Glucocorticoids regulate the ontogenetic transition of adrenergic receptor subtypes in rat liver

Evaluation of liver glycogen catabolism during hypercortisolism induced by the administration of dexamethasone in rats

BACKGROUND

The contribution of liver glycogen catabolism to hyperglycemia and glucose intolerance induced by pharmacological hypercortisolism were investigated.

METHODS

For this purpose, adult male Wistar rats that received 1.0 mg/kg dexamethasone (DEX) ip at 8:00 a.m. (DEX group) or saline (CON group) once a day for 5 consecutive days were compared.

RESULTS

Experimental hypercortisolism was confirmed by higher (p<0.05) glycemia, lower (p<0.05) body weight and glucose intolerance. In the fed state, the basal glycogen catabolism and the glucagon (1 nM) and epinephrine (2 μM) induced glycogen catabolism were similar between the groups. The activation of glycogen catabolism induced by phenylephrine (2 μM) and isoproterenol (20 μM) were increased (p<0.05) and decreased (p<0.05), respectively, in DEX rats. Furthermore, DEX rats exhibited higher (p<0.05) glycogen catabolism during the infusion of cAMP (3 μM). However, during the infusion of cAMP (15 μM), 6MB-cAMP (3 μM) or cyanide (0.5 mM), the intensification of glycogen breakdown was similar. Thus, in general, hypercortisolism does not influence the basal glycogen catabolism and the liver responsiveness to glycogenolytic agents in the fed state. In contrast with fed state, fasted rats (DEX group) showed a more intense (p<0.05) basal glycogen catabolism.

CONCLUSION

The contribution of glycogen catabolism to hyperglycemia during hypercortisolism depends of the nutritional status, starting from a negligible participation in the fed state up to a significant contribution in the fasted state.

Prenatal dexamethasone 'programmes' hypotension, but stress-induced hypertension in adult offspring

Low birth weight in humans is predictive of hypertension in adult life. Although the mechanisms underlying this link remain unknown, fetal overexposure to glucocorticoids has been implicated. We previously showed that prenatal dexamethasone (DEX) exposure in the rat lowers birth weight and programmes adult hypertension. The current study aimed to further investigate the nature of this hypertension and to elucidate its origins. Unlike previous studies, we assessed offspring blood pressure (BP) with radiotelemetry, which is unaffected by stress artefacts of measurement. We show that prenatal DEX during the last week of pregnancy results in offspring of low birth weight (14% reduction) that have lower basal BP in adulthood ( approximately 4-8 mmHg lower); with the commonly expected hypertensive phenotype only being noted when these offspring are subjected to even mild disturbance or a more severe stressor (up to 30 mmHg higher than controls). Moreover, DEX-treated offspring sustain their stress-induced hypertension for longer. Promotion of systemic catecholamine release (amphetamine) induced a significantly greater rise of BP in the DEX animals (77% increase) over that observed in the vehicle controls. Additionally, we demonstrate that the isolated mesenteric vasculature of DEX-treated offspring display greater sensitivity to noradrenaline and other vasoconstrictors. We therefore conclude that altered sympathetic responses mediate the stress-induced hypertension associated with prenatal DEX programming.

Antenatal dexamethasone and renal vascular resistance in preterm infants

OBJECTIVE

To compare vascular resistance, renal volume and insulin levels in preterm infants with and without antenatal steroids.

METHODS

We studied 61 preterm infants (37 with (group A) and 24 without antecedent of antenatal steroids (group B)). We measured insulin levels at birth in cord blood samples. Ultrasound measurements to evaluate renal arteries resistance index and renal volume were performed during the first 72 h of birth.

RESULTS

Preterm infants from group A tended to have lower weight and gestational age than those from group B. Resistance index in renal arteries was lower in preterm infants with steroid therapy compared with group B (right renal artery 0.73 vs. 0.80; P=0.001, and left renal artery 0.75 vs. 0.79; P=0.01, respectively). Renal volume and insulin levels were not different between the groups. In the multiple regression analysis for factors associated with resistance index of renal arteries, only antenatal steroids use was included in the model (R2=0.13; P=0.003 and R2=0.10; P=0.01 for left and right renal arteries, respectively).

CONCLUSION

Antenatal dexamethasone in preterm infants during the first 72 h of birth seems to decrease resistance index in renal arteries without affecting renal volume and insulin levels.

Effects of dexamethasone and colostrum feeding on mRNA levels and binding capacities of beta-adrenergic receptors in the liver of neonatal calves

Glucocorticoids increase plasma glucose concentrations in neonatal calves, but not hepatic gluconeogenic enzyme mRNA levels and activities. Catecholamines, too, enhance plasma glucose levels and regulate hepatic glucose metabolism. We have measured hepatic mRNA levels of beta-adrenergic receptors and beta-adrenergic receptor binding in neonatal calves on day 5 of life. Calves were fed either colostrums (C) or an isoenergetic milk-based formula (F), and in each feeding group, half of the calves were treated with dexamethasone (DEXA; 30 microg/(kg body weightday)). Abundance of mRNA was highest (P < 0.01) for beta2-adrenergic receptors and was higher (P < 0.01) for beta1- than for beta3-adrenergic receptors. DEXA treatment decreased (P < 0.05) beta1- and beta2-adrenergic receptor mRNA levels. Beta3-adrenergic receptor mRNA levels were higher (P < 0.05) in colostrum- than in formula-fed calves. Competitive binding revealed highest affinities for alprenolol, propranolol (both beta1- and beta2-antagonists), and ICI-188,551 (beta2-antagonist), which did not significantly differ from each other. Atenolol (beta1-antagonist) up to 10(-5) M did not displace (3H)-CGP-12177 from receptors. Competitive binding for adrenaline was best fitted by a two-receptor model. DEXA decreased (P < 0.05) (3H)-CGP-12177 binding capacities, whereas binding affinity of (3H)-CGP-12177 was not affected by DEXA or different feeding. Binding sites correlated positively with mRNA levels of beta2-adrenergic receptors (r = 0.56; P < 0.01). In conclusion, beta2-adrenergic receptors were the dominant subtype in the hepatic tissue. Feeding did not significantly affect beta2-adrenergic binding sites. However, DEXA decreased beta2-adrenergic binding sites and this was regulated at the transcriptional level.

Prenatal stress alters cardiovascular responses in adult rats

Environmental factors in early life are clearly established risk factors for cardiovascular disease in later life. Most studies have focused on nutritional programming and analysed basal cardiovascular parameters rather than responses. In the present study we have investigated whether prenatal stress has long-term effects on cardiovascular responses in adult offspring. Female pregnant Sprague-Dawley rats were subjected to stress three times daily from day 15 to day 21 of gestation. Litters from stressed and control females were cross-fostered at birth to control for mothering effects. When the offspring were 6 months old, blood pressure was measured in the conscious rats through implanted catheters at rest, during restraint stress and during recovery. Basal haemodynamic parameters were similar in the different groups but the pattern of cardiovascular responses during stress and recovery differed markedly between prenatally stressed (PS) and control animals. PS rats had higher and longer-lasting systolic arterial pressure elevations to restraint stress than control animals. They also showed elevated systolic and diastolic blood pressure values during the recovery phase. PS rats demonstrated a greater increase in blood pressure variability compared with control animals during exposure to restraint stress, and showed more prolonged heart rate responses to acute stress and delayed recovery than controls. There was no effect of prenatal stress on baroreflex regulation of heart rate. PS females showed a greater increase in systolic arterial pressure and blood pressure variability and delayed heart rate recovery following return to the home cage then did PS males. These findings demonstrate for the first time that prenatal stress can induce long-term, sex-related changes in the sensitivity of the cardiovascular system to subsequent stress.

Developmental toxicity of terbutaline: critical periods for sex-selective effects on macromolecules and DNA synthesis in rat brain, heart, and liver

beta-Adrenoceptors (betaARs) control cell replication/differentiation, and during development, signaling is not subject to desensitization. We examined the effects of terbutaline, a beta(2)AR agonist used as a tocolytic, on development in rat brain regions and peripheral tissues with high betaAR concentrations. Prenatal terbutaline (gestational days 17-20) decreased cell numbers (DNA content) in the fetal brain and liver. Early postnatal exposure (PN2-5) reduced DNA synthesis in early-developing brain regions of females, with sensitization of the effect upon repeated terbutaline administration; after multiple terbutaline injections, DNA content was reduced in male cerebellum. The cerebellum was targeted later (PN11-14), exhibiting decreased DNA synthesis in both sexes; in contrast, cardiac DNA synthesis decreased after one injection but increased after the fourth daily injection. Our results suggest that excessive betaAR stimulation by terbutaline alters cell development in brain regions and peripheral tissues, with the net effect depending on sex and the timing of exposure. These effects may contribute to neuropsychiatric, cognitive, cardiovascular, and metabolic abnormalities reported in the offspring of women treated with beta-agonist tocolytics.

Beta-adrenoceptor control of G protein function in the neonate: determinant of desensitization or sensitization

Neonatal beta-adrenoceptors (beta-ARs) are resistant to agonist-induced desensitization. We examined the functioning of G(i) and G(s) after repeated administration of beta-AR agonists to newborn rats. Isoproterenol (beta(1)/beta(2) agonist) obtunded G(i) function in the heart but not the liver; in contrast, terbutaline, a beta(2)-selective agonist, enhanced G(i) function. Isoproterenol, but not terbutaline, increased membrane-associated G((s)alpha), which would enhance receptor function. In addition, isoproterenol increased and terbutaline maintained the proportion of the short-splice (S) variant of G((s)alpha) in the membrane fraction; G((s)alpha)S is functionally more active than the long-splice variant. Either isoproterenol or terbutaline treatment increased G((s)alpha) in the cytosolic fraction, a characteristic usually associated with desensitization in the adult. Decreased G(i) activity, coupled with increased membrane-associated G((s)alpha) concentrations and maintenance or increases in membrane G((s)alpha)S, provide strong evidence that unique effects on G protein function underlie the ability of the immature organism to sustain beta-AR cell signaling in the face of excessive or prolonged stimulation; these mechanisms also contribute to tissue selectivity of the effects of beta-agonists with divergent potencies toward different beta-AR subtypes.

Are developing beta-adrenoceptors able to desensitize? Acute and chronic effects of beta-agonists in neonatal heart and liver

During fetal and neonatal development, beta-adrenergic receptors (beta-ARs) appear to be resistant to desensitization by beta-agonist drugs. To determine the mechanisms underlying the regulatory differences between adults and neonates, we administered isoproterenol, a mixed beta(1)/beta(2)-AR agonist, and terbutaline, a beta(2)-selective agonist. Effects were examined in the ensuing 4 h after a single injection, or after the last of four daily injections. We prepared cell membranes from heart (predominantly beta(1)-ARs) and liver (predominantly beta(2)-ARs) and assessed signal transduction in the adenylyl cyclase (AC) pathway. In the first few hours after a single administration of isoproterenol to adult rats, cardiac beta-ARs showed activation of G proteins (elevated AC response to forskolin) and desensitization of beta-AR-mediated responses; after the fourth injection, heterologous desensitization emerged, characterized by a loss of signaling mediated either through beta-ARs or glucagon receptors. Terbutaline evoked an increase in the forskolin response but no desensitization of receptor-mediated responses. When we gave the same treatments to neonatal rats, we observed cardiac G protein activation, but there was neither homologous nor heterologous desensitization of beta-ARs or glucagon receptors. In the adult liver, isoproterenol and terbutaline both failed to evoke desensitization, regardless of whether the drugs were given once or for 4 days. In neonates, however, acute or chronic treatment elicited homologous desensitization of beta-AR-mediated AC signaling, while sensitizing the response to glucagon. These results show that neonatal beta-ARs are inherently capable of desensitization in some, but not all, cell types; cellular responses can be maintained through heterologous sensitization of signaling proteins downstream from the receptor. Differences from adult patterns of response are highly tissue selective and are likely to depend on ontogenetic differences in subtypes of beta-ARs and AC.

Glucocorticoid programming of the fetus; adult phenotypes and molecular mechanisms

It has been long recognised that the glucocorticoid administration to pregnant mammals (including humans) reduces offspring birth weight. Epidemiologically, low weight or thinness at birth is associated with an increased risk of cardiovascular and metabolic disorders in adult life. So, does fetal exposure to glucocorticoids produce such 'programming' of adult disorders? Here data are reviewed which show, in rodents and other model species, that antenatal exposure to glucocorticoids reduces offspring birth weight and produces permanent hypertension, hyperglycaemia, hyperinsulinaemia, altered behaviour and neuroendocrine responses throughout the lifespan. This occurs with exogenous (dexamethasone) or endogenous glucocorticoids, the latter achieved by inhibiting 11 beta-hydroxysteroid dehydrogenase type 2, the feto-placental enzymic barrier to maternal glucocorticoids. Processes underlying fetal programming include determination of the 'set point' of the hypothalamic-pituitary-adrenal axis and of tissue glucocorticoid receptor expression. Detailed molecular mechanisms are being dissected. Analogous stress axis hyperreactivity occurs in lower birth weight humans and may be an early manifestation and indicate approaches to manipulation or prevention of the phenotype.

Beta-adrenoceptor-mediated cell signaling in the neonatal heart and liver: responses to terbutaline

Terbutaline, a beta(2)-adrenoceptor (beta(2)-AR) agonist, is a widely used tocolytic that also crosses the placenta to stimulate fetal beta-ARs. The current study examines the effects of terbutaline administered to neonatal rats. Terbutaline (10 mg/kg sc) given on postnatal day (PN) 2-5 or PN 11-14 elicited significant downregulation of both cardiac and hepatic beta-ARs, with a much greater effect in the liver. Despite the reduction in cardiac beta-ARs, receptor desensitization was absent as evidenced by the maintained ability of isoproterenol to stimulate adenylyl cyclase (AC) in membrane preparations. The underlying mechanism was dissected by using stimulants that operate at different points in the AC signaling pathway, NaF, forskolin, and Mn(2+). When administered in the early neonatal period, terbutaline failed to evoke any changes in cardiac AC activity; however, treatment on PN 11-14 evoked heterologous sensitization downstream from the receptor, evidenced by increases in the response to NaF and forskolin. In the liver, neonatal terbutaline administration elicited a small (approximately equal to 10%) decrease in the AC response to isoproterenol, an effect much smaller than the downregulation of beta-ARs (>40%). In this tissue, desensitization was again offset by heterologous sensitization of AC signaling. These results indicate that, in the developing organism, beta-AR-mediated cell signaling responses are maintained in the face of receptor downregulation through heterologous induction of downstream signaling elements. These unique responses serve to sustain beta-AR signaling in the perinatal period.

Neonatal chlorpyrifos exposure targets multiple proteins governing the hepatic adenylyl cyclase signaling cascade: implications for neurotoxicity

Chlorpyrifos has been hypothesized to interact with receptors and transduction proteins involved in the production of cyclic AMP, contributing to adverse effects on cell replication and differentiation. We studied the effects of neonatal chlorpyrifos exposure on hepatic adenylyl cyclase (AC) activity, as the liver accumulates the highest concentrations of chlorpyrifos and is the site for generation of its active metabolite, chlorpyrifos oxon. Newborn rats were given 1 mg/kg of chlorpyrifos s.c. on PN1-4. On PN5, 24 h after the last dose, AC catalytic activity was induced as assessed by the response to the direct AC stimulant, Mn(2+). In contrast, AC activation dependent upon interaction of the enzyme with G-proteins (forskolin) did not show any enhancement, suggesting impairment of G-protein function. This conclusion was confirmed by impaired responsiveness to fluoride, which directly activates G-proteins. In addition, the response of AC to hormonal signals was altered in a receptor-selective manner, with an enhanced response to glucagon but not to the beta-adrenoceptor agonist, isoproterenol. The effects of chlorpyrifos on AC signaling displayed a critical developmental period of vulnerability, as treatment of older rats (PN11-14) failed to cause substantial induction of AC or interference with G-protein signaling, although it did still enhance the glucagon response. In all cases, the effects of chlorpyrifos disappeared within a few days of discontinuing treatment. These results stand in contrast to the delayed deterioration of AC signaling seen in the brain after the same chlorpyrifos treatment. The temporal and organ selectivity of chlorpyrifos' effects on the AC cascade suggest that disruption of membrane signaling occurs consequent to selective effects on cell development, rather than representing a direct interaction between chlorpyrifos and signaling proteins.

Ontogenetic aspects of hypertension development: analysis in the rat

In this review, we attempt to outline the age-dependent interactions of principal systems controlling the structure and function of the cardiovascular system in immature rats developing hypertension. We focus our attention on the cardiovascular effects of various pharmacological, nutritional, and behavioral interventions applied at different stages of ontogeny. Several distinct critical periods (developmental windows), in which particular stimuli affect the further development of the cardiovascular phenotype, are specified in the rat. It is evident that short-term transient treatment of genetically hypertensive rats with certain antihypertensive drugs in prepuberty and puberty (at the age of 4-10 wk) has long-term beneficial effects on further development of their cardiovascular apparatus. This juvenile critical period coincides with the period of high susceptibility to the hypertensive effects of increased salt intake. If the hypertensive process develops after this critical period (due to early antihypertensive treatment or late administration of certain hypertensive stimuli, e.g., high salt intake), blood pressure elevation, cardiovascular hypertrophy, connective tissue accumulation, and end-organ damage are considerably attenuated compared with rats developing hypertension during the juvenile critical period. As far as the role of various electrolytes in blood pressure modulation is concerned, prohypertensive effects of dietary Na+ and antihypertensive effects of dietary Ca2+ are enhanced in immature animals, whereas vascular protective and antihypertensive effects of dietary K+ are almost independent of age. At a given level of dietary electrolyte intake, the balance between dietary carbohydrate and fat intake can modify blood pressure even in rats with established hypertension, but dietary protein intake affects the blood pressure development in immature animals only. Dietary protein restriction during gestation, as well as altered mother-offspring interactions in the suckling period, might have important long-term hypertensive consequences. The critical periods (developmental windows) should be respected in the future pharmacological or gene therapy of human hypertension.

Glucocorticoid administration alters nuclear transcription factors in fetal rat brain: implications for the use of antenatal steroids

A recent Consensus Conference endorsed antenatal steroid use in prematurity, but indicated the need for future work on molecular and cellular effects on the developing brain. In the current study, pregnant rats were given dexamethasone during late gestation, in doses spanning those recommended for use, and effects on nuclear transcription factors were evaluated. Within the first hour after a single dose of dexamethasone, and intensifying over 4 h, marked induction of brain c-fos was seen. With repeated administration, c-fos became suppressed in some brain regions, but remained elevated in others. Dexamethasone also elicited suppression of the AP-1 family of nuclear binding proteins, but with a slower time course than seen for c-fos induction. The magnitude of the effects of late gestational exposure to dexamethasone on these transcription factors was comparable to those seen when repeated doses were administered to midgestation embryos in the context of dysmorphogenesis. Similarly, the effects on brain c-fos expression were substantially greater than those in the liver, an archetypal glucocorticoid target tissue. These results indicate that even a single, low dose of glucocorticoids used in late gestation, can disrupt the transcription factors that regulate brain cell differentiation.

Transcription of the beta2-adrenergic receptor gene in rat liver is regulated during early postnatal development by an upstream repressor element

As early postnatal development of the male rat proceeds, there is a decline in transcription of the beta2-adrenergic receptor gene in liver which is associated with a decline in beta2-adrenergic receptor mediated glucose mobilization. In this study, primary cultures of rat hepatocytes transiently transfected with fusion genes containing various segments of beta2-adrenergic receptor gene 5'-flanking DNA fused to a promoterless luciferase reporter gene were used to identify genetic elements that might control beta2-adrenergic receptor gene expression during the first 10 days of postnatal life. We found that 261 bp of beta2-adrenergic receptor gene 5'-flanking region (-372 to -95, start of translation is +1) was sufficient to direct high luciferase expression in fetal day 18 hepatocytes and therefore included the beta2-adrenergic receptor gene promoter. Luciferase activities in fetal day 18 hepatocytes transfected with pbeta2AR(-372/-95), pbeta2AR(-1,335/-95) and pbeta2AR(-3,349/-95) were fourfold greater than that in either postnatal day 5 or postnatal day 10 hepatocytes transfected with the same fusion genes. By use of gel mobility shift assays, we observed increased protein binding to a 50 bp segment (-372 to -323) of the beta2-adrenergic receptor gene 5'-flanking region with nuclear extracts prepared from postnatal day 5 and postnatal day 10 hepatocytes compared to fetal day 18 hepatocytes. These findings suggest the presence of a regulatory element in the 5'-flanking region of the beta2-adrenergic receptor gene that appears to be involved in suppression of transcription of the beta2-adrenergic receptor gene in liver during early postnatal development.

A critical period for the role of thyroid hormone in development of renal alpha-adrenergic receptors

Adrenergic input influences renal cell replication/differentiation and the development of excretory function. Kidney cells make adrenoceptors before the arrival of the majority of nerve terminals, and the current study examines whether thyroid hormone plays a role in receptor development. Propylthiouracil (PTU) was given to pregnant and neonatal rats from gestational d 17 through postnatal d 5, a treatment that obtunds thyroid hormone levels throughout the first 2-3 wk postpartum. The PTU group showed significant deficits in the number of alpha1-receptors, and values resolved to normal in parallel with hormone level recovery. The effects were not secondary to alterations in cell differentiation or growth. as the period of receptor abnormalities did not correspond to that of growth inhibition. Similarly, the effects were selective for the alpha1-receptor, as no comparable effects were seen for total membrane protein or for alpha2-receptors. The role of thyroid hormone in alpha1-receptor ontogeny involved a critical developmental window; later in development neither treatment with PTU nor with large doses of thyroid hormone had any impact on alpha1-receptors. Studies of mRNAs encoding the alpha1-receptor subtypes indicated that hypothyroidism targets the alpha1a-subtype, which has been implicated in the transduction of neurotrophic signals; alpha1a-receptor mRNA also showed the largest proportional developmental increase compared with those encoding other alpha1-subtypes. Accordingly, thyroid hormone is likely to set the stage for the subsequent trophic control of renal development by neural input, and hypothyroidism during this critical window can be expected to result in abnormal renal functional development and increased perinatal risk.

Cellular mechanisms for developmental toxicity of chlorpyrifos: targeting the adenylyl cyclase signaling cascade

Developmental neurotoxicity caused by chlorpyrifos exposure is generally thought to target cholinesterase but chlorpyrifos may also act on cellular intermediates, such as adenylyl cyclase, that serve global functions in the coordination of cell development. In the current study, neonatal rats were exposed to apparently subtoxic doses of chlorpyrifos (no weight loss, no mortality) either on Postnatal Days 1-4 or on Postnatal Days 11-14, and the effects on components of the adenylyl cyclase cascade were evaluated in brain regions that are enriched (forebrain) or sparse (cerebellum) in cholinergic innervation, as well as in a nonneural tissue (heart). In all three, chlorpyrifos evoked deficits in multiple components of the adenylyl cyclase cascade: expression and activity of adenylyl cyclase itself, functioning of G-proteins that link neurotransmitter and hormone receptors to cyclase activity, and expression of neurotransmitter receptors that act through this cascade. Disruption of signaling function was not restricted to transduction of cholinergic signals but rather extended to adrenergic signals as well. In most cases, the adverse effects were not evident during the immediate period of chlorpyrifos administration, but appeared after a delay of several days. These results suggest that chlorpyrifos can affect cell development by altering the activity and reactivity of the adenylyl cyclase signaling cascade, a major control point for trophic regulation of cell differentiation. The effects are not restricted to cholinergic targets, nor even to the central nervous system. Hence, disruption of cell development by chlorpyrifos is likely to be more widespread than previously thought.

Glucocorticoids, feto-placental 11 beta-hydroxysteroid dehydrogenase type 2, and the early life origins of adult disease

Increasing human epidemiological data suggest that events that subtly retard intrauterine growth may determine common disorders, such as hypertension and non-insulin-dependent diabetes, in adult life. The underlying mechanisms are unknown. However, excessive fetal exposure to glucocorticoids retards growth and "programs" adult hypertension in rats. 11 beta-Hydroxysteroid dehydrogenase type 2 (11 beta-HSD2) catalyzes the rapid inactivation of cortisol and corticosterone to inert 11 keto-products. Normally, 11 beta-HSD2 in the placenta and some fetal tissues is thought to protect the fetus from excess maternal glucocorticoids. In both rats and humans there is considerable natural variation in placental 11 beta-HSD2, and enzyme activity correlates with birth weight. Moreover, inhibition of feto-placental 11 beta-HSD2 in the rat reduces birth weight and produces hypertensive and hyperglycaemic adult offspring, many months after prenatal treatment; effects are dependent upon intact maternal adrenals, suggesting a direct action on the fetus or placenta. Maternal protein restriction during pregnancy also produces hypertensive offspring and selectively attenuates placental 11 beta-HSD2 activity. These data suggest that feto-placental 11 beta-HSD2, by regulating fetal exposure to maternal glucocorticoids, crucially determines fetal growth and the programming of later disorders. Deficiency of the barrier to maternal glucocorticoids may represent a common pathway between the maternal environment and feto-placental programming of later disease. These data may, at least in part, explain the human observations linking early life events to the risk of subsequent disease.

Ontogeny of beta-adrenoceptor/adenylyl cyclase desensitization mechanisms: the role of neonatal innervation

The ability of adrenergic stimulation to elicit desensitization of the beta-receptor/adenylyl cyclase signaling cascade is not an inherent property of cells but rather is acquired during the period in which sympathetic innervation develops. This study examines whether innervation provides the signal that enables target cardiac and hepatic cells to learn to desensitize their responses. Neonatal rats were sympathectomized with 6-OHDA on postnatal day 1 and were treated at various ages with a regimen of isoproterenol known to elicit desensitization in adults. In control rats, desensitization first appeared between days 6 and 15. Desensitization was heterologous, involving changes in the efficiency of G-protein coupling, as there were parallel decreases in isoproterenol-stimulated adenylyl cyclase activity, basal activity and fluoride-stimulated activity (maximal G-protein activation) without changes in forskolin-Mn2+-stimulated activity (total cyclase catalytic activity). The lesioned animals showed a delay in the onset of desensitization as isoproterenol did not evoke decreased responsiveness until day 25 in the heart; the liver did not display agonist-induced desensitization even at day 25. The effects of lesioning on development of desensitization were entirely separable from those on regulation of beta-receptors themselves: agonist-induced decreases in receptor binding appeared by day 15 in both control and lesioned animals. Uniquely in the youngest animals (6 days old), isoproterenol treatment produced heterologous sensitization of adenylyl cyclase responses rather than desensitization, with a parallel increase in basal, isoproterenol-, fluoride- and forskolin-Mn2+-stimulated activity; the latter indicates induction of total catalytic activity as the primary mechanism of sensitization. The lesioned neonates did not show sensitization, despite the fact that during this period, sympathetic pathways are not functionally competent. Our results indicate that innervation provides a timing signal for the onset of desensitization capabilities of sympathetic target cells, but is not absolutely required for the cells to learn how to desensitize. Prior to the onset of desensitization, agonists induce sensitization that may be important in preserving physiological responsiveness during ontogenetic surges of adrenergic activity. The absence of sensitization in lesioned animals implies that, before physiological function is completely established, early pioneer synapses provide a trophic signal that enables cells to increase their sensitivity to stimulation during the perinatal transition period.

Inhibition of 11-beta-hydroxysteroid dehydrogenase in pregnant rats and the programming of blood pressure in the offspring

Recent epidemiological studies have linked low birth weight with the later occurrence of cardiovascular and metabolic disorders, particularly hypertension. We have proposed that fetal exposure to excess maternal glucocorticoids may underpin this association. Normally, the fetus is protected from maternal glucocorticoids by placental 11beta-hydroxysteroid dehydrogenase (11beta-HSD). We have previously shown that treatment of pregnant rats with dexamethasone, a synthetic glucocorticoid that is poorly metabolized by the enzyme, reduces birth weight and produces elevated blood pressure in the adult offspring. Moreover, low activity of placental 11beta-HSD correlates with low birth weight in rats. Here, we show that maternal administration of carbenoxolone, a potent inhibitor of 11 beta-HSD, throughout pregnancy leads to reduced birth weight (mean 20 percent decrease) and elevated blood pressures (increase in mean arterial pressure, 9 mm Hg in males, 7 mm Hg in females) in the adult offspring of carbenoxolone-treated rats. This effect requires the presence of maternal adrenal products, as carbenoxolone given to adrenalectomized pregnant rats had no effect on birth weight or blood pressure. These data support the hypothesis that excess exposure of the fetoplacental unit to maternal glucocorticoids reduces birth weight and programs subsequent hypertension and indicate a key role for placental 11beta-HSD in controlling such exposure.

Atypical regulation of hepatic adenylyl cyclase and adrenergic receptors during a critical developmental period: agonists evoke supersensitivity accompanied by failure of receptor down-regulation

Ordinarily, beta-adrenergic receptors and responses linked to the receptors increase with development but in the liver, beta-receptors are higher in the fetus and neonate than in adulthood. We examined how hepatic beta-receptor signaling mediated through adenylyl cyclase is regulated in rats of different ages. In each case, animals were pretreated with isoproterenol for 4 d, and on the 5th d, hepatic membrane preparations were examined for adenylyl cyclase activity and receptor binding capabilities. Uniquely in 6-d-old animals, the cyclase response to isoproterenol was enhanced by chronic pretreatment, caused by heterologous sensitization mediated through effects on total catalytic activity (increased response to forskolin-Mn2+) and on G-protein coupling (enhanced effect of fluoride and increased GTP dependence of basal activity). Isoproterenol pretreatment failed to cause beta-receptor down-regulation in 6-d-old animals, but by 15 d of age, down-regulation was detected along with slight desensitization of the cyclase response. However, at 25 d, neither effect was present. In adulthood, repeated isoproterenol administration failed to cause cyclase desensitization but did reduce beta-receptor numbers; the loss of receptors was still unusual in that beta-receptor down-regulation could be achieved with either isoproterenol or with methoxamine, an alpha-receptor agonist. The results indicate that, early in development, hepatic beta-receptor-mediated responses are enhanced, not desensitized, after chronic stimulation. These effects would foster responsiveness of hepatic gluconeogenesis in the face of the massive adrenergic stimulation associated with the transition from fetal to neonatal life. In adulthood, when receptor numbers are far lower than in the neonate, the inability to desensitize the signaling cascade despite receptor down-regulation would serve to maintain the response to catecholamines.

Purification of 11 beta-hydroxysteroid dehydrogenase type 2 from human placenta utilizing a novel affinity labelling technique

11 beta-hydroxysteroid dehydrogenase type 2 (11 beta-HSD2) efficiently inactivates potent glucocorticoid hormones (cortisol and corticosterone), leaving aldosterone unmetabolized. Abundant 11 beta-HSD2 activity in human placenta plays a central role in controlling fetal glucocorticoid exposure, which if excessive is harmful and may predispose to low birth weight and hypertension in adulthood. Similar 11 beta-HSD2 activity in the distal nephron protects mineralocorticoid receptors from glucocorticoids and appears to be important in normal blood pressure control. We have purified human placental 11 beta-HSD2 16000-fold, to homogeneity, and determined over 100 residues of the internal amino acid sequence. Purification was assisted by a novel technique allowing highly specific (single spot on two-dimensional electrophoresis) photoaffinity labelling of active 11 beta-HSD2 in crude tissue extracts by its glucocorticoid substrates. This work reveals that 11 beta-HSD2 is a member of the short-chain alcohol dehydrogenase superfamily (apparent monomer M(r) approximately 40,000). It is a very basic (apparent pI = 9.1) intrinsic membrane protein, requiring as yet undefined membrane constituents for full stability. Affinity chromatography and affinity labelling studies suggest that 11 beta-HSD2 has a compulsory ordered mechanism, with NAD+ binding first, followed by a conformational change allowing glucocorticoid binding with high affinity.

Placental 11 beta-hydroxysteroid dehydrogenase and the programming of hypertension

Excessive foetal exposure to glucocorticoids retards growth and "programmes" adult hypertension in rats. Placental 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD), which catalyses the conversion of corticosterone and cortisol to inert 11 keto-products, normally protects the foetus from excess maternal glucocorticoids. In both rats and humans there is considerable natural variation in placental 11 beta-HSD, and enzyme activity correlates with birth weight. Moreover, inhibition of placental 11 beta-HSD in the rat reduces birth weight and produces hypertensive adult offspring, many months after prenatal treatment with enzyme inhibitors; these effects are dependent upon maternal adrenal products. These data suggest that placental 11 beta-HSD, by regulating foetal exposure to maternal glucocorticoids, crucially determines foeto-placental growth and the programming of hypertension. Maternal protein restriction during pregnancy also produces hypertensive offspring and selectively attenuates placental 11 beta-HSD activity. Thus, deficiency of the placental barrier to maternal glucocorticoids may represent a common pathway between the maternal environment and foeto-placental programming of later disease. These data may, at least in part, explain the human epidemiological observations linking early life events to the risk of subsequent hypertension. The recent characterization, purification and cDNA cloning of a distinct human placental 11 beta-HSD (type 2) will aid the further study of these intriguing findings.

Neural input and the development of adrenergic intracellular signaling: neonatal denervation evokes neither receptor upregulation nor persistent supersensitivity of adenylate cyclase

In the adult, denervation of adrenergic target tissues leads to compensatory upregulation of receptor sites and to supersensitive responses. When 6-hydroxydopamine (6-OHDA) was given to neonatal rats, cardiac beta-receptors failed to show significant upregulation throughout the first five postnatal weeks and alpha 1-receptors were unchanged except at 35 days of age, despite 70-95% depletion of norepinephrine. The failure to upregulate could not be attributed to the high background level of receptor expression commensurate with ontogenetic increases in receptor numbers, since the same deficiency was seen in the liver, a tissue in which beta-receptors decline with development; liver alpha 1-receptors also failed to upregulate after neonatal denervation. Examination of the linkage of beta-receptors to adenylate cyclase indicated major differences from mature regulatory mechanisms, as denervation supersensitivity was completely absent (liver) or emerged only transiently several weeks after 6-OHDA treatment (heart). In the heart, there was evidence for a defect in the G-protein-dependent component of the receptor/cyclase linkage that could contribute to the delayed appearance of supersensitivity. Because the fundamental patterns of receptor ontogeny and of adenylate cyclase responsiveness are still present after neonatal denervation, it is unlikely that neural input provides the major impetus for basal development. However, adult-type regulation of receptors and responses did not emerge even after a prolonged period; thus, neural input during a critical developmental stage may be required for the cell to learn how to adjust receptor expression and the receptor/cyclase link in response to stimulation.

Association of hepatic beta 2-adrenergic receptor gene transcript destabilization during postnatal development in the Sprague-Dawley rat with a M(r) 85,000 protein that binds selectively to the beta 2-adrenergic receptor mRNA 3'-untranslated region

In the liver, transcript destabilization contributes to the decrease in steady-state levels of beta 2-adrenergic receptor mRNA that occurs during early postnatal development in the rat. From genomic DNA, polymerase chain reaction (PCR) was used to amplify a 718-basepair (bp) fragment of the beta 2-adrenergic receptor gene including the entire 3'-untranslated region. Results from SDS-gel electrophoresis and autoradiography demonstrated a M(r) 85,000 cellular factor present in postnatal day 60, but not fetal day 18 rat liver that was ultraviolet (UV) light-crosslinked to in vitro transcribed beta 2-adrenergic receptor RNA 3'-untranslated region. Unlabeled beta 2-adrenergic receptor RNA 3'-untranslated region, but not mouse beta-actin RNA, competed with labeled beta 2-adrenergic receptor RNA 3'-untranslated region for binding to the M(r) 85,000 protein. Cross-linking of the beta 2-adrenergic receptor RNA 3'-untranslated region to the M(r) 85,000 protein was inhibited by the ribohomopolymer poly(U), with poly(A), poly(C) and poly(G) having little or no effect. Thus, a M(r) 85,000 protein has been identified in adult male rat liver that may interact with U-rich sequences in the 3'-untranslated region of the beta 2-adrenergic receptor mRNA and may account for the decreased stability of hepatic beta 2-adrenergic receptor gene transcripts that occurs during development.

Transcriptional and posttranscriptional regulation of hepatic beta 2-adrenergic receptor gene expression during development

Hepatic responsiveness to beta 2-adrenergic stimulation is dynamically regulated during early development as well as following hepatic injury and disease. In the present study, the molecular mechanisms that underlie the decline in the steady-state levels of hepatic beta 2-adrenergic receptor mRNA that occurs during development in the male rat were investigated. As determined by nuclear run-on assays, an age-associated reduction in beta 2-adrenergic receptor gene transcription was observed. The transcription rate of the beta 2-adrenergic receptor gene in postnatal day 18 liver was approximately 50% lower than that of fetal liver. Stability of beta 2-adrenergic receptor gene transcripts was highest (t1/2 approximately 6h) in hepatocytes isolated from fetal rats and was lowest (t1/2 approximately 6h) in hepatocytes from postnatal day 14 rats. In fetal hepatocytes, but not postnatal day 2 hepatocytes, cycloheximide appeared to stabilize beta 2-adrenergic receptor gene transcripts in the presence of actinomycin D. These findings establish the molecular basis of reduced steady-state levels of beta 2-adrenergic receptor mRNA in liver during early postnatal development and suggest multilevel regulatory control of hepatic beta 2-adrenergic receptor gene expression.

Effects of fetal dexamethasone exposure on postnatal control of cardiac adenylate cyclase: beta-adrenergic receptor coupling to Gs regulatory protein

In the adult, glucocorticoids have been shown to upregulate beta-adrenergic control of adenylate cyclase by a variety of mechanisms; glucocorticoids are also thought to play a role in development of cardiac adrenergic function. In the current study, pregnant rats were given 0.2 mg/kg of dexamethasone on gestational days 17, 18, and 19 and the effects on the development of cardiac beta-receptors and their linkage to the stimulatory G-protein, Gs, were examined at 4 days postpartum. beta-Receptor numbers and affinity were unaffected by dexamethasone exposure, nor was there any change in the ability of the GTP analog, Gpp(NH)p, to shift the affinity state of the receptor. Addition of Gpp(NH)p to cardiac membranes enhanced basal and isoproterenol-stimulated adenylate cyclase activity, but the total response to isoproterenol, with or without Gpp(NH)p, represented a very small fraction of total enzymatic activity. Quantitative analysis of Gs indicated no changes attributable to dexamethasone treatment. Although prenatal dexamethasone has been shown to increase adenylate cyclase reactivity to beta-adrenergic input, the effect appears to be at the level of the catalytic subunit of adenylate cyclase, rather than at receptor or G-protein stages.

Fetal dexamethasone exposure interferes with establishment of cardiac noradrenergic innervation and sympathetic activity

Endogenous glucocorticoids provide natural differentiation signals for adrenergic neurons, and exposure to high exogenous steroid levels thus disrupts the timing of neuronal maturation. In the current study, pregnant rats were given 0.05, 0.2, or 0.8 mg/kg dexamethasone on gestational days 17, 18, and 19, and the effects on development of cardiac sympathetic function were assessed postnatally in the offspring. Dexamethasone produced a dose-dependent retardation of body and heart weight gains; at the highest dose, heart weight deficits were smaller than those for body weight, producing a relative cardiomegaly. The weight effects were accompanied by abnormalities of noradrenergic innervation, as assessed with measurements of norepinephrine levels and turnover. Norepinephrine levels were significantly reduced at all doses of dexamethasone, with the magnitude of effect exceeding that on heart or body weights; thus the levels were reduced even when corrected for tissue weight (ng norepinephrine/g heart weight). Norepinephrine turnover, a measure of neuronal impulse activity, showed delayed development at the lowest dose of dexamethasone and displayed profound suppression throughout development at the higher doses. Adverse effects of dexamethasone on norepinephrine turnover were still apparent in young adulthood, despite the recovery of weight variables to within 15% of normal values. In light of the release of steroids during maternal stress and the use of steroids in the therapy of neonatal respiratory distress, developing adrenergic neurons are likely to be targeted for adverse effects even when standard growth indices have normalized.

Glucocorticoid regulation of G-protein subunits in neonatal liver

The effect of dexamethasone administration in vivo on the steady-state levels of G-protein subunits in liver of neonatal rabbits was investigated using specific antibodies to each subunit as well as bacterial toxin-mediated ADP-ribosylation assays. Parallel measurements were also made of the activity of adenylyl cyclase, as influenced by a variety of activators. Dexamethasone administration modulated the levels of G-protein subunits in liver in an age-dependent and subunit-specific manner but not in 24-h-old newborns. The inductive effect of dexamethasone was observed in animals older than 24 h, the greatest effect being on 2- to 3-day-old neonates. In 48-h-old animals the alpha-subunits Gs alpha-1, Gs alpha-2, Gi alpha and the beta-subunit G beta increased 2.0-, 2.1-, 4.3- and 2.8-fold, respectively, compared to the control. The increases were much less for older animals. Dexamethasone treatment also modulated effector-mediated stimulation of adenylyl cyclase activity in vitro and mimicked its effects on G-protein levels; the greatest increase (approximately 2-fold) in the activation of adenylyl cyclase occurred in membranes isolated from 2- to 3-day-old animals. In older animals there was either no effect of dexamethasone or a decrease in activity. The degree of change in enzyme activity paralleled the change in the amount of Gs alpha rather than of Gi alpha or G beta. These results suggest development-dependent regulation of hepatic G-proteins by glucocorticoids.