Encyclopedia of Health and Medicine

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inhibin A peptide HORMONE the corpus luteum in
ovulating women and the TESTES in men produce
that stops the HYPOTHALAMUS from secreting
GONADOTROPIN-RELEASING HORMONE (GNRH). This in
turn stops the PITUITARY GLAND from secreting
LUTEINIZING HORMONE (LH) and FOLLICLE-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 (SPERM production) in men and
likely has additional roles in OVULATION. Some
research suggests inhibin may serve as a marker to
indicate an emerging OVARIAN CANCER or PROSTATE
CANCER. With the cessation of ovulation a woman’s
OVARIES no longer produces inhibin, so inhibin is
no longer present in the BLOOD circulation of postmenopausal
women. Inhibin production returns,
however, when there is an ovarian cancer. Conversely,
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 HORMONE the ISLETS OF LANGER-
HANS in the PANCREAS produce 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 (carbohydrate
METABOLISM). Glucose, a basic sugar molecule,
is a primary energy source for many cellular
activities. When insulin binds with insulin receptors
on a cell membrane, the cell allows glucose
molecules to enter. Insulin facilitates lipid (fatty
acid) metabolism, stimulates the LIVER to convert
excess glucose into the intermediary storage form
glycogen, and facilitates the conversion of amino
acids to proteins for building new MUSCLE tissue.
Insulin also participates in cell activities related to
growth.
The beta cells of the islets of Langerhans synthesize
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 longterm
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-
EROSCLEROSIS and CORONARY ARTERY DISEASE (CAD).
Insufficient insulin production results in DIABETES,
for which insulin is available as an injectable pharmaceutical
as HORMONE THERAPY. Most forms of
insulin available today are recombinant constructions
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 metabolic
syndrome X or syndrome X, in which the cells
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