I

inhibin A peptide HORMONEthe corpus luteum in
ovulating women and the TESTESin men produce
that stops the HYPOTHALAMUS from secreting
GONADOTROPIN-RELEASING HORMONE(GNRH). This in
turn stops the PITUITARY GLAND from secreting
LUTEINIZING HORMONE(LH) andFOLLICLE-STIMULATING
HORMONE(FSH), halting the subsequent cascade of
sex hormones from the gonads (sex glands).
Researchers do not yet fully understand the full
range of inhibin’s actions, though it influences
spermatogenesis (SPERMproduction) in men and
likely has additional roles in OVULATION. Some
research suggests inhibin may serve as a marker to
indicate an emerging OVARIAN CANCERor PROSTATE
CANCER. With the cessation of ovulation a woman’s
OVARIESno longer produces inhibin, so inhibin is
no longer present in the BLOODcirculation of post-
menopausal women. Inhibin production returns,
however, when there is an ovarian cancer. Con-
versely, inhibin levels appear to drop in men who
have BENIGN PROSTATIC HYPERTROPHY (BPH) or
prostate cancer.
For further discussion of inhibin within the
context of the endocrine system’s structure and
function please see the overview section “The
Endocrine System.”
See also FERTILITY; PREGNANCY; TESTOSTERONE.

insulin A peptide HORMONEthe ISLETS OFLANGER-
HANSin the PANCREASproduce that is essential for
the body to use GLUCOSE. Insulin has numerous
roles in the body, the best known of which is the
regulation of glucose levels in the BLOOD(carbohy-
drate METABOLISM). Glucose, a basic sugar mole-
cule, is a primary energy source for many cellular
activities. When insulin binds with insulin recep-
tors on a cell membrane, the cell allows glucose

molecules to enter. Insulin facilitates lipid (fatty

acid) metabolism, stimulates the LIVERto convert

excess glucose into the intermediary storage form

glycogen, and facilitates the conversion of amino

acids to proteins for building new MUSCLEtissue.

Insulin also participates in cell activities related to

growth.

The beta cells of the islets of Langerhans synthe-

size insulin in response to declining glucose levels

in the blood. The release of insulin allows cells to

accept glucose and at the same time directs the liver

to begin converting glucose to glycogen for storage.

Insulin also slows the conversion of fatty acids to

glycogen, a process intended to conserve the long-

term energy resources of the body (fat). These

functions become less efficient in INSULIN RESISTANCE,

a condition in which the cells are slow to bind with

insulin. Disturbances of insulin sensitivity can

allow lipids to accumulate in the blood circulation,

contributing to cardiovascular diseases such as ATH-

EROSCLEROSISandCORONARY ARTERY DISEASE(CAD).

Insufficient insulin production results in DIABETES,

for which insulin is available as an injectable phar-

maceutical as HORMONE THERAPY. Most forms of

insulin available today are recombinant construc-

tions engineered in the laboratory to precisely

match the molecular structure and actions of

endogenous human insulin.

For further discussion of insulin within the

context of the endocrine system’s structure and

function please see the overview section “The

Endocrine System.”

See also CORTISOL; DIGESTIVE ENZYMES; DIGESTIVE

HORMONES.

insulin resistance A condition, also called meta-

bolic syndrome X or syndrome X, in which the cells

148