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

II. Anterior Lobe and Intermediate Lobe
577
Pig NSCELTNITITVEKEECNFCISINTTWCAGYCYTRDLVYKDPARPNIQKTCTFKELVYET 60
Cattle R----------------G---------------------R--------------------
Sheep R----------------S-------------------------------A----------
Goat R----------------S-------------------------------A----------
Horse ----------A------G------------------------------------------
Human ----------AI-----R---------------------------K--------------
Mouse H---------S------R----------------------------T--V----------
Rat
Dog
H---------S------R----------E-----------------T--V----------
AG-----V--A------R----V-A----------------A---S-------R--A---
***** ** ***** **** * *** ********** * *** ** *** ** ***
Pig VKVPGCAHHADSLYTYPVATECHCGKCDSDSTDCTVRGLGPSYCSFSEMKE 111
Cattle ------------------------S---------------------R-I--
Sheep ----------------------------R------------------DIR-
Goat -------R--------------------R-------------------IR-
Horse --------------------A------N------------------GD---
Human -R------------------Q-------------------------G----
Mouse -RL----R-S-----------------------------------------
Rat
Dog
IRL----R-S------------------------------------G----
-R-----R-----H-----------R---------------G----G-PQQ
**** * *** ****** *** * ************ ****
FIGURE 18-10 Sequence comparison of the β -subunit of FSH. See the legend for Figure 18-6 .
Goat
Sheep
Cattle
Pig
Horse
Human
Mouse
Rat
Dog
c . Secretion
As with other pituitary hormones, the gonadotropins are
released in a pulsatile fashion, stimulated by pulses of the
hypothalamic gonadotropin-releasing hormone GnRH.
The release of GnRH in seasonal breeders is tightly regulated
by the length of the photoperiod and the subsequent
release of melatonin by the pineal gland ( Thiery et al. ,
2002 ). Pulsatile GnRH secretion has been demonstrated in
sheep ( Clarke and Cummins, 1982 ) and the horse ( Irvine
and Alexander, 1994 ). GnRH can be used for estrus induction
and prevention in the dog ( Gobello, 2007 ). In contrast
to single administration of GnRH, continuous administration
of GnRH agonists reversibly suppresses gonadotropin
secretion because of down-regulation/desensitization of
GnRH receptors. Effect of dose and duration of treatment
with a GnRH-agonist resulting in a switch from stimulated
LH and FSH release toward down-regulation has been
reported recently for the dog ( Concannon et al. , 2006 ).
In the dog, LH and FSH pulses coincide during different
phases of the estrus cycle although the longer duration
of FSH pulses may be due to the difference in plasma halflife.
During progression from early to late anestrus basal
plasma FSH increase without a concomitant increase in
basal LH concentrations ( Kooistra and Okkens, 2001a ;
Okkens and Kooistra, 2006 ). During the early follicular
phase, the LH secretion has frequent increases of short
duration ( Kooistra et al. , 1999 ). The preovulatory FSH
in female dogs may start a few hours earlier or may coincide
with the LH surge, which is associated with the highest
plasma estradiol-17 β concentrations and the start of
increasing plasma progesterone concentrations ( de Gier
et al. , 2006 ). During the mid- to late-luteal phase, basal LH
concentrations in pregnant bitches were higher with lower
peak frequency and height in comparison to nonpregnant
bitches. Plasma FSH concentrations were higher during
pregnancy ( Onclin et al., 2002 ). Chronic administration of
the synthetic progestin MPA has no effect on plasma LH
concentrations, but, like in pregnancy, increased plasma
FSH concentrations are found ( Beijerink et al. , 2007 ).
Administration of dopamine-agonists to shorten the
anestrus interval results in a rapid rise in basal FSH concentration
( Okkens and Kooistra, 2006 ), whereas the serotonin
antagonist metergoline, which lowers also plasma
prolactin concentrations, does not affect the LH and FSH
secretion patterns in the dog ( Beijerink et al. , 2004 ).
Low plasma estrogen concentrations inhibit the GnRH
and LH release through a negative feedback system,
whereas high concentrations of estrogens may exert a positive
feedback by stimulating GnRH release. Kiss1 neurons
in the periventricular nucleus are thought to be involved in
the estrogen and progesterone-induced LH surge ( Smith
et al. , 2006 ). In the male, LH stimulates the synthesis and
release of testosterone in the Leydig cells of the testis, which
may in turn exert a negative feedback on LH secretion.
This feedback may depend on aromatization of testosterone
to estradiol in the brain. In the male dog, both testosterone
and estradiol are the major inhibitors of LH and
FSH release ( Winter et al. , 1982 ).
Finally, FSH can also be regulated by more specific
stimulation through activin or by prevention of activin
binding and thus inhibition of downstream signaling by
inhibin and follistatin ( Gregory and Kaiser, 2004 ; Gregory
et al. , 2005 ). Other members of the TGF β family such as
the bone morphogenic protein-6, BMP-7, and BMP-15 can
also exert an effect on FSH secretion.
d . Action
FSH stimulates in the female ovary the folliculogenesis
and ripening of the ovaries to the antral follicle stage. LH
stimulates changes in the ovarian follicle resulting in ovulation
and maintenance of the corpus luteum ( Hunter et al. ,
2004 ). LH stimulates the production of the androgen
precursor androstenedione that is converted, by FSH
stimulation, to estradiol-17 β and during the luteal phase