Biology Reference
In-Depth Information
mineralocorticoids, glucocorticoids, and sex steroids) are released
immediately and not stored intracellularly. They are synthesized in
response to an appropriate signal in different organs from a common
precursor (cholesterol) and exert their action by diffusing through the
plasma membrane of target cells and binding to intracellular receptors.
The fatty acid derivatives include the prostaglandins, derived from the
20-carbon fatty acid arachidonic acid. Prostaglandins function
through paracrine regulation and are the targets of the nonsteroidal,
anti-inflammatory drugs, such as aspirin or ibuprofen. The amines (such
as catecholamines and thyroid hormones) are amino acid derivatives,
mostly synthesized from tyrosine residues, although melatonin
is produced from tryptophan. Catecholamines (such as dopamine or
norepinephrine) are the neurotransmitters of the autonomic nervous
system, whereas the thyroid hormones affect almost every tissue,
exerting growth, cardiovascular, and metabolic effects. Melatonin is an
important regulator of circadian rhythms.
Once released, hormones are transported in the bloodstream, either free
or bound to specialized carrier proteins. Even though only free
hormones exert the effect, carrier proteins are important because they
effectively modify the apparent kinetics of the active hormone. After
reaching their target cells, hormones bind to specific receptors, which are
protein molecules located on the membrane or inside the cell. This
initiates a cascade of events culminating in a biological response specific
to the target tissue.
We now provide some background about the mechanisms controlling
the production of two pituitary hormones, LH and GH, which we shall
be examining. In both cases, their production is controlled by
neurohormones produced by the hypothalamus. We begin with LH.
Neurosecretory cells of the hypothalamus produce gonadotropin-
releasing hormone (GnRH)—a short, 10-amino-acid peptide. The GnRH
is transported through the portal veins to the anterior lobe of the
pituitary, where the GnRH binds to its receptors on the pituitary cells.
These cells respond by producing the gonadotropins LH and the follicle-
stimulating hormone (FSH). LH and FSH enter the bloodstream from the
pituitary, travel to the gonads, and subsequently exert their effects
upon their target cells. Although they were identified and named in
females, both LH and FSH also function in males. As in this chapter we
shall be looking at the effects of these hormones on female fertility,
however, we now focus on their activity in women.
In the ovary, FSH and LH stimulate the development of ovarian follicles,
the production of the steroid hormones estrogen and progesterone, and
ovulation. The ovaries regulate the production of FSH by releasing
inhibin, which inhibits FSH production by the pituitary. The ovaries also
exert control through the production of estrogen, which affects both
