Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

II. Anterior Lobe and Intermediate Lobe
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and differentiation of mammary gland tissue in the bitch
( De Coster et al. , 1983 ; Kooistra and Okkens, 2001b, 2002 ;
Okkens et al. , 1985 ). In goats with pseudopregnancy, characterized
by hydrometra and the presence of a persistent
corpus luteum, no correlation was found with the plasma
PRL concentration. PRL does not play a crucial role in the
etiology of pseudopregnancy in the goat ( Hesselink et al. ,
1995 ; Kornalijnslijper et al. , 1997 ).
The effect of the lighting regime on PRL secretion in
rams is thought to be a direct effect of melatonin on the
pituitary gland ( Lincoln and Clarke, 1994 ). In dogs, PRL
secretion has a circannual rhythmicity (i.e., the months with
longer daylight had significantly higher PRL concentrations
than the months with the shortest) ( Corrada et al. , 2003,
2006 ). There are also ultradian gender-related differences
(i.e., higher basal levels were found in females compared
with males) ( Corrada et al. , 2003, 2006 ). In male dogs, PRL
secretion has been reported relatively constant ( Koch et al. ,
2006 ) or pulsatile in nature ( Corrada et al. , 2003, 2006 )
with a distinct breed difference (i.e., serum PRL concentrations
in beagles were significantly higher than in crossbreeds
and German shepherds) ( Corrada et al. , 2003, 2006 ).
Important modulating factors in the control of PRL
secretion are estrogens, especially 17- β-estradiol. Estrogens
modulate the TRH receptor levels in the pituitary ( Lean
et al. , 1977 ) and cause a biphasic increase in transcription
of the prolactin gene ( Gorski et al. , 1985 ). From experiments
in rats, it is concluded that progesterone may also
stimulate the release of PRL ( Deis and Alonso, 1985 ).
Estradiol rapidly induces an enhanced PRL response to
TRH in dogs, without changing basal PRL levels ( Rutteman
et al. , 1987 ). Subsequent administration of medroxyprogesterone
acetate did not further affect these findings.
Neurogenic factors also influence PRL secretion.
Milking and suckling are almost immediately followed
by PRL release. Removal of litters from their dams, for
example, piglets from sows ( Bevers et al. , 1978 ), results in
a rapid decline in PRL levels in plasma. Following return
of the litters, PRL concentrations rise again.
d . Action
PRL is produced mainly by the lactotrope cells of the anterior
pituitary and, in mammals, its most apparent function
is the regulation of lactation. In addition, PRL has been
attributed an important role in reproduction, as was illustrated
by the observation that PRL / mice not only have
defects in mammopoiesis but are also sterile ( Horseman
et al. , 1997 ). Although the bulk of PRL circulating in serum
is produced by the lactotrope cells of the pituitary, PRL
is also expressed extrapituitary by various tissues including
uterine decidualized endometrial cells and leukocytes
(Ben-Jonathan et al. , 1996 ). The most familiar role of PRL
in mammals is stimulation of mammary gland growth and
lactation. PRL increases mitosis of mammary gland epithelial
cells not only during development but also during
pregnancy and lactation. It has been relatively difficult
to demonstrate in vitro effects of PRL on cell proliferation
in mammary glands ( Friesen et al. , 1985 ). This can
be explained by the increasing evidence that the growthpromoting
effect of PRL has much in common with that of
GH (i.e., intermediate factors comparable to IGF are required
for effects on cell proliferation) ( Nicoll et al. , 1985 ).
PRL has a wide variety of other physiological actions
among vertebrates. It may affect water and electrolyte
balance, metabolism, gonadal function, and behavior. Of
these, the effect on the ovary has received much attention.
PRL has a luteotrophic effect in some animals, such as
rodents, sheep, and ferrets ( McNeilly et al. , 1982 ), but not
in the cow ( Bevers and Dieleman, 1987 ). Bovine follicles
do not bind (ovine) PRL ( Bevers et al. , 1987 ).
The reciprocal relationship between PRL and LH has
been well established in several species, including the sow
( Bevers et al. , 1983 ) and the cow ( Dieleman et al. , 1986 ).
This can explain the reduced fertility during lactation that
is known in many species. It has been suggested that PRL
may also interfere directly at the ovarian level ( McNeilly
et al. , 1982 ). These effects of PRL also appear to play a
role in the maintenance of the long interestrous interval in
the bitch. Treatment of bitches with the dopamine agonist
bromocriptine results in considerable shortening of the
interestrous interval ( Okkens et al. , 1985 ).
Mammalian maternal behavior in several species consists
of nest building and caring for offspring. Prolactin-induced
maternal behavior has been established in some animals.
However, the primary role of PRL in inducing mammalian
maternal behavior has recently been questioned ( Scapagini
et al. , 1985 ). There appears to be diversity in the hormonal
basis of maternal behavior, and in some species estrogens
and progesterone play a crucial role ( Rosenblatt, 1984 ).
e . Disease
Prolactinomas are the most common hormonally active pituitary
tumors in humans. There is a marked female preponderance,
and prolactinoma is relatively rare in men. Mixed
GH-PRL adenomas are known to occur in a substantial
number of patients with acromegaly, and the concomitant
production and secretion of PRL by corticotroph adenomas
is not unusual ( Ishibashi and Yamaji, 1985 ). Pituitary PRL
production is under tonic inhibitory control by hypothalamic
dopamine, such that in humans ’ pituitary macroadenomas
other than prolactinomas cause pituitary stalk interruption
and also produce hyperprolactinemia. There have been no
well-documented reports on the occurrence of prolactinomas
in animals, although in dogs with pituitary-dependent hyperadrenocorticism
(PDH), plasma PRL concentrations and the
PRL response to stimulation were higher than those in healthy
control dogs, probably as a result of cosecretion with ACTH
by the transformed corticotropic cells ( Meij et al. , 1997a ).
Thus, there do not seem to be pathological hyperprolactinemical
states in domestic animals that require treatment