Erythropoietin inhalation enhances adult canine alveolar-capillary formation following pneumonectomy.

Paracrine erythropoietin (EPO) signaling in the lung recruits endothelial progenitor cells, promotes cell maturation and angiogenesis, and is upregulated during canine postpneumonectomy (PNX) compensatory lung growth. To determine whether inhalational delivery of exogenous EPO augments endogenous post-PNX lung growth, adult canines underwent right PNX and received, via a permanent tracheal stoma, weekly nebulization of recombinant human EPO-containing nanoparticles or empty nanoparticles (control) for 16 wk. Lung function was assessed under anesthesia pre- and post-PNX. The remaining lobes were fixed for detailed morphometric analysis. Compared with control treatment, EPO delivery significantly increased serum EPO concentration without altering systemic hematocrit or hemoglobin concentration and abrogated post-PNX lipid oxidative stress damage. EPO delivery modestly increased post-PNX volume densities of the alveolar septum per unit of lung volume and type II epithelium and endothelium per unit of septal tissue volume in selected lobes. EPO delivery also augmented the post-PNX increase in alveolar double-capillary profiles, a marker of intussusceptive capillary formation, in all remaining lobes. EPO treatment did not significantly alter absolute resting lung volumes, lung and membrane diffusing capacities, alveolar-capillary blood volume, pulmonary blood flow, lung compliance, or extravascular alveolar tissue volumes or surface areas. Results established the feasibility of chronic inhalational delivery of growth-modifying biologics in a large animal model. Exogenous EPO selectively enhanced cytoprotection and alveolar angiogenesis in remaining lobes but not whole-lung extravascular tissue growth or resting function; the nonuniform response contributes to structure-function discrepancy, a major challenge for interventions aimed at amplifying the innate potential for compensatory lung growth.

In vivo imaging of canine lung deformation: effects of posture, pneumonectomy, and inhaled erythropoietin.

Mechanical stresses on the lung impose the major stimuli for developmental and compensatory lung growth and remodeling. We used computed tomography (CT) to noninvasively characterize the factors influencing lobar mechanical deformation in relation to posture, pneumonectomy (PNX), and exogenous proangiogenic factor supplementation. Post-PNX adult canines received weekly inhalations of nebulized nanoparticles loaded with recombinant human erythropoietin (EPO) or control (empty nanoparticles) for 16 wk. Supine and prone CT were performed at two transpulmonary pressures pre- and post-PNX following treatment. Lobar air and tissue volumes, fractional tissue volume (FTV), specific compliance (Cs), mechanical strains, and shear distortion were quantified. From supine to prone, lobar volume and Cs increased while strain and shear magnitudes generally decreased. From pre- to post-PNX, air volume increased less and FTV and Cs increased more in the left caudal (LCa) than in other lobes. FTV increased most in the dependent subpleural regions, and the portion of LCa lobe that expanded laterally wrapping around the mediastinum. Supine deformation was nonuniform pre- and post-PNX; strains and shear were most pronounced in LCa lobe and declined when prone. Despite nonuniform regional expansion and deformation, post-PNX lobar mechanics were well preserved compared with pre-PNX because of robust lung growth and remodeling establishing a new mechanical equilibrium. EPO treatment eliminated posture-dependent changes in FTV, accentuated the post-PNX increase in FTV, and reduced FTV heterogeneity without altering absolute air or tissue volumes, consistent with improved microvascular blood volume distribution and modestly enhanced post-PNX alveolar microvascular reserves.NEW & NOTEWORTHY Mechanical stresses on the lung impose the major stimuli for lung growth. We used computed tomography to image deformation of the lung in relation to posture, loss of lung units, and inhalational delivery of the growth promoter erythropoietin. Following loss of one lung in adult large animals, the remaining lung expanded and grew while retaining near-normal mechanical properties. Inhalation of erythropoietin promoted more uniform distribution of blood volume within the remaining lung.

Inhalational delivery of induced pluripotent stem cell secretome improves postpneumonectomy lung structure and function.

Cell-free secretory products (secretome) of human induced pluripotent stem cells (iPSCs) have been shown to attenuate tissue injury and facilitate repair and recovery. To examine whether iPSC secretome facilitates mechanically induced compensatory responses following unilateral pneumonectomy (PNX), litter-matched young adult female hounds underwent right PNX (removing 55%-58% of lung units), followed by inhalational delivery of either the nebulized-conditioned media containing induced pluripotent stem cell secretome (iPSC CM) or control cell-free media (CFM); inhalation was repeated every 5 days for 10 treatments. Lung function was measured under anesthesia pre-PNX and 10 days after the last treatment (8 wk post-PNX); detailed quantitative analysis of lung ultrastructure was performed postmortem. Pre-PNX lung function was similar between groups. Compared with CFM control, treatment with iPSC CM attenuated the post-PNX decline in lung diffusing capacity for carbon monoxide and membrane diffusing capacity, accompanied by a 24% larger postmortem lobar volume and distal air spaces. Alveolar double-capillary profiles were 39% more prevalent consistent with enhanced intussusceptive angiogenesis. Frequency distribution of the harmonic mean thickness of alveolar blood-gas barrier shifted toward the lowest values, whereas alveolar septal tissue volume and arithmetic septal thickness were similar, indicating septal remodeling and reduced diffusive resistance of the blood-gas barrier. Thus, repetitive inhalational delivery of iPSC secretome enhanced post-PNX alveolar angiogenesis and septal remodeling that are associated with improved gas exchange compensation. Results highlight the plasticity of the remaining lung units following major loss of lung mass that are responsive to broad-based modulation provided by the iPSC secretome.NEW & NOTEWORTHY To examine whether the secreted products of human induced pluripotent stem cells (iPSCs) facilitate innate adaptive responses following loss of lung tissue, adult dogs underwent surgical removal of one lung, then received repeated administration of iPSC secretory products via inhalational delivery compared with control treatment. Inhalation of iPSC secretory products enhanced capillary formation and beneficial structural remodeling in the remaining lung, leading to improved lung function.

Assessing recruitment of lung diffusing capacity in exercising guinea pigs with a rebreathing technique.

Noninvasive techniques for assessing cardiopulmonary function in small animals are limited. We previously developed a rebreathing technique for measuring lung volume, pulmonary blood flow, diffusing capacity for carbon monoxide (Dl(CO)) and its components, membrane diffusing capacity (Dm(CO)) and pulmonary capillary blood volume (Vc), and septal volume, in conscious nonsedated guinea pigs at rest. Now we have extended this technique to study guinea pigs during voluntary treadmill exercise with a sealed respiratory mask attached to a body vest and a test gas mixture containing 0.5% SF(6) or Ne, 0.3% CO, and 0.8% C(2)H(2) in 40% or 98% O(2). From rest to exercise, O(2) uptake increased from 12.7 to 25.5 ml x min(-1) x kg(-1) while pulmonary blood flow increased from 123 to 239 ml/kg. The measured Dl(CO), Dm(CO), and Vc increased linearly with respect to pulmonary blood flow as expected from alveolar microvascular recruitment; body mass-specific relationships were consistent with those in healthy human subjects and dogs studied with a similar technique. The results show that 1) cardiopulmonary interactions from rest to exercise can be measured noninvasively in guinea pigs, 2) guinea pigs exhibit patterns of exercise response and alveolar microvascular recruitment similar to those of larger species, and 3) the rebreathing technique is widely applicable to human ( approximately 70 kg), dog (20-30 kg), and guinea pig (1-1.5 kg). In theory, this technique can be extended to even smaller animals provided that species-specific technical hurdles can be overcome.

Shifting sources of functional limitation following extensive (70%) lung resection.

We previously found that, following surgical resection of approximately 58% of lung units by right pneumonectomy (PNX) in adult canines, oxygen-diffusing capacity (Dl(O(2))) fell sufficiently to become a major factor limiting exercise capacity, although the decline was mitigated by recruitment, remodeling, and growth of the remaining lung units. To determine whether an upper limit of compensation is reached following the loss of even more lung units, we measured pulmonary gas exchange, hemodynamics, and ventilatory power requirements in adult canines during treadmill exercise following two-stage resection of approximately 70% of lung units in the presence or absence of mediastinal distortion. Results were compared with that in control animals following right PNX or thoracotomy without resection (Sham). Following 70% lung resection, peak O(2) uptake was 45% below normal. Ventilation-perfusion mismatch developed, and pulmonary arterial pressure and ventilatory power requirements became markedly elevated. In contrast, the relationship of Dl(O(2)) to cardiac output remained normal, indicating preservation of Dl(O(2))-to-cardiac output ratio and alveolar-capillary recruitment up to peak exercise. The impairment in airway and vascular function exceeded the impairment in gas exchange and imposed the major limitation to exercise following 70% resection. Mediastinal distortion further reduced air and blood flow conductance, resulting in CO(2) retention. Results suggest that adaptation of extra-acinar airways and blood vessels lagged behind that of acinar tissue. As more lung units were lost, functional compensation became limited by the disproportionately reduced convective conductance rather than by alveolar diffusion disequilibrium.

Predicting diffusive alveolar oxygen transfer from carbon monoxide-diffusing capacity in exercising foxhounds.

Although lung diffusing capacity for carbon monoxide (DL(CO)) is a widely used test of diffusive O2 transfer, few studies have directly related DL(CO) to O2-diffusing capacity (DL(O2)); none has used the components of Dl(CO), i.e., conductance of alveolar membrane and capillary blood, to predict DL(O2) from rest to exercise. To understand the relationship between DL(CO) and DL(O2) at matched levels of cardiac output, we analyzed cumulative data from rest to heavy exercise in 43 adult dogs, with normal lungs or reduced lung capacity following lung resection, that were studied by two techniques. 1) A rebreathing (RB) technique was used to measure Dl(CO) and pulmonary blood flow at two O2 tensions, independent of O2 exchange. DL(CO) was partitioned into CO-diffusing capacity of alveolar membrane and pulmonary capillary blood volume using the Roughton-Forster equation and converted into an equivalent DL(O2), [DL(O2)(RB)]. 2) A multiple inert-gas elimination technique (MIGET) was used to measure ventilation-perfusion distributions, O2 and CO2 exchange under hypoxia, to derive DL(O2) [DL(O2)(MIGET)] by the Lilienthal-Riley technique and Bohr integration. For direct comparisons, DL(O2)(RB) was interpolated to the cardiac output measured by the Fick principle corresponding to DL(O2)(MIGET). The DL(O2)-to-DL(CO) ratio averaged 1.61. Correlation between DL(O2)(RB) and DL(O2)(MIGET) was similar in normal and post-resection groups. Overall, DL(O2)(MIGET) = 0.975 DL(O2)(RB); mean difference between the two techniques was under 5% for both animal groups. We conclude that, despite various uncertainties inherent in these two disparate methods, the Roughton-Forster equation adequately predicts diffusive O2 transfer from rest to heavy exercise in canines with normal, as well as reduced, lung capacities.

Acclimatization of low altitude-bred deer mice ( Peromyscus maniculatus) to high altitude.

A colony of deer mice subspecies ( Peromyscus maniculatus sonoriensis) native to high altitude (HA) has been maintained at sea level for 18-20 generations and remains genetically unchanged. To determine if these animals retain responsiveness to hypoxia, one group (9-11 wk old) was acclimated to HA (3,800 m) for 8 wk. Age-matched control animals were acclimated to a lower altitude (LA; 252 m). Maximal O2 uptake (V̇o2max) was measured at the respective altitudes. On a separate day, lung volume, diffusing capacity for carbon monoxide (DLCO), and pulmonary blood flow were measured under anesthesia using a rebreathing technique at two inspired O2 tensions. The HA-acclimated deer mice maintained a normal V̇o2max relative to LA baseline. Compared with LA control mice, antemortem lung volume was larger in HA mice in a manner dependent on alveolar O2 tension. Systemic hematocrit, pulmonary blood flow, and standardized DLCO did not differ significantly between groups. HA mice showed a higher postmortem alveolar-capillary hematocrit, larger alveolar ducts, and smaller distal conducting structures. In HA mice, absolute volumes of alveolar type I epithelia and endothelia were higher whereas that of interstitia was lower than in LA mice. These structural changes occurred without a net increase in whole-lung septal tissue-capillary volumes or surface areas. Thus, deer mice bred and raised to adulthood at LA retain phenotypic plasticity and adapt to HA without a decrement in V̇o2max via structural (enlarged airspaces, alveolar septal remodeling) and nonstructural (lung expansion under hypoxia) mechanisms and without an increase in systemic hematocrit or compensatory lung growth. NEW & NOTEWORTHY Deer mice ( Peromyscus maniculatus) are robust and very active mammals that are found across the North American continent. They are also highly adaptable to extreme environments. When introduced to high altitude they retain remarkable adaptive ability to the low-oxygen environment via lung expansion and remodeling of existing lung structure, thereby maintaining normal aerobic capacity without generating more red blood cells or additional lung tissue.

Alveolar diffusion-perfusion interactions during high-altitude residence in guinea pigs.

We previously reported in weanling guinea pigs raised at high altitude (HA; 3,800 m) an elevated lung diffusing capacity estimated by morphometry from alveolar-capillary surface area, harmonic mean blood-gas barrier thickness, and pulmonary capillary blood volume (Vc) compared with litter-matched control animals raised at an intermediate altitude (IA; 1,200 m) (Hsia CCW, Polo Carbayo JJ, Yan X, Bellotto DJ. Respir Physiol Neurobiol 147: 105-115, 2005). To determine if HA-induced alveolar ultrastructural changes are associated with improved alveolar function, we measured lung diffusing capacity for carbon monoxide (DLCO), membrane diffusing capacity for carbon monoxide (DMCO), Vc, pulmonary blood flow, and lung volume by a rebreathing technique in litter-matched male weanling Hartley guinea pigs raised at HA or IA for 4 or 12 mo. Separate control animals were also raised and studied at sea level (SL). Resting measurements were obtained in the conscious nonsedated state. In HA animals compared with corresponding IA or SL controls, lung volume and hematocrit were significantly higher while pulmonary blood flow was lower. At a given pulmonary blood flow, DLCO and DMCO were higher in HA-raised animals than in control animals without a significant change in Vc. We conclude that 1) HA residence enhanced physiological diffusing capacity corresponding to that previously estimated on the basis of structural adaptation, 2) adaptation in diffusing capacity and its components should be interpreted with respect to pulmonary blood flow, and 3) this noninvasive rebreathing technique could be used to follow adaptive responses in small animals.

Developmental signals do not further accentuate nonuniform postpneumonectomy compensatory lung growth.

Mechanical forces imposed on lung tissue constitute major stimuli for normal lung development and postpneumonectomy (PNX) compensatory growth and remodeling. Superimposing developmental signals on PNX signals augments compensatory alveolar growth but exaggerates airway-parenchymal dissociation (i.e., dysanaptic lung growth); the latter tends to offset benefits derived from the former. In adult dogs after PNX, lobar expansion and growth of the remaining lobes were markedly non-uniform (Ravikumar et al. J Appl Physiol 97:1567-1574, 2004). We hypothesized that superimposing developmental and post-PNX signals further accentuates nonuniformity of lobar growth. We used high-resolution computed tomography (HRCT) to follow regional lung expansion and growth in foxhounds undergoing right PNX at 2.5 mo of age compared with litter-matched control (Sham) animals; scans were performed 4 and 10 mo following surgery, i.e., before and after somatic maturity. Air and tissue volumes were measured in each lobe; tissue volume estimated by HRCT includes air-free tissue and blood in small vessels <1 mm. Interlobar nonuniformity of tissue volume was absent at 4 mo but evident 10 mo after PNX; growth of the remaining left lower lobe gradually lagged behind other lobes. At maturity, nonuniformity of lobar growth in pneumonectomized puppies was similar to that previously reported in pneumonectomized adults. We conclude that superimposing developmental and post-PNX signals enhances some aspects of compensatory lung growth and remodeling without altering its nonuniform spatial distribution.

The canine spleen in oxygen transport: gas exchange and hemodynamic responses to splenectomy.

In athletic animals the spleen induces acute polycythemia by dynamic contraction that releases red blood cells into the circulation in response to increased O(2) demand and metabolic stress; when energy demand is relieved, the polycythemia is rapidly reversed by splenic relaxation. We have shown in adult foxhounds that splenectomy eliminates exercise-induced polycythemia, thereby reducing peak O(2) uptake and lung diffusing capacity for carbon monoxide (DL(CO)) as well as exaggerating preexisting DL(CO) impairment imposed by pneumonectomy (Dane DM, Hsia CC, Wu EY, Hogg RT, Hogg DC, Estrera AS, Johnson RL Jr. J Appl Physiol 101: 289-297, 2006). To examine whether the postsplenectomy reduction in DL(CO) leads to abnormalities in O(2) diffusion, ventilation-perfusion inequality, or hemodynamic function, we studied these animals via the multiple inert gas elimination technique at rest and during exercise at a constant workload equivalent to 50% or 80% of peak O(2) uptake while breathing 21% and 14% O(2) in balanced order. From rest to exercise after splenectomy, minute ventilation was significantly elevated with respect to O(2) uptake compared with exercise before splenectomy; cardiac output, O(2) delivery, and mean pulmonary and systemic arterial blood pressures were 10-20% lower, while O(2) extraction was elevated with respect to O(2) uptake. Ventilation-perfusion inequality was unchanged, but O(2) diffusing capacities of lung (DL(O2)) and peripheral tissue during exercise were lower with respect to cardiac output postsplenectomy by 32% and 25%, respectively. The relationship between DL(O2) and DL(CO) was unchanged by splenectomy. We conclude that the canine spleen regulates both convective and diffusive O(2) transport during exercise to increase maximal O(2) uptake.

Residence at 3,800-m altitude for 5 mo in growing dogs enhances lung diffusing capacity for oxygen that persists at least 2.5 years.

Mammals native to high altitude (HA) exhibit larger lung volumes than their lowland counterparts. To test the hypothesis that adaptation induced by HA residence during somatic maturation improves pulmonary gas exchange in adulthood, male foxhounds born at sea level (SL) were raised at HA (3,800 m) from 2.5 to 7.5 mo of age and then returned to SL prior to somatic maturity while their littermates were simultaneously raised at SL. Following return to SL, all animals were trained to run on a treadmill; gas exchange and hemodynamics were measured 2.5 years later at rest and during exercise while breathing 21% and 13% O(2). The multiple inert gas elimination technique was employed to estimate ventilation-perfusion (Va/Q) distributions and lung diffusing capacity for O(2) (Dl(O(2))). There were no significant intergroup differences during exercise breathing 21% O(2). During exercise breathing 13% O(2), peak O(2) uptake and Va/Q distributions were similar between groups but arterial pH, base excess, and O(2) saturation were higher while peak lactate concentration was lower in animals raised at HA than at SL. At a given exercise intensity, alveolar-arterial O(2) tension gradient (A-aDo(2)) attributable to diffusion limitation was lower while Dlo(2) was 12-25% higher in HA-raised animals. Mean systemic arterial blood pressure was also lower in HA-raised animals; mean pulmonary arterial pressures were similar. We conclude that 5 mo of HA residence during maturation enhances long-term gas exchange efficiency and Dl(O(2)) without impacting Va/Q inequality during hypoxic exercise at SL.

Long-term enhancement of pulmonary gas exchange after high-altitude residence during maturation.

In a previous study, our laboratory showed that young dogs born at sea level (SL) and raised from 2.5 mo of age to beyond somatic maturity at a high altitude (HA) of 3,100 m show enhanced resting lung function (Johnson RL Jr, Cassidy SS, Grover RF, Schutte JE, and Epstein RH. J Appl Physiol 59: 1773-1782, 1985). To examine whether HA-induced adaptation improves pulmonary gas exchange during exercise and whether adaptation is reversible when animals return to SL before somatic maturity, we raised 2.5-mo-old foxhounds at HA (3,800 m) for 5 mo (to age 7.5 mo) before returning them to SL. Lung function was measured under anesthesia 1 mo and 2 yr after return to SL and during exercise approximately 1 yr after return. In animals exposed to HA relative to simultaneous litter-matched SL controls, resting circulating blood and erythrocyte volumes, lung volumes, septal volume estimated by a rebreathing technique, and lung tissue volume estimated by high-resolution computed tomography scan were persistently higher. Lung diffusing capacity, membrane diffusing capacity, and pulmonary capillary blood volume estimated at a given cardiac output were significantly higher in animals exposed to HA, whereas maximal oxygen uptake and hematocrit were similar between groups. We conclude that relatively short exposure to HA during somatic maturation improves long-term lung function into adulthood.

Splenectomy impairs diffusive oxygen transport in the lung of dogs.

The spleen acts as an erythrocyte reservoir in highly aerobic species such as the dog and horse. Sympathetic-mediated splenic contraction during exercise reversibly enhances convective O2 transport by increasing hematocrit, blood volume, and O2-carrying capacity. Based on theoretical interactions between erythrocytes and capillary membrane (Hsia CCW, Johnson RL Jr, and Shah D. J Appl Physiol 86: 1460-1467, 1999) and experimental findings in horses of a postsplenectomy reduction in peripheral O2-diffusing capacity (Wagner PD, Erickson BK, Kubo K, Hiraga A, Kai M, Yamaya Y, Richardson R, and Seaman J. Equine Vet J 18, Suppl: 82-89, 1995), we hypothesized that splenic contraction also augments diffusive O2 transport in the lung. Therefore, we have measured lung diffusing capacity (DL(CO)) and its components during exercise by a rebreathing technique in six adult foxhounds before and after splenectomy. Splenectomy eliminated exercise-induced polycythemia, associated with a 30% reduction in maximal O2 uptake. At any given pulmonary blood flow, DL(CO) was significantly lower after splenectomy owing to a lower membrane diffusing capacity, whereas pulmonary capillary blood volume changed variably; microvascular recruitment, indicated by the slope of the increase in DL(CO) with respect to pulmonary blood flow, was also reduced. We conclude that splenic contraction enhances both convective and diffusive O2 transport and provides another compensatory mechanism for maintaining alveolar O2 transport in the presence of restrictive lung disease or ambient hypoxia.

Perfusion-related stimuli for compensatory lung growth following pneumonectomy.

Following pneumonectomy (PNX), two separate mechanical forces act on the remaining lung: parenchymal stress caused by lung expansion, and microvascular distension and shear caused by increased perfusion. We previously showed that parenchymal stress and strain explain approximately one-half of overall compensation; the remainder was presumptively attributed to perfusion-related factors. In this study, we directly tested the hypothesis that perturbation of regional pulmonary perfusion modulates post-PNX lung growth. Adult canines underwent banding of the pulmonary artery (PAB) to the left caudal (LCa) lobe, which caused a reduction in basal perfusion to LCa lobe without preventing the subsequent increase in its perfusion following right PNX while simultaneously exaggerating the post-PNX increase in perfusion to the unbanded lobes, thereby creating differential perfusion changes between banded and unbanded lobes. Control animals underwent sham pulmonary artery banding followed by right PNX. Pulmonary function, regional pulmonary perfusion, and high-resolution computed tomography of the chest were analyzed pre-PNX and 3-mo post-PNX. Terminally, the remaining lobes were fixed for detailed morphometric analysis. Results were compared with corresponding lobes in two control (Sham banding and normal unoperated) groups. PAB impaired the indices of post-PNX extravascular alveolar tissue growth by up to 50% in all remaining lobes. PAB enhanced the expected post-PNX increase in alveolar capillary formation, measured by the prevalence of double-capillary profiles, in both unbanded and banded lobes. We conclude that perfusion distribution provides major stimuli for post-PNX compensatory lung growth independent of the stimuli provided by lung expansion and parenchymal stress and strain.

Lack of response to all-trans retinoic acid supplementation in adult dogs following left pneumonectomy.

We showed previously that removing 55-58% of the lung by right pneumonectomy (R-PNX) in adult dogs triggers compensatory growth of the remaining lung, but removing 42-45% of the lung by left PNX (L-PNX) does not. We also showed that, following R-PNX, supplemental all-trans retinoic acid (RA) selectively enhances alveolar capillary endothelial cell volume (Yan X, Bellotto DJ, Foster DJ, Johnson RL, Jr., Hagler HH, Estrera AS, and Hsia CC. J Appl Physiol 96: 1080-1089, 2004). We hypothesized that RA supplementation might enhance compensation following L-PNX and tested this hypothesis by administering RA (2 mg.kg(-1).day(-1), 4 days/wk) or placebo orally to litter-matched adult foxhounds for 4 mo following L-PNX. Resting lung function was measured under anesthesia. Air and tissue volumes of the remaining lung were assessed by high-resolution computed tomography scan and by detailed postmortem morphometric analysis of the fixed lung. There was no significant difference in resting lung function, lung volume, alveolar structure, or septal ultrastructure between RA and placebo treatment groups. We conclude that RA supplementation does not induce post-PNX compensatory lung growth in the absence of existing cellular growth activities initiated by other primary signals.

Dysanaptic growth of conducting airways after pneumonectomy assessed by CT scan.

In immature dogs after pneumonectomy (PNX), pulmonary viscous resistance is persistently elevated predominantly as a result of a high airway resistance (Raw). We examined the anatomical basis for this observation by using computerized tomography scans obtained from foxhounds 4-10 mo after right PNX. Airways of the left lower lobe were followed from generations z = 0 (trachea) to z = 12. By 4 mo post-PNX, airway length increased significantly relative to sham-operated dogs, but airway cross-sectional area (CSA) did not. By 10 mo post-PNX, average airway CSA was 24% above that in controls. Theoretically, the increased airway length and CSA should reduce lobar Raw by 50%. However, post-PNX airway dilatation did not normalize total CSA, and estimated resistance due to turbulence and convective acceleration increased threefold; i.e., the 50% reduction in lobar Raw would be offset by the loss of four of seven lobes. Thus the expected reduction in work of breathing in the whole animal is only ~30%, consistent with previously measured work of breathing in pneumonectomized dogs. We conclude that airway structure adapts slowly and incompletely, resulting in limited functional compensation.

Density-dependent reduction of nitric oxide diffusing capacity after pneumonectomy.

Airway lengthening after pneumonectomy (PNX) may increase diffusive resistance to gas mixing (1/D(G)); the effect is accentuated by increasing acinar gas density but is difficult to detect from lung CO-diffusing capacity (Dl(CO)). Because lung NO-diffusing capacity (Dl(NO)) is three- to fivefold that of Dl(CO), whereas 1/D(G) for NO and CO are similar, we hypothesized that a density-dependent fractional reduction would be greater for Dl(NO) than for Dl(CO). We measured Dl(NO) and Dl(CO) at two tidal volumes (Vt) and with three background gases [helium (He), nitrogen (N(2)), and sulfur hexafluoride (SF(6))] in immature dogs 3 and 9 mo after right PNX (5 and 11 mo of age). At maturity (11 mo), background gas density had no effect on Dl(NO), Dl(CO), or Dl(NO)-to-Dl(CO) ratio in sham controls. In PNX animals, Dl(NO) declined 25-50% in SF(6) relative to He and N(2), and Dl(NO)/Dl(CO) declined approximately 50% in SF(6) relative to He at a Vt of 15 ml/kg, consistent with a significant 1/D(G). At 5 mo of age, Dl(NO)/Dl(CO) declined 25-45% in SF(6) relative to He and N(2) in both groups, but Dl(CO) increased paradoxically in SF(6) relative to N(2) or He by 20-60%. Findings suggest that SF(6), besides increasing 1/D(G), may redistribute ventilation and/or enhance acinar penetration of the convective front.

Retinoic acid-induced alveolar cellular growth does not improve function after right pneumonectomy.

To determine whether all-trans retinoic acid (RA) treatment enhances lung function during compensatory lung growth in fully mature animals, adult male dogs (n = 4) received 2 mg x kg(-1) x day(-1) po RA 4 days/wk beginning the day after right pneumonectomy (R-PNX, 55-58% resection). Litter-matched male R-PNX controls (n = 4) received placebo. After 3 mo, transpulmonary pressure (TPP)-lung volume relationship, diffusing capacities for carbon monoxide and nitric oxide, cardiac output, and septal volume (V(tiss-RB)) were measured under anesthesia by a rebreathing technique at two lung volumes. Lung air and tissue volumes (V(air-CT) and V(tiss-CT)) were also measured from high-resolution computerized tomographic (CT) scans at a constant TPP. In RA-treated dogs compared with controls, TPP-lung volume relationships were similar. Diffusing capacities for carbon monoxide and nitric oxide were significantly impaired at a lower lung volume but similar at a high lung volume. Whereas V(tiss-RB) was significantly lower at both lung volumes in RA-treated animals, V(air-CT) and V(tiss-CT) were not different between groups; results suggest uneven distribution of ventilation consistent with distortion of alveolar geometry and/or altered small airway function induced by RA. We conclude that RA does not improve resting pulmonary function during the early months after R-PNX despite histological evidence of its action in enhancing alveolar cellular growth in the remaining lung.

Regional lung growth following pneumonectomy assessed by computed tomography.

After pneumonectomy (PNX), mechanical strain on the remaining lung is greatly increased. To assess whether remaining lobes expand uniformly after left or right PNX (removing 42 and 58% of lung mass, respectively), we performed high-resolution computed tomography (CT) scans at 45 ml/kg above end-expiratory lung volume on adult male foxhounds after left or right PNX, which were compared with adult Sham controls. Air and tissue volumes were separately measured in each lobe. After left PNX, air and tissue volumes in the right upper and cardiac lobes increased approximately 2.2-fold above and below the heart, whereas volumes in right middle and lower lobes did not change significantly. After right PNX, air and tissue volumes in the left upper and middle lobes increased 2.3- to 2.7-fold across the midline anterior to the heart, whereas the left lower lobe expanded approximately 1.9-fold posterior to the heart. Regional changes in volume density of tissue post-PNX estimated by CT scan parallel postmortem estimates by morphometric analyses. Data indicate heterogeneous regional distribution of mechanical lung strain, which could influence the differential cellular compensatory response following right and left PNX.

Defining a stimuli-response relationship in compensatory lung growth following major resection.

Major lung resection is a robust model that mimics the consequences of loss-of-functioning lung units. We previously observed in adult canines, following 42% and 58% lung resection, a critical threshold of stimuli intensity for the initiation of compensatory lung growth. To define the range and limits of this stimuli-response relationship, we performed morphometric analysis on the remaining lobes of adult dogs, 2-3 years after surgical removal of ∼ 70% of lung units in the presence or absence of mediastinal shift. Results were expressed as ratios to that in corresponding control lobes. Lobar expansion and extravascular tissue growth (∼ 3.8- and ∼ 2.0-fold of normal, respectively) were heterogeneous; the lobes remaining next to the diaphragm exhibited a greater response. Tissue growth and capillary formation, indexed by double-capillary profiles, increased, regardless of mediastinal shift. Septal collagen fibers increased up to 2.7-fold, suggesting a greater need for structural support. Compared with previous cohorts following less-extensive resection, tissue volume and gas-exchange surface areas increased significantly only in the infracardiac lobe following 42% resection, exceeded two- to threefold in all lobes following 58% resection, and then exhibited diminished gains following ∼ 70% resection. In contrast, alveolar-capillary formation increased with incremental resection without reaching an upper limit. Overall structural regrowth was most vigorous and uniform following 58% resection. The diminishment of gains in tissue growth, following ∼ 70% resection, could reflect excessive or maldistributed mechanical stress that threatens septal integrity. Results also suggest additional independent stimuli of alveolar-capillary formation, possibly related to the postresection augmentation of regional perfusion.