Prostaglandins and Cytokines
Prostaglandins are derivatives of
long-chain unsaturated fatty acids that
were discovered in seminal fluid in the
1930s. At first they were thought to be
produced only by the prostate gland
(hence the name) but now have been
found in virtually all mammalian tissues.
Prostaglandins act as local hormones
that have diverse actions on
many different tissues, making generalizations
about their effects difficult.
Many of their effects, however, involve
smooth muscle. In some tissues prostaglandins
regulate vasodilation or vasoconstriction
by their action on smooth
muscle in walls of blood vessels.
They are known to stimulate contraction
of uterine smooth muscle during
childbirth. There also is evidence that
overproduction of uterine prostaglandins
is responsible for painful symptoms
of menstruation (dysmenorrhea)
experienced by many women. Several
inhibitors of prostaglandins that provide
relief from these symptoms have
now been approved as medicines.
Among other actions of prostaglandins
is their intensification of pain in damaged
tissues, mediation of the inflammatory
response, and involvement in
For some years we have known that
cells of the immune system communicate
with each other and that this communication
was crucial to the immune
response. Now we understand that a
large group of polypeptide hormones
called cytokines mediate
communication between cells participating
in the immune response.
Cytokines can affect the cells that
secrete them, affect nearby cells, and
like other hormones, they can affect
cells in distant locations. Their target
cells bear specific receptors for the
cytokine bound to the surface membrane.
Cytokines coordinate a complex
network, with some target cells being
activated, stimulated to divide and
often to secrete their own cytokines.
The same cytokine that activates some
cells may suppress division of other
target cells. Cytokines also are
involved in formation of blood, and
more recently, their role in regulation
of energy balance by the central nervous
systems is being explored.