Page 316 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

cholesterol via pregnenolone (! p. 295). It is to epileptic fits and exerts thermogenic action,
produced in the corpus luteum, ovarian fol- i.e., it raises the basal body temperature
licles and placenta (! p. 304) of the female, (! p. 298). In addition, a decrease in the pro-
and in the adrenal cortex of males and females. gesterone concentration is also believed to be
Like cortisol, most circulating progesterone is responsible for the mood changes and depres-
bound to cortisol-binding globulin (CBG = sion observed before menstruation (premen-
transcortin). Like estradiol (E 2), most pro- strual syndrome, PMS) and after pregnancy
gesterone is broken down during its first pass (postpartum depression).
through the liver, so oral doses of progesterone ! In the kidneys, progesterone slightly inhibits the
are almost completely ineffective. Pregnane- effects aldosterone, thereby inducing increased NaCl
diol is the most important degradation product excretion.
of progesterone.
Actions of progesterone. The main functions Prolactin and Oxytocin
of progesterone are to prepare the female geni-
tal tract for implantation and maturation of The secretion of prolactin (PRL) is inhibited
the fertilized ovum and to sustain pregnancy by prolactin-inhibiting hormone (PIH =
(! see table). Progesterone counteracts many dopamine) and stimulated by thyroliberin
of the effects induced by estrogens, but various (TRH) (! p. 270). Prolactin increases the hy-
effects of progesterone depend on the prepara- pothalamic secretion of PIH in both men and
tory activity or simultaneous action of estro- women (negative feedback control). Con-
gens. During the follicular phase, for example, versely, estradiol (E 2) and progesterone inhibit Estrogens
estrogens increases the density of pro- PIH secretion (indirectly via transmitters, as
gesterone receptors, while simultaneous observed with Gn-RH; see above). Con-
estrogen activity is needed to induce mam- sequently, prolactin secretion rises signifi-
mary growth (see below). cantly during the second half of the menstrual
! The uterus is the chief target organ of pro- cycle and during pregnancy. Prolactin (to-
gesterone. Once estrogen induces endometrial gether with estrogens, progesterone, glucocor-
thickening, progesterone stimulates growth of ticoids and insulin) stimulate breast enlarge-
the uterine muscle (myometrium), restruc- ment during pregnancy and lactogenesis after
tures the endometrial glands (! p. 298), alters parturition. In breast-feeding, stimulation of
the blood supply to the endometrium, and the nerve endings in the nipples by the suck-
changes the glycogen content. This represents ling infant stimulates the secretion of prolactin
the transformation from a proliferative en- (lactation reflex). This also increases release of
dometrium to a secretory endometrium, with oxytocin which triggers milk ejection and in-
a peak occurring around day 22 of the cycle. creases uterine contractions, thereby increas-
Progesterone later plays an important role in ing lochia discharge after birth. When the
the potential implantation (nidation) of the mother stops breast-feeding, the prolactin
fertilized ovum because it reduces myometrial levels drop, leading to the rapid stoppage of
activity (important during pregnancy), nar- milk production.
rows the cervical os, and changes the con- Hyperprolactinemia. Stress and certain drugs in-
sistency of the cervical mucous plug so that it hibit the secretion of PIH, causing an increase in pro-
becomes virtually impregnable to sperm. lactin secretion. Hypothyroidism (! p. 288) can also
! Progesterone inhibits the release of LH during the lead to hyperprolactinemia, because the associated
luteal phase. The administration of gestagens like increase in TRH stimulates the release of prolactin.
progesterone during the follicular phase inhibits ovu- Hyperprolactinemia inhibits ovulation and leads to
lation. Together with its effects on the cervix (see galactorrhea, i.e., the secretion of milk irrespective of
above) and its inhibitory effect on capacitation pregnancy. Some women utilize the anti-ovulatory
(! p. 302), progesterone can therefore have a con- effect of nursing as a natural method of birth control,
traceptive effect (“mini pill”). which is often but not always effective.
! High levels of progesterone have an anes-
thetic effect on the central nervous system. 303
Progesterone also increases the susceptibility
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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