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Concentrations of gonadotropins, estradiol and progesterone in sows selected on an index of ovulation rate and embryo survival
Animal Reproduction Science 54 Ž1998. 31–43
Concentrations of gonadotropins, estradiol and progesterone in sows selected on an index of ovulation rate and embryo survival D.V. Mariscal a,1, E.G.M. Bergfeld a,2 , A.S. Cupp a,3, a,5 F.N. Kojima 4 , K.E. Fike a , T. Sanchez , M.E. Wehrman a,6 , ´ a a R.K. Johnson , R.J. Kittok , J.J. Ford b, J.E. Kinder a,) a
Department of Animal Science, 224J, Institute of Agricultural and Natural Res., PO Box 830908, UniÕersity of Nebraska, Lincoln, NE 68583-0908, USA b Roman L. Hruska Meat Animal Research Center, USDA, ARS, Clay Center, NE 68933-0166, USA Accepted 17 August 1998
Abstract The objective of this study was to determine concentrations of follicle stimulating hormone ŽFSH., luteinizing hormone ŽLH., progesterone ŽP4 . and 17b-estradiol ŽE 2 . in sows from a line selected on an index which emphasized ovulation rate ŽSelect. and from a control line. A further classification of the sows in each line was made according to the estimated number of ovulations during an estrous cycle. Sows in the Select line were ranked into a high ŽHI. or low group ŽLI. when their estimated number of ovulations were 25 or more and 14 to 15, respectively. Sows of the control line were classified into groups as high ŽHC. or low ŽLC. when the estimated values for ovulation rate were 14–15 and 8–9 ovulations, respectively. Blood samples were collected every 12 h during a complete estrous cycle and samples were analyzed for concentrations of FSH and LH. Samples collected every 24 h were assayed for P4 and E 2 . Mean concentrations of FSH, LH, P4 and E 2 did not differ Ž P ) 0.10. between lines or between HI and LI or HC and LC
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Corresponding author. Tel.: q1-402-472-6438; Fax: q1-402-472-6362; Email: [email protected] Present address: Departamento de Zootecnia, Universidad Autonoma Chapingo, Mexico. ´ 2 Present address: American Society of Animal Science, 111 N. Dunlap Ave., Savoy, IL 61874, USA. 3 Present address: Center for Reproductive Biology, Department of Genetics and Cell Biology, Washington State University, Pullman, WA 99164-4234, USA. 4 Also corresponding author. Present address: Department of Animal Science, University of Missouri, Columbia, MO 65211, USA. 5 Present address: Colegio de Posgraduados, Montecillos, Edo. Mexico. 6 Present address: Trans Ova Genetics, Chillicothe, MO 64601, USA. 1
0378-4320r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 4 3 2 0 Ž 9 8 . 0 0 1 4 1 - 9
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D.V. Mariscal et al.r Animal Reproduction Science 54 (1998) 31–43
groups. Selection of pigs for ovulation rate and embryonal survival did not affect concentrations of FSH, LH, P4 and E 2 in sows during the estrous cycle. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Pig, ovary; Ovulation; LH; FSH; Estradiol; Estrous cycle
1. Introduction Litter size is a trait that influences economic efficiency of pig production. Litter size, a trait of low heritability, is sex limited in expression and direct selection for it in pigs has been relatively ineffective ŽJohnson, 1992.. Selection for increased ovulation rate was effective in increasing litter size in mice ŽBradford, 1968; Land and Falconer, 1969; Bindon et al., 1984. and to a lesser extent in pigs ŽCunningham et al., 1979; Lamberson et al., 1991.. An index of ovulation rate and embryonal survival was used to select the sows used in the present study because optimum weighting of these two components was calculated to be effective in increasing litter size ŽJohnson et al., 1984.. Greater circulating concentrations of follicle stimulating hormone ŽFSH. throughout the estrous cycle in pigs selected for ovulation rate than in pigs from an unselected control line was thought to be a major contributing factor to that greater ovulation rate ŽKelly et al., 1988.. The primary objective of the present study was to determine whether plasma concentrations of FSH during the estrous cycle of sows are always greater in lines of pigs selected on the basis of an index which emphasized ovulation rate as compared with sows of a control line. A secondary objective was to determine if there were differences in circulating luteinizing hormone ŽLH., progesterone and estrogen between lines of pigs selected on the basis of an index which emphasized ovulation rate as compared to those of a control line. A third objective was to determine if sows from the index ŽI. and control ŽC. lines with similar ovulation rates had similar concentrations of FSH, LH, progesterone and estrogen. 2. Materials and methods 2.1. Management of experimental animals Nonlactating, postpartum sows Ž13–15 months of age. from the tenth generation of a line selected on an index that emphasized increased ovulation rate and from an unselected control line were used. The base population was a composite with 50% of each of the Landrace and Large White breeds. Selection was based on counts of corpora lutea and number of fetuses by laparotomy at 50 days of gestation in gilts at 7–9 months of age ŽNeal et al., 1989.. Mean ovulation rate was 20.5 and 13.8 at Generation 10 in Index and Control lines, respectively. The sows used in this experiment were sampled from the tenth generation. Only high indexing Index line gilts Ž53 to 162 total. based on number of corpora lutea and fetuses at 50 days of gestation were selected to be farrowed. Thirty nine randomly selected Control sows farrowed in Generation 10.
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The sample of sows used in this experiment Ž12 Index and 12 Control. were selected samples from all sows available. Ovulation rates of these sows and all relatives back to generation 0 were used to calculate estimated breeding values ŽEBV. for each sow. The six sows with the highest and lowest EBV in each line were used. Ovulation rates for Index sows with a high EBV ŽHI. were 25 or more, and rates for sows with the low EBV ŽLI. were 15 or less. High ŽHC. and low ŽLC. Control sows had 15 or more and 9 or less, respectively. These selections were conducted to estimate the effects of long term selection on endocrine profiles, and whether sows from the Index ŽI. and Control ŽC. lines with similar ovulation rates ŽLI and HC. have different endocrine profiles. Stage of the estrous cycle of sows was synchronized with a synthetic progestin ŽRegumate, 15 mg daily per sow for 18 days. approximately 14 days after the first estrus, postweaning. Sows were subsequently cannulated as described by Ford and Maurer Ž1978., approximately 7 days before the first estimated period of estrus following the synchronized estrus. The animals were individually housed and fed according to recommendations of the NRC Ž1988.. Detection of behavioral estrus was accomplished by use of a mature boar twice daily at 0600 and 1800 h. The first day of estrus Žstanding reflex. was considered as day 0 of the estrous cycle. Blood samples were collected from each sow every 12 h during a complete estrous cycle, starting 5 days before the anticipated period of behavioral estrus. The blood samples Ž10 ml. were collected in tubes with 7.0 mg of EDTA and centrifuged at 3000 = g for 20 min. Plasma fractions from each sample were collected and stored at y208C. Concentrations of FSH and LH were determined in all samples and 17-estradiol ŽE 2 . and progesterone ŽP4 . were assayed in samples collected every 24 h. Sows were slaughtered during the luteal phase of the estrous cycle subsequent to the estrous cycle when blood samples were collected. Reproductive tracts were collected at the time of slaughter and the number of corpora lutea were determined by visual observation. 2.2. Hormones assays For the FSH RIA, USDA-398-04P Žanti-pFSH. was used as the first antibody and USDA-pFSH-I-1 were used was used as radio labeled ligand and standard hormone ŽMariscal et al., 1996.. These products were kindly furnished by Dr. D.J. Bolt ŽReproduction Laboratory, BARC-East, Beltsville, MD.. The intra- and inter-assay coefficients of variation were 4.1 and 14.3%, respectively. Assay sensitivity was 44.3 pgrml. Concentrations of LH in plasma were determined by using purified porcine LH ŽLER-786-3. provided by Dr. L.E. Reichert Jr. ŽDept. of Biochemistry, Albany Medical College, Albany, NY. as radio labeled ligand and standards ŽNiswender et al., 1970; Wolfe et al., 1989.. Dr. G.D. Niswender ŽDept. of Physiology, Colorado State Univ., Fort Collins, CO. provided the rabbit anti-porcine LH Ža566.. The intra- and interassay coefficients of variation were 2.8 and 9.2%, respectively. Assay sensitivity was 153 pgrml. Concentrations of E 2 in the samples were determined by RIA using antisera to E 2 ŽLilly, lot a 022367, Lilly Research Laboratories, Indianapolis, IN. and radioactive
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tracer Ž125 I-E 2 , Amersham, Arlington Heights, IL. were used. Concentrations of progesterone in plasma Žextracted. were assayed by using the antisera at a dilution of 1:1 600 000. Samples were subjected to assays in which both unknowns and standard curves were subjected to the extraction process. The assay was validated comparing standards that were unextracted, extracted and extracted with the addition of charcoal extracted barrow plasma. Parallelism of the standard curves and samples containing high and low amounts of E 2 at 400, 200, 100, 50, 25 and 12.5 ml were evaluated by Allfit and found to be parallel ŽDeLean et al., 1978.. Charcoal-extracted barrow plasma was added to standards because this negated the need to evaluate amount of recovery of samples. Intra- and interassay coefficients of variation were 7.2 and 8.6%, respectively. Assay sensitivity was 0.6 pgrml. Plasma P4 was measured by RIA. Progesterone-11-hemisuccinate-TME kindly provided by Dr. A. Belanger ŽLe Centre Hospitalier de I’Universite´ Laval, Quebec, Canada.
Table 1 Mean concentrationsa of FSH and LH in different periods of the estrous cycle in sows from generation 10 of a line selected on an index of ovulation rate and embryonal survival and ranked into a high ŽHI. b or low ŽLI. c estimated ovulation rate group and from an unselected control line and ranked into a high ŽHC. d or low ŽLC. e estimated ovulation rate group Periods of the estrous cycle 1f
2g
3h
4i
5j
X "SEM
FSH (ng r ml) HI LI HC LC Mean k
1.1 0.9 1.0 1.4 1.1C
1.8 1.7 1.5 2.3 1.8 A
1.3 1.4 1.2 1.4 1.3 B
1.2 1.1 1.0 1.2 1.1C
0.6 0.2 0.3 0.4 0.4 D
1.20".14 1.05".13 0.97".15 1.33".14
LH (ng r ml) HI LI HC LC Mean k
2.7 2.4 3.0 3.3 2.8 A
1.0 1.4 1.2 1.1 1.2 B
1.0 1.3 1.3 0.9 1.1B
0.9 1.1 1.0 1.0 1.0 B
1.0 1.1 0.9 1.0 1.0 B
1.32".12 1.45".12 1.49".14 1.44".13
a
Least square means. 25 or more ovulations. c 14 to 15 ovulations. d 14 to 15 ovulations. e 8 to 9 ovulations. f Period of blood collection during days 0 to 1 of estrous cycle; day 0 s first day of estrus. g Period of blood collection during days 2 to 4 of estrous cycle. h Period of blood collection during the first half of luteal phase of estrous cycle, considering luteal phase from day 5 to the day when P4 reached baseline Ž X s 7 days.. i Period of blood collection during second half of luteal phase of estrous cycle Ž X s 7 days.. j Period of blood collection from day when P4 reached baseline after luteolysis to the day prior to estrus Ž X s 2.1 days.. k Mean per period of blood sampling. A,B,C Numbers with different superscripts within rows differ Ž P - 0.05.. b
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was used as radio labeled ligand and P4 ŽSigma Chemical, St. Louis, MO. as standards. This conjugate was labeled with 125 I. A monoclonal antibody against progesterone-11BSA ŽLot a 02-9b4-94; BiosPacific, Emeryville, CA. was used as first antibody. Sheep anti-mouse gamma globulin ŽLot a 20711; Pelfreeze, Rogers, AR. was used as second antibody. Assay determinations of P4 in 0.004, 0.008, 0.02, 0.03 and 0.3 ml of plasma from each of three independent samples were highly correlated Ž r s 99.. Recovery of added mass Ž3.9, 7.8 and 15.5 pg P4 . from 1 ml plasma averaged 90 P 14%. Intra- and interassay coefficients of variation averaged 6.9 and 9.5%, respectively. Assay sensitivity was 96 pgrml. 2.3. Statistical analyses All hormone data were analyzed with mixed model procedures using Proc Mixed of SAS Ž1992.. The fitted model included the fixed effects of line, ovulation rate, and
Table 2 Mean concentrationsa of progesterone ŽP4 . and 17b-estradiol ŽE 2 . in different periods of the estrous cycle in sows from generation 10 of a line selected on an index of ovulation rate and embryonal survival and ranked into a high ŽHI. b or low ŽLI. c estimated ovulation rate group and from an unselected control line and ranked into a high ŽHC. d or low ŽLC. e estimated ovulation rate group Periods of the estrous cycle 1f
2g
3h
4i
5j
X "SEM
P4 (ng r ml) HI LI HC LC Mean k
2.1 0.6 0.8 1.2 1.2 D
41.8 14.1 15.8 15.2 21.7 B
106.1 45.3 56.7 42.6 62.7 A
92.0 46.0 54.5 42.0 58.6 A
9.3 4.2 6.8 3.4 5.9 C
50.2"12 22.0"12 29.9"14 20.9"13
E2 (pg r ml) HI LI HC LC Mean k
32.7 15.8 30.3 29.5 27.1A
3.6 5.5 3.9 3.8 4.2 B
5.8 5.6 4.5 9.8 6.4 B
8.4 6.8 7.6 8.8 7.9 B
108.4 39.8 46.4 31.6 56.5 A
31.8"8 14.7"8 18.6"10 16.7"9
a
Least square means. 25 or more ovulations. c 14 to 15 ovulations. d 14 to 15 ovulations. e 8 to 9 ovulations. f Period of blood collection during days 0 to 1 of estrous cycle; day 0 s first day of estrus. g Period of blood collection during days 2 to 4 of estrous cycle. h Period of blood collection during the first half of luteal phase of estrous cycle, considering luteal phase from day 5 to the day when P4 reached baseline Ž X s 7 days.. i Period of blood collection during second half of luteal phase of estrous cycle Ž X s 7 days.. j Period of blood collection from the day when P4 reached baseline after luteolysis to the day prior to estrus Ž X s 2.1 days.. k Mean per period of blood sampling. A,B,C,D Numbers with differing superscripts within rows differ Ž P - 0.05.. b
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period of estrous cycle. To account for the covariance between observations from the same sow at different periods, different options of covariance structures for residuals available in the repeated statement of Proc Mixed were considered and the model with the best fit was chosen to analyze the data. Means were compared using the predicted differences tables provided by the Proc Mixed procedure of SAS Ž1992.. Hormone data were compared at five different periods over the estrous cycle considering the profile of P4 in circulation. The periods were established as follows: Period 1 s time of behavioral estrus Ždays 0 to 1.; Period 2 s days 2 to 4; Period 3 s first half of luteal phase Žconsidering luteal phase from days 5 to the day when concentration of P4 returned to basal concentrations at the time of luteal regression.; Period 4 s the second half of luteal phase; and Period 5 s from the day when concentra-
Fig. 1. Concentrations of progesterone, 17b-estradiol, LH and FSH during an entire estrous cycle in a representative sow selected on an index of ovulation rate and embryo survival and ranked into a high estimated ovulation rate group ŽHI..
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tions of P4 reached basal concentrations at the time of luteal regression through the day before estrus.
3. Results 3.1. OÕulation rate One sow from the LC group failed to exhibit behavioral estrus during the experimental period and was excluded from the experiment. Data from one sow were excluded
Fig. 2. Concentrations of progesterone, 17b-estradiol, LH and FSH during an entire estrous cycle in a representative sow selected on an index of ovulation rate and embryo survival and ranked into a low estimated ovulation rate group ŽLI..
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from the analyses because she had large variations in secretion of hormones and had an abnormally long estrous cycle Ž35 days.. With this exclusion, length of the estrous cycle was not significantly different among sows of the various groups. Mean ovulation rate Ž"SEM., determined by counting number of corpora lutea present during the estrous cycle after the blood samples were collected, were 51 " 20.82, 17 " 2.24, 22 " 7.16 and 12 " 0.22, respectively, for sows of the HI, LI, HC and LC groups. 3.2. Hormone concentrations Concentrations of FSH were similar Ž P ) 0.10. among sows of the four groups. However, a tendency for a line by ovulation rate interaction Ž P - 0.09. was detected.
Fig. 3. Concentrations of progesterone, 17b-estradiol, LH and FSH during an entire estrous cycle in a representative sow from an unselected line and ranked into a high estimated ovulation rate group ŽHC..
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Greater concentrations of FSH were detected in sows of the LC group than in sows of the HC, HI and LI groups. Overall mean concentrations of FSH were not significantly different and concentrations of LH, P4 , and E 2 were not different Ž P ) 0.10. among sows of the I and C lines or groups ŽH and L. when compared at the various periods of the estrous cycle. Concentration of gonadotropins and steroid hormones varied Ž P - 0.05. across the five periods of the estrous cycle. Data for mean concentrations of the gonadotropins ŽFSH and LH. and gonadal hormones ŽP4 and E 2 . at the different periods of the estrous cycle are included in Tables 1 and 2, respectively. Concentrations of FSH were greater Ž P - 0.05. during Period 2 than Period 1, decreased and remained fairly constant during Periods 3 and 4 and decreased further during Period 5. Concentrations of LH in plasma were greatest Ž P - 0.05. during Period
Fig. 4. Concentrations of progesterone, 17b-estradiol, LH and FSH during an entire estrous cycle in a representative sow from an unselected line and ranked into a low estimated ovulation rate group ŽLC..
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1, decreased and remained relatively constant during the remainder of the estrous cycle. Concentrations of P4 were lower Ž P - 0.05. during behavioral estrus and after luteolysis. During days 2 to 4, concentrations of P4 began to increase and were greatest during the luteal phase. Concentrations of E 2 in plasma were greatest Ž P - 0.05. after luteolysis and before behavioral estrus. At onset of behavioral estrus, concentrations of E 2 decreased and a further decrease occurred during the luteal phase of the estrous cycle. One sow from the HI group had much greater concentrations of P4 during Periods 3 Ž351 ngrml. and 4 Ž269 ngrml. and also E 2 during Period 5 Ž456 pgrml. when compared with mean concentrations of P4 and E 2 during respective periods for the other sows of the HI group. Without data from this sow, corresponding concentrations of P4 and E 2 were 57 ngrml, and 39 pgrml, respectively. However, patterns of P4 and E 2 during the estrous cycle from this sow were similar to those of other sows from the HI group. The inference of the statistical analyses of the P4 and E 2 data did not change with or without data from this sow included. Therefore, values from this sow were included in the statistical analyses. Patterns of secretion of P4 , E 2 , FSH and LH for a representative sow of each group are shown in Figs. 1–4.
4. Discussion Concentrations of FSH during the estrous cycle in sows from the index selected line did not differ from sows in the control line. These results are consistent with those reported previously ŽHunter et al., 1993, 1996. in which prolific Meishan gilts and sows had concentrations of FSH similar to Large White females. No differences in FSH were observed during the periovulatory period in ewes from breeds with low and high ovulation rates ŽScaramuzzi et al., 1993.. In contrast, Kelly et al. Ž1988. and Knox Ž1992. observed that sows selected for high ovulation rate had greater concentrations of FSH than those of a control line. Inconsistency between results of studies may indicate that when different systems of selection were used different physiological control mechanisms may have affected ovulation rate. Collectively, the data from the various studies suggest that differences in rate of ovulation may be affected by variation in circulating concentrations of FSH ŽKelly et al., 1988. and Žor. follicular sensitivity to gonadotropins, variation in the biopotency of gonadotropins ŽAdams et al., 1988. or altered autocrine and paracrine factors in the ovary ŽDriancourt et al., 1990.. Concentrations of FSH were greater during the secondary increases as expected because it has been shown that the secondary increase of FSH release may be greater and more prolonged than the primary preovulatory surge of FSH in pigs ŽVan de Wiel et al., 1981; Kelly et al., 1988.. The physiological significance of this increase is not well understood but it has been implicated in recruitment of ovarian follicles from the resting pool of follicles in the ovary ŽKelly et al., 1988.. In contrast, Knox et al. Ž1991. failed to identify a difference in the rate of ovulation when the secondary surge of FSH was partially suppressed using charcoal extracted follicular fluid. The amount of FSH in circulation during the secondary increase has been associated with differences in ovulation rates of ewes ŽCahill et al., 1981; Lahlou-Kassi et al., 1984..
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The pattern of LH secretion during the estrous cycle was similar between lines in the present study and the preovulatory surge of LH occurred during Period 1 corresponding to the time of behavioral estrus ŽVan de Wiel et al., 1981; Kelly et al., 1988; Knox et al., 1991.. Likewise, no differences in LH were observed between Meishan and Large White females ŽHunter et al., 1993, 1996.. Concentrations of P4 in the two lines followed the pattern reported in several studies ŽHenricks et al., 1972; Parvizi et al., 1976; Van de Wiel et al., 1981; Kelly et al., 1988.. Although line differences were detected in plasma P4 concentrations, samples were collected too infrequently to determine possible differences in the immediate increase in P4 that occurs after ovulation and appears to be associated with greater embryonic survival ŽJindal et al., 1997.. Plasma concentrations of E 2 did not differ between the two lines of sows, a finding which agrees with those of the previous studies using females with high and low ovulation rates ŽGuthrie et al., 1974; Kelly et al., 1988; Hunter et al., 1993.. Hunter et al. Ž1993. reported similar concentrations of E 2 in Meishan and Large White gilts that had similar ovulation rates. In a second study where ovulation was greater in Meishan than in Large White sows, there was an association between ovulation rates and concentrations of E 2 ŽHunter et al., 1996.. Thus, in Meishan females more ovulatory follicles produce greater circulating concentrations of E 2 ; yet, Meishan females appear to metabolize E 2 more rapidly than Large White females ŽTilton et al., 1994.. In the current study, the peak concentrations of E 2 occurred before the surge of LH when lowest concentrations of FSH were observed. The relative contributions of E 2 and inhibin to negative feedback regulation of FSH secretion have not been defined in detail in pigs. Both are produced by developing follicles during the follicular phase of the estrous cycle, and inhibin has been implicated to have a greater influence on FSH secretion during this time than E 2 ŽHasegawa et al., 1988; Trout et al., 1992.. Collectively, all the studies evaluating reproductive hormones during the periovulatory period have indicated that genetic lines with differing ovulation rates may or may not have different hormone concentrations.
5. Conclusions Selection based on an index which emphasized increased ovulation rate did not affect the concentrations and pattern of secretion of FSH, LH, P4 and E 2 in sows. Sows with similar ovulation rates from the select and control line also had similar concentrations of FSH, LH, P4 and E 2 and concentrations of these hormones in the blood were not related to ovulation rate. Other factors such as interovarian components may be implicated in regulation of ovulation rate in this line.
Acknowledgements We thank Dr. D.J. Bolt for providing porcine antisera and FSH, and Bruce Melson and Felipe Rodriguez for the assistance in laboratory and statistical analyses, respec-
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tively. This paper is published as paper no. 11913, Nebraska Agricultural Research Division. Mention of a trade name, proprietary product or specific equipment does not constitute a guarantee or warranty by the USDA and does not imply approval to the exclusion of other products that may be suitable.
References Adams, T.E., Quirke, J.F., Hanrahan, J.P., Adams, B.M., Watson, J.G., 1988. Gonadotrophin secretion during the periovulatory period in Galway and Finnish Landrace ewes selected for high ovulation rate. Reprod. Fertil. 83, 575–584. Bindon, B.M., Piper, L.R., Thimonier, J., 1984. Preovulatory LH characteristics and time of ovulation in the prolific Booroola Merino ewe. J. Reprod. Fertil. 71, 519–523. Bradford, G.E., 1968. Selection for litter size in mice in the presence and absence of gonadotropin treatment. Genetics 58, 283–295. Cahill, L.P., Saumande, J., Ravault, J.P., Blanc, M., Thimonier, J., Mariana, J.C., Mauleon, P., 1981. Hormonal and follicular relationships in ewes of high and low ovulation rates. J. Reprod. Fertil. 62, 141–150. Cunningham, P.J., England, M.E., Young, L.D., Zimmerman, D.R., 1979. Selection for ovulation rate in swine: correlated response in litter size and weight. J. Anim. Sci. 48, 509–516. DeLean, A., Munson, P.J., Rodbard, D., 1978. Simultaneous analysis of families of sigmoidal curves: application to bioassay, radioligand assay and physiological dose–response curves. Am. J. Physiol. 235, E97–E102. Driancourt, M.A., Castonguay, F., Bindon, B.M., Piper, L.R., Quirke, J.F., Hanrahan, J.P., 1990. Ovarian follicular dynamics in lines of sheep ŽFinn, Merinos. selected on ovulation rate. J. Anim. Sci. 68, 2034–2041. Ford, J.J., Maurer, R.R., 1978. Simple technique for chronic venous catheterization of swine. Lab. Anim. Sci. 28, 615–618. Guthrie, H.D., Henricks, D.M., Handlin, D.L., 1974. Plasma hormone levels and fertility in pigs induced to ovulate with PMSG. J. Reprod. Fertil. 41, 361–370. Hasegawa, Y., Miyamoto, K., Iwamura, S., Igarashi, M., 1988. Changes in serum concentrations of inhibin in cyclic pigs. J. Endocrinol. 118, 211–219. Henricks, D.M., Guthrie, H.D., Handlin, D.L., 1972. Plasma estrogen, progesterone and luteinizing hormone levels during the estrous cycle in pigs. Biol. Reprod. 6, 210–218. Hunter, M.G., Biggs, C., Foxcroft, G.R., McNeilly, A.S., Tilton, J.E., 1993. Comparison of endocrinology and behavioral events during the periovulatory period in Meishan and Large-White hybrid gilts. J. Reprod. Fertil. 97, 475–480. Hunter, M.G., Picton, H.M., Biggs, C., Mann, G.E., McNeilly, A.S., Foxcroft, G.R., 1996. Periovulatory endocrinology in high ovulating Meishan sows. J. Endocrinol. 150, 141–147. Jindal, R., Cosgrove, J.R., Foxcroft, G.R., 1997. Progesterone mediates nutritionally induced effects on embryonic survival in gilts. J. Anim. Sci. 75, 1063–1070. Johnson, R.K., 1992. Selection for fertility in swine. Pig News Inf. 13, 59–61. Johnson, R.K., Zimmerman, D.R., Kittok, R.J., 1984. Selection for components of reproduction in swine. Livest. Prod. Sci. 11, 541–558. Kelly, C.R., Socha, T.E., Zimmerman, D.R., 1988. Characterization of gonadotropic and ovarian steroid hormones during the periovulatory period in high ovulating select and control line gilts. J. Anim. Sci. 66, 1462–1474. Knox, R.V., 1992. Follicle maturation, ovulation rate and early embryonic development in pigs as influenced by natural and induced changes in gonadotropins during the estrous cycle. PhD dissertation, University of Nebraska-Lincoln. Knox, R.V., Naber, C.H., Zimmerman, D.R., 1991. Follicle stimulating hormone ŽFSH. during the secondary surge in gilts as influenced by administration of porcine follicular fluid ŽpFF.. J. Anim. Sci. 69, 761–769.
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Lahlou-Kassi, Schams, D., Glatzel, P., 1984. Plasma gonadotrophin concentrations during oestrous cycle and after ovariectomy in two breeds of sheep with low and high fecundity. J. Reprod. Fertil. 70, 165–173. Lamberson, W.R., Johnson, R.K., Zimmerman, D.R., Long, T.E., 1991. Direct responses to selection for increased litter size, decreased age at puberty, or random selection following selection for ovulation rate in swine. J. Anim. Sci. 69, 3129–3143. Land, R.B., Falconer, D.S., 1969. Genetic studies of ovulation rate in the mouse. Genet. Res. ŽCambridge. 13, 25–46. Mariscal, D.V., Wolfe, P.L., Bergfeld, E.G., Cupp, A.S., Kojima, N.K., Fike, K.E., Sanchez, T.E., Wehrman, ´ M.E., Johnson, R.K., Kittok, R.J., Ford, J.J., Kinder, J.E., 1996. Comparison of circulating concentrations of reproductive hormones in boars selected for size of testes or number of ovulations and embryonal survival to concentrations in respective control lines. J. Anim. Sci. 74, 1905–1914. Neal, S.M., Johnson, R.K., Kittok, R.J., 1989. Index selection for components of litter size in swine: response to five generations of selection. J. Anim. Sci. 67, 1933–1945. Niswender, G.D., Reichert, L.E. Jr., Zimmerman, D.R., 1970. Radioimmunoassay of serum levels of LH throughout the estrous cycle in pigs. Endocrinology 87, 576–580. NRC, 1988. Nutrient Requirements of Swine, 9th edn. National Academy Press, Washington, DC. Parvizi, N., Elsaesser, F., Smidt, D., Ellendorff, F., 1976. Plasma luteinizing hormone and progesterone in the adult female pig during the oestrous cycle, late pregnancy and lactation, and after ovariectomy and pentobarbitone treatment. J. Endocrinol. 69, 193–203. SAS, 1992. Changes and Enhancements. Technical Report P-229, SASrSTAT Software. Scaramuzzi, R.J., Adams, N.R., Baird, D.T., Campbell, B.K., Downing, J.A., Findlay, J.K., Henderson, K.M., Martin, G.B., McNatty, K.P., McNeilly, A.S., Tsonis, C.G., 1993. A model for follicle selection and the determination of ovulation rate in the ewe. Reprod. Fertil. Dev. 5, 459–478. Tilton, J.E., Biggs, C., Hunter, M.G., Foxcroft, G.R., 1994. Gonadotropin secretion in ovariectomized Chinese Meishan and hybrid Large White gilts; responses to challenges with estradiol benzoate, gonadotropin-releasing hormone, or porcine follicular fluid. Biol. Reprod. 51, 963–970. Trout, W.E., Killen, J.H., Christenson, R.K., Schanbacher, B.D., Ford, J.J., 1992. Effects of weaning on concentrations of inhibin in follicular fluid and plasma of sows. J. Reprod. Fertil. 94, 107–114. Van de Wiel, D.F.M., Erkens, J., Koops, W., Vos, E., Van Landeghem, A.A.J., 1981. Periestrus and midluteal time courses of circulating LH, FSH, prolactin, estradiol-17b and progesterone in the domestic pig. Biol. Reprod. 24, 223–233. Wolfe, P.L., Wolfe, M.W., Stumpf, T.T., Stotts, J.A., Day, M.L., Kittok, R.J., Johnson, R.K., Kinder, J.E., 1989. Control of luteinizing hormone in postpubertal boars with large testes. J. Anim. Sci. 67, 1334–1340.
Concentrations of gonadotropins, estradiol and progesterone in sows selected on an index of ovulation rate and embryo survival D.V. Mariscal a,1, E.G.M. Bergfeld a,2 , A.S. Cupp a,3, a,5 F.N. Kojima 4 , K.E. Fike a , T. Sanchez , M.E. Wehrman a,6 , ´ a a R.K. Johnson , R.J. Kittok , J.J. Ford b, J.E. Kinder a,) a
Department of Animal Science, 224J, Institute of Agricultural and Natural Res., PO Box 830908, UniÕersity of Nebraska, Lincoln, NE 68583-0908, USA b Roman L. Hruska Meat Animal Research Center, USDA, ARS, Clay Center, NE 68933-0166, USA Accepted 17 August 1998
Abstract The objective of this study was to determine concentrations of follicle stimulating hormone ŽFSH., luteinizing hormone ŽLH., progesterone ŽP4 . and 17b-estradiol ŽE 2 . in sows from a line selected on an index which emphasized ovulation rate ŽSelect. and from a control line. A further classification of the sows in each line was made according to the estimated number of ovulations during an estrous cycle. Sows in the Select line were ranked into a high ŽHI. or low group ŽLI. when their estimated number of ovulations were 25 or more and 14 to 15, respectively. Sows of the control line were classified into groups as high ŽHC. or low ŽLC. when the estimated values for ovulation rate were 14–15 and 8–9 ovulations, respectively. Blood samples were collected every 12 h during a complete estrous cycle and samples were analyzed for concentrations of FSH and LH. Samples collected every 24 h were assayed for P4 and E 2 . Mean concentrations of FSH, LH, P4 and E 2 did not differ Ž P ) 0.10. between lines or between HI and LI or HC and LC
)
Corresponding author. Tel.: q1-402-472-6438; Fax: q1-402-472-6362; Email: [email protected] Present address: Departamento de Zootecnia, Universidad Autonoma Chapingo, Mexico. ´ 2 Present address: American Society of Animal Science, 111 N. Dunlap Ave., Savoy, IL 61874, USA. 3 Present address: Center for Reproductive Biology, Department of Genetics and Cell Biology, Washington State University, Pullman, WA 99164-4234, USA. 4 Also corresponding author. Present address: Department of Animal Science, University of Missouri, Columbia, MO 65211, USA. 5 Present address: Colegio de Posgraduados, Montecillos, Edo. Mexico. 6 Present address: Trans Ova Genetics, Chillicothe, MO 64601, USA. 1
0378-4320r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 4 3 2 0 Ž 9 8 . 0 0 1 4 1 - 9
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groups. Selection of pigs for ovulation rate and embryonal survival did not affect concentrations of FSH, LH, P4 and E 2 in sows during the estrous cycle. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Pig, ovary; Ovulation; LH; FSH; Estradiol; Estrous cycle
1. Introduction Litter size is a trait that influences economic efficiency of pig production. Litter size, a trait of low heritability, is sex limited in expression and direct selection for it in pigs has been relatively ineffective ŽJohnson, 1992.. Selection for increased ovulation rate was effective in increasing litter size in mice ŽBradford, 1968; Land and Falconer, 1969; Bindon et al., 1984. and to a lesser extent in pigs ŽCunningham et al., 1979; Lamberson et al., 1991.. An index of ovulation rate and embryonal survival was used to select the sows used in the present study because optimum weighting of these two components was calculated to be effective in increasing litter size ŽJohnson et al., 1984.. Greater circulating concentrations of follicle stimulating hormone ŽFSH. throughout the estrous cycle in pigs selected for ovulation rate than in pigs from an unselected control line was thought to be a major contributing factor to that greater ovulation rate ŽKelly et al., 1988.. The primary objective of the present study was to determine whether plasma concentrations of FSH during the estrous cycle of sows are always greater in lines of pigs selected on the basis of an index which emphasized ovulation rate as compared with sows of a control line. A secondary objective was to determine if there were differences in circulating luteinizing hormone ŽLH., progesterone and estrogen between lines of pigs selected on the basis of an index which emphasized ovulation rate as compared to those of a control line. A third objective was to determine if sows from the index ŽI. and control ŽC. lines with similar ovulation rates had similar concentrations of FSH, LH, progesterone and estrogen. 2. Materials and methods 2.1. Management of experimental animals Nonlactating, postpartum sows Ž13–15 months of age. from the tenth generation of a line selected on an index that emphasized increased ovulation rate and from an unselected control line were used. The base population was a composite with 50% of each of the Landrace and Large White breeds. Selection was based on counts of corpora lutea and number of fetuses by laparotomy at 50 days of gestation in gilts at 7–9 months of age ŽNeal et al., 1989.. Mean ovulation rate was 20.5 and 13.8 at Generation 10 in Index and Control lines, respectively. The sows used in this experiment were sampled from the tenth generation. Only high indexing Index line gilts Ž53 to 162 total. based on number of corpora lutea and fetuses at 50 days of gestation were selected to be farrowed. Thirty nine randomly selected Control sows farrowed in Generation 10.
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The sample of sows used in this experiment Ž12 Index and 12 Control. were selected samples from all sows available. Ovulation rates of these sows and all relatives back to generation 0 were used to calculate estimated breeding values ŽEBV. for each sow. The six sows with the highest and lowest EBV in each line were used. Ovulation rates for Index sows with a high EBV ŽHI. were 25 or more, and rates for sows with the low EBV ŽLI. were 15 or less. High ŽHC. and low ŽLC. Control sows had 15 or more and 9 or less, respectively. These selections were conducted to estimate the effects of long term selection on endocrine profiles, and whether sows from the Index ŽI. and Control ŽC. lines with similar ovulation rates ŽLI and HC. have different endocrine profiles. Stage of the estrous cycle of sows was synchronized with a synthetic progestin ŽRegumate, 15 mg daily per sow for 18 days. approximately 14 days after the first estrus, postweaning. Sows were subsequently cannulated as described by Ford and Maurer Ž1978., approximately 7 days before the first estimated period of estrus following the synchronized estrus. The animals were individually housed and fed according to recommendations of the NRC Ž1988.. Detection of behavioral estrus was accomplished by use of a mature boar twice daily at 0600 and 1800 h. The first day of estrus Žstanding reflex. was considered as day 0 of the estrous cycle. Blood samples were collected from each sow every 12 h during a complete estrous cycle, starting 5 days before the anticipated period of behavioral estrus. The blood samples Ž10 ml. were collected in tubes with 7.0 mg of EDTA and centrifuged at 3000 = g for 20 min. Plasma fractions from each sample were collected and stored at y208C. Concentrations of FSH and LH were determined in all samples and 17-estradiol ŽE 2 . and progesterone ŽP4 . were assayed in samples collected every 24 h. Sows were slaughtered during the luteal phase of the estrous cycle subsequent to the estrous cycle when blood samples were collected. Reproductive tracts were collected at the time of slaughter and the number of corpora lutea were determined by visual observation. 2.2. Hormones assays For the FSH RIA, USDA-398-04P Žanti-pFSH. was used as the first antibody and USDA-pFSH-I-1 were used was used as radio labeled ligand and standard hormone ŽMariscal et al., 1996.. These products were kindly furnished by Dr. D.J. Bolt ŽReproduction Laboratory, BARC-East, Beltsville, MD.. The intra- and inter-assay coefficients of variation were 4.1 and 14.3%, respectively. Assay sensitivity was 44.3 pgrml. Concentrations of LH in plasma were determined by using purified porcine LH ŽLER-786-3. provided by Dr. L.E. Reichert Jr. ŽDept. of Biochemistry, Albany Medical College, Albany, NY. as radio labeled ligand and standards ŽNiswender et al., 1970; Wolfe et al., 1989.. Dr. G.D. Niswender ŽDept. of Physiology, Colorado State Univ., Fort Collins, CO. provided the rabbit anti-porcine LH Ža566.. The intra- and interassay coefficients of variation were 2.8 and 9.2%, respectively. Assay sensitivity was 153 pgrml. Concentrations of E 2 in the samples were determined by RIA using antisera to E 2 ŽLilly, lot a 022367, Lilly Research Laboratories, Indianapolis, IN. and radioactive
D.V. Mariscal et al.r Animal Reproduction Science 54 (1998) 31–43
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tracer Ž125 I-E 2 , Amersham, Arlington Heights, IL. were used. Concentrations of progesterone in plasma Žextracted. were assayed by using the antisera at a dilution of 1:1 600 000. Samples were subjected to assays in which both unknowns and standard curves were subjected to the extraction process. The assay was validated comparing standards that were unextracted, extracted and extracted with the addition of charcoal extracted barrow plasma. Parallelism of the standard curves and samples containing high and low amounts of E 2 at 400, 200, 100, 50, 25 and 12.5 ml were evaluated by Allfit and found to be parallel ŽDeLean et al., 1978.. Charcoal-extracted barrow plasma was added to standards because this negated the need to evaluate amount of recovery of samples. Intra- and interassay coefficients of variation were 7.2 and 8.6%, respectively. Assay sensitivity was 0.6 pgrml. Plasma P4 was measured by RIA. Progesterone-11-hemisuccinate-TME kindly provided by Dr. A. Belanger ŽLe Centre Hospitalier de I’Universite´ Laval, Quebec, Canada.
Table 1 Mean concentrationsa of FSH and LH in different periods of the estrous cycle in sows from generation 10 of a line selected on an index of ovulation rate and embryonal survival and ranked into a high ŽHI. b or low ŽLI. c estimated ovulation rate group and from an unselected control line and ranked into a high ŽHC. d or low ŽLC. e estimated ovulation rate group Periods of the estrous cycle 1f
2g
3h
4i
5j
X "SEM
FSH (ng r ml) HI LI HC LC Mean k
1.1 0.9 1.0 1.4 1.1C
1.8 1.7 1.5 2.3 1.8 A
1.3 1.4 1.2 1.4 1.3 B
1.2 1.1 1.0 1.2 1.1C
0.6 0.2 0.3 0.4 0.4 D
1.20".14 1.05".13 0.97".15 1.33".14
LH (ng r ml) HI LI HC LC Mean k
2.7 2.4 3.0 3.3 2.8 A
1.0 1.4 1.2 1.1 1.2 B
1.0 1.3 1.3 0.9 1.1B
0.9 1.1 1.0 1.0 1.0 B
1.0 1.1 0.9 1.0 1.0 B
1.32".12 1.45".12 1.49".14 1.44".13
a
Least square means. 25 or more ovulations. c 14 to 15 ovulations. d 14 to 15 ovulations. e 8 to 9 ovulations. f Period of blood collection during days 0 to 1 of estrous cycle; day 0 s first day of estrus. g Period of blood collection during days 2 to 4 of estrous cycle. h Period of blood collection during the first half of luteal phase of estrous cycle, considering luteal phase from day 5 to the day when P4 reached baseline Ž X s 7 days.. i Period of blood collection during second half of luteal phase of estrous cycle Ž X s 7 days.. j Period of blood collection from day when P4 reached baseline after luteolysis to the day prior to estrus Ž X s 2.1 days.. k Mean per period of blood sampling. A,B,C Numbers with different superscripts within rows differ Ž P - 0.05.. b
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was used as radio labeled ligand and P4 ŽSigma Chemical, St. Louis, MO. as standards. This conjugate was labeled with 125 I. A monoclonal antibody against progesterone-11BSA ŽLot a 02-9b4-94; BiosPacific, Emeryville, CA. was used as first antibody. Sheep anti-mouse gamma globulin ŽLot a 20711; Pelfreeze, Rogers, AR. was used as second antibody. Assay determinations of P4 in 0.004, 0.008, 0.02, 0.03 and 0.3 ml of plasma from each of three independent samples were highly correlated Ž r s 99.. Recovery of added mass Ž3.9, 7.8 and 15.5 pg P4 . from 1 ml plasma averaged 90 P 14%. Intra- and interassay coefficients of variation averaged 6.9 and 9.5%, respectively. Assay sensitivity was 96 pgrml. 2.3. Statistical analyses All hormone data were analyzed with mixed model procedures using Proc Mixed of SAS Ž1992.. The fitted model included the fixed effects of line, ovulation rate, and
Table 2 Mean concentrationsa of progesterone ŽP4 . and 17b-estradiol ŽE 2 . in different periods of the estrous cycle in sows from generation 10 of a line selected on an index of ovulation rate and embryonal survival and ranked into a high ŽHI. b or low ŽLI. c estimated ovulation rate group and from an unselected control line and ranked into a high ŽHC. d or low ŽLC. e estimated ovulation rate group Periods of the estrous cycle 1f
2g
3h
4i
5j
X "SEM
P4 (ng r ml) HI LI HC LC Mean k
2.1 0.6 0.8 1.2 1.2 D
41.8 14.1 15.8 15.2 21.7 B
106.1 45.3 56.7 42.6 62.7 A
92.0 46.0 54.5 42.0 58.6 A
9.3 4.2 6.8 3.4 5.9 C
50.2"12 22.0"12 29.9"14 20.9"13
E2 (pg r ml) HI LI HC LC Mean k
32.7 15.8 30.3 29.5 27.1A
3.6 5.5 3.9 3.8 4.2 B
5.8 5.6 4.5 9.8 6.4 B
8.4 6.8 7.6 8.8 7.9 B
108.4 39.8 46.4 31.6 56.5 A
31.8"8 14.7"8 18.6"10 16.7"9
a
Least square means. 25 or more ovulations. c 14 to 15 ovulations. d 14 to 15 ovulations. e 8 to 9 ovulations. f Period of blood collection during days 0 to 1 of estrous cycle; day 0 s first day of estrus. g Period of blood collection during days 2 to 4 of estrous cycle. h Period of blood collection during the first half of luteal phase of estrous cycle, considering luteal phase from day 5 to the day when P4 reached baseline Ž X s 7 days.. i Period of blood collection during second half of luteal phase of estrous cycle Ž X s 7 days.. j Period of blood collection from the day when P4 reached baseline after luteolysis to the day prior to estrus Ž X s 2.1 days.. k Mean per period of blood sampling. A,B,C,D Numbers with differing superscripts within rows differ Ž P - 0.05.. b
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period of estrous cycle. To account for the covariance between observations from the same sow at different periods, different options of covariance structures for residuals available in the repeated statement of Proc Mixed were considered and the model with the best fit was chosen to analyze the data. Means were compared using the predicted differences tables provided by the Proc Mixed procedure of SAS Ž1992.. Hormone data were compared at five different periods over the estrous cycle considering the profile of P4 in circulation. The periods were established as follows: Period 1 s time of behavioral estrus Ždays 0 to 1.; Period 2 s days 2 to 4; Period 3 s first half of luteal phase Žconsidering luteal phase from days 5 to the day when concentration of P4 returned to basal concentrations at the time of luteal regression.; Period 4 s the second half of luteal phase; and Period 5 s from the day when concentra-
Fig. 1. Concentrations of progesterone, 17b-estradiol, LH and FSH during an entire estrous cycle in a representative sow selected on an index of ovulation rate and embryo survival and ranked into a high estimated ovulation rate group ŽHI..
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tions of P4 reached basal concentrations at the time of luteal regression through the day before estrus.
3. Results 3.1. OÕulation rate One sow from the LC group failed to exhibit behavioral estrus during the experimental period and was excluded from the experiment. Data from one sow were excluded
Fig. 2. Concentrations of progesterone, 17b-estradiol, LH and FSH during an entire estrous cycle in a representative sow selected on an index of ovulation rate and embryo survival and ranked into a low estimated ovulation rate group ŽLI..
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from the analyses because she had large variations in secretion of hormones and had an abnormally long estrous cycle Ž35 days.. With this exclusion, length of the estrous cycle was not significantly different among sows of the various groups. Mean ovulation rate Ž"SEM., determined by counting number of corpora lutea present during the estrous cycle after the blood samples were collected, were 51 " 20.82, 17 " 2.24, 22 " 7.16 and 12 " 0.22, respectively, for sows of the HI, LI, HC and LC groups. 3.2. Hormone concentrations Concentrations of FSH were similar Ž P ) 0.10. among sows of the four groups. However, a tendency for a line by ovulation rate interaction Ž P - 0.09. was detected.
Fig. 3. Concentrations of progesterone, 17b-estradiol, LH and FSH during an entire estrous cycle in a representative sow from an unselected line and ranked into a high estimated ovulation rate group ŽHC..
D.V. Mariscal et al.r Animal Reproduction Science 54 (1998) 31–43
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Greater concentrations of FSH were detected in sows of the LC group than in sows of the HC, HI and LI groups. Overall mean concentrations of FSH were not significantly different and concentrations of LH, P4 , and E 2 were not different Ž P ) 0.10. among sows of the I and C lines or groups ŽH and L. when compared at the various periods of the estrous cycle. Concentration of gonadotropins and steroid hormones varied Ž P - 0.05. across the five periods of the estrous cycle. Data for mean concentrations of the gonadotropins ŽFSH and LH. and gonadal hormones ŽP4 and E 2 . at the different periods of the estrous cycle are included in Tables 1 and 2, respectively. Concentrations of FSH were greater Ž P - 0.05. during Period 2 than Period 1, decreased and remained fairly constant during Periods 3 and 4 and decreased further during Period 5. Concentrations of LH in plasma were greatest Ž P - 0.05. during Period
Fig. 4. Concentrations of progesterone, 17b-estradiol, LH and FSH during an entire estrous cycle in a representative sow from an unselected line and ranked into a low estimated ovulation rate group ŽLC..
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1, decreased and remained relatively constant during the remainder of the estrous cycle. Concentrations of P4 were lower Ž P - 0.05. during behavioral estrus and after luteolysis. During days 2 to 4, concentrations of P4 began to increase and were greatest during the luteal phase. Concentrations of E 2 in plasma were greatest Ž P - 0.05. after luteolysis and before behavioral estrus. At onset of behavioral estrus, concentrations of E 2 decreased and a further decrease occurred during the luteal phase of the estrous cycle. One sow from the HI group had much greater concentrations of P4 during Periods 3 Ž351 ngrml. and 4 Ž269 ngrml. and also E 2 during Period 5 Ž456 pgrml. when compared with mean concentrations of P4 and E 2 during respective periods for the other sows of the HI group. Without data from this sow, corresponding concentrations of P4 and E 2 were 57 ngrml, and 39 pgrml, respectively. However, patterns of P4 and E 2 during the estrous cycle from this sow were similar to those of other sows from the HI group. The inference of the statistical analyses of the P4 and E 2 data did not change with or without data from this sow included. Therefore, values from this sow were included in the statistical analyses. Patterns of secretion of P4 , E 2 , FSH and LH for a representative sow of each group are shown in Figs. 1–4.
4. Discussion Concentrations of FSH during the estrous cycle in sows from the index selected line did not differ from sows in the control line. These results are consistent with those reported previously ŽHunter et al., 1993, 1996. in which prolific Meishan gilts and sows had concentrations of FSH similar to Large White females. No differences in FSH were observed during the periovulatory period in ewes from breeds with low and high ovulation rates ŽScaramuzzi et al., 1993.. In contrast, Kelly et al. Ž1988. and Knox Ž1992. observed that sows selected for high ovulation rate had greater concentrations of FSH than those of a control line. Inconsistency between results of studies may indicate that when different systems of selection were used different physiological control mechanisms may have affected ovulation rate. Collectively, the data from the various studies suggest that differences in rate of ovulation may be affected by variation in circulating concentrations of FSH ŽKelly et al., 1988. and Žor. follicular sensitivity to gonadotropins, variation in the biopotency of gonadotropins ŽAdams et al., 1988. or altered autocrine and paracrine factors in the ovary ŽDriancourt et al., 1990.. Concentrations of FSH were greater during the secondary increases as expected because it has been shown that the secondary increase of FSH release may be greater and more prolonged than the primary preovulatory surge of FSH in pigs ŽVan de Wiel et al., 1981; Kelly et al., 1988.. The physiological significance of this increase is not well understood but it has been implicated in recruitment of ovarian follicles from the resting pool of follicles in the ovary ŽKelly et al., 1988.. In contrast, Knox et al. Ž1991. failed to identify a difference in the rate of ovulation when the secondary surge of FSH was partially suppressed using charcoal extracted follicular fluid. The amount of FSH in circulation during the secondary increase has been associated with differences in ovulation rates of ewes ŽCahill et al., 1981; Lahlou-Kassi et al., 1984..
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The pattern of LH secretion during the estrous cycle was similar between lines in the present study and the preovulatory surge of LH occurred during Period 1 corresponding to the time of behavioral estrus ŽVan de Wiel et al., 1981; Kelly et al., 1988; Knox et al., 1991.. Likewise, no differences in LH were observed between Meishan and Large White females ŽHunter et al., 1993, 1996.. Concentrations of P4 in the two lines followed the pattern reported in several studies ŽHenricks et al., 1972; Parvizi et al., 1976; Van de Wiel et al., 1981; Kelly et al., 1988.. Although line differences were detected in plasma P4 concentrations, samples were collected too infrequently to determine possible differences in the immediate increase in P4 that occurs after ovulation and appears to be associated with greater embryonic survival ŽJindal et al., 1997.. Plasma concentrations of E 2 did not differ between the two lines of sows, a finding which agrees with those of the previous studies using females with high and low ovulation rates ŽGuthrie et al., 1974; Kelly et al., 1988; Hunter et al., 1993.. Hunter et al. Ž1993. reported similar concentrations of E 2 in Meishan and Large White gilts that had similar ovulation rates. In a second study where ovulation was greater in Meishan than in Large White sows, there was an association between ovulation rates and concentrations of E 2 ŽHunter et al., 1996.. Thus, in Meishan females more ovulatory follicles produce greater circulating concentrations of E 2 ; yet, Meishan females appear to metabolize E 2 more rapidly than Large White females ŽTilton et al., 1994.. In the current study, the peak concentrations of E 2 occurred before the surge of LH when lowest concentrations of FSH were observed. The relative contributions of E 2 and inhibin to negative feedback regulation of FSH secretion have not been defined in detail in pigs. Both are produced by developing follicles during the follicular phase of the estrous cycle, and inhibin has been implicated to have a greater influence on FSH secretion during this time than E 2 ŽHasegawa et al., 1988; Trout et al., 1992.. Collectively, all the studies evaluating reproductive hormones during the periovulatory period have indicated that genetic lines with differing ovulation rates may or may not have different hormone concentrations.
5. Conclusions Selection based on an index which emphasized increased ovulation rate did not affect the concentrations and pattern of secretion of FSH, LH, P4 and E 2 in sows. Sows with similar ovulation rates from the select and control line also had similar concentrations of FSH, LH, P4 and E 2 and concentrations of these hormones in the blood were not related to ovulation rate. Other factors such as interovarian components may be implicated in regulation of ovulation rate in this line.
Acknowledgements We thank Dr. D.J. Bolt for providing porcine antisera and FSH, and Bruce Melson and Felipe Rodriguez for the assistance in laboratory and statistical analyses, respec-
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tively. This paper is published as paper no. 11913, Nebraska Agricultural Research Division. Mention of a trade name, proprietary product or specific equipment does not constitute a guarantee or warranty by the USDA and does not imply approval to the exclusion of other products that may be suitable.
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