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Division of Endocrinology and Metabolism, Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15213
Address all correspondence and requests for reprints to: Dr. Stephen J. Winters, Division of Endocrinology and Metabolism, Department of Medicine, University of Pittsburgh, Montefiore N919, 200 Lothrop Street, Pittsburgh, Pennsylvania 15213. E-mail: winters{at}med1.dept-med.pitt.edu
The mechanisms by which gonadal steroids regulate gonadotropin
secretion remain incompletely understood. As previous studies suggest
that the pituitary actions of testosterone (T) and estradiol (E) differ
in male primates and rodents, we compared the effects of 10
nM T, 0.1 nM E, and 10 nM
dihydrotestosterone (DHT) on the LH response to hourly pulses of GnRH
as well as the GnRH receptor (GnRH-R) and LH subunit messenger RNA
(mRNA) levels in dispersed pituitary cells from intact male monkeys and
rats. T suppressed (P < 0.01) and E increased
(P < 0.05) GnRH-stimulated LH secretion by rat
pituitary cells. With monkey pituitary cells, on the other hand, there
was no significant effect of either T or DHT on GnRH-stimulated LH
secretion. In E-treated monkey cells, a period of initial enhancement
(P < 0.05) was followed by significant suppression
(P < 0.05) of LH secretion. GnRH-R mRNA was
unchanged by T or E in either rat or monkey cells. T suppressed LHß
(P < 0.01) and
-subunit (P
< 0.01) mRNAs, whereas E increased
-subunit (P
< 0.01), but did not alter LHß mRNA levels in rat cells. In monkey
cells, however, neither T nor E affected LHß or
-subunit mRNA
levels significantly. Our results identify different regulatory
mechanisms by which testicular steroid hormones control LH secretion by
the pituitary in male primates and rodents. We propose that the primary
site of androgen negative feedback in the male primate is to restrain
GnRH pulsatile secretion, whereas in the male rat T also decreases
gonadotropin synthesis and secretion by directly affecting the
pituitary. E suppresses GnRH-stimulated LH secretion in the primate
pituitary, but amplifies the action of GnRH in the rat. Our data also
reveal that the action of T to suppress LH secretion and subunit mRNA
in male rats is not through decreased GnRH-R gene expression.
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