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-
-
Reprod. Fertil. Dev., 1992, 4 , 283-8
Embryo-derived Platelet Activating Factor
Chris O'Neill Human Reproduction Unit, Departments of Physiology and Obstetrics and Gynaecology, The University of Sydney, Royal North Shore Hospital of Sydney, St Leonards, NSW 2065, Australia.
Abstract Platelet-activating factor (PAF) is secreted by the preimplantation embryo of a number of species. The role of this secretion is yet to be fully elucidated. Evidence to date indicates that it has an important function as an autocrine stimulant of embryonic metabolism, growth and viability. Production of PAF by embryos appears to be severely compromised in vitro. This may be a major cause of reduced embryo viability following embryo culture and it may also help to explain the controversy in the literature regarding the production of PAF by embryos. PAF also alters several aspects of maternal physiology during early pregnancy including endometrial prostaglandin secretion. The significance of these changes remains to be defined.
Extra keywords: preimplantation embryo, PAF.
Introduction The preimplantation embryo produced I-o-alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet-activating factor; PAF; paf-acether; AGEPC). PAF is the first biologically active phospholipid described. It is unusual in that it contains an ether-linked long-chain hydrocarbon on the C1 carbon of the glycerol backbone, thus making it an ether phospholipid. PAF was first described in association with the generation of allergic and anaphylactic reactions. Upon recognition of its antigen, IgE-sensitized basophils cause extensive platelet activation; a soluble mediator causes this activation (Benveniste et al. 1972). In 1979, two groups independently described the chemical structure of the mediator (Demopoulous et al. 1979; Blank 1979). Since that time PAF has been shown to be produced by a large number of cell types under different conditions, although a physiological function for PAF has not been clearly elucidated. Its actions in pregnancy may fulfil such a physiological role (O'Neill 1991).
Production of PAF by Embryos The production of PAF by the early embryo was first detected in mice by the observation of a mild thrombocytopaenia throughout the first week of pregnancy (O'Neill 1985~).This was due solely to the presence of the embryo (O'Neill 1985b) and PAF was isolated and characterized from embryo-conditioned culture medium (O'Neill 1985~).These observations have been independently confirmed in several laboratories (Roberts et al. 1987; Elias et al. 1989; Keefer et al. 1989; Kodama et al. 1989). 1031-3613/92/030283$05.00
Chris O'Neill
A feature of PAF production by embryos of a variety of species in vitro was its variability and the fact that a significant proportion of embryos failed to produce PAF (O'Neill et al. 1987; Collier et al. 1988, 1990; Battye et al. 1991~).It was also observed that PAF secretion appeared to decrease with increasing time in culture (Ryan et al. 1989). Although the production of PAF by the embryo has been independently confirmed (mouse: Kodama et a[. 1989; human; Punjabi et al. 1990), there have also been some reports claiming that embryos do not produce PAF (mouse: Smal et al. 1990; human: Amiel et al. 1989). This may be due to the variability of PAF production by embryos in vitro and also to the significant technical challenge of successfully extracting, purifying and assaying PAF. Each of the negative reports has utilized unconventional extraction procedures. Such conflicting reports are not unique to PAF production by embryos; indeed, measurement of PAF secretion by a variety of cell types in culture is subject to conflicting reports. Recent studies in vitro have shown that relatively minor changes in the culture environment can enhance or suppress the secretion of PAF (Leyravaud and Benveniste 1989). Because of the difficulties associated with measuring PAF secretion, the synthesis of specific, competitive PAF-receptor antagonists has given a considerable impetus to PAF research. Pharmacological Studies
PAF antagonists and inhibitors have been used to determine the role of PAF in early pregnancy (Acker et al. 1988; Spinks and O'Neill 1988; Ando et al. 1990; Milligan and Finn 1990; Spinks et al. 1990). Administration of a variety of PAF antagonists reduced the success rate of mouse and rat embryo implantation (reviewed O'Neill et al. 1988). The contragestational effects of the antagonist SRI 63-441 in the mouse did not appear to be manifested by altering the endocrine profiles of treated animals or by direct cytotoxic effects on the embryo (Spinks and O'Neill 1988). However, dose-related responses to PAF antagonists have not been achieved (O'Neill et al. 1988) (low concentrations of antagonists inhibiting implantation but higher concentrations having no effect). The reasons for this result have not yet been established, but might conceivably include: some unique feature of the role of PAF in early pregnancy; the manner by which the embryo processes PAF or its antagonists; or the multiple inhibitory effects that PAF-receptor antagonists are known to exert over cellular metabolism of PAF. The failure by Milligan and Finn (1990) to confirm the contragestational action of a PAF antagonist might be explained by the very narrow dose ranges in which a variety of PAF antagonists are active in the mouse model (O'Neill et al. 1988) and the testing of only one dose of the antagonist without confirmation of its biological potency in the strain of mouse used. Despite this lack of dose-responsiveness in the contragestational action of PAF antagonists, studies using low-dose antagonists (in the dose range where an inhibitory effect on pregnancy was displayed) suggested that the antagonists exerted their effect by acting on the embryo rather than by inhibiting the capacity of the uterus to support implantation (Spinks et al. 1990). Effects of PAF on the Embryo
A number of lines of evidence support a direct effect of PAF on the embryo: (I) PAF increased the utilization of carbohydrate substrates by the preimplantation embryo from at least the two-cell stage of development in mice (Ryan et al. 1989, 1990a), human (O'Neill et al. 1989) and sheep (Ryan 1990). This was a specific effect of PAF in mice, being inhibited by a PAF-receptor antagonist (SRI 63 441) (Ryan et al. 1990~).(2) Mouse two-cell embryos grown in the presence of PAF from 72 h onwards contained 12% more cells (P < 0.05) than those grown in the absence of PAF (Ryan et al. 1990b). (3) PAF stimulated the mitotic index of mouse preimplantation embryos in vitro. The effect was apparently specific since it was attenuated by the PAF-receptor antagonist, WEB 2086. PAF appeared
Embryo-derived PAF
to affect the rate at which cells enter metaphase rather than the total number of cells that enter metaphase (O'Neill et al. 1990). ( 4 ) Mouse (Ryan et al. 1990b) and human (O'Neill et al. 1989) embryos grown in the presence of PAF in vitro had a substantially higher implantation and development rate than similar embryos grown in the absence of exogenous PAF. For mice, this effect was specific since the increase was blocked by a specific PAFreceptor antagonist (SRI 63 441). Taken together, these pharmacological and in vitro studies provide evidence for a direct effect of PAF on preimplantation embryos which results in their increased viability and pregnancy potential. Whether the mitogenic effects are secondary to the enhancement of metabolism, or a specific action of PAF to trigger commitment to cell division, remains to be determined. The inhibition of the beneficial effects of PAF on the mouse embryo by receptor antagonists (SRI 63 441 and WEB 2086) suggests that the actions of PAF are receptor mediated. An embryonic PAF receptor is yet to be identified, however. Since PAF is secreted from the 1-cell stage of preimplantation embryo development, it is the earliest putative mediator of embryo development yet to be identified. Its variable production in vitro, together with its apparent beneficial effect on the pregnancy potential of the embryos, suggests that its production may, under some circumstances, be a rate-limiting factor to the successful establishment of pregnancy. Extensive study of the control of embryo-derived PAF production, the mechanism of its action on the embryo and its mode of signal transduction is thus warranted.
Maternal Effects of PAF Because of the potent actions of PAF on many cell types, it would be expected that its secretion by the embryo would have significant effects on the female reproductive tract. As noted earlier, a mild transient thrombocytopaenia occurs during early pregnancy presumably as a consequence of blood platelet activation in the reproductive tract by PAF. Whether such extensive localized platelet activation has any beneficial effects for the embryo or reproductive tract in the preparation for the establishment of pregnancy remains to be demonstrated. The array of very potent biological mediators released by platelets upon their activation, however, makes it attractive to speculate that they would have such a role (for further discussion see Burton et al. 1991). A recent preliminary study (Stein and O'Neill 1991) has shown that there is reduced vascularity (48%, P < 0.001) in the mouse oviduct on Day 2 of pregnancy compared with pseudopregnant animals and this was prevented by a PAF receptor antagonist. Such an observation is entirely consistent with the known actions of PAF on capillary beds. PAF is known to interact with the arachidonic acid cascade in a number of tissue types and recent studies have shown in vitro that PAF causes modifications to the secretion of prostaglandins by luteal phase endometrium (human: Smith and Kelly 1988; bovine: Gross et al. 1990). At least in the cow, these changes were shown to be specific (Gross et al. 1990). In sheep, PAF was shown to inhibit oxytocin-induced secretion of prostaglandin (PG)F2, in situ and attenuates oxytocin-induced polyphosphate inositol production by endometrial tissue in vitro (Battye et al. 1991b). Whether embryo-derived PAF has similar effects remains to be definitively proven. In particular, a role for PAF in the suppression of maternal luteolytic PGF,, secretion depends on the demonstration that PAF is secreted by the embryo or maternal tissue at the time of 'maternal recognition of pregnancy'; this is yet to be established.
Uterine Production of PAF Another possible source of PAF in early pregnancy is the endometrium (for review see Harper 1989). Uterine PAF increases throughout the luteal phase (rabbit: Angle et al. 1988; human: Alecozay et al. 1989). Its concentration in the tissues is apparently under endocrine
Chris O'Neill
control (Angle et al. 1988). The role of this endometrial PAF has not yet been defined; in the eutherian mammals there is no evidence that it is secreted by uterine cells. If PAF is not secreted by the endometrium it presumably does not play a direct role in communication within the female reproductive tract. There have been recent reports (Stewart et al. 1989; Stewart and Phillips 1989), however, that PAF retained within cells may have a messenger role at that site; perhaps this is the role of endometrial PAF. Conclusions
Evidence of a role for PAF in the establishment of mammalian pregnancy is accumulating. Elucidation of its role and sources of production is complicated by the difficulties of PAF extraction and assay procedures. There appear to be some unusual aspects of the pharmacology of PAF antagonists in early pregnancy, making a pharmacological approach difficult to interpret. The considerable variability of PAF production by embryos in vitro, its declining production with increased time in vitro, together with the apparent role of PAF as an autocrine stimulant of embryo development, all serve to suggest that embryoderived PAF production may often limit the fertility of mammals. This hypothesis warrants investigation. Acknowledgment This work was supported by grants from the National Health and Medical Research Council of Australia. References Acker, G . , Hecquet, F., Etienne, A., Braquet, P., and Mencia-Huerta, J. M. (1988). Role of platelet activating activity (PAF) in the ovoimplantation in the rat: effect of the specific PAF-acether antagonist, BN 52021. Prostaglandins 35, 233-41. Alecozay, A. A., Casslen, B. G., Riehl, R. M., Deleon, F. D., Harper, M. J. K., Silvia, M., Nouchi, T. A., and Hanahan, D. J. (1989). Platelet activating factor in human luteal phase endometrium. Biol. Reprod. 41, 578-86. Amiel, M. L., Duquenne, C., Benveniste, J., and Testart, J. (1989). Platelet aggregating activity in human culture media free of PAF-acether. Hum. Reprod. (Oxford) 4, 327-30. Ando, M., Suginami, H., and Matsuura, S. (1990). Pregnancy suppression by a platelet activating factor antagonist, ONO-6240, in mice. Asia-Oceania J. Obstet. Gynaecol. 16, 169-74. Angle, M. J., Jones, J. A., McManus, L. M., Pinckard, R. N., and Harper, M. J. K. (1988). Platelet activating factor in the rabbit uterus during early pregnancy. J. Reprod. Fertil. 83, 711-22. Battye, K. M., Ammit, A. J., O'Neill, C., Evans, G. (1991~).Production of platelet-activating factor by the preimplantation sheep embryo. J. Reprod. Fertil. 93, 507-14. Battye, K. M., O'Neill, C., and Evans, G. (1991b). Platelet activating factor (PAF) suppresses oxytocin-stimulated phosphatidylinositol turn over in ovine endometrium. Proc. Endocr. Soc. Aust. 34, abstract 21. Benveniste, J., Henson, P. M., and Cochrane, C. G. (1972). Leukocyte-dependent histamine release from rabbit platelets. The role of IgE, basophils and platelet-activating factor. J. Exp. Med. 136, 1356-64. Blank, M. L., Snyder, F., Byers, L. W., Brooks, B., and Muirhead, E. E. (1979). Antihypertensive activity in an alkyl ether analog of phosphatidylcholine. Biochem. Biophys. Res. Commun. 90, 1194-200. Burton, G., O'Neill, C., and Saunders, D. M. (1991). Platelets and pregnancy. In 'The Platelet in Health and Disease'. (Ed. C. P . Page.) pp. 191-209. (Backwell: London.) Collier, M., O'Neill, C., Ammit, J., and Saunders, D. M. (1988). Biochemical and pharmacological characterisation of human embryo-derived platelet activating factor. Hum. Reprod. (Oxford) 3, 993-8. Collier, M., O'Neill, C., Ammit, A. J., and Saunders, D. M. (1990). Measurement of human embryoderived platelet-activating factor (PAF) using a quantitative bioassay of platelet aggregation. Hum. Reprod. (Oxford) 5, 323-8.
Chris O'Neill
Ryan, J. P., Spinks, N. R., O'Neill, C., and Wales, R. G. (1990b). Implantation potential and fetal viability of mouse embryos cultured in media supplemented with platelet activating factor. J. Reprod. Fertil. 89, 309-1 5 . Smal, M. A., Dziadek, M., Cooney, S. J., Attard, M., and Baldo, B. A. (1990). Examination for platelet activating factor production by preimplantation mouse embryos using a specific radioimmunoassay. J. Reprod. Fertil. 90, 419-25. Smith, S. K., and Kelly, R. W. (1988). Effect of platelet-activating factor on the release of PGF-2a and PGE2 by separated cells of human endometrium. J. Reprod. Fertil. 82, 271-6. Spinks, N. R., and O'Neill, C. (1988). Antagonists of embryo-derived platelet activating factor prevent implantation of mouse embryos. J. Reprod. Fertil. 84, 89-98. Spinks, N. R., Ryan, J. P., and O'Neill, C. (1990). Antagonists of embryo-derived platelet activating factor act by inhibiting the ability of the mouse embryo to implant. J. Reprod. Fertil. 88, 241-8. Stein, B . A., and O'Neill, C. (1991). Reduced vascularity in the mouse fallopian tube in early pregnancy. Proceedings of 6th Symposium Australian and New Zealand Microcirculation Soc. Abstr. 15. Stewart, A. G., Dubbin, P. N., Harris, T., and Dusting, G. J. (1989). Evidence for an intracellular action of platelet-activating factor in bovine culture aortic endothelial cells. Br. J. Pharmacol. 96, 503-5. Stewart, A. G., and Phillips, W. A. (1989). Intracellular platelet activating factor regulates eicosanoid generation in guinea pig resident peritoneal macrophages. Br. J. Pharmacol. 98, 141-8.
Manuscript received 15 November 1991; revised and accepted 24 March 1992
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Reprod. Fertil. Dev., 1992, 4 , 283-8
Embryo-derived Platelet Activating Factor
Chris O'Neill Human Reproduction Unit, Departments of Physiology and Obstetrics and Gynaecology, The University of Sydney, Royal North Shore Hospital of Sydney, St Leonards, NSW 2065, Australia.
Abstract Platelet-activating factor (PAF) is secreted by the preimplantation embryo of a number of species. The role of this secretion is yet to be fully elucidated. Evidence to date indicates that it has an important function as an autocrine stimulant of embryonic metabolism, growth and viability. Production of PAF by embryos appears to be severely compromised in vitro. This may be a major cause of reduced embryo viability following embryo culture and it may also help to explain the controversy in the literature regarding the production of PAF by embryos. PAF also alters several aspects of maternal physiology during early pregnancy including endometrial prostaglandin secretion. The significance of these changes remains to be defined.
Extra keywords: preimplantation embryo, PAF.
Introduction The preimplantation embryo produced I-o-alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet-activating factor; PAF; paf-acether; AGEPC). PAF is the first biologically active phospholipid described. It is unusual in that it contains an ether-linked long-chain hydrocarbon on the C1 carbon of the glycerol backbone, thus making it an ether phospholipid. PAF was first described in association with the generation of allergic and anaphylactic reactions. Upon recognition of its antigen, IgE-sensitized basophils cause extensive platelet activation; a soluble mediator causes this activation (Benveniste et al. 1972). In 1979, two groups independently described the chemical structure of the mediator (Demopoulous et al. 1979; Blank 1979). Since that time PAF has been shown to be produced by a large number of cell types under different conditions, although a physiological function for PAF has not been clearly elucidated. Its actions in pregnancy may fulfil such a physiological role (O'Neill 1991).
Production of PAF by Embryos The production of PAF by the early embryo was first detected in mice by the observation of a mild thrombocytopaenia throughout the first week of pregnancy (O'Neill 1985~).This was due solely to the presence of the embryo (O'Neill 1985b) and PAF was isolated and characterized from embryo-conditioned culture medium (O'Neill 1985~).These observations have been independently confirmed in several laboratories (Roberts et al. 1987; Elias et al. 1989; Keefer et al. 1989; Kodama et al. 1989). 1031-3613/92/030283$05.00
Chris O'Neill
A feature of PAF production by embryos of a variety of species in vitro was its variability and the fact that a significant proportion of embryos failed to produce PAF (O'Neill et al. 1987; Collier et al. 1988, 1990; Battye et al. 1991~).It was also observed that PAF secretion appeared to decrease with increasing time in culture (Ryan et al. 1989). Although the production of PAF by the embryo has been independently confirmed (mouse: Kodama et a[. 1989; human; Punjabi et al. 1990), there have also been some reports claiming that embryos do not produce PAF (mouse: Smal et al. 1990; human: Amiel et al. 1989). This may be due to the variability of PAF production by embryos in vitro and also to the significant technical challenge of successfully extracting, purifying and assaying PAF. Each of the negative reports has utilized unconventional extraction procedures. Such conflicting reports are not unique to PAF production by embryos; indeed, measurement of PAF secretion by a variety of cell types in culture is subject to conflicting reports. Recent studies in vitro have shown that relatively minor changes in the culture environment can enhance or suppress the secretion of PAF (Leyravaud and Benveniste 1989). Because of the difficulties associated with measuring PAF secretion, the synthesis of specific, competitive PAF-receptor antagonists has given a considerable impetus to PAF research. Pharmacological Studies
PAF antagonists and inhibitors have been used to determine the role of PAF in early pregnancy (Acker et al. 1988; Spinks and O'Neill 1988; Ando et al. 1990; Milligan and Finn 1990; Spinks et al. 1990). Administration of a variety of PAF antagonists reduced the success rate of mouse and rat embryo implantation (reviewed O'Neill et al. 1988). The contragestational effects of the antagonist SRI 63-441 in the mouse did not appear to be manifested by altering the endocrine profiles of treated animals or by direct cytotoxic effects on the embryo (Spinks and O'Neill 1988). However, dose-related responses to PAF antagonists have not been achieved (O'Neill et al. 1988) (low concentrations of antagonists inhibiting implantation but higher concentrations having no effect). The reasons for this result have not yet been established, but might conceivably include: some unique feature of the role of PAF in early pregnancy; the manner by which the embryo processes PAF or its antagonists; or the multiple inhibitory effects that PAF-receptor antagonists are known to exert over cellular metabolism of PAF. The failure by Milligan and Finn (1990) to confirm the contragestational action of a PAF antagonist might be explained by the very narrow dose ranges in which a variety of PAF antagonists are active in the mouse model (O'Neill et al. 1988) and the testing of only one dose of the antagonist without confirmation of its biological potency in the strain of mouse used. Despite this lack of dose-responsiveness in the contragestational action of PAF antagonists, studies using low-dose antagonists (in the dose range where an inhibitory effect on pregnancy was displayed) suggested that the antagonists exerted their effect by acting on the embryo rather than by inhibiting the capacity of the uterus to support implantation (Spinks et al. 1990). Effects of PAF on the Embryo
A number of lines of evidence support a direct effect of PAF on the embryo: (I) PAF increased the utilization of carbohydrate substrates by the preimplantation embryo from at least the two-cell stage of development in mice (Ryan et al. 1989, 1990a), human (O'Neill et al. 1989) and sheep (Ryan 1990). This was a specific effect of PAF in mice, being inhibited by a PAF-receptor antagonist (SRI 63 441) (Ryan et al. 1990~).(2) Mouse two-cell embryos grown in the presence of PAF from 72 h onwards contained 12% more cells (P < 0.05) than those grown in the absence of PAF (Ryan et al. 1990b). (3) PAF stimulated the mitotic index of mouse preimplantation embryos in vitro. The effect was apparently specific since it was attenuated by the PAF-receptor antagonist, WEB 2086. PAF appeared
Embryo-derived PAF
to affect the rate at which cells enter metaphase rather than the total number of cells that enter metaphase (O'Neill et al. 1990). ( 4 ) Mouse (Ryan et al. 1990b) and human (O'Neill et al. 1989) embryos grown in the presence of PAF in vitro had a substantially higher implantation and development rate than similar embryos grown in the absence of exogenous PAF. For mice, this effect was specific since the increase was blocked by a specific PAFreceptor antagonist (SRI 63 441). Taken together, these pharmacological and in vitro studies provide evidence for a direct effect of PAF on preimplantation embryos which results in their increased viability and pregnancy potential. Whether the mitogenic effects are secondary to the enhancement of metabolism, or a specific action of PAF to trigger commitment to cell division, remains to be determined. The inhibition of the beneficial effects of PAF on the mouse embryo by receptor antagonists (SRI 63 441 and WEB 2086) suggests that the actions of PAF are receptor mediated. An embryonic PAF receptor is yet to be identified, however. Since PAF is secreted from the 1-cell stage of preimplantation embryo development, it is the earliest putative mediator of embryo development yet to be identified. Its variable production in vitro, together with its apparent beneficial effect on the pregnancy potential of the embryos, suggests that its production may, under some circumstances, be a rate-limiting factor to the successful establishment of pregnancy. Extensive study of the control of embryo-derived PAF production, the mechanism of its action on the embryo and its mode of signal transduction is thus warranted.
Maternal Effects of PAF Because of the potent actions of PAF on many cell types, it would be expected that its secretion by the embryo would have significant effects on the female reproductive tract. As noted earlier, a mild transient thrombocytopaenia occurs during early pregnancy presumably as a consequence of blood platelet activation in the reproductive tract by PAF. Whether such extensive localized platelet activation has any beneficial effects for the embryo or reproductive tract in the preparation for the establishment of pregnancy remains to be demonstrated. The array of very potent biological mediators released by platelets upon their activation, however, makes it attractive to speculate that they would have such a role (for further discussion see Burton et al. 1991). A recent preliminary study (Stein and O'Neill 1991) has shown that there is reduced vascularity (48%, P < 0.001) in the mouse oviduct on Day 2 of pregnancy compared with pseudopregnant animals and this was prevented by a PAF receptor antagonist. Such an observation is entirely consistent with the known actions of PAF on capillary beds. PAF is known to interact with the arachidonic acid cascade in a number of tissue types and recent studies have shown in vitro that PAF causes modifications to the secretion of prostaglandins by luteal phase endometrium (human: Smith and Kelly 1988; bovine: Gross et al. 1990). At least in the cow, these changes were shown to be specific (Gross et al. 1990). In sheep, PAF was shown to inhibit oxytocin-induced secretion of prostaglandin (PG)F2, in situ and attenuates oxytocin-induced polyphosphate inositol production by endometrial tissue in vitro (Battye et al. 1991b). Whether embryo-derived PAF has similar effects remains to be definitively proven. In particular, a role for PAF in the suppression of maternal luteolytic PGF,, secretion depends on the demonstration that PAF is secreted by the embryo or maternal tissue at the time of 'maternal recognition of pregnancy'; this is yet to be established.
Uterine Production of PAF Another possible source of PAF in early pregnancy is the endometrium (for review see Harper 1989). Uterine PAF increases throughout the luteal phase (rabbit: Angle et al. 1988; human: Alecozay et al. 1989). Its concentration in the tissues is apparently under endocrine
Chris O'Neill
control (Angle et al. 1988). The role of this endometrial PAF has not yet been defined; in the eutherian mammals there is no evidence that it is secreted by uterine cells. If PAF is not secreted by the endometrium it presumably does not play a direct role in communication within the female reproductive tract. There have been recent reports (Stewart et al. 1989; Stewart and Phillips 1989), however, that PAF retained within cells may have a messenger role at that site; perhaps this is the role of endometrial PAF. Conclusions
Evidence of a role for PAF in the establishment of mammalian pregnancy is accumulating. Elucidation of its role and sources of production is complicated by the difficulties of PAF extraction and assay procedures. There appear to be some unusual aspects of the pharmacology of PAF antagonists in early pregnancy, making a pharmacological approach difficult to interpret. The considerable variability of PAF production by embryos in vitro, its declining production with increased time in vitro, together with the apparent role of PAF as an autocrine stimulant of embryo development, all serve to suggest that embryoderived PAF production may often limit the fertility of mammals. This hypothesis warrants investigation. Acknowledgment This work was supported by grants from the National Health and Medical Research Council of Australia. References Acker, G . , Hecquet, F., Etienne, A., Braquet, P., and Mencia-Huerta, J. M. (1988). Role of platelet activating activity (PAF) in the ovoimplantation in the rat: effect of the specific PAF-acether antagonist, BN 52021. Prostaglandins 35, 233-41. Alecozay, A. A., Casslen, B. G., Riehl, R. M., Deleon, F. D., Harper, M. J. K., Silvia, M., Nouchi, T. A., and Hanahan, D. J. (1989). Platelet activating factor in human luteal phase endometrium. Biol. Reprod. 41, 578-86. Amiel, M. L., Duquenne, C., Benveniste, J., and Testart, J. (1989). Platelet aggregating activity in human culture media free of PAF-acether. Hum. Reprod. (Oxford) 4, 327-30. Ando, M., Suginami, H., and Matsuura, S. (1990). Pregnancy suppression by a platelet activating factor antagonist, ONO-6240, in mice. Asia-Oceania J. Obstet. Gynaecol. 16, 169-74. Angle, M. J., Jones, J. A., McManus, L. M., Pinckard, R. N., and Harper, M. J. K. (1988). Platelet activating factor in the rabbit uterus during early pregnancy. J. Reprod. Fertil. 83, 711-22. Battye, K. M., Ammit, A. J., O'Neill, C., Evans, G. (1991~).Production of platelet-activating factor by the preimplantation sheep embryo. J. Reprod. Fertil. 93, 507-14. Battye, K. M., O'Neill, C., and Evans, G. (1991b). Platelet activating factor (PAF) suppresses oxytocin-stimulated phosphatidylinositol turn over in ovine endometrium. Proc. Endocr. Soc. Aust. 34, abstract 21. Benveniste, J., Henson, P. M., and Cochrane, C. G. (1972). Leukocyte-dependent histamine release from rabbit platelets. The role of IgE, basophils and platelet-activating factor. J. Exp. Med. 136, 1356-64. Blank, M. L., Snyder, F., Byers, L. W., Brooks, B., and Muirhead, E. E. (1979). Antihypertensive activity in an alkyl ether analog of phosphatidylcholine. Biochem. Biophys. Res. Commun. 90, 1194-200. Burton, G., O'Neill, C., and Saunders, D. M. (1991). Platelets and pregnancy. In 'The Platelet in Health and Disease'. (Ed. C. P . Page.) pp. 191-209. (Backwell: London.) Collier, M., O'Neill, C., Ammit, J., and Saunders, D. M. (1988). Biochemical and pharmacological characterisation of human embryo-derived platelet activating factor. Hum. Reprod. (Oxford) 3, 993-8. Collier, M., O'Neill, C., Ammit, A. J., and Saunders, D. M. (1990). Measurement of human embryoderived platelet-activating factor (PAF) using a quantitative bioassay of platelet aggregation. Hum. Reprod. (Oxford) 5, 323-8.
Chris O'Neill
Ryan, J. P., Spinks, N. R., O'Neill, C., and Wales, R. G. (1990b). Implantation potential and fetal viability of mouse embryos cultured in media supplemented with platelet activating factor. J. Reprod. Fertil. 89, 309-1 5 . Smal, M. A., Dziadek, M., Cooney, S. J., Attard, M., and Baldo, B. A. (1990). Examination for platelet activating factor production by preimplantation mouse embryos using a specific radioimmunoassay. J. Reprod. Fertil. 90, 419-25. Smith, S. K., and Kelly, R. W. (1988). Effect of platelet-activating factor on the release of PGF-2a and PGE2 by separated cells of human endometrium. J. Reprod. Fertil. 82, 271-6. Spinks, N. R., and O'Neill, C. (1988). Antagonists of embryo-derived platelet activating factor prevent implantation of mouse embryos. J. Reprod. Fertil. 84, 89-98. Spinks, N. R., Ryan, J. P., and O'Neill, C. (1990). Antagonists of embryo-derived platelet activating factor act by inhibiting the ability of the mouse embryo to implant. J. Reprod. Fertil. 88, 241-8. Stein, B . A., and O'Neill, C. (1991). Reduced vascularity in the mouse fallopian tube in early pregnancy. Proceedings of 6th Symposium Australian and New Zealand Microcirculation Soc. Abstr. 15. Stewart, A. G., Dubbin, P. N., Harris, T., and Dusting, G. J. (1989). Evidence for an intracellular action of platelet-activating factor in bovine culture aortic endothelial cells. Br. J. Pharmacol. 96, 503-5. Stewart, A. G., and Phillips, W. A. (1989). Intracellular platelet activating factor regulates eicosanoid generation in guinea pig resident peritoneal macrophages. Br. J. Pharmacol. 98, 141-8.
Manuscript received 15 November 1991; revised and accepted 24 March 1992
