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Regular Article |
a Animal Reproduction Laboratory, Texas A&M University Agricultural Research Station, Beeville, Texas 78102
b Center for Animal Biotechnology and Genomics and
c Department of Animal Science, Texas A&M University, College Station, Texas 77843
d Department of Animal Science, University of Missouri, Columbia, Missouri 65211
ABSTRACT
In the present study, we tested the hypothesis that short-term fasting would reduce leptin gene expression, circulating leptin, and LH pulsatility in prepubertal heifers in association with a decrease in circulating concentrations of insulin and insulin-like growth factor I (IGF-I). Twelve prepubertal crossbred heifers (mean ± SD = 315 ± 5 kg body weight) were assigned randomly to one of two treatments in two replicates: 1) control; normal feed consumption (n = 6) and 2) fasted; 48 h of total feed restriction (n = 6). Blood samples were collected at 15-min intervals for 8 h on Days 0 and 2 of the experiment and twice on Day 1. Subcutaneous fat samples were collected before treatment onset (Day -1) and at the end of the intensive blood sampling on Day 2. Acute feed restriction markedly reduced leptin mRNA in adipose tissue (P < 0.01) and circulating concentrations of leptin (P < 0.05), IGF-I (P < 0.01), and insulin (P = 0.05) as compared with controls on Day 2. Moreover, the treatment x day interaction (P < 0.076) and within-day contrasts (expressed as a percentage of Day 0 values) revealed that the mean frequency of LH pulses in the fasted group was lower (P < 0.06) than in controls on Day 2. Neither mean concentrations of growth hormone (GH) nor GH secretory dynamics were affected by acute feed restriction. Fasting-mediated decreases in leptin gene expression and circulating leptin, in association with reductions in secretion of IGF-I, insulin, and LH, provide a basis for investigating leptin as a hormone signaling energy status to the central reproductive axis in cattle.
anterior pituitary, gene regulation, growth hormone, insulin, leptin, luteinizing hormone, neuropeptides/neurotransmitters, pituitary, pituitary hormones, puberty
INTRODUCTION
Nutrition plays a major role in controlling reproductive processes. However, the physiological mechanisms through which nutrition mediates its effects are not well understood. Clearly, changes in the availability of nutrients are perceived by the hypothalamus and influence gonadotropin secretion via effects on hypothalamic GnRH release [1, 2]. Nutritional signaling pathways engendering both neural and endocrine components have been implicated in this process [3–7]. Recently, leptin, a hormone synthesized and secreted mainly by adipocytes and positively correlated with body mass index in humans [8], has been proposed as a metabolic signal for reproduction. In rodents and humans, short-term fasting reduces leptin synthesis and secretion [9–12] and suppresses circulating LH [13, 14]. Moreover, leptin treatment prevented the decrease in circulating LH induced by 48-h fasting in ovariectomized, estradiol-implanted mice [15, 16].
Whether leptin plays a central role in regulating reproduction in cattle has not been determined. However, body energy reserves have a profound influence on reproduction in this species, including the process of sexual maturation in heifers [7]. Therefore, an important first step in defining leptin's actions would be to determine if leptin gene expression and circulating leptin are affected by nutritional perturbations in cattle. If so, are these changes associated with other metabolic sequelae known to influence the hypothalamic-pituitary axis? For example, acute feed restriction results in a reduction in circulating insulin, glucose, and insulin-like growth factor I (IGF-I) in cattle [17, 18]. Moreover, heifers fed to gain weight at low rates had lower concentrations of LH, IGF-I, insulin, and glucose than those gaining at a higher rate, and puberty was delayed [19].
We tested the hypothesis that short-term fasting would reduce leptin gene expression in adipose tissue and circulating concentrations of leptin in prepubertal heifers coincident with 1) declines in circulating insulin and IGF-I and 2) a heightened restraint of the prepubertal pattern of LH secretion.
MATERIALS AND METHODS
All animal-related procedures employed in this study were approved by the Institutional Agricultural Animal Care and Use Committee (IAACUC) of The Texas A&M University System (protocol 8-122).
Animal Model
Twelve prepubertal (Brahman x Hereford, F1) heifers 11–12 mo of age and weighing (mean ± SD) 315 ± 5 kg were used for this study. Average body condition score was 5.5 ± 0.2 on a scale of 1 to 9 (1 = emaciated; 9 = obese). Heifers were maintained in pens measuring 25 x 9 m, with pubertal status assessed on a continual basis using serum progesterone concentrations in twice weekly blood samples. Blood sampling for progesterone analysis began 2 mo before the beginning of the experiment and continued until puberty in all heifers was confirmed after completion of the experiment. Concentrations of progesterone did not exceed 0.2 ng/ml at any time before or during the experiment. On average, heifers reached puberty 64.5 ± 14.9 days after completion of the study, based upon occurrence of serum progesterone values 
1 ng/ml.
Animal Procedures
Each heifer was assigned to a treatment in one of two replicates (n = 6 heifers/treatment) conducted approximately 1 wk apart during March: 1) control, Coastal bermudagrass hay and water ad libitum plus 3.6 kg daily of a concentrate formulated to promote 0.68 kg gain/day (n = 3 heifers/replicate); and 2) fasted, 48-h total feed restriction and free access to water (n = 3 heifers/replicate). On the day before the onset of treatments (Day -1), a subcutaneous fat sample lateral to the tail head was collected by aseptic biopsy performed under epidural anesthesia using 2% lidocaine HCl. Adipose tissue samples were snap frozen in liquid nitrogen and stored at -80°C until Northern blot analysis for leptin mRNA. At the same time (Day -1), jugular catheters (polyethylene tubing, 1.4 mm inside diameter, 1.9 mm outside diameter; Becton Dickinson, Parsippany, NJ) were placed into each heifer for intensive blood sampling. Intensive sampling was performed while heifers were loosely tethered in a stanchion inside the Animal Reproduction Laboratory. Otherwise, heifers remained in outside pens as previously described. Blood was collected at 15-min intervals for 8 h on Days 0 and 2 after control heifers had received their morning feed, with twice daily samples collected approximately 8 h apart on Day 1. Samples were placed immediately on ice, and plasma was harvested by centrifugation within 2 h of collection. Plasma was stored at -20°C until RIA of hormones. A second subcutaneous fat biopsy was performed at the end of the experiment (Fig. 1).
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Radioimmunoassays
Circulating concentrations of LH and growth hormone (GH) were determined as previously described [20, 21] for samples collected at 15-min intervals for 8 h on Days 0 and 2. Plasma concentrations of IGF-I were determined by RIA as previously described [21] in samples collected every hour for 8 h on Days 0 and 2 and in two samples collected 8 h apart on Day 1. Plasma concentrations of insulin were determined as previously described [22] in three samples collected every 4 h on Days 0 and 2 and in two samples collected 8 h apart on Day 1. Plasma concentrations of progesterone (twice weekly samples) were determined with the Coat-A-Count direct assay (Diagnostic Products, Los Angeles, CA) as previously reported [23], and serum estradiol-17ß (twice daily samples) was assayed in extracted samples [24]. Intra- and interassay coefficients of variation (CVs) for the above assays were 9–12% and 10–21%, respectively. Plasma concentrations of leptin were determined in one assay in the same samples collected for assay of insulin. Sensitivity of the assay for values reported herein was 0.3 ng/ml, with an intraassay CV of 5.1%. The RIA is highly specific for ovine leptin, and a detailed summary of its validation for use in bovine serum has been published [25]. Recombinant ovine leptin, which was produced and purified by SDS-PAGE to >98% purity [25], was utilized both as the iodinated tracer and as the standard. This preparation exhibits 97% homology to bovine leptin [26]. An ovine leptin antiserum was utilized [25], and dilutions of bovine plasma were parallel (r = 0.99) to the ovine standard. Recovery of recombinant ovine leptin added to bovine serum (endogenous concentration = 2 ng/ml) to yield low (3 ng/ml) or high (7 ng/ml) final concentrations was 101% ± 10%.
Northern Blot Analysis
Total cellular RNA was isolated from 0.6 g of subcutaneous adipose tissue using the Totally RNA kit (Ambion, Austin, TX) according to the manufacturer's instructions with one modification; excess lipids from tissue homogenates were removed using chloroform:isoamyl (24:1) extraction. Sixteen micrograms of RNA was loaded on 1% agarose gels, separated by electrophoresis, and transferred onto nylon membranes. Ultraviolet transillumination of ethidium bromide-stained RNA was used to quantify 18S rRNA bands using a Fluor-S MultiImager System (Bio-Rad Laboratories, Hercules, CA). Blots were hybridized with a random-primed 32P-labeled DNA probe generated from a 350-bp ovine leptin cDNA [27] (GenBank accession U62123). Hybridization signals were quantitated with an Instant Imager (Packard Instrument Co., Downers Grove, IL) and normalized with 18S rRNA.
Statistical Analysis
Mean concentrations of LH, GH, leptin, IGF-I, insulin, and estradiol-17ß were analyzed by ANOVA for repeated measures using the SAS PROC GLM procedure [28]. Treatment, day, replicate, heifer (treatment x replicate), and appropriate interactions were sources of variation. Both visual inspection and a pulse detection algorithm [29] were used to determine the frequency and amplitude of LH and GH pulses. In all cases, both LH and GH pulses were visually unambiguous (rapid increase above baseline within not more than two samples and exponential decay), and results of statistical detection were identical to those determined visually. Because of random differences in pulse frequency between groups on Day 0, both analysis of covariance (ANCOVA) and transformation of pulse frequency values to a percentage of Day 0 values were used to compare treatment means on Day 2. When a significant treatment x day interaction was detected by standard ANOVA, either the least significant means or t-test procedure of SAS was used to compare means within day. Leptin mRNA data were transformed to percentage of Time 0 (Day -1) and analyzed by the t-test procedure of SAS. Serum progesterone values <1 ng/ml were considered indicative of the absence of a corpus luteum.
RESULTS
One control heifer developed a respiratory infection during the experiment and was eliminated from all analyses. Plasma concentrations of progesterone were <0.2 ng/ml throughout the experiment in all heifers. Serum estradiol averaged 0.5 ± 0.04 pg/ml in both groups during the experiment and was not influenced by dietary treatment, day of the experiment, or the interaction of these two variables.
Leptin Gene Expression and Circulating Leptin
Representative blots of leptin mRNA in adipose tissue from four control and four fasted heifers are shown in Figure 2A. Computerized image analyses of the blot normalized to 18S rRNA showed that fasting reduced (P < 0.01) leptin mRNA expression in adipose tissue by 42% on Day 2 (Fig. 2A). Circulating concentrations of leptin declined (P < 0.05) within the fasted group between Days 0 and 2 (Fig. 2B) and were lower (P < 0.05) in the fasted than in the control group on Day 2.
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Plasma Insulin, IGF-I, and GH
Plasma concentrations of insulin were reduced (P = 0.05) by short-term fasting (Fig. 3A) on Day 2 relative to controls. Within the fasted group, plasma insulin had declined (P < 0.05) 47% by Day 2, whereas in controls plasma insulin actually increased (P < 0.05). A 50% reduction (P < 0.01) in circulating concentrations of IGF-I was also observed in the fasted group on Day 2 as compared with controls (Fig. 3B). Mean plasma concentrations, pulse amplitude, and frequency of GH pulses determined in blood samples collected at 15-min intervals for 8 h on Days 0 and 2 were not affected by short-term fasting (Table 1).
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Pulsatile Release of LH
The frequencies of LH pulses in control and fasted animals did not differ on Day 0 (P > 0.10; Fig. 4). However, a 65% reduction (P < 0.01) in the frequency of LH pulses/8 h occurred in the fasted group between Day 0 (2.3 ± 0.4 pulses/8 h) and Day 2 (0.8 ± 0.3 pulses/8 h), whereas pulse frequency in the control group did not change (1.5 ± 0.3 pulses/8 h versus 1.4 ± 0.3 pulses/8 h). Because of a tendency for pulse frequency in the fasted group to be greater than that for the controls on Day 0 (Fig. 4), we examined main effects of treatment using two approaches. In the first method, we employed an ANCOVA to adjust for differences between groups before fasting on Day 0, and in the second method, we converted values on Day 2 to a percentage of those on Day 0. After adjustment for differences on Day 0 using the ANCOVA, a treatment x day interaction (P < 0.076, Fig. 4) remained, indicating that values for fasted heifers (0.8 ± 0.3 pulses/8 h) were lower than those of controls (1.4 ± 0.3 pulses/8 h) on Day 2. Contrasts of values on Day 2 expressed as a percentage of those on Day 0 confirmed a lower (P < 0.06) pulse frequency in fasted than in control heifers. In both fasted and control heifers, neither mean plasma concentrations of LH (2.9 ± 0.2 ng/ml and 2.8 ± 0.2 ng/ml) nor amplitude of LH pulses (5.1 ± 0.3 ng/ml and 5.6 ± 0.5 ng/ml) were affected by treatment, day, or the interaction of these two variables.
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DISCUSSION
Using highly specific reagents for ovine/bovine leptin, we found that leptin gene expression and circulating leptin are markedly reduced in the fasted prepubertal heifer. These observations provide a basis for examining leptin as a metabolic signal regulating the central reproductive axis of cattle. Tsuchiya et al. [30] also observed a decrease in leptin gene expression in subcutaneous adipose tissue of cattle fasted for 2 days, but effects on circulating leptin, gonadotropin secretion, or metabolic changes were not reported. In rodents, leptin plays a significant role in regulating gonadotropin secretion. Genetically obese (ob/ob) mice, which are leptin deficient due to a mutation in the gene encoding leptin, are infertile and have atrophic reproductive organs [31]. Treatment of female ob/ob mice with leptin restored circulating concentrations of LH and development of reproductive tissues to normal [31]. In genetically normal mice [32, 33] and rats [34], leptin administration advanced the onset of puberty. Reports in mice [9], rats [10], hamsters [11], and humans [12] indicate that leptin synthesis/secretion is acutely responsive to changes in nutritional status. In addition, the fasting-induced decrease in LH secretion was prevented by leptin treatment in mice and rats fasted for 48 h [15, 16]. Using a multispecies RIA (Multispecies Leptin Assay; Linco Research, St. Louis, MO) employing human leptin as tracer and standard hormone, we previously found no correspondence between fasting-induced decreases in leptin gene activity and plasma concentrations of leptin [35]. However, using a highly specific ovine RIA validated for determination of bovine leptin [25], we were able to detect significant reductions in circulating concentrations of leptin in heifers fasted for 48 h. The basis for the discrepancy between the two assays appears to be related to assay sensitivity. The ruminant RIA is approximately 3.8 times more sensitive than the human assay, as evidenced by the fact that up to 2.5 ng of human leptin is required to displace iodinated ovine leptin from anti-ovine leptin antibodies [25]. With leptin values in the current study averaging 0.9 to 1.5 ng/ml, the ruminant-specific assay was clearly more valuable for detecting nutritionally mediated changes.
It has been proposed that the effects of leptin on gonadotropin secretion might be mediated in part by neuropeptide Y (NPY). Neuropeptide Y has been shown to regulate gonadotropin release by inhibiting LH secretion in ewes [36], and feed-restricted, ovariectomized ewes had increased NPY mRNA in the hypothalamus [37]. Recently, we have shown that intracerebroventricular infusions of NPY suppress LH secretion in ovariectomized, estradiol-implanted cows in association with reductions in amplitude and frequency of cerebrospinal fluid GnRH pulses [38]. Moreover, leptin receptor mRNA has been detected in the hypothalamus of the ewe [39] colocalizing with NPY on neurons located in the arcuate nucleus of the hypothalamus [40]. Currently, we are investigating the effects of cerebroventricular infusion of leptin on patterns of LH secretion. Preliminary results show that centrally infused recombinant ovine leptin increases mean concentrations of LH in ovariectomized, estradiol-implanted cows (unpublished data).
Our observation of a reduction in mean concentrations of insulin after fasting (Day 2) is similar to that reported by McCann and Hansel [17] in feed-restricted heifers. The role of insulin in regulating hypothalamic-pituitary function is controversial. Daniel et al. [41] reported an increase in mean concentrations of LH with chronic lateral cerebroventricular infusions of insulin or insulin plus glucose in feed-restricted ewes. However, results of an earlier study of feed-restricted ewes failed to support these observations [42]. Moreover, when insulin was centrally infused during a period of increased feed intake, a decrease in mean concentration and frequency of LH pulses was observed. Therefore, although a decline in serum insulin was observed in the current study, it is not clear whether insulin directly modulates gonadotropin secretion under fasting conditions. Nevertheless, insulin appears to play a role in regulating synthesis and secretion of leptin. Adipose tissue fragments cultured in the presence of insulin increased both synthesis and secretion of leptin into the medium [43]. Conversely, synthesis and secretion of insulin may be affected by leptin; leptin receptors have been found in insulin-secreting cells of the pancreas, and leptin inhibited insulin secretion in perfused rat pancreas [44]. In the current study, fasting caused coincident decreases in insulin, leptin gene expression, and circulating leptin.
Insulin-like growth factor I has been associated with pubertal development in cattle. Yelich et al. [19] reported an increase in circulating concentrations of IGF-I in heifers with the approach of puberty. Heifers fed to gain 0.23 kg/day had lower serum concentrations of IGF-I than heifers fed to gain 1.36 kg/day, and puberty was delayed in heifers fed to gain at the slower rate. However, at puberty, circulating concentrations of IGF-I did not differ between the two groups. Moreover, heifers immunized against GH-releasing factor had decreased circulating concentrations of IGF-I and puberty was delayed [45]. In vitro studies have shown that IGF-I induces an increase in GnRH-stimulated LH release from rat anterior pituitary cell cultures [46, 47]. Receptors with high specificity for IGF-I are located in the median eminence of the brain in rats [48], and 125I-labeled IGF-I was found to cross the blood-brain barrier and accumulate in the paraventricular nucleus and median eminence [49]. Hiney et al. [50] observed an increase in LH secretion and an advancement in the onset of puberty in prepubertal rats that received infusions of IGF-I into the third ventricle. Collectively, these studies suggest that IGF-I may act centrally in the hypothalamus and/or pituitary to modulate gonadotropin secretion. Chronic or acute feed restriction in cattle leads to a reduction in circulating concentrations of IGF-I [18, 21, 51]. In agreement with a previous report [18], we observed a marked reduction in the circulating concentrations of IGF-I with short-term fasting.
In our study, GH secretion was not affected by short-term fasting. Mean concentrations, pulse amplitudes, and frequencies of GH pulses did not change in fasted heifers, suggesting that GH is not acutely regulated by energy status in ruminants. These results support those of earlier studies showing that GH secretion was unaffected when metabolism of oxidizable fuels (glucose and fatty acids) was acutely blocked by 2-deoxyglucose and methyl palmoxirate in feed-restricted sheep [52].
Historically, short-term restriction of nutrients has been considered less disruptive to the central reproductive axis in ruminants than in monogastric animals. Acute fasting in humans [13] and rodents [14] causes immediate decreases in mean concentrations of LH. In the male rhesus monkey, mean concentrations of LH were diminished by skipping a single daily meal [53]. In contrast, studies in ruminants [17, 54] have failed to show changes in LH secretion in mature cyclic females after 2 days of fasting. In our study, we observed a decline in the frequency of LH pulses in relatively lean, growing, prepubertal heifers fasted for 48 h. This finding suggests that hypothalamic-pituitary function of immature female cattle is exquisitely sensitive to acute perturbations in energy status. However, mean concentrations of LH were not affected. An integral component of sexual maturation in heifers, and in other mammals, is a marked increase in the frequency of LH pulses, which reflects a decrease in negative feedback sensitivity to estradiol [55]. When frequency increases from a prepubertal low (0.1 to 0.2 pulses/h) to >1 pulse/h, functional increases in mean LH concentrations can be expected. However, the magnitude of the negative change in frequency observed in the current study would not be sufficient to affect LH concentrations [55]. Therefore, failure to detect such changes was not surprising.
The results of the present study demonstrate that short-term fasting is able to induce a decrease in synthesis of leptin in adipose tissue and in circulating leptin of prepubertal heifers. This effect occurs coincident with reductions in circulating concentrations of insulin and IGF-I and a diminished frequency of LH pulses. We report here for the first time a strong association between synthesis of a putative hormonal messenger of nutrient status, the circulating protein product of this messenger, and gonadotropin secretion in cattle. These observations, which are similar to those in mice and rats, provide a strong impetus to consider further the role of leptin in the regulatory cascade linking energy balance to the central reproductive axis in cattle.
ACKNOWLEDGMENTS
We acknowledge the National Pituitary Hormone Program for pituitary hormones, and Dr. Jerry Reeves for the LH antisera. We also acknowledge with gratitude the excellent technical assistance of Mark Besancon, Melvin Davis, Randle Franke, Marsha Green, Charles Johnson, Michelle Stamm, and Brad Thedin.
FOOTNOTES
First decision: 6 December 1999.
1 Supported by Project H-6881 of the Texas Agricultural Experiment Station. 
2 Correspondence: G.L. Williams, Animal Reproduction Laboratory, Texas A&M University Agricultural Research Station, 3507 Hwy 59 E, Beeville, TX 78102-9410. FAX: 361 358 4930; glw{at}fnbnet.net
Accepted: February 15, 2000.
Received: November 9, 1999.
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