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K. R. Foresman and J. C. Daniel Jr

Summary. Plasma levels of progesterone were determined from serial samples taken from 2 black bears over 3 consecutive fall periods. Each animal was pregnant during the 1st and 3rd years. Variations in progesterone levels were seen between animals and within each animal between pregnancies. Average baseline levels during the mid-to-late preimplantation period were 5·0–12·5 ng/ml and increased 2–3-fold at the approximate time of implantation. Values during non-pregnancy were detectable but much lower (0·6–2·5 ng/ml). The observed pattern of progesterone secretion in this species appears consistent with that reported for other species exhibiting obligate delayed implantation.

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The effects of the rabbit uterine fluid component, blastokinin (BKN), and rabbit serum on nucleic acid synthesis in rabbit and mink blastocysts were examined. [3H] Thymidine incorporation was not enhanced by BKN in rabbit embryos explanted on the 3rd to 5th days post coitum, but thymidine uptake was somewhat greater in 4- and 5-day blastocysts in serum-containing medium than in the controls in defined medium. [3H] Uridine incorporation was inhibited slightly by serum and significantly stimulated by BKN in 5-day rabbit blastocysts, but not in younger ones. Rabbit serum inhibited uridine uptake in mink blastocysts, while BKN had no effect.

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J. C. Daniel Jr, Amy E. Jetton and Beverly S. Chilton

Summary. The direct effect of prolactin on uteroglobin production and on uterine endometrial oestrogen and progesterone receptor concentrations was tested by using ovariectomized rabbits (at least 12 weeks) treated with prolactin; prolactin + progesterone; prolactin + oestradiol + progesterone; oestradiol + progesterone; or progesterone alone. Prolactin treatment produced a significant (P < 0·05) increase in the concentration of cytosolic oestrogen and progesterone receptors, restoring the concentrations to values found at oestrus. However, the concentration of nuclear receptors remained low. In the remaining treatment categories there was no significant (P > 0·05) increase in the concentration of oestrogen and progesterone receptors compared with those in ovariectomized controls. However, the sequential treatment of ovariectomized animals with prolactin + progesterone stimulated uteroglobin production to a concentration equal to that found in intact rabbits on the 5th day of pregnancy. This was not achieved by prolactin or progesterone alone or with oestradiol. These results suggest that prolactin acts as an essential factor in the rabbit uterine response to progesterone, perhaps by the modulation of progesterone receptor activity.

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G. W. Randall, J. C. Daniel Jr and B. S. Chilton

Summary. The effect of prolactin on uteroglobin production by immature rabbits was studied with neonatal (1 day old) and juvenile (14 days old) does. The animals were divided into 11 treatment groups for each age category and exposed to a 9-day injection protocol. Each day the animals received a subcutaneous injection of oestradiol-17β and/or ovine prolactin and/or progesterone, or were sham-injected. Juvenile animals, which received 100 μg oestradiol/kg 24 h−1, plus progesterone or plus prolactin and progesterone, produced detectable amounts of uteroglobin in the uterine secretions (0·034 ± 0·010 mg uteroglobin/mg total protein and 0·098 ± 0·031 mg uteroglobin/mg total protein, respectively). None of the animals in the other juvenile treatment groups or any of the neonatal groups produced uteroglobin.

From this survey it was apparent that uteroglobin secretion could be induced by exogenous oestradiol and progesterone in rabbits treated as early as 14 days of age, and that the added supplementation of prolactin enhanced the response to the ovarian steroids. As a result, additional juvenile animals were injected with 100 μg oestradiol ± prolactin + progesterone and the effects of these two treatments were quantitated as follows: uteroglobin mRNA levels by slot-blot hybridization; endometrial surface area by computerized image analysis; and oestrogen, progesterone and prolactin receptors by immunocytochemistry. Prolactin modified the response of the juvenile rabbit uterus to oestradiol + progesterone for all parameters tested.

Keywords: prolactin; uteroglobin; immunocytochemistry; receptors; rabbit

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J. C. Daniel Jr, S. C. Juneja, S. P. Taylor, P. B. Lonergan, P. K. Sullivan and B. S. Chilton

Summary. Ovariectomized rabbits from different breeders were treated at different times of the year with prolactin alone or with progesterone and the production of uteroglobin by the uterus was studied. There were seasonal, strain and dose variables in the uterine response to prolactin and progesterone. Treatment with prolactin (at 1 mg/day) plus progesterone generally induced higher levels of uteroglobin production than did treatment with progesterone alone. The differences were greatest in the winter for Tennessee animals and in the spring for animals from the New Mexico and North Carolina colonies.

Ovariectomy produced a decrease (P < 0·01) in the concentration of cytosolic oestrogen and progesterone receptors, and prolactin treatment restored the concentration to oestrous control values. However, there were no seasonally dependent changes in the concentration of the receptors for any of the treatment groups.

Increased doses of prolactin (2 mg/day) induced high levels of uteroglobin production and new proteins to appear in uterine secretions of long-term ovariectomized rabbits but much lower levels (10–11%) when given to pregnant does. Additional ovulations were also noted plus adverse effects on the embryos.

Keywords: prolactin; rabbit uterus; uteroglobin; progesterone; seasonal variability