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PHYSIOLOGY, ENDOCRINOLOGY, AND REPRODUCTION |
* Department of Animal and Poultry Science, and Department of Pathobiology, Ontario Veterinary College, University of Guelph, Ontario, Canada N1G 2W1
1 Corresponding author: gbedecar{at}uoguelph.ca
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: hen • serial blood sampling • gonadotropin
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are key mediators of reproduction in both birds and mammals, and their biosynthesis and release is tightly regulated by several factors, including gonadotropin-releasing hormone (GnRH). Gonadotropin-releasing hormone is a decapeptide synthesized by the hypothalamus, released into the hypophyseal portal vascular system, and carried by the blood to the anterior pituitary, where it binds to its specific G-coupled receptors. Different forms of GnRH have been isolated in vertebrates and birds, and 3 types of GnRH (chicken GnRH-I, chicken GnRH-II, and an immunoreactive form similar to lamprey GnRH-III) have been reported in avian species (King and Millar, 1982a,b; Miyamoto et al., 1982, 1984; Bentley et al., 2004). Although their relative role is still controversial, all 3 GnRH have been shown to stimulate the synthesis and release of LH in the chicken, with GnRH-II being the most potent (Proudman et al., 2005). However, localization (Katz et al., 1990; Sharp et al., 1990) and active immunization studies (Sharp et al., 1990) suggest that GnRH-I is the form acting on the pituitary to stimulate the reproductive axis.
In mammals, GnRH release is pulsatile, and continuous infusion down-regulates the synthesis and release of gonadotropins (Belchetz et al., 1978; Marshall and Kelch, 1986; Gharib et al., 1990). It has been shown that varying GnRH pulse frequencies differentially regulate the synthesis and release of LH and FSH, with low and high GnRH pulse frequency preferentially stimulating FSH and LH, respectively (Wildt et al., 1981; Haisenleder et al., 1991; Kaiser et al., 1997; Burger et al., 2002; Bédécarrats and Kaiser, 2003).
In immature as well as laying turkey hens, the secretion of LH, progesterone, and testosterone is pulsatile (Chapman et al., 1994; Yang et al., 1997; Liu et al., 2001a,b, 2002). Similarly, in male broiler breeders, LH secretion is also pulsatile, and LH pulses correlate with testosterone episodes, whereas FSH pulses are unique (Vizcarra et al., 2004). In vitro, perfusion of hypothalamic sections also results in GnRH pulsatile release (Li et al., 1994), and stimulation of diced pituitary glands with pulsatile GnRH at a frequency over 1 pulse per 30 min results in desensitization (Guémené and Williams, 1992,1999). In laying birds, ovulation occurs daily and is triggered by a preovulatory LH surge (Furr et al., 1973; Lague et al., 1975; Etches and Cunningham, 1977; Proudman et al., 1984; Liu et al., 2004b). However, blind surges also occur in between preovulatory surges, and it has, therefore, been difficult to detect individual LH pulsatility in actively laying hens (Yang et al., 1997). To the best of our knowledge, no data are currently available regarding LH pulsatility in older hens. Therefore, in this study, we determined the LH concentration in blood samples collected from 83-wk-old broiler breeders every 10 min over a 6-h period using our VAP serial bleeding technique.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Implantation of VAP
The VAP used in this study (Rat-O-port model ROP–3.5H) were purchased from Access Technologies (Skokie, IL). They were composed of a polysulfone reservoir chamber connected to a silicone catheter (0.6-mm i.d. x 1.1-mm o.d. x 15 cm; Figure 1A
). Birds were fasted for several hours before surgery, and 15 to 20 min before anesthesia, a 2 mg/kg dose of butorphanol was given i.m. as pre-medication analgesic. Anesthesia was induced with 4.0% isoflurane in an oxygen mixture using a face mask and was further maintained with a 1 to 1.5% mix using an endotracheal tube. Once anaesthetized, birds were placed on a warmed oat bag in a left-lateral recumbence position. Body temperature, respiratory rates, and heart rates were monitored continuously during the entire surgical procedure. The neck area was cleaned with 4% chlorhexidine, feather tracts over the right jugular were parted, and few feathers were occasionally plucked. After sterilization of the area with 70% ethanol, a 2- to 3-cm skin incision was made over the jugular furrow. The jugular vein was then carefully exteriorized from its surrounding connective tissue and from the vagus nerve as well as other small blood vessels. Encircling sutures (5 polydioxanone surgical sutures) were placed around the jugular vein, and the cranial suture was tied off to occlude blood flow. While pressure was applied on the caudal end to prevent blood backflow, a 0.5- to 0.8-mm transverse venotomy was performed using fine iris scissors, and the catheter tip of a previously flushed (heparinized saline, 1 IU/mL) VAP was advanced through the vein to the junction of the anterior vena cava and the right atrium. The caudal suture was then tightened, and the catheter was securely sutured above and below the venotomy site to prevent any movement (Figure 1, panel B). To test the effectiveness of the VAP, a blood sample was withdrawn through the chamber, and the port was flushed with heparinized saline. To fix the VAP chamber in its final location under the neck skin, an s.c. pocket was made dorsal to the skin incision on the lower right side of the neck, and the chamber was sutured in place to the underlying neck muscle using Surgipro 4-0 (Tyco Healthcare, Mansfield, MA). A short loop of catheter was left loose between the extra vascular portion and the chamber to allow for neck movement, stretching, and to prevent dislodgement of the catheter. The skin was sutured by simple continuous pattern using 4-0 polydioxanone surgical sutures. A schematic representation of a VAP in place is shown in Figure 1, panel C.
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Remote Serial Blood Collection Procedure
For blood collection, right-angle "Huber" point needles (22 gauge) were connected to Tygon microtubing (50 cm long, 4 mm in diameter; Access Technologies). Prior to use, the line and needle were flushed with heparinized saline. The needle was then inserted through the skin and the septum into the port’s chamber. The extension line was passed through the top of the cage and connected to a 3-way stopcock valve with a luer lock. The second opening of the valve was connected to a 20-mL syringe reservoir containing heparin saline used to flush the line after each blood withdrawal, and the third opening of the valve was used for blood collection. A schematic representation of a VAP with the extension line is shown in Figure 2, panel A. For each sample collection, excess saline was first removed from the line, 1 mL of blood was withdrawn using 1.2-mL lithium–heparin blood S-monovette collection tubes (Sarstedt, Nümbrecht, Germany), and the line was then flushed with heparinized saline. Blood cells were separated by centrifugation (1,000 x g, 15 min), and plasma samples were stored at –80°C for analyses. Blood cells were resuspended in an equivalent volume of saline and returned to the hen of origin. During a 6-h serial sampling protocol (blood collection every 10 min), reconstituted cells from 9 consecutive samples were returned to the hen every 90 min. At the end of 6 h of blood collection, 250 mg of ampicillin were infused to the bird through the VAP. During serial blood sampling, birds were unrestrained, free to move in their pen, and had free access to food and water (Figure 2, panel B).
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Iodination was done by mixing 5 µg of chicken LH (USDA-cLH-1–3) in 10 µL of distilled water with 20 µL of 0.25 M sodium phosphate buffer (pH 7.4.), 10 µL of freshly prepared chloramine T (58 µg/mL in sodium phosphate buffer, pH 7.4), and 5 µL of Na 125I (500 µCi). After 10 min of incubation, this mixture was transferred to a Sephadex G-25 column (Sigma-aldrich, St. Louis, MO) to separate radio-labeled LH from free 125I.
Standards were prepared by serial dilutions of USDA-cLH-K-3 (ranging from 0.031 to 8 ng/tube) in 500 µL of PBS-BSA, and plasma samples (100 µL) were adjusted to 500 µL with the same buffer. Antiserum against LH (USDA-AcLH-5; final dilution of 1:20 000) and 30,000 cpm of iodinated LH were then added, and assay tubes were incubated for 48 h at 4°C. Secondary antibody (goat anti-rabbit gamma globulin) was added at a 1:20 final dilution, and assay tubes were incubated for an additional 16 h at 4°C. On the final day, 3 mL of PBS was added to each tube (except total counts), and samples were centrifuged at 1,900 x g for 30 min. Pellets were resuspended in an additional 1 mL of PBS, and tubes were centrifuged for 15 min at 1,900 x g. Finally, radioactivity was measured in pellets using a gamma counter. Each sample was assayed in duplicate in a single assay with an intraassay CV of 17%.
Statistical Analysis
Luteinizing hormone pulsatility was analyzed by cluster analysis (Veldhuis and Johnson, 1986) using Pulse_XP, an integrated software package (hormone pulsatility data analysis, Version 1.0001, University of Virginia, Charlottesville). Cluster sizes were 2 by 1 and a t-statistic of 2. For the variance model, each time point minimum-detectable concentration for the assay was set at 0.0325 ng, and a CV of 17%, as well as the average concentrations of duplicates for particular time points, were used.
Differences in the pulse width, amplitude, and increase above the basal among the 5 hens were analyzed by 1-way ANOVA followed by Tukey’s posthoc test if needed. Results shown in Table 1 are expressed as mean ± SD.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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. After surgery, none of the birds showed any sign of infection or postsurgical complication, and, with the exception of 1 hen that developed a leg injury after 3.5 mo, all other birds were kept for 5 mo before being euthanized. The absence of BW loss, despite feed restriction (Table 2), is also an indicator of low stress and good general health during the experimental period.
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Effectiveness of VAP was evaluated weekly by withdrawal of a blood sample, and to avoid catheter occlusion, we found it crucial to flush the port with heparinized saline weekly. As a result, VAP remained functional for at least 3 mo. Thereafter, withdrawal occlusions started to occur more frequently but could generally be resolved by additional flushing.
The effectiveness of the VAP and the collection system were further tested during serial blood collection (every 10 min for 6 h). About 92% of the expected samples were collected successfully. The lack of success with the remaining % was mainly due to occasional occlusions from vascular collapse or due to the bird pecking at the collection line. To minimize these failures, blood collection and line flushing had to be performed very slowly to prevent the vein from collapsing or rupturing. To protect the extension line from pecking when full of blood, a flexible, thicker rubber tube was placed around the line. Alternatively, a protective collar could also be placed around the neck of the hen with the wide opening facing the back of the bird. In this setting, the extension line is taped to the edge of the collar to prevent movement. However, we suggest it is put in place 1 wk before sampling to allow the bird time to adapt to the collar. Being able to reintroduce the red blood cells during the serial collection prevented hypoxia and hypovolemia, and no signs of distress or discomfort were observed throughout the procedure. Because the extension line was simply disconnected by removing the right angle needle from the port’s chamber, minimal skin damage was inflicted, and the hens could be returned to a group environment if necessary.
LH Pulsatility in Older Broiler Breeders
Laying percentages were calculated as number of eggs laid per month divided by the number of days in the month multiplied by 100 and are presented in Table 3. Laying status the day before and after serial blood sampling is also presented in Table 3. Hens at 0 and 8% laying rates didn’t lay eggs during the days surrounding blood collection, whereas the hen at 12% laying rate laid 1 egg 5 d after blood collection, and hens at 56 and 64% laying rates laid eggs during the days surrounding, as well as the day of, blood collection.
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. Running bars on top of each graph represent pulses’ number and width. Results are also summarized in Table 1
. Overall, circulating LH patterns appeared to be pulsatile in all birds with a frequency of 5 to 6 pulses for 6 h for bird 1 (8% laying), bird 2 (12% laying), bird 4 (0% laying), and bird 5 (56% laying). Bird 3 (64% laying) displayed lower numbers of peaks (4 peaks) compare with birds 1, 2, 4, and 5. To define a peak, the software needs adjacent values on each side and, in the case of bird 3, missing points resulted in 2 high values interpreted as 1 peak or low values interpreted as no peak. Similarly, missing values for this bird also affected the width of the peaks.
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. Although the average pulse height appeared higher in birds that laid eggs around the time of blood collection (birds 3 and 5) than in hens with low laying rates, the difference was not significant. Baseline levels were below 0.5 ng/mL for the out-of-lay hen and hens at 8 and 12% laying rates, whereas they were from 0.5 to 1.0 ng/mL in hens at 56 and 64% laying rates. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Vascular access ports were first used in chickens by Ruschkowski et al. (1993) for collecting blood samples every 2 h over a 24- to 26-h period. However, surgery was performed under local anesthesia, and the port implanted was significantly larger than the one used in our study. We used general anesthesia, because it is a safer and less stressful procedure. We also used an extension line attached to a right angle needle, which allowed us to perform blood collections on unrestrained birds, helping to reduce stress levels even further. This is critical, because stress has been shown to affect the release of most hormones (Johnson, 1981; Laakso et al., 1984; White and Etches, 1984; Craig and Craig, 1985; Beuving and Vonder, 1986; Taher et al., 1986; Vachon and Moreau, 2001). Catheterization (Cravener and Vasilatos-Younken, 1989) and cannulation (Chapman et al., 1994; Liu et al., 2004a) have also been routinely used to perform serial blood sampling in chickens. However, these techniques often result in infection or loss of access to the blood due to clot formation, even after flushing with heparin and SCG (NaCl, sodium citrate, gentamicin; Liu et al., 2004a). For single samples, the extension line was not required, and the procedure was quicker than venipuncture, with no damage to the vasculature, thus reducing the stress to the bird. When a catheter or cannula is used, birds have to be kept isolated in individual cages to prevent damaging or removing the cannula, leading to uncontrolled bleeding and infections. Because VAP are located under the skin, birds can be kept in collective floor pens between collections without risk and can be moved to individual cages at the time of collection. Furthermore, cannulas inserted through the jugular vein can be used only for a relatively short period due to occlusions by thrombus formation (Liu et al., 2004a). The materials used in VAP were well tolerated and production of thrombus was significantly reduced compared with cannulas, thus minimizing the occurrence of blood clotting. In our experiment, all the implanted ports remained functional for at least 3 mo, and, overall, when compared with other techniques, VAP greatly facilitated serial or single blood sampling. In addition, although not performed in this study, it has previously been shown that VAP can be removed safely from the bird once the experiment is completed (Ruschkowski et al., 1993). However, we cannot exclude the possible development of valvular endocarditis over a longer period, a common condition reported in humans and animals that undergo catheterization on the jugular vein (Saito et al., 2001; Bortone et al., 2004).
LH Pulsatility in Old Broiler Breeders
To the best of our knowledge, this study is the first to evaluate LH profiles in older broiler breeders. Egg production naturally declines with advancing duration of the reproductive period (Robinson et al., 1990; Yu et al., 1992; Grossman et al., 2000; Grossman and Koops, 2001; Liu et al., 2001b; Robinson et al., 2001), and birds used in our study can be considered old under industry standards, with low overall egg production.
We clearly showed that circulating LH levels are pulsatile, with a frequency of about 5 to 6 pulse per 6 h in old broiler breeders. Furthermore, the frequency did not seem to depend on the egg-laying rate around the time of blood collection. In turkey hens, although a drastic increase in LH baseline occurs around stimulation, no major difference in LH pulse frequency was observed (Chapman et al., 1994). However, age rather than laying rate might be a factor influencing pulse frequency if we compare our results to data collected in younger layers (Liu et al., 2004b) or turkeys (Yang et al., 1997). Interestingly, although not statistically significant, pulse amplitude was higher in birds laying more eggs around the time of blood collection than the birds laying low numbers of eggs. Because we bled the birds for only for 6 h, we could not evaluate the effect of the preovulatory surges in those hens. However, unlike in photostimulated turkey hens (Chapman et al., 1994) and actively laying birds (Yang et al., 1997; Liu et al., 2004b), "blind surges" were distinct and could easily be identified. Similarly to what was reported in previous studies (Chapman et al., 1994; Liu et al., 2001a), LH baseline levels increased with laying activity from below 0.5 ng/mL in hens with lower laying rates to 0.5 to 1.0 ng/mL in hens with higher laying rates. Interestingly, LH basal levels were 4 to 5 times lower than that observed in broiler breeders at the peak of lay (Liu et al., 2004b). It is worth mentioning that basal levels in these "peak of lay" hens were similar to the maximum amplitude of pulses observed in our experiment.
The use of VAP appears to be a safe and reliable technique for serial bleeding, and, in combination with the remote collection line, it can reduce the stress associated with handling and restraint. In old broiler breeders (84 wk of age), rhythmic fluctuations in circulating LH levels suggest that LH release is pulsatile, with an average frequency of 1 pulse per h. Although no significant differences were observed, birds laying more eggs had the tendency to display higher amplitude and baseline than out-of-lay birds.
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ACKNOWLEDGMENTS |
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Received for publication January 30, 2006. Accepted for publication April 11, 2006.
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