430 H. Y. Al-Matubsi and R. J. Faircloughinhibiting prostaglandin synthesis using ﬁnadyne, which is were placed in heparinized glass tubes and the catheter wasan inhibitor of the cyclo-oxygenase pathway. reﬁlled with heparinized saline (50 iu ml–1). Ovarian venous blood was collected using the method described by McCracken et al. (1969). Approximately 5 ml Materials and Methods ovarian blood was allowed to drain freely every 30 minExperimental animals from the open end of the catheter into heparinized 15 ml graduated centrifuge tubes. The time taken to collect this Border Leicester Merino ewes (n = 9) were prepared sample was measured using a stopwatch. Three millilitres ofwith ovarian autotransplants as described by Goding et al. ovarian venous blood were collected at alternate 15 min(1967). The ewes were housed individually in metabolic intervals after collection of each 5 ml sample aftercages in a temperature-controlled room (20 C) and were fed oestradiol injection. Thus, samples were collected everyonce a day with 800 g of a pelleted ration consisting of 15 min for determination of hormone concentrations andhammer milled lucerne (60%) and oats (40%). Water was every 30 min for determination of blood ﬂow (Lamsa et al.,available ad libitum. The study was carried out at CSIRO 1989). The blood samples were centrifuged at 1900 g forDivision of Animal Production, Australia. All protocols 15 min. Plasma was collected and stored at –20 C untilwere approved by the Animal Experimentation Ethics assayed for oxytocin and progesterone (ovarian venousCommittees of Victoria University of Technology and the plasma) or PGFM and oestradiol (jugular venous plasma) byCSIRO Division of Animal Production. radioimmunoassay (RIA). Blood ﬂow (ml min–1) was calculated by measuring the time taken to collect a knownExperimental design volume of ovarian venous blood. The packed cell volume As ewes with autotransplanted ovaries do not naturally (PCV) was determined at 1 h intervals and the plasma ﬂowundergo oestrous cycles, oestrus was induced synchro- (ml min–1) was calculated by multiplying the blood ﬂow bynously by two injections of 125 µg synthetic PGF2α 100-PCV divided by 100. The secretion rate of oxytocin and(Estrumate; ICI, Sydney) given 15 days apart. After the progesterone (ng min–1) was obtained by multiplying thesecond injection, oestrus was detected by inspection twice plasma ﬂow (ml min–1) by the concentration of hormone ina day for the presence of crayon marks after mating with a the ovarian venous plasma (ng ml–1).ram ﬁtted with a sire-o-sine harness (Radford et al., 1960).The day that the ewes displayed oestrous behaviour Hormone analysiswas designated day 0. On day 15 of the cycle, all ewes PGFM assay. Plasma PGFM concentrations werewere injected i.m. with 50 µg oestradiol benzoate measured by RIA (Burgess et al., 1990) with a sensitivity of(Intervet, Sydney) in peanut oil. In addition, four of 8 pg ml–1. The intra- and interassay coefﬁcients of variationthese ewes were injected i.m. with 2.2 mg kg–1 of the were 8 and 11%, respectively.prostaglandin synthetase inhibitor, ﬁnadyne (AllhankTrading Company, Melbourne) at 3 h intervals starting at Oestradiol assay. The concentration of oestradiol wasthe time of oestradiol injection. The remaining ﬁve ewes measured in peripheral blood plasma by RIA (Burgess et al.,received vehicle only. 1990) with a sensitivity of 7 pg ml–1. The samples were measured in a single assay and the intra-assay coefﬁcient ofCannulation of jugular and ovarian veins variation was 4.7%. Cannulations were carried out under local anaesthesia(10% lignocaine hydrochloride spray: xylocaine; Astra Progesterone assay. Progesterone was assayed in 100 µlPharmaceuticals, Sydney) as described by McCracken et al. ovarian plasma extracted with 2 ml n-hexane (Crown(1969) at least 24 h before the start of blood sampling. In Scientiﬁc, Victoria) according to the method of Rice et al.brief, a polyvinyl catheter was inserted into the jugular vein (1986). The sensitivity of the assay was 0.25 ng ml–1. Theexteriorized in the skin loop to cannulate the ovarian vein. samples were measured in a single assay and the intra-assayThe tip of the catheter was positioned at the junction of the coefﬁcient of variation was 7%.ovarian and jugular veins. An additional polyvinyl catheter(50 cm) was inserted into the contralateral jugular vein. The Oxytocin assay. Plasma oxytocin concentrations werecatheters were ﬁlled with heparinized saline (1000 iu ml–1). measured by RIA as described by Al-Matubsi et al. (1997). The sensitivity of the assay was 16 pg ml–1, and the intra-Blood sampling and interassay coefﬁcients of variation were 6 and 11.9%, respectively. On day 15 after oestrus, 5 ml and 3 ml samples of bloodwere collected from the ovarian and contralateral jugular Statistical analysisveins, respectively, at 30 min intervals for 6 h beforeoestradiol or ﬁnadyne injections and subsequently at Statistically signiﬁcant pulses of ovarian vein oxytocin15 min intervals for up to 9 h after injection. The blood and jugular vein PGFM were determined using a Pulsarsamples (3 ml) collected from the contralateral jugular vein program (Merriam and Wachter, 1982). Assay variability
Effects of ﬁnadyne on oestradiol-induced secretion of oxytocin and PGF2α during late oestrus 431 (a) was estimated by regression analysis of the standard 3000 60 500 deviation for duplicate determinations and the mean at 55 a 2500 50 400 each point. Baseline was calculated representing the 45 2000 40 contribution of long-term trends (15 h) but not ﬂuctuations 35 300 of shorter duration (30 min). The amplitudes of the ovarian 1500 30 25 oxytocin and peripheral PGFM pulses were calculated by 200 1000 20 subtracting baseline values. The resulting values were then 15 500 10 100 rescaled in terms of standard deviation units by dividing the 5 rescaled values by an estimate of assay variability. The 0 0 0 –6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6 7 8 9 amplitude of the rescaled pulses was identiﬁed by applying height and duration criteria speciﬁed by user-deﬁned cut-off (b) points [G(n)] for pulses. These calculations were repeated 3000 60 500 55 until two iterations produced the same values for pulses or 2500 50 400 until the preset limit of six iterations was completed. The 45 2000 40 quadratic (a), linear (b), and constant (c) terms for Pulsar 35 300 were as follows: for oxytocin: a = 0.00, b = 11.91 and 1500 30 25 200 c = 0.00; and for PGFM: a = 0.00, b = 11.17 and c = 0.00. 1000 20 15 The following G(n) values were selected for both oxytocin 500 10 100 and PGFM pulses: G(1) = 6.5, G(2) = 4.45, G(3) = 3.25, 5 0 0 0 G(4) = 2.57 and G(5) = 2.05. Coincident episodes in the –6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6 7 8 9 secretion of oxytocin and PGFM were deﬁned as those that (c) showed an increase in the value of the PGFM pulse at the same time as a deﬁned oxytocin pulse. The plasmaProgesterone (ng min–1) 3000 60 500 55 a secretion rates of oxytocin and concentrations of PGFM 2500 50 PGFM (pg ml–1) 400 Oxytocin (ng min–1) 45 pulses were expressed in ng min–1 and pg ml–1, respectively, 2000 40 35 300 and the duration of that pulse was designated as τ being the 1500 30 number of minutes between the last time point before and 25 200 1000 20 the ﬁrst time point after a signiﬁcant increase in hormone 15 100 concentration as detected by the Pulsar program. The area 500 10 5 under the signiﬁcant ovarian oxytocin and peripheral PGFM 0 0 0 pulses was then calculated for each ewe and was expressed –6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6 7 8 9 as (ng min–1) τ and (pg ml–1) τ, respectively. The overall (d) mean concentration, pulse amplitude and duration of the 3000 60 500 55 ab pulse, and the area under the pulse were obtained using the 2500 50 400 Pulsar analysis program. The values were expressed as 45 2000 40 mean SEM. Individual characteristics of these responses 35 300 and the differences in concentrations of oestrogen and 1500 30 25 200 progesterone were compared using a Student’s unpaired t 1000 20 test. The number of ewes that showed pulses of oxytocin 15 500 10 100 and PGFM after oestradiol or oestradiol plus ﬁnadyne 5 0 0 0 injections was compared using a chi-squared test. –6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6 7 8 9 (e) Results 3000 60 a 500 55 The progesterone secretion rate of oestradiol-treated ewes 2500 50 45 400 (679.04 ± 87.98 ng min–1) was not signiﬁcantly different 2000 40 from that in the oestradiol–ﬁnadyne-treated ewes 35 300 1500 30 (762.77 141.76 ng min–1). Progesterone secretion re- 25 200 mained high during the sampling period, indicating the 1000 20 15 presence of a functional corpus luteum in both groups (Figs 100 500 10 1 and 2). In both treated groups, circulating concentrations 5 0 0 0 –6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6 7 8 9 treated with oestradiol only on day 15 after oestrus. a and b indicate Time (h) signiﬁcant episodes in secretion of ovarian oxytocin and PGFM,Fig. 1. Oxytocin ( ) and progesterone ( ) secretion rates into respectively. : Identiﬁes synchronous episodes of secretion ofovarian venous plasma and concentrations of peripheral 13,14- both compounds. ⇓: Indicates time of oestradiol injection and ↓dihydro-15-keto PGF2α (PGFM; ) from individual ewes (a–e) indicates times of injection of ﬁnadyne vehicle (control).
432 H. Y. Al-Matubsi and R. J. Fairclough (a) The effect of intramuscular injections of oestradiol only 3000 60 b 500 and oestradiol plus ﬁnadyne on peripheral PGFM 55 2500 50 concentrations and ovarian oxytocin secretion are shown 400 45 (Figs 1 and 2, respectively). The mean basal ovarian 2000 40 35 300 oxytocin secretion rate for oestradiol–ﬁnadyne-treated ewes 1500 30 (0.47 0.09 ng min–1) was not signiﬁcantly different from 25 200 1000 20 that in oestradiol-treated ewes (0.50 0.16 ng min–1). 15 500 10 100 During the ﬁrst 6 h of the sampling period, before the 5 oestrogen and ﬁnadyne injections, the mean amplitude 0 0 0 –6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6 7 8 9 (6.57 1.39 ng min–1) and the mean area under the curve (2.33 0.88 ng min–1) τ for the ovarian oxytocin pulses in (b) oestradiol–ﬁnadyne-treated ewes were not signiﬁcantly 3000 60 500 55 a different from those in oestradiol-treated ewes 2500 50 400 (10.17 3.23 ng min–1 and 10.68 3.31 ng min–1 τ, 45 2000 40 respectively). 35 300 Administration of oestradiol plus ﬁnadyne to ovarian 1500 30 25 200 autotransplanted ewes on day 15 of the oestrous cycle 1000 20 signiﬁcantly (P < 0.05) reduced the number of ewesProgesterone (ng min–1) 15 500 10 100 PGFM (pg ml–1) showing pulses of oxytocin (n = 0 versus n = 5) and PGFM Oxytocin (ng min–1) 5 0 0 0 (n = 0 versus n = 5) when compared with ewes treated with –6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6 7 8 9 oestradiol only. None of the oestradiol–ﬁnadyne-treated (c) ewes showed signiﬁcant pulses in ovarian oxytocin 3000 60 500 secretion after injection. In oestradiol-treated ewes, at least 55 one detectable pulse of ovarian oxytocin was observed after 2500 50 45 400 oestrogen injection. The mean amplitude (17.7 7.29 ng 2000 40 35 300 min–1) of these pulses was not signiﬁcantly different from 1500 30 those measured before oestrogen injection (10.17 25 200 1000 20 3.23 ng min–1). However, a signiﬁcant (P < 0.05) increase 15 100 in the area under the curve for ovarian oxytocin secretion pulses (30.57 7.3 ng min–1) τ was observed after 500 10 5 0 0 0 oestrogen injection when compared with samples collected –6 –5 –4 –3 –2 –1 0 before injection (10.68 3.31 ng min–1) τ. In these ewes, 1 2 3 4 5 6 7 8 9 (d) the ovarian oxytocin pulses were detected at a mean of 3000 60 a 500 5.05 0.37 h and the mean inter-pulse interval was 55 2500 50 3.36 0.45 h. In oestradiol-treated ewes, administration of 400 45 oestrogen signiﬁcantly (P < 0.05) increased the duration of 2000 40 35 300 ovarian oxytocin pulses (54 5.5 versus 26.25 7.2 min) 1500 30 compared with corresponding values measured before 25 200 1000 20 oestrogen injection. 15 None of the oestradiol–ﬁnadyne-treated ewes and all of 500 10 100 5 the ewes treated with oestradiol only showed signiﬁcant 0 0 0 pulses of PGFM in peripheral plasma after oestradiol or –6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6 7 8 9 ﬁnadyne injections. Mean basal circulating concentrations Time (h) of PGFM were signiﬁcantly (P < 0.05) different inFig. 2. Oxytocin ( ) and progesterone ( ) secretion rates into oestradiol–ﬁnadyne-treated ewes (14.55 3.0 pg ml–1)ovarian venous plasma and concentrations of peripheral 13,14-dihydro-15-keto PGF2α (PGFM; ) from individual ewes (a–d) compared with ewes that received oestrogen onlytreated with oestradiol–ﬁnadyne on day 15 after oestrus. a and b (28.45 2.10 pg ml–1) over the sampling period.indicate signiﬁcant episodes in secretion of ovarian oxytocin and In oestradiol-treated ewes, at least one detectable pulsePGFM, respectively. : Identiﬁes synchronous episodes of in plasma PGFM concentration was observed aftersecretion of both compounds. ⇓ and ↓: Indicate times of oestradiol injection. The mean amplitude of these pulsesand ﬁnadyne injections, respectively. (237.18 43.13 pg ml–1) was not signiﬁcantly different from those measured before oestrogen injectionof oestradiol were signiﬁcantly (P < 0.001) higher during (176.16 68.37 pg ml–1). However, there was a signiﬁcantthe 9 h after oestradiol injection compared with the 6 h (P < 0.05) increase in the area under the curveperiod before oestradiol injection (21.48 1.14 versus (1062.11 309.67 versus 302.65 128.91 pg ml–1) τ and9.99 0.89 and 18.66 1.29 versus 10.66 1.0 pg ml–1, duration of the pulse (109.5 16.65 versus 36 6 min) ofrespectively). the PGFM response measured in peripheral plasma
Effects of ﬁnadyne on oestradiol-induced secretion of oxytocin and PGF2α during late oestrus 433collected after oestrogen injection compared with samples the absence of luteal oxytocin release. Hooper et al. (1986)collected before injection. Plasma PGFM pulses were reported that, in ewes, 56% of oxytocin pulses wereobserved in all ewes treated with oestradiol only at > 4 h coincident with pulses in uterine PGF2α and 97% of allafter injection. pulses of uterine PGF2α release were accompanied or During the sampling period, 62.5% of ovarian oxytocin followed by pulses of oxytocin in the ovarian vein. In thepulses were associated with, or preceded, the increase in present study, the percentage of PGFM pulses that occurredperipheral PGFM concentrations. In contrast, 46.15% of the immediately before or coincided with a signiﬁcant increaseplasma PGFM pulses occurred immediately before or in ovarian oxytocin pulses was decreased (46.15%) bycoincided with a signiﬁcant increase in the ovarian administration of oestradiol. Thus, the present studyoxytocin pulses. reafﬁrms the ﬁndings of Zhang et al. (1991), who reported similar effects of oestradiol administered to ewes treated with either sham or X-irradiated ovarian follicles. Discussion The mechanism by which oestrogen stimulates ovarianIn this study, ovarian autotransplanted ewes were used as a oxytocin and uterine PGF2α release is not fully understood.model to determine whether oestrogen acts to stimulate Oestrogen may act indirectly, perhaps via the uterus, torelease of ovarian oxytocin directly or indirectly via release release PGF2α, which, in turn, could stimulate ovarianof PGF2α, which in turn stimulates ovarian oxytocin. The oxytocin release. Such a mechanism of action is unlikely toconcentrations of oxytocin in ovarian venous plasma were have occurred in the present study as PGF2α would need to20–1403 pg ml–1 in the present study, which are similar to act through the systemic circulation to stimulate ovarianthose detected by Hooper et al. (1986) in the utero–ovarian oxytocin and it has been shown that 99% of PGF2α isvein (50–1499 pg ml–1) and were much higher than those in cleared from blood after one passage through the lungsperipheral plasma (20–220 pg ml–1; Hooper et al., 1986). (Piper et al., 1970). However, using ewes with ovarianTogether, these observations indicate that, in the present autotransplants does not necessarily preclude the possibilitystudy, oxytocin in ovarian venous plasma represents luteal that the effects of oestrogen and ﬁnadyne are mediatedrather than posterior pituitary secretion. through uterine release of PGF2α, as PGFM is known to As would be expected in ovarian autotransplanted ewes stimulate luteal oxytocin–neurophysin secretion (Watkins(Goding et al., 1967), the secretion of progesterone and Moore, 1987). Another possibility is that the corpusremained high in both groups, indicating that the corpus luteum of ewes bearing ovarian autotransplants becomesluteum of the transplanted ovary is maintained despite hypersensitive to low concentrations of PGF2α in theintermittent surges of peripheral plasma PGFM in all ewes absence of normal basal concentrations from the adjacentbefore oestradiol injection and in ewes treated with uterine horn. Such hypersensitivity may allow the corpusoestradiol only after injection. Our observation that the luteum to release luteal oxytocin in response to evenadministration of oestradiol can induce the simultaneous low concentrations of PGF2α that escape degradation byrelease of ovarian oxytocin and uterine PGF2α in ovarian the lungs. An alternative site of oestradiol action mayautotransplanted ewes after a latency period of 4 h in all be directly on the ovary to induce ovarian oxytocinewes treated with oestradiol only is in agreement with the release. Oestradiol (Glass et al., 1984) and PGF2α (Fitzstudy of Al-Matubsi et al. (1997). et al., 1982) receptors have been reported in large luteal In the present study, synchronous pulses of ovarian cells, which are the sites of oxytocin synthesis (Rodgersoxytocin and uterine PGF2α were observed during the ﬁrst et al., 1983). Infusion of oestrogen into the corpus luteum6 h of the sampling period before oestradiol treatment. causes luteal regression (Cook et al., 1974) and lutealHowever, this did not affect subsequent synchronous cells from sheep (Tsai and Wiltbank, 1997) and cowssecretion of these hormones after oestradiol treatment. (Milvae and Hansel, 1983; Tsai et al., 1996) can produceThus, the uterine refractoriness to ovarian oxytocin release prostaglandins, such as PGF2α, PGE2 and PGI2. On the basisand uterine PGF2α secretion can be eliminated as a reason of these ﬁndings it is possible that oestrogen stimulatesfor variability in timing of the response. The results from the release of ovarian oxytocin through luteal prostaglandinspresent study and other studies (Hooper et al., 1986; Al- (or some other metabolite of arachidonic acid) (CookeMatubsi et al., 1998) demonstrate that oxytocin pulses in and Ahmed, 1998). However, the physiological role ofutero–ovarian or ovarian venous plasma frequently occur in luteal prostaglandins during the oestrous cycle and thethe absence of any signiﬁcant increase in utero–ovarian mechanisms controlling its production remain to bePGF2α or peripheral PGFM concentrations and indicate that elucidated.ovarian oxytocin can occur independently of uterine Thus, in intact ewes, the initiation of the arachidonicPGF2α. In contrast, Lamsa et al. (1989) observed that uterine acid cascade is of importance for the secretion of oxytocinPGF2α secretion into the utero–ovarian vein begins to after oestrogen treatment.increase before the discharge of luteal oxytocin. Mann(1999) demonstrated that normal frequency of episodes of The authors would like to thank J. Downing for helping withPGF2α release, with lower amplitude and of longer cannulation of the animals and K. Tellbach for assisting with theduration, can occur at the anticipated time of luteolysis in collection of blood samples.
434 H. Y. Al-Matubsi and R. J. Fairclough References McCracken JA, Uno A, Goding JR, Ichikawa Y and Baird DT (1969) The in- vivo effects of sheep pituitary gonadotrophins on the secretion ofAl-Matubsi HY, Downing J, Jenkin G and Fairclough R (1997) Effect of steroids by the autotransplanted ovary of the ewe Journal of oestradiol on ovarian oxytocin secretion rate and luteolysis in the ewe Endocrinology 45 425–440 after ovarian auto-transplantation Reproduction, Fertility and Mann GE (1999) The role of luteal oxytocin in episodic secretion of Development 9 683–688 prostaglandin F2α at luteolysis in the ewe Animal Reproduction ScienceAl-Matubsi HY, Downing J, Jenkin G and Fairclough R (1998) Stimulation 57 167–175 of ovarian oxytocin secretion and uterine prostaglandin release by Merriam GR and Wachter KW (1982) Algorithms for the study of episodic exogenous progesterone early in the cycle of the ovarian auto- hormone secretion American Journal of Physiology 243 E310–E318 transplanted ewe Journal of Reproduction and Fertility 112 279–288 Milvae RA and Hansel W (1983) Prostacyclin, prostaglandin F2α andBurgess KM, Ralph MM, Jenkin G and Thorburn GD (1990) Effect of progesterone production by bovine luteal cells during the estrous cycle oxytocin and oestradiol on uterine prostaglandin release in non- Biology of Reproduction 29 1063–1068 pregnant and early pregnant ewes Biology of Reproduction 42 822–833 Piper PJ, Vane JR and Wyllie JH (1970) Inactivation of prostaglandins by theCook B, Karsch FJ, Foster DL and Nalbandov AV (1974) Estrogen-induced lungs Nature 225 600–604 luteolysis in the ewe: possible sites of action Endocrinology 94 Radford HH, Watson RH and Wood GF (1960) A crayon and associated 1197–1201 harness for the detection of mating under ﬁeld conditions AustralianCooke RG and Ahmed N (1998) Delayed luteolysis after intra-uterine Veterinary Journal 36 57–66 infusion of nordihydroguaiaretic acid in the ewe Animal Reproduction Rice GE, Jenkin G and Thorburn GD (1986) Comparison of particle- Science 52 113–121 associated progesterone and oxytocin in the ovine corpus luteumFitz TA, Mayan MH, Sawyer HR and Niswender GD (1982) Journal of Endocrinology 108 109–116 Characterization of two steroidogenic cell types in the ovine corpus Richard S and Zingg HH (1990) The human oxytocin gene promoter is luteum Biology of Reproduction 27 703–711 regulated by estrogens Journal of Biological Chemistry 265 6098–6103Flint APF and Sheldrick EL (1982) Ovarian secretion of oxytocin is Rodgers RJ, O’Shea JD, Frindly JK, Flint APF and Sheldrick EL (1983) Large stimulated by prostaglandin Nature 297 587–588 luteal cells the source of oxytocin in the sheep Endocrinology 113Glass JD, Fitz TA and Niswender GD (1984) Cytosolic receptor for 2302–2304 oestradiol in the corpus luteum of the ewe: variation throughout the Tsai SJ and Wiltbank MC (1997) Prostaglandin F2α induces expression of oestrous cycle and distribution between large and small steroidogenic prostaglandin G/H synthase-2 in the ovine corpus luteum, a potential cell types Biology of Reproduction 31 967–974 positive feedback loop during luteolysis Biology of Reproduction 57Goding JR, McCracken JA and Baird DT (1967) The study of ovarian 1016–1022 function in the ewe by means of a vascular autotransplantation Tsai SJ, Wiltbank MC and Bodensteiner KJ (1996) Distinct mechanisms technique Journal of Endocrinology 39 37–52 regulate induction of messenger ribonucleic acid for prostaglandin G/HHooper SB, Watkins WB and Thorburn GD (1986) Oxytocin, oxytocin- synthase-2, PGE (EP3) receptor, and PGF2α receptor in bovine associated neurophysin and prostaglandin concentrations in the utero- preovulatory follicles Endocrinology 137 3348–3355 ovarian vein in pregnant and non pregnant sheep Endocrinology 119 Watkins WB and Moore LG (1987) Effect of systemic intravenous infusion 2590–2597 of PGF2α and 13,14-dihydro-15-keto-PGF2α on the release of oxytocin-Jacobs DSC, Edgerton LA, Silvia WL and Schillo KK (1988) Effect of an associated neurophysin from the ovary in the ewe Journal of estrogen antagonist (tamoxifen) on cloprostenol-induced luteolysis in Reproduction and Fertility 80 105–112 heifers Journal of Animal Science 66 735–742 Zhang J, Weston PG and Hixon JE (1991) Inﬂuence of oestradiol on theKarsch FJ, Noveroske JW, Roche JF, Norton HW and Nalbandov AV (1970) secretion of oxytocin and prostaglandin during luteolysis in the ewe Maintenance of ovine corpora lutea in the absence of ovarian follicles Biology of Reproduction 45 395–403 Endocrinology 87 1228–1236Lamsa JC, Knot SJ, Eldering JA, Nay MG and McCracken JA (1989) Prostaglandin F2α stimulated release of ovarian oxytocin in the sheep in vivo: threshold and dose dependency Biology of Reproduction 40 Received 22 May 2000. 1215–1223 Accepted 16 October 2000.