Effects of blood flow on lung ACE kinetics: evidence for microvascular recruitment.

The parameter Amax/Km (product of reactant enzyme mass in perfused microvessels and the constant kcat/Km), calculated from in vivo assays of pulmonary endothelial ectoenzymes (e.g., angiotensin-converting enzyme, ACE), can provide estimates of the perfused pulmonary microvascular surface area (PMSA) in the absence of enzyme dysfunction. We examined the relationship between PMSA and pulmonary blood flow (Qb) in anesthetized rabbits placed on total heart bypass, using [3H]benzoyl-Phe-Ala-Pro (BPAP) as the ACE substrate. When Qb was increased from 250 to 1,100 ml/min, at zone 3 conditions, pulmonary arterial pressure increased, pulmonary vascular resistance (PVR) decreased, and Amax/Km increased linearly, reflecting increasing PMSA. When only the left lung was perfused, increasing Qb from 250 to 636 +/- 17 ml/min (the last value representing fully recruited and/or distended vascular bed), PVR decreased, while Amax/Km increased. When Qb was further increased to 791 +/- 44 ml/min, both PVR and Amax/Km remained unchanged, confirming the lack of additional changes in PMSA. We conclude that Amax/Km provides a sensitive indication of PMSA, because it 1) increases with increasing Qb and decreasing PVR, 2) reaches a maximum at Qb values that correspond to the minimal values in PVR, and 3) like PVR, did not change with further increases in Qb. Compared with predicted changes in PMSA produced by either microvascular recruitment alone or distension alone, our data indicate that recruitment is a larger contributor to the observed increase in PMSA.     

Use of norepinephrine uptake to measure lung capillary recruitment with exercise

We used the multiple indicator-dilution technique with norepinephrine, a vascular endothelium surface marker, to study the pulmonary vascular changes in awake exercising dogs. The vascular space tracers, labeled erythrocytes and albumin, and a water space tracer, 1,8-octanediol, were injected with the norepinephrine, and right atrium-aortic root dilution curves were obtained in nine dogs, at rest and at two increasing levels of exercise. Extravascular lung water multiple tracer dilutional estimates increased with flow and rapidly approached a maximal asymptotic value representing 75% of the postmortem lung weight. The ratio of the extravascular lung water measured in this way to that measured gravimetrically also increased, to reach an asymptotic proportion of close to 100%. The transit time-defined central vascular space increased linearly with flow; the ratio of lung tissue space to lung vascular space, therefore, decreased with increasing flow. The mean tracer upslope norepinephrine extractions at rest and at the two levels of exercise were 17 +/- 1.2, 14 +/- 0.8, and 15 +/- 0.8% (SE). With the use of the Crone approximation, we computed permeability-surface area products for norepinephrine; these increased linearly with flow. If permeability does not change, the increase in the permeability-surface area product with flow can be attributed to capillary recruitment. We conclude that when all lung tissue has become accessible to 1,8-octanediol delivered via the perfused vascular space, there is nevertheless further recruitment, with increase in flow, of vascular surface that can extract norepinephrine.     

Effects of alveolar pressure on lung angiotensin-converting enzyme function in vivo

Angiotensin-converting enzyme lines the luminal surface of pulmonary capillary endothelial cells. The metabolism of its synthetic substrate, 3H-benzoyl-L-phenylalanyl-L-alanyl-L-proline ([3H]BPAP) has been used as an indicator of pulmonary microvascular function. Because the flow-volume status of the pulmonary capillaries is dependent on intra-alveolar pressure, we have studied the effects of airway pressure on endothelial plasmalemmal angiotensin-converting enzyme function in rabbit lungs in vivo. Static inflation of the lungs to a pressure of 0 or 5 Torr did not change percent transpulmonary metabolism and Amax/Km ratio (defined as E X Kcat/Km and thus, under normal conditions, an indirect measure of perfused endothelial luminal surface area) compared with control measurements during conventional mechanical ventilation. When the inflation pressure was increased to 10 Torr, percent metabolism of [3H]BPAP remained unaltered but Amax/Km decreased to 60% of the control value. This decrease was in close relation to the decrease in pulmonary blood flow. Addition of 5 cmH2O positive end-expiratory pressure (PEEP) to the mechanical ventilation also decreased Amax/Km values and pulmonary blood flow but did not influence percent metabolism of [3H]BPAP. These results suggest that the detected alterations in apparent enzyme kinetics were more likely due to hemodynamic changes than to alterations in angiotensin-converting enzyme function. Thus high static alveolar pressures as well as PEEP probably reduced the fraction of perfused microvessels as reflected in changes in Amax/Km ratios. This information should prove useful in interpreting the response of pulmonary endothelial enzymes to injury.     

Effects of acid-base imbalance on pulmonary angiotensin-converting enzyme in vivo

The effects of acid-base balance disturbances on pulmonary endothelial angiotensin-converting enzyme (ACE) were studied in anesthetized mechanically ventilated rabbits. Enzyme function was estimated from [3H]benzoyl-Phe-Ala-Pro ([3H]BPAP) utilization under first-order reaction conditions during a single transpulmonary passage and expressed as 1) substrate metabolism (M), 2) Amax/Km (Amax being equal to the product of enzyme mass and the constant of product formation), and 3) (Amax/Km)/100 ml blood flow. When respiratory acidosis/alkalosis was produced by altering respiratory rate at constant airway pressure, substrate (BPAP) utilization varied proportionally to arterial pH and inversely proportionally to arterial PCO2 (PaCO2) (P less than 0.05). Percent BPAP metabolism (%M) ranged from 92 +/- 3 (respiratory alkalosis) to 85 +/- 3 (normal), 82 +/- 3 (respiratory acidosis), and 78 +/- 2% (severe respiratory acidosis). Amax/Km similarly decreased from 899 +/- 129 to 825 +/- 143, 601 +/- 74, and 450 +/- 34 ml/min, respectively, and (Amax/Km)/100 ml blood flow was reduced from 176 +/- 26 to 131 +/- 22, 111 +/- 12, and 97 +/- 5, respectively. However, when respiratory acidosis/alkalosis was produced by altering both respiratory rate and airway pressure, no changes were observed in either %M, Amax/Km or (Amax/Km)/100 ml blood flow. Similarly metabolic alkalosis or acidosis did not alter M, Amax/Km or (Amax/Km)/100 ml blood flow. These results indicate that pulmonary endothelial ACE function can be affected by acid-base disturbances, probably indirectly through changes in perfused microvascular surface area.     

Effect of flow and surface area on angiotensin-converting enzyme activity in rabbit lungs

Pulmonary angtiotensin-converting enzyme (ACE) is located on the luminal surface of pulmonary microvasculature. Multiple indicator-dilution techniques have been used to measure pulmonary ACE activity in vivo and in isolated lungs. These studies suggest that ACE activity is depressed in several forms of acute lung injury. Depression of ACE activity may reflect impaired substrate delivery to enzyme sites because of flow-related reduction of perfused surface area. To assess the role of altered microvascular flow and surface area in the measurement of ACE activity, we utilized similar techniques to estimate the apparent Km and Vmax of pulmonary ACE in isolated, Krebs-perfused rabbit lungs. Km is an estimate of the affinity of a synthetic ACE substrate, [3H]benzoyl-phenyl-alanyl-alanyl-proline ([3H]BPAP), for ACE and should not be influenced by the rate of substrate delivery to luminal enzyme sites. Conversely, Vmax is an index of the number of ACE sites and should be influenced by perfusion changes that alter the number of perfused sites (recruitment or derecruitment). When isolated lungs were subjected to physiological maneuvers designed to increase or decrease perfused surface area, apparent Vmax increased or decreased respectively. Apparent Km was not altered by these maneuvers. Km and Vmax were independent of changes in perfusion rate when surface area was held constant. Thus these parameters should be useful in evaluating perfusion changes in normal and injured lungs.     

Early pulmonary endothelial enzyme dysfunction after phorbol ester in conscious rabbits

We investigated changes in angiotensin converting-enzyme (ACE) activity before and at 5, 15, 60, and 240 min after 20 micrograms phorbol myristate acetate/kg body wt iv in conscious rabbits. ACE activity was estimated in vivo from the single-pass transpulmonary metabolism of the synthetic substrate [3H]benzoyl-Phe-Ala-Pro [( 3H]BPAP) under first-order reaction conditions. Within 5 min after PMA administration, all animals developed profound granulocytopenia (15% of control) and moderate thrombocytopenia (57% of control), both lasting for the duration of the experiment. Concomitantly, there was a significant decrease in the transpulmonary metabolism of [3H]BPAP and the calculated apparent first-order reaction constant Amax/Km of ACE for [3H]BPAP. No histological evidence of lung injury was observed at these times. Since a concomitant fall in the permeability surface area product for urea was also observed, we considered that the apparent decline in ACE activity might have resulted from a reduction in perfused endothelial surface area. To resolve this, we studied the effect of PMA on the Km (a measure of enzyme affinity for its substrate) and Amax (a derivative of Vmax that is dependent upon total enzyme present and thus capillary surface area) of ACE and 5'-nucleotidase for [3H]BPAP and [14C]AMP, respectively. A significant increase in Km for both enzymes was observed at 1 h after PMA, whereas Amax was unaffected, suggesting that low-dose PMA may indeed produce endothelial cell enzyme dysfunction independent of its effect on capillary surface area. These results provide evidence of pulmonary capillary functional injury before or in the absence of structural endothelial damage.     

Kinetics of pulmonary angiotensin-converting enzyme and 5'-nucleotidase in vivo

Angiotensin-converting enzyme and 5'-nucleotidase line the luminal surface of pulmonary microvascular endothelium and participate in the synthesis and/or degradation of potent vasoactive substances. We applied Michaelis-Menten kinetics in simultaneous estimations of apparent constants Km and Amax (product of Vmax and microvascular plasma volume) of these two enzymes for the substrates 3H-labeled benzoyl-Phe-Ala-Pro and 14C-labeled 5'-AMP, respectively, in vivo. Values of angiotensin-converting enzyme for benzoyl-Phe-Ala-Pro (Km = 10-11 microM; Amax = 12-13 mumol X min-1) were somewhat higher than published estimates in vitro and changed predictably in response to the known enzyme inhibitor captopril. Kinetic values of 5'-nucleotidase for 5'-AMP (Km = 3-4 microM; Amax = 3-4 mumol/min) were substantially lower than those reported in vitro but also responded predictably to the competitive inhibitor of 5'-nucleotidase, adenosine 5'-[alpha, beta-methylene]diphosphate. These data offer in vivo estimates of enzyme kinetics that are useful in revealing enzyme behavior in their normal physiological environment and provide means of evaluating the action of pharmacological, physiological, and pathological modulators of enzyme activity, in vivo.     

Comparison of labeled propanediol and urea as markers of lung vascular injury

The purpose of these studies was a comparison of [14C]urea (U) and 1,3-[14C]propanediol (Pr) as measures of lung vascular permeability-surface area (PS) under base-line conditions and after lung injury caused by alloxan infusion in isolated perfused dog lungs. Indicator mixtures of 125I-albumin, 51Cr-red blood cells, 3HOH, and U or Pr were injected under base-line conditions, after 1.2 g of alloxan, and after an additional 0.8 g of alloxan. Indicator-dilution curves were analyzed from sampled outflow blood to provide PS, the square root of effective extravascular diffusivity multiplied by exchange surface area (D1/2S), and extravascular lung water (EVLW) from the tracer mean transit times (VW). Results show that alloxan increases PS and D1/2S for U, D1/2S for Pr, and VW and EVLW by desiccation. All indicator-dilution parameters correlate significantly with alloxan dose. Interpretation of Pr transport suggests that materials with lipid and hydrophilic pathways might be used in conjunction with U to minimize the effects of surface area changes and increase the sensitivity of these tracers to permeability alteration. In addition Pr may be a useful alternative to U as a marker of vascular damage.     

Pulmonary angiotensin-converting enzyme kinetics after acute lung injury in the rabbit

Depression of lung endothelial cell metabolic function may be an early and sensitive indicator of lung damage. When such functions are measured in vivo, substrates injected usually must be limited to "trace" doses due to the significant hemodynamic effects of high doses of substrate. Under first-order conditions (i.e., trace doses) the enzyme or transport system rate constant Vmax/Km may be calculated, but independent estimates of each variable (Vmax and Km) are not available. We therefore used multiple indicator-dilution methods and higher substrate concentrations to apply a mathematical model, based on saturable kinetics that yield independent estimates of the apparent kinetic parameters Vmax and Km for pulmonary angiotensin-converting enzyme (ACE). We used the ACE substrate, [3H]benzoyl-phenylalanyl-alanyl-proline ([3H]BPAP) and made these measurements and also estimates of serotonin [5-hydroxytryptamine (5-HT)] removal, before and after acute lung injury induced by intratracheal administration of phorbol myristate acetate (PMA). PMA significantly depressed the percent 5-HT removal (62 +/- 3 to 44 +/- 4%) and BPAP percent metabolism (74 +/- 2 to 66 +/- 2), when trace amounts of either compound were injected as a bolus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of hypoxia on the longitudinal distribution of pulmonary vascular resistance

We have examined the influence of hypoxia on the longitudinal distribution of vascular resistance and intravascular pressure in isolated cat lungs using the low-viscosity bolus technique. Hypoxia increased total vascular resistance, decreased total lung blood volume, and moved the maximum local resistance downstream away from the main pulmonary artery. The circumference of the main pulmonary artery was increased and the extravascular lung water (double indicator dilution technique) was decreased by hypoxia. Thus, it would appear that distension of the large pulmonary arteries and a decrease in the amount of lung tissue perfused contributed to the change in resistance distribution brought about by hypoxia.     

Pulmonary blood volume and edema in postpneumonectomy lung growth in rats

After pneumonectomy in young animals, the contralateral lung undergoes compensatory growth and generally attains the same weight and air space volume as both lungs in age-matched controls. In this study, we determined the contribution of lung edema and increased blood volume to the weight gain in rats. Three weeks after pneumonectomy (n = 18) or sham pneumonectomy (n = 17), the pulmonary blood volume and the extravascular water and albumin were evaluated by use of 51Cr-labeled erythrocytes and 125I-labeled albumin. The air space volume, blood-free lung weights, and DNA and protein content were also compared. The data show that the total pulmonary blood volumes and the blood volume per gram of blood-free dry lung were similar in pneumonectomized and age-matched sham controls. The total extravascular albumin and the extravascular albumin per gram of blood-free dry lung were also similar as well as the extravascular lung water, wet-to-dry weight ratios, DNA and protein content, and air space volumes. These data indicate that the increased weight of the postpneumonectomy lung was due to cellular and stromal proliferation. The blood volume and interstitial fluid increased in proportion to the increase in lung parenchyma. Neither vascular congestion nor increased extravascular protein and water contributed to the observed weight gain.     

Effect of isoproterenol or exercise on pulmonary lymph flow and hemodynamics

Experiments were performed to determine whether different methods of increasing cardiac output would have similar effects on lung lymph flow, and to assess the contribution of the microvasculature (fluid-exchanging vessels) to the total calculated pulmonary vascular resistance. Yearling unanesthetized sheep with chronic vascular catheters and lung lymph fistulas underwent intravenous infusions of isoproterenol at 0.2 micrograms X kg-1. min-1 (n = 8) or were exercised on a treadmill (n = 16). Both isoproterenol and exercise increased cardiac output, lowered calculated total pulmonary and systemic vascular resistances, and had no effect on the calculated pulmonary microvascular pressure. Isoproterenol infusions did not affect lung lymph flow, whereas exercise increased lung lymph flow in proportion to the increase in cardiac output. We conclude that 1) the sheep has a different pulmonary hemodynamic response to exercise than dogs and man, 2) the microvasculature is recruited during exercise-induced but not isoproterenol-induced increases in cardiac output, and 3) the microvasculature represents only a small proportion of the total calculated pulmonary vascular resistance.     

Use of diazepam for interpreting changes in extravascular lung water.

Estimates of extravascular lung water volume (Qew) by use of the multiple indicator-dilution method with a hydrophilic indicator such as tritiated water, along with a vascular reference indicator, depend not only on tissue hydration but also on tissue perfusion. Separation of these effects might be facilitated if both hydrophilic and lipophilic indicators were used, with the assumption that the extravascular volume accessible to the lipophilic indicator would be independent of hydration. We found that in isolated perfused dog lung lobes the extravascular volume accessible to the lipophilic amine [14C]diazepam (Qed) was inversely proportional to the albumin concentration of the perfusate. This suggested that while the bolus was in the lungs, only a small fraction of the diazepam was in the aqueous phase of either lung tissue or perfusate. Changing the flow rate over a fairly wide range had little influence on the pattern of the tritiated water or [14C]diazepam effluent concentration curves when time was normalized to the lobar mean transit time. This suggests that the association of the diazepam with both the plasma albumin and the lipoid fraction of the tissue was in very rapid equilibrium on the time scale of a single pass through the lung lobe and that there was little barrier to its diffusion to and from the tissue. When the extravascular water volume was increased by either raising the hydrostatic pressure or instilling saline into the airways, both Qew and Qew/Qed increased.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thromboxane A2 and prostacyclin do not modulate pulmonary hemodynamics during exercise in sheep

The purpose of this study was to determine the role of thromboxane and prostacyclin in modulating pulmonary hemodynamics during maximal cardiopulmonary stress in the healthy lung. We studied 11 yearling sheep in paired studies during progressive maximal treadmill exercise with and without meclofenamate (n = 5), ibuprofen (n = 6), or UK38485 (n = 2). We also studied five sheep during hypoxia and hypoxic exercise, and six sheep during prolonged steady-state treadmill exercise for 45-60 min with and without drug treatment. We measured the metabolites of thromboxane A2 (thromboxane B2, TxB2) and prostacyclin (6-ketoprostaglandin F1 alpha, 6-keto-PGF1 alpha) in blood plasma and lung lymph in each protocol. We found that progressive exercise significantly reduced pulmonary vascular resistance but that cyclooxygenase or thromboxane synthesis blockade did not alter the change. Plasma TxB2 rose minimally but significantly during maximal exercise, but 6-keto-PGF1 alpha did not change. During continuous hypoxia, exercise reduced pulmonary vascular resistance nearly to base-line levels, but the degree of reduction was also unchanged by drug treatment. There were also no significant changes in lymph or plasma TxB2 or 6-keto-PGF1 alpha during 45-60 min of continuous moderate exercise. We conclude that neither TxB2 nor prostacyclin modulate pulmonary hemodynamics in the normal lung during maximal exercise, prolonged moderate exercise, or exercise-induced reductions in vascular resistance during hypoxia.     

Protective effect of endogenous beta-adrenergic tone on lung fluid balance in acute bacterial pneumonia in mice

Some investigators have reported that endogenous beta-adrenoceptor tone can provide protection against acute lung injury. Therefore, we tested the effects of beta-adrenoceptor inhibition in mice with acute Escherichia coli pneumonia. Mice were pretreated with propranolol or saline and then intratracheally instilled with live E. coli (10(7) colony-forming units). Hemodynamics, arterial blood gases, plasma catecholamines, extravascular lung water, lung permeability to protein, bacterial counts, and alveolar fluid clearance were measured. Acute E. coli pneumonia was established after 4 h with histological evidence of acute pulmonary inflammation, arterial hypoxemia, a threefold increase in lung vascular permeability, and a 30% increase in extravascular lung water as an increase in plasma catecholamine levels. beta-Adrenoceptor inhibition resulted in a marked increase in extravascular lung water that was explained by both an increase in lung vascular permeability and a reduction in net alveolar fluid clearance. The increase in extravascular lung water with propranolol pretreatment was not explained by an increase in systemic or vascular pressures. The increase in lung vascular permeability was explained in part by anti-inflammatory effects of beta-adrenoceptor stimulation because plasma macrophage inflammatory protein-2 levels were higher in the propranolol pretreatment group compared with controls. The decrease in alveolar fluid clearance with propranolol was explained by a decrease in catecholamine-stimulated fluid clearance. Together, these results indicate that endogenous beta-adrenoceptor tone has a protective effect in limiting accumulation of extravascular lung water in acute severe E. coli pneumonia in mice by two mechanisms: 1) reducing lung vascular injury and 2) upregulating the resolution of alveolar edema.     

Extravascular water content of heart and lungs after acute myocardial ischemia

We measured the extravascular water content of hearts and lungs of anesthetized dogs subjected to one of the following protocols: a)sham operation, b) circumflex artery ligation, c) increased left atrial pressure (Pla), or d) increased Pla and circumflex artery ligation. After 4 h, extravascular water of the heart and lungs increased significantly in the three experimental groups when compared with values from sham-operated dogs. After circumflex artery ligation, extravascular heart water increased 29% and lung water 8%, although Pla and calculated pulmonary microvascular pressure (Pmv) did not change. Extravascular heart water also rose 30% after increasing Pla from 23 to 37 cm H2O by inflating a left atrial baloon. In these dogs, extravascular lung water increased as a hyperbolic function of Pmv. Increasing Pla to 20 cm H2O in dogs with coronary artery ligation resulted in a 16% increase in heart water. Also at each Pmv, extravascular lung water was greater in dogs with coronary artery ligation than in dogs without. These data indicate that the increased extravascular lung water after coronary artery ligation cannot be explained solely by hemodynamic mechansims. We suggest that acute myocardial ischemia contributes to an increase in vascular permeability in the heart and lungs.     

The response of the pulmonary circulation to exercise during normoxia and hypoxia following pneumonectomy in the adult sheep

Pneumonectomy approximately halves the available pulmonary vascular bed. It is unknown whether the remaining lung has sufficient vascular reserve to cope with increased blood flow under stressful conditions without demonstrating abnormal pulmonary hemodynamics. To investigate this question, unanesthetized ewes with vascular catheters had hemodynamics assessed before and after a left pneumonectomy. Subsequently, on different days, the sheep were exercised on a treadmill under normoxic and hypobaric hypoxic (430 mmHg) (1 mmHg = 133.3 Pa) conditions. Pneumonectomy itself increased mean pulmonary arterial pressure by 4 mmHg. During normoxic or hypoxic exercise, the pneumonectomized sheep demonstrated a pulmonary hemodynamic response similar to normal sheep with two lungs. The pressure-flow relation for the right lung suggested the vascular reserve of the lung was not exceeded during exercise in the pneumonectomized sheep. Eighteen to 70 days after pneumonectomy there was no evidence of right ventricular hypertrophy, but there were small increases in the number of muscularized vessels less than 50 microns diameter and in the amount of muscle in normally muscularized pulmonary arteries. This study demonstrates that pneumonectomy slightly increases mean pulmonary arterial pressure. However, there is sufficient vascular reserve in the remaining lung to permit a normal hemodynamic response to exercise-induced increased blood flow even under hypoxic conditions.     

Short-term hypoxic exposure at rest and during exercise reduces lung water in healthy humans.

Hypoxia and hypoxic exercise increase pulmonary arterial pressure, cause pulmonary capillary recruitment, and may influence the ability of the lungs to regulate fluid. To examine the influence of hypoxia, alone and combined with exercise, on lung fluid balance, we studied 25 healthy subjects after 17-h exposure to 12.5% inspired oxygen (barometric pressure = 732 mmHg) and sequentially after exercise to exhaustion on a cycle ergometer with 12.5% inspired oxygen. We also studied subjects after a rapid saline infusion (30 ml/kg over 15 min) to demonstrate the sensitivity of our techniques to detect changes in lung water. Pulmonary capillary blood volume (Vc) and alveolar-capillary conductance (D(M)) were determined by measuring the diffusing capacity of the lungs for carbon monoxide and nitric oxide. Lung tissue volume and density were assessed using computed tomography. Lung water was estimated by subtracting measures of Vc from computed tomography lung tissue volume. Pulmonary function [forced vital capacity (FVC), forced expiratory volume after 1 s (FEV(1)), and forced expiratory flow at 50% of vital capacity (FEF(50))] was also assessed. Saline infusion caused an increase in Vc (42%), tissue volume (9%), and lung water (11%), and a decrease in D(M) (11%) and pulmonary function (FVC = -12 +/- 9%, FEV(1) = -17 +/- 10%, FEF(50) = -20 +/- 13%). Hypoxia and hypoxic exercise resulted in increases in Vc (43 +/- 19 and 51 +/- 16%), D(M) (7 +/- 4 and 19 +/- 6%), and pulmonary function (FVC = 9 +/- 6 and 4 +/- 3%, FEV(1) = 5 +/- 2 and 4 +/- 3%, FEF(50) = 4 +/- 2 and 12 +/- 5%) and decreases in lung density and lung water (-84 +/- 24 and -103 +/- 20 ml vs. baseline). These data suggest that 17 h of hypoxic exposure at rest or with exercise resulted in a decrease in lung water in healthy humans.     

Edema from cyclooxygenase products of endogenous arachidonic acid in isolated lung

We infused A23187, a calcium ionophore, into the pulmonary circulation of dextran-salt-perfused isolated rabbit lungs to release endogenous arachidonic acid. This led to elevations in pulmonary arterial pressure and to pulmonary edema as measured by extravascular wet-to-dry weight ratios. The increase in pressure and edema was prevented by indomethacin, a cyclooxygenase enzyme inhibitor, and by 1-benzylimidazole, a selective inhibitor of thromboxane (Tx) A2 synthesis. Transvascular flux of 125I-albumin from vascular to extravascular spaces of the lung was not elevated by A23187 but was elevated by infusion of oleic acid, an agent known to produce permeability pulmonary edema. We confirmed that A23187 leads to elevations in cyclooxygenase products and that indomethacin and 1-benzylimidazole inhibit synthesis of all cyclooxygenase products and TxA2, respectively, by measuring perfusate levels of prostaglandin (PG) I2 as 6-ketoprostaglandin F1 alpha, PGE2, and PGF2 alpha and TxA2 as TxB2. We conclude that release of endogenous pulmonary arachidonic acid can lead to pulmonary edema from conversion of such arachidonic acid to cyclooxygenase products, most notably TxA2. This edema was most likely from a net hydrostatic accumulation of extravascular lung water with an unchanged permeability of the vascular space, since an index of permeability-surface area product (i.e., transvascular albumin flux) was not increased.     

Effects of gas composition and pH on kinetics of lung angiotensin-converting enzyme

Given the pH dependence of enzymes in general and the potential importance of a blood and alveolar gas composition dependency on the interpretation of changes in the hydrolysis of angiotensin-converting enzyme (ACE) substrates by pulmonary endothelial ACE, we examined the influence of Pco2 and Po2 on the hydrolysis of a synthetic ACE substrate (benzoyl-phenylalanyl-alanyl-proline, BPAP) on passage through isolated rabbit lungs. Perfusate pH values of about 7.1, 7.4, and 7.9 were obtained by ventilating the lungs with gas containing different CO2 concentrations and Po2 values of approximately 110 and approximately 10 Torr were obtained by varying the concentration of O2 in the ventilating gas mixture. In the range studied neither acidosis nor alkalosis produced any significant changes in BPAP hydrolysis or in the kinetic parameters, Vmax and Km, for the hydrolysis process. On the other hand, a reduction in BPAP hydrolysis was detected when the Po2 was reduced from 110 to 10 Torr. The Vmax for BPAP hydrolysis by the lung was inversely correlated with the magnitude of the hypoxic vasoconstriction that occurred, suggesting that the reduced BPAP hydrolysis with hypoxia was due to the loss of perfused surface area due to the vasoconstriction. The results suggest that correlations between Pco2 and/or pH and whole-lung ACE activity that might occur in diseased lungs do not imply causalty. The hemodynamic consequences of changing Po2 (i.e., hypoxic vasoconstriction) may alter whole-organ ACE activity in the sense of changing the perfused surface area (i.e., the amount of ACE in contact with flowing perfusate).