the maximum percentage decline in FSH and the interval to FSH nadir were related to the peak Ž
Ž .
plasma concentrations of E maximum decline in FSH s 11.17 q 1.564 peak E
y 0.009
2 2
Ž .
2 2
. Ž Ž
. peak E
, R s 0.75; P - 0.01 , hours to FSH nadir s 10.628 q 1.486 hours to peak E y
2 2
Ž .
2 2
. 0.0282 hours to peak E
, R s 0.22; P - 0.05 . Concentrations of FSH increased as E
2 2
declined from its peak value, irrespective of maximum value achieved. It was concluded that the intramuscular administration of ODB in oil to ovariectomised heifers given a PRID results in
higher plasma concentrations of E and causes a greater reduction in FSH than administration
2
topically by intravaginal gelatine capsule. E transiently suppresses FSH in ovariectomised
2
heifers, and the magnitude of the suppression is dose-dependent; however FSH concentrations begin to increase 1–2 days after ODB administration while concentrations of E were declining
2
but still high. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Cattle; Endocrinology; Ovariectomy; Oestradiol; FSH
1. Introduction
Follicle growth in cattle is a highly regulated process under the control of FSH, LH and local intra-follicular growth factors. There are recurrent waves of follicular growth
Ž throughout the oestrous cycle Savio et al., 1988; Sirois and Fortune, 1988; Ginther et
. Ž
. al., 1989 , most of pregnancy Ginther et al., 1996a , and they resume early in the
Ž .
postpartum period Murphy et al., 1990; Savio et al., 1990; Stagg et al., 1995 . Each wave begins with the emergence of a cohort of oestrogen active follicles of 3–5 mm in
Ž diameter, preceded by an increase in concentrations of FSH Adams et al., 1992; Bo et
. al., 1993; Sunderland et al., 1994; Stagg et al., 1998 . This is followed by the selection
Ž .
of generally one dominant follicle DF , which occurs in association with declining FSH Ž
. concentrations Sunderland et al., 1994; Mihm et al., 1997 . Finally, the fate of the DF is
Ž .
determined by LH pulse frequency, where infrequent pulses 1 every 3–4 h lead to DF Ž
. atresia while more frequent pulses 1 per hour leads to ovulation. Thus, each wave has
an inherent life span, determined by the interval from wave emergence to dominance and the duration of dominance. The duration of dominance is dependent on the number
Ž .
of waves 2 or 3 during the oestrous cycle, and the influence of nutrition, lactation and Ž
. suckling during the postpartum period Roche et al., 1999 on wave dynamics.
The factors that regulate the recurrent FSH increases are mainly of ovarian origin, Ž
. Ž
. particularly from the DF Ginther et al., 1996b with oestradiol E
one of the key
2
intra-follicular components involved. The relationship between systemic E and FSH
2
concentrations however, is unclear and the reported effects of E on FSH are inconsis-
2
Ž .
tent. Bolt et al., 1990 reported that 10 mg of oestradiol 17b in 5 ml of sterile corn oil administered intramuscularly, either alone or in combination with norgestomet, sup-
pressed plasma FSH in ovariectomised heifers for at least 64 h. Kesner and Convey Ž
. 1982 also reported a suppressive effect of 1 mg of E 17b in 3 ml safflower seed oil
Ž .
Ž .
injected intramuscularly on FSH as did Price and Webb 1988 and Barnes et al. 1990 , where E
was administered in crystalline form in a subcutaneous implant to intact
2
Ž .
Ž heifers. Wolfe et al., 1992 found a decrease in FSH when high doses subcutaneous
. implants of E17b were administered to ovariectomised heifers but no effect on FSH
Ž .
following lower doses. However, Schoenemann et al., 1985 found little or no effect of Ž
. E
1 mg E17b in 2 ml safflower oil on FSH in ovariectomised animals.
2
The ability to exogenously regulate FSH concentrations in cattle is important to synchronise follicle wave development in both cyclic and anoestrous cattle. Although
progress has been made in recent years on improving the efficiency of oestrous synchronisation regimens, variability in oestrous response and pregnancy rates after
treatment are still major limitations preventing its greater use in cattle. It is necessary to control follicle wave dynamics as well as luteal function in order to have a ‘healthy’ DF
Ž .
present at the end of treatment Mihm et al., 1994 . When used together, E and
2
Ž .
progesterone can synchronise follicle waves, with variable success Bo et al., 1995 . However, the optimum dose of E necessary to suppress FSH is not clear, neither is the
2
effect of route of administration of E on blood concentrations of E known. Hence, the
2 2
aim of this experiment was to determine the effect of dose and route of E administra-
2
tion at time of progesterone administration on circulating concentrations of FSH, in order to select optimum dose to use in subsequent studies to manipulate follicle waves.
In this experiment, ovariectomised animals given progesterone were used to prevent confounding effects of endogenous E production.
2
2. Materials and methods
