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Plasma luteinizing hormone during pregnancy in the pig
Animal Reproduction Science, 5 (1982/1983) 213--218
213
Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands
PLASMA LUTEINIZING HORMONE DURING PREGNANCY IN THE PIG A. ZIECIK 1, J.E. TILTON, R. WEIGL and G.L. WILLIAMS
North Dakota State University, Fargo, ND 58105 (U.S.A.) ~Institute of Animal Physiology and Biochemistry, University of Agriculture and Technology, 10-718 Olsztyn (Poland) (Accepted 11 October 1982)
ABSTRACT
Ziecik, A., Tilton, J.E., Weigl, R. and Williams, G.L., 1983. Plasma luteinizing hormone during pregnancy in the pig. Anim. Reprod. Sci., 5: 213--218. Three chronically catheterized Duroc gilts were used to characterize the pattern of plasma LH in the systemic circulation during pregnancy. Blood samples were collected four times daily (08.00, 12.00, 16.00 and 20.00 h) from the second day of estrus until day 7 postpartum in one pig and to 108 and 98 days of gestation in the remaining two. The concentration of plasma LH fluctuated in a pulsatile manner throughout the studied periods of gestation in all three pigs, with decreasing amplitude towards parturition. Significant correlations between the decline of LH levels and the day o f pregnancy were found, and the equations for the linear regression lines are presented. It is suggested that the level of LH in early and mid-pregnancy mimics LH concentrations in the midluteal phase of the estrous cycle.
INTRODUCTION
A few studies of porcine luteinizing hormone (LH) during early pregnancy have been reported and certain aspects of these studies are contradictory. Plasma LH concentrations during early gestation in domestic pigs reported by Guthrie et al. (1972) and Ziecik et al. (1982) are different from those described by Tillson et al. (1970). Data concerning LH level during mid gestation are limited (Ziecik et al., 1982). Parvizi et al. (1976), using miniature pigs, observed low plasma LH concentration throughout late pregnancy and showed increases in LH 3 weeks before parturition. The purpose of this experiment was to quantify LH concentration during the entire period of gestation in the same chronically catheterized pigs. MATERIALS AND METHODS
The experiment was conducted with three Duroc gilts. The pigs were mated naturally approximately 4 h after the onset of estrus and again the
0378-4320/83/0000--0000/$03.00
© 1983 Elsevier Scientific Publishing Company
214 morning of the next day, designated Day 1. Four days before sampling was begun a cannula was inserted into the vena cava via a cephalic vein exteriorized by passage under the skin to the back to facilitate blood collection. Blood samples (3 ml) were collected four times daffy at 08.00, 12.00, 16.00 and 20.00 h from the second day of estrus to 7 days postpartum in one pig (No. 1) and to 108 (No. 2) and 98 (No. 3) days of pregnancy in the remaining pigs because of obstruction of the cannula. Parturition occurred in the all pigs at the normal term (114--117 days). The heparinized plasma supernatant was removed and stored a t - - 2 0 ° C until analyzed. LH concentration was estimated by a homologous double-antibody radioimmunoassay with porcine LH-GPZ-1 (biological p o t e n c y 0.62 × NIH-LH-S1) used as a standard and for iodination, as described by Ziecik et al. (1978). Anti-LH serum (Ziecik et al., 1979) was used at a final dilution of 1 : 200 000. The sensitivity of the assay was 0.1 ng LH/ml. The within-assay coefficient of variation was 4.6% and the between-assay coefficient was 11.3%. Data were analyzed by split-plot analysis o f variance appropriate for repeated measurements in individual animals (Gill and Hafs, 1971). Hours of collection were used as the first variable and 12-day periods of pregnancy as the second. Similarly, mean amplitudes of LH pulses were determined for the same periods in each animal and used to compare amplitudes of pulses among 12
215
8]
ptg No 1
io,3~ [g,~
:]
II
---- ptg No 2 .......... pLg No3
5 4 3
,
2
'
~
i
t
12
c
-r 5'
3 2" 1
i i , , , ,
,,,,
,,-,
,r,,
" ' ' r ' ' ' ,
" " , " ' , " ' ,
" ' ,
" ' ,
25
r,,,
2~
" ' "
''''
'''1
,r,,
"''1
" ' ' l ' "
'
'
"
'
"
' "
"r''"
"
36
48
60
72
G-
54321 49 •
/
!
•
73
84
97 Deys
108 of pregnancy
parturition
96
+3
Fig. 1. Plasma LH fluctuations throughout gestation in three pigs. Day I is the second day of estrus.
1.10 2.25 1.25
2.20 2.58 1.33
1 2 3
1 2 3
16.00
20.00
1.50 a
1.11 1.29 2.21
1.15 1.59 1.46
1.82 2.56 1.41
1.14 1.41 0.95
13--24
1.36 b
0.49 1.50 1.57
1.80 1.62 1.42
1.00 1.75 1.21
0.94 1.74 1.36
25--36
1.34 b
0.37 1.28 1.67
0.67 1.62 2.04
0.57 2.02 1.22
2.41 0.84 1.39
37--48
1.12 b
0.80 1.05 1.44
0.27 2.27 1.74
0.28 1.94 1.19
0.18 1.20 1.16
49--60
1.17 b
1.13 1.66 1.21
1.76 1.27 1.16
1.32 1.21 0.79
0.82 0.95 0.77
61--72
1.25 b
2.49 1.07 0.82
2.67 0.79 0.47
2.24 0.52 0.47
2.09 0.85 0.57
73--84
a, b, c, d - - M e a n s w i t h a c o m m o n s u p e r s c r i p t d o n o t d i f f e r s i g n i f i c a n t l y (P ~ 0.05). d - - n = 96.
1.94 a
2.10 2.90 1.59
1 2 3
12.00
Average
1.45 2.86 1.67
1--12
Days o f p r e g n a n c y
1 2 3
Pig No.
08.00
(h)
Blood sample collection
0.75 c
1.12 0.80 0.68
1.27 0.48 0.51
1.13 0.43 0.48
1.17 0.52 0.44
85--96
0.48 d
0.58 0.42 --
0.60 0.48 --
0.47 0.47 --
0.40 0.44 --
97--108
M e a n p l a s m a L H c o n c e n t r a t i o n ( n g / m l ) in 1 2 - - d a y p e r i o d s o f p r e g n a n c y f o r e a c h t i m e o f c o l l e c t i o n f r o m individual pigs (n = 1 2 ) a n d average p l a s m a L H level for t h r e e pigs (n = 1 4 4 ) a t c h o s e n i n t e r v a l ( 1 2 d a y s ) f o r g r o u p i n g t h e d a t a . M e a n s a n d averages S.E.M. varied f r o m 7.1 t o 3 6 . 6 a n d f r o m 5.8 t o 7.4%, r e s p e c t i v e l y , o f t h e values s h o w n .
TABLE I
t~
217 DISCUSSION Following the preovulatory LH surge in the pig, the LH concentrations remain low until Day 8 when high episodic surges are observed (Wilfinger, 1974). These midluteal phase LH surges terminate at Day 14 of the estrous cycle and thereafter lower concentrations of plasma LH persist. Sampling four times daily in our study clearly showed that the level of LH in early and mid pregnancy is similar to the LH concentrations in the midluteal phase of the estrous cycle. This does not eliminate the possibility that "pulses" observed in our investigation consist of a few peaks. Large episodic surges of LH with a duration of 2--4 h and a frequency varying between six and eight peaks per 24 h in the luteal phase of the estrous cycle have been reported by Van de Wiel et al. (1981). Pulses occurring in the midluteal phase of the cycle and in the first half of pregnancy (Fig. 1) reached approximately 30--50% of the amplitude of the preovulatory surges (Tilton et ah, 1982). In general, our results confirmed earlier observations (Guthrie et al., 1972; Ziecik et al., 1982) which demonstrated high concentrations of LH in early pregnancy and the pulsatile manner of secretion in mid (Ziecik et al., 1982) and late (Parvizi et al., 1976) gestation. This study did n o t confirm the early results of Tillson et al. (1970) indicating that levels of LH in peripheral plasma of nonpregnant sows tended to be higher than those in pregnant. Our results suggest that the level of LH in early and mid pregnancy mimics the LH concentrations in the midluteal phase of the estrous cycle. It is n o t self evident that LH is the only hormone involved in luteal function during pregnancy (Saunders et al., 1980). Putative luteotropin does n o t cross-react with porcine LH in radioimmunoassay but treatment with antibodies to LH resulted in atrophy of corpora lutea and complete loss of embryos in pregnant gilts (Spies et al., 1967). The theory of maternal recognition of pregnancy in pigs (Bazer and Thatcher, 1977) is based upon evidence that the blastocysts synthesize estrogens, which block the movement of the pig uterine luteolysin to the uterine venous drainage. Estrogens, which are luteotropic in the intact pig (Kidder et al., 1955; Gardner et al., 1963), fail to maintain corpora lutea in gilts following hypophysial stalk transection (Anderson et al., 1967) and in in vitro studies (Watson and Maule Walker, 1978). The luteotropic role of estrogens in the pig during pregnancy may be a stimulus to increased LH receptor concentrations because estradiol benzoate appears to increase the number of unoccupied LH receptors independently of the uterus (H.A. Garverick, personal communication, 1981). On the other hand, increasing levels of estrogens during gestation (Robertson and King, 1974) can exert an inhibitory effect on the basal LH concentration, and on the amplitude of LH pulses. This could explain the decrease in LH concentrations during pregnancy observed in our investigation. The drop in LH concentrations shown here may lead to a parallel decline in the concentration of progesterone (Guthrie et al., 1972; H. Krzymowska, A. Ziecik, unpublished observations).
218 REFERENCES
Anderson, L.L., Dyck, G.W., Mori, H., Henricks, D.M. and Melampy, R.M., 1967. Ovarian function in pigs following hypophysial stalk transection or hypophysectomy. Am. J. Physiol., 212: 1188--1194. Bazer, F.W. and Thatcher, W.W., 1977. Theory of maternal recognition of pregnancy in swine based on estrogen controlled endocrine versus exocrine secretion of prostaglandin F2a by the uterine endometrium. Prostaglandins, 14: 397--401. Gardner, M.L., First, N.L. and Casida L.E., 1963. Effect of exogenous estrogens on corpus luteum maintenance in gilts. J. Anita. Sci., 22: 132--134. Gill, J.L. and Hafs, H.D., 1971. Analysis of repeated measurements of animals. J. Anim. Sci., 33: 331--336. Guthrie, H.D., Henricks, D.M. and Handlin, D.L., 1972. Plasma estrogen, progesterone and LH prior to estrus and during early pregnancy in pigs. Endocrinology, 91 : 675--679. Kidder, W.E., Casida, L.E. and Grummer, R.H., 1955. Some effects of estrogen injections on the estrual cycle of gilts. J. Anim. Sci., 14: 470--474. Parvizi, N., Elsaesser, F., Smidt, D. and Ellendorff, F., 1976. Plasma luteinizing hormone and progesterone in the adul t female pig during the oestrus cycle, late pregnancy and lactation, and after ovariectomy and pentobarbitone treatment. J. Endocrinol., 69: 193--203. Robertson, H.A. and King, G.J., 1974. Plasma concentrations of progesterone, oestrone, oestradiol-17~ and of oestrone sulphate in the pig at implantation, during pregnancy and at parturition. J. Reprod. Fertil., 40: 133--141. Saunders, P.T.K., Ziecik, A.J. and Flint, A.P.F., 1980. Gonadotrophin-like substance in pig placenta and embryonic membranes. J. Endocrinol., 85: 25P. Spies, H.G., Slyter, A.L. and Quadri, S.K., 1967. Regression of corpora lutea in pregnant gilts adminstered antiovine LH rabbit serum. J. Anim. Sci., 26: 768--771. Tillson, S.A., Erb, R.E. and Niswender, G.D., 1970. Comparison of luteinizing hormone and progesterone in blood and metabolites of progesterone in urine of domestic sows during the estrous cycle and early pregnancy. J. Anim. Sci., 30: 795--805. Tilton, J.E., Foxcroft, G.R., Ziecik, A.J., Coombs, S.L. and Williams, G.L., 1982. Time of the preovulatory LH surge in the gilt and sow relative to the onset of behavioral estrus. Teriogenology (in press). Van de Wiel, D.F.M., Erkens, J., Koops, W., Vos, E. and Van Landeghem, A.A.J., 1981. Periestrus and midluteal time courses of circulating LH, FSH, prolactin, estradiol-17~ and progesterone in the domestic pig. Biol. Reprod., 24: 223--233. Watson, J. and Maule Walker, F.M., 1978. Progesterone secretion bY the corpus luteum of the early pregnant pig during superfusion in vitro with PGF-2a, LH and oestradiol. J. Reprod. Fertil., 52: 209--212. Wilfinger, W.W., 1974. Plasma concentrations of luteinizing hormone, follicle stimulating hormone and prolactin in ovariectomized, hysterectomized and intact swine. Ph.D. Thesis, University of Maryland, College Park, MD. Ziecik, A., Goralska, M., Krzymowski, T. and Pogorzelski, K., 1978. Isolation and purification of porcine LH for radioimmunoassay and radioreceptor assay. Bull. Acad. Pol. Sci., CI.II. Ser. Sci. Biol., 26: 739--744. Ziecik, A., Krzymowska, H. and Sobczak, E., 1979. Production of antibodies against the porcine luteinizing hormone by two immunization methods. Endokrinologie, 74: 304--308. Ziecik, A., Krzymowska, H. and Tilton, J.E., 1982. Porcine LH levels during estrous cycle, gestation, parturition and early lactation. J. Anita. Sci., 54: 1221--1226.
213
Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands
PLASMA LUTEINIZING HORMONE DURING PREGNANCY IN THE PIG A. ZIECIK 1, J.E. TILTON, R. WEIGL and G.L. WILLIAMS
North Dakota State University, Fargo, ND 58105 (U.S.A.) ~Institute of Animal Physiology and Biochemistry, University of Agriculture and Technology, 10-718 Olsztyn (Poland) (Accepted 11 October 1982)
ABSTRACT
Ziecik, A., Tilton, J.E., Weigl, R. and Williams, G.L., 1983. Plasma luteinizing hormone during pregnancy in the pig. Anim. Reprod. Sci., 5: 213--218. Three chronically catheterized Duroc gilts were used to characterize the pattern of plasma LH in the systemic circulation during pregnancy. Blood samples were collected four times daily (08.00, 12.00, 16.00 and 20.00 h) from the second day of estrus until day 7 postpartum in one pig and to 108 and 98 days of gestation in the remaining two. The concentration of plasma LH fluctuated in a pulsatile manner throughout the studied periods of gestation in all three pigs, with decreasing amplitude towards parturition. Significant correlations between the decline of LH levels and the day o f pregnancy were found, and the equations for the linear regression lines are presented. It is suggested that the level of LH in early and mid-pregnancy mimics LH concentrations in the midluteal phase of the estrous cycle.
INTRODUCTION
A few studies of porcine luteinizing hormone (LH) during early pregnancy have been reported and certain aspects of these studies are contradictory. Plasma LH concentrations during early gestation in domestic pigs reported by Guthrie et al. (1972) and Ziecik et al. (1982) are different from those described by Tillson et al. (1970). Data concerning LH level during mid gestation are limited (Ziecik et al., 1982). Parvizi et al. (1976), using miniature pigs, observed low plasma LH concentration throughout late pregnancy and showed increases in LH 3 weeks before parturition. The purpose of this experiment was to quantify LH concentration during the entire period of gestation in the same chronically catheterized pigs. MATERIALS AND METHODS
The experiment was conducted with three Duroc gilts. The pigs were mated naturally approximately 4 h after the onset of estrus and again the
0378-4320/83/0000--0000/$03.00
© 1983 Elsevier Scientific Publishing Company
214 morning of the next day, designated Day 1. Four days before sampling was begun a cannula was inserted into the vena cava via a cephalic vein exteriorized by passage under the skin to the back to facilitate blood collection. Blood samples (3 ml) were collected four times daffy at 08.00, 12.00, 16.00 and 20.00 h from the second day of estrus to 7 days postpartum in one pig (No. 1) and to 108 (No. 2) and 98 (No. 3) days of pregnancy in the remaining pigs because of obstruction of the cannula. Parturition occurred in the all pigs at the normal term (114--117 days). The heparinized plasma supernatant was removed and stored a t - - 2 0 ° C until analyzed. LH concentration was estimated by a homologous double-antibody radioimmunoassay with porcine LH-GPZ-1 (biological p o t e n c y 0.62 × NIH-LH-S1) used as a standard and for iodination, as described by Ziecik et al. (1978). Anti-LH serum (Ziecik et al., 1979) was used at a final dilution of 1 : 200 000. The sensitivity of the assay was 0.1 ng LH/ml. The within-assay coefficient of variation was 4.6% and the between-assay coefficient was 11.3%. Data were analyzed by split-plot analysis o f variance appropriate for repeated measurements in individual animals (Gill and Hafs, 1971). Hours of collection were used as the first variable and 12-day periods of pregnancy as the second. Similarly, mean amplitudes of LH pulses were determined for the same periods in each animal and used to compare amplitudes of pulses among 12
215
8]
ptg No 1
io,3~ [g,~
:]
II
---- ptg No 2 .......... pLg No3
5 4 3
,
2
'
~
i
t
12
c
-r 5'
3 2" 1
i i , , , ,
,,,,
,,-,
,r,,
" ' ' r ' ' ' ,
" " , " ' , " ' ,
" ' ,
" ' ,
25
r,,,
2~
" ' "
''''
'''1
,r,,
"''1
" ' ' l ' "
'
'
"
'
"
' "
"r''"
"
36
48
60
72
G-
54321 49 •
/
!
•
73
84
97 Deys
108 of pregnancy
parturition
96
+3
Fig. 1. Plasma LH fluctuations throughout gestation in three pigs. Day I is the second day of estrus.
1.10 2.25 1.25
2.20 2.58 1.33
1 2 3
1 2 3
16.00
20.00
1.50 a
1.11 1.29 2.21
1.15 1.59 1.46
1.82 2.56 1.41
1.14 1.41 0.95
13--24
1.36 b
0.49 1.50 1.57
1.80 1.62 1.42
1.00 1.75 1.21
0.94 1.74 1.36
25--36
1.34 b
0.37 1.28 1.67
0.67 1.62 2.04
0.57 2.02 1.22
2.41 0.84 1.39
37--48
1.12 b
0.80 1.05 1.44
0.27 2.27 1.74
0.28 1.94 1.19
0.18 1.20 1.16
49--60
1.17 b
1.13 1.66 1.21
1.76 1.27 1.16
1.32 1.21 0.79
0.82 0.95 0.77
61--72
1.25 b
2.49 1.07 0.82
2.67 0.79 0.47
2.24 0.52 0.47
2.09 0.85 0.57
73--84
a, b, c, d - - M e a n s w i t h a c o m m o n s u p e r s c r i p t d o n o t d i f f e r s i g n i f i c a n t l y (P ~ 0.05). d - - n = 96.
1.94 a
2.10 2.90 1.59
1 2 3
12.00
Average
1.45 2.86 1.67
1--12
Days o f p r e g n a n c y
1 2 3
Pig No.
08.00
(h)
Blood sample collection
0.75 c
1.12 0.80 0.68
1.27 0.48 0.51
1.13 0.43 0.48
1.17 0.52 0.44
85--96
0.48 d
0.58 0.42 --
0.60 0.48 --
0.47 0.47 --
0.40 0.44 --
97--108
M e a n p l a s m a L H c o n c e n t r a t i o n ( n g / m l ) in 1 2 - - d a y p e r i o d s o f p r e g n a n c y f o r e a c h t i m e o f c o l l e c t i o n f r o m individual pigs (n = 1 2 ) a n d average p l a s m a L H level for t h r e e pigs (n = 1 4 4 ) a t c h o s e n i n t e r v a l ( 1 2 d a y s ) f o r g r o u p i n g t h e d a t a . M e a n s a n d averages S.E.M. varied f r o m 7.1 t o 3 6 . 6 a n d f r o m 5.8 t o 7.4%, r e s p e c t i v e l y , o f t h e values s h o w n .
TABLE I
t~
217 DISCUSSION Following the preovulatory LH surge in the pig, the LH concentrations remain low until Day 8 when high episodic surges are observed (Wilfinger, 1974). These midluteal phase LH surges terminate at Day 14 of the estrous cycle and thereafter lower concentrations of plasma LH persist. Sampling four times daily in our study clearly showed that the level of LH in early and mid pregnancy is similar to the LH concentrations in the midluteal phase of the estrous cycle. This does not eliminate the possibility that "pulses" observed in our investigation consist of a few peaks. Large episodic surges of LH with a duration of 2--4 h and a frequency varying between six and eight peaks per 24 h in the luteal phase of the estrous cycle have been reported by Van de Wiel et al. (1981). Pulses occurring in the midluteal phase of the cycle and in the first half of pregnancy (Fig. 1) reached approximately 30--50% of the amplitude of the preovulatory surges (Tilton et ah, 1982). In general, our results confirmed earlier observations (Guthrie et al., 1972; Ziecik et al., 1982) which demonstrated high concentrations of LH in early pregnancy and the pulsatile manner of secretion in mid (Ziecik et al., 1982) and late (Parvizi et al., 1976) gestation. This study did n o t confirm the early results of Tillson et al. (1970) indicating that levels of LH in peripheral plasma of nonpregnant sows tended to be higher than those in pregnant. Our results suggest that the level of LH in early and mid pregnancy mimics the LH concentrations in the midluteal phase of the estrous cycle. It is n o t self evident that LH is the only hormone involved in luteal function during pregnancy (Saunders et al., 1980). Putative luteotropin does n o t cross-react with porcine LH in radioimmunoassay but treatment with antibodies to LH resulted in atrophy of corpora lutea and complete loss of embryos in pregnant gilts (Spies et al., 1967). The theory of maternal recognition of pregnancy in pigs (Bazer and Thatcher, 1977) is based upon evidence that the blastocysts synthesize estrogens, which block the movement of the pig uterine luteolysin to the uterine venous drainage. Estrogens, which are luteotropic in the intact pig (Kidder et al., 1955; Gardner et al., 1963), fail to maintain corpora lutea in gilts following hypophysial stalk transection (Anderson et al., 1967) and in in vitro studies (Watson and Maule Walker, 1978). The luteotropic role of estrogens in the pig during pregnancy may be a stimulus to increased LH receptor concentrations because estradiol benzoate appears to increase the number of unoccupied LH receptors independently of the uterus (H.A. Garverick, personal communication, 1981). On the other hand, increasing levels of estrogens during gestation (Robertson and King, 1974) can exert an inhibitory effect on the basal LH concentration, and on the amplitude of LH pulses. This could explain the decrease in LH concentrations during pregnancy observed in our investigation. The drop in LH concentrations shown here may lead to a parallel decline in the concentration of progesterone (Guthrie et al., 1972; H. Krzymowska, A. Ziecik, unpublished observations).
218 REFERENCES
Anderson, L.L., Dyck, G.W., Mori, H., Henricks, D.M. and Melampy, R.M., 1967. Ovarian function in pigs following hypophysial stalk transection or hypophysectomy. Am. J. Physiol., 212: 1188--1194. Bazer, F.W. and Thatcher, W.W., 1977. Theory of maternal recognition of pregnancy in swine based on estrogen controlled endocrine versus exocrine secretion of prostaglandin F2a by the uterine endometrium. Prostaglandins, 14: 397--401. Gardner, M.L., First, N.L. and Casida L.E., 1963. Effect of exogenous estrogens on corpus luteum maintenance in gilts. J. Anita. Sci., 22: 132--134. Gill, J.L. and Hafs, H.D., 1971. Analysis of repeated measurements of animals. J. Anim. Sci., 33: 331--336. Guthrie, H.D., Henricks, D.M. and Handlin, D.L., 1972. Plasma estrogen, progesterone and LH prior to estrus and during early pregnancy in pigs. Endocrinology, 91 : 675--679. Kidder, W.E., Casida, L.E. and Grummer, R.H., 1955. Some effects of estrogen injections on the estrual cycle of gilts. J. Anim. Sci., 14: 470--474. Parvizi, N., Elsaesser, F., Smidt, D. and Ellendorff, F., 1976. Plasma luteinizing hormone and progesterone in the adul t female pig during the oestrus cycle, late pregnancy and lactation, and after ovariectomy and pentobarbitone treatment. J. Endocrinol., 69: 193--203. Robertson, H.A. and King, G.J., 1974. Plasma concentrations of progesterone, oestrone, oestradiol-17~ and of oestrone sulphate in the pig at implantation, during pregnancy and at parturition. J. Reprod. Fertil., 40: 133--141. Saunders, P.T.K., Ziecik, A.J. and Flint, A.P.F., 1980. Gonadotrophin-like substance in pig placenta and embryonic membranes. J. Endocrinol., 85: 25P. Spies, H.G., Slyter, A.L. and Quadri, S.K., 1967. Regression of corpora lutea in pregnant gilts adminstered antiovine LH rabbit serum. J. Anim. Sci., 26: 768--771. Tillson, S.A., Erb, R.E. and Niswender, G.D., 1970. Comparison of luteinizing hormone and progesterone in blood and metabolites of progesterone in urine of domestic sows during the estrous cycle and early pregnancy. J. Anim. Sci., 30: 795--805. Tilton, J.E., Foxcroft, G.R., Ziecik, A.J., Coombs, S.L. and Williams, G.L., 1982. Time of the preovulatory LH surge in the gilt and sow relative to the onset of behavioral estrus. Teriogenology (in press). Van de Wiel, D.F.M., Erkens, J., Koops, W., Vos, E. and Van Landeghem, A.A.J., 1981. Periestrus and midluteal time courses of circulating LH, FSH, prolactin, estradiol-17~ and progesterone in the domestic pig. Biol. Reprod., 24: 223--233. Watson, J. and Maule Walker, F.M., 1978. Progesterone secretion bY the corpus luteum of the early pregnant pig during superfusion in vitro with PGF-2a, LH and oestradiol. J. Reprod. Fertil., 52: 209--212. Wilfinger, W.W., 1974. Plasma concentrations of luteinizing hormone, follicle stimulating hormone and prolactin in ovariectomized, hysterectomized and intact swine. Ph.D. Thesis, University of Maryland, College Park, MD. Ziecik, A., Goralska, M., Krzymowski, T. and Pogorzelski, K., 1978. Isolation and purification of porcine LH for radioimmunoassay and radioreceptor assay. Bull. Acad. Pol. Sci., CI.II. Ser. Sci. Biol., 26: 739--744. Ziecik, A., Krzymowska, H. and Sobczak, E., 1979. Production of antibodies against the porcine luteinizing hormone by two immunization methods. Endokrinologie, 74: 304--308. Ziecik, A., Krzymowska, H. and Tilton, J.E., 1982. Porcine LH levels during estrous cycle, gestation, parturition and early lactation. J. Anita. Sci., 54: 1221--1226.