OVXE Group Research Articles

Genistein and Silicon Synergistically Protects Against Ovariectomy-Induced Bone Loss Through Upregulating OPG/RANKL Ratio.

We have reported that genistein (Gen) and silicon (Si) have synergistic effects on ovariectomy-induced bone loss in rat; however, the potential mechanisms behind this effect were not fully clarified yet. This study was performed to evaluate the bone protective mechanisms of concomitant intake of genistein and silicon in ovariectomized rat by OPG/RANKL axis. Three-month-old Sprague-Dawley female rats were subjected to ovariectomy (OVX) or sham surgery; after surgery, the OVX rats were randomly divided into five groups: OVX-Gen, OVX-Si, OVX-Gen-Si, OVX-E, and OVX. Genistein, silicon, and 17β-estradiol supplementation were started after ovariectomy and continued for 10weeks. The results showed that genistein and silicon treatment increased the bone mineral density (BMD) of ovariectomized rats. In addition, the BMD of the tibia and femur were highest in the OVX-Gen-Si group compared with OVX-Gen and OVX-Si group (p < 0.05). After 10weeks treatment with genistein and silicon, the bone structure of ovariectomized rats was recovered, there was no difference of bone histomorphometric parameters between OVX-Gen-Si, OVX-E, and SHAM group (p > 0.05), and there was no difference in the concentration of serum ALP, Ca, P, OPG, and RANKL between OVX-Gen-Si, SHAM, and OVX-E groups (p > 0.05). RT-PCR showed that genistein and silicon treatment could effectively increase the OPG mRNA expression and decreased the RANKL mRNA expression compared to that of the OVX group (p < 0.05), the OPG/RANKL mRNA ratios were significantly decreased in the OVX group (p < 0.05), and it was nearly to normal in the OVX-Gen-Si group. Immunohistochemical staining results showed that genistein and silicon supplementation could effectively increase the protein expression of OPG and decrease the protein expression of RANKL in bone tissues; there were no significant differences in OPG and RANKL positive expression areas between OVX-Gen-Si, SHAM, and OVX-E group (p > 0.05). The results above indicate that genistein and silicon supplementation can effectively reduce RANKL, increase OPG levels, and OPG/RANKL ratios in the serum and bone tissue of ovariectomized rats; this is the main mechanism by which genistein and silicon play a bone protective role in ovariectomized rats.

Effects of estrogen status in osteocyte autophagy and its relation to osteocyte viability in alveolar process of ovariectomized rats

Estrogen maintains osteocyte viability, whereas its deficiency induces osteocyte apoptosis. As autophagy is important for osteocyte viability, we hypothesized whether the anti-apoptotic effect of estrogen is related to autophagy in osteocytes. Thirty adult female rats were sham-operated (SHAM) or ovariectomized (OVX). After three weeks, twelve rats of SHAM and OVX groups were killed before treatment (basal period), whereas the remaining rats received estrogen (OVXE) or vehicle (OVX) for 45 days. Fragments of maxilla containing alveolar process of the first molars were embedded in paraffin or Araldite. Paraffin-sections were stained with hematoxylin/eosin for histomorphometry, or subjected to the silver impregnation method for morphological analysis of osteocyte cytoplasmic processes. Autophagy was analyzed by immunohistochemical detections of beclin-1, MAP-LC3α and p62, whereas apoptosis was evaluated by immunohistochemical detections of cleaved caspase-3 and BAX, TUNEL (Terminal deoxynucleotidyl transferase dUTP nick end labeling) method and by ultrastructural analysis. Araldite-semithin sections were subjected to the Sudan-black method for detection of lipids. OVX-basal group showed high frequency of caspase-3-, TUNEL- and p62-positive osteocytes accompanied with low frequency of beclin-1- and MAP-LC3α-positive osteocytes. At 45 days, OVXE group exhibited higher number of osteocytes, higher frequency of beclin-1- and MAP-LC3α-positive osteocytes, and lower frequency of caspase-3, BAX-, TUNEL- and p62-positive osteocytes than OVX group. Significant reduction in bone area was observed in the OVX compared to OVXE and SHAM groups. The highest frequency of Sudan-Black-positive osteocytes and osteocytes with scarce cytoplasmic processes, or showing apoptotic features were mainly observed in OVX groups. Our results indicate that estrogen deficiency decreases autophagy and increases apoptosis, whereas estrogen replacement enhances osteocyte viability by inhibiting apoptosis and maintaining autophagy in alveolar process osteocytes. These results suggest that the anti-apoptotic effect of estrogen may be, at least in part, related to autophagy regulation in osteocytes.

Synergistic Effects of Genistein and Zinc on Bone Metabolism and the Femoral Metaphyseal Histomorphology in the Ovariectomized Rats.

In this study, we evaluated the synergistic effects of genistein and zinc on bone metabolism, bone mineral density, and the femoral metaphyseal histomorphology in the ovariectomized rats. Sixty female Sprague-Dawley rats, aged 13weeks, were divided into sham-operated group (SHAM), ovariectomized group (OVX), genistein-treated group (OVX-Gen), zinc-treated group (OVX-Zn), genistein combined zinc-treated group (OVX-Gen-Zn), and 17β-estradiol-treated group (OVX-E) (n=10). Genistein, zinc, and 17β-estradiol injections were started immediately after OVX and continued for 10weeks. The OVX-Gen group was subcutaneous injections of genistein (5mg/kg once every day). The OVX-Zn group was given intragastric ZnSO4 (0.25mg/kg once every day) after bilateral ovariectomy. The OVX-Gen-Zn group was subcutaneous injections of genistein (5mg/kg), at the same time intragastric ZnSO4 (0.25mg/kg once every day). OVX-E group were intraperitoneally injected with 17β-estradiol (10μg/kg) once every other day. The results showed that genistein and zinc did not alter body weight in OVX rats. The soluble ZnSO4 had no effect to uterus weight loss. Genistein and zinc supplementation significantly increased the BMD of the femur and lumbar spine in OVX rats (P<0.05, vs. OVX control group). The BMD of the lumbar spine and femur in the OVX-Gen-Zn group is higher than that in the OVX-Gen and OVX-Zn groups (P<0.05), and there were no differences of BMD among the OVX-Gen-Zn, OVX-E, and SHAM groups (P>0.05). The femoral metaphyseal morphology and bone histomorphometric parameters revealed that the trabecular volume and thickness in the OVX-Gen-Zn and OVX-E groups were similar to that of SHAM group, and the OVX-Gen-Zn group had more trabecular volume than the OVX-Gen and OVX-Zn groups. Serum zinc was found to be significantly reduced in the OVX group when compared with SHAM. Serum zinc levels were enhanced in the OVX-Zn, OVX-Gen-Zn, and OVX-E groups and restored to normal levels. Serum calcium and serum phosphorus in the OVX-Gen-Zn group was greater than that in the OVX-Gen and OVX-Zn groups (P<0.05); the serum calcium, phosphorus, and ALP in the OVX-Gen-Zn group were similar to that of SHAM and OVX-E groups (P>0.05). The OVX group had higher osteocalcin, CTX, and RANKL than the SHAM group (P<0.01). They were lower in the OVX-Gen and OVX-Zn groups (P<0.05), and they were significantly lower in the OVX-Gen-Zn and OVX-E groups, than in the OVX group. There were no significant differences of osteocalcin, CTX, and RANKL among the OVX-Gen-Zn, OVX-E, and SHAM groups (P>0.05). OPG and OPG/RANKL ratios were significantly higher in the OVX-Gen and OVX-Zn groups than that in the OVX group (P<0.05). The OPG and OPG/RANKL ratio in the OVX-Gen-Zn group is higher than that in the OVX-Gen and OVX-Zn groups (P<0.05, vs. OVX-Gen group; P<0.01, vs. OVX-Zn group). The differences of the OPG and OPG/RANKL ratios between the OVX-Gen-Zn, OVX-E, and the SHAM groups were not significant (P>0.05). The results above indicate that the genistein and zinc have synergistic effects on both stimulation of bone formation and suppression of bone resorption in ovariectomized rats, thereby increasing bone mass.

Effect of 17ß-estradiol on zinc content of hippocampal mossy fibers in ovariectomized adult rats

Sex hormones such as estrogen (17ß-estradiol) may modulate the zinc content of the hippocampus during the female estrous cycle. The mossy fiber system is highly plastic in the adult brain and is influenced by multiple factors including learning, memory, and stress. However, whether 17ß-estradiol is able to modulate the morphological plasticity of the mossy fibers throughout the estrous cycle remains unknown. Ovariectomized (Ovx) female 70- to 90-day-old Sprague-Dawley rats without or with estrogen supplement (OvxE) were compared with control rats in three stages of the estrous cycle: diestrus, proestrus, and estrus. The brain tissue from each of the five groups was processed with Timm's silver sulfide technique using the Image J program to measure the mossy fiber area in the stratum lucidum of CA3. Total zinc in the hippocampus was measured using Graphite Furnace Atomic Absorption Spectrophotometry. Two additional (Ovx and OvxE) groups were examined in spatial learning and memory tasks using the Morris water maze. Similar increases in total zinc content and mossy fiber area were observed. The mossy fiber area decreased by 26 ± 2 % (difference ± SEM percentages) in Ovx and 23 ± 4 % in estrus as compared to the proestrus group and by 18 ± 2 % in Ovx compared to OvxE. Additionally, only the OvxE group learned and remembered the task. These results suggest that estradiol has a significant effect on zinc content in hippocampal CA3 during the proestrus stage of the estrous cycle and is associated with correct performance in learning and memory.

Characteristics of contractile activity in the renal artery of ovariectomized rats

The incidence of cardiovascular disease is markedly lower in cycling, pre-menopausal women and post-menopausal women receiving estrogen than in men or untreated post-menopausal women. Clinical studies demonstrate a protective role of estrogen in hormone replacement therapy in terms of reducing cardiovascular risk. However, the benefits of hormone replacement therapy in cardiovascular disease remain unclear. We investigated the effects of estrogen on the contractile responses of the renal artery of ovariectomized Wistar rats (OVX) compared to both ovariectomized 17 beta-estradiol-treated rats (OVXE) and sham-operated (control) rats. Isometric contraction of renal artery was recorded with a strain gauge transducer. The maximum contractile response of the renal artery smooth muscle to KCl (80 mM) in the OVXE group was significantly higher than that in both the control and OVX groups. The phenylephrine (PE) concentration-response curves in all three groups indicated a greater sensitivity at lower concentrations of PE following treatment with 100 microM L-arginine methyl ester (L-NAME). The EC50 values for PE in the three groups were 2 times lower in the presence of L-NAME than those lacking exposure to L-NAME. The EC50 value for PE in the OVX group was approximately 3 times lower in the presence of L-NAME than in those lacking exposure to L-NAME and 100 nM BMY 7378, an alpha 1D-adrenoceptor antagonist. The rate of relaxation of the PE-induced contraction (T1/2) was significantly reduced in the OVX group relative to both the control and OVXE groups. T1/2 values after treatment with 100 microM L-NAME were slower than those lacking exposure to L-NAME in all groups. Further, the T1/2 value of the OVX group was 2 times greater than that of the control; this change was reversed in the OVXE group. In conclusion, our results suggest that estrogen regulates contraction and relaxation in the renal artery via NO synthase activity and alteration of the Ca2+ transport systems.

Effects of teriparatide and alendronate on bone mineral density of osteoporotic rats

To investigate the effects of teriparatide (hPTH1-34, PTH) and alendronate (Alen) on bone turnover rate and bone mineral density (BMD) of ovariectomized (OVX) osteoporotic rats. 70 female 6-month-old Wistar rats were randomly divided into 7 groups: (1) baseline group: killed immediately as baseline controls; (2) sham operation group: injected subcutaneously with normal saline (NS) as normal controls; (3) OVXb group: underwent ovarietomy (OVX) and killed 6 weeks after OVX as pre-therapeutic controls; (4) OVXe group: injected with NS subcutaneously and then sacrificed 14 weeks after OVX as controls by the end of treatment; (5) PTH group: PTH 40 microg.kg(-1).d(-1) was administered; (6) Alen group: Alen 100 microg.kg(-1).d(-1) was administered; (7) A + P group: PTH 40 microg.kg(-1).d(-1) and Alen 100microg.kg(-1).d(-1) were administered. In groups 4 approximately 7, different medicines were injected subcutaneously QD 5 times per week from the 6th week to the 14th week after OVX and then the rats were killed and their right femurs, lumbar vertebrae, and samples of blood and urine were collected. Absorptometry was used to measure the BMD of the right femur and lumbar vertebrae. The serum calcium, phosphate, creatinine, alanine transaminase, and alkaline phosphatase (ALP) activity were measured by automatic biochemical analysis. The bone resorption marker urine deoxypyridinoline/creatinine (UDpd/Cr) level was measured by enzyme-linked immuosorbent assay. Six weeks after OVX the ALP and UDpd/Cr levels in the OVXb group were 101 U/L +/- 59 U/L and (118 +/- 32) x 10(-6) respectively, both significantly higher than those of the baseline group (58 U/L +/- 10 U/L and (48 +/- 34) x 10(-6) respectively, both P < 0.01) and the BMD results of the OVXb group were all significantly lower than those in the baseline group (all P < 0.01), which indicated that an OVX osteoporotic rat model was established successfully. The ALP and UDpd/Cr levels of the Alen group were 61 U/L +/- 28 U/L and (17 +/- 39) x 10(-6), significantly lower than those of the PTH group 120 U/L +/- 36 U/L and (111 +/- 26) x 10(-6) respectively, both P < 0.01) and the UDpd/Cr levels of the A + P group were between those of the Alen group and those of the PTH group. The BMD levels of the femur and lumbar vertebrae of the PTH, Alen, and A + P groups were all significantly higher than those of the control groups (all P < 0.01), and were similar to or higher than those of the sham operation group without significant differences between the PTH and Alen groups. The BMD level of the lumber vertebrae of the A + P group were significantly higher than those in the PTH group (all P < 0.05), and the femoral BMD results of the A + P group were significantly higher than those in the PTH and Alen groups (all P < 0.01). PTH and Alen are all effective on osteoporosis. In particular, the combination of PTH and Alen is more effective on increasing the BMD level.

Effect of 17ß-estradiol or alendronate on the bone densitometry, bone histomorphometry and bone metabolism of ovariectomized rats

The objective of the present study was to evaluate the effect of 17beta-estradiol or alendronate in preventing bone loss in 3-month-old ovariectomized Wistar rats. One group underwent sham ovariectomy (control, N = 10), and the remaining three underwent double ovariectomy. One ovariectomized group did not receive any treatment (OVX, N = 12). A second received subcutaneous 17beta-estradiol at a dose of 30 microg/kg for 6 weeks (OVX-E, N = 11) and a third, subcutaneous alendronate at a dose of 0.1 mg/kg for 6 weeks (OVX-A, N = 8). Histomorphometry, densitometry, osteocalcin and deoxypyridinoline measurements were applied to all groups. After 6 weeks there was a significant decrease in bone mineral density (BMD) at the trabecular site (distal femur) in OVX rats. Both alendronate and 17beta-estradiol increased the BMD of ovariectomized rats, with the BMD of the OVX-A group being higher than that of the OVX-E group. Histomorphometry of the distal femur showed a decrease in trabecular volume in the untreated group (OVX), and an increase in the two treated groups, principally in the alendronate group. In OVX-A there was a greater increase in trabecular number. An increase in trabecular thickness, however, was seen only in the OVX-E group. There was also a decrease in bone turnover in both OVX-E and OVX-A. The osteocalcin and deoxypyridinoline levels were decreased in both treated groups, mainly in OVX-A. Although both drugs were effective in inhibiting bone loss, alendronate proved to be more effective than estradiol at the doses used in increasing bone mass.

Dose-response effects of estradiol implants on bone mineral density in ovariectomized ewes.

In a longitudinal in vivo study, we studied the effect of two different doses of 17 beta-estradiol (E2) administered in the form of a subcutaneous implant, on bone mineral density (BMD) of the lumbar vertebrae (L4, L5, L4-L6/L5-L7), the calcaneus (CAL) and the distal radius (DR) in ovariectomized (OVX) ewes. The BMD of various regions of the femur, tibia and humerus were studied at autopsy. Skeletally mature ewes (n = 45) were divided into four groups: sham operated (n = 12), OVX (n = 15), OVX plus one E2 implant (OVXE, n = 12) and OVX plus two E2 implants (OVX2E, n = 6). BMD of L4, L5, L4-L6/L5-L7, CAL and DR was determined at 0, 6 and 12 months using dual-energy X-ray absorptiometry. In-vivo precision of BMD for the last three lumbar vertebrae ranged from 1.4-4.3%, and 1.5% and 3.5% for CAL and DR respectively. In the in vivo study, there were no significant changes in the mean BMD in the sham group at any time point (each group served as its own control). In the OVX group, mean BMD was significantly lower at L5 and DR at 6 months and significantly lower at L4 at 12 months. In the OVXE group, the mean BMD was significantly higher at L5, CAL and DR at 12 months. In the OVX2E group, BMD was significantly higher at CAL but significantly lower at L4 at 12 months. None of the treatments produced significant changes of mean BMD of L4-L6/L5-L7 at any time point.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of 17 beta-estradiol on distribution of pituitary isoforms of luteinizing hormone and follicle-stimulating hormone during the follicular phase of the bovine estrous cycle.

The objective of this study was to examine the influence of 17 beta-estradiol (E2) on distribution of LH and FSH isoforms during the follicular phase of the bovine estrous cycle prior to the preovulatory surges of LH and FSH. On Day 16 of the estrous cycle (Day 0 = estrus), intact controls (CONT; n = 4) were treated with prostaglandin F2 alpha (PGF2 alpha) to induce luteal regression and initiation of the follicular phase. Other cows were also treated with PGF2 alpha and either ovariectomized (OVX; n = 5) or ovariectomized and given E2 implants (OVXE; n = 6) to mimic the pattern of increasing E2 concentrations during the follicular phase of the estrous cycle. Pituitaries were collected 40 h after treatment with PGF 2 alpha or ovariectomy (0 h). Aliquots of pituitary extracts were chromatofocused on pH 10.5-4.0 gradients. The LH resolved into thirteen isoforms (designated A-L and S, beginning with the most basic form) while FSH resolved into nine isoforms (designated I-IX, beginning with the most basic form). The percentage of LH as isoform F (elution pH = 9.32 +/- 0.01) was greater (p < 0.05) in the OVX group (48.5%) than in the OVXE group (45.0%). LH isoforms I (elution pH = 6.98 +/- 0.01) and J (elution pH = 6.48 +/- 0.01) were more abundant (p < 0.05) in cows from the OVXE (2.3 and 5.8%, respectively) than the OVX group (1.4 and 3.7%, respectively). Distribution of LH isoforms in cows from the three groups did not differ (p > 0.10).(ABSTRACT TRUNCATED AT 250 WORDS)

Increasing Concentrations of 17β-Estradiol Has Differential Effects on Secretion of Luteinizing Hormone and Follicle-Stimulating Hormone and Amounts of Mrna for Gonadotropin Subunits During the Follicular Phase of the Bovine Estrous Cycle1

The hypothesis tested was that 17 beta-estradiol (E2) would increase amounts of mRNA for alpha, LH beta, and FSH beta subunits during the follicular phase of the estrous cycle prior to the preovulatory surge of gonadotropins in cows. On Day 16 (Day 0 = estrus) of the estrous cycle, all cows were treated with prostaglandin F2 alpha (PGF2 alpha). Cows served as intact controls (CONT, n = 4) were ovariectomized (OVX, n = 5), or were ovariectomized and administered E2 (OVXE, n = 6) in increasing doses starting at the time of treatment with PGF2 alpha. Cows were bled for 6 h before and for 40 h after PGF2 alpha treatment to characterize pulsatile secretion of LH and FSH. Forty hours after PGF2 alpha treatment, pituitaries were collected for evaluation of amounts of mRNA for alpha, LH beta, and FSH beta subunits. Amplitude of LH pulses was greater (p < 0.05) in cows from the OVXE than from the CONT group. Concentrations of FSH were greater in cows from both the OVXE and OVX (p < 0.01) groups than from the CONT group. Amounts of mRNA for alpha and FSH beta subunits were greater (p < 0.01) in pituitaries of cows from the OVX than from the CONT or OVXE groups. Amounts of mRNA for LH beta subunit in pituitaries of cows from the OVX group tended to be greater (p < 0.08) than from the CONT group. Cows in the OVXE group tended (p < 0.08) to have greater amounts of mRNA for FSH beta subunit than did CONT cows.(ABSTRACT TRUNCATED AT 250 WORDS)

Steady-state amounts of alpha- and luteinizing hormone (LH) beta-subunit messenger ribonucleic acids are uncoupled from pulsatility of LH secretion during sexual maturation of the heifer.

Our primary objective for this study was to determine whether steady-state amounts of alpha- and LH beta-subunit mRNAs in the anterior pituitary are altered during sexual maturation in the bovine female. A secondary objective was to determine whether 17 beta-estradiol (E2) alters amounts of LH subunit mRNAs before onset of puberty. Heifers (7 mo old) were assigned to one of three treatments: 1) ovariectomized (OVX, n = 16); 2) OVX and administered E2 (OVXE, n = 16); or 3) ovary-intact (INTACT, n = 20). Pituitaries were collected at an estimated 120 days before onset of puberty (prepuberty) or 25 days before onset of puberty (peripuberty). Six INTACT heifers were used to determine time of puberty during the experimental period, and their pituitaries were collected 40 h after administration of prostaglandin F2 alpha (postpubertal INTACT group). Relative amounts of mRNAs for LH subunits in each pituitary were determined by Northern analysis and scanning densitometry. Amounts of alpha- and LH beta-subunit mRNAs were lower in pituitaries of INTACT heifers and OVXE heifers, regardless of stage of sexual maturation, than in those of OVX heifers. Amounts of alpha-subunit mRNA were similar in OVXE and INTACT heifers regardless of stage of sexual maturation. Amounts of LH beta-subunit mRNA did not change during sexual maturation in heifers in the INTACT group. Concentrations of E2 were higher and LH beta-subunit mRNA were lower in heifers from the prepubertal OVXE group than in heifers in all other treatment groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of estrous cycle, estrogen and progesterone administration on the antidromic and orthodromic reaction threshold of medial preoptic neurons in the rat hypothalamus.

In the present study, attempts were made to evaluate the effects of the estrous cycle and the administration of estradiol (Ovx-E) or progesterone (Ovx-P) after ovariectomy on the threshold of antidromic (AD) and orthodromic (OD) responses of neurons in the medial preoptic area (POA) induced by electrical stimulation of the median eminence-arcuate nucleus (ARC) of the hypothalamus. Single units were evaluated with extracellular recording. The threshold of AD and OD response in POA neurons declined during estrus and proestrus of the estrous cycle and in the Ovx-E group, but increased in the Ovx-P group and during diestrus. The number of neurons with the inhibitory OD response increased during proestrus and in the Ovx-E group, but decreased during diestrus. The neurons with the excitatory OD response increased in the Ovx-P group and during diestrus, and decreased during proestrus. The threshold of the inhibitory and excitatory OD response decreased during proestrus, estrus and in the Ovx-E group, but increased in the Ovx-P group. It is apparent from these studies that estrogen appears to activate the neurons of the POA receiving inhibitory synaptic input via the axonal collateral branches of the ARC neurons, and that progesterone seems to activate the neurons of the POA receiving excitatory synaptic input.

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

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<0.05) as the interval from time of ovariectomy increased. Cows in the OVX-E group had a higher mean concentration of LH than cows in the OVX group at 30, 44 and 58 days postpartum (P<0.05). Frequency of LH pulses did not differ between cows in the OVX and OVX-E groups at any period. Data from this experiment support the concept that suckling is acting in a chronic fashion to inhibit LH secretion during the postpartum period. In the absence of ovaries, chronic administration of exogenous estradiol in low concentrations has a positive effect on secretion of LH in the postpartum cow.