Concomitance of luteinizing hormone and progesterone oscillations during the transition from preluteolysis to luteolysis in cattle

Loss of luteal sensitivity to luteinizing hormone underlies luteolysis in cattle: A hypothesis

By virtue of the secretion of progesterone (P4), corpus luteum (CL) is important not only for normal cyclicity but also for conception and continuation of pregnancy in female mammals. Luteolysis (also called luteal regression) is defined as loss of the capacity to synthesize and secrete P4 followed by the demise of the CL. There is strong evidence that sequential pulses of prostaglandin F2α (PGF) secreted from the uterus near the end of luteal phase induces luteolysis in farm animals. Loss of luteal sensitivity to luteinizing hormone (LH) at the end of menstrual cycle has been reported to be critical for initiation of luteolysis in primates, however this has not been investigated in farm animals. A closer observation of the published real-time profiles of circulating hormones (P4, LH, and PGF) and their inter-relationships around the time of the beginning of spontaneous luteolysis in cattle revealed- 1) A natural pulse of PGF causes a transient P4 suppression lasting a couple of hours followed by a rebound in P4 concentration, 2) The P4 secretions that occur in response to LH pulses before the beginning of luteolysis (i.e., preluteolysis) either fail or do so to a lesser extent during luteolysis indicating a loss of sensitivity to LH, and 3) The loss of sensitivity coincides with the beginning of luteolysis (i.e., transition), and apparently luteolysis does not initiate until there is loss of sensitivity to LH. The CL is sensitive to LH during preluteolysis, and the LH-stimulated P4-dependent and/or independent local survival mechanisms maintain the steroidogenic capability and viability of the CL until the very end of preluteolysis. Luteolysis does not appear to initiate with the PGF pulse(s) that occur during this period. With the loss of sensitivity to LH at the transition, however, a progressive decline in P4 begins initiating luteolysis. Also, the survival mechanisms become compromised making the CL less viable. The uterine PGF pulses that occur after the beginning of luteolysis induces increase in the local luteolytic factors, which contribute to further luteolysis, more importantly, structural luteolysis with ultimate demise of the CL. Therefore, I hypothesize that the loss of luteal sensitivity to LH underlies luteolysis in cattle. The hypothesis not only unifies the basic mechanism of luteolysis in a farm animal and primates but also provides a perspective to view luteolysis as a process rather than a factor-mediated event. A novel unified working model for luteolysis in a farm animal and primates is described. A better understanding of the luteal physiology including how responsiveness to LH diminishes in aging CL would help in the development of novel strategies in modulating CL structure-function to improve and/or control fertility in humans as well as in animals.

Regression of corpus luteum in cetaceans: A systematic review

Functional and structural change of corpus luteum through the cascade of several genes in the ovary leads to ovulation and pregnancy. In most mammals, the absence of pregnancy leads to the disintegration of the corpus luteum. In the ovary of cetaceans, the regression of the corpus luteum gets delayed and persists on the surface as scars (corpus albicans). The database on luteolysis of mammals was collected and examined to know the mechanisms involved in the corpus luteum regression of cetaceans. Surprisingly, there existed no data on the concerned topic. Some past findings reported the persistence of ovarian scars through the entire life span, while few reported the regression. Also, those investigations were about the physiology and histology of corpus luteum regression. The pathways and the genes involved in the regression of the cetacean corpus luteum remain unexplored. This review is all about the regression of corpus luteum and recommends gene-based evolutionary studies in the future to resolve the existing theories on ovarian scar persistence in cetaceans.

Profiles of prostaglandin F2α metabolite in dairy cattle during luteal regression and pregnancy: implications for corpus luteum maintenance†

Mechanisms of bovine corpus luteum (CL) maintenance during the second month of pregnancy have not been adequately investigated, despite significant reproductive losses. In the first month, interferon-tau is believed to suppress oxytocin-stimulated prostaglandin F2α (PGF) production, yet there are conflicting reports of circulating PGF metabolite (PGFM). In this study, characterization of PGFM and P4 occurred through continuous bihourly blood sampling in cows undergoing CL regression (day 18-21, n = 5), and during the first (day 18-21, n = 5) and second month (day 47-61; n = 16) of pregnancy. Cattle in the second month were assigned to control (n = 8) or oxytocin treatment (n = 8; three pulses to mimic luteolysis) to evaluate if oxytocin receptors were active. All cows but one (which had elevated PGFM prior to oxytocin treatment) maintained the pregnancy. Basal PGFM concentrations were low (11.6 ± 0.7 pg/mL) in the first month but increased 2.54-fold in the second month. Few (0.26 ± 0.12 pulses/day) PGFM pulses with low peak concentrations (28.8 ± 3.1 pg/mL) were observed during the first month of pregnancy, similar to cows not undergoing regression. However, in the second month, frequency (1.10 ± 0.26 pulses/day) and peak concentration (67.2 ± 5.0 pg/mL) of PGFM pulses increased, displaying similar frequency but lower peak PGFM than seen in regression (1.44 ± 0.14 pulses/day; 134.5 ± 18.9 pg/mL). Oxytocin treatment increased likelihood of PGFM pulses post-treatment and increased peak concentration (89.7 ± 10.1 pg/mL) in cows during the second month. Thus, cows have more PGFM pulses during second than first month of pregnancy, possibly induced by endogenous oxytocin, indicating suppression of PGF production is an important mechanism for CL maintenance during first but not second month of pregnancy.

Ultrasonography-accessed luteal size endpoint that most closely associates with circulating progesterone during the estrous cycle and early pregnancy in beef cows

The aim was to evaluate the associations between circulating P4 concentrations, corpus luteum (CL) size (diameter, area or volume) and blood perfusion (BP) in cows. In Experiment 1, Pearson's correlations (P < 0.05) with P4 concentrations were observed during CL development (D8) for total area (TA; r = 0.76), luteal area (ACL; r = 0.72), total and luteal diameter (TD and DCL respectively; r = 0.46). During mid-late diestrus, there was a positive correlation (P < 0.05) only at D15 with TA and ACL (r > 0.60), TD, total volume (TV) and luteal volume (VCL; r > 0.434). During luteal regression, the correlation was only observed at D18 for ACL (r = 0.478) and D20 with several variables. In Experiment 2, CL weight and ACL had the greatest correlation with P4 (r > 0.6). In Experiment 3, TA and ACL were the variables that were most closely correlated with serum P4 concentrations at D7 in recipient cows. Correlation coefficients were greater for luteal measurements when there were compact compared with cavitary CLs. In Experiment 4, there was no correlation (P > 0.05) between P4 and any of the variables measured on D4 and D7 in recipient cows detected in estrus. On D18 to D20, all CL characteristics were correlated (P < 0.05) with plasma P4, and luteal BP and BP area were more closely (P < 0.05) correlated than ACL. In conclusion, CL perimeter area measurements had the greatest association with luteal function during CL development; whereas for BP there was a greater correlation with P4 than luteal size during luteolysis.

Stimulation of LH, FSH, and luteal blood flow by GnRH during the luteal phase in mares

A study was performed on the effect of a single dose per mare of 0 (n = 9), 100 (n = 8), or 300 (n = 9) of GnRH on Day 10 (Day 0 = ovulation) on concentrations of LH, FSH, and progesterone (P4) and blood flow to the CL ovary. Hormone concentration and blood flow measurements were performed at hours 0 (hour of treatment), 0.25, 0.5, 1, 2, 3, 4, and 6. Blood flow was assessed by spectral Doppler ultrasonography for resistance to blood flow in an ovarian artery before entry into the CL ovary. The percentage of the CL with color Doppler signals of blood flow was estimated from videotapes of real-time color Doppler imaging by an operator who was unaware of mare identity, hour, or treatment dose. Concentrations of LH and FSH increased (P < 0.05) at hour 0.25 and decreased (P < 0.05) over hours 1 to 6; P4 concentration was not altered by treatment. Blood flow resistance decreased between hours 0 and 1, but the decrease was greater (P < 0.05) for the 100-μg dose than for the 300-μg dose. The percentage of CL with blood flow signals increased (P < 0.05) between hours 0 and 1 with no significant difference between the 100- and 300-μg doses. The results supported the hypothesis that GnRH increases LH concentration, vascular perfusion of the CL ovary, and CL blood flow during the luteal phase; however, P4 concentration was not affected.

Relationships among nitric oxide metabolites and pulses of a PGF2α metabolite during and after luteolysis in mares

Hourly circulating concentrations of a PGF2α metabolite (PGFM), progesterone (P4), and LH were obtained from a reported project, and concentrations of nitric oxide (NO) metabolites (NOMs; nitrates and nitrites) were determined in eight mares. Unlike the reported project, hormone concentrations were normalized to the peak of the first PGFM pulse of luteolysis (early luteolysis), second PGFM pulse (late luteolysis), and a pulse after luteolysis. The duration of luteolysis was 23.1 ± 1.0 hours, and the peak of the first and second PGFM pulses occurred 6.5 ± 0.9 and 14.8 ± 0.8 hours after the beginning of luteolysis. Concentration of P4 decreased progressively within and between the PGFM pulses Changes were not detected in LH concentration in association with the PGFM pulses. Concentration of NOMs was greater (P < 0.05) at the peak of the PGFM pulse during early luteolysis (88.8 ± 15.0 μg/mL) than during late luteolysis (58.8 ± 9.0 μg/mL). Concentration of NOMs began to decrease (P < 0.05) 4 hours before the peak of the PGFM pulse of early luteolysis. Concentration began to increase (P < 0.05) an hour after the peak of the PGFM pulse of late luteolysis. An NOM decrease and increase was not detected during the PGFM pulse after luteolysis. On a temporal basis, results indicated that NO either is not required for luteolysis in mares or has a role in or responds only during late luteolysis. A caveat is that the relative contribution of the CL versus other body tissues to circulating concentrations of NOMs in mares has not been determined.

Circulating nitric oxide metabolites during luteolysis and the effect of luteinizing hormone on circulating nitric oxide metabolites in heifers

Temporal relationships among circulating concentrations of nitric oxide metabolites (NOM), progesterone (P4), and luteinizing hormone (LH) within the hours of a PGFM pulse were studied during luteolysis in heifers. The peak of a PGFM pulse was designated Hour 0. All of the following increases and decreases were significant. Within a spontaneous PGFM pulse (experiment 1; n = 7), concentrations of P4 and LH decreased between Hours -1 and 0 and increased between Hours 0 and 1; NOM increased between Hours -1 and 2. In experiment 2, PGFM pulses were simulated by intrauterine infusion of PGF2α (PGF group, n = 6), and another group was also treated with acyline to inhibit LH secretion (acyline-PGF group, n = 6). Averaged over the two groups, concentration of P4 decreased between Hours -2 and 0, increased (rebounded) between Hours 0 and 1, and decreased after Hour 2. In the PGF group, concentration of LH decreased between Hours -2 and -0.5 and increased between Hour 0 and Hour 1.5 to a maximum at Hour 1.5; NOM decreased between Hours -2 and -1.5 and increased between Hours 0 and 1.5. In the acyline-PGF group, the effect of hour was not significant for concentrations of LH and NOM. The absence of an increase in NOM concentration when LH was inhibited is a novel finding. The hypotheses were supported that concentrations of LH and NOM are temporally related, and LH has a role in the increase in NOM within the hours of a PGFM pulse.

A novel monoclonal antibody-based enzyme-linked immunosorbent assay to determine luteinizing hormone in bovine plasma

The development of a novel enzyme-linked immunosorbent assay (ELISA) for determining luteinizing hormone (LH) in bovine plasma is described. Anti-bovine LH (bLH) monoclonal antibodies (mAbs) were produced and characterized. One mAb recognizing the bLH β subunit was used for immunoaffinity purification of substantial amounts of biologically active bLH from pituitary glands. The purified bLH in combination with 2 anti-bLH β subunit mAbs was used to develop a sandwich ELISA, which satisfied all the criteria required to investigate LH secretory patterns in the bovine species. The ELISA standard curve was linear over the range 0.05 to 2.5 ng/mL, and the assay proved suitable for measuring bLH in plasma without any prior treatment of samples. Cross-reactivity and recovery tests confirmed the specificity of the method. The intra- and inter-assay coefficients of variation ranged between 3.41% and 9.40%, and 9.29% and 15.84%, respectively. The analytical specificity of the method was validated in vivo by provocative tests for LH in heifers, using the LH releasing peptide gonadotropin-releasing hormone. In conclusion, the adoption of mAbs for this ELISA for coating the wells and labeling, combined with the easy one-step production of reference bLH, ensures long-term continuity in large-scale measurements of LH in the bovine species.

Dynamics of circulating progesterone concentrations before and during luteolysis: a comparison between cattle and horses

The profile of circulating progesterone concentration is more dynamic in cattle than in horses. Greater prominence of progesterone fluctuations in cattle than in horses reflect periodic interplay in cattle between pulses of a luteotropin (luteinizing hormone; LH) and pulses of a luteolysin (prostaglandin F2alpha; PGF2alpha). A dose of PGF2alpha that induces complete regression of a mature corpus luteum with a single treatment in cattle or horses is an overdose. The overdose effects on the progesterone profile in cattle are an immediate nonphysiological increase taking place over about 30 min, a decrease to below the original concentration, a dose-dependent rebound 2 h after treatment, and a progressive decrease until the end of luteolysis. An overdose of PGF2alpha in horses results in a similar nonphysiological increase in progesterone followed by complete luteolysis; a rebound does not occur. An overdose of PGF2alpha and apparent lack of awareness of the rebound phenomenon has led to faulty interpretations on the nature of spontaneous luteolysis. A transient progesterone suppression and a transient rebound occur within the hours of a natural PGF2alpha pulse in cattle but not in horses. Progesterone rebounds are from the combined effects of an LH pulse and the descending portion of a PGF2alpha pulse. A complete transitional progesterone rebound occurs at the end of preluteolysis and the beginning of luteolysis and returns progesterone to its original concentration. It is proposed that luteolysis does not begin in cattle until after the transitional rebound. During luteolysis, rebounds are incomplete and gradually wane. A partial rebound during luteolysis in cattle is associated with a concomitant increase in luteal blood flow. A similar increase in luteal blood flow does not occur in mares.

Changes in LH pulsatility profiles in dairy heifers during exposure to oestrous urine and vaginal mucus

Difficulty in observing oestrus is a problem for many dairy farmers performing AI. Finding ways to synchronize oestrous cycles or strengthen display of oestrus without hormonal treatments would be of great interest because many consumers object to the use of exogenous hormones on healthy animals. Modification of reproductive cycles through chemical communication has been reported in several species including cattle. LH is an important regulator of the follicular phase and could possibly be subject to pheromonal influence. This study focuses on the effect of volatile compounds from oestrous substances on LH pulsatility preceding the preovulatory LH surge in cattle. Four heifers of the Swedish Red breed were kept individually in isolation. Exposure to water during the control cycle (CC), and bovine oestrous urine and vaginal mucus during the treated cycle (TC), started simultaneously with induction of oestrus. Blood sampling at 15-min intervals started 37 h after administration of PGF_2α and continued for 8 h. Monitoring of reproductive hormones, visual oestrus detection and ultrasonographic examination of the ovaries continued until ovulation had occurred. The mean concentration of LH at pulse nadir was significantly higher during TC (2.04 ± 0.18 ng/ml) than during CC (1.79 ± 0.16 ng/ml), and peak amplitude was significantly higher during CC (Δ1.03 ± 0.09) than during TC (Δ0.87 ± 0.09). No other parameters differed significantly between the two cycles. We conclude that the difference in LH pulsatility pattern may be an effect of exposing heifers to oestrous vaginal mucus and/or urine and that the mechanism behind this needs further investigation.

Induction of PGFM pulses and luteolysis by sequential estradiol-17β treatments in heifers

The effects of sequential induction of PGFM pulses by estradiol-17β (E2) on prominence of PGFM pulses and progesterone (P4) concentration were studied in heifers. Three treatments of vehicle (n = 12) or E2 (n = 12) at doses of 0.05 or 0.1 mg were given at 12-h intervals beginning on Day 15 postovulation. Blood samples were collected every 12 h from Days 13-24 and hourly for 12 h after the first and third treatments. On Day 15, all heifers were in preluteolysis and on Day 16 were in preluteolysis in the vehicle-treated heifers (n = 11) and either preluteolysis (n = 4) or luteolysis (n = 8) in the E2-treated heifers. Peak concentration of induced PGFM pulses during preluteolysis on Day 15 was greater (P < 0.04) than for pulses during preluteolysis on Day 16. The interval from ovulation to the beginning of luteolysis was shorter (P < 0.04) in the E2-treated heifers than in the vehicle-treated heifers. An E2-induced PGFM pulse was less prominent (P < 0.008) in heifers in temporal association with a transient resurgence in P4 than in heifers with a progressive P4 decrease. The hypothesis that repeated E2 exposure stimulates increasing prominence of PGFM pulses was not supported. Instead, repeated exposure reduced the prominence of PGFM pulses, in contrast to the stimulation from the first E2 treatment. Reduced prominence of a PGF(2α) pulse during luteolysis can lead to a transient resurgence in P4 concentration.

Luteolysis and associated interrelationships among circulating PGF2α, progesterone, LH, and estradiol in mares

The changing concentrations and temporal relationships among a PGF2α metabolite (PGFM), progesterone (P(4)), LH, and estradiol-17β (E(2)) before, during, and after luteolysis were studied in 10 mares. Blood samples were collected every hour for ≥4 d beginning on day 12 after ovulation. The luteolytic period extended from a decrease in P(4) at a common transitional hour (Hour 0) at the end of preluteolysis and beginning of luteolysis to a defined ending when P(4) reached 1 ng/mL. The length of luteolysis was 22.9 ± 0.9 h, contrasting with 2 d in published P(4) profiles from sampling every 6 to 24 h. In mares with complete data for Hours -40 to -2 (n = 6), PGFM concentrations remained below assay sensitivity (n = 2) or two or three small pulses (peak, 29 ± 4 pg/mL) occurred. During luteolysis, the pulses became more prominent (peak, 193 ± 36 pg/mL). Rhythmicity of PGFM pulses was not detected by a pulsatility program during preluteolysis but was detected in seven of nine mares during luteolysis and postluteolysis combined. The nadir-to-nadir interval for LH pulses and the peak-to-peak interval between adjacent pulses were longer (P < 0.05) during preluteolysis than during luteolysis (nadir to nadir, 5.2 ± 0.3 h vs 3.6 ± 0.4 h; peak to peak, 9.4 ± 1.0 h vs 4.7 ± 0.5 h). Unlike reported findings in cattle, concentrations of P(4) decreased linearly within the hours of each PGFM pulse during luteolysis, and a positive effect of an LH pulse on P(4) and E(2) concentration was not detected. The reported balancing of P(4) concentrations between a negative effect of PGF2α and a positive effect of LH in heifers was not detected in mares.

Effect of dose of estradiol-17β on prominence of an induced 13,14-dihydro-15-keto-PGF(2α) (PGFM) pulse and relationship of prominence to progesterone, LH, and luteal blood flow in heifers

Various doses of estradiol-17β (E(2)) were used in heifers to induce a pulse of 13,14-dihydro-15-keto-prostaglandin F(2α) (PGFM). The effect of E(2) concentration on the prominence of PGFM pulses and the relationship between prominence and intrapulse concentration of progesterone (P(4)), LH, and luteal blood flow were studied. A single dose of 0 (vehicle), 0.01, 0.05, or 0.1 mg of E(2) was given (n = six/group) 14 d after ovulation. Blood samples were collected, and luteal blood flow was evaluated hourly for 10 h after the treatment. The 0.05-mg dose increased and the 0.1-mg dose further increased the prominence of the induced PGFM pulse, compared with the 0.0-mg dose and the 0.01-mg dose. The PGFM pulses were subdivided into three different prominence categories (<50, 50 to 150, and >150 pg/mL at the peak). In the 50 to 150 category, P(4) concentration increased (P < 0.05) between -2 h and 0 h (0 h = peak of PGFM pulse). In the >150 category, P(4) decreased (P < 0.05) between -1 h and 0 h, LH increased (P < 0.05) at 1 h, and luteal blood flow apparently decreased (P < 0.05) at 2 h of the PGFM pulse. The novel results supported the following hypotheses: (1) an increase in E(2) concentration increases the prominence of a PGFM pulse, and (2) greater prominence of a PGFM pulse is associated with a greater transient intrapulse depression of P(4) at the peak of the PGFM pulse. In addition, the extent of the effect of prostaglandin F(2α) on the increase in LH and changes in blood flow within the hours of a PGFM pulse was related positively to the prominence of the PGFM pulse.

Effect of luteinizing hormone oscillations on progesterone concentrations based on treatment with a gonadotropin-releasing hormone antagonist in heifers

Close temporality has been reported between the episodic secretion of luteinizing hormone (LH) and progesterone (P4) during the midluteal phase and preceding the beginning of luteolysis in cattle. In the present studies, the relationship between LH and P4 was examined by blocking LH oscillations with the gonadotropin-releasing hormone (GnRH) antagonist, acyline. In a titration study, the minimal single acyline dose for blocking LH oscillations in heifers was 3 μg/kg. The main experiment compared LH and P4 concentrations and oscillations between a group treated with acyline on day 15 after ovulation (n = 8) and a control group (n = 4). Concentrations of P4 in blood samples collected every 8 h on days 13 to 18 indicated that acyline treatment did not alter the time that luteolysis began or the length of the luteolytic process. In blood samples collected every hour for 24 h beginning at the hour of treatment, acyline reduced the LH concentrations and blocked LH oscillations. The hourly LH means were 0.06 to 0.08 ng/mL, comparable to the mean concentration at the nadirs of LH oscillations in controls (0.07 ng/mL). During the hourly sampling, the GnRH antagonist produced the following P4 responses: (1) lower P4 concentrations, (2) fewer and reduced prominence of P4 oscillations, and (3) increased length and variability in the interval between the peaks of P4 oscillations. Results indicated that LH oscillations affect both the prominence and the rhythmicity of P4 oscillations during preluteolysis but not the onset and length of luteolysis.