Fluid reabsorption and glucose consumption in edematous rat lungs

Continuous Endogenous Exhaled CO Monitoring by Laser Spectrometer in Human EVLP Before Lung Transplantation

Endogenous production of carbon monoxide (CO) is affected by inflammatory phenomena and ischemia-reperfusion injury. Precise measurement of exhaled endogenous CO (eCO) is possible thanks to a laser spectrometer (ProCeas® from AP2E company). We assessed eCO levels of human lung grafts during the normothermic Ex-Vivo Lung Perfusion (EVLP). ProCeas® was connected in bypass to the ventilation circuit. The surgical team took the decision to transplant the lungs without knowing eCO values. We compared eCO between accepted and rejected grafts. EVLP parameters and recipient outcomes were also compared with eCO values. Over 7 months, eCO was analyzed in 21 consecutive EVLP grafts. Two pairs of lungs were rejected by the surgical team. In these two cases, there was a tendency for higher eCO values (0.358 ± 0.52 ppm) compared to transplanted lungs (0.240 ± 0.76 ppm). During the EVLP procedure, eCO was correlated with glucose consumption and lactate production. However, there was no association of eCO neither with edema formation nor with the PO₂/FiO₂ ratio per EVLP. Regarding post-operative data, every patient transplanted with grafts exhaling high eCO levels (>0.235 ppm) during EVLP presented a Primary Graft Dysfunction score of 3 within the 72 h post-transplantation. There was also a tendency for a longer stay in ICU for recipients with grafts exhaling high eCO levels during EVLP. eCO can be continuously monitored during EVLP. It could serve as an additional and early marker in the evaluation of the lung grafts providing relevant information for post-operative resuscitation care.

How to measure alterations in alveolar barrier function as a marker of lung injury

The alveolar capillary membrane maintains the proper water and solute content of the epithelial lining fluid at the alveolar air-liquid interface, which is critical for adequate gas exchange in the lung. This is possible due to the alveolar fluid clearance (AFC) capacity of this membrane that assists in the removal of salt and water from the alveolar air spaces. The alveolar capillary membrane also provides a barrier that restricts the passage of proteins and water from the interstitial and vascular compartments into the alveolar air spaces. This restricted passage is due to the presence of tight junctions between adjacent alveolar epithelial cells. Severe injury to the alveolar epithelial/endothelial membrane results in increased protein permeability and impairment of AFC, which leads to the formation of protein-rich edema with the consequent deterioration of gas exchange. Many animal models of lung injury, focused on damage of the alveolar-capillary membrane, assess the AFC capacity and the barrier function. We describe a simple method to assess the AFC rate in normal and pathological conditions in mice. We also describe two complementary methods to assess the alveolar-capillary barrier function, which require measuring the concentration of endogenous plasma proteins in bronchoalveolar lavage fluid and detection of tight-junction proteins in lung tissue by immunofluorescence.

Role of endothelin-1 in acute lung injury

The alveolar-capillary membrane serves as a barrier that prevents the accumulation of fluid in the alveolar space and restricts the diffusion of large solutes while facilitating an efficient gas exchange. When this barrier becomes dysfunctional, patients develop acute lung injury (ALI), which is characterized by pulmonary edema and increased lung inflammation that leads to a life-threatening impairment of gas exchange. In addition to the increase of inflammatory cytokines, plasma levels of endothelin-1 (ET-1), which is a primarily endothelium-derived vasoconstrictor, are increased in patients with ALI. As patients recover, ET-1 levels decrease, which suggests that ET-1 may not only be a marker of endothelial dysfunction but may have a role in the pathogenesis of ALI. While pulmonary edema accumulates, alveolar fluid clearance (AFC) is of critical importance, as failure to return to normal clearance is associated with poor prognosis in patients with pulmonary edema. AFC involves active transport mechanisms where sodium (Na(+)) is actively transported from the alveolar airspaces, across the alveolar epithelium, and into the pulmonary circulation, which creates an osmotic gradient that is responsible for the clearance of lung edema. In this article, we review the relevance of ET-1 in the development of ALI, not only as a vasoconstrictor molecule but also by inhibiting AFC via the activation of endothelial ET-B receptors and generation. Furthermore, this review highlights the therapeutic role of drugs such as beta-adrenergic agonists and, in particular, of endothelin receptor antagonists in patients with ALI.

Glucose transport in the lung and its role in liquid movement

Glucose concentration in the liquid present in the alveolar/airway lumen is the consequence of the balance between removal by lung epithelial cells and entry from the plasma or lung interstitium through the paracellular pathway. Glucose removal is mediated by active, Na(+) -dependent, cotransport and results in transepithelial Na(+) transport and liquid absorption in animals with significant rates of luminal glucose uptake and when luminal glucose concentration is high enough. Cotransport kinetics predicted a low luminal glucose concentration at the steady state, and foetal lung fluid and adult alveolar epithelial lining fluid glucose concentrations were indeed found lower than plasma. When luminal glucose concentration is low, the glucose-dependent part of transepithelial Na(+) transport is abated and alveolar liquid clearance reduced. A means to refuel this mechanism of liquid absorption would be to increase glucose entry in alveolar spaces through an increase in paracellular permeability. This hypothesis was modelled, and experimental data were found to acceptably agree with predictions.

Single dexamethasone injection increases alveolar fluid clearance in adult rats

OBJECTIVE

Epithelial Na+ channels and Na+/K+-adenosine triphosphatase (ATPase) in alveolar epithelium have a very important role in the absorption of excessive fluid from the alveolar space. We examined whether single dexamethasone injection at therapeutic doses would modulate lung epithelial Na+ channels and Na+/K+-ATPase and increase alveolar fluid clearance in adult rats.

DESIGN

Controlled laboratory study.

SETTING

University research laboratory.

SUBJECTS

Adult male Sprague-Dawley rats (n = 138).

INTERVENTIONS

Rats were intraperitoneally injected with dexamethasone at a dose ranging from 0.02 to 2.0 mg/kg, and allowed free access to food and water.

MEASUREMENTS AND MAIN RESULTS

Alveolar fluid clearance was determined by measuring the increase in albumin concentration in the lung instillate solution. We discovered a significant increase in alveolar fluid clearance at 48 and 72 hrs after dexamethasone treatment. The effect of dexamethasone was dose dependent. In addition, increased alveolar fluid clearance was associated with a faster recover from hypoxemia, which was induced by filling the alveolar space with instillate solution. The dexamethasone-induced increase in alveolar fluid clearance was inhibited by amiloride and ouabain. Quantitative reverse transcriptase-polymerase chain reaction showed that dexamethasone treatment increased lung beta-epithelial Na+ channel mRNA levels. The expression of gamma-epithelial Na+ channel mRNA was also increased slightly. In contrast, alpha-epithelial Na+ channel mRNA levels did not differ from control levels. There was no change in alpha1- or beta1-Na+/K+-ATPase mRNA levels over 72 hrs after dexamethasone treatment. However, we found that lung Na+/K+-ATPase hydrolytic activity, determined by monitoring the ouabain-sensitive ATPase hydrolysis, was increased at 48 and 72 hrs after dexamethasone treatment.

CONCLUSIONS

Single dexamethasone injection at therapeutic doses is capable of modulating lung epithelial Na+ channels and Na+/K+-ATPase and increase alveolar fluid clearance, thereby accelerating recovery from pulmonary edema.

Mechanisms of pulmonary edema clearance during acute hypoxemic respiratory failure: role of the Na,K-ATPase

Pulmonary edema is the hallmark of acute respiratory distress syndrome. It occurs when the permeability of the alveolar-capillary barrier is increased, causing alveolar flooding and impaired gas exchange. The mechanisms of alveolar fluid resorption are different from those of alveolar edema formation. Alveolar fluid resorption into the vessels is brought about mainly by active transport of sodium ions (Na+) out of the alveolar spaces with water following the osmotic gradient. Na+ transport across the alveolar epithelium, and thus alveolar fluid resorption, is regulated by apical Na+ channels, the basolateral sodium potassium-adenosine triphosphatase (Na,K-ATPase), and possibly chloride channels. The Na,K-ATPase has been localized to the alveolar epithelium and the importance of its role in contributing to lung edema clearance has been demonstrated. In models of lung injury, several reports have shown that catecholamines such as isoproterenol and dopamine up-regulate Na+ channels and the Na,K-ATPase giving rise to increased alveolar fluid resorption. Although recombinant gene technology is not yet a therapeutic option for the treatment of pulmonary edema, several experimental studies have reported that overexpression of Na,K-ATPase genes causes increased fluid resorption during hyperoxic lung injury. There is significant evidence that fluid clearance is impaired in patients with lung injury. Therapeutic strategies aimed at increasing the ability of alveolar epithelium to resorb the edema should lead to benefits for patients with acute respiratory distress syndrome.

Early changes in alveolar fluid clearance by nitric oxide after endotoxin instillation in rats

Alveolar fluid clearance may be inhibited and/or stimulated under pathologic conditions. We examined the early change of alveolar fluid clearance after endotoxin instillation in adult rats. We employed electron paramagnetic resonance nitric oxide (NO) trapping technique with iron complex with N,N-diethyldithiocarbamate as an NO trapping agent. We found that lung NO signals reached the highest magnitude by 6 hours after endotoxin instillation. NO production was accompanied by increases in lung cyclic guanosine monophosphate levels. Alveolar fluid clearance decreased significantly 6 hours after the administration of the endotoxin and increased further at 24 hours. These changes were shown to be related to the function of amiloride-sensitive sodium ion channels. Treatment with gadolinium chloride and aminoguanidine significantly decreased lung NO and cyclic guanosine monophosphate levels and completely ameliorated the decrease in alveolar fluid clearance. In addition, the increase in alveolar fluid clearance at 24 hours returned to normal levels after treatment with gadolinium chloride and aminoguanidine. We found immunoreactive inducible nitric oxide synthase to be abundantly expressed in the cytoplasm of alveolar macrophages. Our results suggest that alveolar endotoxin inhibits alveolar fluid clearance at 6 hours by NO. NO is produced via inducible NO synthase in endotoxin-stimulated alveolar macrophages and was also shown to increase alveolar fluid clearance at 24 hours.

Prolonged isoproterenol infusion impairs the ability of beta(2)-agonists to increase alveolar liquid clearance

We determined if prolonged isoproterenol (Iso) infusion in rats impaired the ability of the beta(2)-adrenergic agonist terbutaline to increase alveolar liquid clearance (ALC). We infused rats with Iso (at rates of 4, 40, or 400 microg.kg(-1).h(-1)) or vehicle (0.001 N HCl) for 48 h using subcutaneously implanted miniosmotic pumps. After this time, the rats were anesthetized, and ALC was determined (by mass-balance after instillation of Ringer lactate containing albumin into the lungs) under baseline conditions and after terbutaline administration. Baseline and terbutaline-stimulated ALC in vehicle-infused rats averaged, respectively, 19.6 +/- 1.2% (SE) and 44.7 +/- 1.5%/h. The ability of terbutaline to increase ALC was eliminated at 400 microg.kg(-1).h(-1)Iso, inhibited by 26% at 40 microg.kg(-1).h(-1) Iso, and was not affected by 4 microg.kg(-1).h(-1) Iso. beta-adrenergic receptor (betaAR) density of freshly isolated alveolar epithelial type II (ATII) cells from Iso-infused rats was reduced by the 40 and 400 microg.kg(-1).h(-1) infusion rates. These data demonstrate that prolonged exposure to beta-agonists can impair the ability of beta(2)-agonists to stimulate ALC and produce ATII cell betaAR downregulation.

Beta1-adrenergic agonist is a potent stimulator of alveolar fluid clearance in hyperoxic rat lungs

Because it was still uncertain whether a stimulation of beta1-adrenoceptors accelerated alveolar fluid clearance in hyperoxic lung injury, the effect of denopamine, a selective beta1-adrenergic agonist, on alveolar fluid clearance was determined in rats exposed to 93% oxygen for 48 and 56 h. Alveolar fluid clearance was measured by the progressive increase in the concentration of Evans blue labeled albumin instilled into the alveolar spaces over 1 h at 37 degrees C in isolated rat lungs. The principle results were as follows: 1) Although lung water volume increased in rats exposed to hyperoxia for 48 and 56 h, basal alveolar fluid clearance did not change for up to 56 h; 2) Denopamine increased alveolar fluid clearance in rats exposed to hyperoxia as well as in rats without exposure to hyperoxia; 3) Denopamine primarily increased amiloride-insensitive alveolar fluid clearance in rats exposed to hyperoxia; 4) The potency of denopmaine was similar to that of terbutaline, a selective beta2-adrenergic agonist. In summary, denopamine is a potent stimulator of alveolar fluid clearance in rats exposed to hyperoxia.

Denopamine, a beta(1)-adrenergic agonist, increases alveolar fluid clearance in ex vivo rat and guinea pig lungs

The effect of denopamine, a selective beta(1)-adrenergic agonist, on alveolar fluid clearance was determined in both ex vivo rat and guinea pig lungs. Alveolar fluid clearance was measured by the progressive increase in the concentration of Evans blue-labeled albumin over 1 h at 37 degrees C. Denopamine (10(-6) to 10(-3) M) increased alveolar fluid clearance in a dose-dependent manner in ex vivo rat lungs. Denopamine also stimulated alveolar fluid clearance in guinea pig lungs. Atenolol, a selective beta(1)-adrenergic antagonist, and amiloride, a sodium channel inhibitor, inhibited denopamine-stimulated alveolar fluid clearance. The potency of denopamine was similar to that of similar doses of isoproterenol or terbutaline. Short-term hypoxia (100% nitrogen for 1-2 h) did not alter the stimulatory effect of denopamine. Denopamine (10(-4), 10(-3) M) increased intracellular adenosine 3',5'-cyclic monophosphate levels in cultured rat alveolar type II cells. In summary, denopamine, a selective beta(1)-adrenergic agonist, stimulates alveolar fluid clearance in both ex vivo rat and guinea pig lungs.

Alveolar epithelial barrier. Role in lung fluid balance in clinical lung injury

Several studies have established that transport of sodium from the air spaces to the lung interstitium is a primary mechanism driving alveolar fluid clearance, although further work is needed to determine the role of chloride in vectorial fluid transport across the alveolar epithelium. Although there are significant differences among species in the basal rates of sodium and fluid transport, the basic mechanism seems to depend on sodium uptake by channels on the apical membrane of alveolar type II cells, followed by extrusion of sodium on the basolateral surface by Na,K-ATPase. This process can be upregulated by several catecholamine-dependent and independent mechanisms. The identification of water channels expressed in lung, together with the high water permeabilities, suggest a potential role for channel-mediated water movement between the air space and capillary compartments, although definitive evidence will depend on the results of transgenic mouse knock-out studies. The application of this new knowledge regarding salt and water transport in alveolar epithelium in relation to pathologic conditions has been successful in clinically relevant experimental studies, as well as in a few clinical studies. The studies of exogenous and endogenous catecholamine regulation of alveolar fluid clearance are a good example of how new insights into the basic mechanisms of alveolar sodium and fluid transport can be translated to clinically relevant experimental studies. Exogenous catecholamines can increase the rate of alveolar fluid clearance in several species, including the human lung, and it is also apparent that release of endogenous catecholamines can upregulate alveolar fluid clearance in animals with septic or hypovolemic shock. It is possible that therapy with beta-adrenergic agonists might be useful to accelerate the resolution of alveolar edema in some patients. In some patients, the extent of injury to the alveolar epithelial barrier may be too severe for beta-adrenergic agonists to enhance the resolution of alveolar edema, although some experimental studies indicate that alveolar fluid clearance can be augmented in the presence of moderately severe lung injury. A longer-term upregulation of alveolar epithelial fluid transport might be achieved by strategies that accelerate the proliferation of alveolar type II cells repopulating the injured epithelium in clinical lung injury. More clinical research is needed to evaluate the strategies that can upregulate alveolar epithelial fluid transport with both short-term therapy (i.e., beta-agonists) and more sustained, longer-term effects of epithelial mitogens such as keratinocyte growth factor. These approaches may be useful in reducing mortality in the acute respiratory distress syndrome.

Effects of EP4 solution and LPD solution vs Euro-Collins solution on Na(+)/K(+)-ATPase activity in rat alveolar type II cells and human alveolar epithelial cell line A549 cells

BACKGROUND

Intact alveolar epithelial Na(+)/K(+)- adenosinetriphosphatase (ATPase) function is important in preventing alveolar fluid accumulation after lung transplantation. We examined whether the type of preservation solution used influences Na(+)/K(+)-ATPase activity in alveolar epithelial cells.

METHODS

Rat alveolar type II cells were preserved with EP4, low-potassium dextran (LPD), or Euro-Collins solution at 7 degrees C for 5 and 20 hours. To assess cell toxicity, we measured cell viability and lactate dehydrogenase release. Na(+)/K(+)-ATPase activity was measured as ouabain-sensitive ATPase hydrolysis. We also examined the effect of terbutaline (10(-3) mol/liter) and dibutyryl cyclic adenosine monophosphate (dbcAMP) (10(-3) mol/liter) on Na(+)/K(+)-ATPase activity in A549 cells preserved for 5 hours.

RESULTS

All solutions caused significant damage of rat alveolar type II cells at 20 hours. However, Na(+)/K(+)-ATPase activity was preserved at normal levels with EP4 and LPD over 20 hours. Terbutaline and dbcAMP significantly increased Na(+)/K(+)-ATPase activity in A549 cells preserved with EP4 and LPD solutions for 5 hours. However, we observed no activation in the cells preserved with Euro-Collins solution. We found no significant difference in intracellular cAMP levels after terbutaline challenge among the types of preservation solution.

CONCLUSIONS

We conclude that extracellular-type solutions such as EP4 and LPD may be preferable for maintaining not only the basal activity but also the ability to activate Na(+)/K(+)-ATPase in response to beta-adrenergic agonists, in alveolar epithelial cells.

Effect of epinephrine on alveolar liquid clearance in the rat

Endogenous epinephrine has been found to increase alveolar liquid clearance (ALC) in several pulmonary edema models. In this study, we infused epinephrine intravenously for 1 h in anesthetized rats to produce plasma epinephrine concentrations commonly observed in this species under stressful conditions and measured ALC by mass balance. Epinephrine increased ALC from 31.5 +/- 3.2 to 48.9 +/- 1.1 (SE)% of the instilled volume (P < 0.05). The increased ALC was prevented by either propranolol or amiloride. To determine whether ALC returns to normal after plasma epinephrine concentration normalizes, we measured ALC 2 h after stopping an initial 1-h epinephrine infusion and found ALC to be at baseline values. Finally, to determine whether desensitization of the liquid clearance response occurs, we evaluated the effects of both repeated 1-h infusions and a continuous 4-h infusion of epinephrine on ALC and found no reduction in ALC under either condition. We conclude that epinephrine increases ALC by stimulating beta-adrenoceptors and sodium transport, that the increase is reversible once plasma epinephrine concentration normalizes, and that desensitization of the ALC response does not appear to occur after 4 h of continuous epinephrine exposure.

Effects of intraalveolar oxygen concentration on alveolar fluid absorption and metabolism in isolated rat lungs

We evaluated the effects of intraalveolar oxygen concentration on alveolar fluid absorption and metabolism in isolated rat lungs. Alveolar fluid absorption was determined by measuring increase in albumin concentration in the instillate solution during 2 h of incubation. Oxidative phosphorylation was assessed by gas analysis of the solution. Glycolysis was assessed by determining glucose escape and lactate release in the solution. We found that alveolar fluid absorption did not change under hyperoxic and hypoxic experimental environments (range 100-10% oxygen). Glycolysis was reduced under hyperoxia and stimulated under hypoxia, however, lung ATP content did not change. When oxidative phosphorylation was inhibited by NaCN, both alveolar fluid absorption and lung ATP content were reduced. Our data indicate that isolated rat lungs maintain optimal energy production for alveolar fluid absorption by stimulating glycolysis, even though glycolysis alone is not enough. We conclude that alveolar fluid absorption determined in isolated rat lungs is not influenced by intraalveolar oxygen concentration in the range above 10% oxygen.

Transalveolar fluid absorption ability in rat lungs preserved with Euro-Collins solution and EP4 solution

BACKGROUND

Pulmonary reimplantation response, presenting lung edema, is a major obstacle of lung transplantation. Transalveolar fluid absorption mechanism, regulated by active transalveolar Na+ transport via Na+ channel and Na+-K+-ATPase, is considered to be essential for resolution of lung edema. We investigated the effect of lung preservation on this fluid transport mechanism.

METHODS

The rat lungs were flushed and preserved with either EP4 solution (EP4) or Euro-Collins solution (EC). First, we determined the basal transalveolar fluid movement by calculating alveolar fluid clearance (AFC) after pulmonary flushing, 24- and 72-hr preservation. Then, we assessed the effects of Na+ channel blocker, amiloride, and Na+-K+-ATPase inhibitor, ouabain, on AFC after 24-hr preservation. We further measured lung ATP content and Na+-K+-ATPase activity after 24-hr preservation to evaluate cellular metabolism and enzymatic activity during preservation.

RESULTS

We found that the lungs preserved with EC showed significantly lower AFC and less inhibitory effects of both blockers than with EP4 after 24-hr preservation. Na+-K+-ATPase activity was significantly lower with EC than with EP4, even though lung ATP content was not affected by preservation solution.

CONCLUSIONS

EP4 preservation provides a better environment for maintaining transalveolar fluid absorption mechanism than EC preservation. Therefore, lung preservation with EP4 may ensure more reliable ability in resolving pulmonary edema.

Dose-response relationship between plasma epinephrine concentration and alveolar liquid clearance in dogs

Previously, alveolar liquid clearance (ALC) was observed to increase in a canine model of neurogenic pulmonary edema (NPE) by adrenal epinephrine (S. M. Lane, K. C. Maender, N. E. Awender, and M. B. Maron. Am. J. Respir. Crit. Care Med. 158: 760-768, 1998). In this study the dose-response relationship between plasma epinephrine concentration and ALC was determined in anesthetized dogs by infusing epinephrine to produce plasma concentrations of 256 +/- 37, 1,387 +/- 51, 15,737 +/- 2,161, and 363,997 +/- 66,984 (SE) pg/ml (n = 6 for each concentration) for 4 h and measuring the resultant ALC. The latter was determined by mass balance after instillation of autologous plasma into a lower lung lobe. These plasma concentrations produced ALCs of 14.3 +/- 1.2, 20.5 +/- 1.9, 30.1 +/- 1.5, and 37.9 +/- 2.7% of the instilled volume, respectively. ALC after the lowest infusion rate was not different from that previously observed under baseline conditions (14.1 +/- 2.1%), whereas in a previous study of NPE, plasma epinephrine concentration increased to 7,683 +/- 687 pg/ml and ALC was 30.4 +/- 1.6%. These data indicate that, during recovery from canine NPE, ALC is not maximally stimulated and suggest that it might be possible to pharmacologically produce further increases in the rate of resolution of this form of edema.

The regulation of lung liquid absorption by endogenous cAMP in postnatal sheep lungs perfused in situ

1. The lungs of two groups of lambs aged 0-2 weeks and 6-12 weeks were artificially perfused in situ with warmed and oxygenated sheep blood. The airspaces of the lungs were filled with liquid containing an impermeant tracer to allow estimation of net liquid movement across the pulmonary epithelium at rest and after administration of certain drugs. 2. Dibutyryl cAMP (dB-cAMP, 10-4 M) stimulated the rate of lung liquid (LL) absorption in the lungs of four neonatal sheep aged 9-12 days, from -1.43 +/- 0.2 to -2.75 +/- 0.3 ml h-1 (kg body wt)-1 (P < 0.05, comparison of regression lines by Student's t test), but had no effect in four juvenile sheep aged 6-12 weeks (P > 0.10). 3. Theophylline, a non-selective phosphodiesterase (PDE) inhibitor (5 x 10-4 M), increased LL absorption from a resting rate of -1.55 +/- 0.3 to -3.62 +/- 0.5 ml h-1 kg-1 in the lungs of four sheep aged 1-12 days and from -1.47 +/- 0.3 to -3.73 +/- 0.4 ml h-1 kg-1 in four sheep aged 6-12 weeks (P < 0.05, Student's paired t test). 4. The beta-adrenergic antagonist sotalol (10-4 M) reduced LL absorption rate from -1.47 +/- 0.1 to -1.22 +/- 0.1 ml h-1 kg-1 (P < 0.01) in the lungs of four sheep aged 4-13 days, while theophylline given after sotalol had no effect. In four sheep aged 6-12 weeks, sotalol had no effect on LL absorption rate, whereas theophylline given after sotalol increased LL absorption rate from -1.06 +/- 0.1 to -1.92 +/- 0.2 ml h-1 kg-1 (P < 0.05). 5. The A1/A2 purinergic receptor blocker 7-(beta-chloroethyl) theophylline (CET; given at 5 x 10-6 M and 10-4 M) had no effect on LL absorption rate in the lungs of four sheep aged 6-12 weeks, confirming that theophylline produced its effect of increasing LL absorption by inhibiting PDE hydrolytic activity. 6. The selective PDE IV (cAMP-specific) PDE inhibitor rolipram was given in the perfused lungs of seven sheep aged 6-12 weeks at doses between 10-8 and 10-4 M, increasing LL absorption rate at concentrations of 10-6 M and above; the half-maximal effective concentration was estimated to be 5.9 x 10-7 M. 7. Rolipram (10-5 M) increased LL absorption rate from -1.99 +/- 0.2 to -3.18 +/- 0.5 ml h-1 kg-1 in the perfused lungs of four sheep aged 6-11 days, and from -1.21 +/- 0.4 to -3.45 +/- 0.3 ml h-1 kg-1 in the perfused lungs of four sheep aged 6-12 weeks (P < 0.05). Sotalol (10-4 M) reduced LL absorption rate from -3.39 +/- 0.8 to -2. 18 +/- 0.4 ml h-1 kg-1 (P < 0.05) in four sheep aged 10-14 days, while rolipram given after sotalol had no effect. In four sheep aged 6-12 weeks, sotalol had no effect on resting LL absorption rate, whereas rolipram given after sotalol increased absorption rate from -1.27 +/- 0.1 to -2.02 +/- 0.6 ml h-1 kg-1 (P < 0.05). 8. We conclude that cAMP mediates a component of LL absorption postnatally, and that while beta-adrenergic stimulation was the sole source of endogenous cAMP in neonates, this was not the case in juveniles, in whom cAMP originated, at least in part, from other sources.

Effects of ATP-sensitive potassium channel opener on potassium transport and alveolar fluid clearance in the resected human lung

Since the effect of an ATP-sensitive potassium channel (KATP channel) opener on the function of alveolar epithelial cells is unknown, the effect of YM934, a newly synthesized KATP channel opener, on potassium influx into the alveolar spaces and alveolar fluid clearance was determined in the resected human lung. An isosmolar albumin solution with a low potassium concentration was instilled into the distal airspaces of resected human lungs. Alveolar fluid clearance was measured by the progressive increase in alveolar protein concentration. Net potassium transport was measured by the change in potassium concentration and alveolar fluid volume. YM934 (10(-4) M) increased net influx of potassium by 140% into the alveolar spaces and also increased alveolar fluid clearance by 60% in the experiments with a potassium concentration of 0.3 mEq/1. Glibenclamide (10(-4) M), a KATP channel blocker, inhibited the YM934-increased influx of potassium transport and the increase in alveolar fluid clearance. Also amiloride (10(-5) M), an inhibitors of apical sodium uptake, blocked the YM934 stimulated increase in net alveolar fluid clearance. These results indicate that a KATP channel opener can effect potassium transport and net vectorial fluid movement across the human alveolar epithelium.

New in situ mouse model to quantify alveolar epithelial fluid clearance

Because the availability of transgenic mice makes it possible to examine the contribution of single genes to in vivo function, we developed a simple in situ mouse model that can be used to quantify isosmolar alveolar epithelial fluid clearance (AFC). Mice were killed, a tracheostomy was done, and then a test solution of a 5% isosmolar albumin solution with 0.1 micro Ci of 125I-labeled albumin was instilled via the trachea into the distal air spaces of both lungs. After instillation, the lungs were inflated to 7 cmH2O with 100% O2 and maintained at 37 degrees C by placing the animals under an infrared lamp. AFC was measured by the progressive increase in concentration of labeled and unlabeled protein over 1 h. The results indicated the following. 1) Basal, unstimulated AFC in mouse lungs was significantly faster than in ex vivo rat lungs (27 +/- 5% in in situ mice vs. 11 +/- 3% in ex vivo rat lungs; P < 0.05). 2) Comparison of equivalent doses (10(-4) M) of beta-adrenergic agonist (isoproterenol) and beta2-adrenergic agonists (terbutaline and salmeterol) indicated that stimulated clearance occurred only in presence of isoproterenol. 3) Because atenolol, a specific beta1-antagonist, abolished the effect of isoproterenol, the beta-adrenergic stimulation appears to be mediated by beta1-receptors. The rate of AFC in nonperfused mouse lungs was significantly faster than in prior studies of nonperfused lungs in rats and sheep. Interestingly, the stimulated clearance rate in mice was similar to the fast rates of AFC that we recently reported in patients recovering from hydrostatic pulmonary edema. This in situ model is a unique experimental preparation that can be readily used to quantify isosmolar epithelial fluid clearance in mice.

Perfusion technique determines alveolar fluid resorption rate in the isolated perfused rat lung

The isolated perfused lung (IPL) preparation is a well-established model for the study of alveolar epithelial sodium transport. We noted that preparations of normal fluid-filled rat lungs with recirculated perfusate reproducibly lost weight, whereas preparations in which the perfusate was discarded after a single pass through the lungs had a variable and lesser weight change. To confirm this, we performed IPL experiments by using male Sprague-Dawley specific-pathogen-free rats (175-225 g). In 10 IPLs, perfusate initially was discarded after passing through the lungs and then was recirculated continuously. During the single-pass period, the rate of weight change was +0.7 +/- 2.0 mg/min compared with -9.0 +/- 1.3 mg/min for the recirculating period. Adenosine 3',5'-cyclic monophosphate (cAMP) accumulated during recirculation. The weight loss induced by recirculation was reproduced by perfusion with 8-bromoadenosine 3',5'-cyclic monophosphate or terbutaline in single-pass fashion and blocked when the kinase inhibitor H-8 or phosphodiesterase was present in the recirculating perfusate. In summary, perfusate recirculation in the IPL stimulates fluid resorption at least partially via cAMP. This should be factored into the design and interpretation of IPL experiments.

Dopamine stimulates sodium transport and liquid clearance in rat lung epithelium

Pulmonary edema clearance is driven primarily by active sodium transport out of the alveoli, mediated predominantly by apical sodium channels and the basolateral NA,K-ATPase. We postulated that dopamine, analogous to its effects in other transporting epithelia, could regulate these sodium transport mechanisms and affect lung liquid clearance. We therefore studied the effects of dopamine on sodium transport and liquid clearance in isolated perfused rat lungs. Instillation of dopamine into the airways caused a dose-dependent increase in liquid clearance from isolated rat lungs of up to 33% above control values at 10(-8) to 10(-4) M concentrations. 10(-6) M amiloride, which selectively inhibits apical sodium channels, decreased basal liquid clearance by 34% but did not inhibit the dopamine-mediated stimulation of lung liquid clearance. Instillation of 10(-4) M amiloride into rat airways, which inhibits other sodium transport mechanisms non-selectively, decreased basal lung liquid clearance by 49% and inhibited the dopamine-mediated stimulation of lung liquid clearance. Perfusion of rat lungs with 5 x 10(-4) M ouabain to specifically inhibit Na,K-ATPase reduced both basal clearance (by 55%) and the dopamine-stimulated increase in lung fluid clearance. Conceivably, the stimulation of lung liquid clearance by dopamine is due to a modulation of Na,K-ATPase in the pulmonary epithelium.

Protein kinase A phosphorylation and G protein regulation of type II pneumocyte Na+ channels in lipid bilayers

Protein kinase A (PKA)- and G protein-mediated regulation of immunopurified adult rabbit alveolar epithelial type II (ATII) cell proteins that exhibit amiloride-sensitive Na+ channel activity was studied in planar lipid bilayers and freshly isolated ATII cells. Addition of the catalytic subunit of PKA + ATP increased single channel open probability from 0.42 +/- 0.05 to 0.82 +/- 0.07 in a voltage-independent manner, without affecting unitary conductance. This increase in open probability of the channels was mainly due to a decrease in the time spent by the channel in its closed state. The apparent inhibition constant for amiloride increased from 8.0 +/- 1.8 microM under control conditions to 15 +/- 3 microM after PKA-induced phosphorylation; that for ethylisopropylamiloride increased from 1.0 +/- 0.4 to 2.0 +/- 0.5 microM. Neither pertussis toxin (PTX) nor guanosine 5'-O-(3-thiotriphosphate) affected ATII Na+ channel activity in bilayers. Moreover, PTX failed to affect amiloride-inhibitable 22Na+ uptake in freshly isolated ATII cells. In vitro, ADP ribosylation induced by PTX revealed the presence of a specifically ribosylated band at 40-45 kDa in the total solubilized ATII cell protein fraction, but not in the immunopurified fraction. Moreover, the immunopurified channel was downregulated in response to guanosine 5'-O-(3-thiotriphosphate)-mediated activation of the exogenous G alpha(i-2), but not G(oA), G alpha(i-1), or G alpha(i-3), protein added to the channel. This effect occurred only in the presence of actin. These results suggest that amiloride-sensitive Na+ channels in adult alveolar epithelia regulated by PKA-mediated phosphorylation also retain the ability to be regulated by G alpha([i-2), but not G alpha([i-1) or G alpha(i-3), protein.

Alveolar liquid clearance in multiple nonperfused canine lung lobes

We evaluated the ability of canine isolated nonperfused lung lobes to absorb fluid from their air spaces by simultaneously measuring alveolar liquid clearance (ALC) in three lobes removed from the same dog. Autologous plasma was instilled in the air spaces of each lobe, and the increase in plasma protein concentration resulting from fluid reabsorption was used to calculate ALC. ALC after 4 h was 16.5 +/- 0.6% (SE) of the instilled fluid volume under baseline conditions and was 30.2 +/- 1.3% after terbutaline (10(-5) M) administration. These values were similar to those previously reported for intact dogs. Propranolol (10(-4) M) and ouabain (10(-3) M) reduced ALC in terbutaline-stimulated lobes to 20.4 +/- 0.8 and 3.9 +/- 1.4%, respectively. There was no significant difference in ALC among the three lobes under either baseline conditions or after terbutaline administration. These data indicate that the sodium and water transport mechanisms of the canine alveolar epithelium remain viable during 4 h of nonperfusion and that there are no intrinsic differences in the transport properties of individual lung lobes. The ability to study several lobes simultaneously without the need for perfusion will allow for the design of experiments in which multiple interventions can be studied by using lung lobes from the same animal.

Terbutaline stimulates alveolar fluid resorption in hyperoxic lung injury

Alveolar fluid resorption occurs by active epithelial sodium transport and is accelerated by terbutaline in healthy lungs. We investigated the effect of terbutaline on the rate of alveolar fluid resorption from rat lungs injured by hyperoxia. Rats exposed to > 95% O2 for 60 h, sufficient to increase wet-to-dry lung weight and cause alveolar edema, were compared with air-breathing control rats. After anesthesia, the animals breathed 100% O2 for 10 min through a tracheostomy. Ringer solution was instilled into the alveoli, and the steady-state rate of volume resorbed at 6 cmH2O pressure was measured via a pipette attached to the tracheostomy tubing. Ringer solution in some animals contained terbutaline (10(-3) M), ouabain (10(-3) M), or both. Normoxic animals resorbed 49 +/- 6 microliters.kg-1.min-1; ouabain reduced this by 39%, whereas terbutaline increased the rate by 75%. The effect of terbutaline was blocked by ouabain. Hyperoxic animals absorbed 78 +/- 9 microliters.kg-1.min-1; ouabain reduced this by 44%. Terbutaline increased the rate by a mean of 39 microliters.kg-1.min-1, similar to the absolute effect seen in the normoxic group, and this was blocked by ouabain. Terbutaline accelerates fluid resorption from both normal and injured rat lungs via its effects on active sodium transport.

Reconstitution of immunopurified alveolar type II cell Na+ channel protein into planar lipid bilayers

Low-amiloride-affinity (L-type) Na+ channels have been functionally and immunologically localized to alveolar type II (ATII) cells. Purified rabbit ATII epithelial cells were isolated by elastase digestion and solubilized with 3-[(3-cholamidopropyl)dimethyl-ammonio]-1-propanesulfonate. The solubilized proteins were purified by ion-exchange chromatography, followed by immunoaffinity purification over a column to which rabbit polyclonal antibodies raised against purified bovine renal Na+ channel protein were bound. The proteins eluted from the immunoaffinity column were assayed for specific binding of [3H]Br-benzamil and reconstituted into planar lipid bilayers. Sequential purification steps gave a final enrichment in specific [3H]Br-benzamil binding of > 2,000 compared with the homogenate. Single-channel currents of 25 pS were recorded from the immunopurified rabbit ATII cell protein. Addition of the catalytic subunit of protein kinase A (PKA) plus ATP to the presumed cytoplasmic side of the bilayer resulted in a significant increase in the single-channel open probability (Po), from 0.40 +/- 0.14 to 0.8 +/- 0.12, without altering single-channel conductance. The addition of amiloride or ethylisopropyl amiloride (EIPA) to the side opposite that in which PKA acts reduced Po with no change in single-channel conductance. Rabbit ATII Na+ channels in bilayers had an inhibitory constant for amiloride of 8 microM and 1 microM for EIPA. These data confirm the presence of L-type Na+ channels in adult mammalian ATII cells.

Effect of metabolic inhibitors on Na+ transport in isolated perfused rat lungs

Alveolar fluid absorption is a process driven by transepithelial alveolar Na+ transport. Since lungs produce significant amounts of lactate under anaerobic but also under aerobic conditions, glycolysis may conceivably contribute to producing the energy needed for transepithelial Na+ transport and fluid absorption. The effects of inhibition of oxidative phosphorylation or glycolysis on alveolar Na+ transport, fluid absorption, and preservation of alveolar epithelial barrier properties were examined using isolated, fluid-filled rat lungs. Basal lung lactate production was 65 +/- 1.0 mumol/h/g dry wt in the presence of 10 mmol/liter glucose. When oxidative phosphorylation was inhibited with rotenone, cyanide, or the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP), lung lactate production increased 5- to 7-fold within 30 min (P < 0.001). No significant decrease in alveolar Na+ transport was observed over 1 h, whereas a 3-fold increase in passive epithelial permeability was observed. With rotenone and CCCP, but not cyanide, fluid absorption from airspaces was decreased but never abolished. Inhibition of aerobic glycolysis with iodoacetate did not significantly affect alveolar Na+ transport or fluid absorption. In the presence of isoproterenol or dibutyryl cyclic adenosine monophosphate (cAMP) + isobutylmethylxanthine, which have previously been shown to stimulate alveolar Na+ transport, lung lactate production increased 2-fold (P < 0.001). Inhibition of glycolysis depressed stimulated alveolar Na+ and fluid transports (P < 0.001). Inhibition of ion transport by ouabain or amiloride decreased lung lactate production (P < 0.001) under stimulated but not under unstimulated conditions. These observations suggest that glycolysis does not significantly contribute to energy provision for alveolar epithelial Na+ transport in lungs under basal, aerobic conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Mechanisms and regulation of ion transport in adult mammalian alveolar type II pneumocytes

The adult alveolar epithelium consists of type I and type II (ATII) pneumocytes that form a tight barrier, which severely restricts the entry of lipid-insoluble molecules from the interstitial to the alveolar space. Current in vivo and in vitro evidence indicates that the alveolar epithelium is also an absorptive epithelium, capable of transporting Na+ from the alveolar lumen, which is bathed by a small amount of epithelial lining fluid, to the interstitial space. The in situ localization of Na(+)-K(+)-ATPase activity in ATII cells and the fact that these cells are involved in a number of crucial functions, such as surfactant secretion and alveolar remodeling after injury, led investigators to examine their transport characteristics. Radioactive flux studies, in both freshly isolated and cultured cells, and bioelectric measurements in ATII cells grown on porous supports indicate that they transport Na+ according to the Koefoed-Johnsen and Ussing model of epithelial transport. Na+ enters the apical membrane, because of the favorable electrochemical gradient, through Na+ cotransporters, a Na(+)-H+ antiport, and cation channels and is pumped across the basolateral membrane by a ouabain-sensitive Na(+)-K+ pump. Na+ transport is enhanced by substances that increase intracellular adenosine 3',5'-cyclic monophosphate. In addition to Na+ transporters, ATII cells contain several transporters that regulate their intracellular pH, including a H(+)-ATPase, which may explain the low pH of the epithelial lining fluid. The absorptive properties of ATII cells may play an important role in regulating the degree of alveolar fluid in health and disease.

Luminal glucose enhances transepithelial Na+ and fluid transports in rat lungs

The effects of luminal glucose on transepithelial Na+ and fluid transports were investigated in rat lungs. Two preparations were used: isolated, perfused lungs and lungs in situ perfused with blood (cross-circulations), a situation more comparable to that existing in vivo. Unidirectional (efflux from air spaces, J(out)) and net (Jnet) Na+ fluxes and fluid absorption from air spaces were estimated in lungs filled with bicarbonate-buffered solutions containing 10 mmol/l of either mannitol, glucose or alpha-methyl-D-glucopyranoside, or 0.1 mmol/l phlorizin in the presence of glucose. In the presence of mannitol J(out) was estimated to be 7.8 +/- 1.02 pmol cm-2 s-1 in isolated lungs and 9.2 +/- 0.97 pmol cm-2 s-1 in lungs in situ, and Jnet 1.0 +/- 0.33 and 2.5 +/- 0.35 pmol cm-2 s-1, respectively. When glucose replaced mannitol J(out) (+ 30% + 40%), Jnet (+200% + 300%) and fluid absorption (+ 100% + 400%) were enhanced in both preparations. Substituting methyl glucoside for mannitol increased Na+ and fluid absorption rates to the same extent as glucose. Phlorizin, in the presence of glucose, reduced Na+ and fluid transports to values similar to those observed in the presence of mannitol. These changes did not result from modifications of the paracellular permeabilities (assessed with [3H]mannitol).(ABSTRACT TRUNCATED AT 250 WORDS)