M. L. Day

Influence of prepubertal ovariectomy and estradiol replacement therapy on secretion of luteinizing hormone before and after pubertal age in heifers

Abstract Secretion of luteinizing hormone (LH) and effects of estradiol were evaluated during and after the prepubertal decline in negative feedback of estradiol on secretion of LH. Prepubertal heifers (269 ± 4 days of age; n=10) were ovariectomized on February 6, 1981 (Day 0). Five ovariectomized heifers were administered a subcutaneous implant on Day 0 which provided physiological serum concentrations of estradiol (OVX-E 2 ). The remaining 5 heifers were not implanted (OVX). A second estradiol implant was administered to OVX-E 2 heifers on Day 164 (n=3) or Day 206 (n=2) of the study. Blood samples were collected sequentially (every 12 min for 8 hr) at approximately two week intervals from Days 0 to 232 of the experiment. The experimental period spanned from approximately 100 days before (269 days of age) to 100 days after (501 days of age) the expected age at puberty. Mean serum concentration of LH and frequency of LH pulses increased rapidly from Days 0 to 36 in OVX heifers and were followed by a further gradual rise in pulse frequency (Day 50 to 232) and a reciprocal decline in mean LH and pulse amplitude. The rapid post-ovariectomy increase in secretion of LH was blocked by estradiol in OVX-E 2 heifers. All characteristics (mean, frequency and amplitude) of secretion of LH increased gradually during the experimental period in OVX-E 2 heifers (Days 0 to 232). Mean concentration and amplitude of pulses were higher in OVX-E 2 than in OVX heifers by Days 148 and 134, respectively. These differences were maintained for the remainder of the experimental period. No acute effects of the second estradiol implant on secretion of LH were observed in OVX-E 2 heifers. Results of this study indicate that long-term changes in secretion of LH occur following prepubertal ovariectomy in heifers and suggest that the previously documented prepubertal decline in negative feedback of estradiol on secretion of LH is followed by a period of positive feedback after pubertal age is surpassed.

Effects of suckling and low doses of estradiol on luteinizing hormone secretion during the postpartum period in beef cows

Abstract An experiment was conducted to test if suckling acutely suppressed circulating levels of LH during the postpartum period in beef cows. In addition, the influence of exogenous administration of low concentrations of estradiol on LH secretion during the postpartum period was evaluated. Twelve mature cows were randomly assigned before parturition to one of three treatments. Four intact cows were used as controls (INT). Eight cows were ovariectomized within the first 7 days following parturition. Four of these cows received a silastic 17β-estradiol implant subcutaneously at the time of ovariectomy (OVX-E); the remaining four cows received no further treatment (OVX). All cows were allowed to nurse one calf for 30 min daily between 1200 and 1230 hours for the duration of the experiment. Blood samples were collected at 12 min intervals for 6 hr before and 6 hr after suckling on days 9, 30, 44 and 58 postpartum. Mean interval (mean ± SE) to the first increase in peripheral progesterone concentrations indicative of the onset of ovarian luteal activity was detected in INT cows 37 ± 4.9 days postpartum. An acute effect of suckling on LH secretion did not occur in INT and OVX cows but mean LH concentrations were reduced in OVX-E cows following suckling on days 44 and 58. Mean LH concentrations remained low in INT cows; whereas, in OVX and OVX-E cows LH concentrations increased linearly (P

Circulating concentrations of 17-estradiol influence pattern of LH in circulation of cows☆

The objective of the research was to determine the relationship between circulating 17 beta-estradiol (E2) and secretion of luteinizing hormone (LH) in cows. A second objective was to determine if response to E2 was influenced by interval between ovariectomy and the start of E2 treatment. Thirty-one nulliparous cows 3 yr of age were randomly assigned to a 2 x 4 factorial arrangement of treatments. Sixteen cows were ovariectomized at 18 mo of age (long term), and the other 15 cows were ovariectomized at 36 mo of age (short term). At the time of ovariectomy of cows in the short term group, 11 cows in the short term group and 12 cows in the long term group were implanted subcutaneously with 1, 2 or 4 polydimethylsiloxane capsules containing E2. The other eight cows served as non-implanted controls (n = 4-short term, n = 4-long term). All cows were fitted with jugular vein catheters on day 29 of treatment, and on day 30 blood samples were collected at 12-min intervals for 6 hr. At the end of 6 hr, luteinizing hormone-releasing hormone (LHRH) was administered and blood sampling continued at 12-min intervals for an additional hour. Serum was analyzed for LH and E2. Variables of LH secretion analyzed were mean concentration, frequency of pulses, amplitude of pulses and maximum concentration after LHRH. There were no significant interactions for any of the variables of LH among cows ovariectomized for the long and short term. There was a significant linear increase in mean concentration of LH with increased circulating concentration of E2.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of 17β-estradiol on the preovulatory surge of LH in the bovine female☆

The hypothesis tested was that increasing concentration of 17beta-estradiol (E(2)) subsequent to luteolysis stimulates the preovulatory surge of LH and that a decline in E(2) after the initial rise is not necessary to trigger the preovulatory surge of LH in the bovine female. Beef cows were synchronized to Day 16 of the estrous cycle. At Hour 0, all cows were ovariectomized and received one of four E(2) treatments: 1) luteal phase E(2) (LE; n=5), 2) increasing then decreasing E(2) (DE; n=5), 3) increasing and subsequent maintenance of high E(2) (IE; n=4), and 4) no E(2) (NE; n=3). Cows in the LE group received one E(2) implant at Hour 0 which provided low concentrations of E(2). Cows in the DE group received one E(2) implant at 0, 8, 16, 24, 32 and 40 hours; implants were subsequently removed at 8-hour intervals, thus mimicking the preovulatory rise and fall of E(2). Cows in the IE group were treated with the same regimen of E(2) implants as cows of the DE group, except that no E(2) implants were removed. Blood samples were collected at Hour 0 and at hourly intervals from Hour 2 through 80, for serum LH and E(2) quantification. The number of cows responding with a surge of LH was 0/3, 0/5, 4/5 and 3/4 for the NE, LE, DE and IE treatments, respectively. The proportion of cows responding with an LH surge was different (P<0.01) when data for cows in the NE and LE groups were pooled and compared with the pooled data of cows in the DE and IE groups. The mean time of the LH surge was not different (P>0.80) for cows responding with an LH surge (DE and IE treatments). Thus, increased levels of E(2) greater than luteal phase concentrations are needed to initiate preovulatory surges of LH, and it appears that concentrations of E(2) need to reach a certain level but do not need to decrease after this initial rise to stimulate a surge release of LH.

Luteinizing hormone and progesterone concentrations in serum of heifers administered a short half-life prostaglandin (PGF2α) or long half-life prostaglandin analogue (fenprostalene) on days six or eleven of the estrous cycle☆

Two trials were conducted to measure the progesterone (P(4)) decline and luteinizing hormone (LH) surge in serum subsequent to administration of a short half-life (short t (1 2 )) prostaglandin (PGF(2alpha)) or a long half-life (long t (1 2 )) prostaglandin analogue (fenprostalene) on Days 6 or 11 of the estrous cycle. Twenty-five crossbred Shorthorn and five Hereford heifers with a mean weight of 331.4 +/- 29.8 kg were used in both trials. The heifers were randomly allotted to receive either a short t (1 2 ) or long t (1 2 ) prostaglandin treatment on Day 6 or 11 of the estrous cycle. A crossover design for the main effect, treatment (type of prostaglandin), was conducted. Heifers that received PGF(2alpha) in Trial I were given fenprostalene in Trial II and vice versa. Stage of the estrous cycle (day) was the same for each heifer in both trials. Stage of estrous cycle was standardized to either Day 6 or 11 by administering Syncro-Mate B (SMB). Blood was collected every hour for 80 h post injection to quantify LH and P(4) concentrations. There were no significant differences (P > 0.05) between the short t (1 2 ) or long t (1 2 ) for either P(4) or LH profiles. In addition, no differences were detected between stages of the estrous cycle for the timing of the preovulatory surge of LH after prostaglandin administration.

Effects of copulation on timing of the LH surge following synchronization of estrus in the bovine.

Forty-four crossbred postpubertal bovine females were used to study how mating with a bull affected estradiol-17beta (E(2)) secretion and timing of the preovulatory LH surge. Estrous cycles were synchronized with two injections of prostaglandin-F(2alpha) (PGF(2alpha)) 11 d apart. Females were either isolated from males (NE) or exposed to epididectomized bulls (BE) after the second PGF(2alpha) injection. Females exposed to bulls were allowed to mate once and then were separated from the bull. Blood samples were collected at 2-h intervals from the second PGF(2alpha) injection until 12-h post injection to monitor progesterone (P(4)) and luteinizing hormone (LH) concentrations and at hourly intervals from 12 h to 60 h post-injection to monitor LH secretion and timing of the preovulatory LH surge. Samples were also collected at 4-h intervals until 60 h post-injection to monitor estrogen (E(2)) secretion. LH surges were detected in 16 and 14 of 22 females from the BE and NE groups, respectively, during the 60-h period after PGF(2alpha) injection Mean P(4) concentrations and time of P(4) decline to <1 ng/ml were not different between the two treatment groups (P>0.30). Mean E(2) concentration during the 60-h sampling period was different (P<0.003) between BE and NE groups, and a significant treatment effect (P<0.002) occurred 48 h, 52 h and 60 h after the second PGF(2alpha) injection. However, mean LH concentration before the LH surge, duration of the LH surge and peak LH concentration during the surge were not different between the BE and NE groups (P>0.40). Mean time for the second PGF(2alpha) injection to the beginning of the LH surge was 51.6 +/- 1.5 h (X +/- S E) for the females not exposed to bulls and 48.5 +/- 1.4 h for females exposed to bulls (P>0.14). In this study, the presence of and/or mating by a bull did not affect LH secretion or timing of the preovulatory LH surge after PGF(2alpha) administration.