Pharmacology and physiology of ovine corticosteroid receptors

Preterm birth affects both surfactant synthesis and lung liquid resorption actors in fetal sheep

INTRODUCTION

After birth, the lungs must resorb the fluid they contain. This process involves multiple actors such as surfactant, aquaporins and ENaC channels. Preterm newborns often exhibit respiratory distress syndrome due to surfactant deficiency, and transitory tachypnea caused by a delay in lung liquid resorption. Our hypothesis is that surfactant, ENaC and aquaporins are involved in respiratory transition to extrauterine life and altered by preterm birth. We compared these candidates in preterm and term fetal sheeps.

MATERIALS AND METHODS

We performed cesarean sections in 8 time-dated pregnant ewes (4 at 100 days and 4 at 140 days of gestation, corresponding to 24 and 36 weeks of gestation in humans), and obtained 13 fetal sheeps in each group. We studied surfactant synthesis (SP-A, SP-B, SP-C), lung liquid resorption (ENaC, aquaporins) and corticosteroid regulation (glucocorticoid receptor, mineralocorticoid receptor and 11-betaHSD2) at mRNA and protein levels.

RESULTS

The mRNA expression level of SFTPA, SFTPB and SFTPC was higher in the term group. These results were confirmed at the protein level for SP-B on Western Blot analysis and for SP-A, SP-B and SP-C on immunohistochemical analysis. Regarding aquaporins, ENaC and receptors, mRNA expression levels for AQP1, AQP3, AQP5, ENaCα, ENaCβ, ENaCγ and 11βHSD2 mRNA were also higher in the term group.

DISCUSSION

Expression of surfactant proteins, aquaporins and ENaC increases between 100 and 140 days of gestation in an ovine model. Further exploring these pathways and their hormonal regulation could highlight some new explanations in the pathophysiology of neonatal respiratory diseases.

Chronic maternal hypercortisolemia in late gestation alters fetal cardiac function at birth

Studies in our laboratory have shown that modest chronic increases in maternal cortisol concentrations over the last 0.20 of gestation impair maternal glucose metabolism and increase the incidence of perinatal stillbirth. Previous studies had found that an increase in maternal cortisol concentrations from 115 to 130 days of gestation in sheep increased both proliferation in fetal cardiomyocytes and apoptosis in the fetal cardiac Purkinje fibers. We hypothesized that the adverse effects of excess cortisol may result in defects in cardiac conduction during labor and delivery. In the present study, we infused cortisol (1 mg·kg^-1·day^-1) into late gestation pregnant ewes and continuously monitored fetal aortic pressure and ECG through labor and delivery. We found that, although the fetuses of cortisol infused ewes had normal late gestation patterns of arterial pressure and heart rate, there was a significant decrease in fetal aortic pressure and heart rate on the day of birth, specifically in the final hour before delivery. Significant changes in the fetal ECG were also apparent on the day of birth, including prolongation of the P wave and P-R interval. We speculate that chronic exposure to glucocorticoids alters cardiac metabolism or ion homeostasis, contributing to cardiac dysfunction, precipitated by active labor and delivery.

Hypothalamic-Pituitary--Adrenal Axis-Feedback Control

The hypothalamo-pituitary-adrenal axis (HPA) is responsible for stimulation of adrenal corticosteroids in response to stress. Negative feedback control by corticosteroids limits pituitary secretion of corticotropin, ACTH, and hypothalamic secretion of corticotropin-releasing hormone, CRH, and vasopressin, AVP, resulting in regulation of both basal and stress-induced ACTH secretion. The negative feedback effect of corticosteroids occurs by action of corticosteroids at mineralocorticoid receptors (MR) and/or glucocorticoid receptors (GRs) located in multiple sites in the brain and in the pituitary. The mechanisms of negative feedback vary according to the receptor type and location within the brain-hypothalmo-pituitary axis. A very rapid nongenomic action has been demonstrated for GR action on CRH neurons in the hypothalamus, and somewhat slower nongenomic effects are observed in the pituitary or other brain sites mediated by GR and/or MR. Corticosteroids also have genomic actions, including repression of the pro-opiomelanocortin (POMC) gene in the pituitary and CRH and AVP genes in the hypothalamus. The rapid effect inhibits stimulated secretion, but requires a rapidly rising corticosteroid concentration. The more delayed inhibitory effect on stimulated secretion is dependent on the intensity of the stimulus and the magnitude of the corticosteroid feedback signal, but also the neuroanatomical pathways responsible for activating the HPA. The pathways for activation of some stressors may partially bypass hypothalamic feedback sites at the CRH neuron, whereas others may not involve forebrain sites; therefore, some physiological stressors may override or bypass negative feedback, and other psychological stressors may facilitate responses to subsequent stress.

Mineralocorticoid effects in the late gestation ovine fetal lung

This study was designed to determine the effects of corticosteroids at MR in the late‐gestation fetal lung. Since both the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR) are expressed at relatively high levels in the fetal lung, endogenous corticosteroids may act at MR as well as GR in the preterm fetal lung. The GR agonist, betamethasone, the MR agonist, aldosterone, or both were infused intravenously for 48 h in ovine fetuses of approximately 130 days gestation. Effects on airway pressures during stepwise inflation of the in situ lung, expression of ENaC alpha (SCNN1A), ENaC beta (SCNN1B), and Na,K ATPase (ATP1A1), and elastin and collagen content were determined after the infusions. We found that aldosterone significantly reduced the airway pressure measured during the initial step in inflation of the lung, although aldosterone had no overall effect on lung compliance, nor did aldosterone induce expression of ENaCα, ENaCβ or Na,K ATPaseα1. Betamethasone significantly increased expression of the epithelial sodium channel (ENaC) subunit mRNAs, and collagen and elastin content in the lungs, although this dose of betamethasone also had no effect on lung compliance. There was no synergy between effects of the MR and GR agonists. Transcriptomic analysis suggested that although aldosterone did not alter genes in pathways related to epithelial sodium transport, aldosterone did alter genes in pathways involved in cell proliferation in the lungs. The results are consistent with corticosteroid‐induced fluid reabsorption at birth through GR rather than MR, but suggest that MR facilitates lung maturation, and may contribute to inflation with the first breaths via mechanisms distinct from known aldosterone effects in other epithelia.

Infusion of the mineralocorticoid receptor agonist, aldosterone, to the ovine fetus resulted in reduced airway pressures with initial lung inflation. However, aldosterone did not alter lung surfactant or epithelial sodium transport genes which are classical MR gene targets. Transcriptomic analysis revealed an aldosterone effect on genes related to cell cycle, suggesting that MR have a role distinct form that of GR in the maturing lung.

A role for mineralocorticoid receptors in the physiology of the ovine fetus: effects on ACTH and lung liquid composition

In the human and ovine fetus, the presence of 11β-hydroxysteroid dehydrogenase 1 allows cortisol and other corticosteroids to act at mineralocorticoid receptors (MRs) in lung and brain. To test the physiologic role of MRs in the late gestation fetus, fetal lambs were infused with a specific MR antagonist for 12 h. Infusion of the MR antagonist significantly increased plasma ACTH and cortisol concentrations. Infusion of the MR antagonist also significantly increased fetal Pco2 and hematocrit, and decreased fetal pH, but did not alter fetal heart rate or blood pressure. Infusion of the MR antagonist altered the ratio of Na⁺ to K⁺ in lung fluid but did not alter the rate of production of lung liquid or the expression of the epithelial sodium channel α or of the Na,K ATPaseα1 in lung. These results suggest that corticosteroids act at MR to regulate ACTH and blood volume and modulate lung fluid composition in the fetus, but basal levels of corticosteroids do not alter lung liquid production rate through effects on MR.

Differential effects of mineralocorticoid blockade on the hypothalamo-pituitary-adrenal axis in pregnant and nonpregnant ewes

During pregnancy, plasma ACTH and cortisol are chronically increased; this appears to occur through a reset of hypothalamo-pituitary-adrenal (HPA) activity. We have hypothesized that differences in mineralocorticoid receptor activity in pregnancy may alter feedback inhibition of the HPA axis. We tested the effect of MR antagonism in pregnant and nonpregnant ewes infused for 4 h with saline or the MR antagonist canrenoate. Pregnancy significantly increased plasma ACTH, cortisol, angiotensin II, and aldosterone. Infusion of canrenoate increased plasma ACTH, cortisol, and aldosterone in both pregnant and nonpregnant ewes; however, the temporal pattern of these responses differed between these two reproductive states. In nonpregnant ewes, plasma ACTH and cortisol transiently increased at 1 h of infusion, whereas in pregnant ewes the levels gradually increased and were significantly elevated from 2 to 4 h of infusion. MR blockade increased plasma aldosterone from 2 to 4 h in the pregnant ewes but only at 4 h in the nonpregnant ewes. In both pregnant and nonpregnant ewes, the increase in plasma aldosterone was significantly related to the timing and magnitude of the increase in plasma potassium. The results indicate a differential effect of MR activity in pregnant and nonpregnant ewes and suggest that the slow changes in ACTH, cortisol, and aldosterone are likely to be related to blockade of MR effects in the kidney rather than to effects of MR blockade in hippocampus or hypothalamus.

Na+,K+-ATPase activity and subunit protein expression: ontogeny and effects of exogenous and endogenous steroids on the cerebral cortex and renal cortex of sheep

We examined the effects of development, exogenous, and endogenous glucocorticoids on Na(+),K(+)-ATPase activity and subunit protein expression in ovine cerebral cortices and renal cortices. Ewes at 60%, 80%, and 90% gestation, newborns, and adults received 4 dexamethasone or placebo injections. Cerebral cortex Na(+),K(+)-ATPase activity was higher (P < .05) in placebo-treated newborns than fetuses of placebo-treated ewes and adults, α(1)-expression was higher at 90% gestation than the other ages; α(2)-expression was higher in newborns than fetuses; α(3)-expression was higher in newborns than 60% gestation; β(1)-expression was higher in newborns than the other ages, and β(2)-expression higher at 60% than 80% and 90% gestation, and in adults. Renal cortex Na(+),K(+)-ATPase activity was higher in placebo-treated adults and newborns than fetuses. Cerebral cortex Na(+),K(+)-ATPase activity was higher in dexamethasone- than placebo-treated adults, and α(1)-expression higher in fetuses of dexamethasone- than placebo-treated ewes at 60% and 80% gestation. Renal cortex Na(+),K(+)-ATPase activity and α(1)-expression were higher in fetuses of dexamethasone- than placebo-treated ewes at each gestational age, and β(1)-expression was higher in fetuses of dexamethasone- than placebo-treated ewes at 90% gestation and in dexamethasone- than placebo-treated adults. Cerebral cortex Na(+),K(+)-ATPase activity, α(1)-expression, β(1)-expression, and renal cortex α(1)-expression correlated directly with increases in fetal cortisol. In conclusion, Na(+),K(+)-ATPase activity and subunit expression exhibit specific developmental patterns in brain and kidney; exogenous glucocorticoids regulate activity and subunit expression in brain and kidney at some ages; endogenous increases in fetal cortisol regulate cerebral Na(+),K(+)-ATPase, but exogenous glucocorticoids have a greater effect on renal than cerebral Na(+),K(+)-ATPase.

Expression of ENaC subunits, chloride channels, and aquaporins in ovine fetal lung: ontogeny of expression and effects of altered fetal cortisol concentrations

Transition of the epithelium of the fetal lung from fluid secretion to fluid reabsorption requires changes in the expression of ion channels. Corticosteroids regulate expression of several of these channels, including the epithelium sodium channel (ENaC) subunits and aquaporins (AQP). We investigated the ontogenetic changes in these ion channels in the ovine fetal lung during the last half of gestation, a time of increasing adrenal maturation. Expression of the mRNAs for the chloride channels, cystic fibrosis transmembrane conductance regulator (CFTR), and chloride channel 2 (CLCN2) decreased with age. Expression of mRNAs for AQP1, AQP5, and for subunits of ENaC (alpha, beta, gamma) increased with age. In the fetal sheep the expression of ENaCbeta mRNA was dramatically higher than the expression of ENaCalpha or ENaCgamma, but expression of ENaCbeta protein decreased with maturation, although the ratio of the mature (112 kDa) to immature (102 kDa) ENaCbeta protein increased with age, particularly in the membrane fraction. In contrast, ENaCalpha mRNA and protein both increase with maturation, and the mature form of ENaCalpha (68 kDa) predominates at all ages. A modest increase in fetal cortisol, within the range expected to occur naturally in late gestation but prior to active labor, increased ENaCalpha mRNA but not ENaCbeta, ENaCgamma, or AQP mRNAs. We conclude that in the ovine fetal lung, appearance of functional sodium channels is associated with induction of ENACalpha and ENaCgamma, and that ENaCalpha expression may be induced by even small, preterm increases in fetal cortisol.

Is the fetal lung a mineralocorticoid receptor target organ? Induction of cortisol-regulated genes in the ovine fetal lung, kidney and small intestine

BACKGROUND

Lung, kidney and small intestine are involved in fetal volume regulation and amniotic fluid secretion and play a pivotal role in the transition from intrauterine to extrauterine life.

OBJECTIVE

This study was performed to determine the ontogeny of mineralocorticoid receptors (MR) and glucocorticoid receptors (GR), and of MR- and GR-regulated genes and proteins, serum and glucocorticoid-induced kinase (Sgk-1), epithelial sodium channel (ENaC alpha), and Na,K-ATPase alpha1.

METHODS

Lung, renal cortex and medulla, and small intestine were collected from fetuses at 80, 100, 120, 130 and 145 days' gestation and from day 1 and 7 neonatal lambs. Real-time PCR was performed to determine mRNA concentration for MR, GR, the 11 beta-hydroxysteroid dehydrogenases (11 beta-HSD1 and 2), Sgk-1, ENaC alpha, and Na,K-ATPase alpha1. Protein expression of ENaC alpha and Na,K-ATPase alpha1 in whole cell and membrane fractions was determined by immunoblotting.

RESULTS

Expression of corticosteroid-induced genes in renal cortex increases at term; in small intestine the induction occurs postnatally. In contrast, in lung expression of MR and GR mRNAs were greater at 100 days to term than postnatally and 11 beta-HSD1 peaked at 145 days; the corticosteroid-induced genes also increased prenatally: Sgk-1 and ENaC alpha increased by 120 days, peaking at 145 days, and Na,K-ATPase alpha1 was greatest at 130 days.

CONCLUSIONS

The expression of high levels of MR and 11 beta-HSD1 in preterm fetal lung suggest low endogenous fetal cortisol may exert actions at the high affinity MR in vivo, leading to increases in expression of sodium channels important in the regulation of lung liquid secretion and reabsorption.

Regulation of maternal ACTH in ovine pregnancy: does progesterone play a role?

Pregnancy is characterized by increased plasma adrenocorticotropic hormone (ACTH) and cortisol. Studies suggest that progesterone acts as an antagonist at mineralocorticoid receptors. Therefore, we tested the hypothesis that chronic progesterone, produced by treatment of nonpregnant ewes or during pregnancy, will result in increased plasma ACTH relative to the plasma cortisol concentrations. We studied three groups of ewes: ovariectomized nonpregnant, nonpregnant treated with progesterone, and pregnant ewes. In two series of studies, ewes were adrenalectomized and replaced with 0.35 mg x kg(-1) x day(-1) or 0.5 mg x kg(-1) x day(-1) cortisol. In both studies, aldosterone was infused at 3 microg x kg(-1) x day(-1). In the first study, additional infusions of cortisol over 24 h were used to increase daily replacement doses to 0.5, 1, or 1.5 mg x kg(-1) x day(-1), and intact pregnant and nonpregnant ewes were studied with infusions of cortisol at 0, 0.5, and 1 mg x kg(-1) x day(-1). In adrenalectomized ewes chronically replaced to 0.35 mg x kg(-1) x day(-1) cortisol, plasma ACTH concentrations were decreased significantly in the nonpregnant progesterone-treated ewes compared with the ovariectomized nonpregnant ewes. With 0.5 mg x kg(-1) x day(-1) cortisol, plasma ACTH levels were greater in pregnant ewes than in nonpregnant ewes with or without progesterone. Overall plasma ACTH levels at 0.35 mg x kg(-1) x day(-1) were significantly related to the plasma protein concentration, suggesting that the ACTH levels in the hypocorticoid ewes are most closely related to plasma volume. Across all steroid doses, ACTH was positively related to plasma proteins and progesterone, and negatively related to cortisol. We conclude that increased progesterone does not alter the feedback relation of cortisol to ACTH, but may modulate ACTH indirectly through plasma volume.

Cardiac corticosteroid receptors mediate the enlargement of the ovine fetal heart induced by chronic increases in maternal cortisol

Previous studies have demonstrated that modest, physiologically relevant increases in maternal cortisol in late gestation result in enlargement of the fetal heart. In this study, we investigated the role of mineralocorticoid receptor (MR) or glucocorticoid receptor (GR) in this enlargement. Ewes with single fetuses were randomly assigned at approximately 120 days of gestation to one of four groups: maternal cortisol infusion (1 mg/kg per day, cortisol); maternal cortisol infusion with fetal intrapericardial infusion of the MR antagonist (MRa) potassium canrenoate (600 microg/day; cortisol+MRa); maternal cortisol infusion with fetal intrapericardial infusion of the GR antagonist (GRa) mifepristone (50 microg/day, cortisol+GRa); and maternal saline infusion (control). At approximately 130 days of gestation, fetal heart to body weight ratio and right ventricular (RV) and left ventricular (LV) free wall thicknesses were increased in the cortisol group when compared with control group. Fetal hearts from the cortisol+MRa group weighed significantly less, with thinner LV, RV, and interventricular septum walls, when compared with the cortisol group. Fetal hearts from the cortisol+GRa group had significantly thinner RV walls than the cortisol group. Fetal arterial pressure and heart rate were not different among groups at 130 days. Picrosirius red staining of fetal hearts indicated that the increased size was not accompanied by cardiac fibrosis. These results suggest that physiologic increases in maternal cortisol in late gestation induce fetal cardiac enlargement via MR and, to a lesser extent, by GR, and indicate that the enlargement is not secondary to an increase in fetal blood pressure or an increase in fibrosis within the fetal heart.

Increased maternal cortisol in late-gestation ewes decreases fetal cardiac expression of 11beta-HSD2 mRNA and the ratio of AT1 to AT2 receptor mRNA

Moderately elevated maternal cortisol levels late in gestation cause enlargement of the fetal sheep heart. We have used quantitative real-time PCR to examine expression of candidate genes in fetal hearts from mothers in whom cortisol levels were increased (by infusion of 1 mg cortisol.kg(-1).day(-1)) or decreased (by adrenalectomy and replacement to 0.5 mg cortisol.kg(-1).day(-1)) from 115 to 130 days gestation. Control ewes were not treated with steroid. Expression of mineralocorticoid receptor (MR), glucocorticoid receptor (GR), 11beta-hydroxysteroid dehydrogenases 1 and 2 (11beta-HSD1 and -2), IGF I and II, IGF receptors 1 and 2 (IGF-1R and IGF-2R), endothelial nitric oxide synthase, VEGF, myotrophin, angiotensinogen, the angiotensin receptors 1 and 2 (AT1R and AT2R), and the angiotensin converting enzymes 1 and 2 were measured. MR mRNA abundance in fetal hearts was found to be similar to that in adult kidney and hippocampus. Although there were no significant changes in most genes, 11beta-HSD2 and IGF-1R expression were significantly decreased in the high cortisol group and 11beta-HSD2 expression negatively correlated to left ventricular wall thickness. There was also a significant change in the ratio of AT receptor expression, with increased AT2R and decreased AT1R in the high cortisol group. MR, GR, and 11beta-HSD1 immunoreactivity was found in cardiomyocytes and cardiac blood vessels in 126-128 day fetal sheep; in contrast 11beta-HSD2 staining was predominantly in blood vessels. These results indicate that cortisol could indeed act in the fetal heart to induce enlargement and suggest that the renin-angiotensin system may play a role.

Fetal fornix transection and gestation length in sheep

Experiments in several species indicate that the hippocampus influences hypothalamo-pituitary-adrenal (HPA) axis function. In fetal sheep, simultaneous ACTH and cortisol rises over the last 30 days of gestation peak at term and are necessary for birth. We hypothesized that if the fetal hippocampal formation is functional in late gestation, loss of hippocampal input to the HPA axis following fetal fornix transection would change gestation length in comparison to controls. At 118-121 days of gestation (dG), stereotaxic technique was used in fetal sheep to sham transect (SHAM; n = 8) or transect (FXTX; n = 6) the dorsal fornix at the level of the hippocampal commissure. No differences were found between SHAM and FXTX fetuses in daily hormone profiles over the last week of gestation or in gestation length (148.0 +/- 1.2 vs. 149.0 +/- 0.4 dG, respectively). We conclude that the fetal hippocampus is immature in late gestation and we speculate that an immature hippocampus is necessary for the loss of negative feedback control that gives rise to the long term, simultaneous increases in ACTH and cortisol that are indispensable for labor and delivery at term in sheep.

Differential effects of prenatal exposure to dexamethasone or cortisol on circulatory control mechanisms mediated by angiotensin II in the central nervous system of adult sheep

Prenatal exposure to elevated maternal glucocorticoids (dexamethasone (DEX) or cortisol (CORT)) for 2 days early in pregnancy can 'programme' alterations in adult offspring of sheep, including elevated arterial pressure. DEX treatment also results in greater angiotensin II type 1 (AT1) receptor expression in the medulla oblongata in late gestation fetuses than in saline (SAL)- or CORT-exposed animals. We hypothesized that this would result in functional changes in brainstem angiotensinergic control of cardiovascular function in DEX- but not CORT-exposed animals. To test this hypothesis, cardiovascular responses to intracerebroventricular (I.C.V.) angiotensin II were examined in adult male offspring exposed to DEX (0.48 mg h(-1); n = 7), CORT (5 mg h(-1), n = 6) or SAL (n = 9) from 26 to 28 days of gestation. Increases in mean arterial pressure during i.c.v. infusion of angiotensin II (1 or 10 microg h(-1)) were significantly greater in the DEX group (10 +/- 1 mmHg at 1 microg h(-1)) compared with SAL (6 +/- 1 mmHg) or CORT (6 +/- 1 mmHg) animals (P < 0.05). I.C.V. infusion of the AT1 antagonist losartan significantly decreased cardiac output and heart rate in DEX animals, but not in SAL or CORT animals. Thus, increased expression of brainstem AT1 receptor mRNA after prenatal DEX is associated with increased responsiveness of cardiovascular control to activation of brain AT receptors by exogenous and endogenous angiotensin II. The altered role of the brain RAS in sheep exposed prenatally to DEX was not observed in sheep exposed prenatally to cortisol, suggesting these two glucocorticoids have distinct programming actions.

Genomic analysis of neuroendocrine development of fetal brain-pituitary-adrenal axis in late gestation

The present study was performed to identify the changes in genomic expression of critical components of the hypothalamus-pituitary-adrenal (HPA) axis in the second half of gestation in fetal sheep. We isolated mRNA from pituitary, hypothalamus, hippocampus, and brain stem in fetal sheep at 80, 100, 120, 130, and 145 days of gestation and 1 and 7 days after delivery (n = 4-5/group). Using real-time RT-PCR, we measured mRNA expression levels of glucocorticoid receptor (GR), mineralocorticoid receptor (MR), serum- and glucocorticoid-induced kinase-1 (sgk1), proopiomelanocortin (POMC), CRF, and arginine vasopressin (AVP). Both MR and GR were highly expressed in pituitary and hippocampus; in all tissues GR was more highly expressed than MR. AVP was more highly expressed than CRF in hypothalamus. MR, GR, and sgk1 expression were increased postnatally in brain stem, and sgk1 expression was increased postnatally in hypothalamus. GR expression was reduced in pituitary in term fetuses compared with younger ages. Hypothalamic CRF expression was increased at the end of gestation compared with younger ages, and AVP expression was increased in newborn lambs. Pituitary POMC was increased at 100 days of gestation compared with 80 days; hypothalamic POMC was increased at 120 days. Overall, the results demonstrate the expression of both MR and GR in brain regions important for control of the HPA axis. Decreases in expression of GR in pituitary at the end of gestation might contribute to the decreased corticosteroid negative feedback sensitivity at term in this species.

Glucocorticoid programming of adult disease

Fetal exposure to elevated levels of glucocorticoids can occur naturally when maternal glucocorticoids are elevated in times of stress or when exogenous glucocorticoids are administered. Epidemiological studies and animal models have shown that, whereas short-term benefits may be associated with fetal glucocorticoid exposure, long-term deleterious effects may arise. This review compares the effects of exposure to natural versus synthetic glucocorticoids and considers the ways in which the timing of the exposure and the sex of the fetus may influence outcomes. Some of the long-term effects of glucocorticoid exposure may be explained by epigenetic mechanisms.

Mineralocorticoid receptor expression in late-gestation ovine fetal lung

Changes in adrenal corticosteroid secretion result in changes in lung liquid production in the late-gestation fetus. To test for the presence of mineralocorticoid receptor (MR) in fetal pulmonary epithelium, lungs from fetal sheep of 120 to 130 days' gestation (term about 148 days) were collected and frozen for identification of mRNA for MR in homogenates by reverse transcriptase polymerase chain reaction (RT-PCR) or for determination of 3H-cortisol binding at MR. Other samples of fetal lungs were fixed for localization of MR and Na+, K+ adenosine triphosphatase (ATPase) alpha by immunohistochemistry. MR mRNA was identified in lung tissue from fetuses and newborn lambs, but not from pregnant ewes; MR-regulated genes, including SGK1 and ENaCalpha were also expressed in fetal and newborn lungs. Immunoreactive MR was found in pulmonary epithelial cells and to be colocalized with Na+, K+ ATPase alpha in many sites. These results indicate that the molecular apparatus for mineralocorticoid-stimulated lung liquid reabsorption is present in epithelium by 120 days' gestation.

Pituitary expression of type I and type II glucocorticoid receptors during chicken embryonic development and their involvement in growth hormone cell differentiation

Glucocorticoids can induce somatotroph differentiation in vitro and in vivo during chick embryonic and rat fetal development. In the present study, we identified the nuclear receptors involved in somatotroph differentiation and examined their ontogeny and cellular distribution during pituitary development in the chicken embryo. Several steroids were tested for their ability to induce GH cell differentiation. Only glucocorticoids and aldosterone were effective at low nanomolar concentrations, suggesting involvement of both type I (mineralocorticoid) and type II (glucocorticoid) receptors (MR and GR, respectively). ZK98299 and spironolactone (GR and MR antagonists, respectively) when used alone were unable to block corticosterone or aldosterone (2 nm)-induced somatotroph differentiation. However, ZK98299 and spironolactone in combination abolished corticosterone or aldosterone (2 nm)-induced somatotroph differentiation. When used separately, both antagonists attenuated induction of GH mRNA by corticosterone. Spironolactone alone blocked somatotroph differentiation induced by 0.2 nm corticosterone or aldosterone, indicating that corticosteroids at subnanomolar concentrations act only through the MR. GR protein was detected in pituitary extracts as early as embryonic d 8, whereas MR protein was readily detectable only around d 12. GR were expressed in greater than 95% of all pituitary cells, whereas MR were expressed in about 40% of all pituitary cells. Dual-label immunofluorescence revealed that the majority of somatotrophs on d 12 expressed MR. Given the high affinity of corticosteroids for MR and that corticosteroid concentrations during embryonic development are in the subnanomolar range, expression of MR may constitute a significant developmental event during somatotroph differentiation.