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Biology of Reproduction, Vol 23, 415-422, Copyright © 1980 by Society for the Study of Reproduction
1 Department of Physiology and Biophysics, West Virginia University, Morgantown, WV 26506. Two effects of estradiol on tonic LH secretion in the ewe were observed when the steroidal
milieu of the estrous cycle was reproduced in the absence of the ovaries by the administration of
estradiol and progesterone in Silastic implants. One effect of estradiol was revealed in a model
corresponding to the follicular phase, namely following a precipitous decrease in circulating progesterone. Under these conditions, estradiol diminished the magnitude of the increase in tonic LH
secretion initiated by progesterone withdrawal. Importantly, basal estradiol levels were fully
effective in this regard; increments above basal concentrations produced no further significant
inhibition of LH. The second effect of estradiol was revealed under conditions resembling the
luteal phase, namely in the presence of graded increases in serum progesterone. Estradiol greatly
enhanced the ability of all doses of progesterone to suppress LH. Again, higher estradiol concentrations did not appear to be more effective in this regard than basal concentrations.
2 Department of Physiology and Biophysics, University of Kentucky, Lexginton, KY 40536.
3 Department of Physiology, University of Pittsburgh, Pittsburgh, PA 15261.
4 Reproductive Endocrinology Program, Departments of Physiology,
Obstetrics and Gynecology, and Biological Sciences,
The University of Michigan,
Ann Arbor, Michigan 48109
5 Reproductive Endocrinology Program,
Departments of Pathology, and Biological Sciences, The University of Michigan, Ann Arbor, Michigan 48109
Note:
ACKNOWLEDGMENTS
We wish to thank Douglas Doop for his assistance
with the animal experimentation, Barbara Glover for
performing radioimmunoassays, Dr. G. D. Niswender
for antisera used in radioimmunoassays. Dr. L. E.
Reichert, Jr., for purified ovine LH used for iodination. and Dr. S. Webel for the progesterone implants
used to synchronize estrus.
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