placenta, blood-brain barrier, brain, red cells,

kidneys, colon, many other organs

GLUT 2 B-cell glucose sensor; transport out of intestinal

and renal epithelial cells

12–20 B cells of islets, liver, epithelial cells of small in-

testine, kidneys

GLUT 3 Basal glucose uptake <1 Brain, placenta, kidneys, many other organs

GLUT 4 Insulin-stimulated glucose uptake 5 Skeletal and cardiac muscle, adipose tissue,

other tissues

GLUT 5 Fructose transport 1–2 Jejunum, sperm

GLUT 6 None — Pseudogene

GLUT 7 Glucose 6-phosphate ransporter in endoplasmic

reticulum

— Liver,? other tissues

a
The K
m
is the glucose concentration at which transport is half-maximal.
Modified from Stephens JM, Pilch PF: The metabolic regulation and vesicular transport of GLUT 4, the major insulin-responsive glucose transporter. Endocr Rev 199 5 ;16: 5 29.

FIGURE 21–4
Cycling of GLUT 4 transporters through endosomes in insulin-sensitive tissues.
Activation of the insulin receptor causes
activation of phosphatidylinositol 3-kinase, which speeds translocation of the GLUT 4-containing endosomes into the cell membrane. The GLUT 4
transporters then mediate glucose transport into the cell.

Insulin receptor

Fusion

Glucose transport

Phosphatidylinositol

3-kinase

Internalization