Androstenedione metabolism in the late gestation sheep fetus

We have determined metabolic parameters for androstenedione (A) in chronically catheterized late gestation (day 130) sheep fetuses. The MCR (MCRA) was 3210 +/- 229 (SEM, n = 12) ml/min, the fetal arterial whole blood concentration of A [A] was 65 +/- 5 pg/ml, and the blood production rate (PRA) was 204 +/- 20 ng/min. Pulsatile administration of ACTH in amounts that raised fetal arterial plasma cortisol concentrations by 5- to 7-fold increased [A] to 154 +/- 20 pg/ml and PRA to 471 +/- 31 ng/min with no change in MCRA. In the presence of metopirone to block fetal adrenal cortisol output, ACTH treatment still provoked elevations in [A] (to 198 +/- 23 pg/ml) and PRA (539 +/- 158 ng/min), without altering MCRA. The major radiolabeled product in blood of infused [3H]A was [3H]testosterone; smaller amounts of phenolic steroids were formed. Extensive metabolism of [3H]A occurred in whole blood in vitro. The major product was [3H]testosterone; the 17-oxidoreductase activity was associated with the red blood cells. Umbilical vein [A] was greater than umbilical artery [A]; ACTH treatment increased [A] in both vessels. Concomitant metopirone abolished the arteriovenous difference by eliminating the ACTH-induced increase in venous [A], although arterial [A] rose significantly. The venous [A] and the arteriovenous gradient were restored with exogenous glucocorticoid treatment to the fetus. Collagenase-dispersed fetal adrenal cells secreted A. Adrenal cells from fetuses pretreated with ACTH in vivo had higher basal and ACTH-induced output of A in vitro than cells from fetuses pretreated with saline in vivo. We conclude that the MCRA in fetal sheep is extremely high, in part due to conversion of A to testosterone in fetal blood. The elevated PRA after ACTH plus metopirone and the lack of an umbilical arteriovenous gradient of [A] in this, but not other groups of fetuses, suggests a source of A production independent of the cortisol-induced changes in the placenta. Direct evidence is provided for fetal adrenal secretion of A which is enhanced by ACTH pretreatment of the fetus in vivo and for the utilization of circulating A in the fetus as a precursor for estrogen in both fetal and maternal compartments.     

Proopiomelanocortin processing in the anterior pituitary of the ovine fetus after lesion of the hypothalamic paraventricular nucleus

The hypothalamic-pituitary-adrenocortical axis plays an essential role in the maturation of fetal organs and, in sheep, birth. Lesioning the paraventricular nucleus (PVN) in fetal sheep prevents adrenocortical maturation and parturition without altering plasma immunoreactive ACTH concentrations. The purpose of this study was to determine the effect of PVN lesion on anterior pituitary processing of proopiomelanocortin (POMC) to ACTH, plasma concentrations of ACTH and ACTH precursors (POMC; 22-kDa proACTH), and expression of subtilisin-like prohormone convertase 3 (SPC3) in corticotropes in fetal sheep. PVN lesion did not affect anterior pituitary POMC and 22-kDa proACTH levels, whereas ACTH was significantly affected. The ACTH precursor (POMC plus 22-kDa proACTH) to ACTH ratio in the anterior pituitary was significantly increased after PVN lesion. Post-PVN lesion, fetal plasma ACTH(1-39), was below the limit of detection, whereas ACTH precursors (POMC plus 22-kDa proACTH) were not affected. In the inferior region of the anterior pituitary, 40-50% of corticotropes had detectable SPC3 hybridization signal, and PVN lesion did not change the extent of colocalization of POMC and SPC3, or SPC3 mRNA levels within corticotropes. Neither the percent of corticotropes in the superior region containing SPC3 hybridization (7-12%) or hybridization signal strength was altered in response to PVN lesion. In conclusion, the fetal PVN is necessary for sustaining adequate anterior pituitary processing of POMC to ACTH and ACTH release needed for maturing the adrenal cortex in the sheep fetus.     

Adrenal TGFbeta1 mRNA levels fall during late gestation and are not regulated by cortisol in the sheep fetus

During mammalian development there are periods when the fetal adrenal is either relatively refractory or increasingly sensitive to trophic stimulation. This pattern of regulation of adrenal growth and function ensures that the fetal lungs, liver, brain and kidney are exposed in a programmed temporal sequence to the genomic actions of circulating glucocorticoids. The factors which act to maintain periods of adrenal quiescence are not known. In the present study we have measured the level of messenger RNA (mRNA) expression of a putative inhibitor of adrenal steroidogenesis, transforming growth factor beta 1 (TGFbeta1), and a key steroidogenic enzyme, cytochrome P450 17alpha hydroxylase (CYP17), during periods of adrenal quiescence and activation in the sheep fetus. We have also investigated the relative roles of the fetal hypothalamic-pituitary axis and cortisol in the regulation of expression of adrenal TGFbeta1 and CYP17 mRNA during late gestation. Adrenal expression of TGFbeta1 was greatest at around 100 days gestation, at a time when the fetal sheep adrenal is relatively refractory to trophic stimulation and there was an inverse relationship between the expression of TGFbeta1 and CYP17 mRNA in the adrenal gland during the peripartum period. Whilst disconnection of the fetal hypothalamic-pituitary disconnection (HPD) axis resulted in a decrease in adrenal CYP 17 mRNA expression, there was no effect of fetal HPD, with or without cortisol replacement, on adrenal TGFbeta1 mRNA expression in late gestation. Thus TGFbeta1 may play a role in inhibiting adrenal steroidogenesis and ensuring that the adrenal remains relatively refractory to trophic stimulation during mid gestation. The maintenance of low adrenal TGFbeta1 expression during late gestation is not dependent, however, on stimulation by the fetal hypothalamic-pituitary axis.     

Infusion of N-proopiomelanocortin-(1-77) increases adrenal weight and messenger ribonucleic acid levels of cytochrome P450 17alpha-hydroxylase in the sheep fetus during late gestation

In the sheep there is a rapid increase in fetal adrenal growth and steroidogenesis during the last 10-15 days gestation (term = 147+/-3 days gestation). In the rat, peptides derived from the N-terminal region of POMC play a role in compensatory adrenal growth and in potentiation of ACTH-induced steroidogenesis. We therefore investigated the effects of infusion of bovine N-POMC-(1-77) and its biosynthetic derivative, N-POMC-(1-49) on adrenal growth and on the expression of adrenal steroidogenic enzymes in the late gestation sheep fetus. Twenty-seven pregnant ewes were used in this study. Fetal vascular catheters were inserted between 116-125 days gestation, and purified bovine N-POMC-(1-77) (2 microg/ml x h), N-POMC-(1-49) (2 microg/ml x h) and saline were each infused for 48 h between 136 and 138 days gestation. Intrafetal infusion of N-POMC-(1-77) resulted in an increased adrenal/fetal body weight ratio (94.6+/-5.7 mg/kg) compared with that in saline-infused (75.6+/-1.8 mg/kg), but not N-POMC-(1-49)-infused (82.7+/-6.1 mg/kg), fetal sheep. The ratio of CYP17 messenger RNA (mRNA) to 18S ribosomal RNA was also significantly higher in fetal adrenals ofthe N-POMC-(1-77)-infused group (49.1+/-4.7) compared with that in either the N-POMC-(1-49)-infused (20.4+/-6.4) or saline-infused (15.2+/-4.4) group. There was no difference, however, in the ratios of adrenal CYP11A1 mRNA/3beta-hydroxysteroid dehydrogenase/delta5,delta4-isomerase mRNA and CYP21A1 mRNA/18S ribosomal RNA among the N-POMC-(1-77)-, N-POMC-(1-49)-, and saline-infused groups. There was also no significant change in either plasma cortisol or ACTH concentrations in response to the infusion of either N-POMC-(1-77) or N-POMC-(1-49). In summary, intrafetal infusion of N-POMC-(1-77) stimulated fetal adrenal growth and resulted in a specific increase in adrenal CYP17 gene expression in late gestation. N-POMC-(1-77) may therefore play a modulatory role in the increase in fetal adrenal growth and steroidogenesis that occurs before birth.     

Effect of fetal adrenalectomy on messenger ribonucleic acid for proopiomelanocortin in the anterior pituitary and for corticotropin-releasing hormone in the paraventricular nucleus of the ovine fetus

In the ovine fetus, adrenalectomy at 90-120 days gestational age (dGA) results in a gradual increase in basal concentrations of fetal plasma ACTH beginning at approximately 122 dGA. Bilateral adrenalectomy at 116-119 dGA also results in an increase in POMC mRNA in the fetal pituitary. It is not known whether both the paraventricular nuclei (PVN) of the hypothalamus and the anterior pituitary of the ovine fetus are responsive in late gestation to the removal of cortisol negative feedback. The purpose of this study was to determine the subsequent effect of fetal adrenalectomy at 118-121 dGA on the CRH mRNA content in fetal PVN and on POMC mRNA in the fetal anterior pituitary at 134 dGA. Mature Rambouellet-Columbia cross-bred ewes (n = 10), bred on a single occasion only and carrying fetuses of known gestational ages, were used. Both fetal adrenal glands were exposed via a retroperitoneal approach and removed [adrenalectomized (ADX); n = 5]. In control fetuses (CONT; n = 5) adrenal glands were exposed and isolated, but not removed. At 134 dGA, fetal plasma cortisol concentrations were significantly greater in CONT fetuses (7.2 +/- 2.5 ng/ml) than in ADX fetuses (mean +/- SD, 1.97 +/- 0.9 ng/ml; P less than 0.025). At 134 dGA the fetal PVN was removed by micropunching, and the anterior pituitary was separated from neurointermediate and posterior lobes after necropsy. Total RNA was prepared by the guanidium isothiocyanate-cesium chloride method and subjected to Northern analysis using specific cDNA probes to CRH and POMC. After autoradiography, quantification of mRNA was performed by scanning densitometry. Quantities of specific hybridization signal for POMC and CRH were normalized to the content of actin mRNA in each individual sample. RNA prepared from PVN exhibited a single specifically hybridizing band for CRH of approximately 1300 nucleotides. RNA prepared from anterior pituitary exhibited a single specifically hybridizing band for POMC at approximately 1300 nucleotides. Anterior pituitary POMC mRNA was significantly increased (P less than 0.025) in ADX fetuses (236 +/- 32% of CONT). CRH mRNA in PVN was greater in ADX fetuses than in CONT fetuses (P less than 0.05; mean +/- SEM, 179 +/- 21% of CONT). Adrenalectomy in fetal sheep significantly increased expression of CRH and POMC. We conclude that the increased levels of mRNA for CRH and POMC indicate that both the fetal PVN (CRH) and the anterior pituitary (POMC) are responsive to removal of the primary source of circulating glucocorticoid at this gestational age.(ABSTRACT TRUNCATED AT 400 WORDS)     

Neuropeptide Y in the sheep fetus: effects of acute hypoxemia and dexamethasone during late gestation

Plasma concentrations of neuropeptide Y (NPY) were measured in pregnant ewes and their fetuses under basal conditions and in response to acute hypoxemia during late gestation. The effects of fetal treatment with dexamethasone on these NPY responses were also examined. Under general anesthesia, 10 Welsh Mountain ewes and their fetuses were chronically instrumented between 117-120 days gestation (dGA; term is approximately 145 dGA) with vascular and amniotic catheters, and an ultrasonic probe around a femoral artery of each fetus. At 124 dGA, five fetuses were continuously infused i.v. with dexamethasone for 48 h at a rate of 1.73 +/- 0.16 microg x kg(-1) x h(-1) while the remaining five fetuses received vehicle at the same rate. At 126 dGA, 45 h from the onset of either infusion, 1 h of materno-fetal hypoxemia was induced by reducing maternal FiO2. During normoxia, maternal plasma NPY concentrations were three times those measured in fetal plasma in both groups. During hypoxemia, PaO2 fell to similar levels in the control and dexamethasone-treated groups in both mothers and fetuses. In control animals, there was a significant increase in the NPY concentration in fetal, but not maternal, plasma during hypoxemia. Fetal treatment with dexamethasone significantly enhanced the fetal NPY response to acute hypoxemia but had no effects on basal NPY levels in the fetal or maternal plasma or on the maternal response to acute hypoxemia. These data show: 1) differences between the maternal and fetal plasma NPY response to maternal inhalation hypoxia; 2) that NPY may play a role in mediating fetal defense responses to acute hypoxemia; and 3) that fetal exposure to glucocorticoids modifies the fetal plasma NPY response to acute hypoxemia.     

Proenkephalin messenger RNA is expressed both in the rat anterior and posterior pituitary.

The presence of proenkephalin (PENK)-derived opioid peptides in the pituitary gland is well known. However, the cellular sources of their biosynthetic origin in all three pituitary lobes are less clear. In this study we identified the potential sites of synthesis by localizing the mRNA coding for PENK in the rat pituitary gland using in situ hybridization histochemistry. Numerous cells containing PENK mRNA were detected throughout the anterior lobe. Although suggested by previous reports, no mRNA signal could be detected in the intermediate lobe. Surprisingly, high levels of PENK mRNA were found in the posterior lobe. The cellular distribution in the neural lobe implies that pituicytes, a special class of glial cells, may express PENK mRNA.     

Regulation of proopiomelanocortin messenger ribonucleic acid levels in the ovine fetal anterior pituitary in vitro

Parturition in sheep is dependent upon maturation of the fetal hypothalamo-pituitary-adrenocortical (HPA) axis. Anterior pituitary expression of the ACTH precursor, proopiomelanocortin (POMC), increases during the final days of gestation in spite of exponentially increasing fetal plasma cortisol levels. Lesion of the hypothalamic paraventricular nucleus prevents the late gestation increase in POMC mRNA. The purpose of this study was to examine glucocorticoid, corticotropin releasing factor (CRF) and arginine vasopressin (AVP) regulation of POMC mRNA levels in fetal anterior pituitary corticotropes in vitro and to address potential interactions between glucocorticoids and neuropeptides in regulating POMC. Anterior pituitaries from fetal sheep at two gestational ages (dGA; 118–125 dGA, n=9; 140–144 dGA, n=7) were enzymatically dispersed. POMC mRNA levels were determined at 24, 48 and 72 h post-dispersion. CRF, AVP and dexamethasone (DEX) regulation of POMC mRNA were determined at 24 and 72 h post-dispersion. The capacity of CRF and AVP to modulate DEX suppression of POMC mRNA levels was also examined. POMC mRNA was elevated at 24 h (P<0.01) and 48 h (P<0.05) post-dispersion compared to 0 h (immediately post-dispersion) in 140–144 dGA but not 118–125 dGA corticotropes. DEX suppressed POMC mRNA in a dose-dependent manner (when administered at 24 h post-dispersion) in the 140–144 dGA anterior pituitary cells but not 118–125 dGA anterior pituitary cells. Administration of DEX (10 nM) at 0 h prevented the increase in POMC mRNA levels observed at 24 h post dispersion in the 140–144 dGA group. Neither CRF nor AVP (administered at either 24 or 72 h post-dispersion) altered POMC mRNA levels in either 118–125 or 140–144 dGA anterior pituitary cells. Continuous exposure of anterior pituitary cells with either CRF or AVP (50 pM) through 96 h increased (P<0.05) POMC mRNA. No synergistic or additive effects were observed with CRF and AVP. Four hour pretreatment with CRF but not AVP (100 nM at 24 h post-dispersion) attenuated (P<0.05) DEX suppression of POMC mRNA levels in 140–144 dGA corticotropes. In conclusion, our results indicate that direct glucocorticoid suppression of POMC expression in fetal sheep initiates between 120 and 140 dGA, coincident with the period of gestation when fetal plasma cortisol is exponentially rising. Further, while short duration exposure of fetal corticotropes to either CRF or AVP had no effect on POMC mRNA, CRF appears capable of interfering with glucocorticoid suppression of POMC mRNA. The latter observation provides a potential mechanism via which the fetal PVN may counter rising fetal plasma cortisol concentrations resulting in the previously observed late gestation increase in anterior pituitary POMC mRNA.     

Subpopulations of corticotrophs in the sheep pituitary during late gestation: effects of development and placental restriction

The prepartum surge in fetal plasma cortisol is essential for the normal timing of parturition in sheep and may result from an increase in the ratio of ACTH to proopiomelanocortin (POMC) in the fetal circulation. In fetuses subjected to experimental induction of placental restriction, the prepartum surge in fetal cortisol is exaggerated, whereas pituitary POMC mRNA levels are decreased, and in vitro, unstimulated ACTH secretion is elevated in corticotrophs nonresponsive to CRH. We therefore investigated the changes in the relative proportions of cells expressing POMC, ACTH, and the CRH type 1 receptor (CRHR(1)) shortly before birth and during chronic placental insufficiency. Placental restriction (PR) was induced by removal of the majority of placental attachment sites in five ewes before mating. Pituitaries were collected from control and PR fetal sheep at 140 d (control, n = 4; PR, n = 4) and 144 d (control, n = 6; PR, n = 4). Pituitary sections were labeled with specific antisera raised against POMC, ACTH, and CRHR(1). Three major subpopulations of corticotrophs were identified that expressed POMC + ACTH + CRHR(1), ACTH + CRHR(1), or POMC only. The proportion of pituitary corticotrophs expressing POMC + ACTH + CRHR(1) decreased (P < 0.05) between 140 (control, 60 +/- 1%; PR, 66 +/- 4%) and 144 (control, 45 +/- 2%; PR, 56 +/- 6%) d. A significantly higher (P < 0.05) proportion of corticotrophs expressed POMC + ACTH + CRHR(1) in the pituitary of the PR group compared with controls. This study is the first to demonstrate subpopulations of corticotrophs in the fetal sheep pituitary that differentially express POMC, ACTH, and CRHR(1) and the separate effects of gestational age and placental restriction on these subpopulations of corticotrophs.     

Insulin-induced hypoglycemia increases proopiomelanocortin messenger ribonucleic acid levels in rat anterior pituitary gland

To study the effect of acute stress on ACTH secretion and synthesis in rat pituitary and hypothalamus, ACTH content and POMC mRNA levels (measured by use of Northern blot analysis) in these tissues as well as the levels of ACTH in plasma and those of CRF in the hypothalamus were determined after insulin-induced hypoglycemia. Plasma ACTH levels increased at 30 and 60 min. ACTH levels in the anterior pituitary lobe (AP) decreased at 30 min, and then returned to control levels at 60 min. No change was seen in the intermediate-posterior pituitary (IP) or the hypothalamus after insulin injection. CRF levels decreased at 30 and 60 min, then returned to control levels at 90 min in the medial basal hypothalamus, including the median eminence. Hybridization with a cDNA probe revealed a single size class of POMC mRNA in AP, IP, and hypothalamus, and the size of POMC mRNA in these tissues did not change during the experimental period. POMC mRNA levels in AP increased at 60 min and reached a peak at 120 min, but those in IP and hypothalamus did not change. These results suggest that 1) insulin-induced hypoglycemia stimulates both secretion and synthesis of ACTH (at least by increasing POMC mRNA levels) in the AP, and 2) the levels of ACTH and POMC mRNA in the IP and hypothalamus are not affected by insulin-induced hypoglycemia.     

Intrafetal insulin-like growth factor-I infusion stimulates adrenal growth but not steroidogenesis in the sheep fetus during late gestation

We investigated the effects of an intrafetal infusion of IGF-I on adrenal growth and expression of the adrenal steroidogenic and catecholamine-synthetic enzyme mRNAs in the sheep fetus during late gestation. Fetal sheep were infused for 10 d with either IGF-I (26 microg/kg.h; n = 14) or saline (n = 10) between 120 and 130 d gestation, and adrenal glands were collected for morphological analysis and determination of the mRNA expression of steroidogenic and catecholamine-synthetic enzymes. Fetal body weight was not altered by IGF-I infusion; however, adrenal weight was significantly increased by 145% after IGF-I infusion. The density of cell nuclei within the fetal adrenal cortex (the zona glomerulosa and zona fasciculata), and within the adrenaline synthesizing zone of the adrenal medulla, was significantly less in the IGF-I-infused fetuses compared with the saline-infused group. Thus, based on cell-density measurements, there was a significant increase in cell size in the zona glomerulosa and zona fasciculata of the adrenal cortex and in the adrenaline-synthesizing zone of the adrenal medulla. There was no effect of IGF-I infusion on the adrenal mRNA expression of the steroidogenic or catecholamine-synthetic enzymes or on fetal plasma cortisol concentrations. In summary, infusion of IGF-I in late gestation resulted in a marked hypertrophy of the steroidogenic and adrenaline-containing cells of the fetal adrenal in the absence of changes in the mRNA levels of adrenal steroidogenic or catecholamine-synthetic enzymes or in fetal plasma cortisol concentrations. Thus, IGF-I infusion results in a dissociation of adrenal growth and function during late gestation.     

Stress-induced alterations in the levels of messenger RNA coding for proopiomelanocortin and prolactin in rat pituitary

Acute stress promotes the secretion of prolactin (PRL) and of proopiomelanocortin (POMC)-derived peptides, adrenocorticotropic hormone and beta-endorphin, from the pituitary into the systemic circulation. The present study evaluates the influence of recurrent stress upon the biosynthetic activity of cells secreting these hormones in the rat. Chronic, intermittent, electrical foot-shock (3 mA,1 s duration, every 5 s for 30 min, twice daily) over a period of 1, 3 or 7 days caused an increase in messenger ribonucleic acid (mRNA) levels coding for POMC in the anterior pituitary. Maximally elevated mRNA levels were achieved after 3 days treatment (about 80% in excess of control values) which showed no further change at 7 days. These elevated levels of POMC mRNA were associated with increased levels of immunoreactive (ir)-beta-endorphin in the adenohypophysis following 7 days of stress treatment. In contrast, this treatment did not significantly alter mRNA levels coding for PRL in the anterior pituitary. Similarly, POMC mRNA levels in the intermediate/posterior pituitary were also not significantly altered during exposure to repeated stress. Similar changes in the biosynthesis of the pituitary hormones were seen in rats suffering from chronic arthritic pain for 3 weeks: there was an approximately 80% increase in POMC mRNA levels in the anterior pituitary which was associated with an increase in the levels of ir-beta-endorphin in this lobe and an increase in the plasma levels of ir-beta-endorphin. In contrast, there were no changes in the levels of mRNA coding for PRL in the adenohypophysis.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chromogranin A messenger RNA expression in the rat anterior pituitary is permissively regulated by the adrenal gland

The influence of the adrenal gland on the expression of chromogranin A in the anterior pituitary was studied in the rat. Adrenalectomy caused a progressive and pronounced (20% of control levels at day 10 after adrenalectomy) decrease of Chromogranin A mRNA levels in anterior pituitary. Daily injection of dexamethasone (15 micrograms/animal, s.c.) fully reversed the postadrenalectomy decrement in chromogranin A mRNA levels. Chromogranin A protein content, however, was unchanged 10 days after adrenalectomy. In contrast, pro-opiomelanocortin mRNA levels were significantly elevated after adrenalectomy and restored to normal by dexamethasone, with a time course similar to the changes in chromogranin A mRNA levels. These data demonstrate that the adrenal gland permissively regulates chromogranin A expression in the anterior pituitary, at a pretranslational locus, and that this regulation is probably mediated by glucocorticoids.     

Vasopressin mediates mitogenic responses to adrenalectomy in the rat anterior pituitary

To determine whether increased vasopressinergic activity during chronic stress or adrenalectomy mediates trophic changes in the corticotroph, we examined the effect of peripheral V1 receptor blockade in rats, using the antagonist, dGly[Phaa1,D-tyr(et), Lys, Arg]vasopressin (VP), on the number of pituitary cells taking up bromodeoxyuridine (BrdU) and cells containing immunoreactive ACTH (irACTH). Adrenalectomy significantly increased the number of BrdU- and ACTH-labeled cells at 3 and 6 d, and a much larger increase was observed at 28 d. Minipump infusion of V1 antagonist for 28 d, at doses blocking the increases in ACTH and corticosterone induced by exogenous VP, prevented the increases in BrdU incorporation, but not irACTH cells observed 28 d after adrenalectomy. Unexpectedly, colocalization of BrdU with ACTH-positive cells was minor (about three cells per pituitary section), and this was unaffected by adrenalectomy or V1 antagonist infusion. In contrast, adrenalectomy for 6 or 14 d failed to increase BrdU incorporation or irACTH cells in V1b receptor knockout mice while inducing the expected increase in wild-type mice. The data show that VP is required for pituitary mitogenesis after adrenalectomy but, at least in rats, not for increasing the number of corticotrophs. The lack of colocalization of ACTH in mitotic cells suggests that recruitment of corticotrophs during adrenalectomy occurs from undifferentiated cells.     

Rat brain regional preproenkephalin A messenger RNA levels are altered in genetic hypertension

Considerable neuroanatomical and pharmacological evidence suggests that preproenkephalin A-derived peptides, particularly methionine-enkephalin, are involved in regulation of the cardiovascular system in both physiological and pathological states. In this study, we used a rat preproenkephalin A complementary DNA to determine whether proenkephalin A-derived peptides participate in the pathogenesis of hypertension as reflected by brain regional messenger RNA levels. Complementary DNA clones of the rat preproenkephalin A mRNA and rat small myelin basic protein mRNA were hybridized to total RNA extracted from hypothalamus, pons-medulla, thoracic cord, midbrain, and cerebellum of 3 1/2-week-old and 12-week-old Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. In 3 1/2-week and 12-week animals there were no differences in the levels of myelin basic protein messenger RNA between the two groups in any brain region. At 3 1/2 weeks, preproenkephalin A mRNA levels did not differ between normotensive and hypertensive strains. In contrast, at 12 weeks preproenkephalin A mRNA levels were increased in hypothalamus, midbrain, thoracic cord, and cerebellum of SHR relative to WKY. Preproenkephalin A mRNA was significantly reduced in the pons-medulla of SHR relative to WKY. Our findings provide evidence that alterations in brain regional preproenkephalin A mRNA levels are associated with the development of spontaneous hypertension in the rat.     

In vivo evidence for increased adrenal sensitivity to adrenocorticotropin-(1-24) in the lamb fetus late in gestation

Plasma concentrations of immunoreactive ACTH and cortisol were monitored daily in chronically cannulated sheep fetuses during the last 3 weeks of gestation. A prepartum increase in fetal plasma cortisol occurred without a concomitant rise in fetal plasma ACTH. When fetal lambs were injected with various doses of ACTH-(1-24) and the plasma cortisol responses were integrated over time, the resulting changes in the log dose-response curves indicate that fetal adrenal sensitivity increases late in gestation. Thus, the marked rise in fetal plasma cortisol before birth can be explained, at least in part, by an increase in fetal adrenal sensitivity to ACTH.