Georges Saumon

Significance of active ion transport in transalveolar water absorption: a study on isolated rat lung.

1. Experiments were performed on isolated rat lungs perfused with Ringer solutions containing red cells. The goal was to clarify the role of active transport of Na+ for the absorption of fluid across the alveolar membrane, and to characterize active and passive pathways. 2. Partially degassed lungs were filled with 5 ml of an isotonic Ringer solution containing 125I‐labelled albumin in order to calculate the fluid movement, and 22Na+ or 36Cl‐ for measurement of ion fluxes. Passive non‐electrolyte permeability was determined in all experiments using [3H]mannitol. 3. The average rate of fluid absorption in phosphate‐buffered instillates was 134 nl/s (S.E., 18.5; n = 14). With ouabain (10(‐4) M) in the perfusate the fluid absorption rate fell to 57 nl/s (S.E., 8.2; n = 18). Amiloride (10(‐3)‐10(‐4) M) in the instillate reduced the absorption to 75 nl/s (S.E., 8.6; n = 16). These results show that fluid absorption depends on transcellular transport of Na+ and that alveolar epithelial cells have a Na+ entry system in the luminal membrane and a Na+‐K+ pump in the abluminal membrane. 4. The transcellular ion transport operates in parallel with a paracellular, passive leak that allows mannitol to pass with a permeability surface area product of 1.2 X 10(‐4) ml/s, corresponding to a permeability coefficient of 2.4 X 10(‐8) cm/s, assuming an alveolar surface area of 5000 cm2. 5. The passive fluxes of Na+ were 9.4 pmol/(cm2s) (S.E., 1.3; n = 25) in the direction from alveoli to perfusate and 8.0 pmol/(cm2s) (S.E., 0.86; n = 6) from perfusate to plasma. The passive fluxes of Cl‐ in the two directions were not significantly different either. Thus the transalveolar electrical potential difference is too small to affect ion movements measurably. 6. The passive permeability to Na+ was 6.7 X 10(‐8) cm/s and to Cl‐ was 10.2 X 10(‐8) cm/s (alveolar surface area assumed to be 5000 cm2). The ratio of the permeabilities is close to the ratio of the diffusion coefficients in free solution, suggesting a neutral or weakly charged paracellular channel. 7. We conclude that the alveolar epithelium performs solute‐coupled fluid transport from alveoli to plasma, and that it shows many features that are common to other fluid‐transporting epithelia; with an approximate surface area of 100 m2 in humans it constitutes one of the largest epithelial surfaces in the body.(ABSTRACT TRUNCATED AT 400 WORDS)

Fluid absorption by rat lung in situ: pathways for sodium entry in the luminal membrane of alveolar epithelium.

1. The purpose of the investigation was to characterize the luminal membrane and the paracellular pathway of rat lung alveolar epithelium. Experiments were performed on lungs in situ instilled with isotonic, buffered Ringer solution and perfused with blood from a donor rat using cross‐circulation technique. 2. The rate of active Na+ transport was 4.4 pmol/(cm2s). The fluid absorption was 156 nl/s, and was unaffected by the presence of protein in the instillate (166 nl/s). In the absence of Na+, fluid absorption was zero. Amiloride (10(‐3) M) reduced fluid absorption by 60%. Amiloride, combined with absence of D‐glucose, arrested fluid absorption completely. Phloridzin at the luminal side reduced fluid absorption whilst phloretin had no effect. Amiloride together with phloridzin (10(‐3) M) also arrested absorption. Thus, there are two entry systems for Na+ in the luminal membrane: Na+ channels and a Na+‐D‐glucose symport. These results show that alveolar fluid absorption is due to cellular activity. 3. Substitution of Cl‐ with gluconate not only stopped fluid absorption, but led to slight reversal of net fluid movement. 4. Passive unidirectional flux of Na+, determined with 22Na+, was 9.9 pmol/(cm2s) and that of Cl‐, determined with 36Cl‐, was 12.4 pmol/(cm2s). These fluxes were based on an assumed alveolar surface area of 5000 cm2. Transference numbers calculated from these figures are close to those in free solution, suggesting a neutral or weakly charged intercellular junctional pathway. The D‐mannitol permeability in the paracellular pathway was 1.7 X 10(‐8) cm/s. 5. It is a consequence of the proposed mechanism for fluid absorption that it becomes inoperative if the normally high reflexion coefficients for Na+ and Cl‐ are lowered in pathological states. In such conditions pulmonary oedema may develop depending on the net balance of passive mechanical and colloid‐osmotic forces. 6. An explanation of the reversal of fluid transport at the time of birth is presented.

Ventilator-induced lung injury.

During mechanical ventilation, high end-inspiratory lung volume (whether it be because of large tidal volume (VT) and/or high levels of positive end-expiratory pressure) results in a permeability type pulmonary oedema, called ventilator-induced lung injury (VILI). Previous injury sensitises lung to mechanical ventilation. This experimental concept has recently received a resounding clinical illustration after a 22% reduction of mortality was observed in acute respiratory distress syndrome patients whose VT had been reduced. In addition, it has been suggested that repetitive opening and closing of distal units at low lung volume could induce lung injury but this notion has been challenged both conceptually and clinically after the negative results of the Acute Respiratory Distress Syndrome clinical Network Assessment of Low tidal Volume and Elevated end-expiratory volume to Obviate Lung Injury (ARDSNet ALVEOLI) study. Experimentally and clinically, involvement of inflammatory cytokines in VILI has not been unequivocally demonstrated. Cellular response to mechanical stretch has been increasingly investigated, both on the epithelial and the endothelial side. Lipid membrane trafficking has been thought to be a means by which cells respond to stress failure. Alterations in the respiratory system pressure/volume curve during ventilator-induced lung injury that include decrease in compliance and position of the upper inflection point are due to distal obstruction of airways that reduce aerated lung volume. Information from this curve could help avoid potentially harmful excessive tidal volume reduction.

cAMP and beta-adrenergic stimulation of rat alveolar epithelium. Effects on fluid absorption and paracellular permeability.

The absorption of fluid (bicarbonate-buffered Ringer with 10 mmol/l glucose) instilled into rat lungs is a Na+-coupled process that takes place through two apical transport systems: an amiloride-sensitive Na+ transport and a Na+-glucose co-transport. Fluid absorption in isolated, perfused rat lungs and the permeability to 3H-mannitol of alveolar epithelium were studied in control conditions and during stimulation of the alveolar epithelium by cAMP or isoproterenol. cAMP led to a threefold increase in the rate of fluid absorption and to an increase in the paracellular permeability. A similar response was found following beta-adrenergic stimulation obtained with isoproterenol in the perfusate. The increase in fluid transport was due to enhancement of the amiloride-sensitive component of Na+ transport. The Na+-glucose co-transport which accounts for about 60% of fluid absorption in control conditions was depressed, possibly as a consequence of a depolarization of the apical alveolar cell membrane. Fluid absorption was reduced by 40% by apical amiloride (10(-4) mol/l) in control lungs and to an even larger extent in isoproterenol-stimulated lungs; it was completely abolished by amiloride in cAMP stimulated lungs. Since the Na+-glucose co-transport was still operative, this suggests that a secretory process was triggered. This was confirmed in experiments in which both kinds of transport were inhibited with a combination of amiloride and glucose-free Ringer. In these conditions fluid balance was zero in unstimulated lungs whilst fluid entry into alveoli was observed in isoproterenol and cAMP stimulated lungs.

Mechanical ventilation and hemorrhagic shock-resuscitation interact to increase inflammatory cytokine release in rats.

Objective:To determine whether hemorrhagic shock and resuscitation (HSR) and high lung stress during mechanical ventilation interact to augment lung and systemic inflammatory responses and whether their sequence affects these responses. Design:Prospective, randomized, controlled animal study. Setting:Research laboratory. Subjects:Fifty-six male Wistar rats. Interventions:Controls were immediately killed after anesthesia. High lung stress was produced by mechanical ventilation with high tidal volume of 30 mL/kg and no positive end-expiratory pressure (HV) for 2 hrs. HSR consisted of lessening systemic arterial pressure to 30 mm Hg for 1 hr followed by reinjection of the withdrawn blood. Experimental groups consisted of HSR only and HSR preceded or followed by HV or conventional mechanical ventilation. Measurements and Main Results:Interleukin-1β, interleukin-6, and macrophage inhibitory protein 2 were determined in lung homogenate, bronchoalveolar lavage fluid, and plasma. HV ventilation alone did not increase plasma or lung cytokine content compared with controls. HSR significantly increased all mediators in lungs and plasma but not macrophage inhibitory protein 2 in plasma. Conventional ventilation, applied either before or after HSR, did not influence lung or systemic mediator release, whereas HV significantly increased mediator release when combined with HSR whatever the sequence of injuries. Lung mediator content was significantly higher in animals ventilated with HV before the HSR stress than in animals submitted to HSR and then ventilated with HV. Plasma macrophage inhibitory protein 2 concentrations followed the same pattern. Conclusions:This study shows that HSR and high lung tissue stress interact to increase lung and systemic release of inflammatory mediators in a way that depends on their sequence. Previous injury may sensitize lungs to inadequate mechanical ventilation, but inadequate mechanical ventilation may also sensitize lungs to postoperative complications.

Evidence-based medicine or fuzzy logic: what is best for ARDS management?

Treatment of acute respiratory distress syndrome (ARDS) is a challenge for most ICU physicians. Independently of the severity of the patients condition, the difficulty stems from our uncertainty on two important points: (a) Has ARDS prognosis improved over the years? (b) Have enough studies of sufficient quality been carried out to know what is the best (or the least deleterious) way of managing such patients? Laudable efforts have been made to clarify these questions. In this issue of Intensive Care Medicine, Rossaint and colleagues [I] present an overview of the available science on the symptomatic management of ARDS. The value of this review does not disspell some frustration about ARDS management. This is for two reasons. First, what are the respective roles of randomized controlled studies (the gold standard for proponents of evidence-based medicine) and of the rest of scientific litterature, i.e., clinical physiological studies, historical clinical series, clinical case series, and physiological studies (modeling and animal studies) in improving symptomatic treatment? Second, does considering ARDS as a syndrome and not a variety of diseases direct enough attention to causal treatment? Our opinion about these questions we express in terms of two questions and three propositions: • Questions

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.

Perflubron dosing affects ventilator-induced lung injury in rats with previous lung injury.

Objectives:Randomized controlled trials of partial liquid ventilation in acute respiratory distress syndrome have been negative. Reasons for this failure may reside in the use of too large doses of perfluorocarbon. The objective was to evaluate whether various doses of perflubron affect ventilation-induced injury in edematous lungs in different ways. Design:Prospective, controlled animal study. Setting:Research laboratory of a university. Subjects:Male Wistar rats weighing 300 ± 20 g. Interventions:Separate groups of rats were injected with α-naphtylthiourea to produce mild permeability pulmonary edema. They were then given 0, 7 (low), 13 (moderate), or 20 mL/kg (near functional residual capacity) perflubron doses and mechanically ventilated with a large (33 mL/kg) tidal volume for 15 mins. Measurements and Main Results:125I-albumin distribution space was used to assess lung microvascular permeability. Quasi-static respiratory system pressure-volume curves were analyzed. Administration of low and moderate perflubron doses significantly improved respiratory mechanics and reduced the ventilator-induced permeability alterations to the level observed in rats that were not ventilated. By contrast, a perflubron dose that was near functional residual capacity increased end-inspiratory plateau pressure and aggravated the permeability alterations due to high tidal volume ventilation. Conclusions:Near functional residual capacity but not low perflubron dose worsens ventilation-induced lung injury of preinjured lungs. This may provide some explanation for the negative results of the recent clinical trials, and it stresses the importance of the amount of perflubron used for partial liquid ventilation.

Real Time Lung Imaging for the Detection of Lung Injury and Alveolar Fluid Movement During Mechanical Ventilation

Experimental ventilator-induced lung injury (VILI) is characterized by alterations in alveolar epithelial and microvascular permeability that favors the systemic dissemination of lung borne cytokines or bacteria. Animal models of VILI have been shown relevant to patient care and outcome and help explaining why most patients with the acute respiratory distress syndrome do not die from respiratory failure but from multiple organ dysfunction. Recent experimental studies also showed that adverse ventilator patterns may propel airway secretions and bacteria to previously healthy lung regions. Noninvasive imaging techniques were used for years to study the net rate of protein flow across the pulmonary microvascular endothelium and the alveolar epithelium in vivo, during normal breathing and lung inflation. More recently, the two-way protein fluxes across the alveolo-capillary barrier and the intra-pulmonary dispersion of alveolar edema have been monitored during mechanical ventilation. These experiments have provided new insights on the mechanisms of experimental VILI that may be of clinical value. This review will describe the evolution of these techniques and their main physiological and pharmacological applications in the era of VILI.