J. Anim. Sci. 2005. 83:1832-1838
© 2005 American Society of Animal Science
ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
Fish meal supplementation alters uterine prostaglandin F2
synthesis in beef heifers with low luteal-phase progesterone1,2
N. E. Wamsley*,
P. D. Burns
,3,
T. E. Engle* and
R. M. Enns*
* Department of Animal Sciences, Colorado State University, Fort Collins 80523; and
and
Department of Biological Sciences, University of Northern Colorado, Greeley 80639
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Abstract
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The objective of the current study was to evaluate the effect of 
-3 fatty acids in fish meal on mitigating uterine PGF2
synthesis in heifers with low luteal-phase concentrations of progesterone. Animals were individually fed a corn silage-based diet supplemented with fish meal (5% of DMI; n = 12) or corn gluten meal (6% of DMI; n = 13). Estrous cycles were synchronized using PGF2 beginning on d 25 of supplementation. Random heifers from each supplement group (n = 6 fish meal, and n = 7 corn gluten meal) were given three additional i.m. injections of PGF2 (25 mg) at 12-h intervals beginning at 0600 on d 3 after estrus to induce formation of corpora lutea that secrete lower concentrations of progesterone. Jugular blood samples were collected daily commencing on d 1 and continuing through d 16 of the estrous cycle to determine serum progesterone concentrations. Oxytocin was administered i.v. (100 IU) to heifers on d 16 after estrus to stimulate uterine PGF2 synthesis. Before statistical analyses, heifers were sorted to either normal or low luteal-phase progesterone as determined from serum progesterone on d 9 of the estrous cycle. After sorting, treatment groups consisted of 1) normal luteal progesterone + fish meal (n = 6); 2) low luteal progesterone + fish meal (n = 6); 3) normal luteal progesterone + corn gluten meal (n = 6); and 4) low luteal progesterone + corn gluten meal (n = 7). Serum concentrations of the PGF2 metabolite following oxytocin stimulation tended (P = 0.09) to be greater in heifers with low luteal-phase progesterone compared with heifers with normal luteal-phase progesterone. Fish meal supplementation mitigated this response in heifers with low luteal-phase progesterone (P < 0.05), but had no effect on heifers with normal luteal-phase progesterone. In conclusion, the -3 fatty acids in fish meal seem to decrease uterine PGF2 synthesis in heifers with low luteal-phase serum concentrations of progesterone.
Key Words: Fish Meal • Heifers • Progesterone • Prostaglandin
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Introduction
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A delayed increase in postovulatory or low luteal-phase progesterone after mating has been associated with poor embryonic development and fertility (Mann and Lamming, 1995
; Larson et al., 1997; Hommeida et al., 2004). Administration of progesterone early after mating often improves embryonic development (Garrett et al., 1988) and fertility (Mann and Lamming, 2001). Thus, low progesterone following mating may result in slower-developing embryos, leading to inadequate control of uterine PGF2 release during the period of maternal recognition of pregnancy (d 14 to 20 after breeding). Furthermore, studies show that uteri from cows exposed to lower serum progesterone after mating produce greater amounts of uterine PGF2 in response to acute stimuli such as oxytocin (Mann and Lamming, 2001). Therefore, controlling uterine PGF2 synthesis during this critical period may improve fertility in mated cows that have low concentrations of serum progesterone.
The -3 fatty acids have been reported to alter PG biosynthesis in a number of cells and tissues (Weber and Sellmayer, 1991). Fish meal is a rich source of -3 fatty acids; these fatty acids can escape ruminal biohydrogenation (Ashes et al., 1992) and become incorporated in uterine endometrium (Burns et al., 2003). Recent studies indicate that -3 fatty acids alter PGF2 biosynthesis in bovine endometrial cells in vitro (Mattos et al., 2003), and fish meal supplementation decreases oxytocin-induced uterine PGF2 synthesis in vivo (Mattos et al., 2002). Therefore, the objective of this study was to investigate the ability of fish meal to decrease PGF2 synthesis in heifers with low luteal-phase progesterone.
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Materials and Methods
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Animal and General Procedures
The Colorado State University Animal Care and Use Committee approved all animal procedures described herein. Twenty-five nulliparous Angus heifers (407.5 ± 41 kg) were transported approximately 160 km from the Colorado State University Beef Improvement Center at Saratoga, WY, to the Agricultural Research Development and Education Center feedlot facility at Fort Collins, CO, and housed in individual pens (2 x 12.5 m). Animals were stratified by BW and assigned randomly to one of two isoenergetic and isonitrogenous experimental diets (Table 1). Heifers were individually fed a corn silage-based diet and supplemented with menhaden fish meal (n = 12; 5% of DMI; SeaLac, Omega Protein, Hammond, LA) or corn gluten meal (n = 13; 6% of DMI). Experimental diets were fed as a total mixed ration once daily in the morning. Random samples of the experimental diets were collected, and fatty acid composition was evaluated using GLC (Table 1). After 25 d on the experimental diets, estrous cycles were synchronized. All heifers were given a single 25-mg injection of PGF2 (Lutalyse, Pharmacia and Upjohn Co., Kalamazoo, MI). Heifers were observed for estrous behavior twice daily at 12-h intervals. Heifers not detected in estrus within 6 d were administered a second dose of PGF2. Beginning at 0600 h on d 3 after estrus, randomly selected heifers from each treatment diet group (n = 6 fish meal, and n = 7 corn gluten meal) were given three 25-mg PGF2 i.m. injections at 12-h intervals to induce the formation of corpora lutea that secrete low levels of luteal-phase progesterone (Beal et al., 1980), thereby creating a 2 x 2 factorial arrangement of dietary treatments and progesterone concentration.
Blood Sampling and Hormonal Analyses
Jugular blood samples were collected daily beginning on d 1 and continuing through d 16 of the estrous cycle and analyzed for progesterone concentrations. Samples were allowed to clot on ice and were centrifuged within 2 h of collection at 1,500 x g at 4°C for 20 min. Serum was harvested and stored at –20°C until determination of progesterone. Samples were assayed for progesterone in duplicate using a double-antibody RIA as described by Niswender (1973). Intra- and interassay CV were 16.1 and 17.1%, respectively, for three serum pools across three assays. Sensitivity of the assay was 0.04 ng/mL.
On d 16 after estrus, each heifer was challenged with oxytocin to stimulate uterine PGF2 synthesis and release. Jugular blood samples were collected at –60, –30, – 15, and 0 min before oxytocin to establish basal PGF2 release. Oxytocin (100 IU) was administered i.v. immediately after the 0 time point collection, and jugular samples were collected at 15, 30, 45, 60, 90, and 120 min after oxytocin administration to determine oxytocin-induced PGF2 release. Serum was harvested as described previously and stored at –20°C until determination of 13,14-dihydro-15-keto-PGF2 (PGFM; the stable metabolite of PGF2). Samples were assayed for PGFM in duplicate using a double-antibody RIA as described by Silvia and Niswender (1984). Intra- and interassay CV were 8.6 and 7.8%, respectively, for three serum pools across two assays. Sensitivity of the assay was 0.05 ng/mL.
Fatty Acid Analysis
Jugular blood samples were collected from eight randomly selected heifers from each supplement group (n = 4 normal luteal, and n = 4 low luteal-phase progesterone group) on d 0 (immediately before the start of the study). Samples were then collected from the same heifers every 7 d thereafter for 35 d. Samples were collected into tubes containing EDTA, placed on ice, transported to the laboratory, and centrifuged within 1 h of collection at 1,500 x g at 4°C for 20 min. Plasma was harvested and stored at –80°C until analyzed for plasma -3 fatty acid composition using GLC.
Long-chain fatty acids in feed and plasma samples were methylated as described by Burns et al. (2003). An Agilent 6890 series gas chromatograph (Agilent Technologies, Wilmington, DE) equipped with a 6B90 series injector and flame ionization detector was used to assess fatty acid composition of feed and plasma samples as previously described (Burns et al., 2003).
Statistical Analyses
Before statistical analyses, progesterone samples were assayed, and data were plotted for each animal. It was clear that administration of PGF2 on d 3 after estrus induced low luteal-phase progesterone in some but not all PGF2-injected animals. Similarly, it was evident that some of the noninjected animals exhibited concentrations of progesterone indicative of a low luteal phase. Of those PGF2-treated heifers with apparently low luteal-phase progesterone, concentrations of progesterone were beginning to peak by d 9 after estrus. Therefore, heifers were sorted to either a low or normal luteal-phase group (within dietary supplement groups) using the following criteria: 1) mean concentrations of progesterone were established on d 9 after estrus for all heifers treated with PGF2 on d 3 after estrus and for all noninjected heifers; 2) if a heifer treated with PGF2 on d 3 after estrus exhibited d 9 progesterone concentrations greater than 1 SD above the established d-9 mean for the low PGF2-injected group, then the heifer was sorted to the normal luteal-phase noninjected group; 3) in contrast, if a noninjected heifer exhibited d-9 progesterone concentrations lower than 1 SD below the established d-9 mean for the normal treatment group, the heifer was sorted to the low-luteal-phase PGF2-injected group. Thus, after sorting, treatment groups consisted of 1) normal luteal progesterone + fish meal (n = 6); 2) low luteal progesterone + fish meal (n = 6); 3) normal luteal progesterone + corn gluten meal (n = 6); and 4) low luteal progesterone + corn gluten meal (n = 7).
Effects of dietary supplement on serum concentrations of progesterone and plasma -3 fatty acids were analyzed using Proc Mixed of SAS (SAS Inst., Inc., Cary, NC) with repeated measures. A heterogeneous autoregressive covariance structure was used to account for heterogeneous variances among repeated measures. The statistical model included day, dietary supplement, luteal-phase progesterone concentration, and all possible two- and three-way interactions. Heifer within treatment (dietary supplement x luteal-phase progesterone concentration) was used as a random variable in the statistical model. Before statistical analysis of serum concentrations of PGFM, data were examined for normality using the Proc Univariate of SAS. It was determined that the data were not normally distributed, and they were therefore transformed using the log10 of the serum concentration of PGFM estimate. The effects of dietary supplement on basal PGFM and oxytocin-stimulated PGFM were analyzed as described previously, but included time in the model. A spatial covariance structure was used in the model to account for unequal sampling periods and heterogeneous variances among repeated measures. If main effects or interactions were significant (P < 0.05), means were separated using the PDIFF option of SAS.
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Results and Discussion
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Effect of dietary supplementation on plasma eicosapentaenoic and docosahexaenoic fatty acids is shown in Figure 1. Plasma eicosapentaenoic and docosahexaenoic fatty acids did not differ between heifers with normal luteal-phase progesterone and those with low luteal-phase progesterone; however, there was a main effect of diet and a diet x time interaction for plasma eicosapentaenoic and docosahexaenoic fatty acids (P < 0.01). Plasma eicosapentaenoic and docosahexaenoic fatty acids did not differ between heifers supplemented with fish meal or those supplemented with corn gluten meal on d 0 of the experiment or on d 7 of supplementation. However, plasma eicosapentaenoic and docosahexaenoic fatty acids were greater (P < 0.01) for heifers supplemented with fish meal on d 14 of supplementation and remained greater throughout the remainder of the study.
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Figure 1. Plasma concentrations of eicosapentaenoic acid (A) and docosahexaenoic acid (B) over the first 35 d of supplementation in heifers supplemented with corn gluten meal (n = 4 low progesterone; n = high progesterone) and fish meal (n = 4 low progesterone; n = 4 high progesterone). Day 0 was immediately before supplementation was initiated. Fish meal supplementation increased plasma concentration of eicosapentaenoic and docosahexaenoic acid on d 14 through 35 compared with the concentrations in heifers fed corn gluten meal (*within day of supplementation; P < 0.05). Values are expressed as means ± SEM of the percentage of total fatty acid content.
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Long-chain -3 fatty acids can escape microbial bio-hydrogenation and become incorporated into tissues (Ashes et al., 1992). Fish meal is abundant in the -3 fatty acids eicosapentaenoic and docosahexaenoic fatty acids, and supplementation of heifers with fish meal resulted in greater plasma eicosapentaenoate and docosahexaenoate than in heifers receiving corn gluten meal. These results agree with previous work conducted in our laboratory, wherein fish meal supplementation resulted in greater concentrations of plasma eicosapentaenoic and docosahexaenoic fatty acids (Burns et al., 2002, 2003).
Effects of dietary supplementation on serum concentrations of progesterone are shown in Figure 2. Heifers assigned to the normal luteal-phase treatment group had greater (P < 0.01) serum concentrations of progesterone than heifers in the low luteal-phase group. There was no effect of diet or diet x luteal-phase interaction on serum concentrations of progesterone (Figure 2).
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Figure 2. Serum concentrations of progesterone in heifers assigned to normal luteal-phase progesterone supplemented with corn gluten meal (n = 6), low luteal-phase progesterone supplemented with corn gluten meal (n = 7), normal luteal-phase progesterone supplemented with fish meal (n = 6), or low luteal-phase progesterone supplemented with fish meal (n = 6). There was no effect of diet (P = 0.89) or diet x luteal-phase progesterone (P = 0.87) on serum concentrations of progesterone. Serum concentrations of progesterone were greater for heifers assigned to normal luteal-phase progesterone treatment group than for heifers assigned to low luteal-phase progesterone treatment group (P < 0.01). Values are expressed as means ± SEM.
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Hinckley et al. (1996) reported that eicosapentaenoic and docosahexaenoic acids decreased basal and LH-induced progesterone synthesis in cultured bovine luteal cells in vitro. The inclusion of fish meal in the diet increased plasma eicosapentaenoic and docosahexaenoic acids in the present study; however, this increase in eicosapentaenoic and docosahexaenoic acid did not affect serum concentrations of progesterone. These results agree with previous studies, in which fish meal supplementation had no effect on serum progesterone concentrations (Burns et al., 2002; Mattos et al., 2002). The discrepancy between in vitro and in vivo studies is unknown and warrants further investigation.
Effect of dietary supplementation on basal (–60, –30, –15, and 0 min time points) and oxytocin-stimulated (15, 30, 45, 60, and 120 min after oxytocin) PGFM secretion is shown in Figure 3. There was no effect of diet or luteal phase on basal secretion of PGFM. However, there tended (P = 0.09) to be a diet x luteal phase interaction for basal concentration of PGFM. Heifers with low luteal-phase progesterone and supplemented with corn gluten meal tended (P = 0.09) to have greater basal concentrations of PGFM than heifers in the other treatment groups. Serum concentrations of PGFM following oxytocin administration tended (P = 0.09) to be greater for heifers with low luteal-phase progesterone compared with heifers with normal luteal-phase progesterone. There was a significant (P < 0.05) diet x luteal-phase interaction for serum concentration of PGFM. Serum concentrations of PGFM following oxytocin administration did not differ among heifers supplemented with corn gluten meal and with normal luteal-phase progesterone, heifers supplemented with fish meal and with low luteal-phase progesterone, or heifers supplemented with fish meal and with normal luteal-phase progesterone. In contrast, heifers with low luteal-phase progesterone that were supplemented with corn gluten meal secreted greater amounts of PGF2 in response to oxytocin (P < 0.05) than heifers in the other three treatment groups.
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Figure 3. Serum concentrations of prostaglandin F2 metabolite (PGFM) before (basal release; –60, –30, –15, and 0 min) and after oxytocin administration (oxytocin-induced uterine release; 15, 30, 45, 60, 90, and 120 min) in heifers with normal luteal-phase progesterone supplemented with corn gluten meal (n = 6), low luteal-phase progesterone supplemented with corn gluten meal (n = 7), normal luteal-phase progesterone supplemented with fish meal (n = 6), and low luteal-phase progesterone supplemented with fish meal (n = 6) on d 16 of the estrous cycle. Oxytocin (100 IU; i.v.) was administered immediately after collection of the 0 time point. There was no effect of dietary supplement (P = 0.40) or luteal-phase progesterone (P = 0.65) on basal concentrations of PGFM. There tended (P = 0.09) to be an effect of dietary supplement x luteal-phase progesterone on basal concentrations of PGFM. Basal concentrations of PGFM tended to be greater for heifers with low progesterone and supplemented with corn gluten meal than for the other treatment groups. There was no effect of dietary supplement on oxytocin-induced uterine PGFM release (P = 0.31); however, there tended to be an effect of luteal-phase progesterone (P = 0.09) and an effect of dietary supplement x luteal-phase interaction (P < 0.05). Oxytocin-induced uterine PGFM release was greater in heifers with low luteal-phase progesterone and supplemented with corn gluten meal than for the other treatment groups. Values are expressed as means ± SEM.
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In the current study, oxytocin-induced uterine PGF2 release was greatest in cows that had low luteal-phase progesterone and were supplemented with corn gluten meal. These results are in agreement with other studies, in which low luteal-phase progesterone resulted in a uterine environment that secreted greater PGF2 in response to oxytocin (Mann and Lamming, 1995, 2001). Fish meal supplementation mitigated oxytocin-induced uterine PGF2 synthesis in heifers with lower luteal-phase progesterone, which agrees with other reports, in which fish meal supplementation decreased oxytocin-induced uterine PGF2 synthesis (Thatcher et al., 1997; Mattos et al., 2002). However, in the present study, fish meal supplementation did not mitigate oxytocin-induced PGF2 synthesis in heifers with normal luteal-phase progesterone. The mechanism(s) by which fish meal mitigate(s) uterine PGF2 synthesis, especially in animals with low luteal-phase progesterone, is unknown.
The -3 fatty acids in fish meal are likely responsible for mitigating uterine PGF2 synthesis. These fatty acids have been reported to inhibit PG synthesis in a number cell types and tissues (Weber and Sellmayer, 1991). The -3 fatty acids have been reported to inhibit cyclooxygenase gene expression in several tissues that secrete PG in response to acute stimuli (Gilbert et al., 1999a,b; Obata et al., 1999). However, the -3 fatty acids do not seem to affect cyclooxygenase gene expression in bovine endometrium cells (Mattos et al., 2001). We have reported that fish meal supplementation increases eicosapentaenoic acid, while decreasing arachidonic acid in uterine endometrium (Burns et al., 2003). Therefore, fish meal supplementation may result in less available arachidonic acid for PGF2 synthesis, causing decreased PGF2 synthesis following oxytocin stimulation as observed in the present study and reported by others (Thatcher et al., 1997; Mattos et al., 2002). It also is possible that eicosapentaenoic and docosahexaenoic acids decrease PGF2 synthesis by inhibiting cyclooxygenase activity. In rat hepatoma cells, both of these fatty acids cause a reduction in cyclooxygenase activity (Larsen et al., 1997). Clearly, fish meal supplementation mitigates uterine PGF2 synthesis, especially in animals with low luteal-phase progesterone. The mechanism(s) by which the -3 fatty acids in fish meal regulate(s) PG biosynthesis is still unknown and requires further investigation.
Fish meal supplementation has been shown to improve reproductive performance in lactating dairy and beef cows (Armstrong et al., 1990; Burke et al., 1997; Burns et al. 2002). In the present study, 42% of the heifers spontaneously developed corpora lutea that secreted low concentrations of progesterone. Exposing the uterus to low concentrations of progesterone can lead to poor embryonic development (Garrett et al., 1988) and increased risk of failure of maternal recognition of pregnancy (Mann and Lamming, 2001; Hommeida et., 2004). We suggest that the -3 fatty acids in fish meal may decrease the strength of the luteolytic signal in cows that spontaneously develop corpora lutea that secrete low levels of progesterone during the period of maternal recognition of pregnancy. Decreased uterine PGF2 secretion during this critical period may save slow-developing embryos that otherwise would not properly signal maternal recognition of pregnancy.
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Implications
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The addition of fish meal to the diet suppressed oxytocin-induced uterine prostaglandin F2 secretion in heifers that had low, but not in heifers that had normal, luteal-phase progesterone. Fish meal supplementation may improve fertility in cows with low luteal-phase progesterone following mating by suppressing uterine prostaglandin F2 release during the period of maternal recognition of pregnancy.
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Footnotes
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1 This research was supported in part by a grant from the Colorado State Univ. Agric. Exp. Stn. 
2 The authors thank J. Johnson of Omega Protein (Hammond, LA) for the donation of fish meal and inputs on the design of this experiment.
3 Correspondence: 2536 Biological Sciences (phone: 970-351-2695; fax: 970-351-2335; e-mail: patrick.burns{at}unco.edu).
Received for publication December 16, 2004.
Accepted for publication April 16, 2005.
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Abstract
Introduction
