Development of the lung liquid reabsorptive mechanism in fetal sheep: synergism of triiodothyronine and hydrocortisone

Additional effects using progestins in mares on levels of thyroid hormones and steroids in neonates

The risk of pregnancy loss in mares leads to the use of exogenous hormones to help pregnancy maintenance. The objective was to evaluate the proportion of thyroid hormones and steroids in neonates, in the following postpartum period, born to mares fed with synthetic progesterone and to verify the existence of a correlation between the level of progesterone between mother and neonate. Twenty-seven mares and their foals were used. The animals were divided into 5 experimental groups: group 1 (control, without hormonal supplementation), group 2 (random samples fed to 120 days of pregnancy with long-term progesterone), group 3 (mares fed with short-term progesterone as of 280.º day of pregnancy), group 4 (mares fed with long-term progesterone as of 280.º day of pregnancy) and group 5 (mares fed with synthetic hormone [altrenogest] as of 280.º day of pregnancy). The animal's blood collection took place immediately after parturition for the hormonal measurement. The hormones measured in neonates were total T3, free T4, TSH, progesterone and cortisone. In mares, only levels of progesterone. The groups of neonates showed no difference on levels of total T3, free T4, TSH and progesterone. There was no difference on levels of progesterone in mares among the groups. Neonates from groups 4 and 5 had higher and lower cortisone levels, respectively. No neonate showed clinical change. There was also no correlation between levels of progesterone in mares and foals. Thus, hormonal supplementation with long-term progesterone as of 280 days of pregnancy leds to an increase in the neonate's cortisone levels, in the meantime, supplementation with altrenogest as of 280 days of pregnancy caused a decrease on cortisone levels in foals, despite clinical signs have not been observed on these animals.

mTOR signalling, embryogenesis and the control of lung development

The existence of a nutrient sensitive "autocatakinetic" regulator of embryonic tissue growth has been hypothesised since the early 20th century, beginning with pioneering work on the determinants of foetal size by the Australian physiologist, Thorburn Brailsford-Robertson. We now know that the mammalian target of rapamycin complexes (mTORC1 and 2) perform this essential function in all eukaryotic tissues by balancing nutrient and energy supply during the first stages of embryonic cleavage, the formation of embryonic stem cell layers and niches, the highly specified programmes of tissue growth during organogenesis and, at birth, paving the way for the first few breaths of life. This review provides a synopsis of the role of the mTOR complexes in each of these events, culminating in an analysis of lung branching morphogenesis as a way of demonstrating the central role mTOR in defining organ structural complexity. We conclude that the mTOR complexes satisfy the key requirements of a nutrient sensitive growth controller and can therefore be considered as Brailsford-Robertson's autocatakinetic centre that drives tissue growth programmes during foetal development.

Thyroid hormones in fetal growth and prepartum maturation

The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), are essential for normal growth and development of the fetus. Their bioavailability in utero depends on development of the fetal hypothalamic-pituitary-thyroid gland axis and the abundance of thyroid hormone transporters and deiodinases that influence tissue levels of bioactive hormone. Fetal T4 and T3 concentrations are also affected by gestational age, nutritional and endocrine conditions in utero, and placental permeability to maternal thyroid hormones, which varies among species with placental morphology. Thyroid hormones are required for the general accretion of fetal mass and to trigger discrete developmental events in the fetal brain and somatic tissues from early in gestation. They also promote terminal differentiation of fetal tissues closer to term and are important in mediating the prepartum maturational effects of the glucocorticoids that ensure neonatal viability. Thyroid hormones act directly through anabolic effects on fetal metabolism and the stimulation of fetal oxygen consumption. They also act indirectly by controlling the bioavailability and effectiveness of other hormones and growth factors that influence fetal development such as the catecholamines and insulin-like growth factors (IGFs). By regulating tissue accretion and differentiation near term, fetal thyroid hormones ensure activation of physiological processes essential for survival at birth such as pulmonary gas exchange, thermogenesis, hepatic glucogenesis, and cardiac adaptations. This review examines the developmental control of fetal T4 and T3 bioavailability and discusses the role of these hormones in fetal growth and development with particular emphasis on maturation of somatic tissues critical for survival immediately at birth.

Trials with thyroid hormone in preterm infants: clinical and neurodevelopmental effects

A large number of articles exist on thyroid hormone function and its clinical correlates, but only a few exist on trials with thyroid hormones in premature infants. Most of these trials had clinical short-term endpoints, while only one trial had a long-term neurodevelopmental endpoint. None of the trials reported changes in mortality and morbidities. A trend toward a lower occurrence of patent ductus arteriosus is found in thyroid hormone treated infants. A gestational age-dependent effect of thyroxine on neurodevelopmental outcome was found in post-hoc subgroup analyses up until the age of 10 years. Thyroxine treatment was associated with improved mental, motor, and neurological outcomes in infants <28 weeks gestation, but with worse mental and neurological outcome in infants of 29 weeks gestation. Future trials should focus on neurodevelopmental outcomes. Continuous administration of thyroid hormone may be more effective than bolus administration.

Triiodo-L-thyronine rapidly stimulates alveolar fluid clearance in normal and hyperoxia-injured lungs

RATIONALE

Edema fluid resorption is critical for gas exchange and requires active epithelial ion transport by Na, K-ATPase and other ion transport proteins.

OBJECTIVES

In this study, we sought to determine if alveolar fluid clearance (AFC) is stimulated by 3,3',5 triiodo-L-thyronine (T(3)).

METHODS

AFC was measured in in situ ventilated lungs and ex vivo isolated lungs by instilling isosmolar 5% bovine serum albumin solution with fluorescein-labeled albumin tracer and measuring the change in fluorescein isothiocyanate-albumin concentration over time.

MEASUREMENTS AND MAIN RESULTS

Systemic treatment with intraperitoneal injections of T(3) for 3 consecutive days increased AFC by 52.7% compared with phosphate-buffered saline-injected control rats. Membranes prepared from alveolar epithelial cells from T(3)-treated rats had higher Na, K-ATPase hydrolytic activity. T(3) (10(-6) M), but not reverse T(3) (3,3',5' triiodo-L-thyronine), applied to the alveolar space increased AFC by 31.8% within 1.5 hours. A 61.5% increase in AFC also occurred by airspace instillation of T(3) in ex vivo isolated lungs, suggesting a direct effect of T(3) on the alveolar epithelium. Exposure of rats to an oxygen concentration of greater than 95% for 60 hours increased wet-to-dry lung weights and decreased AFC, whereas the expression of thyroid receptor was not markedly changed. Airspace T(3) rapidly restored the AFC in rat lungs with hyperoxia-induced lung injury.

CONCLUSIONS

Airspace T(3) rapidly stimulates AFC by direct effects on the alveolar epithelium in rat lungs with and without lung injury.

Developmental regulation of lumenal lung fluid and electrolyte transport

In the fetus, there is a net secretion of liquid (LL) by the lung as a result of active transport of chloride ions. The rate of secretion and the resulting volume of LL are vital for normal lung growth but how volume is sensed and how secretion may be regulated are still unknown. Towards term under the influence of thyroid and adrenocorticoid hormones, the epithelial sodium channel (ENaC) is increasingly expressed in the pulmonary epithelium. Adrenaline released by the fetus during labour activates ENaC and produces rapid absorption of liquid in preparation for air breathing; absence of ENaC is incompatible with survival. There may be other mechanisms involved in aiding liquid clearance including changes in epithelial permeability, an effect of oxygen on both ENaC and Na/K ATPase and perhaps the influence of additional hormones on ENaC activity. Some time after birth there are further developmental changes with the appearance of other cation channels (CNG1 and perhaps NSCC) which contribute to the liquid absorptive side of the balance existing across the epithelium between secretion and absorption to produce essentially almost no net liquid movement in the postnatal lung. The evidence for these processes is discussed and areas of uncertainty indicated.

Developmental control of iodothyronine deiodinases by cortisol in the ovine fetus and placenta near term

Preterm infants have low serum T4 and T3 levels, which may partly explain the immaturity of their tissues. Deiodinase enzymes are important in determining the bioavailability of thyroid hormones: deiodinases D1 and D2 convert T4 to T3, whereas deiodinase D3 inactivates T3 and produces rT3 from T4. In human and ovine fetuses, plasma T3 rises near term in association with the prepartum cortisol surge. This study investigated the developmental effects of cortisol and T3 on tissue deiodinases and plasma thyroid hormones in fetal sheep during late gestation. Plasma cortisol and T3 concentrations in utero were manipulated by exogenous hormone infusion and fetal adrenalectomy. Between 130 and 144 d of gestation (term 145+/-2 d), maturational increments in plasma cortisol and T3, and D1 (hepatic, renal, perirenal adipose tissue) and D3 (cerebral), and decrements in renal and placental D3 activities were abolished by fetal adrenalectomy. Between 125 and 130 d, iv cortisol infusion raised hepatic, renal, and perirenal adipose tissue D1 and reduced renal and placental D3 activities. Infusion with T3 alone increased hepatic D1 and decreased renal D3 activities. Therefore, in the sheep fetus, the prepartum cortisol surge induces tissue-specific changes in deiodinase activity that, by promoting production and suppressing clearance of T3, may be responsible for the rise in plasma T3 concentration near term. Some of the maturational effects of cortisol on deiodinase activity may be mediated by T3.

Developmental acquisition of T3-sensitive Na-K-ATPase stimulation by rat alveolar epithelial cells

Late in gestation, the developing air space epithelium switches from chloride and fluid secretion to sodium and fluid absorption. Absorption requires Na-K-ATPase acting in combination with apical sodium entry mechanisms. Hypothyroidism inhibits perinatal fluid resorption, and thyroid hormone [triiodothyronine (T3)] stimulates adult alveolar epithelial cell (AEC) Na-K-ATPase. This study explored the developmental regulation of Na-K-ATPase by T3 in fetal rat distal lung epithelial (FDLE) cells. T3 increased Na-K-ATPase activity in primary FDLE cells from gestational day 19 [both primary FDLE cells at embryonic day 19 (E19) and the cell line FD19 derived from FDLE cells at E19]. However, T3 did not increase the Na-K-ATPase activity in less mature FDLE cells, including primary E17 and E18 FDLE cells and the cell line FD18 (derived from FDLE cells at E18). Subsequent experiments assessed the T3 signal pathway to define whether it was similar in the late FDLE and adult AEC and to determine the site of the switch in responsiveness to T3. As in adult AEC, in the FD19 cell line, the phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin blocked the T3-induced increase in Na-K-ATPase activity and plasma membrane quantity. T3 caused a parallel increase in phosphorylation of Akt at Ser473 in FDLE cells from E19, but not from E17 or E18. In the FD18 cell line, transient expression of a constitutively active mutant of the PI3K catalytic p110 subunit significantly augmented the Na-K-ATPase activity and the cell surface expression of Na-K-ATPase alpha(1) protein. In conclusion, FDLE cells from E17 and E18 lacked T3-sensitive Na-K-ATPase activity but acquired this response at E19. The developmental stimulation of Na-K-ATPase by T3 in rat FDLE cells requires activation of PI3K, and the acquisition of T3 responsiveness may be at PI3K or upstream in the signaling pathway.

Congenital diaphragmatic hernia prevents absorption of distal air space fluid in late-gestation rat fetuses

We hypothesized that congenital diaphragmatic hernia (CDH) may decrease distal air space fluid absorption due to immaturity of alveolar epithelial cells from a loss of the normal epithelial Na+ transport, as assessed by amiloride and epithelial Na+ channel (ENaC) and Na-K-ATPase expression, as well as failure to respond to endogenous epinephrine as assessed by propranolol. Timed-pregnant dams were gavage fed 100 mg of nitrofen at 9.5-day gestation to induce CDH in the fetuses, and distal air space fluid absorption experiments were carried out on 22-day gestation (term) fetuses. Controls were nitrofen-exposed fetuses without CDH. Absorption of distal air space fluid was measured from the increase in 131I-albumin concentration in an isosmolar, physiological solution instilled into the developing lungs. In controls, distal air space fluid absorption was rapid and mediated by β-adrenoceptors as demonstrated by reversal to fluid secretion after propranolol. Normal lung fluid absorption was also partially inhibited by amiloride. In contrast, CDH fetuses continued to show lung fluid secretion, and this secretion was not affected by either propranolol or amiloride. CDH lungs showed a 67% reduction in α-ENaC and β-ENaC expression, but no change in α1-Na-K-ATPase expression. These studies demonstrate: 1) CDH delays lung maturation with impaired distal air space fluid absorption secondary to inadequate Na+ uptake by the distal lung epithelium that results in fluid-filled lungs at birth with reduced capacity to establish postnatal breathing, and 2) the main stimulus to lung fluid absorption in near-term control fetuses, elevated endogenous epinephrine levels, is not functional in CDH fetuses.

C-type natriuretic peptide and its receptor are downregulated in pulmonary epithelium following birth

C-type natriuretic peptide (CNP) is a member of the natriuretic peptide family and acts through the membrane bound guanylyl cyclase linked natriuretic peptide receptor B (NPR-B) to increase intracellular cGMP. Activation of the CNP/NPR-B pathway in pulmonary epithelium has been linked to the inhibition of amiloride-sensitive sodium absorption and to the stimulation of the cystic fibrosis transmembrane conductance regulator (CFTR). Given the importance of ion movement across the pulmonary epithelium of the fetal and newborn lung, we sought to examine the expression of CNP and NPR-B in pulmonary epithelium of the developing fetal lamb and following the transition to air breathing. Lambs were sacrificed at 100 and 136 days of gestation and at 3 days, and 4 weeks after full term delivery. Lung sections were immunostained for CNP and NPR-B. At 100 days of gestation, staining for CNP and NPR-B was absent within all pulmonary epithelium. At 136 days of gestation, prominent staining for both CNP and NPR-B was seen within alveolar type II cells, non-ciliated cells of the distal airways (Clara cells), and ciliated epithelium of the upper airways. At both 3 days and 4 weeks following birth, staining for CNP and NPR-B was absent in alveolar type II cells, ciliated bronchial epithelium and was markedly reduced in Clara cells. The presence of CNP and NPR-B within the pulmonary epithelium in the nearterm fetal period and its rapid downregulation following birth suggests that CNP may contribute to the maintenance of the fluid-filled lung through the regulation of trans-epithelial ion flux.

Development of a lung slice preparation for recording ion channel activity in alveolar epithelial type I cells

Background

Lung fluid balance in the healthy lung is dependent upon finely regulated vectorial transport of ions across the alveolar epithelium. Classically, the cellular locus of the major ion transport processes has been widely accepted to be the alveolar type II cell. Although evidence is now emerging to suggest that the alveolar type I cell might significantly contribute to the overall ion and fluid homeostasis of the lung, direct assessment of functional ion channels in type I cells has remained elusive.

Methods

Here we describe a development of a lung slice preparation that has allowed positive identification of alveolar type I cells within an intact and viable alveolar epithelium using living cell immunohistochemistry.

Results

This technique has allowed, for the first time, single ion channels of identified alveolar type I cells to be recorded using the cell-attached configuration of the patch-clamp technique.

Conclusion

This exciting new development should facilitate the ascription of function to alveolar type I cells and allow us to integrate this cell type into the general model of alveolar ion and fluid balance in health and disease.

Spectrum of ion channels in alveolar epithelial cells: implications for alveolar fluid balance

The efficient transition from placental to atmospheric delivery of oxygen at birth is critically dependent on rapid reabsorption of fetal lung fluid. In the perinatal period, this process is driven by active transepithelial sodium transport and is almost exclusively dependent on expression and modulation of the amiloride-sensitive epithelial sodium channel (ENaC). However, later in development, the amiloride sensitivity of the reabsorptive response, which must be sustained to keep the lungs effectively dry, wanes as a function of postnatal age. This Featured Topic (Experimental Biology Meeting, Washington, DC, April, 2004) presented exciting new evidence to demonstrate that, in addition to ENaC, the adult alveolar epithelium expresses a plethora of amiloride-insensitive ion channels, including cystic fibrosis transmembrane conductance regulator, proton channels, voltage-dependent potassium channels, and cyclic nucleotide-gated cation channels. Furthermore, important evidence for selective modulation of ENaC subunits in the lung in response to cardiovascular disease was demonstrated. Finally, it is clear that newly emerging models of human alveolar epithelium in combination with the novel lung slice electrophysiological preparation will ensure that the ascription of function to specific ion channels in the in situ human lung will soon be a real possibility.

Developmental Regulation of Lung Liquid Transport

The developing distal lung epithelium displays an evolving liquid transport phenotype, reflecting a changing and dynamic balance between Cl- ion secretion and Na+ ion absorption, which in turn reflects changing functional requirements. Thus in the fetus, Cl--driven liquid secretion predominates throughout gestation and generates a distending pressure to stretch the lung and stimulate growth. Increasing Na+ absorptive capacity develops toward term, anticipating the switch to an absorptive phenotype at birth and beyond. There is some empirical evidence of ligand-gated regulation of Cl- transport and of regulation via changes in the driving force for Cl- secretion. Epinephrine, O2, glucocorticoid, and thyroid hormones interact to stimulate Na+ absorption by increasing Na+ pump activity and apical Na+ conductance (GNa+) to bring about the switch from net secretion to net absorption as lung liquid is cleared from the lung at birth. Postnatally, the lung lumen contains a small Cl--based liquid secretion that generates a surface liquid layer, but the lung retains a large absorptive capacity to prevent alveolar flooding and clear edema fluid. This review explores the mechanisms underlying the functional development of the lung epithelium and draws upon evidence from classic integrative physiological studies combined with molecular physiology approaches.

Thyroid hormone stimulates Na-K-ATPase activity and its plasma membrane insertion in rat alveolar epithelial cells

Na-K-ATPase protein is critical for maintaining cellular ion gradients and volume and for transepithelial ion transport in kidney and lung. Thyroid hormone, 3,3',5-triiodo-l-thyronine (T3), given for 2 days to adult rats, increases alveolar fluid resorption by 65%, but the mechanism is undefined. We tested the hypothesis that T3 stimulates Na-K-ATPase in adult rat alveolar epithelial cells (AEC), including primary rat alveolar type II (ATII) cells, and determined mechanisms of the T3 effect on the Na-KATPase enzyme using two adult rat AEC cell lines (MP48 and RLE-6TN). T3 at 10-8 and 10-5 M increased significantly hydrolytic activity of Na-K-ATPase in primary ATII cells and both AEC cell lines. The increased activity was dose dependent in the cell lines (10-9-10-4 M) and was detected within 30 min and peaked at 6 h. Maximal increases in Na-K-ATPase activity were twofold in MP48 and RLE-6TN cells at pharmacological T3 of 10-5 and 10-4 M, respectively, but increases were statistically significant at physiological T3 as low as 10-9 M. This effect was T3 specific, because reverse T3 (3,3',5'-triiodo-l-thyronine) at 10-9-10-4 M had no effect. The T3-induced increase in Na-K-ATPase hydrolytic activity was not blocked by actinomycin D. No significant change in mRNA and total cell protein levels of Na-K-ATPase were detected with 10-9-10-5 M T3 at 6 h. However, T3 increased cell surface expression of Na-K-ATPase alpha1- or beta1-subunit proteins by 1.7- and 2-fold, respectively, and increases in Na-K-ATPase activity and cell surface expression were abolished by brefeldin A. These data indicate that T3 specifically stimulates Na-K-ATPase activity in adult rat AEC. The upregulation involves translocation of Na-K-ATPase to plasma membrane, not increased gene transcription. These results suggest a novel nontranscriptional mechanism for regulation of Na-K-ATPase by thyroid hormone.

O2 can raise fetal pneumocyte Na+ conductance without affecting ENaC mRNA abundance

In fetal pneumocytes, increasing P(O(2)) can raise apical Na(+) conductance (G(Na(+))) and increase the abundance of epithelial Na(+) channel subunit (alpha-, beta-, and gamma-ENaC) mRNA, suggesting that the rise in G(Na(+)), which may be important to the perinatal maturation of the lung, reflects O(2)-evoked ENaC gene expression. However, we now show that physiologically relevant increases in P(O(2)) do not affect alpha-, beta-, and gamma-ENaC mRNA abundance in pneumocytes maintained (approximately 48 h) in hormone-free medium or in medium supplemented with dexamethasone and tri-iodothyronine, although the response does persist in cells maintained in medium containing a complex mixture of hormones/growth factors. However, parallel electrometric studies revealed clear increases in G(Na(+)) under all tested conditions and so it is now clear that O(2)-evoked increases in G(Na(+)) can occur without corresponding increases in ENaC mRNA abundance. It is therefore unlikely that this rise in G(Na(+)) is secondary to O(2)-evoked ENaC gene expression.

Pulmonary effects of triiodothyronine (T3) and hydrocortisone (HC) supplementation in preterm infants less than 30 weeks gestation: results of the THORN trial--thyroid hormone replacement in neonates

The THORN trial was a multicenter, randomized, double-blind, placebo-controlled clinical trial to test the hypothesis that administration of triiodothyronine (T(3)) and hydrocortisone would decrease mortality and respiratory morbidity in preterm infants of less than 30 wk gestation. Two hundred fifty-three infants were randomized to receive either 6 micro g.kg(-1).d(-1) of T(3) with 1 mg.kg(-1).d(-1) of hydrocortisone or 5% dextrose (placebo) as a continuous i.v. infusion for 7 d. The dose was halved on d 5. Our first primary outcome was death or ventilator dependence at 1 wk, and the second was death or oxygen dependence at 2 wk. The overall mortality rate for both groups was 11.4%. Relative risk of death or ventilator dependence at 1 wk, treated versus placebo, was 0.87, p = 0.2, and death or oxygen dependence at 2 wk, 1.00, p = 0.9. We examined the relationship between free T(3) (FT(3)) and free thyroxine (FT(4)) levels in the first 7 d and the primary outcome death or ventilator dependence at 1 wk in all 253 babies. We found significant positive correlations of p = 0.05 for FT(3) and p = 0.002 for FT(4). Thus the higher the FT(3) and FT(4) levels, the better the outcome. No beneficial effects of T(3) and hydrocortisone were shown. In this study, although FT(3) levels were doubled by the treatment infusion, FT(4) levels were significantly suppressed. The lack of any beneficial effect of T(3) in our study may be explained by suppression of FT(4) in the treatment group.

Invited review: Clearance of lung liquid during the perinatal period

At birth, the distal lung epithelium undergoes a profound phenotypic switch from secretion to absorption in the course of adaptation to air breathing. In this review, we describe the developmental regulation of key membrane transport proteins and the way in which epinephrine, oxygen, glucocorticoids, and thyroid hormones interact to bring about this crucial change in function. Evidence from molecular, transgenic, cell culture, and whole lung studies is presented, and the clinical consequences of the failure of the physiological mechanisms that underlie perinatal lung liquid absorption are discussed.

Transepithelial sodium and water transport in the lung. Major player and novel therapeutic target in pulmonary edema

Active transepithelial transport of sodium from the airspaces to the lung interstitium is a primary mechanism driving alveolar fluid clearance. This mechanism depends on sodium uptake by amiloride-sensitive sodium channels on the apical membrane of alveolar type II cells followed by extrusion of sodium on the basolateral surface by the Na-K-ATPase. Injury to the alveolar epithelium can disrupt the integrity of the alveolar barrier or downregulate ion transport pathways thus reducing net alveolar fluid reabsorption, and enhancing the extent of alveolar edema. Endogenous catecholamines upregulate alveolar fluid clearance in several experimental models of acute lung injury, but this upregulation is short-term and often not sufficient to counterbalance alveolar flooding. There is new evidence, however, that pharmacological treatment with beta-adrenergic agonists and/or epithelial growth factors may induce a more sustained stimulation of alveolar fluid reabsorption and in turn facilitate recovery from experimental pulmonary edema. Similar results have been achieved experimentally by gene transfer enhancing the abundance of sodium transporters in the alveolar epithelium. Clinical studies show that impaired alveolar fluid transport mechanisms contribute to the development, severity and outcome of pulmonary edema in humans. Very recent data suggest that mechanisms that augment transepithelial sodium transport and enhance the clearance of alveolar edema may lead to more effective prevention or treatment for pulmonary edema and acute lung injury.

Estimation of lung liquid production in fetal sheep with blue dye dextran and radioiodinated serum albumin

Lung liquid production and reabsorption rates and lung volumes were measured in 99 fetal sheep (119-148 days of gestation) by indicator-dilution methods with the simultaneous use of blue dye dextran (BDD) and radioiodinated serum albumin (RISA). There were no significant differences between rates of lung liquid production or reabsorption by the two methods (n = 71 pairs; paired t-test; Wilcoxon test; ANOVA); this was equally true for rates in milliliters per hour or milliliters per kilogram body weight per hour and was independent of age. Volumes measured by both methods showed a close linear relationship (r = 0.97; for slope P < 0.0001; n = 99), whether expressed as milliliters or milliliters per kilogram body weight. Either method could give the higher volume. Values differed by only approximately 4%, independent of age or parameter (ml or ml/kg body wt; volumes regressed to original volume, or as measured in untreated control hours). However, this small difference was significant by paired t-test or Wilcoxon test when all data were combined irrespective of age; it was not significant after allowance for gestational age (two-way ANOVA). Both indicators showed the same increase in lung volume toward birth and the same fall when related to body weight (slopes significant P = 0.0003-0.0004; r = 0.93). Two-way ANOVA showed that the declines were significant (P = 0.003). The data suggest that 1) there was no significant difference in production or reabsorption rates measured by BDD or RISA, 2) differences in volumes measured by the two indicators were only significant if gestational age was ignored and were too small to have physiological importance, and 3) although BDD and RISA each may have methodological weaknesses, for purposes of measuring lung liquid volumes both are sufficiently accurate and reproducible to obtain meaningful physiological results.

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.

A longitudinal assessment of thyroid hormone concentrations in preterm infants younger than 30 weeks' gestation during the first 2 weeks of life and their relationship to outcome

OBJECTIVE

This trial examined the effects of triiodothyronine (T3) and hydrocortisone on lung disease. We present here the placebo data as this provides a natural history of thyroid hormone changes in this group of very preterm infants. We also examined the relationship between thyroid hormone levels and the outcome death and ventilator dependence at 2 weeks.

METHODS

Plasma-free T3 (FT3), free thyroxine (FT4), total T3, total T4, and thyroid-stimulating hormone were measured prospectively in preterm infants who were <30 weeks' gestation during the first 14 days of life. The data were obtained from the placebo arm of a multicenter, randomized, double-blind, clinical trial of T3 and hydrocortisone, called the THORN Trial.

RESULTS

A total of 128 infants were recruited into the placebo group. The mean FT3 level at <5 hours of age was 4.9 pmol/L and remained below this level. FT4 levels decreased from 15 pmol/L to 9.7 pmol/L at 7 days and then increased to 11.0 pmol/L by day 14. Total T3 and total T4 levels fell after 5 hours of age and reached a minimum on day 3. Thyroid-stimulating hormone levels fell markedly from 9.2 mU/L to 1.8 mU/L at 72 hours and then increased to approximately 4 mU/L. We found that all thyroid hormones but particularly FT3 and FT4 hormones were highly significantly related to outcome. The lower the hormone levels, the worse the outcome (death or ventilator dependence at 2 weeks of age).

CONCLUSION

1) Thyroid hormone levels in preterm infants <30 weeks were much lower than in term infants, 2) the postnatal surge of thyroid hormones normally seen at 24 to 48 hours of age in term infants did not occur in our group of preterm infant, and 3) low FT3 and FT4 levels are associated with higher mortality and severity of lung disease.

Lung growth for beginners

Lung growth occurs as a series of tightly regulated events commencing in the embryo and continuing post-natally. It depends on a number of factors, including developmental, genetic and environment ones. Abnormalities of any of these factors may have a profound influence on lung growth. The causes of developmental abnormalities of the lung such as lung cysts and congenital diaphragmatic hernia are poorly understood, but may result from a combination of genetic and environmental factors. Normal fetal breathing movements and an adequate balance between the production of fetal lung fluid and drainage of this fluid are both essential for normal fetal lung growth. It seems that fetal breathing movements are necessary to maintain sufficient pressure within the airways and perhaps to directly stimulate lung growth via induction of mitogenic activity. The volume of intra-pulmonary fluid is regulated by the resistance of the upper airway and by contractions of the diaphragm. Increased drainage of the amniotic fluid, another essential factor for normal lung growth and development, will result in marked pulmonary hypoplasia as may occur with pre-term rupture of the membranes and with fetal renal disease. Perhaps the most important factor for adverse lung growth is pre-term delivery of the infant from any cause including intra-uterine infection. Both ante- and post-natal factors, including mechanical ventilation and oxygen therapy, will affect normal alveolization. In this review, particular attention is paid to breathing movements and the balance between fluid production and drainage.

Dexamethasone and thyroid hormone pretreatment upregulate alveolar epithelial fluid clearance in adult rats

The in vivo effect of 48-h glucocorticoid and thyroid hormone 3,3', 5-triiodine-L-thyronine (T(3)) pretreatment on alveolar epithelial fluid transport was studied in adult rats. An isosmolar 5% albumin solution was instilled, and alveolar fluid clearance was studied for 1 h. Compared with controls, dexamethasone pretreatment increased alveolar fluid clearance by 80%. T(3) pretreatment stimulated alveolar fluid clearance by 65%, and dexamethasone and T(3) had additive effects (132%). Propranolol did not inhibit alveolar fluid clearance in either group, indicating that stimulation was not secondary to endogenous beta-adrenergic stimulation. With the use of bromodeoxyuridine in vivo labeling, there was no evidence of cell proliferation. Alveolar fluid clearance was partially inhibited by amiloride in all groups. Fractional amiloride inhibition was greater in dexamethasone- and dexamethasone-plus-T(3)-pretreated rats than in control animals, but less in T(3)-pretreated rats. In summary, pretreatment with dexamethasone, T(3), or both in combination upregulate in vivo alveolar fluid clearance similarly to short-term beta-adrenergic stimulation. The effects are mediated partly by increased amiloride-sensitive Na(+) transport, because the stimulated alveolar fluid clearance was more amiloride sensitive than in control rats. These observations may have clinical relevance because glucocorticoid therapy is commonly used with acute lung injury.

Errors in estimating lung liquid volume in fetal lambs when using radiolabeled serum albumin and blue dextran

Fetal lung liquid volume is usually determined by using radio-iodinated serum albumin (RISA) or blue dextran (BD) as volume tracers. We tested the reliability of both tracers at 124 (G124) and 142 days of gestation (G142; term = G147) when the labels were employed simultaneously. We measured the proportion of label bound reversibly to the lung, or apparently lost from the lung compartment, by washing out the lung with saline and 5% albumin. At G124, volume estimates with the two labels were similar. At G142, the volume estimate with BD (36.3 +/- 8.7 ml/kg of body wt) was higher (P < 0. 05) than with RISA (22.3 +/- 3.5 ml/kg). This difference resulted from reversible binding of BD, because 5% albumin washout released 38.5 +/- 4.0% of the BD added at the start of the experiment but a lesser amount of RISA (9.8 +/- 0.7%; P < 0.05). At G142, when RISA was used alone, its reversible binding was 1.3 +/- 0.2%. Background absorbance increased during experiments, giving rise to an apparent increase in BD concentration. We conclude that RISA is an effective tracer for lung liquid volume determination in the fetal lamb, whereas our findings of substantial epithelial binding of BD and large changes in background absorbance demonstrate that, under the conditions of our experiments, BD is a poor tracer close to term.

Significance of ion transport during lung development and in respiratory disease of the newborn

Active ion transport plays a critical role in the liquid movement across the fetal and perinatal lung epithelium. The fetal lung liquid production is coupled with active secretion of Cl- into the luminal space. The potential for fluid absorbing mechanisms related to active Na+ transport from the apical to the basolateral side of the epithelium appears near the end of gestation. At birth there is a dramatic change of environment with commencement of air-breathing, sudden increase in oxygen partial pressure (PO2) and profound changes in the pulmonary circulation. A concurrent switch from fluid secretion to maintenance of low amounts of alveolar fluid is another major physiological adjustment taking place in the perinatal distal lung epithelium. The fluid-absorbing mechanism is a result of a well-synchronized co-operation between the basolateral membrane Na-K-ATPase and the apical membrane Na+ channels and it promotes salt and water movement from the airspace. Inability of the fetal lung epithelium to switch from fluid secretion to Na+ transport-dependent absorption seems to be an important factor adversely contributing to the respiratory distress of the newborn premature infant.

G protein-coupled prostaglandin receptor modulates conductive Na+ uptake in lung apical membrane vesicles

Because G protein-regulated cation channels in type II pneumocytes constitute the most likely pathway for alveolar Na+ entry, we explored the hypothesis that a G protein-coupled prostaglandin (PG) E2 receptor controls perinatal lung alveolar Na+ transport. [3H]PGE2 binding to the alveolar apical membrane was trypsin sensitive and showed a rank order of competitive inhibition: PGE2 = PGE1 > PGD2 > PGF2 alpha. Kinetic analysis demonstrated both high-affinity [dissociation constant (KD) = 2.1 +/- 0.7 nM; maximal binding (Bmax) = 27 +/- 7 fmol/mg protein] and low-affinity (KD = 28 +/- 2 nM; Bmax = 265 +/- 29 fmol/mg protein) binding sites. Modulation of high-affinity GTPase activity identified a similar potency order (IC50 = 11 mM for PGF2 alpha vs. 10-50 microM for other PGs), suggesting that the receptors are G protein coupled. Finally, 1 microM PGE2 (approximately IC25) increased conductive 22Na+ uptake into membrane vesicles only in the presence of 100 microM intravesicular GTP. The KD value for the high-affinity binding site together with the rank order of PG effect on ligand binding and G protein function places this PG receptor in the EP3 subtype, whereas Na+ uptake studies suggest that it helps maintain perinatal lung Na+ homeostasis.

Alveolar epithelial fluid clearance is mediated by endogenous catecholamines at birth in guinea pigs

Transition from placental to pulmonary oxygenation at birth depends on a rapid removal of fetal lung fluid from the developing alveoli. Alveolar fluid clearance was examined in ventilated, anesthetized developing guinea pigs of the ages newborn, 2-d-old, 5-d-old, 30-d-old, and 60-d-old (adult). An isosmolar 5% albumin solution was instilled into the lungs of the guinea pigs; the guinea pigs were then studied for 1 h. Alveolar fluid clearance was measured from the increase in alveolar protein concentration as water was reabsorbed. Newborn guinea pigs had a very high alveolar fluid clearance rate that declined rapidly within the first 5 postnatal days towards adult levels. The high alveolar fluid clearance at birth was apparently mediated by the beta-adrenergic system as demonstrated by the elevated plasma epinephrine levels and the increased sensitivity to inhibition by the beta-adrenergic antagonist propranolol immediately after birth. Surprisingly, exogenous addition of epinephrine was not able to stimulate alveolar fluid clearance in the newborn lung, but exogenous epinephrine stimulation increased over time to adult levels. The elevated alveolar fluid clearance at birth was associated with a significantly greater amiloride sensitivity in the newborn guinea pig lung. Northern blot analysis of distal lung tissue as well as isolated alveolar epithelial type II cells showed and confirmed higher levels of the alpha-subunit of the epithelial sodium channel mRNA in the newborn lung that rapidly tapered off toward adult levels. In conclusion, these data demonstrate the importance of the beta-adrenergic system and amiloride-sensitive sodium transporting pathways for clearance of fetal lung fluid at birth.

Massive decline in lung liquid before vaginal delivery at term in the fetal lamb

OBJECTIVE

Our aim was to determine the volume of liquid remaining in the lungs of the fetal lamb just before a normal vaginal delivery at term to assess the extent to which an excess of liquid in the airspaces might contribute to the respiratory morbidity that accompanies elective cesarean delivery.

STUDY DESIGN

The volume of liquid in the future airspace of the lungs was determined at the end of labor in eight fetal lambs at term from the dilution of an impermeable tracer (125I-labeled human serum albumin) mixed into the liquid. This volume was compared with that measured in a second group of 10 fetal lambs studied 7 days before the expected date of delivery (term = 147 days).

RESULTS

The volume of lung liquid present at the end of labor was 6.8 +/- 1.0 ml x kg(-1) (n = 8) compared with 28.2 +/- 1.8 ml x kg(-1) (n = 10) in the second group of lambs studied before the onset of labor at 140 days of gestation.

CONCLUSION

Our results indicate that the bulk (>75%) of the liquid that fills the lungs of the fetal lamb at 140 days of gestation is cleared at some time before normal term birth, suggesting that the adverse respiratory impact of elective cesarean delivery may be largely explained by denying the fetus this important adaptive mechanism.

The amiloride-sensitive Na+ channel: from primary structure to function

Three homologous subunits of the amiloride-sensitive Na+ channel, entitled alpha, beta, and gamma, have been cloned either from distal colon of a steroid-treated rat or from human lung. The alpha, beta, and gamma subunits have similarities with degenerins, a family of proteins found in the mechanosensory neurons of the nematode Caenorhabditis elegans. All these proteins are characterized by the presence of a large extracellular domain, located between two transmembrane alpha-helices, and by short NH2 and COOH terminal cytoplasmic segments. They constitute the first members of a new gene super-family of ionic channels. The epithelial Na+ channel is specifically expressed at the apical membrane of Na(+)-reabsorbing epithelial cells. Its activity is controlled by several distinct hormones, especially by corticosteroids. These hormones act either transcriptionally (such as aldosterone in distal colon, or glucocorticoids in lung) and/or post-transcriptionally (such as aldosterone in kidney). Recent works have provided new insights in the function of that important osmoregulatory system.

Lung-liquid production in vitro by lungs from fetal guinea pigs: effects of amiloride on responses to aldosterone

Lungs from near-term fetal guinea pigs (62 ± 2 days of gestation) were supported in vitro for 3 h; lung-liquid production was monitored by a dye-dilution method based on Blue Dextran 2000. Untreated preparations produced fluid at 1.26 ± 0.14 mL∙kg−1 body mass∙h−1, with no significant change over the ensuing hours (ANOVA, regression analysis; n = 16). Experimental preparations received aldosterone at plasma concentrations reported to be present at birth. Aldosterone produced rapid, significant reductions in fluid production, and occasionally reabsorptions, which persisted beyond treatment. Reductions during treatment were as follows: 10−8 M aldosterone, 90.8 ± 4.9% (P < 0.001; n = 4); 2 × 10−9 M aldosterone, 64.1 ± 16.6% (P < 0.05–0.001; n = 6), and 7 × 10−10 M aldosterone, 48.6 ± 11.7% (P < 0.005–0.001; n = 6). The linear log dose response curve (r = 0.99) showed a theoretical threshold at 3.4 × 10−11 M aldosterone. Responses to 7 × 10−10 M aldosterone were abolished by 10−6 M amiloride. At the highest concentration of aldosterone (10−8 M), 10−6 M amiloride significantly reduced responses, and the changes were no longer significant by ANOVA. At both high and low aldosterone concentrations, responses with amiloride were significantly lower than those without amiloride (ANOVA, P < 0.03–0.04). Amiloride controls and untreated preparations showed no significant changes in fluid production. It is concluded that aldosterone at plasma concentrations present at birth can cause reductions in lung-liquid production or reabsorption through effects on amiloride-sensitive Na+ channels, and that the responses are remarkably rapid.

Effect of lung liquid volume on respiratory performance after caesarean delivery in the lamb

1. The volume of liquid in the lungs of the fetal lamb is reported to fall in the final days of gestation and during labour itself. We aimed to test the hypothesis that this fall in liquid volume adapts the lungs for air breathing and pulmonary gas exchange. 2. In twelve chronically catheterized fetal lambs we measured lung liquid volume at 140 days gestation (term is 147 days) and then delivered the fetuses by Caesarean section under maternal spinal anaesthesia. In five fetuses we removed approximately half the liquid contained in the lungs just before delivery (experimental group) while the remaining seven fetuses were delivered without change to their lung liquid (control group). 3. Lambs born with reduced lung liquid volume improved their arterial blood gas and acid-base status more quickly than lambs born without alteration to lung liquid. 4. Carotid arterial blood gas values in the first 60 min of postnatal life were significantly related to the volume of liquid present in the lungs at birth, with higher arterial partial pressure of oxygen (Pa,02) and arterial oxygen saturation (Sa,02) and lower arterial partial pressure of carbon dioxide (Pa,CO2) levels being associated with lower lung liquid volumes. 5. We conclude that postnatal gas exchange is enhanced by a reduction in the volume of liquid remaining in the lungs when breathing starts.

Differing patterns of mechanical response to direct fetal hormone treatment

A single combined intramuscular dose of betamethasone and l-thyroxine (T4) or placebo was injected into the shoulder of fetal lambs 48 hours prior to delivery at days 121 (n = 14), 128 (n = 25) or 135 (n = 20) of gestation. Respiratory mechanics were calculated using multiple linear regression analysis. Both respiratory system resistance (RRS) and elastance (ERS) decreased approximately 4 fold between gestational days 121 (D121) and 135 (D135). Both variables were also reduced by hormone treatment. Reduction in ERS was due to a reduction in both lung (EL) and chest wall (EW) components. In absolute terms EW decreased with gestational age; however, EW as a proportion of total elastance (% EW) increased. Inclusion of a volume-dependent elastance term in the multiple linear regression model enabled us to separate total elastance into volume-independent (E1) and volume-dependent (E2V) components. E1 decreased almost 8-fold compared with only a 2.5-fold fall in E2V between D121 and D135. %E2, the proportion of ERS which is volume-dependent and which provides an index of overventilation, doubled over this time period. Hormone treatment affected E1 and E2V components equally hence %E2 was not altered. Both excised lung volume and end expiratory alveolar volume increased with gestational age and with treatment. The response to treatment was qualitatively similar at each of the gestational ages examined, however, for all mechanics variables, except resistance and E1, the magnitude of response to treatment was significantly smaller in D135 animals compared with other age groups.

Fetal lung liquid: a major determinant of the growth and functional development of the fetal lung

1. During fetal life the lung develops as a liquid-filled organ. This liquid is produced by the fetal lung and leaves via the trachea from where it is either swallowed or enters the amniotic sac. Fetal lung liquid plays a crucial role in the growth and development of the lungs by maintaining them in a distended state. It is now recognized that the retention of liquid within the future airways is required to maintain the lungs at an appropriate level of expansion in order to stimulate their growth. Indeed, it is likely that most, if not all, of the conditions and malformations that lead to inadequate growth of the fetal lung do so by reducing the volume of lung liquid and hence the degree of lung expansion. 2. The volume of fetal lung liquid is principally regulated by the resistance to lung liquid efflux through the fetal upper airway and by the presence of diaphragmatic activity associated with fetal breathing movements (FBM). During non-breathing periods, the relatively high resistance offered by the upper airway to the efflux of lung liquid opposes the loss of liquid from the lung, thereby maintaining fetal lung expansion. During episodes of FBM, when the larynx is actively dilated and the resistance to lung liquid efflux is reduced, lung liquid leaves the lungs at an increased rate. However, selective inhibition of diaphragmatic muscle activity in the foetus leads to a reduction in lung liquid volume, rather than an increase. This finding indicates that during periods of FBM, rhythmical contractions of the diaphragm retard the loss of lung liquid and help to maintain lung expansion when the upper airway resistance is reduced. It is now apparent that the maintenance of lung expansion by FBM is the basis for their role in promoting fetal lung growth. 3. Successful transition from intra-uterine to extra-uterine life is dependent upon the clearance of liquid from the fetal lungs at the time of birth so that the lungs may effectively function as an organ of gas exchange.(ABSTRACT TRUNCATED AT 400 WORDS)

Hydrocortisone promotes the maturation of Na(+)-dependent ion transport across the fetal pulmonary epithelium

The pulmonary epithelium's change from Cl(-)-dependent fluid secretion to Na(+)-dependent fluid absorption in late gestation appears to be important in the transition of the lung from a fluid-filled organ in utero to an air-filled organ after birth. This maturational process may be regulated in part by hormones. We examined the effects of hydrocortisone on ion transport across monolayer cultures of distal pulmonary epithelial cells isolated from the fetal rat. Hydrocortisone pretreatment enhanced terbutaline (10(-5) M) stimulation of short-circuit current (Isc) but only in monolayers derived from immature fetal cells (day 18 of gestation), stimulating basal Isc by 270% in control monolayers and by 329% in hydrocortisone-pretreated monolayers. Amiloride (10(-4) M) inhibited terbutaline-stimulated Isc by different amounts, depending on gestational age and pretreatment; Isc fell 40% in control monolayers derived from immature cells, 68% in hydrocortisone-pretreated monolayers from immature fetal cells, and approximately 70% in both control and hydrocortisone-pretreated monolayers from mature fetal cells (day 21 of gestation). The basal Isc in monolayers derived from immature cells was also variably inhibited by amiloride with Isc decreasing 26% in control monolayers and 57% in hydrocortisone-pretreated monolayers. The differential responses of terbutaline-stimulated Isc to benzamil, dimethylamiloride, and bumetanide suggested that Isc sensitivity to amiloride was dependent predominantly on Na+ channel activity regardless of gestational age or pretreatment. We conclude that hydrocortisone promotes the maturation of transepithelial Na+ transport in fetal rat lung epithelium by altering Na+ entry into the cell through Na+ channels. Hydrocortisone also enhances beta-adrenergic agonist stimulation of ion transport.

Effects of hormones on potential difference and liquid balance across explants from proximal and distal fetal rat lung

1. Fetal rat tracheas and lung buds form liquid-filled cysts in submersion culture. The volume that accumulates in cysts is driven by active Cl- secretion. 2. We examined the effects, on these explants, of hormones that induce liquid absorption by fetal sheep lung in vivo. Explants were impaled with microelectrodes to measure potential difference (PD). Liquid was estimated from explant weight. 3. Water/dry weight ratio of lung buds and tracheas after 8 days in culture averaged 12 and 22. Exposure of triiodothyronine (T3) and hydrocortisone (HC) followed by a physiological dose of adrenaline on day 7 for 24 h or by a maximal dose of terbutaline on day 8 for 4 h induced a 35% decrease in water/dry weight ratio of distal buds but not tracheas. No hormone, or combination of two hormones, affected the ratio for tracheas or lung buds. 4. Basal PDs of tracheas (18.9 mV) and lung buds (3.7 mV) were increased about 50% by terbutaline. The terbutaline response was inhibited by bumetanide, but not by amiloride injected into the cysts. 5. T3 and HC pretreatment reduced basal PD by one-third. Subsequent exposure to terbutaline raised the PD of hormone-pretreated lung buds by more than 150%, a response that was blocked by amiloride, but was antagonized minimally by bumetanide. Responses of hormone-pretreated tracheas were not different from those of untreated tracheas. 6. We conclude that: (a) absorption of liquid from lung buds is driven by an amiloride-sensitive process (active Na+ transport?) and (b) only distal lung contributes to the adrenaline-sensitive reabsorptive process required for perinatal adaptation to air breathing.

Myocardial contractile protein ATPase activities in adrenalectomized and thyroidectomized rats

This report compares the effects of adrenalectomy and thyroidectomy, with and without hormone replacement, on loss of contractile protein ATPase activities. The rationale for this study was derived from the similarities in their intracellular receptors, mechanisms of action, and the large number of proteins regulated by both hormones. Rats were adrenalectomized, thyroidectomized, or both, and were subsequently treated for 6 weeks with hydrocortisone, triiodothyronine, or saline. Sham-operated rats were given saline for the same period of time. Six weeks of adrenal insufficiency resulted in diminished enzymatic activity of myofibrillar, Ca(2+)-activated myosin ATPase, and actin-activated myosin ATPase fractions. Treatment with hydrocortisone prevented the decline in enzymatic activity due to adrenalectomy. Likewise, thyroidectomy caused a loss of enzymatic activity which was prevented by treatment with triiodothyronine. The full deleterious effect of combined ablation could be partially prevented by treatment with either hydrocortisone or triiodothyronine, but the latter was most effective. The results suggest that hydrocortisone and triiodothyronine each had significant positive effects in the presence of the other, but not in its absence, on the activity of myofibrillar Ca(2+)-dependent Mg-ATPase and Ca(2+)-activated myosin ATPase. The effects of these two hormones on actin-activated myosin ATPase activity were more independent of each other. We conclude that the actions of thyroid and glucocorticoid hormones on the heart are interrelated and that optimum myocardial function results from their combined action.

Liquid flow across the epithelium of the artificially perfused lung of fetal and postnatal sheep

1. The lungs of five fetal (133-140 days gestation) and thirty-four postnatal (2-240 days) sheep were artificially perfused in situ with warmed and oxygenated sheep blood. In postnatal animals the airspace of the lung was filled with liquid similar in composition to fetal lung liquid. In fetal and postnatal animals luminal liquid volume was measured by the impermeant tracer technique. 2. Under resting conditions the pulmonary epithelium of fetal animals secreted liquid at a mean (+/- S.E.M.) rate of 2.0 (+/- 0.4) ml (kg body weight)-1 h-1, those of postnantal animals absorbed liquid at -1.8 (+/- 0.2) ml (kg body weight)-1 h-1. 3. Addition of 2,4-dinitrophenol to achieve a concentration of 1.5 x 10(-3) M in the perfusing blood in postnatal animals caused complete cessation of liquid absorption. 4. Light and electron microscopic examination of the lung after periods of up to 6 h of artificial perfusion showed no evidence of epithelial damage. From 3 h onwards, liquid accumulation was evident in the perivascular spaces. 5. Addition of adrenaline to the perfusate in fetal animals caused absorption of liquid to occur at a mean rate of -2.9 (+/- 1.3) ml (kg body weight)-1 h-1. In postnatal animals adrenaline caused the rate of liquid absorption to increase from a mean rate of -1.4 (+/- 0.2) to -2.2 (+/- 0.3) ml (kg body weight)-1 h-1. 6. In the fetus addition of amiloride (0.8 x 10(-4) M) to the luminal fluid blocked adrenaline-induced liquid absorption and caused secretion to occur at 1.3 (+/- 0.3) ml (kg body weight)-1 h-1. 7. In postnatal animals the response to amiloride was age dependent. In newborn lambs (2-14 days) amiloride blocked liquid absorption and caused secretion of liquid to occur in seven out of eight animals at a mean rate of 0.9 (+/- 0.3) ml (kg body weight)-1 h-1 (n = 8). In older animals (15-240 days) the characteristic response to amiloride was slowing of the rate of liquid absorption (mean rate of absorption,-0.2 (+/- 0.09) ml (kg body weight)-1 h-1, n = 18) with liquid secretion being seen in only three of eighteen animals.(ABSTRACT TRUNCATED AT 400 WORDS)