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Matrix metalloproteinases (MMP) and tissue inhibitors of MMP (TIMP) regulate tissue remodeling events necessary for ovulation. Thus, changes in MMP and TIMP expression and protein enzyme activity were examined in vivo and in vitro during follicular development and atresia in the horse. Equine granulosa cells and follicular fluid from medium (15 to 29 mm) healthy and atretic follicles and from large (>30 mm) healthy and preovulatory follicles were collected by transvaginal aspiration. The cells were either snap-frozen (in vivo study) or cultured for 48 h (in vitro study) to determine gene expression and protein enzyme activity of MMP-2 and MMP-9 and TIMP-1 and TIMP-2. Concentrations of progesterone and estradiol were determined by RIA in follicular fluid and conditioned media and were used along with follicle dynamics to classify follicles. In vivo, expression of MMP-2 and TIMP-2 was increased (P < 0.05) in large-preovulatory follicles, whereas TIMP-1 was decreased. The ratio of MMP-2:TIMP-2 expression was decreased (P < 0.05) in medium-healthy and large-preovulatory follicles, whereas the MMP-9:TIMP-1 ratio was increased only in large-preovulatory follicles compared with large-healthy follicles. Estradiol was greatest (P < 0.05) in the fluid of large-healthy and large-preovulatory follicles. However, medium-atretic follicles were associated with the least estradiol concentrations, both in vivo and in vitro. Progesterone concentrations were greatest (P < 0.05) in large-preovulatory follicles both in vivo and in vitro. In healthy follicles in vivo, the diameter was correlated with estradiol concentration, the estradiol:progesterone ratio, MMP-9 and TIMP-1 expression, and MMP-2 and MMP-9 protein activity. In contrast to in vivo studies, the ratio of MMP-9:TIMP-1 expression was increased (P < 0.05) in medium-healthy follicles; TIMP-2 expression decreased in large-preovulatory follicles in vitro. In addition, MMP-9 protein activity was decreased (P < 0.05) in the media samples of cells from large-healthy follicles compared with those from medium-healthy follicles. These results indicate that changes in MMP-2 and MMP-9 activities may be essential to the tissue reorganization necessary for ovulation in the equine ovary.

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Recent studies associate obesity and insulin resistance in horses with development of abnormal reproductive function and debilitating laminitis. The factors contributing to insulin resistance in obese horses are unknown. However, human studies provide evidence that elevated inflammatory cytokines such as tumor necrosis factor alpha (TNFalpha), IL1, and IL6 play direct roles in development of obesity-associated insulin resistance. Thus, inflammation may be a key link between obesity and insulin resistance in horses. The aim of the current investigation was to examine possible relationships between obesity, inflammatory cytokines, and insulin sensitivity (IS) in the horse. Age was recorded and BCS and percent body fat (% FAT) were determined as measures of obesity in 60 mares. In addition, blood mRNA expression of IL1, IL6, and TNFalpha and circulating concentrations of TNFalpha protein (TNFp) were determined in each mare. Finally, fasted concentrations of insulin were determined, and IS was determined using the hyperinsulinemic, euglycemic clamp. Significant correlations between several variables provided evidence for the design of 4 population regression models to estimate relationships between measures of obesity, inflammatory factors, and IS in the sample population. The results of these analyses revealed that IS decreased as BCS and % FAT increased (P < 0.001) in the sample population. Additionally, increased IL1 (P < 0.05) and TNFp (P < 0.01) were associated with decreased IS. However, increased TNFalpha (P < 0.001) was associated with decreased IS only in mares 20 yr of age and older. Increased BCS and % FAT were associated with increased expression of TNFalpha (P = 0.053) and IL1 (P < 0.05), and increased TNFp (P < 0.05). Surprisingly, increased BCS and % FAT were associated with decreased IL6 expression (P = 0.05) in mares <20 yr of age. Finally, evaluation of the influence of obesity and inflammatory cytokines on IS within the same model suggested that BCS and % FAT (P < 0.001) with TNFalpha [mRNA (P = 0.07) and protein (P < 0.05)] are inversely associated with IS independently of one another. Combined, these results provide the first evidence associating obesity with increased inflammatory factors in the horse. Furthermore, the results suggest that an interrelationship exists among obesity, inflammatory cytokines, and IS in the horse and emphasize the need for further studies to elucidate the nature of these relationships.

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In mares, obesity is associated with continuous reproductive activity during the non-breeding season. To investigate the effect of obesity and associated alterations in metabolic parameters on the oestrous cycle, two related studies were conducted. In Experiment 1, obese (body condition score > 7) mares were fed ad libitum or were moderately feed restricted during the late summer and autumn months. Feed restriction did not alter the proportion of mares entering seasonal anoestrus. However, obese mares exhibited a significantly longer duration of the oestrous cycle, significant increases in circulating concentrations of leptin and insulin, and decreased insulin sensitivity and concentrations of thyroxine compared with feed-restricted mares throughout the experiment. Experiment 2 was designed to investigate the effects of administration of the insulin-sensitising drug metformin hydrochloride on insulin sensitivity and the characteristics of the oestrous cycle in obese mares. In a dose-response trial, metformin increased insulin sensitivity after 30 days following administration of 3 g day(-1), but not 6 or 9 g day(-1), compared with controls receiving vehicle only. However, there were no differences in insulin sensitivity or oestrous cycle characteristics between control and metformin-treated groups when the 3 g day(-1) dose was tested for a longer period of 2 months. These results demonstrate that obesity is associated with aberrations in the oestrous cycle and perturbations in several markers of metabolic status. The results also indicate that metformin is not an effective long-term monotherapy for increasing insulin sensitivity in horses at the doses tested. Additional studies are needed to examine possible effects of increasing insulin sensitivity on reproductive activity in obese mares.

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Consumption of wild-type (toxic) endophyte-infected tall fescue (E+) by horses during late gestation is known to adversely affect pregnancy outcome; however, little is known of the potential disruptive consequences of E+ consumption by mares during the critical phases of placentation and fetal development in early pregnancy. The objective of this study was to evaluate the detrimental effects of feeding E+ to mares during early gestation. Mares (n = 12) paired by stage of gestation (d 65 to 100) were assigned to diets (six per diet) consisting of endophyte-free (E-) or E+ tall fescue seed (50% E- or E+ tall fescue seed, 45% sweet feed, and 10% molasses fed at 1.0% of BW/d). Mares also had ad libitum access to E+ or E- annual ryegrass hay, and were fed diets for 10 d. Following removal from the tall fescue diet on d 11, mares were placed on common bermudagrass pasture and monitored until d 21. Morning and evening rectal temperatures were recorded and daily blood samples were collected for progesterone and prolactin (PRL) analyses, whereas samples for 3,4-dihydroxyphenyl acetic acid (a catecholamine metabolite) analysis were collected on alternate days. For clinical chemistry analysis, blood samples were collected on d 0, 5, 10 and 21. Daily urine samples were collected for ergot alkaloid analysis, and ultrasonography was performed for presence of echogenic material in fetal fluids. Rectal temperatures (E+ 37.76+/-0.03; E- 37.84+/-0.03 degrees C) and serum PRL concentrations (E+ 14.06< or =0.76; E- 12.11+/-0.76 ng/mL) did not differ (P = 0.96) between treatments. Measuring the change in basal serum concentration from d 0 over time, progesterone concentrations did not differ (-0.64 +/-1.49 and -0.55+/-1.47 ng/mL for E+ and E- mares, respectively). There was no negative pregnancy outcome, and ultrasonography indicated no increase in echogenic material in fetal fluids. Plasma 3,4-dihydroxyphenyl acetic acid concentrations decreased (P < 0.05) in E+ compared with E- mares (2.1+/-0.14 and 4.4+/0.43 ng/mL, respectively). Urinary ergot alkaloid concentration was greater (P < 0.01) in mares consuming E+ compared with E- (532.12+/- 52.51 and 13.36+/-2.67 ng/mg of creatinine, respectively). Although no fetal loss was observed during the current study, elevated concentrations of urinary ergot alkaloid were consistent with depressed endogenous catecholamine activity, suggestive of an endocrine disruptive effect of hypothalamic origin.

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Peripheral insulin resistance is the failure of proper cellular glucose uptake in response to insulin. Insulin resistance and hyperinsulinemia are associated with several disease states in the horse and reproductive function disturbances in humans, including polycystic ovarian syndrome. To test the hypothesis that insulin resistance (IR) and hyperinsulinemia disrupt the estrous cycle in mares, two experiments were conducted to first develop a model to induce IR and to then examine the effect of this model on the duration of the estrous cycle. In Exp. 1, a hyperinsulinemic-euglycemic clamp (HEC) procedure was performed on seven mares to determine insulin sensitivity before and immediately following infusion of a heparinized lipid solution. The HEC procedure was repeated 1 wk after lipid infusion. Mares developed IR following the lipid infusion (P < 0.05), and some individuals maintained IR for up to 1 wk. Mares also exhibited increased blood insulin both immediately following treatment and 1 wk later (P < 0.05). In Exp. 2, induction of insulin resistance by lipid solution was not accompanied by changes in circulating concentrations of luteinizing hormone, and duration of the luteal phase, compared with the duration of untreated luteal phases. Nonetheless, lipid infusion and the resultant insulin resistance were associated with an increased interovulatory period (P < 0.05), and peak concentrations of progesterone (P < 0.05) were higher during the treated vs. untreated luteal phases of the estrous cycle. The results from the preliminary study suggest that infusion of a lipid solution may induce transient insulin resistance and hyperinsulinemia. The resulting insulin resistance and hyperinsulinemia may modify characteristics of the estrous cycle, perhaps at the level of the ovary.

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A small percentage of mature mares continue to exhibit estrous cyclicity during the non-breeding season which is of interest because of the importance of timing of the breeding season to the equine breeding industry. Previously, it was demonstrated that the continuation of estrous cycles was more likely to occur in mature than young mares. Additionally, an apparent association exists between elevated body fat and increased circulating concentrations of leptin, and the occurrence of estrous cycles during the non-breeding season. Two experiments were conducted to test the hypothesis that pharmacological manipulation of body fat and leptin by administration of the beta(2)-adrenergic receptor agonist clenbuterol, with and without continuous treatment with melatonin, would increase the proportion of mares entering anestrus or advance its timing. In Experiment 1, eight mature mares were administered clenbuterol (3.2 microg/kg) daily from October through January and six mares were untreated. In Experiment 2, eight mares were administered clenbuterol daily from June through December, eight mares were treated with continuous release melatonin implants, and seven mares were treated with both clenbuterol and melatonin. Melatonin treatment was included to confirm previous results that constant treatment with melatonin did not advance the timing of anestrus. In both studies, leptin concentrations (P<0.01, both experiments) and body fat percentage (P<0.01, Experiment 1; P<0.05, Experiment 2) declined in response to daily clenbuterol treatment. Unexpectedly, however, this decrease was attenuated in mares also treated with melatonin (P<0.05). Although treatment with clenbuterol was associated with a decline in body fat and circulating concentrations of leptin, the timing or proportion of mares exhibiting anestrus was not modified (P>0.05). The results demonstrate the manipulation of body condition via pharmacological intervention does not modify the mechanisms controlling seasonal anestrus in the mare.

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Twelve untrained aged mares were used to determine whether 7 days of light exercise improved peripheral tissue insulin sensitivity. Mares were divided into obese-exercised (n = 3), obese-sedentary (n = 3), lean-exercised (n = 3) and lean-sedentary (n = 3) groups. The exercised groups were worked at a trot to a heart rate (HR) of not more than 140 beats/min for 30 min in a round pen. Each group was subjected to 3 euglycaemic hyperinsulinaemic clamps: prior to exercise (P), 24 h following the seventh exercise training session (E) and 9 days postexercise training (PE). Prior to exercise training, the mares in the obese group were confirmed insulin-resistant compared to the mares in the lean group. There was no change in bodyweight or body condition in the obese or lean groups throughout the study. Glucose infusion rate (GIR) was higher (P<0.05) on E compared to P days in the obese-exercised and lean-exercised groups. Insulin sensitivity returned to pre-exercise values by 9 days postexercise in the obese-exercised and lean-exercised groups. The results of this study suggest that improvement in insulin sensitivity occurs in obese mares without a long interval of exercise training and in the absence of a change in bodyweight.

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The seasonal nature of reproductive activity in mares is widely accepted and considerable attention has focused on the mechanisms that lead to the initiation of the breeding season. In contrast, considerably less information is available about the termination of the breeding season. It is interesting to note that each winter a sub-population of mares continues to undergo oestrous cyclicity during the non-breeding season. Continuation of reproductive activity during the winter occurs most frequently in mares that maintain a non-pregnant condition in successive years. The maintenance of a non-pregnant condition in successive years leads to an increase in the percentage of total body fat and it has been proposed that the degree of adiposity may be a determinant of reproductive activity during the winter months. To investigate this hypothesis we have manipulated fat stores by either pharmacological treatments or feed restriction. The studies described in this review demonstrate that manipulation of body fat during the autumn months fails to modify the mechanisms that lead to anoestrus or the proportion of mares that continues to show oestrous cyclicity during the winter months. On the basis of these and related studies two hypotheses are presented that may serve as a template for future work. The first hypothesis proposes that one aspect of the long-term regulation of seasonal reproductive rhythms in mares, specifically anoestrus, may reflect recognition of the availability of metabolic fuels before perception of a change in photoperiod. Alternatively, energy availability may need to reach a critical value before a presumptive inhibitory daylength signal initiates termination of the breeding season. This review describes previous and current studies that have led to development of these proposals.

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In a variety of species, short-term feed restriction leads to rapid changes in the reproductive axis and reduces serum levels of leptin. Two experiments were performed to test the hypothesis that a single day of feed restriction in aged and young mares would cause a suppression of the gonadotropins and serum leptin concentrations. The estrous cycles of 12 aged (>eight years; Exp. 1) and eight young (

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In seasonally breeding species photoperiodic information is thought to be conveyed to the reproductive and prolactin axis via changes in circulating concentrations of melatonin. For some species, a constant melatonin stimulus is perceived as a short day, whereas in others no photoperiodic information is provided. In the mare, a preliminary study demonstrated that constant administration of melatonin did not modify prolactin secretion, suggesting that this treatment regimen failed to provide photoperiodic information. To further investigate this proposal and to investigate an alternative explanation, namely a seasonal variation in response to melatonin, 4 experiments were performed. In experiments 1-3, the effects of constant administration of melatonin on prolactin secretion were investigated. In each study the time of treatment initiation varied beginning before the summer solstice, (May 9; Exp. 1), at the autumnal equinox (Sept. 21; Exp. 2) or the winter solstice (Dec. 21; Exp. 3). In Experiment 4, melatonin was administered as a timed daily injection (5 PM) for 6 months, beginning at the summer solstice (June 21). Constantly elevated physiological concentrations of melatonin (expts. 1-3) and an extended nighttime elevation of melatonin (exp. 4) suppressed prolactin concentrations only during the spring and early summer months (April-August). At other times during the year prolactin concentrations were similar to untreated mares. In the presence of a continuous melatonin implant the circannual rhythm of prolactin secretion was not disturbed. The results suggest that the prolactin axis of the mare is sensitive to an inhibitory melatonin signal during a restricted period of time and that at other times is refractory to this signal.

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The objectives of this study were to compare the timing and mechanisms controlling the onset of anestrus in young and mature mares treated either continuously with melatonin and in those that remained untreated. Changes in body weight, subcutaneous body fat measured to provide an estimate of total body fat, and circulating concentrations of leptin were compared throughout the 1-yr experimental period. The results demonstrate that in young mares the timing of anestrus occurs significantly earlier in the year than in mature mares and that mature mares are more likely to exhibit continuous reproductive activity during the nonbreeding season. The propensity of mature mares to exhibit this phenomenon is not modified by continuous treatment with melatonin but is associated with higher mean circulating concentrations of leptin, body weight, and estimated percent of body fat. In both young and mature mares, body weight, percent of body fat, and circulating concentrations of leptin are higher during summer than winter months. We conclude that, in the mare, the reproductive response to a decrease in photoperiod or a presumptive inhibitory melatonin signal is modified by energy availability, which may be signaled to the hypothalamus-pituitary axis via a change in the circulating concentration of leptin. An additional observation confirmed that the prolactin axis is responsive to continuous treatment with melatonin but that a suppression of prolactin secretion is limited to the spring months.

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This paper describes attempts to initiate equine adipocyte cultures from necropsy cases with varying intervals from time of death to isolation and culture. Equine adipocytes were isolated from 21 necropsy cases, regardless of the interval from time after death to establishment in primary ceiling cultures. However, while all cultures produced adipocytes, only 2 attempts to produce long-term equine adipocyte cultures from the subcutaneous rump fat depots were successful and not contaminated. Findings from these experiments indicate that it is possible to collect and culture equine adipocytes from necropsy cases with varying intervals of time of death to culturing provided that the issue of contamination is addressed. Viable cells were produced from tissue with an interval of 38.5 hours as well as 45 minutes. This result encourages the continuation of research using equine necropsy cases as a source of adipose tissue.

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The metabolic effects of short-term feed restriction and dietary calorie source were studied in horses receiving high-roughage or high-concentrate diets. Four Thoroughbred geldings were assigned to four treatment groups in a 4 x 4 Latin square experiment. The four treatments were 1) a nutritionally adequate high-roughage ration (70% roughage, 30% concentrate; AHR), 2) a nutritionally adequate high-concentrate ration (40% roughage, 60% concentrate; AHC), 3) 70% of the intake of the AHR diet (RHR), and 4) 70% of the intake of the AHC diet (RHC). Diets AHR and AHC were designed to meet the caloric need of horses undergoing moderately intense work. Blood samples were taken on the first 7 d of each period for analysis of serum T4 and T3 concentrations. On d 9 of each feeding period, each horse was fed 1.0 kg of oats as the morning meal. Jugular blood was sampled before and immediately after, as well as at 30 min after, completion of the meal and subsequently every hour for 7 h. Daily serum T4 and T3 concentrations were not affected by day, feeding level, or diet composition. Meal feeding produced an increase (P < 0.01) in T4 and T3 concentrations when horses were adapted to the AHR and AHC diets but not the RHR or RHC diets. Thyroxine concentrations were lowest (P < 0.05) when horses were adapted to the AHC diet. Glucose (P < 0.05), insulin (P < 0.01), and NEFA (P < 0.01) concentrations were higher in response to the meal when horses received RHR than for the other diets. These results indicate that nutrient restriction alters responses to meal feeding in horses and that this response may also be affected by the dietary roughage:concentrate ratio.

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In the mare, endogenous opioids have been implicated in the suppression of gonadotropin secretion during seasonal anestrus (AN). The present study tested whether continuation of reproductive activity during the nonbreeding season (NBS) reflects the absence of a seasonal shift in opioid tone compared to what occurs in AN mares. During the NBS, 11 AN and 8 luteal-phase mares received 0.1, 0.05, 0. 025 mg/kg naloxone (NAL) or vehicle on alternate days. Whereas cycling mares responded to all dosages of NAL, AN mares responded only to the higher dosages for FSH, and LH failed to increase at any dosage employed. During the breeding season (BS), the response to these dosages of NAL was reevaluated in 12 mares in the luteal phase of a synchronized estrous cycle. Although there was no difference between cycling mares during the breeding and nonbreeding seasons in FSH response, those mares that had cycled during the NBS showed a greater LH response to 0.05 mg/kg NAL than mares during the BS. From these data, we conclude that opioid tone is lower during the BS than during AN and that this shift in inhibitory tone does not occur in mares that cycle during the NBS. Thus, reduced opioid tone may play a role in the mechanisms controlling the nonseasonal exhibition of estrous cycles in the mare.

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The reproductive tracts of nine thoroughbred mares were examined by ultrasound to determine the day of ovulation (day 0). Mares were fitted with intercavernous sinus cannulae on the day before the start of sample collection of pituitary venous effluent rich in oxytocin. Intercavernous sinus blood samples were collected for at least 36 h at 5 min intervals beginning at noon on day 13 (n = 2), day 15 (n = 5) or day 16 (n = 2) after ovulation. Concentrations of oxytocin and 13,14-dihydro-15-keto prostaglandin F2 alpha (PGFM) in plasma were determined by radioimmunoassay. Three high-magnitude surges of PGFM (> 1 ng ml-1) were found in these samples. Three high magnitude pulses of oxytocin (> 200 pg ml-1) were also observed, one associated with each of the PGFM surges. In each of these cases, the oxytocin pulse appeared to follow or coincide with the onset of the PGFM surge. Lower magnitude pulses of both hormones were detected throughout the bleeding period in every mare. The average interval between these pulses was 122.3 min for oxytocin and 121.0 min for PGFM. The interval between pulses for individual mares varied from 90 to 199 min for oxytocin, and from 87 to 213 min for PGFM. However, there was no correlation between PGFM and oxytocin pulse intervals among mares. Within each mare, there was no discernable association between low magnitude pulses of oxytocin and PGFM. From these data, it was concluded that high-magnitude surges of PGF2 alpha are associated with similar surges of oxytocin from the posterior pituitary gland, and that PGF2 alpha may induce their secretion. The posterior pituitary gland also appears to secrete oxytocin in a pulsatile manner at a frequency of approximately 1 pulse every 2 h but these pulses do not appear to be associated with the low magnitude pulses of PGF2 alpha secreted from the uterus.

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This study investigated the hypothesis that for a subpopulation of horse mares continuation of estrous cycles during the nonbreeding season may be attributed to continued stimulatory glutamatergic activity on GnRH-secreting neurons. The gonadotropin response to the glutamatergic agonist N-methyl-DL-aspartic acid (NMA) was compared in cycling and anestrous mares during the nonbreeding season. It was anticipated that the gonadotropin response to NMA in cycling mares would be attenuated, compared with that of anestrous mares. The experiment used 16 anestrous mares and 15 mares that cycled during the nonbreeding season. The effect of NMA on prolactin secretion was also evaluated. In addition, the seasonal rhythm of prolactin secretion was compared in anestrous and cycling mares during October-April. In cycling mares, the response to NMA was dependent on the stage of the cycle, and a significantly (p < 0.05) larger proportion responded during the luteal phase (6 of 8), compared with the follicular phase (1 of 7 mares). The proportion of anestrous mares that responded to NMA was similar to that of cycling mares during the luteal phase, but larger than during the follicular phase. In anestrous and cycling mares, NMA suppressed prolactin secretion, and in both groups prolactin secretion decreased during the nonbreeding season. Thus, we conclude that differences in reproductive activity in mares during the nonbreeding season are unlikely to reflect a change in glutamatergic activity.

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The purpose of this experiment was to investigate whether N-methyl-D,L-aspartate stimulated gonadotropin secretion in mares and to determine the response in two experimental paradigms where gonadotropin secretion is low or elevated. In Experiment 1, conducted during the breeding season (summer), eight long-term ovariectomized mares were treated daily for 21 d with progesterone plus estradiol (n = 4) or oil vehicle. Beginning on Day 14, each mare received, in a randomized design on alternate days, an intravenous injection of either 0, 0.5, 1.0, or 5.0 mg/kg NMA. Treatment with NMA was not accompanied by a change in gonadotropin release in vehicle-treated ovariectomized mares. In steroid-treated mares, gonadotropin values were suppressed compared with vehicle controls, but NMA only induced a significant increase in follicle-stimulating hormone (FSH) and only at the highest dose. In Experiment 2, the effect of the administration of NMA was compared in intact anestrous mares (n = 4) and long-term ovariectomized mares (n = 4) during the nonbreeding season. At the highest doses (1.0 and 5.0 mg/kg), NMA induced a significant increase in luteinizing hormone (LH) and FSH in intact mares. In contrast, pretreatment concentrations of LH and FSH were higher in long-term ovariectomized mares and NMA only increased LH, but not FSH, at the highest dose. The results demonstrate that the administration of NMA stimulates the release of LH and FSH in mares. During anestrus, ovarian intact mares appear more responsive than long-term ovariectomized mares. At high doses, NMA may induce differential release of the gonadotropins in long-term ovariectomized mares.

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Twelve horse mares were used in a repeated-measures design consisting of 3 replicates of 4 mares each. On Day 6 following ovulation, luteolysis was initiated with an i.m. injection of prostaglandin F2 alpha (PGF2 alpha; Lutalyse, 10 mg). Either 12 (-12) or 36 (-36) h before PGF2 alpha (PRE), blood samples were collected simultaneously from the intercavernous sinus (ICS) and jugular (JUG) vein at 10-min intervals for an 8-h period. Pituitary capacity to exogenous GnRH (2 micrograms/kg BW, i.v.) was evaluated at the alternate time point within this period. Frequent sampling and GnRH challenge were repeated 36 (+36) or 60 (+60) h after PGF2 alpha (POST). Progesterone reached a nadir 48 h after PGF2 alpha, while the first significant elevation in estradiol (p < 0.05) was not detected until 156 h. PRE LH and FSH pulse frequencies in ICS samples were 0.6 +/- 0.3 and 1.8 +/- 0.8 pulses/8 h, respectively. Three high-amplitude, concurrent ICS pulses of LH and FSH were observed during this period. These were associated with elevated gonadotropin levels in the corresponding JUG samples. In contrast, low-amplitude ICS pulses at this time were predominately releases of FSH alone that were not detectable in the JUG. Following luteolysis, POST gonadotropin pulse frequencies in the ICS increased (p < 0.001) to 7.4 +/- 0.4 and 6.8 +/- 0.5 pulses/8 h for LH and FSH, respectively. Concurrent LH and FSH releases of relatively low amplitude characterized 87.7% of the POST ICS pulses, none of which were measurable in corresponding JUG samples.(ABSTRACT TRUNCATED AT 250 WORDS)

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The effects of dietary energy and recombinant bovine somatotropin (bST) on pattern of LH release, follicular development, and onset of puberty were studied in 40 Angus heifers. At 7 mo of age, heifers were assigned to a 2 x 2 factorial experiment; the main effects were dietary energy (high [HDE]: 14.15 Mcal of ME/d or moderate [MDE]: 10.84 Mcal of ME/d) and somatotropin (bST; 350 mg every 2 wk or vehicle). Beginning at 9 mo of age, heifers were observed twice daily for estrous activity. From 10.5 to 12 mo of age, five heifers from each treatment group were selected for weekly ultrasound examination of ovarian structures and biweekly sequential blood sampling to determine concentrations of LH. Somatotropin treatment altered intermediary metabolism in a manner consistent with enhanced accretion of lean tissue and decreased deposition of fat. The HDE heifers were younger (P < .001) at puberty than the MDE heifers, but BW at puberty was not different among treatment groups. Age and body weight at puberty were not affected by bST. Frequency of LH pulses increased within the 10.5 to 12 mo of age sampling window in HDE-treated heifers but not in MDE heifers (dietary energy x month of age; P < .02). Secretion of LH was unaffected by bST. Ovaries of bST-treated heifers tended (P < .09) to have fewer follicles > 5 mm in diameter than those of vehicle-treated heifers. We conclude that chronic treatment with bST did not alter age at puberty or pattern of LH release in heifers and that even modest differences in energy intake influence the timing of the prepubertal increase in pulsatile LH release.

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