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Biology of Reproduction, Vol 25, 832-841, Copyright © 1981 by Society for the Study of Reproduction
1 Animal Reproduction Laboratory, Department of Animal Science,
Michigan State University, East Lansing, MI 48824 Changes in concentrations of LH and progesterone in serum and specific binding of 125I-hCG to
crude membrane preparations of luteal cells were examined during spontaneous regression (Group
1), prostaglandin-induced regression (Group 2), and growth (Group 3) of bovine corpora lutea
(CL). Specific binding of 125I-hCG (presumably by the LH receptor) to luteal homogenates was
validated using Scatchard analysis and shown to be specific for LH/hCG with high affinity (Ka =
2.18 ± 0.76 x 1010 M-1). All cattle had estrous cycles synchronized with a synthetic prostaglandin
F2 During spontaneous regression (Group 1), Day 19 to Day 21 of the estrous cycle, serum progesterone, specific binding of hCG, and CL weight decreased (P<0.05) concomitantly. No significant
change
in LH occurred prior to Day 21. During PG-induced regression of corpora lutes (Group 2),
0 to 36 h after PG, the amount of hCG binding, weights of corpora lutea and concentrations of
serum progesterone decreased significantly and remained at low levels through 60 h. However,
serum progesterone decreased to basal levels 12 h earlier than did hCG binding. In contrast, serum
LH increased by 12 h (P<0.05) and remained elevated through 60 h after PG. Correlations between
variables measured during spontaneous and induced luteal regression were similar. During CL development (Group 3), 1.9 to 4.5 days after the preovulatory LH surge, serum
progesterone, specific binding of hCG, and CL weight increased coincidently (P<0.05). Maximum
levels of serum progesterone and binding of hCG were reached at Day 8.3. However, between Days
8.3 and 12.4 post-LH surge, serum progesterone and CL weight did not change (P>0.05), while
binding of hCG to corpora lutea decreased (P<0.05). From these data we suggest that synchronization of estrus using two injections of prostaglandin
results in short-term endocrine changes that are similar to changes seen during the late luteal phase
of the estrous cycle. Since serum progesterone declined to basal levels prior to binding of hCG to
corpora lutea during PG-induced regression, other events such as decreased adenylate cyclase
activity may be more closely associated with decreased function of corpora lutea than decreases in
LH receptor during regression of corpora lutea. In addition, we conclude that changes in numbers
of LH receptors may be more closely associated to progesterone secretion in early luteal development (<4.5 days post-LH surge) than during late luteal development (4.5 to 12.4 days post-LH
surge). This suggests that with an increase in LH binding sites, the early developing corpus luteum
may become more responsive to basal levels of serum LH causing an increase in progesterone
secretion.
2 Agricultural Institute, Grange, County Meath, Ireland
, cloprostenol (PG). Group 1 had ovaries removed on Day 17, 18, 19, on estrus, or 24 h after
estrus. Group 2 had ovaries removed at 0, 12, 24, 48, 60, or 72 h after PG. Group 3 had ovaries
removed at 5, 7, 9, 11, 13, or 15 days after PG. Blood samples were taken from all animals at 8 h
intervals up to time of ovariectomy for measurement of serum progesterone and LH and at 2 h
intervals around estrus to determine time of preovulatory LH surge. Data from heifers in Group 3
were grouped for analysis according to days after preovulatory LH surges.
Note:
ACKNOWLEDGMENTS
We are very grateful to the National Pituitary
Agency, NIAMDD, for supplying the hCG (CR-119)
and various NIH bovine hormone preparations used;
Mr. D. Kinsley, Mr. D. Prendiville, and Ms. A.
McArthur for assistance in performing the animal
experiments; and Joanna Gruber for the preparation
of this manuscript.
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