J. Anim. Sci. 2002. 80:3277-3285
© 2002 American Society of Animal Science
High versus low body condition in mares: Interactions with responses to somatotropin, GnRH analog, and dexamethasone1
L. R. Gentry,
D. L. Thompson, Jr.2,
G. T. Gentry, Jr.,
K. A. Davis and
R. A. Godke
Department of Animal Science, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge 70803-4210
2 Correspondence:
phone: 225-578-3445; fax: 225-578-3279; e-mail:
dthompson{at}agctr.lsu.edu.
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Abstract
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Mares that had previously been fed to attain body condition scores (BCS) of 7.5 to 8.5 (high) or 3.0 to 3.5 (low) were used to determine the interaction of BCS with the responses to 1) administration of equine somatotropin (eST) daily for 14 d beginning January 20 followed by administration of GnRH analog (GnRHa) daily for 21 d and 2) 4-d treatment with dexamethasone later in the spring when mares in low BCS had begun to ovulate. The majority of mares with high BCS continued to cycle throughout the winter, as evidenced by larger ovaries (P < 0.002), more corpora lutea (P < 0.05), greater progesterone concentrations during eST treatment (P < 0.04), and more (P < 0.05) large- and medium-sized follicles. Treatment with eST alone or in combination with GnRHa had no effect (P > 0.05) on ovarian activity or ovulation. Plasma leptin concentrations were greater (P < 0.002) in mares with high BCS; however, there was no effect (P > 0.10) of eST treatment. Plasma IGF-I concentrations were greater (P < 0.0001) in mares treated with eST compared with mares given vehicle, and mares with high BCS had greater IGF-I (P < 0.02) and LH concentrations (P < 0.02) than mares with low BCS. Plasma leptin concentrations in mares with high BCS were increased (P < 0.001) within 12 h of dexamethasone treatment; the leptin response (P < 0.001) in mares with low BCS was greatly reduced (P < 0.001) and transient. Glucose and insulin concentrations also increased (P < 0.0001) after dexamethasone treatment in both groups, and the magnitude of the response was greater (P < 0.0001) in mares with high BCS than in mares with low BCS. In summary, low BCS in mares was associated with a consistent seasonal anovulatory state that was affected little by eST and GnRHa administration. In contrast, all but one mare with high BCS continued to experience estrous cycles and(or) have abundant follicular activity on their ovaries. The IGF-I response to eST treatment was also reduced in mares with low BCS, as was the basal leptin concentration and leptin response to dexamethasone. Although low BCS and leptin concentrations were associated with inactive ovaries during winter and early spring, mares with low BCS eventually ovulated in April and May while leptin concentrations remained low.
Key Words: Body Condition • Dexamethasone • Leptin • Mares • Somatotropin
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Introduction
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Inadequate nutrition and(or) reduced body fat are associated with impaired reproductive efficiency in farm animals (Short and Bellows, 1971; Selk et al., 1988; Schillo, 1992). A similar relationship has been noted for horses (Henneke et al., 1983, 1984; Hines et al., 1987).
Leptin is an adipocyte-derived protein suggested to be the signal between body condition (percent body fat) in an animal and the hypothalamus (Houseknecht et al., 1998). In other species, leptin concentrations vary directly with percentage of body fat (Prolo et al., 1998; Chilliard et al., 2000). In horses, McManus and Fitzgerald (2000) reported that mares deprived of feed for 24 h had decreased leptin concentrations, and Fitzgerald and McManus (2000) described a profound seasonal variation in leptin concentrations in mature mares that was barely discernable in young mares. Moreover, Cartmill et al. (2001) reported that the glucocorticoid analog, dexamethasone, is a potent stimulator of leptin secretion in geldings.
Somatotropin is involved with growth of many tissues in the body but also seems to have a role in reproduction in cattle (Lucy, 2000), sheep (Joyce et al., 1998, 2000), swine (Echternkamp et al., 1994) and horses (Cochran et al., 1999a,b). Cochran et al. (1999a) reported that treatment of seasonally anovulatory mares with recombinant equine somatotropin (eST) potentiated the ovarian response to a subthreshold GnRH analog (GnRHa) regimen, resulting in large ovarian follicles and ovulation.
Based on the potential interactions of body condition, leptin, eST, GnRH, and dexamethasone in horses, the present experiment was designed to test the hypothesis that nutritional restriction leading to low body condition would alter 1) the responses to eST and GnRHa treatment during the seasonal anovulatory period and 2) the response to dexamethasone treatment later in the year when all mares had experienced at least one ovulation.
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Materials and Methods
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Animals and Establishment of Body Condition.
Twenty-four light horse mares, 3 to 19 yr of age, with a mean BW of 496 kg and a mean body condition score (BCS) of 7 (6.5 to 8.0; Henneke et al., 1983) were allotted to two groups of 12 such that breed type, age, and BW were evenly distributed across groups. One group was then randomly selected to be the high BCS group and the other the low BCS group. Beginning in September, mares in the high BCS group were allowed free choice access to dormant Alicia bermudagrass and winter ryegrass pastures and were supplemented with a good quality bermudagrass hay as needed. Mares in the low BCS group were slowly limit-grazed for decreasing periods of time on equivalent pastures until they reached the desired BCS of 3.0 to 3.5, at which time they were grazing approximately 2 h/d. Also, mares were group fed a good quality bermudagrass hay as needed to help maintain the mares at the desired BCS. Weight data showing changes that occurred over time are presented elsewhere (Gentry et al., 2002). All mares had free access to water and a trace-mineralized salt block (Champions Choice; Cargill, Inc., Minneapolis, MN). Mares were housed at the Louisiana State University Agricultural Center, Idlewild Research Station in Clinton, Louisiana, and were maintained on routine herd health and deworming regimens (Evans, 2001).
Determination of Body Condition Scores and Stage of Cyclicity.
Body weights and BCS were recorded weekly throughout the experiment. Body condition score was determined each time by the same technician by visual appraisal and palpation of the six areas suggested by Henneke et al. (1983). Once the mares approached their target BCS (December), ovarian examinations by transrectal ultrasonography (Aloka 550V with 5 MHz linear-array transducer; Aloka Science and Humanity, Wallingford, CT), and estrus detection with a stallion was initiated. Mares were checked for estrus and their ovaries assessed for ovarian size, follicle number and size, and number of corpora lutea every 72 h. Ovarian scores were assigned based on rectal palpation of ovarian size: 1) 5 mm or less; 2) 6 to 10 mm; 3) 11 to 25 mm; 4) 26 to 40 mm; and 5) greater than 40 mm. Follicles were assigned to categories based on their size (small, 10 mm or less; medium, 11 to 19 mm; large, 20 mm or larger). When a mare achieved a follicle of at least 30 mm, her ovaries were examined daily until the follicle ovulated or regressed to below 20 mm. Hormonal and reproductive data collected from September through January are presented elsewhere (Gentry et al., 2002).
Somatotropin and Gonadotropin-Releasing Hormone Assay Treatments.
Beginning January 20, one-half (n = 6) of the mares with low BCS and one-half (n = 6) of the mares with high BCS were randomly allotted to receive either daily injections of eST (25 µg/kg BW; EquiGen, BresaGen Ltd., Adelaide, Australia) or daily injections of vehicle for a period of 14 d. At the end of the 14-d eST treatment, all mares in both groups received daily injections of a GnRHa (50 ng/kg BW; des-Gly10, [D-His(Bzl)6]-LHRH ethylamide; Sigma Chemical, St. Louis, MO; L-2761) for 21 d or until ovulation occurred. Teasing with the stallion and assessments of ovarian function were continued throughout the eST and GnRHa treatment phases. Once it was determined that a mare had ovulated, GnRHa injections for that mare were stopped.
Blood samples were collected daily throughout the treatment injection period by jugular venipuncture into two 7-mL evacuated tubes, one containing potassium oxalate and sodium fluoride for measurement of glucose and the other containing sodium heparin for the measurement of all other hormones (Vacutainer; Becton and Dickinson, Franklin Lakes, NJ). Samples were centrifuged at 1,500 x g at 4°C for 15 min and the plasma harvested and stored at -15°C. Concentrations of LH (Thompson et al., 1983a), FSH (Thompson et al., 1983b), somatotropin (Thompson et al., 1992), IGF-I (Sticker et al., 1995), insulin (DePew et al., 1994), and leptin (McManus and Fitzgerald, 2000) were determined by RIA previously validated for horse plasma. Progesterone concentrations were determined by RIA using commercially available reagents (Diagnostic Systems Laboratories, Inc., Webster, TX). Glucose concentrations were determined colorimetrically (Tech. Bull. No. 315; Sigma). Intra- and interassay coefficients of variation and assay sensitivities were 6%, 9%, and 0.2 ng/mL, respectively, for LH; 7%, 11%, and 1.4 ng/mL, respectively, for FSH; 8%, 11%, and 0.5 ng/mL, respectively, for somatotropin; 5%, 12%, and 8 ng/mL, respectively, for IGF-1; 5%, 8%, and 0.1 ng/mL, respectively, for insulin; 4%, 8%, and 0.8 ng/mL, respectively, for leptin; 5%, 8%, and 0.05 ng/mL, respectively, for progesterone; and 2%, 5%, and 0.4 mM, respectively, for glucose.
Dexamethasone Treatment.
After the GnRHa-treatment period ended (February 24), mares were monitored every 3 d (blood sampling for progesterone analysis, estrus detection with a stallion, and ultrasound examinations) until the mares with low BCS ovulated. Once it was determined that a mare in the low BCS group had ovulated, she was paired with a mare from the high BCS group that had ovulated about the same time, and then both mares were immediately started on a regimen of 4 d of daily dexamethasone (125 µg/kg BW; i.m.) administration. Jugular blood samples were collected at 0800 (immediately before treatment) and 2000 during treatment and then once daily at 0800 for 4 more days. Blood samples were handled as described previously and stored at -15°C until analyzed for plasma concentrations of leptin, insulin, and glucose.
Statistical Analyses.
Data were analyzed by the GLM procedure of SAS (SAS Inst. Inc., Cary, NC) as a completely randomized design with a 2 x 2 factorial arrangement of treatments (Steel and Torrie, 1980) with repeated measures (split-plot; Gill and Hafs, 1971). Main effects of BCS and eST treatment and their interaction were tested with the mare within treatment interaction, and time and its interactions with the other main effects were tested with residual error. Likewise, main effects of BCS and its interaction with eST treatment after dexamethasone administration were tested with the mare within treatment interaction, and time and its interactions with the other main effects were tested with residual error. Also, the time effect was partitioned into two main phases, the eST-treatment phase and the GnRHa-treatment phase, for comparison. Differences between or among groups within each time period were assessed by the LSD test (Steel and Torrie, 1980).
Due to the large difference in mean leptin concentrations between mares with high vs low BCS, a separate ANOVA was performed on the data of the mares with low BCS to determine their response to dexamethasone treatment. These data were analyzed by ANOVA with time, mare, and the interaction of time by mare as sources of variation; the interaction served as the error term for testing effects of time.
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Results
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Mean BW and BCS for the mares in the high vs low groups at the onset of the eST-treatment phase on January 20 were 512 vs 412 kg (SEM = 9.7 kg) and 8.7 vs 3.7 (SEM = 0.12), respectively. Data collected during November and December showed that seven mares with high BCS continued to experience estrous cycles throughout the winter months without going anestrous, one mare with high BCS went into anestrus (mid-November until late spring), and four mares experienced a brief period with low progesterone concentrations (late December to mid-February), but continued to have several large follicles on their ovaries at ultrasound examination. In comparison, all 12 mares with low BCS had entered anestrus by the end of November. Toward the end of GnRHa treatment, two mares with low BCS did ovulate, one that had received eST and one that had not, however, they both reverted back to an anestrous state when GnRHa treatment was halted. At the onset of eST treatment on January 20, the number of anovulatory mares was 12 in the low BCS group and 5 in the high BCS group (2 receiving eST and 3 receiving vehicle).
Treatment with Equine Somatotropin and Gonadotropin-Releasing Hormone Analog.
Although there was no effect of eST treatment, mares with high BCS had larger ovaries (P < 0.002; Figure 1a
) and more corpora lutea (P < 0.0003; data not shown) than mares with low BCS. Analysis of progesterone data showed that mares with high BCS had greater (P < 0.04) progesterone concentrations (Figure 1b) during eST treatment but were not different (P > 0.05) by d 6 of GnRHa treatment. On average, mares with high BCS had more (P < 0.02) large follicles than mares with low BCS (1.85 vs 0.76; SEM = 0.34), and the number of large follicles was greater (P < 0.002) during the GnRHa-treatment period (Figure 2a) than during the eST-treatment period. More (P < 0.05) medium-sized follicles were present toward the end of the experiment than at the beginning, and mares with high BCS had more (P < 0.05) medium follicles than mares with low BCS (Figure 2b). The number of small follicles did not differ between groups (P > 0.05; data not shown).
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Figure 1. Mean ovarian score (a) and plasma progesterone concentrations (b) in mares either well fed (high BCS) or feed-restricted to achieve low body condition (low BCS) by the start of 14 d of equine somatotropin (eST) treatment on January 20; all mares were subsequently treated with a GnRH analog (GnRHa) for 21 d. Pooled SEM were 0.12 for ovarian score and 1.3 ng/mL for progesterone concentrations. The vertical lines in each graph indicate the LSD value (P < 0.05) for comparisons between groups for each time period. Day 0 represents the first day of eST treatment.
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Figure 2. Mean number of large (a; 20 mm or greater) and medium (b; 11 to 19 mm) follicles in mares either well fed (high BCS) or feed-restricted to achieve low body condition (low BCS) by the start of 14 d of equine somatotropin (eST) treatment on January 20; all mares were subsequently treated with a GnRH analog (GnRHa) for 21 d. Pooled SEM were 0.35 and 0.55 for number of large and medium follicles, respectively. The vertical lines in each graph indicate the LSD value (P < 0.05) for comparisons between groups for each time period. Day 0 represents the first day of eST treatment.
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Plasma leptin concentrations (Figure 3) were greater (P < 0.002) in mares with high BCS than in mares with low BCS throughout the experiment, but there was no effect (P > 0.10) of eST treatment. Also, an interaction (P < 0.003) between BCS and phase was detected in that leptin concentrations increased with high BCS throughout the period of eST treatment, peaked during the first week of GnRHa treatment, and declined over time thereafter.
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Figure 3. Mean plasma leptin concentrations in mares either well fed (high BCS) or feed-restricted to achieve low body condition (low BCS) by the start of 14 d of equine somatotropin (eST) treatment on January 20 (d 0); all mares were subsequently treated with a GnRH analog (GnRHa) for 21 d. Pooled SEM was 0.63 ng/mL. Leptin concentrations were greater (P < 0.002) in mares with high BCS relative to mares with low BCS, and were greater (P < 0.003) during GnRHa treatment than during eST treatment. The vertical line indicates the LSD value (P < 0.05) for comparisons between groups for each time period.
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There was an interaction of BCS, eST treatment, and phase (P < 0.05) for somatotropin concentrations (data not shown); however, only isolated differences were noted on d 10 of eST treatment and on d 10 of GnRHa treatment. Plasma IGF-I concentrations (Figure 4) were greater (P < 0.0001) in mares treated with eST relative to mares given vehicle. Moreover, mares of high BCS had greater (P < 0.02) IGF-I concentrations than mares with low BCS. An interaction of BCS, eST treatment, and phase (P < 0.0001) indicated that the rise in IGF-I concentrations occurred during the eST treatment phase and decreased back to pre-eST levels during the GnRHa-treatment phase.
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Figure 4. Mean plasma IGF-I concentrations in mares either well fed (high BCS) or feed-restricted to achieve low body condition (low BCS) by the start of 14 d of equine somatotropin (eST) treatment on January 20 (d 0); all mares were subsequently treated with a GnRH analog (GnRHa) for 21 d. Pooled SEM was 14 ng/mL. There was an effect (P < 0.0001) of eST treatment and an interaction (P < 0.0001) with BCS and time. The vertical line indicates the LSD value (P < 0.05) for comparisons between groups for each time period.
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Mares with high BCS had greater (P < 0.02) daily LH concentrations than mares of low BCS (Figure 5), and the difference was greater (BCS-phase interaction, P < 0.01) during the GnRHa-treatment phase. Minor differences in daily FSH concentrations were noted (data not shown) but were isolated and not consistent among mares within any treatment group.
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Figure 5. Mean plasma LH concentrations in mares either well fed (high BCS) or feed-restricted to achieve low body condition (low BCS) by the start of 14 d of equine somatotropin (eST) treatment on January 20; all mares were subsequently treated with a GnRH analog (GnRHa) for 21 d. Pooled SEM was 0.13 ng/mL. The vertical line indicates the LSD value (P < 0.05) for comparisons between groups for each time period. Day 0 represents the first day of eST treatment.
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Treatment with Dexamethasone.
Dexamethasone treatment was initiated in the first pair of mares on April 2 and in the last pair of mares on May 6, which was 39 and 73 d from the last eST injection. Given the time differential between eST treatment and dexamethasone treatment, it seemed unlikely that there would be any carry-over effects of eST treatment during the dexamethasone trial; an initial ANOVA with previous eST treatment as a factor indicated that this was the case (P > 0.05), and this factor was dropped from the analyses.
Plasma leptin concentrations approximately doubled (P < 0.001) in mares with high BCS within 12 h of onset of dexamethasone treatment, and leptin concentrations continued to rise severalfold until d 3 (Figure 6a). Leptin concentrations in mares with low BCS increased (P < 0.001) after dexamethasone treatment as well (Figure 6b), but the response was quite different from mares with high BCS. First, the mean stimulated concentrations did not exceed 3 ng/mL, and second, the stimulated concentrations returned to pre-dexamethasone concentrations within 24 h after each injection.
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Figure 6. Mean plasma leptin (a) concentrations in mares either well fed (high BCS) or feed-restricted to achieve low body condition (low BCS) during 4 d of dexamethasone treatment (arrows). The means for mares with low BCS are also presented (b) on a scale appropriate for visualization of their changes. Pooled SEM were 3.1and 0.44 ng/mL for (a) and (b), respectively. The vertical lines in each graph indicate the LSD value (P < 0.05) for comparisons between groups for each time period or between time periods for the mares with low BCS. Day 1, the day of first dexamethasone injection, ranged from April 2 to May 6, for the 12 pairs of mares (one of high BCS and one of low BCS, started based on when the low BCS mare ovulated).
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Concentrations of glucose (Figure 7a) and insulin (Figure 7b) also increased (P < 0.001) in response to dexamethasone treatment. The magnitude of the glucose and insulin responses was greater (P < 0.0001) in mares with high BCS relative to mares with low BCS.
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Figure 7. Mean plasma glucose (a) and insulin (b) concentrations in mares either full fed (high BCS) or feed-restricted to achieve low body condition (low BCS) during 4 d of dexamethasone treatment (arrows). Pooled SEM were 0.20 mM and 4.5 ng/mL for glucose and insulin concentrations, respectively. The vertical lines in each graph indicate the LSD value (P < 0.05) for comparisons between groups for each time period. Day 1, the day of first dexamethasone injection, ranged from April 2 to May 6, for the 12 pairs of mares (one of high BCS and one of low BCS, started based on when the low BCS mare ovulated).
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Discussion
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This experiment was in part designed to examine the interaction of body condition, as evidenced by BCS, on the responses to eST and GnRHa treatment in mares during the seasonal anovulatory period. This was based on two experiments of Cochran et al. (1999a), in which treatment of seasonally anovulatory mares with eST significantly increased plasma IGF-I concentrations and the number of small follicles present on the ovaries (both experiments), resulted in greater ovulation rates when mares were administered a subthreshold GnRHa regimen (not sufficient to induce ovulation itself). The mares in those experiments were of "good" BCS, and all mares were anovulatory. Administration of somatotropin has also been shown to have beneficial effects on ovarian activity, particularly increased follicle numbers, in other species. Yearling beef heifers treated with bovine somatotropin for 142 d had more total follicles than heifers receiving vehicle (Tripp et al., 2000). Lucy (2000) reported that administration of recombinant bovine somatotropin increased ovarian follicular development, and it was suggested that this is due to either direct effects of somatotropin (since somatotropin receptors are found within granulosa cells as well as oocytes) or to indirect effects of increased IGF-I and(or) nutrient partitioning that occurs after somatotropin treatment. Buratini et al. (2000) substantiated that finding by showing a twofold increase in IGF-I concentrations and a 36% increase in small follicle numbers in Nelore heifers treated with bovine somatotropin. Scaramuzzi et al. (1999), however, reported that short-term treatment of sheep with ovine GH had no effect on ovulation rate or follicular development. Similarly, Eckery et al. (1994) reported no increase in ovulation rate or number of small antral follicles when ewes were treated with recombinant bGH.
In contrast to the experiments of Cochran et al. (1999a), the profound differences in reproductive status for mares with high vs low BCS in the present experiment precluded direct comparison of the treatment effects, given that most mares with high BCS were already cyclic before eST treatment was initiated. In the present experiment, there was no effect of 14 d of eST treatment on the number of small follicles in either group (high vs low BCS), even though plasma IGF-I concentrations increased 100 to 200% in mares with low BCS and 200 to 300% in mares with high BCS during eST treatment. Unlike Cochran et al. (1999a), eST treatment was not continued in the present experiment during the 21 d of GnRHa treatment and that may account for the lack of response to GnRHa treatment in the mares with low BCS. As reported previously (Smith et al., 1999; Kulinski et al., 2002), the elevated IGF-I concentrations in eST-treated mares decreased to control levels within about 6 to 7 d after cessation of eST injections; thus, any potentiating effects of eST treatment for the GnRHa treatment may have diminished by that time as well.
At the onset of eST treatment in January, leptin concentrations were very low to undetectable in mares with low BCS, which was consistent with studies that show leptin to vary directly with body mass index and percentage body fat (Prolo et al., 1998; Chilliard et al., 2000; Fitzgerald and McManus, 2000). Plasma leptin concentrations were greater in mares with high BCS, and average concentrations were greater during the GnRHa-treatment period than during the eST-treatment period. Although the rise in leptin concentrations seemed to start during the eST-treatment phase, there was no eST effect in the statistical analysis of leptin concentrations; thus, the increase in leptin may have been due to the initiation of GnRHa treatment. However, the rise was short-lived, and the possibility that GnRHa treatment influences leptin secretion needs to be examined further. Hardie et al. (1996) reported that incubation of isolated rat adipocytes with somatotropin or IGF-I in culture had no affect on leptin synthesis or secretion. In an experiment by Houseknecht et al. (2000), somatotropin treatment significantly increased leptin mRNA in steers exhibiting a positive IGF-I response. Also, GnRH administration had no effect on leptin concentrations in hyperandrogenic or healthy women (Maliqueo et al., 2000). Although leptin may stimulate the release of GnRH from the hypothalamus and gonadotropins from the pituitary, circulating leptin is probably not modulated by pulsatile GnRH and(or) LH secretion (Sir-Petermann et al., 1999; Maliqueo et al., 2000).
As reported previously (Smith et al., 1999; Capshaw et al., 2001), administration of eST stimulated plasma IGF-I concentrations severalfold within 7 d. In untreated animals, plasma IGF-I concentrations are indicative of nutritional status and actually decrease in nutrient-restricted animals when plasma somatotropin concentrations are generally increased (Granger et al., 1989; Sticker et al., 1995; Breier, 1999), the result of an "uncoupling" of the normal somatotropin-IGF-I axis (McGuire et al., 1992). The data presented herein show that mares with low BCS can respond to eST with an increase in plasma IGF-I concentrations; however, the response was not as great as in mares with high BCS.
Mares with high BCS had greater average LH concentrations during the experiment, especially during the GnRHa-treatment phase. Given that most of these mares were experiencing estrous cycles, it would be expected that LH concentrations would be greater, even during most of diestrus, compared with seasonally anovulatory mares (Ginther, 1992). The tendency for greater LH concentrations in mares with high BCS during the GnRHa-treatment phase was likely due less to the daily GnRHa injections than to the reduced progesterone concentrations at the same time, indicative of the follicular phase when LH secretion is high (Ginther, 1992). The lack of any effect of eST treatment on LH or FSH concentrations is consistent with the reports of others (Gong et al., 1991; Cochran, 1999b).
Dexamethasone treatment for 4 d increased plasma glucose and insulin concentrations, which was indicative of a hyperglucocorticoid state (Guyton and Hall, 1996). Cartmill et al. (2001) reported previously that this dose of dexamethasone for 4 d increased plasma glucose, insulin, and leptin concentrations in geldings, and the data in the present experiment extend those observations to include mares with high BCS. In contrast, mares with low BCS that did exhibit positive albeit reduced glucose and insulin responses to the dexamethasone had little change in leptin concentrations. Moreover, their leptin response to each injection of dexamethasone was transient, with concentrations returning to baseline within 24 h after each injection. We speculate that 1) the adipose tissue mass in mares with low BCS was so low that dexamethasone stimulation could not sustain a continual rise in leptin concentrations in peripheral blood or that 2) their adipose tissue was relatively unresponsive to the dexamethasone. If the first possibility turns out to be correct, then this regimen of dexamethasone may have potential as a means of estimating body fat stores in horses.
Dexamethasone and insulin have both been reported to stimulate leptin secretion in various nonequine species (Larsson and Ahren, 1996; Miell et al., 1996; Ramsay and White, 2000) and to stimulate leptin secretion (Hardie et al., 1996; Wabitsch et al., 1996; Considine et al., 1997) and mRNA content (Reul et al., 1997; Bradley and Cheatham, 1999) in adipocytes cultured in vitro. Moreover, serum immunoreactive leptin concentrations are increased in humans with Cushing’s syndrome (Leal Cerro et al., 1996), which some have attributed to a direct effect of glucocorticoids on adipocytes (Masuzaki et al., 1997) but others to the associated hyperinsulinemia and(or) impaired insulin sensitivity (Widjaja et al., 1998). Given that glucose and insulin concentrations increased in these mares at the same time as leptin concentrations in response to dexamethasone, it is not possible to determine whether dexamethasone directly stimulated leptin or whether insulin was involved in the response as well.
In conclusion, low BCS in mares resulted in a consistent seasonal anovulatory state that was not reversed by eST and GnRHa administration. In contrast, all but one mare with high BCS continued to experience estrous cycles and(or) have abundant follicular activity on their ovaries. The IGF-I response to eST treatment was also reduced in mares with low BCS, as was the basal plasma leptin concentration and leptin response to dexamethasone. Although low BCS and leptin concentrations were associated with inactive ovaries during winter and early spring, mares with low BCS eventually ovulated in April and May, while leptin concentrations remained low. Thus, further research is needed to determine the role of leptin in the reproductive response to nutrient restriction in mares.
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Implications
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Low body condition scores in these mares resulted in profound effects on hormonal and reproductive characteristics during the seasonal anovulatory period. Further treatment of these thin mares with somatotropin followed by an analog of gonadotropin releasing hormone did not stimulate their ovaries. These mares also have very low concentrations of leptin, a hormone thought to help regulate nutrient intake and coordinate the body’s response to high vs low planes of nutrition. Moreover, mares in good body condition had a much more pronounced increase in leptin concentrations in response to dexamethasone treatment than mares in poor body condition. Together, these results provide possible explanations on how the nutritional status of the mare leads to either the ovulatory, cyclic state, or to anestrus and anovulation.
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Footnotes
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1 Approved for publication by the Director of the Louisiana Agric. Exp. Sta. as manuscript no. 02-11-0110. We thank A. F. Parlow and the NIDDKD, National Hormone and Pituitary Program, Harbor-UCLA Medical Center, Torrance, CA, for reagents. 
Received for publication March 8, 2002.
Accepted for publication July 23, 2002.
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Abstract
Introduction
