on the ovary. Beef heifers received a progestogen ear implant on Day 2 and were assigned Ž
. Ž
. randomly to five groups on Day 3: control sesame seed oil, n s 8 ; 5 mg of E-17b im n s 8 ;
Ž .
0.1 mg of E-17b im n s 8 ; 0.1 mg of E-17b given into the wall of the uterus, near the tip of the Ž
Ž . .
horn ipsilateral to the dominant follicle intrauterine iu , n s 8 ; or 0.1 mg of E-17b given into Ž
Ž . .
the stroma of the ovary, immediately adjacent to the dominant follicle intraovarian io , n s 6 . Ž
. Local iu and io treatments were given via a transvaginal ultrasound-guided needle injection.
Treatment with 5 mg of E-17b im resulted in suppression of the dominant follicle of the first Ž
. follicular wave and early emergence of the second follicular wave P - 0.05 . Diameter profiles
of the dominant follicle in heifers treated with 0.1 mg im or 0.1 mg iu differed from those of control heifers on Day 5, whereas diameter profiles of the dominant follicle in heifers treated with
0.1 mg io did not differ from the controls. Daily changes in diameter of the dominant follicle did
Ž .
not differ among the three groups treated with 0.1 mg of E-17b im, iu and io . Hourly changes in circulating concentrations of FSH and LH were not detected following estradiol treatment either
before or after the results were combined for all estradiol-treated groups. Results are supportive of the hypothesis that the suppressive effect of estradiol in cattle is exerted indirectly through a
systemic route rather than directly at the ovary. Although low plasma concentrations of FSH and LH were not detected, systemic treatments with high E-17b dosages resulted in follicular
suppression whereas local treatments with subminimal dosages, within the ovary bearing the dominant follicle, were without effect. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Heifer; Follicle; Progestogen; Estradiol; Gonadotropins
1. Introduction
The dominant follicle of a follicular wave suppresses the growth of other follicles of Ž
. the wave subordinates and prevents emergence of the next follicular wave until the
Ž dominant follicle of the preceding wave regresses or ovulates Ginther et al., 1989c;
. Adams et al., 1992a, 1993 . The mechanism by which follicular dominance occurs is not
understood. However, it has been shown that the dominant follicle produces large Ž
. Ž
amounts of steroidal i.e., estrogen, androgens and non-steroidal factors i.e., inhibin, .
follistatin, growth factors of which some have been shown to suppress follicular growth Ž
by altering systemic concentrations of FSH Ginther et al., 1989c; Adams et al., 1992a; .
Findlay et al., 1992; Fortune, 1994; Bergfelt et al., 1994 . Intraovarian products may Ž
also act directly or locally on the ovary to alter follicular development Lobb and .
Dorrington 1992; Findlay, 1993 . Experiments in which exogenous estradiol was administered to determine its effect on
Ž .
follicular development Hutz et al., 1988; Engelhart et al., 1989 have indicated follicle regression after treatment. Similarly, we have documented suppression of growth of the
Ž dominant follicle after estradiol treatment to progestogen-implanted cattle Bo et al.,
. 1991, 1995a,b . In this regard, the dominant follicle of a follicular wave gains the
Ž capacity to produce larger quantities of estradiol than subordinate follicles Badinga et
. al., 1992; Bodensteiner et al., 1996 . Perhaps, the degree of estrogen production by the
dominant follicle is part of a mechanism that facilitates dominance by suppressing Ž
. Ž
development of other follicles of the wave either directly i.e., locally or indirectly i.e., .
systemically .
Ž Exogenous estrogens have been shown to induce an LH surge Short et al., 1979;
. Kinder et al., 1991 and it was suggested that LH release may be associated with
induction of follicular atresia. However, results from our laboratory do not support such a hypothesis; estradiol treatment to heifers with progestogen-ear implants or during the
luteal phase of the cycle did not induce an LH surge yet growth of the dominant follicle
Ž .
was suppressed Bo et al., 1993, 1994 . In another laboratory, follicular atresia was detected prior to the estradiol-induced LH release in cows treated on Day 16 of the cycle
Ž .
Engelhart et al., 1989 . Alternatively, estrogen may induce follicular atresia by altering Ž
. tonic LH secretion andror through FSH suppression Price and Webb, 1988 . Estradiol
Ž .
has been reported to decrease LH pulse amplitude in sheep Rawlings et al., 1984 and Ž
. cattle Price and Webb, 1988 and, progesterone or progestogen implants in cattle have
been reported to decrease LH pulse frequency and suppress maximal diameter of the Ž
dominant follicle in a dose-dependent manner Ireland and Roche, 1982; Adams et al., .
1992a; Savio et al., 1993a,b; Stock and Fortune, 1993; Sanchez et al., 1995 . Estrogen- induced suppression of LH may be apparent only during a progestational phase, and
estradiol and progesterone may have a synergistic effect on altering circulating concen- Ž
. trations of FSH. Estradiol alone
Butler et al., 1983; Wolfe et al., 1992 or in
Ž .
combination with progestogen Barnes et al., 1981 has been shown to suppress FSH; Ž
however, suppression was more prolonged in the progestogen-implanted heifers Bolt et .
al., 1990 . It has been suggested that estradiol and progesterone given in combination Ž
. have an additive suppressive effect on both LH and FSH Price and Webb, 1988 .
Treatment of rhesus monkeys with estradiol into the ovary bearing the largest Ž
. dominant
follicle caused follicular atresia without apparently altering circulating Ž
. Ž
. concentrations of LH or FSH Hutz et al., 1988 . Dierschke et al. 1994 also reported
that estrogen induced follicular atresia in monkeys and rats by an action directly within the ovary rather than through gonadotropin suppression. To our knowledge, there are no
reports of a local effect of estrogen on follicular dynamics in cattle.
We hypothesized that the suppressive effect of exogenous estradiol, in combination with progestogen, on ovarian follicles in cattle is mediated systemically rather than
locally. Experiment 1 was designed to determine a minimal effective systemic dose of estradiol for inducing follicular regression in progestogen-implanted heifers. Experiment
2 was designed to utilize a dose just below the minimal effective systemic dose of
Ž . Ž
. estradiol-17b E-17b
subminimal dose; 0.1 mg locally, to determine whether estradiol treatment induces suppression of follicles through a direct action at the level of the
ovary.
2. Materials and methods
