Ž .
higher P - 0.0004
in the first than in the second serial bleeding period in the estradiol-treated group.
4. Discussion
Ž .
In Experiment 1, a single systemic im injection of supraphysiologic doses of E-17b in progestogen-implanted heifers resulted in a dose-dependent increase in circulating
concentrations of estradiol. Concentrations after treatment were higher than expected. However, the first blood sample was taken 2 h after treatment in the present study,
Ž whereas sampling began 6 h after E-17b treatment in the previous study Bo et al.,
. 1994 . Values at 6 h in the present study were similar to those reported previously. The
Ž .
Ž dynamics of E-17b absorption peak at 2 h after treatment and metabolism basal levels
. at 24 to 50 h depending on the dosage used are in agreement with reports from other
Ž .
laboratories Rexroad et al., 1977; Randel et al., 1979 . It would seem that increased Ž
. circulating concentrations of E-17b for a period of 12 h 0.5 or 1.0 mg of E-17b to 24
Ž .
h 5.0 mg of E-17b is adequate to cause suppression of the dominant follicle at this stage of development of the first follicular wave. Elevated concentrations of E-17b for
10 h, as was seen in the 0.1 mg of E-17b group, did not seem to be sufficient to induce follicular suppression. We have shown previously that the administration of 5.0 mg of
Ž E-17b to progestogen-implanted heifers causing significantly elevated circulating E-17b
. concentrations for approximately 42 h will induce follicular regression and emergence
of a new follicular wave, regardless of the stage of development of the dominant follicle Ž
. of the first follicular wave Bo et al., 1994, 1995a,b . Therefore, greater doses or
longer-acting forms of estrogen would not seem to be necessary, and may even result in Ž
delayed andror asynchronous emergence of the new follicular wave Bo et al., 1993, .
1994 . Experiment 2 utilized a subminimal dose of E-17b chosen from Experiment 1 to
examine the local versus systemic effects of estradiol on follicular development. The io Ž
. local treatment dose of 0.1 mg of E-17b raised systemic concentrations of estradiol
Ž .
- 125 pgrml for about 6 h and follicle growth was not suppressed. Conversely, the
Ž .
systemic treatment iu or im with 0.1 mg of E-17b resulted in a greater increase in Ž
. systemic concentrations of estradiol
- 250 pgrml
for a similar period of time Ž
. approximately 6 h and a transient suppression of follicular growth. The intraovarian
dose of 0.1 mg, E-17b and volume of sesame seed oil used as a vehicle was similar to Ž
. that previously used in monkeys Hutz et al., 1988 , but contrary to findings in monkeys,
this treatment did not result in follicular regression in cattle. The dosage used for the local treatment is considerably greater than the E-17b present in the fluid in the
Ž .
dominant follicle, which has been reported to be about 1500 ngrml Singh et al., 1998 . Therefore, the amount given adjacent to the dominant follicle would have been expected
to induce profound follicular suppression compared with systemic treatments if the effects of E-17b were mediated locally.
Injection of 0.1 mg of E-17b into the uterine wall also represented a local treatment. With the close association of the vasculature between drainage of the cranial end of the
Ž .
uterine horn and supply to the ovary Ginther, 1974 , E-17b treatment was expected to elevate estradiol concentrations in the arterial blood supplying the ovary containing the
dominant follicle. Results indicated a slightly greater increase in systemic concentrations of estradiol compared with the intraovarian injection. Furthermore, results indicated a
Ž .
transient suppression of follicular growth for 1 day Day 5 before the diameter of the dominant follicle increased similar to that in the control group. In Experiment 1,
systemic treatment with 0.1 mg of E-17b im did not appear to suppress follicular growth, whereas, in Experiment 2, there was a transient suppression of follicular growth
similar to that following local treatment into the uterine wall. Although the difference between the two experiments with respect to systemic treatment is unknown, the
intrauterine treatment may have been more representative of a systemic than a local treatment.
Differences in the concentrations of circulating E-17b in the 0.1 mg groups may have been due to the volume of sesame seed oil in which E-17b was injected into the ovary
Ž .
0.2 ml versus 1.0 ml for im and iu sites or its absorption from the site of injection. In any case, results support an indirect or systemic route of action. When systemic
concentrations of E-17b were higher, follicular growth was affected, at least transiently, Ž
. and as dose of E-17b and circulating concentrations increased, follicular growth was
more profoundly affected. The effects of 0.5 and 1 mg of E-17b on dominant follicle growth in Experiment 1
further support the notion that the suppressive effect of estradiol is synergistic with progestogenrprogesterone. Similar dosages of estradiol benzoate have been used to
increase synchrony of estrus andror ovulation and fertility when given after prosta- Ž
. glandin treatment Peters et al., 1977 or 24 to 48 h after the removal of progesterone-re-
Ž leasing vaginal devices in cattle McDougall et al., 1992; Day et al., 1997; Fike et al.,
. 1997; Hanlon et. al., 1997; Martinez et al., 1998 . If estrogens given alone had a direct
suppressive effect on ovarian follicles, such treatments would have been expected to result in decreased fertility.
Although results support an indirect or systemic route of follicular suppression by estradiol, hourly changes in circulating concentrations of FSH and LH were not detected
following estradiol treatment. Additional analyses for changes in FSH between serial bleeding periods within each group indicated significantly higher concentrations during
Ž .
the second serial bleeding period Hours 16 to 26 compared with the first serial Ž
. bleeding period Hours 2 to 12 for both control and treated groups. The change in FSH
between the two serial bleeding periods may be attributable to low amplitude surges of FSH that have been detected in plasma samples collected at 4-h intervals during the
Ž .
early luteal phase in cattle Bergfelt et al., 1997 . Regardless, the present results do not Ž
. agree with those reported previously Bo et al., 1994 in which treatment of progesto-
gen-implanted heifers with 5.0 mg of E-17b suppressed plasma FSH for about 24 h and resulted in regression of the dominant follicle. Although comparable results with respect
to alterations in development of the dominant follicle following different doses of estradiol and routes of administration are indicated in the present study, the lack of any
detectable changes in FSH do not allow for a similar conclusion. The difference in FSH results between the previous and present studies may be attributable to the time of
Ž .
treatment Day 1 versus Day 3, respectively andror the FSH assay. In the present
study, samples were collected at a time when systemic FSH would be expected to be at Ž
. basal concentrations Adams et al., 1992a,b . Therefore, the FSH values in the control
group and at the time of treatment with E-17b would have been near the sensitivity of Ž
. the assay, an area of great variability Adams et al., 1992a,b . Furthermore, a recent
report indicated that the bovine antiserum, used herein, was inferior to ovine antiserum Ž
. when used in radioimmunoassays for measuring FSH Crowe et al., 1997 .
Changes in circulating concentrations of LH were similar between the two serial bleeding periods for the control group; however, LH concentrations were significantly
Ž .
higher during the first serial bleeding period Hours 2 to 12 compared with the second Ž
. Hours 16 to 26 in the estradiol-treated groups. Bolus administration of estradiol that
resulted in supraphysiological concentrations in circulating estradiol has been reported to Ž
. exert a biphasic effect on LH secretion Kesner et al., 1981; Butler et al., 1983 ;
secretion was initially inhibited and this was followed by a surge release. In a previous Ž
study, treatment of heifers on Day 1 with 5.0 mg of E-17b without exogenous
. Ž
progestogenrprogesterone was followed by a surge release of LH 16 to 17 h later Bo .
et al., 1994 . Conversely, when the same treatment was done in progestogen-implanted heifers no surge in LH was detected. The progestogen ear implant may have partially
blocked the estradiol-induced LH release, resulting in apparent higher concentrations of LH between 9 and 12 h after treatment. Furthermore, considering that LH was
apparently lower in both estradiol-treated and control groups during the second serial bleeding period, the increasing concentrations of progestogen from the ear implant and
progesterone from the CL may have suppressed LH release. Although increases in progestogenrprogesterone concentrations have been shown to result in decreased LH
Ž .
pulse frequency and follicular suppression Savio et al., 1993b , this effect was not apparent in the present study.
5. Conclusion
