British Journal of Obstetrics and Gynaecology June 1976. Vol83. pp 464-469
MONOAMINE OXIDASE ACTIVITY IN THE FETAL LUNG AND LIVER BY
J. B. JONES* LUCIENNE PAPADAKI AND
JOYCEHUBBARD Departments of Obstetrics and Gynaecology and Gynaecological Pathology The Bland-Sutton Institute of Pathology The Middlesex Hospital Medical School, Ridinghouse Street, London W1P 7LB
Summary Monoamine oxidase (MAO) in lung and liver is important in the degradation of circulating 5-hydroxytryptamine. These sites of M A 0 activity have been investigated histochemically in the human fetus of 12 to 18 weeks gestation. Enzyme activity could be demonstrated in the liver by both tryptamine and adrenaline oxidation. In the lung, M A 0 activity was present only when adrenaline was used as substrate. It may be, therefore, that in the premature baby the capacity of M A 0 to metabolize 5-hydroxytryptamine is not fully developed, which could lead to deleterious effects on pulmonary function. removed by the lungs in one circulation (Vane, 1969). In utero, the fetus is certainly capable of producing considerable amounts of 5-HT (Loose and Paterson, 1966; Jones and Rowsell, 1973). During gestation it is the trophoblastic M A 0 which probably regulates the concentration of free 5-HT within the fetoplacental vasculature (Jones et al, 1974). At birth, however, liver and lung M A 0 must immediately take over the degradation of any circulating 5-HT which is not platelet-bound. Thus, in order to assess the development of M A 0 in these tissues, the M A 0 activity has been investigated histochemically in human fetal lung and liver.
INTRODUCTION THErole of monoamine oxidase (MAO) in the metabolism of 5-hydroxytryptamine (5-HT) is well known. Removal of 5-HT released into the circulation may occur in two different ways. The first is by rapid uptake and storage of the amine in blood platelets (Pletscher, 1968). This process is one of active transport which allows platelets to accumulate 5-HT against a concentration gradient and then to concentrate it in storage granules. Secondly, the M A 0 enzyme systems present in tissue mitochondria are responsible for the oxidative deamination of 5-HT into its urinary metabolite, 5-hydroxyindoleacetic acid. Both lungs and liver play an important role in this process. Of a given amount of 5-HT present in the blood stream, about 65 per cent will be removed by the liver and more than 95 per cent of that remaining will be
METHODS Tissue for histochemical analysis was obtained from fetuses of 12 to 18 weeks gestation. Abortion in all cases was induced by intraamniotic injection of 40 per cent urea followed by prostaglandin E, (Bowen-Simpkins, 1973).
* Present address: Department of Obstetrics and Gynaecology, Royal United Hospital, Coombe Park, Bath.
464
465
MOA IN FETAL LUNG AND LIVER
Fresh samples of lung, liver and placenta were chilled immediately in hexane cooled to -70 "C using an alcohol-solid CO, mixture and stored at this temperature (Chayen et al, 1973). Cryostat sections (10 pm) were cut at -20 "C and mounted on coverslips. Monoamine oxidase was demonstrated histochemically by a method that involves the reduction of a tetrazolium salt (tetranitro-blue tetrazolium) to a coloured formazan deposit (Pearse, 1972; after Glenner et al, 1957). Using tryptamine (2.5 pM),the substrate generally employed for this reaction, the tissue sections were incubated for 30 to 40 minutes at 37 "C.Control sections were also included which had been previously incubated for 15 minutes at 37 "C in reaction medium lacking substrate but containing iproniazid (0-01 M) as an enzyme inhibitor. Since adrenaline may be a more suitable substrate than tryptamine in some tissues (Chayen et al, 1973), the ability of the tissue sections to oxidize adrenaline was also tested by incubation for 15 to 20 minutes in reaction medium containing 5.0 pM adrenaline. In this instance, since amphetamine was found to be a better inhibitor for adrenaline than iproniazid, the control sections were prepared by preincubation for 15 minutes in medium with 0.01 M amphetamine but lacking substrate. To compensate for the drop in pH caused by iproniazid and adrenaline, the reaction media containing these compounds were prepared in buffer at pH 8 . 0 so that the pH of the final complete medium was at the required level (Chayen et al, 1973). All sections were counterstained with methyl green extractedin chloroform. Haematoxylin and eosin stained sections were used for comparison.
RESULTS Monoamine oxidase activity was detected to varying degrees in all three tissues examined and an overall assessment of its histochemical localization and the intensity of the reaction product may be seen in Table I. Enzyme activity was evaluated according to its tissue location, patchy or generalized, and the type of cytoplasmic staining, diffuse or granular. The intensity of the reaction was graded visually, from one to three 'pluses', on the basis of the amount of deposit.
TABLEI Comparative assessment of the histochemical Ioculization of M A 0
Tryptamine Tissue
Type of deposit Generalized granular
lntensity
++ +
Adrenaline Type of deposit Patchy granular
Liver Generalized diffuse __
Lung
Generalized granular
-*
++ +
Patchy granular Patchy granular
Intensity
+++ +
+++
++
Placenta Generalized diffuse
+
* See text. Liver The fetal liver consisted of a branching meshwork of hepatic cell plates separated by sinusoids filled with haemocytoblasts (Fig. 1). M A 0 activity was present in the cytoplasm of all the hepatic cells. Tryptamine oxidation produced an intense generalized granular reaction product (Fig. 2). When adrenaline was used as substrate, there were two types of reaction: a heavy granular deposit occurring in some groups of cells was superimposed on a weaker, diffuse cytoplastic staining appearing in all the hepatic cells (Fig. 3). The former type of granular deposition was random in fashion and bore no relation to the lobular architecturz of the adult liver, which is poorly developed at this stage of gestation.
Lung The fetal lung resembled glandular tissue. The developing bronchiolar tubules, lined by columnar epithelial cells, were surrounded by highly cellular, mesenchymal tissue in which a few pulmonary arterioles and budding capillaries were seen (Fig. 4). In all but one case tryptamine oxidation failed to demonstrate any enzyme
FIG.1 Fetal liver. Haemocytoblasts fill sinusoids separating cords of hepatic celh (arrow). (H. and E. x 600.)
FIG.2 Fetal liver. Abundant M A 0 activity occurs in all the hepatic cells. (Tryptamine oxidation x 600.)
FIG.3 Fetal liver. A diffuse reaction in the hepatic cells together with a heavy granular deposit in a few groups is seen. (Adrenaline oxidation X600.)
FIG.4 Fetal lung. Epithelial tubules lie in an undifferentiated mesenchyme whose cells tend to be concentrated near the tubules (arrow). There are a few scattered capillaries. (H. and E. x 600.)
FIG.5 Fetal lung. There is no enzyme reaction; only the counter-stained nuclei may be seen. (Tryptamine oxidation x 600.)
FIG.6 Fetal lung. Intense M A 0 activity appears in groups of mesenchymal cells in areas where capillaries are found and adjacent to the epithelial tubules. (Adrenaline oxidation x 600.)
468
JONES, PAPADAKI AND HUBBARD
activity (Fig. 5). In the one lung from a fetus of 18 weeks gestation, however, a very faint generalized reaction could be detected but was extremely difficult to assess. Nevertheless, a positive reaction product was obtained in all cases when adrenaline was used as substrate (Fig. 6). This took the form of a patchy granular deposit similar to that seen in the liver, but localized in mesenchymal areas adjacent to the basement membrane of developing bronchioles and capillaries.
Placenta Incubation with both substrates revealed a strong enzyme reaction which was concentrated in the syncytio- and cytotrophoblastic cells with some deposit visible in the stromal cells of the placental villi. Tryptamine oxidation produced a heavy, overall granular deposit ; while with adrenaline, there was a weaker, generalized and more diffuse cytoplasmic staining combined with a patchy granular deposit of moderate intensity. Controls After establishing the critical preincubation times for iproniazid and amphetamine, there was complete inhibition of both tryptamine and adrenaline oxidation. DISCUSSION The first line of defence against circulating 5-HT in the adult is uptake and storage in blood platelets (Pletscher, 1968), where little is metabolized since 5-HT is such a poor substrate for platelet M A 0 (Collins and Sandler, 1971). It is the importance of the liver and especially the lung in the metabolism of vasoactive amines that has been stressed (Vane, 1969). There is a rapid and extensive intrapulmonary and intrahepatic uptake and metabolism of exogenous 5-HT (Drapanas and McDonald, 1963; Alabaster and Bakhle, 1970; Gruby et al, 1971; Gillis et al, 1972). Moreover, there appear to be sufficiently large reserves of M A 0 in both the lung and liver to deal with substantial increases in circulating 5-HT (Davison and Sandler, 1956; Moore and Eiseman, 1966). The liver from fetuses of 12 to 18 weeks gestation contains M A 0 that is capable of oxidizing both tryptamine and adrenaline.
Histochemically the enzyme is located exclusively within the hepatic cells. A similar localization is found in the adult rat liver (Plate XIXc in Pearse, 1972). Since initial uptake of 5-HT appears to be into sinusoidal lining cells (Gershon and Ross, 1966), it seems that there must be intercellular cooperation in its elimination and degradation. At the same stage of fetal development, there is no detectable M A 0 activity in the lung with tryptamine oxidation. The faint, generalized reaction found in the lung from one fetus of 18 weeks gestation, however, suggests that the functional capacity of the enzyme toward this substrate is beginning. On the other hand, with adrenaline as substrate, there is a strong but patchy reaction product confined to mesenchymal areas where sparsely scattered capillaries are found. A proportion of these undifferentiated mesenchymal cells will probably become intrasepta1 fibroblasts (Collet and Des Biens, 1974). Although Tyler and Pearse (1965) found weak M A 0 activity in all the cells of the interalveolar septum of the adult rat, it seems possible that the enzyme is, in fact, located in the intraseptal fibroblasts. Since exogenous 5-HT is retained almost exclusively in the capillary endothelial cells (Strum and Junod, 1972), the situation may be similar to that postulated in the liver, with uptake into one cell but metabolism by another. The identification of an abundance of M A 0 in the placenta is in agreement with previous biochemical and histochemical findings (Sandler and Coveney, 1962; Jones et al, 1974), suggesting an essential role in the metabolism of vasoactive amines within the fetoplacental unit. Moreover, in pre-eclampsia, when M A 0 activity in the placenta is diminished (Sandler and Coveney, 1962), there is an increase in placental and cord blood levels of 5-HT (Senior et al, 1963), together with raised concentrations of 5-hydroxyindoleacetic acid in the amniotic fluid (Loose and Paterson, 1966). The presence of a diffuse as well as a granular reaction product, especially with adrenaline oxidation, is suggestive of more than one form of enzyme. Multiple forms of M A 0 do exist depending on the tissue and species, and differ from each other in their substrate specificities and sensitivities to M A 0 inhibitors (Sandler and
MOA IN FETAL LUNG AND LIVER
Youdim, 1972). Because of this, both tryptamine and adrenaline oxidation of fetal lung and liver was investigated as well as two M A 0 inhibitors, iproniazid and amphetamine. If a comparison can be made between the fetal 5-HT regulatory system and that of the adult, it should be remembered that until birth the pulmonary circulation is largely bypassed via the ductus arteriosus. It is not surprising, therefore, if at 12 to 18 weeks gestation, the fetus has not yet developed a fully operational pulmonary M A 0 system, this function being performed by the fetal liver and the placenta until birth. Further work needs to be done to establish the precise time at which the pulmonary M A 0 system matures. If this should prove to be only towards term, premature birth might adversely affect the infant through inability to metabolize 5-HT after separation of the placenta, thereby allowing the free plasma amine to exert deleterious pharmacological effects.
ACKNOWLEDGEMENTS We are grateful to Dr J. 0. W. Beilby and Mr S. J. Steele for encouragement and advice; to Dr B. J. Large of the University of Leeds Medical School, Department of Pharmacology, for valuable criticism; and to Mr A. L. E. Barron for skilled photography.
REFERENCES Alabaster, V. A., and Bakhle, Y. S. (1970): British Journal of Pharmacology, 40,468. Bowen-Simpkins, P. (1 973) : Journal of Obstetrics and Gynaecology of the British Commonwealth, 80, 824.
469
Chayen, J., Bitensky, L., and Butcher, R. G. (1973): Practical Histochemistry. John Wiley and Sons, London, pp 5 and 172. Collet, A. J., and Des Biens, G. (1974): Anatomicul Record, 119,343. Collins, G. G. S., and Sandler, M. (1971): Biochemical Pharmacology, 20, 289. Davison, A. N., and Sandler, M. (1956): Clinica chimica acta, 1, 450. Drapanas, T.,and McDonald, J. C. (1963): Surgery, Gynecology and Obstetrics, 116, 481. Gershon, M. D., and Ross, L. L. (1966): Journal of Physiology, 186,477. Gillis, C. N., Greene, N. M., Cronan, L. H., and Hamrnond, G. L. (1972): Circulation Research, 30, 666. Glenner, G. G., Burtner, H. J., and Brown, G. W., Jr. (1957) : Journal of Histochemistry and Cytochemistry, 5, 591. Gruby, L. A., Rowlands, C., Varley, B. Q., and Wyllie, J. H. (1971): British Journal of Surgery, 58, 525. Jones, J. B., and Rowsell, A. (1973): Journal of Obstetrics and Gynaecology of the British Commonwea(rlr, 80, 687. Jones, J. B., Madill, G. T., and Pryse-Davies. J. (1974): Journal of Obstetrics and Gynaecology of the British Commonwealth, 81,469. Loose, R., and Paterson, W. G. (1966): JourmI of Obstetrics and Gynaecology of the British Commonwealth, 13, 647. Moore, T. C., and Eiseman, B. (1966): Surgery, 59, 765. Pearse, A. G. E. (1972): Histochemistry, Theoretical and Applied, Volume 2. 3rd edition. Churchill Livingstone, London, pp 863 and 1340. Pletscher, A. (1968) : British Journal of Pharmacology, 32, 1. Sandler, M., and Coveney, J. (1962): Lancet, 1, 1096. Sandler, M., and Youdim, M. B. H. (1972): Pharmacological Reviews, 24, 331. Senior, J. B., Fahim, I., Sullivan, F. M., and Robson, J . M. (1963): Lancet, 2, 553. Strum, J. M., and Junod, A. F. (1972): Journal of Cell Biology, 54, 456. Tyler, W . S., and Pearse, A. G. E. (1965): Thorax, 20, 149. Vane, J . R. (1969): British Journal of Pharmacology, 35, 209.
MONOAMINE OXIDASE ACTIVITY IN THE FETAL LUNG AND LIVER BY
J. B. JONES* LUCIENNE PAPADAKI AND
JOYCEHUBBARD Departments of Obstetrics and Gynaecology and Gynaecological Pathology The Bland-Sutton Institute of Pathology The Middlesex Hospital Medical School, Ridinghouse Street, London W1P 7LB
Summary Monoamine oxidase (MAO) in lung and liver is important in the degradation of circulating 5-hydroxytryptamine. These sites of M A 0 activity have been investigated histochemically in the human fetus of 12 to 18 weeks gestation. Enzyme activity could be demonstrated in the liver by both tryptamine and adrenaline oxidation. In the lung, M A 0 activity was present only when adrenaline was used as substrate. It may be, therefore, that in the premature baby the capacity of M A 0 to metabolize 5-hydroxytryptamine is not fully developed, which could lead to deleterious effects on pulmonary function. removed by the lungs in one circulation (Vane, 1969). In utero, the fetus is certainly capable of producing considerable amounts of 5-HT (Loose and Paterson, 1966; Jones and Rowsell, 1973). During gestation it is the trophoblastic M A 0 which probably regulates the concentration of free 5-HT within the fetoplacental vasculature (Jones et al, 1974). At birth, however, liver and lung M A 0 must immediately take over the degradation of any circulating 5-HT which is not platelet-bound. Thus, in order to assess the development of M A 0 in these tissues, the M A 0 activity has been investigated histochemically in human fetal lung and liver.
INTRODUCTION THErole of monoamine oxidase (MAO) in the metabolism of 5-hydroxytryptamine (5-HT) is well known. Removal of 5-HT released into the circulation may occur in two different ways. The first is by rapid uptake and storage of the amine in blood platelets (Pletscher, 1968). This process is one of active transport which allows platelets to accumulate 5-HT against a concentration gradient and then to concentrate it in storage granules. Secondly, the M A 0 enzyme systems present in tissue mitochondria are responsible for the oxidative deamination of 5-HT into its urinary metabolite, 5-hydroxyindoleacetic acid. Both lungs and liver play an important role in this process. Of a given amount of 5-HT present in the blood stream, about 65 per cent will be removed by the liver and more than 95 per cent of that remaining will be
METHODS Tissue for histochemical analysis was obtained from fetuses of 12 to 18 weeks gestation. Abortion in all cases was induced by intraamniotic injection of 40 per cent urea followed by prostaglandin E, (Bowen-Simpkins, 1973).
* Present address: Department of Obstetrics and Gynaecology, Royal United Hospital, Coombe Park, Bath.
464
465
MOA IN FETAL LUNG AND LIVER
Fresh samples of lung, liver and placenta were chilled immediately in hexane cooled to -70 "C using an alcohol-solid CO, mixture and stored at this temperature (Chayen et al, 1973). Cryostat sections (10 pm) were cut at -20 "C and mounted on coverslips. Monoamine oxidase was demonstrated histochemically by a method that involves the reduction of a tetrazolium salt (tetranitro-blue tetrazolium) to a coloured formazan deposit (Pearse, 1972; after Glenner et al, 1957). Using tryptamine (2.5 pM),the substrate generally employed for this reaction, the tissue sections were incubated for 30 to 40 minutes at 37 "C.Control sections were also included which had been previously incubated for 15 minutes at 37 "C in reaction medium lacking substrate but containing iproniazid (0-01 M) as an enzyme inhibitor. Since adrenaline may be a more suitable substrate than tryptamine in some tissues (Chayen et al, 1973), the ability of the tissue sections to oxidize adrenaline was also tested by incubation for 15 to 20 minutes in reaction medium containing 5.0 pM adrenaline. In this instance, since amphetamine was found to be a better inhibitor for adrenaline than iproniazid, the control sections were prepared by preincubation for 15 minutes in medium with 0.01 M amphetamine but lacking substrate. To compensate for the drop in pH caused by iproniazid and adrenaline, the reaction media containing these compounds were prepared in buffer at pH 8 . 0 so that the pH of the final complete medium was at the required level (Chayen et al, 1973). All sections were counterstained with methyl green extractedin chloroform. Haematoxylin and eosin stained sections were used for comparison.
RESULTS Monoamine oxidase activity was detected to varying degrees in all three tissues examined and an overall assessment of its histochemical localization and the intensity of the reaction product may be seen in Table I. Enzyme activity was evaluated according to its tissue location, patchy or generalized, and the type of cytoplasmic staining, diffuse or granular. The intensity of the reaction was graded visually, from one to three 'pluses', on the basis of the amount of deposit.
TABLEI Comparative assessment of the histochemical Ioculization of M A 0
Tryptamine Tissue
Type of deposit Generalized granular
lntensity
++ +
Adrenaline Type of deposit Patchy granular
Liver Generalized diffuse __
Lung
Generalized granular
-*
++ +
Patchy granular Patchy granular
Intensity
+++ +
+++
++
Placenta Generalized diffuse
+
* See text. Liver The fetal liver consisted of a branching meshwork of hepatic cell plates separated by sinusoids filled with haemocytoblasts (Fig. 1). M A 0 activity was present in the cytoplasm of all the hepatic cells. Tryptamine oxidation produced an intense generalized granular reaction product (Fig. 2). When adrenaline was used as substrate, there were two types of reaction: a heavy granular deposit occurring in some groups of cells was superimposed on a weaker, diffuse cytoplastic staining appearing in all the hepatic cells (Fig. 3). The former type of granular deposition was random in fashion and bore no relation to the lobular architecturz of the adult liver, which is poorly developed at this stage of gestation.
Lung The fetal lung resembled glandular tissue. The developing bronchiolar tubules, lined by columnar epithelial cells, were surrounded by highly cellular, mesenchymal tissue in which a few pulmonary arterioles and budding capillaries were seen (Fig. 4). In all but one case tryptamine oxidation failed to demonstrate any enzyme
FIG.1 Fetal liver. Haemocytoblasts fill sinusoids separating cords of hepatic celh (arrow). (H. and E. x 600.)
FIG.2 Fetal liver. Abundant M A 0 activity occurs in all the hepatic cells. (Tryptamine oxidation x 600.)
FIG.3 Fetal liver. A diffuse reaction in the hepatic cells together with a heavy granular deposit in a few groups is seen. (Adrenaline oxidation X600.)
FIG.4 Fetal lung. Epithelial tubules lie in an undifferentiated mesenchyme whose cells tend to be concentrated near the tubules (arrow). There are a few scattered capillaries. (H. and E. x 600.)
FIG.5 Fetal lung. There is no enzyme reaction; only the counter-stained nuclei may be seen. (Tryptamine oxidation x 600.)
FIG.6 Fetal lung. Intense M A 0 activity appears in groups of mesenchymal cells in areas where capillaries are found and adjacent to the epithelial tubules. (Adrenaline oxidation x 600.)
468
JONES, PAPADAKI AND HUBBARD
activity (Fig. 5). In the one lung from a fetus of 18 weeks gestation, however, a very faint generalized reaction could be detected but was extremely difficult to assess. Nevertheless, a positive reaction product was obtained in all cases when adrenaline was used as substrate (Fig. 6). This took the form of a patchy granular deposit similar to that seen in the liver, but localized in mesenchymal areas adjacent to the basement membrane of developing bronchioles and capillaries.
Placenta Incubation with both substrates revealed a strong enzyme reaction which was concentrated in the syncytio- and cytotrophoblastic cells with some deposit visible in the stromal cells of the placental villi. Tryptamine oxidation produced a heavy, overall granular deposit ; while with adrenaline, there was a weaker, generalized and more diffuse cytoplasmic staining combined with a patchy granular deposit of moderate intensity. Controls After establishing the critical preincubation times for iproniazid and amphetamine, there was complete inhibition of both tryptamine and adrenaline oxidation. DISCUSSION The first line of defence against circulating 5-HT in the adult is uptake and storage in blood platelets (Pletscher, 1968), where little is metabolized since 5-HT is such a poor substrate for platelet M A 0 (Collins and Sandler, 1971). It is the importance of the liver and especially the lung in the metabolism of vasoactive amines that has been stressed (Vane, 1969). There is a rapid and extensive intrapulmonary and intrahepatic uptake and metabolism of exogenous 5-HT (Drapanas and McDonald, 1963; Alabaster and Bakhle, 1970; Gruby et al, 1971; Gillis et al, 1972). Moreover, there appear to be sufficiently large reserves of M A 0 in both the lung and liver to deal with substantial increases in circulating 5-HT (Davison and Sandler, 1956; Moore and Eiseman, 1966). The liver from fetuses of 12 to 18 weeks gestation contains M A 0 that is capable of oxidizing both tryptamine and adrenaline.
Histochemically the enzyme is located exclusively within the hepatic cells. A similar localization is found in the adult rat liver (Plate XIXc in Pearse, 1972). Since initial uptake of 5-HT appears to be into sinusoidal lining cells (Gershon and Ross, 1966), it seems that there must be intercellular cooperation in its elimination and degradation. At the same stage of fetal development, there is no detectable M A 0 activity in the lung with tryptamine oxidation. The faint, generalized reaction found in the lung from one fetus of 18 weeks gestation, however, suggests that the functional capacity of the enzyme toward this substrate is beginning. On the other hand, with adrenaline as substrate, there is a strong but patchy reaction product confined to mesenchymal areas where sparsely scattered capillaries are found. A proportion of these undifferentiated mesenchymal cells will probably become intrasepta1 fibroblasts (Collet and Des Biens, 1974). Although Tyler and Pearse (1965) found weak M A 0 activity in all the cells of the interalveolar septum of the adult rat, it seems possible that the enzyme is, in fact, located in the intraseptal fibroblasts. Since exogenous 5-HT is retained almost exclusively in the capillary endothelial cells (Strum and Junod, 1972), the situation may be similar to that postulated in the liver, with uptake into one cell but metabolism by another. The identification of an abundance of M A 0 in the placenta is in agreement with previous biochemical and histochemical findings (Sandler and Coveney, 1962; Jones et al, 1974), suggesting an essential role in the metabolism of vasoactive amines within the fetoplacental unit. Moreover, in pre-eclampsia, when M A 0 activity in the placenta is diminished (Sandler and Coveney, 1962), there is an increase in placental and cord blood levels of 5-HT (Senior et al, 1963), together with raised concentrations of 5-hydroxyindoleacetic acid in the amniotic fluid (Loose and Paterson, 1966). The presence of a diffuse as well as a granular reaction product, especially with adrenaline oxidation, is suggestive of more than one form of enzyme. Multiple forms of M A 0 do exist depending on the tissue and species, and differ from each other in their substrate specificities and sensitivities to M A 0 inhibitors (Sandler and
MOA IN FETAL LUNG AND LIVER
Youdim, 1972). Because of this, both tryptamine and adrenaline oxidation of fetal lung and liver was investigated as well as two M A 0 inhibitors, iproniazid and amphetamine. If a comparison can be made between the fetal 5-HT regulatory system and that of the adult, it should be remembered that until birth the pulmonary circulation is largely bypassed via the ductus arteriosus. It is not surprising, therefore, if at 12 to 18 weeks gestation, the fetus has not yet developed a fully operational pulmonary M A 0 system, this function being performed by the fetal liver and the placenta until birth. Further work needs to be done to establish the precise time at which the pulmonary M A 0 system matures. If this should prove to be only towards term, premature birth might adversely affect the infant through inability to metabolize 5-HT after separation of the placenta, thereby allowing the free plasma amine to exert deleterious pharmacological effects.
ACKNOWLEDGEMENTS We are grateful to Dr J. 0. W. Beilby and Mr S. J. Steele for encouragement and advice; to Dr B. J. Large of the University of Leeds Medical School, Department of Pharmacology, for valuable criticism; and to Mr A. L. E. Barron for skilled photography.
REFERENCES Alabaster, V. A., and Bakhle, Y. S. (1970): British Journal of Pharmacology, 40,468. Bowen-Simpkins, P. (1 973) : Journal of Obstetrics and Gynaecology of the British Commonwealth, 80, 824.
469
Chayen, J., Bitensky, L., and Butcher, R. G. (1973): Practical Histochemistry. John Wiley and Sons, London, pp 5 and 172. Collet, A. J., and Des Biens, G. (1974): Anatomicul Record, 119,343. Collins, G. G. S., and Sandler, M. (1971): Biochemical Pharmacology, 20, 289. Davison, A. N., and Sandler, M. (1956): Clinica chimica acta, 1, 450. Drapanas, T.,and McDonald, J. C. (1963): Surgery, Gynecology and Obstetrics, 116, 481. Gershon, M. D., and Ross, L. L. (1966): Journal of Physiology, 186,477. Gillis, C. N., Greene, N. M., Cronan, L. H., and Hamrnond, G. L. (1972): Circulation Research, 30, 666. Glenner, G. G., Burtner, H. J., and Brown, G. W., Jr. (1957) : Journal of Histochemistry and Cytochemistry, 5, 591. Gruby, L. A., Rowlands, C., Varley, B. Q., and Wyllie, J. H. (1971): British Journal of Surgery, 58, 525. Jones, J. B., and Rowsell, A. (1973): Journal of Obstetrics and Gynaecology of the British Commonwea(rlr, 80, 687. Jones, J. B., Madill, G. T., and Pryse-Davies. J. (1974): Journal of Obstetrics and Gynaecology of the British Commonwealth, 81,469. Loose, R., and Paterson, W. G. (1966): JourmI of Obstetrics and Gynaecology of the British Commonwealth, 13, 647. Moore, T. C., and Eiseman, B. (1966): Surgery, 59, 765. Pearse, A. G. E. (1972): Histochemistry, Theoretical and Applied, Volume 2. 3rd edition. Churchill Livingstone, London, pp 863 and 1340. Pletscher, A. (1968) : British Journal of Pharmacology, 32, 1. Sandler, M., and Coveney, J. (1962): Lancet, 1, 1096. Sandler, M., and Youdim, M. B. H. (1972): Pharmacological Reviews, 24, 331. Senior, J. B., Fahim, I., Sullivan, F. M., and Robson, J . M. (1963): Lancet, 2, 553. Strum, J. M., and Junod, A. F. (1972): Journal of Cell Biology, 54, 456. Tyler, W . S., and Pearse, A. G. E. (1965): Thorax, 20, 149. Vane, J . R. (1969): British Journal of Pharmacology, 35, 209.
