Lung Liquid Secretion Rate Research Articles

Inhibitor of intramembranous absorption in ovine amniotic fluid

Intramembranous absorption increases during intra-amniotic infusion of physiological saline solutions. The increase may be due partly to the concomitant elevation in fetal urine production as fetal urine contains a stimulator of intramembranous absorption. In this study, we hypothesized that the increase in intramembranous absorption during intra-amniotic infusion is due, in part, to dilution of a nonrenal inhibitor of intramembranous absorption that is present in amniotic fluid. In late-gestation fetal sheep, amniotic fluid volume and the four primary amniotic inflows and outflows were determined over 2-day intervals under three conditions: 1) control conditions when fetal urine entered the amniotic sac, 2) during intra-amniotic infusion of 2 l/day of lactated Ringer solution when urine entered the amniotic sac, and 3) during the same intra-amniotic infusion when fetal urine was continuously replaced with lactated Ringer solution. Amniotic fluid volume, fetal urine production, swallowed volume, and intramembranous absorption rate increased during the infusions independent of fetal urine entry into the amniotic sac or its replacement. Lung liquid secretion rate was unchanged during infusion. Because fetal membrane stretch has been shown not to be involved and because urine replacement did not alter the response, we conclude that the increase in intramembranous absorption that occurs during intra-amniotic infusions is due primarily to dilution of a nonrenal inhibitor of intramembranous absorption that is normally present in amniotic fluid. This result combined with our previous study suggests that a nonrenal inhibitor(s) together with a renal stimulator(s) interact to regulate intramembranous absorption rate and, hence, amniotic fluid volume.

The Effects of in Vivo Pulmonary Oxygenation on Lung Liquid Production in Near‐Term Fetal Sheep

Lung liquid (LL) is secreted into the fetal lung lumen, but it must be rapidly absorbed at birth to allow air breathing. In vitro studies have implicated oxygen as a possible factor causing the switch from secretion to absorption of lung liquid at birth. We developed a technique of oxygenating the fetal lung using liquid ventilation with haemoglobin (Hb) solutions in chronically catheterized fetal lambs (129-140 days gestation; term, 147 days). In some experiments 2,3-diphosphoglycerate (DPG) was added to increase oxygen delivery. LL secretion rate (Jv) was measured using an indicator dilution method. Eighteen fetuses were divided into four groups and ventilated with liquid under the following conditions: (i) Hb with oxygen, (ii) Hb without oxygen, (iii) Hb with DPG and oxygen and (iv) Hb and DPG without oxygen. There was a significant rise (2.6 mmHg, P < 0.02) in fetal arterial Po2 in group iii, but in none of the other groups. In the first 3 h of liquid ventilation there was no difference in Jv between the groups. In group i, during hours 4-6 of liquid ventilation, there was a significant rise in secretion rate from 2.25 +/- 0.88 to 3.74 +/- 0.85 ml h-1 kg-1 (P < 0.001). In group iii, when comparing Jv in the first 3 h of liquid ventilation with that in the following 3 h period of liquid ventilation, a strong trend towards reduction in secretion was observed, falling from 3.03 +/- 0.65 to 0.74 +/- 0.92 ml h-1 kg-1 (three of the four experiments showed a significant decrease in Jv in hours 4-6). These experiments indicate that oxygen delivered to the fetus using liquid ventilation with haemoglobin solutions leads to increased LL secretion when oxygen delivery is small, and suggest there is a decrease in secretion with greater oxygen delivery to the lung.

THE EFFECTS OF IN VIVO PULMONARY OXYGENATION ON LUNG LIQUID PRODUCTION IN NEAR-TERM FETAL SHEEP

Lung liquid (LL) is secreted into the fetal lung lumen, but it must be rapidly absorbed at birth to allow air breathing. In vitro studies have implicated oxygen as a possible factor causing the switch from secretion to absorption of lung liquid at birth. We developed a technique of oxygenating the fetal lung using liquid ventilation with haemoglobin (Hb) solutions in chronically catheterized fetal lambs (129-140 days gestation; term, 147 days). In some experiments 2,3-diphosphoglycerate (DPG) was added to increase oxygen delivery. LL secretion rate (Jv) was measured using an indicator dilution method. Eighteen fetuses were divided into four groups and ventilated with liquid under the following conditions: (i) Hb with oxygen, (ii) Hb without oxygen, (iii) Hb with DPG and oxygen and (iv) Hb with DPG without oxygen. There was a significant rise (2·6 mmHg, P < 0·02) in fetal arterial PO2 in group iii, but in none of the other groups. In the first 3 h of liquid ventilation there was no difference in Jv between the groups. In group i, during hours 4-6 of liquid ventilation, there was a significant rise in secretion rate from 2·25 ± 0·88 to 3·74 ± 0·85 ml h−1 kg−1 (P < 0·001). In group iii, when comparing Jv in the first 3 h of liquid ventilation with that in the following 3 h period of liquid ventilation, a strong trend towards reduction in secretion was observed, falling from 3·03 ± 0·65 to 0·74 ± 0·92 ml h−1 kg−1 (three of the four experiments showed a significant decrease in Jv in hours 4-6). These experiments indicate that oxygen delivered to the fetus using liquid ventilation with haemoglobin solutions leads to increased LL secretion when oxygen delivery is small, and suggest there is a decrease in secretion with greater oxygen delivery to the lung.

Cortisol pretreatment enhances the lung growth response to tracheal obstruction in fetal sheep.

We have investigated whether cortisol pretreatment of sheep fetuses will result in a greater liquid accumulation within the lung and a greater lung growth response to obstruction of the fetal trachea. Chronically catheterized fetal sheep received either 1) a cortisol infusion at an increasing dose (1.5-4.0 mg/day) from days 118 to 127 of gestation; the fetal trachea was then obstructed from days 128 to 131 of gestation (n = 4); 2) a saline infusion from days 118 to 127 of gestation; the fetal trachea was then obstructed from days 128 to 131 of gestation (n = 4); or 3) a saline infusion from days 118 to 127 of gestation with no period of tracheal obstruction (control; n = 4). Fetal tracheal pressures were measured from days 128 to 131 of gestation, whereas lung liquid secretion rates and volumes were measured on days 118, 128, and 131 of gestation. On day 131 of gestation, all fetuses were given an intravenous injection of [3H]thymidine and were killed 8 h later. Cortisol pretreatment increased the volume of liquid that accumulated within the fetal lung from 69.5 +/- 4.1 to 96.1 +/- 14.1 ml/kg after 3 days of tracheal obstruction. Similarly, cortisol pretreatment significantly enhanced the increase in lung DNA content from 257.4 +/- 11.0 to 309.1 +/- 16.3 mg/kg after 3 days of tracheal obstruction. We conclude that pretreatment of fetuses with cortisol increases the volume of liquid that accumulates after tracheal obstruction and, as a result, increases the fetal lung growth response to tracheal obstruction.

Role of the adrenal glands in the maturation of lung liquid secretory mechanisms in fetal sheep.

Our aim was to determine the role of the fetal adrenal glands in the gestational age-related increase in the ability of epinephrine to induce the reabsorption of lung liquid. Fetal lung liquid volumes and secretion rates were measured in five chronically catheterized control and six bilaterally adrenalectomized (ADX) fetal sheep at approximately 5-day intervals from 120 to 144 days of gestation (term approximately 146 days). The ability of epinephrine to induce the reabsorption of fetal lung liquid was then determined on day 144. Fetal adrenalectomy prevented the preparturient increase in fetal plasma cortisol and 3,5,3'-triiodothyronine (T3) concentration and significantly reduced the gestational age-related increase in fetal lung liquid volumes and secretion rates. Close to term (144 days), epinephrine infusions caused a significantly greater rate of lung liquid reabsorption in control (32.2 +/- 4.8 ml/h) compared with ADX (3.7 +/- 0.7 ml/h) fetuses. We conclude that the presence of the fetal adrenal glands is necessary for the age-related increase in 1) the lung liquid secretion rate and 2) the ability of the fetal lung to reabsorb liquid late in gestation. It is likely that cortisol is the active adrenal hormone involved, supporting the theory that cortisol plays a crucial role in the clearance of lung liquid at birth.

Acidaemia enhances the inhibitory effect of hypoxia on fetal lung liquid secretion in sheep.

Previous studies have shown that moderate fetal asphyxia reduces the secretion rate of fetal lung liquid. The present aim was to determine the relative effects of the individual components of asphyxia (hypoxia, hypercapnia and acidaemia) on lung liquid secretion in fetal sheep. Fetal hyperoxia was also studied to determine the extent to which lung liquid secretion is restricted by the relatively low fetal blood PO2. As each manipulation of fetal blood gas tensions and pH treatment produced alterations in more than one aspect of blood composition, data from all treatment groups were combined and a multiple analysis of variance was performed to determine the separate effects of PaO2, PaCO2, SaO2 and pHa. Lung liquid secretion rate was significantly reduced when mean PaO2 values were below 24.5 mmHg (range 12.9-24.3 mmHg). When PaO2 values below 24.5 mmHg occurred in combination with pHa values below 7.275 (range 6.934-7.268) the secretion rates were further reduced. Alterations in pHa alone or in PaCO2 had no significant effect. These results indicate that hypoxia is the principal factor responsible for the inhibition of lung liquid secretion during asphyxia and that acidaemia enhances this inhibition.

Role of fetal breathing movements in control of fetal lung distension

Our aim was to determine the role of fetal breathing movements (FBM) in the maintenance of fetal lung liquid volume. Experiments were performed in 14 chronically catheterized fetal sheep. FBM were selectively abolished for 48 h by the infusion of tetrodotoxin (TTX) onto the phrenic nerves of five fetuses. Lung liquid volumes and secretion rates were measured before each treatment, 46-48 h after the start of the TTX infusion, and 22-24 h after the end of the infusion. Blockade of the phrenic nerves reduced fetal lung liquid volumes from 27.6 +/- 1.9 to 21.8 +/- 2.6 ml/kg and increased lung liquid secretion rates from 3.8 +/- 0.6 to 6.2 +/- 1.1 ml.h-1.kg-1. Control experiments confirmed the lack of effect of TTX infused intravenously and saline infused intrapleurally on changes in fetal lung liquid volume and secretion rate. To measure the static relaxation volume of the fetal lung, in six fetuses we combined skeletal muscle paralysis with bypass of the upper airway for 48 h. This reduced fetal lung liquid volume from 39.1 +/- 3.1 to 23.0 +/- 2.5 ml/kg and increased lung liquid secretion rates from 4.1 +/- 0.7 to 5.8 +/- 0.9 ml.h-1.kg-1. This experiment demonstrates that the fetal lung is normally maintained at a level of expansion that is much greater than its static relaxation volume. We conclude that the volume of luminal liquid in the fetal lungs is dependent on the diaphragmatic contractions associated with FBM. Their effect is to resist the elastic recoil of the fetal lungs, thereby reducing the loss of liquid from the lungs via the trachea.

Amiloride blocks the inhibition of fetal lung liquid secretion caused by AVP but not by asphyxia

We have examined whether the activation of Na+ channels, located on the luminal surface of pulmonary epithelial cells, mediates the inhibitory effects of both arginine vasopressin (AVP) and moderate asphyxia on fetal lung liquid secretion. Lung liquid secretion rates were measured in chronically catheterized fetal sheep during AVP infusions and during periods of asphyxia with and without an Na+ transport blocker (amiloride; 10(-4) M) present in lung liquid. Lung liquid secretion rates were also measured during epinephrine infusions with amiloride present in lung liquid. These secretion rates were compared with measurements made during a preceding control period. Both asphyxia and an infusion of AVP significantly reduced the rate of secretion of fetal lung liquid from 8.4 +/- 1.5 and 18.0 +/- 3.7 to 3.6 +/- 1.0 (P < 0.01) and 5.5 +/- 2.1 ml/h (P < 0.01), respectively. The addition of amiloride to lung liquid did not reverse the inhibitory effects of asphyxia on lung liquid secretion (8.6 +/- 0.8 vs. 0.7 +/- 0.4 ml/h) but did block the inhibitory effects of both epinephrine (14.8 +/- 4.4 vs. 13.8 +/- 3.1 ml/h) and AVP (18.0 +/- 3.7 vs. 19.5 +/- 5.0 ml/h). The addition of amiloride to lung liquid during fetal normoxia did not significantly affect fetal lung liquid secretion rates (8.2 +/- 1.1 vs. 7.4 +/- 0.7 ml/h). We conclude that the inhibitory effect of AVP on fetal lung liquid secretion, like that of epinephrine, involves the activation of luminal surface Na+ channels, whereas the inhibitory effect of asphyxia does not.

The effects of furosemide and bumetanide on lung liquid production by in vitro lungs from fetal guinea pigs

Lungs from fetal guinea pigs of 61 +/- 3 days of gestation were supported in vitro for 3 h, and lung liquid secretion rates were measured by a dye dilution technique based on Blue Dextran 2000. Ten preparations that had received no treatment showed an average secretion rate of 1.12 +/- 0.28 mL.kg-1 body weight.h-1 during the first hour, and there were no significant changes over the following 2 h. In studies of 54 fetal lungs, furosemide, bumetanide, control ethanol carrier, or saline alone were placed in the supporting medium during the middle hour of the 3-h incubations (ABA design). Furosemide at 10(-3) M reduced secretion 83.4 +/- 16.8%; at 10(-4) and 10(-5) M it produced smaller reductions. Bumetanide at 10(-3) M usually produced reabsorption (129.9 +/- 23.0% reduction), at 10(-4) M it reduced secretion 30.9 +/- 11.8%, but at 10(-5) M it was ineffective. Control carrier and saline were without effect. The ability of the loop diuretics to produce reabsorption of fluid in some preparations suggests the unmasking of an active reabsorptive process. The results also suggest that lung liquid secretion in the fetal guinea pig, as in the sheep, is dependent on a Na+ and Cl- cotransport system.

Fluid production by in vitro lungs from fetal guinea pigs

Lungs from fetal guinea pigs (54-67 days of gestation) were supported in vitro, and lung liquid secretion rates were measured by a dye-dilution technique. The average secretion rate in the first hour was 2.14 +/- 0.08 (SE) mL x kg-1 body weight.h-1 (0.21 +/- 0.01 mL/h) (n = 450); this was comparable to intact preparations. In an independent study of 30 lungs, secretion continued unchanged for 3 h, with no significant change in fluid composition. Between 54 days and term, production appeared to fall in terms of millilitres per kilogram per hour. The following agents were placed in the supporting saline during the middle hour of incubation. (i) Sodium iodoacetate: at 10(-4) M this produced a fall in secretion (fall, succeeding hours; 55.4 +/- 23.0 and 64.9 +/- 17.5%; n = 6); at 10(-3) M it stopped secretion (fall, succeeding hours; 87.2 +/- 10.3 and 100%, n = 6). (ii) Ouabain: at 10(-5) M there was no change in production (n = 6); at 10(-4) M, four preparations were unaffected, two reduced production. (iii) Epinephrine (10(-7) M) produced a significant fall in production in all cases (n = 6); in four preparations secretion reduced (average fall, 64.4 +/- 10.8%); in two preparations there was reabsorption (average rate, -1.03 mL.kg-1.h-1). This extends the effect of epinephrine to the guinea pig, and suggests that the in vitro preparation is a useful model for studies of the fetal lung.

Regulation of lung liquid secretion by arginine vasopressin in fetal sheep

We have investigated the influence of gestational age on the inhibition of fetal lung liquid secretion by arginine vasopressin (AVP). In eight fetal sheep, lung liquid secretion rates were measured before and during infusion of AVP (300 mu.kg-1.h-1) at gestational ages between 110 and 148 days. During infusions, the concentration of AVP in fetal plasma increased from less than 8.7 +/- 0.2 pg/ml to 848.7 +/- 75.1 pg/ml. Fetal plasma epinephrine concentrations were not altered during AVP infusion. Infusions of AVP had no effect on fetal lung secretion before 135 days of gestation; they caused a 40.8% inhibition between 136 and 140 days, and at ages greater than 140 days induced an inhibition of 78.4%. In two ewes during labor, AVP infusion caused either a complete inhibition of secretion or reabsorption of lung liquid. The inhibitory effect of AVP increased in an exponential-like fashion with increasing gestational age and appeared to parallel the preparturient rise in fetal plasma cortisol concentrations. Our results indicate that AVP may be involved in the clearance of lung liquid at term and that AVP is unlikely to mediate the inhibitory effect of fetal asphyxia on lung liquid secretion, at least until after 135 days of gestation.

Pulmonary glucose transport in the fetal sheep.

1. In the chronically catheterized sheep fetus between 122 and 143 days gestation the concentration of D-glucose in lung liquid was very low (usually less than 0.01 mM, the lower limit of detection of the analytical method) whereas the mean plasma concentration was 0.19 mM (S.E.M. 0.4, n = 13). 2. When the lung liquid concentration of D-glucose was raised to 1.67-5.00 mM, rapid uptake was observed until the concentration had fallen to its preceding low level. The uptake showed saturation kinetics (Vmax = 2.29-8.78 mumol/min, increasing with gestation; mean Km = 0.14 +/- 0.02 mM, n = 11, no change with gestation). This active uptake of glucose was blocked by phloridzin (10(-4) M). It was associated with a decrease in lung liquid secretion rate from which a change in net sodium flux could be inferred of an order suggesting one-to-one glucose-sodium co-transport. 3. Radiolabelled 3-O-methyl-D-glucose (3-O-meG) - a monosaccharide which is transported but not metabolized - was taken up rapidly from lung liquid and this rapid uptake was inhibited by D-glucose with 50% inhibition at 0.35 mM (+/- 0.08, n = 9). It was also inhibited by phloridzin (10(-4) M). 4. Radiolabelled 2-deoxy-D-glucose - a monosaccharide which is not a substrate for sodium-coupled transport - was taken up only very slowly from lung liquid; the rate of uptake was appropriate for passive diffusional transport and it was unaffected by the addition of D-glucose or phloridzin to lung liquid. 5. Intravenous infusion of D-glucose caused no detectable increase in the concentration of glucose in lung liquid unless phloridzin was added, when a slow increase was observed. 6. In two experiments with active transport blocked by phloridzin in lung liquid (10(-4) M), the rate of entry of labelled 3-O-meG from plasma to lung liquid was measured during intravenous infusion of this tracer for 29 and 23 h. The rates of entry were similar to the rate of efflux of the tracer from lung liquid when uptake was blocked by phloridzin or D-glucose, and similar to the rate expected for a metabolically inert tracer (i.e. it was some two orders of magnitude less than efflux from lung liquid in the absence of an inhibitor).(ABSTRACT TRUNCATED AT 400 WORDS)

Epithelial solute permeability, ion transport and tight junction morphology in the developing lung of the fetal lamb.

1. Experiments were performed on exteriorized fetal lambs of between 69 days' gestation and term (147 days) in order to observe changes in lung volume and lung liquid secretion rate, and to delineate any alterations in solute permeability, ion transport and tight junction morphology in the maturing lung epithelium. Whilst it was technically possible to measure solute permeability as early as 69 days it was not feasible to apply the Ussing flux ratio technique before 84 days.2. Fetal lung liquid volume and secretion rate, when normalized for body weight, increase linearly with gestation, whereas tracheal volume expressed in the same manner remains constant.3. When expressed in terms of pore theory, epithelial permeability to small polar non-electrolytes does not change between 69 days and term (equivalent pore radius 0.66 nm and 0.64 nm respectively).4. In the immature fetus of 69-76 days, mean epithelial tight junction strand number is 8.3, whereas by the end of gestation it has fallen to 4.6.5. The transfer constants (min(-1)) for sodium and chloride movement in the direction lung liquid to plasma are, respectively, some 6 and 4 times greater at 84-87 days than at term.6. As in the mature fetus, the lung epithelium at 84-87 days actively transports chloride from plasma to lung lumen, albeit with a slightly reduced transport e.m.f. Sodium movement does not, at any gestational age, differ from the predictions for passive transfer.7. In lung liquid the concentrations of chloride and potassium increase and that of bicarbonate decreases during gestation, whilst that of sodium does not change. The rises in lung liquid chloride and potassium concentrations follow those in plasma, maintaining plasma/lung liquid ratios of 0.7 and 0.95 respectively. However, plasma bicarbonate remains constant and the plasma/lung liquid ratio for bicarbonate rises from 3 at 69-76 days to 20 near term as the lung liquid bicarbonate falls from 9.8 to under 2 m-mole kg(-1) H(2)O.8. Whereas lung liquid protein concentration remains constant and low at about 0.35 g l.(-1), plasma protein concentration rises from 23 g l.(-1) at 69-76 days to 43 g l.(-1) near term. During the same period arterial blood pressure doubles.