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Adrenocorticotropin and glucocorticoid concentrations in fetal and maternal plasma of rabbit does continuously infused with cortisol from day 21 to day 24 of gestation
Adrenocorticotropin and glucocorticoid concentrations in fetal and maternal plasma of rabbit does continuously infused with cortisol from day 21 to day 24 of gestation PATRICIA M. JACK. PH.D. DAVID M. MAGYAR, D.O. JOHN E. BC:STER, M.D.
LESLEY H. REES. M.D .. M.R.C.P. SALLY RATTER, B.Sc. PETER W. 1\ATHANIELSZ. M.D .. PH.D. Cambridge and London, Enr;land. and 1immue, Califomia Previous studies have suggested that suppression of fetal rabbit adrenocorticotropin (ACTH) secretion results in impaired development of ACTH-specific adenylate cyclase activity. In order to test this hypothesis we measured fetal and maternal plasma ACTH, cortisol, and corticosterone concentrations in control does and does infused with 0.3 mg of cortisol/hour at 21 to 24 days of gestation. This cortisol infusion regimen significantly depressed maternal and fetal plasma ACTH and corticosterone concentrations but did not change fetal serum cortisol concentrations. These findings further support the hypothesis that fetal ACTH plays a role in the normal development of the adenylate cyclase catalytic portion of the ACTH receptor complex and adrenal differentiation during this critical period of fetal life. (AM. J. OesTET. GYNECOL. 133:594, 1979.)
WE HAVE PREVIOUSLY shown that specific adrenocorticotropin (ACTH) sensitive adenylate cyclase activity of the adrenal is minimal at 19 days after coitus in the fetal rabbit. A marked increase in activity occurs between days 22 and 24. At 24 days' gestation the specific ACTH response in comparison with control nonstimulated values was as great as any observed over the period of development studied, up to 90 days after delivery. The development of this adrenocortical cell response coincided with the cytologic differentiation of the fetal adrenocortical cells into steroidogenic cells From the Physiological Laboratory, Cambridge, tht
Department of Obstetrics and G}necolog:y, [lni-oersif) of CalifomW., Los Angeles/Harbor General Hospital. Torrance, and the Department of Chemical Pathology, St. Bartholomews Hospital, London. Supported by the Wei/come Trust. Sponsored by the Society for Gynecologic Investigation. Reprint requests: Dr. Peter W. Nathanielsz, Department of Obstetrics and Gynecology, University of California, Los Angeles/Harbor General Hospital, Torrance, CalifomW. 90509.
594
that commences on day 19 of gestation and is complete hy approximately day 24. 1 Continuous infusion of cortisol into the maternal circulation from 21 to 24 days' gestation suppresses both the increase in .:~~.C1'Hstimulated adenylate q•clase activity and the cytologic differentiation of the fetal adrenocortical cells.' These observations suggest that the administered cortisol had crossed the placenta, entered the fetal circulation, and caused a fall in fetal ACTH secretion. We hypothesized that the reduced fetal ACTH secretion impaired the normal development of the ACTH-sensitive adenylate
cyclase response. The present investigation was undertaken to provide data on fetal hormone levels in mothers infused with both saline and cortisol in order to test the proposed role of ACTH in the stimulation of its own receptor during ontogenesis. Maternal plasma hormone concentrations were also determined.
Material and methods Animals and operative techniques. The 1\ew Zealand White does used were mated on a single occasion. The day of mating was taken to be day I of develop0002-9378/79/060594+04$00.40/0
©
1979 The C.\'. Mosby Co.
Volume 13~l Number 6
Plasma ACTH and glucocorticoid after cortisol infusion
595
Table I. Mean hormone concentrations ± standard errors of the means of maternal and fetal plasma in saline-infused and cortisol-infused mothers. The range of values is given in parentheses AC71f (pg!ml) Group
Saline-infused mothers Cortisol-infused mothers
Maternal 65.8 ± 17.9 (21-145) n 8 3.0 ± 0.9 (<0.1-5.5) n=8
I
Cortisol (ng!ml)
Fetal
Maternal
67.5 ± 8.7 (26-121) n 15 4.2 ± 0.7 (< 1.0-11) n = 24
5.7 ± 1.2 (0.8-16.9) n = 17 38.6 ± 4.6 (12.1-69.5) n 16
ment. Care of animals and surgical procedures for insertion of intra-aortic and inferior vena caval catheters on day 21 of gestation were as described previous1y. 2 Five animals were infused with 0.9~ saline alone (w/v) and nine were infused with 0.9% saline, containing 0.15 mg cortisol per milliliter via the aortic catheter at a rate of 2 ml/hour from the time of completion of the catheterization until the does were killed on day 24 of gestation. This cortisol infusion regimen has been shown to cause premature delivery in about 72 hours. 2 To prevent delivery, progesterone in oil (2 mg daily) was, therefore, administered intramuscularly to all animals. 2 Maternal blood samples were collected from conscious unrestrained does at intervals during the cortisol infusion via the noninfused intravenous catheter, providing potency was maintained. No maternal plasma samples were taken less than 24 hours after the start of the cortisol infusion to the doe. Samples of l ml of whole blood were withdrawn at 12 hour intervals within 15 minutes of 9:00 or 21:00. On day 24, 3~ sodium pentobarbitone (w/v) was administered via the intravenous catheter. When anesthesia had been achieved each fetus was removed individually after isolation of its blood supply from that of its mother, leaving intact the blood supply to the remaining fetuses until they in turn were removed. All does had healthy fetuses that demonstrated active movements at the time of removal from the uterus. During the collection procedure the fetuses remaining in utero were kept warm and moist either within the maternal abdominal cavity or with the aid of saline cloths. Blood was collected by seepage from the neck veins following decapitation, immediately after the exteriorization of the fetus. All blood samples were collected into cold plastic pots which contained heparin and had been kept at -20° C. These pots were then kept in crushed ice until spun at OS C for 10 minutes at 2,000 rpm. The plasma obtained was transferred immediately to polyethylene tubes, frozen, and stored at -20° C. Although the plasma from individual fetuses was treated separately
I
Corticosterone (nglml)
Fetal
Maternal
34.5 ± 6.7 (13.5-61.8) n 8 31.9±5.7 (3.1-73.5) n 17
3.9 ± O.f1 (2.0-7.8) 15 n 1.4 ± 0.3 (<0.4-3.0) 13
1
Fetal 4.1 0.5 ( l.l-6.0) n 8 l.:l ± 0.2 (0.5-2.74) n = 16
wherever possible, due to the small volume of blood available it proved necessary to pool the plasma for some litters. All samples were assayed for ACTH within one month of collection. Chemicals and reagents. 1,2,6,7- 3 H-labeled corticosterone, 89.6 Ci/mmole, and 1,2- 3H-labeled cortisol, 14.3 Ci/mmole, were obtained from New England Nuclear, Boston, Massachusetts. Corticosterone and cortisol antisera were donated by Radioassav Systems Laboratories, Inc., Carson, California. Nonradioactive steroids were obtained from Steraloids, Wilton, New Hampshire. Cortisol for infusion (Efcortelan) was a gift from Glaxo Laboratories Ltd., Greenford, England, and progesterone (Gestone) was donated by Paines and Byrne, Greenford, England. Hormone estimations. Cortisol and corticosleronl'. Cortisol and corticosterone were assayed following chromatography as previously described.a· 4 The within-assay coefficient of variation was 9% tor cortisol and 3% in the case of corticosterone. The between-assay coefficient of variation was 9% for both hormones. Accuracy was evaluated by add· ing known amounts of steroids to aliquots of rabbit plasma stripped of endogenous steroid by charcoal treatment for 2 hours at 50" C. Pool A consisted of the steroid-free rabbit plasma only. Pool B consisted of 2 ng of each steroid per milliliter of plasma. Pool C contained 20 ng of corticosterone and 50 ng of cortisol per milliliter of plasma. The recovery of added steroid ranged from 81 o/c to 10 I%. ACTH. Plasma ACTH concentration in the doe and the fetus was measured by the highly sensitive redox assay. 5 All samples were assayed for ACTH in four different dilutions. Treatment and analysis of data. Since fetal plasma ACTH was suppressed below the limits of sensitivity of the assay in some cortisol infusions, these samples were given the most conservative value, that of the lower limit of the assay, for subsequent analysis. As some of the data were obviously skewed, a .Mann-Whitney U test was used to compare samples in order to avoid the
596 Jack et al.
Am.
M
F
F
M
J.
March 15, 1979 Obstet. Gvnecol.
F
M
20
s c
s
c
s
c
s
c
Fig. 1. A comparison of the mean ACTH (picograms per milliliter), cortisol (nanograms per milliliter), and corticosterone concentrations in saline-infused (open histograms) and cortisol-infused (stippled histograms) animals. The standard errors are represented by vertical bars. For number of determinations (8 to 24) see Table I.
assumptions that they were drawn from normally distributed populations and had equal variances, implicit in the parametric t test. The test was carried out according to Siegel, 6 by computing the normal distribution function, z, by substitution of C (the Mann-Whitney statistic) in a formula with corrections for tied ranks as appropriate. For comparisons in which the direction of the difference was predicted before the study was undertaken, a one-tailed test was used; for other cases a two-tailed test was employed.
Results
Comment
ACTH values. There was no significant difference between fetal and maternal values (t,vo-tai!ed test) in either the saline-infused or cortisol-infused mothers (Table I). Infusion of cortisol into the maternal circulation resulted in a depression of ACTH levels in maternal and fetal plasmas and in most cases the concentration was below the lower limit of sensitivity of the assay for the volumes of plasma available. There was no overlap between the values obtained in either doe or fetus for saline-infused and cortisol-infused animals. A p value of <0.0023 (one-tailed test) was obtained for a comparison of maternal concentrations and a p value of <0.00003 (one-tailed test) was obtained for a comparison of fetal concentrations (Fig. I). Plasma cortisol and corticosterone concentrations. Cortisol was found to be the predominant glucocorticoid in saline-infused mother and fetus. The mean cortisol: corticosterone ratios were 1.46 and 8.5. respectively. Fetal plasma cortisol concentration
plasma corticosterone concentrations were very similar. There was slightly more variation in the cortisol levels; coefficients of variation were 12.8% (mother) and 12.2o/c (fetus). The cortisol infusion regimen elevated the concentration of cortisol in the maternal circulation to seven times the control value (p <0.0002 and p < 0.0005, respectively; one-tailed tests). The resultant cortisol: corticosterone ratios in maternal and fetal plasma, respectively, were 27.5 and 24.5 (Fig. 1).
\Vas
con-
siderably higher than the maternal value in these animals (p < 0.0002; two-tailed test). [n contrast. the
The cortisol infusion regtmen resulted in a stgnificant elevation of maternal plasma cortisol concentration. Cortisol clearly crosses the placenta and depresses fetal levels of ACTH and corticosterone while fetal plasma cortisol remains relatively unchanged. The maintained fetal plasma cortisol may simply reflect an equilibration across the placenta following a marked depression of fetal cortisol in addition to corticosterone. Alternatively the concentration of cortisol observed in the fetal plasma of saline-infused control mothers may represent a threshold value beyond which fetal mechanisms come into play to keep the level within prescribed limits. No data are available in the rabbit fetus regarding the factors affecting plasma clearance of cortisol. An increased clearance rate could result from excretion of the hormone, its sequestration. or its conversion to some other steroid derivative during its passage across the placenta or within the fetal compartment per se. Any metabolite of cortisol formed would appear to be implicated in the regulation of the fetal pituitary ACTH/adrenal loop. Evidence for
Volume 133 Number 6
transposition of steroids has been advanced in the metabolism of the fetal rabbit lung in the period of 21 to 29 days' gestation. 7• R Finally the high fetal plasma cortisol levels of both saline- and cortisol-infused mothers may be a consequence of the stress encountered during the harvesting procedure. This is unlikely in the cortisol-infused animals since the plasma ACTH concentration is depressed. The observed depression of fetal pituitary ACTH presented here supports the initial conjecture that the maternal administration of cortisol causes a fall in fetal ACTH secretion in addition to lowering fetal plasma corticosterone. The reduced ACTH output would be
Plasma ACTH and glucocorticoid after cortisol infusion
expected to cause the arrested cytoiogic differentiation and steroidogenesis in the fetal adrenal that we have previously demonstrated. 1 Studies involving the effects of fetal decapitation and their subsequent reversal by the administration of ACTH at the time of decapitation suggest that fetal plasma ACTH is mainly of pituitary origin. 9 The findings, coupled \AJith the impairment of the fetal adrenal adenylate cyclase responsiveness to ACTH following the maternal administration of cortisol are extra experimental evidence for our postulate that ACTH plays a role in the ontogenetic development of the catalytic portion of its own receptor.
REFERENCES I. Albano, J.D. M., Jack, P. M., Joseph, T., et al.: The development of ACTH-sensitive adenylate cyclase activity in the
foetal rabbit adrenal: A correlated biochemical and morphological study,]. Endocrinol. 71:333, 1976. 2. Nathanielsz, P. W., and Abel, M. A.: Initiation of parturition in the rabbit by maternal and foetal administration of cortisol: Effect of rate and duration of administration and suppression of delivery by progesterone, J. Endocrinol. 57:47, 1973. 3. Manlimos, F. S., and Abraham, G. E.: Chromatographic purification of tritiated steroids prior to use in radioim-munoassay, Anal. Lett. 8:403, 1975. 4. i\braham, G. E., Man!imos, F. S., and Garza, R.: Radioimmunoassay of steroids, in Abraham, G. E., editor: Hand-
597
5.
6. 7. 8. 9.
book of Radioimmunoassay, New York, 1977, Marcel Dekker, Inc., p. 591. Rees, L. H., Ratcliffe, J. .£11~.., Besser, G. ~..1.. et al.: Comparison of the redox assay for ACTH with previous assays, Nature 241:81, 1973. Siegel, S.: Nonparametric Statistics for the Behavioral Sciences, Tokyo, 1956, McGraw-Hill Kogakusha Ltd., p. 116. Giannopoulos, G.: Uptake and metabolism of cortisone and cortisol by the fetal rabbit lung, Steroids 23:845, 1974. Torday,J. S., Olson, E. B., Jr., and First, N. L.: Production of cortisol from cortisone by the isolated, perfused fetal rabbit lung, Steroids 67:869,' 1976. · Jack, P.M. B., and Milner, R. D. G.: Adrenocorticotrophin and the development of insulin secretion in the rabbit foetus,]. Endocrinol. 64:67, 1975.
LESLEY H. REES. M.D .. M.R.C.P. SALLY RATTER, B.Sc. PETER W. 1\ATHANIELSZ. M.D .. PH.D. Cambridge and London, Enr;land. and 1immue, Califomia Previous studies have suggested that suppression of fetal rabbit adrenocorticotropin (ACTH) secretion results in impaired development of ACTH-specific adenylate cyclase activity. In order to test this hypothesis we measured fetal and maternal plasma ACTH, cortisol, and corticosterone concentrations in control does and does infused with 0.3 mg of cortisol/hour at 21 to 24 days of gestation. This cortisol infusion regimen significantly depressed maternal and fetal plasma ACTH and corticosterone concentrations but did not change fetal serum cortisol concentrations. These findings further support the hypothesis that fetal ACTH plays a role in the normal development of the adenylate cyclase catalytic portion of the ACTH receptor complex and adrenal differentiation during this critical period of fetal life. (AM. J. OesTET. GYNECOL. 133:594, 1979.)
WE HAVE PREVIOUSLY shown that specific adrenocorticotropin (ACTH) sensitive adenylate cyclase activity of the adrenal is minimal at 19 days after coitus in the fetal rabbit. A marked increase in activity occurs between days 22 and 24. At 24 days' gestation the specific ACTH response in comparison with control nonstimulated values was as great as any observed over the period of development studied, up to 90 days after delivery. The development of this adrenocortical cell response coincided with the cytologic differentiation of the fetal adrenocortical cells into steroidogenic cells From the Physiological Laboratory, Cambridge, tht
Department of Obstetrics and G}necolog:y, [lni-oersif) of CalifomW., Los Angeles/Harbor General Hospital. Torrance, and the Department of Chemical Pathology, St. Bartholomews Hospital, London. Supported by the Wei/come Trust. Sponsored by the Society for Gynecologic Investigation. Reprint requests: Dr. Peter W. Nathanielsz, Department of Obstetrics and Gynecology, University of California, Los Angeles/Harbor General Hospital, Torrance, CalifomW. 90509.
594
that commences on day 19 of gestation and is complete hy approximately day 24. 1 Continuous infusion of cortisol into the maternal circulation from 21 to 24 days' gestation suppresses both the increase in .:~~.C1'Hstimulated adenylate q•clase activity and the cytologic differentiation of the fetal adrenocortical cells.' These observations suggest that the administered cortisol had crossed the placenta, entered the fetal circulation, and caused a fall in fetal ACTH secretion. We hypothesized that the reduced fetal ACTH secretion impaired the normal development of the ACTH-sensitive adenylate
cyclase response. The present investigation was undertaken to provide data on fetal hormone levels in mothers infused with both saline and cortisol in order to test the proposed role of ACTH in the stimulation of its own receptor during ontogenesis. Maternal plasma hormone concentrations were also determined.
Material and methods Animals and operative techniques. The 1\ew Zealand White does used were mated on a single occasion. The day of mating was taken to be day I of develop0002-9378/79/060594+04$00.40/0
©
1979 The C.\'. Mosby Co.
Volume 13~l Number 6
Plasma ACTH and glucocorticoid after cortisol infusion
595
Table I. Mean hormone concentrations ± standard errors of the means of maternal and fetal plasma in saline-infused and cortisol-infused mothers. The range of values is given in parentheses AC71f (pg!ml) Group
Saline-infused mothers Cortisol-infused mothers
Maternal 65.8 ± 17.9 (21-145) n 8 3.0 ± 0.9 (<0.1-5.5) n=8
I
Cortisol (ng!ml)
Fetal
Maternal
67.5 ± 8.7 (26-121) n 15 4.2 ± 0.7 (< 1.0-11) n = 24
5.7 ± 1.2 (0.8-16.9) n = 17 38.6 ± 4.6 (12.1-69.5) n 16
ment. Care of animals and surgical procedures for insertion of intra-aortic and inferior vena caval catheters on day 21 of gestation were as described previous1y. 2 Five animals were infused with 0.9~ saline alone (w/v) and nine were infused with 0.9% saline, containing 0.15 mg cortisol per milliliter via the aortic catheter at a rate of 2 ml/hour from the time of completion of the catheterization until the does were killed on day 24 of gestation. This cortisol infusion regimen has been shown to cause premature delivery in about 72 hours. 2 To prevent delivery, progesterone in oil (2 mg daily) was, therefore, administered intramuscularly to all animals. 2 Maternal blood samples were collected from conscious unrestrained does at intervals during the cortisol infusion via the noninfused intravenous catheter, providing potency was maintained. No maternal plasma samples were taken less than 24 hours after the start of the cortisol infusion to the doe. Samples of l ml of whole blood were withdrawn at 12 hour intervals within 15 minutes of 9:00 or 21:00. On day 24, 3~ sodium pentobarbitone (w/v) was administered via the intravenous catheter. When anesthesia had been achieved each fetus was removed individually after isolation of its blood supply from that of its mother, leaving intact the blood supply to the remaining fetuses until they in turn were removed. All does had healthy fetuses that demonstrated active movements at the time of removal from the uterus. During the collection procedure the fetuses remaining in utero were kept warm and moist either within the maternal abdominal cavity or with the aid of saline cloths. Blood was collected by seepage from the neck veins following decapitation, immediately after the exteriorization of the fetus. All blood samples were collected into cold plastic pots which contained heparin and had been kept at -20° C. These pots were then kept in crushed ice until spun at OS C for 10 minutes at 2,000 rpm. The plasma obtained was transferred immediately to polyethylene tubes, frozen, and stored at -20° C. Although the plasma from individual fetuses was treated separately
I
Corticosterone (nglml)
Fetal
Maternal
34.5 ± 6.7 (13.5-61.8) n 8 31.9±5.7 (3.1-73.5) n 17
3.9 ± O.f1 (2.0-7.8) 15 n 1.4 ± 0.3 (<0.4-3.0) 13
1
Fetal 4.1 0.5 ( l.l-6.0) n 8 l.:l ± 0.2 (0.5-2.74) n = 16
wherever possible, due to the small volume of blood available it proved necessary to pool the plasma for some litters. All samples were assayed for ACTH within one month of collection. Chemicals and reagents. 1,2,6,7- 3 H-labeled corticosterone, 89.6 Ci/mmole, and 1,2- 3H-labeled cortisol, 14.3 Ci/mmole, were obtained from New England Nuclear, Boston, Massachusetts. Corticosterone and cortisol antisera were donated by Radioassav Systems Laboratories, Inc., Carson, California. Nonradioactive steroids were obtained from Steraloids, Wilton, New Hampshire. Cortisol for infusion (Efcortelan) was a gift from Glaxo Laboratories Ltd., Greenford, England, and progesterone (Gestone) was donated by Paines and Byrne, Greenford, England. Hormone estimations. Cortisol and corticosleronl'. Cortisol and corticosterone were assayed following chromatography as previously described.a· 4 The within-assay coefficient of variation was 9% tor cortisol and 3% in the case of corticosterone. The between-assay coefficient of variation was 9% for both hormones. Accuracy was evaluated by add· ing known amounts of steroids to aliquots of rabbit plasma stripped of endogenous steroid by charcoal treatment for 2 hours at 50" C. Pool A consisted of the steroid-free rabbit plasma only. Pool B consisted of 2 ng of each steroid per milliliter of plasma. Pool C contained 20 ng of corticosterone and 50 ng of cortisol per milliliter of plasma. The recovery of added steroid ranged from 81 o/c to 10 I%. ACTH. Plasma ACTH concentration in the doe and the fetus was measured by the highly sensitive redox assay. 5 All samples were assayed for ACTH in four different dilutions. Treatment and analysis of data. Since fetal plasma ACTH was suppressed below the limits of sensitivity of the assay in some cortisol infusions, these samples were given the most conservative value, that of the lower limit of the assay, for subsequent analysis. As some of the data were obviously skewed, a .Mann-Whitney U test was used to compare samples in order to avoid the
596 Jack et al.
Am.
M
F
F
M
J.
March 15, 1979 Obstet. Gvnecol.
F
M
20
s c
s
c
s
c
s
c
Fig. 1. A comparison of the mean ACTH (picograms per milliliter), cortisol (nanograms per milliliter), and corticosterone concentrations in saline-infused (open histograms) and cortisol-infused (stippled histograms) animals. The standard errors are represented by vertical bars. For number of determinations (8 to 24) see Table I.
assumptions that they were drawn from normally distributed populations and had equal variances, implicit in the parametric t test. The test was carried out according to Siegel, 6 by computing the normal distribution function, z, by substitution of C (the Mann-Whitney statistic) in a formula with corrections for tied ranks as appropriate. For comparisons in which the direction of the difference was predicted before the study was undertaken, a one-tailed test was used; for other cases a two-tailed test was employed.
Results
Comment
ACTH values. There was no significant difference between fetal and maternal values (t,vo-tai!ed test) in either the saline-infused or cortisol-infused mothers (Table I). Infusion of cortisol into the maternal circulation resulted in a depression of ACTH levels in maternal and fetal plasmas and in most cases the concentration was below the lower limit of sensitivity of the assay for the volumes of plasma available. There was no overlap between the values obtained in either doe or fetus for saline-infused and cortisol-infused animals. A p value of <0.0023 (one-tailed test) was obtained for a comparison of maternal concentrations and a p value of <0.00003 (one-tailed test) was obtained for a comparison of fetal concentrations (Fig. I). Plasma cortisol and corticosterone concentrations. Cortisol was found to be the predominant glucocorticoid in saline-infused mother and fetus. The mean cortisol: corticosterone ratios were 1.46 and 8.5. respectively. Fetal plasma cortisol concentration
plasma corticosterone concentrations were very similar. There was slightly more variation in the cortisol levels; coefficients of variation were 12.8% (mother) and 12.2o/c (fetus). The cortisol infusion regimen elevated the concentration of cortisol in the maternal circulation to seven times the control value (p <0.0002 and p < 0.0005, respectively; one-tailed tests). The resultant cortisol: corticosterone ratios in maternal and fetal plasma, respectively, were 27.5 and 24.5 (Fig. 1).
\Vas
con-
siderably higher than the maternal value in these animals (p < 0.0002; two-tailed test). [n contrast. the
The cortisol infusion regtmen resulted in a stgnificant elevation of maternal plasma cortisol concentration. Cortisol clearly crosses the placenta and depresses fetal levels of ACTH and corticosterone while fetal plasma cortisol remains relatively unchanged. The maintained fetal plasma cortisol may simply reflect an equilibration across the placenta following a marked depression of fetal cortisol in addition to corticosterone. Alternatively the concentration of cortisol observed in the fetal plasma of saline-infused control mothers may represent a threshold value beyond which fetal mechanisms come into play to keep the level within prescribed limits. No data are available in the rabbit fetus regarding the factors affecting plasma clearance of cortisol. An increased clearance rate could result from excretion of the hormone, its sequestration. or its conversion to some other steroid derivative during its passage across the placenta or within the fetal compartment per se. Any metabolite of cortisol formed would appear to be implicated in the regulation of the fetal pituitary ACTH/adrenal loop. Evidence for
Volume 133 Number 6
transposition of steroids has been advanced in the metabolism of the fetal rabbit lung in the period of 21 to 29 days' gestation. 7• R Finally the high fetal plasma cortisol levels of both saline- and cortisol-infused mothers may be a consequence of the stress encountered during the harvesting procedure. This is unlikely in the cortisol-infused animals since the plasma ACTH concentration is depressed. The observed depression of fetal pituitary ACTH presented here supports the initial conjecture that the maternal administration of cortisol causes a fall in fetal ACTH secretion in addition to lowering fetal plasma corticosterone. The reduced ACTH output would be
Plasma ACTH and glucocorticoid after cortisol infusion
expected to cause the arrested cytoiogic differentiation and steroidogenesis in the fetal adrenal that we have previously demonstrated. 1 Studies involving the effects of fetal decapitation and their subsequent reversal by the administration of ACTH at the time of decapitation suggest that fetal plasma ACTH is mainly of pituitary origin. 9 The findings, coupled \AJith the impairment of the fetal adrenal adenylate cyclase responsiveness to ACTH following the maternal administration of cortisol are extra experimental evidence for our postulate that ACTH plays a role in the ontogenetic development of the catalytic portion of its own receptor.
REFERENCES I. Albano, J.D. M., Jack, P. M., Joseph, T., et al.: The development of ACTH-sensitive adenylate cyclase activity in the
foetal rabbit adrenal: A correlated biochemical and morphological study,]. Endocrinol. 71:333, 1976. 2. Nathanielsz, P. W., and Abel, M. A.: Initiation of parturition in the rabbit by maternal and foetal administration of cortisol: Effect of rate and duration of administration and suppression of delivery by progesterone, J. Endocrinol. 57:47, 1973. 3. Manlimos, F. S., and Abraham, G. E.: Chromatographic purification of tritiated steroids prior to use in radioim-munoassay, Anal. Lett. 8:403, 1975. 4. i\braham, G. E., Man!imos, F. S., and Garza, R.: Radioimmunoassay of steroids, in Abraham, G. E., editor: Hand-
597
5.
6. 7. 8. 9.
book of Radioimmunoassay, New York, 1977, Marcel Dekker, Inc., p. 591. Rees, L. H., Ratcliffe, J. .£11~.., Besser, G. ~..1.. et al.: Comparison of the redox assay for ACTH with previous assays, Nature 241:81, 1973. Siegel, S.: Nonparametric Statistics for the Behavioral Sciences, Tokyo, 1956, McGraw-Hill Kogakusha Ltd., p. 116. Giannopoulos, G.: Uptake and metabolism of cortisone and cortisol by the fetal rabbit lung, Steroids 23:845, 1974. Torday,J. S., Olson, E. B., Jr., and First, N. L.: Production of cortisol from cortisone by the isolated, perfused fetal rabbit lung, Steroids 67:869,' 1976. · Jack, P.M. B., and Milner, R. D. G.: Adrenocorticotrophin and the development of insulin secretion in the rabbit foetus,]. Endocrinol. 64:67, 1975.