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.

Na+, K+-ATPase activity and subunit isoform protein abundance: effects of antenatal glucocorticoids in the frontal cerebral cortex and renal cortex of ovine fetuses

We examined the effects of single and multiple maternal glucocorticoid courses on cerebral cortical (CC) and renal cortical (RC) Na(+),K(+)-ATPase activity and protein isoform abundance in fetal sheep. Ewes received four dexamethasone or placebo injections in the single course (SC) groups, and the same treatment once a week for five-weeks in the multiple course (MC) groups. CC Na(+),K(+)-ATPase a(2)-abundance was higher (P<0.05) and beta(2)-abundance lower in the SC dexamethasone than placebo group, but Na(+),K(+)-ATPase activity did not change. CC Na(+),K(+)-ATPase activity, a(1)-, beta(1) -, and beta(2)-abundance were lower in the MC dexamethasone than placebo group, but a(2)- and a(3)-abundance did not change. Both dexamethasone courses did not affect CC cell number. RC Na(+),K(+)-ATPase activity, a(1)- and beta(1) -abundance were higher in the MC dexamethasone than placebo group, but did not change in the SC dexamethasone group. We conclude MC, but not a SC of dexamethasone, affect fetal cerebral and renal Na(+),K(+)-ATPase, and MC result in differential effects on Na(+),K(+)-ATPase in these organs.

Effects of postnatal steroids on Na+/K+-ATPase activity and alpha1- and beta1-subunit protein expression in the cerebral cortex and renal cortex of newborn lambs

Na(+)/K(+)-ATPase is a membrane-bound enzyme responsible for Na(+)/K(+) translocation across cell membranes. It is essential for the generation of electrochemical gradients, which control the ionic environment necessary for electrical activity and water and electrolyte balance. Newborn infants who are at risk of developing bronchopulmonary dysplasia (BPD) are frequently treated with corticosteroids. Although these infants are at risk for neurological, water and electrolyte abnormalities, there is little information regarding the effects of clinically relevant doses of corticosteroids on Na(+)/K(+)-ATPase activity and protein isoform expression in the brain and kidney of newborns. In the present study, we examined the effects of dexamethasone on cerebral cortical and renal cortical Na(+)/K(+)-ATPase activity and alpha1- and beta1-protein isoform expression in newborn lambs. Lambs were given four injections of a placebo (n = 11) or one of three different doses of dexamethasone (0.01 mg kg(-1), n = 9; 0.25 mg kg(-1), n = 11; or 0.50 mg kg(-1), n = 9) 12 h apart on Postnatal Days 3 and 4 up to 18 h before harvest of the cerebral cortex and renal cortex. We selected doses in a range to approximate those used to treat infants with BPD. Na(+)/K(+)-ATPase activity was measured in membrane preparations as ouabain-sensitive inorganic phosphate liberation from ATP and alpha1- and beta1-subunit abundance by Western immunoblot. Postnatal treatment of lambs with dexamethasone resulted in a 21.4% increase in Na(+)/K(+)-ATPase activity and a 30.4% increase in catalytic alpha1-protein expression in the cerebral cortex at a dose of 0.50 mg kg(-1) dexamethasone, but not at the lower doses. Dexamethasone treatment was not associated with changes in beta1-isoform expression in the cerebral cortex. In the kidney, dexamethasone treatment was not associated with significant changes in Na(+)/K(+)-ATPase activity or alpha1- or beta1-isoform expression for the doses we examined. Therefore, clinically relevant corticosteroid treatment exerts dose-related, differential organ-specific effects on Na(+)/K(+)-ATPase activity and protein isoform expression in newborn lambs.

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.

Gestational programming: population survival effects of drought and famine during pregnancy

The process whereby a stimulus or stress at a critical or sensitive period of development has long-term effects is termed "programming." Studies in humans and animals convincingly demonstrate that environmental perturbations in utero may permanently change organ structure and metabolism and/or alter homeostatic regulatory mechanisms among the offspring. These programmed changes may be the origins of adult diseases, including cardiovascular disease, obesity, and diabetes. Throughout evolution and development, humans and animals have been exposed to two common environmental stresses, drought and famine. Notably, drought-induced water deprivation is associated with dehydration anorexia and thus a concomitant potential nutrient stress. Our laboratory has performed studies among pregnant rat and sheep in which we simulate drought conditions via maternal dehydration and famine conditions via nutrient restriction. Maternal dehydration results in low-birth-weight offspring, which demonstrate gender-specific plasma hypernatremia and hypertonicity and arterial hypertension. Gestational nutrient restriction also resulted in low-birth-weight offspring. If permitted rapid catch-up growth by nutrient availability, these offspring demonstrate evidence of increased body weight and body fat, and leptin resistance as adults. Conversely, if the catch-up growth is delayed by nutrition restriction, the offspring exhibit normal body weight, body fat, and plasma leptin levels as adults. These studies indicate that osmoregulatory and cardiovascular homeostasis and phenotypic predisposition to obesity may be programmed in utero. Importantly, these results suggest that programming effects may be either potentiated or prevented by interventions during the neonatal period.

Distal air space epithelial fluid clearance in near-term rat fetuses is fast and requires endogenous catecholamines

Knowledge about the conversion of the epithelium in the distal air spaces of the lung from secretion to absorption is imperative to the understanding of postnatal lung development; little such information is available in rats. Distal air space fluid clearance was therefore measured in 21- to 22-day gestation rat fetuses and newborn (40 h) rats. Distal air space fluid clearance was measured from the increase in (131)I-albumin concentration in an isosmolar, physiological solution instilled into the developing lungs. There was no net fluid movement across the distal air space epithelium in the lungs of 21-day gestation fetuses. Twenty-four hours later, distal air space fluid was cleared at a rapid rate in the 22-day gestation fetuses. Within the first 40 h after birth, the rate rapidly declined to adult levels. The high distal air space fluid clearance at 22 days gestation and at 40 h after birth was mediated by beta-adrenergic receptors as demonstrated by elevated plasma epinephrine levels and inhibition by propranolol. Interestingly, the elevated distal air space fluid clearance in the 22-day gestation fetuses was only minimally amiloride sensitive; however, amiloride sensitivity increased over the first 40 h after birth. In conclusion, these studies demonstrate that 1) rapid rates of net alveolar fluid clearance occur late in gestation in the rat and 2) this clearance is driven by elevations of endogenous epinephrine.

Effect of ventilation style on cardiovascular and renal adaptation in preterm newborn lambs

Renal adaptive responses during the 24 h after delivery in term newborn lambs include marked increases in both glomerular filtration rate (GFR) and sodium reabsorption. This study investigated the effects of ventilation style on cardiovascular, renal, and endocrine adaptations in preterm newborn lambs. Lambs (n = 62) were delivered by cesarean section at 131 days gestation (term = 150 days), treated with surfactant, and randomized to one of three ventilation strategies: high-frequency oscillation (12 Hz), high rate (50 breaths/min; tidal volume = 8 ml/kg), or low rate (15 breaths/min; tidal volume = 15 ml/kg). Lambs (5 or 6/group) were ventilated for 2, 5, 10, and 24 h to maintain arterial PCO2 between 45 and 50 mmHg. Plasma vasopressin levels decreased to <25 pg/ml by 10 h, and fractional sodium excretion decreased to <1% by 16 h in all groups. However, cardiac output, renal plasma flow, and GFR values did not change over time for any of the groups. The style of ventilation employed had no measurable effects on overall cardiovascular, renal, or endocrine function. We conclude in ventilated preterm lambs that 1) the ventilation style does not affect the time course for postnatal adaptation, 2) adaptive changes in renal tubular sodium reabsorption are evident by 16 h after birth, and 3) changes in preterm newborn renal sodium reabsorption occur in the absence of postnatal changes in renal plasma flow or GFR.

Direct fetal glucocorticoid treatment alters postnatal adaptation in premature newborn baboons

Abnormalities of premature newborn adaptation after preterm birth result in significant perinatal mortality and morbidity. We assessed the effects of short-term (24 h) fetal betamethasone exposure on preterm newborn baboon pulmonary and cardiovascular regulation and renal sodium handling during the first 24 h after birth. Male fetal baboons (Papio) (124-day gestation, term 185 days) received ultrasound-guided intramuscular injections of saline (n = 5) or betamethasone (0.5 mg/kg; n = 5). Fetuses were cesarean delivered 24 h later, treated with 100 mg/kg surfactant, and ventilated by adjusting peak inspiratory pressures to maintain PCO2 values of 35-50 mmHg for 24 h. Betamethasone- vs. saline-treated mean +/- SE newborn body weights (0.45 +/- 0.02 vs. 0.41 +/- 0.01 kg) were similar. Although prenatal betamethasone did not affect postnatal lung function (PCO2, arterial/alveolar O2 gradient, or dynamic compliance), plasma hormone (cortisol or thyroxine), or catecholamine levels, mean arterial pressure (25 +/- 1 vs. 32 +/- 1 mmHg), plasma sodium concentration (132 +/- 2 vs. 138 +/- 1 meq/l), glomerular filtration rate (0.07 +/- 0.02 vs. 0.16 +/- 0.02 ml.min-1.kg-1), and renal total sodium reabsorption (1.5 +/- 0.5 vs. 16.0 +/- 3.0 mu eq.min-1.kg-1) values were significantly lower in saline-treated than in betamethasone-treated newborns at 24 h. We conclude that despite the fact that there are no pulmonary and endocrine effects, antenatal glucocorticoid exposure alters premature newborn baboon vascular and renal glomerular function and improves sodium reabsorption after preterm delivery.

Glucocorticoid regulation of surfactant components in immature lambs

To assess effects of dose and duration of glucocorticoid exposure on maturation of the fetal lung, we administered single or multiple doses of betamethasone (0.5 mg/kg im) to pregnant sheep for 2 or 21 days before preterm delivery at 125 days of gestation. Lung function (compliance, lung volume at 40 cmH2O pressure, and ventilatory efficiency index) was increased after two to four weekly doses of glucocorticoid (2.5- to 4-fold increase) and after 48 h of exposure (1.4- to 2.3-fold). Total protein of lavage fluid decreased similarly with three doses, four doses, and 48 h of treatment. In lambs with long-term exposure to betamethasone, there was a similar, dose-dependent increase in concentrations of saturated phosphatidylcholine and surfactant proteins A (SP-A) and B (SP-B) (maximal 2- to 3-fold in tissue and 10- to 15-fold in lavage fluid). Levels of SP-A and SP-B were closely correlated in lavage fluid. In animals treated for 48 h, only tissue SP-B was increased (2.7-fold). We conclude that 48 h of glucocorticoid treatment improves lung function in the premature lamb without a detectable increase in lavage surfactant components and that longer exposure to antenatal glucocorticoid increases surfactant lipid and proteins in a coordinated fashion. The enhanced response with repetitive dosing indicates that the process of glucocorticoid-induced lung maturation is either reversible and/or gestational age dependent.