Effect of 5 weeks of oral acetazolamide on patients with pulmonary vascular disease: A randomized, double-blind, cross-over trial

BACKGROUND

The carbonic anhydrase inhibitor acetazolamide stimulates ventilation through metabolic acidosis mediated by renal bicarbonate excretion. In animal models, acetazolamide attenuates acute hypoxia-induced pulmonary hypertension (PH), but its efficacy in treating patients with PH due to pulmonary vascular disease (PVD) is unknown.

METHODS

28 PVD patients (15 pulmonary arterial hypertension, 13 distal chronic thromboembolic PH), 13 women, mean±SD age 61.6±15.0 years stable on PVD medications, were randomised in a double-blind crossover protocol to 5 weeks acetazolamide (250mg b.i.d) or placebo separated by a ≥2 week washout period. Primary endpoint was the change in 6-minute walk distance (6MWD) at 5 weeks. Additional endpoints included safety, tolerability, WHO functional class, quality of life, arterial blood gases, and hemodynamics (by echocardiography).

RESULTS

Acetazolamide had no effect on 6MWD compared to placebo (treatment effect: mean change [95%CI] -18 [-40 to 4]m, p=0.102) but increased arterial blood oxygenation through hyperventilation induced by metabolic acidosis. Other measures including pulmonary hemodynamics were unchanged. No severe adverse effects occurred, side effects that occurred significantly more frequently with acetazolamide vs. placebo were change in taste (22/0%), paraesthesia (37/4%) and mild dyspnea (26/4%).

CONCLUSIONS

In patients with PVD, acetazolamide did not change 6MWD compared to placebo despite improved blood oxygenation. Some patients reported a tolerable increase in dyspnoea during acetazolamide treatment, related to hyperventilation, induced by the mild drug-induced metabolic acidosis. Our findings do not support the use of acetazolamide to improve exercise in patients with PVD at this dosing.

CLINICALTRIALS

GOV IDENTIFIER

NCT02755298.

On the antioxidant properties of erythropoietin and its association with the oxidative-nitrosative stress response to hypoxia in humans

AIM

The aim of this study was to examine if erythropoietin (EPO) has the potential to act as a biological antioxidant and determine the underlying mechanisms.

METHODS

The rate at which its recombinant form (rHuEPO) reacts with hydroxyl (HO˙), 2,2-diphenyl-1-picrylhydrazyl (DPPH˙) and peroxyl (ROO˙) radicals was evaluated in-vitro. The relationship between the erythopoietic and oxidative-nitrosative stress response to poikilocapneic hypoxia was determined separately in-vivo by sampling arterial blood from eleven males in normoxia and following 12 h exposure to 13% oxygen. Electron paramagnetic resonance spectroscopy, ELISA and ozone-based chemiluminescence were employed for direct detection of ascorbate (A(˙-) ) and N-tert-butyl-α-phenylnitrone spin-trapped alkoxyl (PBN-OR) radicals, 3-nitrotyrosine (3-NT) and nitrite (NO2-).

RESULTS

We found rHuEPO to be a potent scavenger of HO˙ (kr = 1.03-1.66 × 10(11) m(-1) s(-1) ) with the capacity to inhibit Fenton chemistry through catalytic iron chelation. Its ability to scavenge DPPH˙ and ROO˙ was also superior compared to other more conventional antioxidants. Hypoxia was associated with a rise in arterial EPO and free radical-mediated reduction in nitric oxide, indicative of oxidative-nitrosative stress. The latter was confirmed by an increased systemic formation of A˙(-) , PBN-OR, 3-NT and corresponding loss of NO2- (P < 0.05 vs. normoxia). The erythropoietic and oxidative-nitrosative stress responses were consistently related (r = -0.52 to 0.68, P < 0.05).

CONCLUSION

These findings demonstrate that EPO has the capacity to act as a biological antioxidant and provide a mechanistic basis for its reported cytoprotective benefits within the clinical setting.

Alveolar Pco2 oscillations and ventilation at sea level and at high altitude

This study examines the potential for a ventilatory drive, independent of mean Pco2, but depending instead on changes in Pco2 that occur during the respiratory cycle. This responsiveness is referred to here as “dynamic ventilatory sensitivity.” The normal, spontaneous, respiratory oscillations in alveolar Pco2 have been modified with inspiratory pulses approximating alveolar Pco2 concentrations, both at sea level and at high altitude (5,000 m, 16,400 ft.). All tests were conducted with subjects exercising on a cycle ergometer at 60 W. The pulses last about half the inspiratory duration and are timed to arrive in the alveoli during early or late inspiration. Differences in ventilation, which then occur in the face of similar end-tidal Pco2 values, are taken to result from dynamic ventilatory sensitivity. Highly significant ventilatory responses (early pulse response greater than late) occurred in hypoxia and normoxia at sea level and after more than 4 days at 5,000 m. The response at high altitude was eliminated by normalizing Po2 and was reduced or eliminated with acetazolamide. No response was present soon after arrival (<4 days) at base camp, 5,000 m, on either of two high-altitude expeditions (BMEME, 1994, and Kanchenjunga, 1998). The largest responses at 5,000 m were obtained in subjects returning from very high altitude (7,100–8,848 m). The present study confirms and extends previous investigations that suggest that alveolar Pco2 oscillations provide a feedback signal for respiratory control, independent of changes in mean Pco2, suggesting that natural Pco2 oscillations drive breathing in exercise.

Travel to high altitude with pre-existing lung disease

The pathophysiology of high-altitude illnesses has been well studied in normal individuals, but little is known about the risks of high-altitude travel in patients with pre-existing lung disease. Although it would seem self-evident that any patient with lung disease might not do well at high altitude, the type and severity of disease will determine the likelihood of difficulty in a high-altitude environment. The present review examines whether these individuals are at risk of developing one of the main forms of acute or chronic high-altitude illness and whether the underlying lung disease itself will get worse at high elevations. Several groups of pulmonary disorders are considered, including obstructive, restrictive, vascular, control of ventilation, pleural and neuromuscular diseases. Attempts will be made to classify the risks faced by each of these groups at high altitude and to provide recommendations regarding evaluation prior to high-altitude travel, advice for or against taking such excursions, and effective prophylactic measures.

An investigation of the role of carbonic anhydrase in aquatic and aerial gas transfer in the African lungfishProtopterus dolloi

Experiments were performed on bimodally breathing African lungfish Protopterus dolloi to examine the effects of inhibition of extracellular vs total (extracellular and intracellular) carbonic anhydrase (CA) activity on pulmonary and branchial/cutaneous gas transfer. In contrast to previous studies on Protopterus, which showed that the vast majority of CO2 is excreted into the water through the gill and/or skin whereas O2 uptake largely occurs via the lung, P. dolloi appeared to use the lung for the bulk of both O2uptake (91.0±2.9%) and CO2 excretion (76.0±6.6%). In support of the lung as the more important site of CO2 transfer,aerial hypercapnia (PCO2=40 mmHg) caused a significant rise in partial pressure of arterial blood CO2(PaCO2) whereas a similar degree of aquatic hypercapnia was without effect on PaCO2. Intravascular injection of low levels (1.2 mg kg-1) of the slowly permanent CA inhibitor, benzolamide, was without effect on red blood cell CA activity after 30 min, thus confirming its suitability as a short-term selective inhibitor of extracellular CA. Benzolamide treatment did not affect CO2 excretion, blood acid–base status or any other measured variable within the 30 min measurement period. Injection of the permeant CA inhibitor acetazolamide (30 mg kg-1) resulted in the complete inhibition of red cell CA activity within 10 min. However, CO2excretion (measured for 2 h after injection) and arterial blood acid–base status (assessed for 24 h after injection) were unaffected by acetazolamide treatment. Intra-arterial injection of bovine CA (2 mg kg-1) caused a significant increase in overall CO2excretion (from 0.41±0.03 to 0.58±0.03 mmol kg-1h-1) and an increase in air breathing frequency (from 19.0±1.3 to 24.7±1.8 breaths min-1) that was accompanied by a slight, but significant, reduction in PaCO2 (from 21.6±1.6 to 19.6±1.8 mmHg).

The findings of this study are significant because they (i) demonstrate that, unlike in other species of African lungfish that have been examined, the gill/skin is not the major route of CO2 excretion in P. dolloi, and (ii) suggest that CO2 excretion in Protopterus may be less reliant on carbonic anhydrase than in most other fish species.

Update: High altitude pulmonary edema

Recent high altitude studies with pulmonary artery (PA) catheterization and broncho-alveolar lavage (BAL) in early high altitude pulmonary edema(HAPE) have increased our understanding of the pathogenetic sequence in HAPE. High preceding PA and pulmonary capillary pressures lead to a non-inflammatory leak of the alveolar-capillary barrier with egress of red cells, plasma proteins and fluid into the alveolar space. The mechanisms accounting for an increased capillary pressure remain speculative. The concept that hypoxic pulmonary vasoconstriction (HPV) is uneven so that regions with less vasoconstriction are over-perfused and become edematous remains compelling but unproved. Also uncertain is the role and extent of pulmonary venoconstriction. With disruption of the normal alveolar-capillary barrier, some individuals may later develop a secondary inflammatory reaction. A high incidence of preceding or concurrent respiratory infection in children with HAPE has been used to support a causative role of inflammation in HAPE. However, alternatively even mild HPV may simply lower the threshold at which inflammation-mediated increases in alveolar capillary permeability cause significant fluid flux into the lung. Other major questions to be addressed in future research are: 1.) What is the mechanism of exaggerated hypoxic pulmonary vasoconstriction? Is there a link to primary pulmonary hypertension? Several observations suggest that susceptibility to HAPE is associated with endothelial dysfunction in pulmonary vessels. This has not yet been studied adequately. 2.) What is the nature of the leak? Is there structural damage, i. e. stress failure, or does stretch cause opening of pores? 3.) What is the pathophysiologic significance of a decreased sodium and water clearance across alveolar epithelial cells in hypoxia? 4.) What is the role of exercise? Do HAPE-susceptible individuals develop pulmonary edema when exposed to hypoxia without exercise? Answers to these questions will increase our understanding of the pathophysiology of HAPE and also better focus research on the genetic basis of susceptibility to HAPE.

Biochemical, histological, and inhibitor studies of membrane carbonic anhydrase in frog gastric acid secretion

Gastric acid secretion is dependent on carbonic anhydrase (CA). To define the role of membrane-bound CA, we used biochemical, histochemical, and pharmacological approaches in the frog (Rana pipiens). CA activity and inhibition by membrane-permeant and -impermeant agents were studied in stomach homogenates and microsomal fractions. H(+) secretion in the histamine-stimulated isolated mucosa was measured before and after mucosal addition of a permeant CA inhibitor (methazolamide) and before and after mucosal or serosal addition of two impermeant CA inhibitors of differing molecular mass: a 3,500-kDa polymer linked to aminobenzolamide and p-fluorobenzyl-aminobenzolamide (molecular mass, 454 kDa). Total CA activity of frog gastric mucosa is 2,280 U/g, of which 10% is due to membrane-bound CA. Membrane-bound CA retains detectable activity below pH 4. Histochemically, there is membrane-associated CA in surface epithelial, oxynticopeptic, and capillary endothelial cells. Methazolamide reduced H(+) secretion by 100%, whereas the two impermeant inhibitors equally blocked secretion by 40% when applied to the mucosal side and by 55% when applied to the serosal side. The presence of membrane-bound CA in frog oxynticopeptic cells and its relative resistance to acid inactivation and inhibition by impermeant inhibitors demonstrate that it subserves acid secretion at both the apical and basolateral sides.

Effects of S-nitrosation of hemoglobin on hypoxic pulmonary vasoconstriction and nitric oxide flux

Free hemoglobin (Hb) augments hypoxic pulmonary vasoconstriction (HPV), ostensibly by scavenging nitric oxide (NO). However, recent evidence suggests that Hb that is S-nitrosated may act as an NO donor and vasodilator. We studied the effects of oxyHb, Hb that is chemically modified to prevent heme binding or oxidation of NO (cyanometHb), and Hb that is S-nitrosated (SNO-Hb and SNO-cyanometHb) on HPV, expired NO (eNO), and perfusate S-nitrosothiol (SNO) concentration in isolated, perfused rabbit lungs. Perfusate containing either 4 microM oxyHb or SNO-Hb increased normoxic pulmonary artery pressure (Ppa), augmented HPV dramatically, and resulted in an 80% fall in eNO in comparison to perfusion with buffer, whereas 4 microM cyanometHb or SNO-cynanometHb had no effect on these variables. Excess glutathione (GSH) added to perfusate containing SNO-Hb resulted in a 20 to 40% fall in the perfusate SNO concentration, with a concomitant increase in metHb content, without affecting Ppa, HPV, or eNO. In conclusion, free Hb augments HPV by scavenging NO, an effect that is not prevented by S-nitrosation. NO released from SNO-Hb in the presence of GSH does not produce measurable vascular effects in the lung or changes in eNO because of immediate oxidation and metHb formation.

Metabolic acidosis

Metabolic acidosis occurs in a number of diseases and even certain normal activities such as heavy exercise. It arises from increased endogenous acid production, exogenous acid (or acid-precursor) administration, base losses, and depression of renal acid secretion. Although the magnitude of acidosis is important, the ultimate pathophysiological impact of any metabolic acidosis is defined by the rate of change and the specific cause of the acidosis. This review discusses whole body, organ, and cellular effects of metabolic acidosis, its diagnosis by pathophysiologic categories, and treatment. The diagnosis is made by a synthesis of the clinical history, physical examination, other hematological values, serum and urinary chemistries, and arterial blood gases and electrolytes. Calculation of the anion and osmolal gaps can be effectively used to further narrow the diagnostic possibilities. Supportive care and therapy directed at the cause of the metabolic acidosis are the mainstays of treatment, since most acidotic states will spontaneously correct once the initiating cause is removed or reversed. Theoretical and clinical evidence are discussed for alkalinizing agents, whose use remains controversial except in the treatment of metabolic acidosis associated with hyperkalemia and certain drug or toxin ingestions.

Hypoxia decreases exhaled nitric oxide in mountaineers susceptible to high-altitude pulmonary edema

An exaggerated hypoxic pulmonary vasoconstriction is essential for development of high-altitude pulmonary edema (HAPE). We hypothesized that susceptibility to HAPE may be related to decreased production of nitric oxide (NO), an endogenous modulator of pulmonary vascular resistance, and that a decrease in exhaled NO could be detected during hypoxic exposure. Therefore, we investigated respiratory tract NO excretion by chemiluminescence and pulmonary artery systolic pressure (Ppa,s) by echocardiography in nine HAPE-susceptible mountaineers and nine HAPE-resistant control subjects during normoxia and acute hypoxia (fraction of inspired oxygen [FI(O2)] = 0.12). The subjects performed oral breathing. Nasally excreted NO was separated from respiratory gas by suction via a nasal mask. In HAPE-susceptible subjects, NO excretion in expired gas significantly decreased (p < 0.05) during hypoxia of 2 h in comparison with normoxia (28 +/- 4 versus 21 +/- 2 nl/min, mean +/- SEM). In contrast, the NO excretion rate of control subjects remained unchanged (31 +/- 6 versus 33 +/- 6 nl/ min, NS). Nasal NO excretion did not differ significantly between groups during normoxia (HAPE-susceptible group, 183 +/- 16 nl/ min; control subjects, 297 +/- 55 nl/min, NS) and was not influenced by hypoxia. The changes in Ppa,s with hypoxia correlated with the percent changes in lower respiratory tract NO excretion (R = -0.49, p = 0.04). Our data provide the first evidence of decreased pulmonary NO production in HAPE-susceptible subjects during acute hypoxia that may contribute among other factors to their enhanced hypoxic pulmonary vascular response.

Role of basolateral carbonic anhydrase in proximal tubular fluid and bicarbonate absorption

Membrane-bound carbonic anhydrase (CA) is critical to renal acidification. The role of CA activity on the basolateral membrane of the proximal tubule has not been defined clearly. To investigate this issue in microperfused rabbit proximal straight tubules in vitro, we measured fluid and HCO(3)(-) absorption and cell pH before and after the extracellular CA inhibitor p-fluorobenzyl-aminobenzolamide was applied in the bath to inhibit only basolateral CA. This inhibitor was 1% as permeant as acetazolamide. Neutral dextran (2 g/dl, molecular mass 70,000) was used as a colloid to support fluid absorption because albumin could affect CO(2) diffusion and rheogenic HCO(3)(-) efflux. Indeed, dextran in the bath stimulated fluid absorption by 55% over albumin. Basolateral CA inhibition reduced fluid absorption ( approximately 30%) and markedly decreased HCO(3)(-) absorption ( approximately 60%), both reversible when CA was added to the bathing solution. In the presence of luminal CA inhibition, which reduced fluid ( approximately 16%) and HCO(3)(-) ( approximately 66%) absorption, inhibition of basolateral CA further decreased the absorption of fluid (to 74% of baseline) and HCO(3)(-) (to 22% of baseline). CA inhibition also alkalinized cell pH by approximately 0.2 units, suggesting the presence of an alkaline disequilibrium pH in the interspace, which would secondarily block HCO(3)(-) exit from the cell and thereby decrease luminal proton secretion (HCO(3)(-) absorption). These data clearly indicate that basolateral CA has an important role in mediating fluid and especially HCO(3)(-) absorption in the proximal straight tubule.

Hemoglobin and red blood cells alter the response of expired nitric oxide to mechanical forces

Expired nitric oxide (NO(e)) varies with hemodynamic or ventilatory perturbations, possibly due to shear stress- or stretch-stimulated NO production. Since hemoglobin (Hb) binds NO, NO(e) changes may reflect changes in blood volume and flow. To determine the role of blood and mechanical forces, we measured NO(e) in anesthetized rabbits, as well as rabbit lungs perfused with buffer, red blood cells (RBCs) or Hb following changes in flow, venous pressure (P(v)), and positive end-expiratory pressure (PEEP). In buffer-perfused lungs decreases in flow and P(v) reduced NO(e), but NO(e) rose when RBCs and Hb were present. These findings are consistent with changes in vascular NO production, whose detection is obscured in blood-perfused lungs by the more dominant effect of Hb NO scavenging. PEEP decreased NO(e) in all perfused lungs but increased NO(e) in live rabbits. The NO(e) fall with PEEP in isolated lungs is consistent with flow redistribution from alveolar septal capillaries to extra-alveolar vessels and decreased surface area or a direct, stretch-mediated depression of lung epithelial NO production. In live rabbits, increased NO(e) may reflect blood flow reduction and decreased Hb NO scavenging and/or autonomic responses that increase NO production. We conclude that blood and systemic responses render it difficult to use NO(e) changes as an accurate measure of lung tissue NO production.

Acetazolamide reduces hypoxic pulmonary vasoconstriction in isolated perfused rabbit lungs

Carbonic anhydrase (CA) may modulate regional blood flow by mediating changes in extra- and intracellular pH. We hypothesized that CA inhibition with acetazolamide would inhibit the kinetics and magnitude of hypoxic pulmonary vasoconstriction (HPV). Isolated rabbit lungs were ventilated and perfused in situ at constant flow, with buffer containing red blood cells. Preparations were sequentially challenged with hypoxic (FI(O(2)) 0.05) and/or hypercapnic (FI(CO(2)) 0.10) gas mixtures for 5 or 10 min. In the experimental groups, acetazolamide (33 microM) was added to the perfusate after establishing baseline responses, and gas challenges were repeated; control groups were studied without acetazolamide. Acetazolamide reduced the increase in pulmonary artery pressure (DeltaPAP) and the rate of pressure rise by approximately 30-50% during hypoxia and combined hypoxia/hypercapnia. The reduction in DeltaPAP occurred for both 5 and 10 min challenges. Acetazolamide did not affect expired nitric oxide concentrations. We conclude that acetazolamide reduces both the magnitude and kinetics of HPV by a mechanism that does not involve nitric oxide.

Pulmonary NO synthase inhibition and inspired CO2: effects on V'/Q' and pulmonary blood flow distribution

Inhaled carbon dioxide decreases ventilation/perfusion ratio (V'/Q') heterogeneity in dogs. The aim of this study was to test whether inhaled CO2 improves the V'/Q' by inhibition of nitric oxide production and whether inhibition of endogenous NO production in the lung alters gas exchange and V'/Q' matching. Eleven healthy dogs were anaesthetized and mechanically ventilated. The multiple inert gas elimination technique (MIGET) was used to measure V'/Q' heterogeneity and regional pulmonary blood flow heterogeneity was assessed in five dogs using fluorescent microspheres. In a separate set of five dogs, exhaled NO levels were measured via chemiluminescence. All dogs were studied before and after 4.8% inspired CO2, and then given the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 10 mg x kg(-1)) via nebulization, after which they were studied again with room air and inhaled CO2. CO2 and L-NAME improved arterial and alveolar oxygen tension, but the improvements with L-NAME did not reach statistical significance. Improved V'/Q' matching, as assessed by the MIGET, occurred under all experimental conditions. Exhaled NO levels were reduced by 40% with CO2 and 70% with L-NAME. The standard deviation of regional pulmonary blood flow assessed via microspheres decreased only with inhaled CO2. Fractal analysis of pulmonary blood flow distributions revealed that regional blood flow was highly correlated with flow to neighbouring pieces of lung in all four conditions with no changes in the fractal dimension. Inspired carbon dioxide improves ventilation perfusion ratio matching and is associated with a more homogeneous distribution of pulmonary blood flow. Although inspired carbon dioxide causes a reduction in exhaled nitric oxide, the differences in pulmonary perfusion distributions found between carbon dioxide and N(omega)-nitro-L-arginine methyl ester suggest that the carbon dioxide effect is not mediated by a reduction in nitric oxide production. The improved ventilation perfusion ratio matching with inhibition of nitric oxide synthase suggests the intriguing possibility requiring further study that endogenous production of nitric oxide in the lung does not subserve ventilation perfusion ratio regulation.

The distribution and physiological significance of carbonic anhydrase in vertebrate gas exchange organs

The enzyme carbonic anhydrase (CA) catalyzes the reversible hydration/dehydration of CO(2) and water, maintaining a near-instantaneous equilibrium among all chemical species involved in the reaction. CA is found in association with all tissue and organ systems involved in the transport and excretion of CO(2), from the site of CO(2) production, metabolically active tissue such as muscle, to circulating red blood cells in the vasculature, to the various organs of gas exchange, the lungs and gills. The presence of the enzyme in every fluid compartment along the pathway of CO(2) transport appears necessary in order to allow the dehydration of HCO(3)(-) to keep pace with the rapid diffusion of CO(2) across biological membranes. Within the actual organ of gas exchange, CA is compartmentalized in multiple subcellular fractions, with the specific subcellular localization determining the enzyme's physiological function.

Effects of the RBC membrane and increased perfusate viscosity on hypoxic pulmonary vasoconstriction

Red blood cells (RBCs) augment hypoxic pulmonary vasoconstriction (HPV) in part by scavenging of nitric oxide (NO) by Hb (Deem S, Swenson ER, Alberts MK, Hedges RG, and Bishop MJ, Am J Respir Crit Care Med 157: 1181-1186, 1998). We studied the contribution of the RBC compartmentalization of Hb to augmentation of HPV and scavenging of NO in isolated perfused rabbit lungs. Lungs were initially perfused with buffer; HPV was provoked by a 5-min challenge with hypoxic gas (inspired O(2) fraction 0.05). Expired NO was measured continuously. Addition of free Hb to the perfusate (0.25 mg/ml) resulted in augmentation of HPV and a fall in expired NO that were similar in magnitude to those associated with a hematocrit of 30% (intracellular Hb of 100 mg/ml). Addition of dextran resulted in a blunting of HPV after free Hb but no change in expired NO. Blunting of HPV by dextran was not prevented by NO synthase inhibition with N(omega)-nitro-L-arginine and/or cyclooxygenase inhibition. RBC ghosts had a mild inhibitory effect on HPV but caused a small reduction in expired NO. In conclusion, the RBC membrane provides a barrier to NO scavenging and augmentation of HPV by Hb. Increased perfusate viscosity inhibits HPV by an undetermined mechanism.

Stress Doppler echocardiography for identification of susceptibility to high altitude pulmonary edema

OBJECTIVE

This prospective single-blinded study was performed to quantitate noninvasive pulmonary artery systolic pressure (PASP) responses to prolonged acute hypoxia and normoxic exercise.

BACKGROUND

Hypoxia-induced excessive rise in pulmonary artery pressure is a key factor in high-altitude pulmonary edema (HAPE). We hypothesized that subjects susceptible to HAPE (HAPE-S) have increased pulmonary artery pressure response not only to hypoxia but also to exercise.

METHODS

PASP was estimated at 45, 90 and 240 min of hypoxia (FiO2 = 12%) and during supine bicycle exercise in normoxia using Doppler-echocardiography in nine HAPE-S and in 11 control subjects.

RESULTS

In the control group, mean PASP increased from 26+/-2 to 37+/-4 mm Hg (deltaPASP 10.3+/-2 mm Hg) after 90 min of hypoxia and from 27+/-4 to 36+/-3 mm Hg (deltaPASP 8+/-2 mm Hg) during exercise. In contrast, all HAPE-S subjects revealed significantly greater increases (p = 0.002 vs. controls) in mean PASP both during hypoxia (from 28+/-4 to 57+/-10 mm Hg, deltaPASP 28.7+/-6 mm Hg) and during exercise (from 28+/-4 to 55+/-11 mm Hg, deltaPASP 27+/-8 mm Hg) than did control subjects. Stress echocardiography allowed discrimination between groups without overlap using a cut off PASP value of 45 mm Hg at work rates less than 150 W.

CONCLUSIONS

These data indicate that HAPE-S subjects may have abnormal pulmonary vascular responses not only to hypoxia but also to supine bicycle exercise under normoxic conditions. Thus, Doppler echocardiography during supine bicycle exercise or after 90 min of hypoxia may be useful noninvasive screening methods to identify subjects susceptible to HAPE.

Diuretic effect of hypoxia, hypocapnia, and hyperpnea in humans: relation to hormones and O(2) chemosensitivity

We studied the contributions of hypoxemia, hypocapnia, and hyperpnea to the acute hypoxic diuretic response (HDR) in humans and evaluated the role of peripheral O(2) chemosensitivity and renal hormones in HDR. Thirteen healthy male subjects (age 19-38 yr) were examined after sodium equilibration (intake: 120 mmol/day) during 90 min of normoxia (NO), poikilocapnic hypoxia (PH), and isocapnic hypoxia (IH) (days 1-3, random order, double blind), as well as normoxic voluntary hyperpnea (HP; day 4), matching ventilation during IH. O(2) saturation during PH and IH was kept equal to a mean level measured between 30 and 90 min of breathing 12% O(2) in a pretest. Urine flow during PH and IH (1.81 +/- 0.92 and 1.94 +/- 1.03 ml/min, respectively) but not during HP (1.64 +/- 0.96 ml/min) significantly exceeded that during NO (control, 1.38 +/- 0.71 ml/min). Urine flow increases vs. each test day's baseline were significant with PH, IH, and HP. Differences in glomerular filtration rate, fractional sodium clearance, urodilatin, systemic blood pressure, or leg venous compliance were excluded as factors of HDR. However, slight increases in plasma and urinary endothelin-1 and epinephrine with PH and IH could play a role. In conclusion, the early HDR in humans is mainly due to hypoxia and hypocapnia. It occurs without natriuresis and is unrelated to O(2) chemosensitivity (hypoxic ventilatory response).

Hemodilution during venous gas embolization improves gas exchange, without altering V(A)/Q or pulmonary blood flow distributions

BACKGROUND

Isovolemic anemia results in improved gas exchange in rabbits with normal lungs but in relatively poorer gas exchange in rabbits with whole-lung atelectasis. In the current study, the authors characterized the effects of hemodilution on gas exchange in a distinct model of diffuse lung injury: venous gas embolization.

METHODS

Twelve anesthetized rabbits were mechanically ventilated at a fixed rate and volume. Gas embolization was induced by continuous infusion of nitrogen via an internal jugular venous catheter. Serial hemodilution was performed in six rabbits by simultaneous withdrawal of blood and infusion of an equal volume of 6% hetastarch; six rabbits were followed as controls over time. Measurements included hemodynamic parameters and blood gases, ventilation-perfusion (V(A)/Q) distribution (multiple inert gas elimination technique), pulmonary blood flow distribution (fluorescent microspheres), and expired nitric oxide (NO; chemoluminescence).

RESULTS

Venous gas embolization resulted in a decrease in partial pressure of arterial oxygen (PaO2) and an increase in partial pressure of arterial carbon dioxide (PaCO2), with markedly abnormal overall V(A)/Q distribution and a predominance of high V(A)/Q areas. Pulmonary blood flow distribution was markedly left-skewed, with low-flow areas predominating. Hematocrit decreased from 30+/-1% to 11+/-1% (mean +/- SE) with hemodilution. The alveolar-arterial PO2 (A-aPO2) difference decreased from 375+/-61 mmHg at 30% hematocrit to 218+/-12.8 mmHg at 15% hematocrit, but increased again (301+/-33 mmHg) at 11% hematocrit. In contrast, the A-aPO2 difference increased over time in the control group (P < 0.05 between groups over time). Changes in PaO2 in both groups could be explained in large part by variations in intrapulmonary shunt and mixed venous oxygen saturation (SvO2); however, the improvement in gas exchange with hemodilution was not fully explained by significant changes in V(A)/Q or pulmonary blood flow distributions, as quantitated by the coefficient of variation (CV), fractal dimension, and spatial correlation of blood flow. Expired NO increased with with gas embolization but did not change significantly with time or hemodilution.

CONCLUSIONS

Isovolemic hemodilution results in improved oxygen exchange in rabbits with lung injury induced by gas embolization. The mechanism for this improvement is not clear.

Mechanisms of improvement in pulmonary gas exchange during isovolemic hemodilution

Severe anemia is associated with remarkable stability of pulmonary gas exchange (S. Deem, M. K. Alberts, M. J. Bishop, A. Bidani, and E. R. Swenson. J. Appl. Physiol. 83: 240-246, 1997), although the factors that contribute to this stability have not been studied in detail. In the present study, 10 Flemish Giant rabbits were anesthetized, paralyzed, and mechanically ventilated at a fixed minute ventilation. Serial hemodilution was performed in five rabbits by simultaneous withdrawal of blood and infusion of an equal volume of 6% hetastarch; five rabbits were followed over a comparable time. Ventilation-perfusion (VA/Q) relationships were studied by using the multiple inert-gas-elimination technique, and pulmonary blood flow distribution was assessed by using fluorescent microspheres. Expired nitric oxide (NO) was measured by chemiluminescence. Hemodilution resulted in a linear fall in hematocrit over time, from 30 +/- 1.6 to 11 +/- 1%. Anemia was associated with an increase in arterial PO(2) in comparison with controls (P < 0.01 between groups). The improvement in O(2) exchange was associated with reduced VA/Q heterogeneity, a reduction in the fractal dimension of pulmonary blood flow (P = 0.04), and a relative increase in the spatial correlation of pulmonary blood flow (P = 0. 04). Expired NO increased with anemia, whereas it remained stable in control animals (P < 0.0001 between groups). Anemia results in improved gas exchange in the normal lung as a result of an improvement in overall VA/Q matching. In turn, this may be a result of favorable changes in pulmonary blood flow distribution, as assessed by the fractal dimension and spatial correlation of blood flow and as a result of increased NO availability.

Effect of thyroplasty on laryngeal airflow

Thyroplasty has virtually replaced Teflon injection as the procedure of choice for treatment of the unilateral paralyzed vocal cord. Previous studies have shown that Teflon injection, by stiffening the vocal cord, decreases the extrathoracic airway obstruction occasionally measured by pulmonary function testing in patients with unilateral vocal cord paralysis. We became interested in the effect of thyroplasty on extrathoracic airflow. In this prospective study, patients underwent prethyroplasty and postthyroplasty pulmonary function testing. Flow volume loops combined with traditional spirometry were used. Postoperative pulmonary function tests were performed at least 2 months after surgery to allow resolution of surgical edema. Our study results support the previous finding that vocal cord paralysis alone causes some degree of extrathoracic obstruction. However, in contrast to Teflon injection, thyroplasty decreased extrathoracic airflow in all but 1 patient, and by criteria based on the ratio of the midexpiratory flow to the midinspiratory flow, caused new postoperative extrathoracic obstruction in 27% of patients. Symptomatic evidence of this obstruction may be more evident in those active patients with more ventilatory demand.

Renal sulfate secretion is carbonic anhydrase dependent in a marine teleost, Pleuronectes americanus

Though chemical assays indicate that carbonic anhydrase (CA) activity is present in marine teleost nephrons, CA inhibitors have no effect on urine pH or bicarbonate excretion, parameters typically CA dependent in almost all vertebrate groups. Because marine teleost renal sulfate secretion is associated with bicarbonate anion exchange, we investigated the effect of CA inhibition on transepithelial sulfate transport by flounder renal tubule primary monolayer cultures (PTC) and on renal sulfate secretion (QSO4) by intact flounder. Both methazolamide and ethoxzolamide (10 microM) inhibited PTC secretory flux by approximately 50%; reabsorptive sulfate flux, Na-dependent glucose transport, and transepithelial electrical resistance were unaffected. A CA inhibitor restricted to the extracellular space (10 microM polyoxyethylene-aminobenzolamide, 3.7 kDa) had no effect on PTC sulfate transport. Intravenous administration of methazolamide reduced QSO4 almost 40% and had no effect on glomerular filtration rate (GFR), urine flow rate, or Pi excretion rate. Serum pH was significantly reduced 0.2 units, whereas urine pH was unchanged. Together, the in vitro and in vivo results indicate that CA facilitates renal sulfate secretion in the seawater teleost.

[Recommendations for sports in chronic obstructive pneumopathy (COPD)]

Although exercise tolerance can be quite limited in patients with COPD due to dyspnea, arterial desaturation, chronic disuse muscular atrophy, and fear of distressing symptoms, regular daily exercise is an important part of optimal disease management. Patients can be provided with simple exercise instructions tailored to their physiological impairment and recreational interests. The benefits of regular exercise observed even in the most severely obstructed patients include desensitization to dyspnea, increased exercise tolerance, improved capacity for daily activities, greater self esteem and independence, and reduced rates of hospitalization. These benefits of exercise training in COPD are best realized within a multidisciplinary program of supervised pulmonary rehabilitation, that also incorporates education about COPD, proper use of medications and oxygen, and psychosocial counseling.

Red-blood-cell augmentation of hypoxic pulmonary vasoconstriction: hematocrit dependence and the importance of nitric oxide

Red blood cells (RBCs) are known to augment hypoxic pulmonary vasoconstriction (HPV). To determine whether this phenomenon is hematocrit (Hct) dependent and related to alterations of either nitric oxide (NO) or adenosine metabolism, we studied mechanically ventilated, pump-perfused lungs from euthanized New Zealand White rabbits. Lungs were perfused in situ in a recirculating manner at constant flow; perfusates consisted of Krebs-Henseleit buffer or buffer plus washed RBCs at a Hct of 10% or 30%. HPV was quantitated as the increase in pulmonary artery pressure (Ppa) from baseline after 5 min of hypoxia. In three experimental sets, we studied the effects of Hct on HPV and expired NO, the effects of nitric oxide synthase (NOS) inhibition, and the effects of adenosine receptor blockade. HPV was greater at a higher Hct, and expired NO varied inversely with Hct and decreased with hypoxia. NOS inhibition eliminated RBC-dependence of HPV. Adenosine-receptor blockade did not affect the RBC-dependence of HPV. We conclude that HPV is dependent on Hct, and that this phenomenon may be related to scavenging of NO but not adenosine by RBCs.

Culture of human main pancreatic duct epithelial cells

Attempts to grow human pancreatic duct epithelial cells in long-term culture have proven difficult. We have developed a system of growing these cells for several passages by adapting methods used to culture dog pancreatic duct cells. Epithelial cells were enzymatically dissociated from the main pancreatic duct and plated onto collagen-coated culture inserts suspended above a human fibroblast feeder layer. After primary culture, the cells were either passaged onto new inserts or plastic tissue culture plates in the absence of collagen. Cells grown on the latter plates were maintained in a serum-free medium. Primary pancreatic duct epithelial cells grow steadily to confluence as a monolayer in the feeder layer system. After primary culture, cells passaged onto new inserts with fresh feeder layer or plastic plates and fed with serum-free medium continued to develop into confluent monolayers for up to four passages. The cells were columnar with prominent apical microvilli, sub-apical secretory vesicles, and lateral intercellular junctions resembling the morphology of normal in vivo epithelial cells. These cells were also positive for cytokeratin 19, 7, and 8 and carbonic anhydrase II, as measured by immunohistochemistry. Metabolically, these cells synthesized and secreted mucin, as measured by incorporation of tritiated N-acetyl-D-glucosamine. In conclusion, we demonstrated that human pancreatic epithelial cells from the main duct can be successfully grown in culture and repeatedly passaged using a feeder layer system, with serum-free medium, and in organotypic cultures.

Lung diffusing capacity and exercise in subjects with previous high altitude pulmonary oedema

Subjects with a history of high-altitude pulmonary oedema (HAPE) have increased pulmonary artery pressure and more ventilation-perfusion (V'A/Q') inhomogeneity with hypoxia and exercise. We used noninvasive methods to determine whether there are differences in the pulmonary diffusing capacity for carbon monoxide (DL,CO) and cardiac output (Q') during exercise, indicative of a more restricted pulmonary vascular bed in subjects with a history of HAPE. Eight subjects with radiographically documented HAPE and five controls with good altitude tolerance had standard pulmonary function testing and were studied during exercise at 30 and 50% of normoxic maximal oxygen consumption (V'O2) at an inspiratory oxygen fraction of 0.14 and 0.21. DL,CO and Q' were measured by CO and acetylene rebreathing techniques. HAPE-resistant subjects had 35% greater functional residual capacity than HAPE-susceptible subjects. Vital capacity and total lung capacity were also 7-10% greater. There were no differences in airflow rates or resting diffusing capacity. However, DL,CO in HAPE-susceptible subjects was lower in hypoxia and with exercise, and showed less increase (32 versus 49%) with the combined stimulus of hypoxic exercise. HAPE-susceptible subjects had smaller increases in stroke volume, Q', and ventilation during exercise. The findings are consistent with lower pulmonary vasoconstriction, greater vascular capacitance and greater ventilatory responsiveness during exercise in subjects who are resistant to high-altitude pulmonary oedema. Their larger lung volumes suggest a constitutional difference in pulmonary parenchyma or vasculature, which may be a determinant of high-altitude pulmonary oedema resistance.

Lung diffusing capacity and exercise in subjects with previous high altitude pulmonary oedema

Subjects with a history of high-altitude pulmonary oedema (HAPE) have increased pulmonary artery pressure and more ventilation-perfusion (V'A/Q') inhomogeneity with hypoxia and exercise. We used noninvasive methods to determine whether there are differences in the pulmonary diffusing capacity for carbon monoxide (DL,CO) and cardiac output (Q') during exercise, indicative of a more restricted pulmonary vascular bed in subjects with a history of HAPE. Eight subjects with radiographically documented HAPE and five controls with good altitude tolerance had standard pulmonary function testing and were studied during exercise at 30 and 50% of normoxic maximal oxygen consumption (V'O2) at an inspiratory oxygen fraction of 0.14 and 0.21. DL,CO and Q' were measured by CO and acetylene rebreathing techniques. HAPE-resistant subjects had 35% greater functional residual capacity than HAPE-susceptible subjects. Vital capacity and total lung capacity were also 7-10% greater. There were no differences in airflow rates or resting diffusing capacity. However, DL,CO in HAPE-susceptible subjects was lower in hypoxia and with exercise, and showed less increase (32 versus 49%) with the combined stimulus of hypoxic exercise. HAPE-susceptible subjects had smaller increases in stroke volume, Q', and ventilation during exercise. The findings are consistent with lower pulmonary vasoconstriction, greater vascular capacitance and greater ventilatory responsiveness during exercise in subjects who are resistant to high-altitude pulmonary oedema. Their larger lung volumes suggest a constitutional difference in pulmonary parenchyma or vasculature, which may be a determinant of high-altitude pulmonary oedema resistance.

Ventilation heterogeneity is increased in hypocapnic dogs but not pigs

Hypocapnia increases ventilation/perfusion (VA/Q) heterogeneity in dogs, possibly by adversely affecting distribution of ventilation through its effects on collateral ventilation. Because pigs lack collateral ventilation, we compared the effects of hypocapnia on ventilation heterogeneity in pentobarbital-anesthetized, mechanically-ventilated dogs and pigs. Simultaneous multiple breath washouts of helium and nitrogen were used to assess the uniformity of the ventilation distribution by the phase III (SnIII) method. Ventilation heterogeneity was partitioned into two components, e.g. convective-dependent inhomogeneity (cdi) and diffusive-convective-dependent inhomogeneity (dcdi). Pulmonary gas exchange was also measured in pigs by the multiple inert gas elimination technique. Ventilation heterogeneity was increased (P < 0.01) in hypocapnic dogs. Inspiration of CO2 decreased ventilation heterogeneity by decreasing dcdi (P < 0.01). In contrast, ventilation heterogeneity was not increased in hypocapnic pigs. However, hypocapnia increased VA/Q heterogeneity by 18% (P < 0.05) in pigs. We conclude that hypocapnia increases ventilation heterogeneity in dogs but not in pigs, most likely related to an interspecies difference in collateral ventilation.

CO2 transport in normovolemic anemia: complete compensation and stability of blood CO2 tensions

Isovolemic hemodilution does not appear to impair CO2 elimination nor cause CO2 retention despite the important role of red blood cells in blood CO2 transport. We studied this phenomenon and its physiological basis in eight New Zealand White rabbits that were anesthetized, paralyzed, and mechanically ventilated at a fixed minute ventilation. Isovolemic anemia was induced by simultaneous blood withdrawal and infusion of 6% hetastarch in sequential stages; exchange transfusions ranged from 15-30 ml in volume. Variables measured after each hemodilution included hematocrit (Hct), arterial and venous blood gases, mixed expired PCO2 and PO2, and blood pressure; also, O2 consumption, CO2 production, cardiac output (Q), and physiological dead space were calculated. Data were analyzed by comparison of changes in variables with changes in Hct and by using the model of capillary gas exchange described by Bidani (J. Appl. Physiol. 70: 1686-1699, 1991). There was complete compensation for anemia with stability of venous and arterial PCO2 between Hct values of 36 +/- 3 and 12 +/- 1%, which was predicted by the mathematical model. Over this range of hemodilution, Q rose 50%, and the O2 extraction ratio increased 61% without a decline in CO2 production or a rise in alveolar ventilation. The dominant compensations maintaining CO2 transport in normovolemic anemia include an increased Q and an augmented Haldane effect arising from the accompanying greater O2 extraction.

Acute mountain sickness is not altered by a high carbohydrate diet nor associated with elevated circulating cytokines

We investigated whether a diet of increased carbohydrate content reduces the symptoms of acute mountain sickness (AMS) and whether concentrations of circulating cytokines rise and correlate with hypoxia and AMS. There were 19 healthy volunteers who ingested in randomized order both a high carbohydrate (68% CHO) or normal carbohydrate (45% CHO) diet for 4 d. On the 4th d, subjects were exposed to 8 h of 10% normobaric oxygen. Each subject completed the Lake Louise Consensus Questionnaire (LLCQ: a questionnaire developed to quantify the common symptoms and consequences of AMS) at the beginning and end of each hypoxic session, at which times venous blood was obtained for the following cytokines: interleukins 1 beta, 6 and 8 (IL-1 beta, IL-6, IL-8) and tumor necrosis factor alpha (TNF-alpha). AMS symptoms did not differ significantly between the diets (LLCQ scores: 68% CHO = 10.1 +/- 3.8 vs. 45% CHO = 10.3 +/- 4.1). Cytokine concentrations did not change with hypoxia on either diet, nor did individual changes correlate with AMS symptoms. We conclude that a high carbohydrate diet for 4 d does not reduce the symptoms of AMS; and plasma cytokine concentrations do not change with hypoxia and the development of AMS and, thus, are not likely mediators of this syndrome.

Dog pancreatic duct epithelial cells: long-term culture and characterization

Epithelial cells, isolated from a normal dog pancreatic duct, were grown on collagen-coated culture inserts suspended above a feeder layer of myofibroblasts. The cells were examined by transmission electron microscopy, immunohistochemistry, cytogenetics, and flow cytometry. In addition, the constitutive and agonist-stimulated mucin secretion of these cells was studied using a [3H]N-acetyl-D-glucosamine labeling assay, and the stimulation of intracellular cAMP was measured. Cells grown on inserts with a feeder layer developed into confluent monolayers consisting of strictly polarized columnar epithelial cells with prominent microvilli, intercellular junctions, and normal chromosomal characteristics. They could be passaged repeatedly without a detectable alteration in their morphology. The cells could also be grown on organotypic cultures, resulting in further differentiated cells simulating in vivo morphology. Immunohistochemistry demonstrated the presence of carbonic anhydrase II in these cells. Cells treated with vasoactive intestinal peptide, epinephrine, and dibutyryl-cAMP demonstrated a marked increase in mucin secretion compared with controls. In parallel experiments, VIP and epinephrine significantly increased intracellular cAMP. In conclusion we have developed a pancreatic epithelial cell preparation with morphology, cytokinetics, chromosomal, and DNA analyses characteristic of normal cells. Similar to normal columnar epithelial cells, these pancreatic duct cells secreted mucin constitutively and responded to agonist by increasing secretion via a cAMP-mediated pathway. They also contained carbonic anhydrase, which indicates that the cells are capable of secreting bicarbonate.

Hypocapnia-induced ventilation/perfusion mismatch: a direct CO2 or pH-mediated effect?

The purpose of this study was to determine whether the increased ventilation/perfusion (VA/Q) mismatch caused by hypocapnic hyperventilation in dogs (J. Appl. Physiol. 1993; 74:1306-1314) is a direct CO2 or a pH-mediated effect. From an initial state of hyperventilated respiratory alkalosis (FIO2 = 0.21, VT = 18 ml/kg, RR = 35), we studied the changes in VA/Q distributions, respiratory gas exchange, and hemodynamics when the acid-base status of the dogs was manipulated by combinations of acid or alkali infusion with or without CO2 inhalation. In this manner, we studied respiratory alkalosis (high pH, low PCO2), normalized acid-base status (normal pH, normal PCO2), metabolic acidosis (low pH, normal PCO2), metabolic alkalosis (high pH, normal PCO2), and a mixed respiratory alkalosis and metabolic acidosis (normal pH, low PCO2). Gas exchange was evaluated using the multiple inert gas elimination technique. PaO2 was reduced and VA/Q heterogeneity was increased in all conditions defined by a high pH, independent of the PCO2 (respiratory alkalosis and metabolic alkalosis). In contrast, PaO2 and VA/Q heterogeneity was unchanged in conditions defined by either a normal or low pH (normalized acid-base status, mixed respiratory alkalosis and metabolic acidosis, and metabolic acidosis). Therefore, we conclude that hypocapnia-induced VA/Q mismatch in hyperventilated dogs is pH-mediated and is not a function of PCO2 per se.

Tracheal gas exchange: perfusion-related differences in inert gas elimination

Exchange of inert gases across the conducting airways has been demonstrated by using an isolated dog tracheal preparation and has been characterized by using a mathematical model (E. R. Swenson, H. T. Robertson, N. L. Polissar, M. E. Middaugh, and M. P. Hlastala, J. Appl. Physiol. 72: 1581-1588, 1992). Theory predicts that gas exchange is both diffusion and perfusion dependent, with gases with a higher blood-gas partition coefficient exchanging more efficiently. The present study evaluated the perfusion dependence of airway gas exchange in an in situ canine tracheal preparation. Eight dogs were studied under general anesthesia with the same isolated tracheal preparation. Tracheal perfusion (Q) was altered from control blood flow (Qo) by epinephrine or papaverine instilled into the trachea and was measured with fluorescent microspheres. Six inert gases of differing blood-gas partition coefficients were used to measure inert gas elimination. Gas exchange was quantified as excretion (E), equal to exhaled partial pressure divided by arterial partial pressure. Data were plotted as ln [E/(l-E)] vs. In (Q/Qo), and the slopes were determined by least squares. Excretion was a positive function of Q, and the magnitude of the response of each gas to changes in Q was similar and highly significant (P < or = 0.0002). These results confirm a substantial perfusion dependence of airway gas exchange.

Acetazolamide slows VA/Q matching after changes in regional blood flow

Inhibition of carbonic anhydrase (CA) by acetazolamide increases ventilation-perfusion (VA/Q) heterogeneity (E. R. Swenson, H. T. Robertson, and M. P. Hlastala. J. Clin. Invest. 92: 702-709, 1993), possibly because of slowing of CO2/H(+)-dependent mechanisms of VA/Q matching with temporal fluctuations of regional ventilation and perfusion. To study this concept, we imposed abrupt changes in regional perfusion by lobar or left main pulmonary artery occlusions (PAOs) in anesthetized mechanically ventilated dogs before and after CA inhibition (20 mg/kg iv acetazolamide). The rate of ventilation redistribution and change in VA/Q distributions with changes in perfusion were measured by planar gamma imaging of the lungs during continuous inhalation of 81mKr gas ventilation scanning and the multiple inert-gas elimination technique. PAO for 5 min caused regional Kr activity to fall by 30 +/- 5% (SD) with a half time (t1/2) of 75 +/- 10 s. With release of the occlusion, counts returned to baseline with t1/2 of 79 +/- 12 s. Acetazolamide increased these respective t1/2 values (161 +/- 16 and 180 +/- 17 s). Consistent with these kinetics, VA/Q mismatch was greater with lobar PAO at 2 min but not at 10 min with CA inhibition compared with that caused by lobar PAO alone. Cyclical lobar PAO and release (10 cycles of 1-min occlusion and 1-min release) caused more VA/Q heterogeneity during CA inhibition. The arterial-to-alveolar inert-gas area difference rose minimally from 0.18 to 0.23 (P < 0.05) with cyclical PAO and from 0.24 to 0.48 (P < 0.01) after CA inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Diuretic effect of acute hypoxia in humans: relationship to hypoxic ventilatory responsiveness and renal hormones

Acute hypoxia causes increased sodium and water excretion. Animal studies suggest that this renal response is largely driven by direct peripheral arterial chemoreceptor stimulation, independent of accompanying changes in ventilation and acid-base status. Whether the diuresis and natriuresis observed in humans made acutely hypoxic are caused by peripheral chemoreceptor stimulation is not known, but, if so, we hypothesized that people with a high ventilatory response to hypoxia (high peripheral chemosensitivity) should have greater diuresis and natriuresis than those with a low ventilatory response to hypoxia. The isocapnic hypoxic ventilatory response (HVR) of 16 subjects on a fixed sodium intake was measured, as were their urinary volume and sodium and bicarbonate losses during 6 h of breathing air (in a normobaric environmental chamber) and, on the subsequent day, 12% O2. The isocapnic HVR correlated positively with hypoxic diuresis (r = 0.87) and natriuresis (r = 0.76). In contrast, the isocapnic HVR did not correlate with bicarbonate excretion, despite the expected respiratory alkalosis of acute hypoxia. The magnitude of diuresis and natriuresis with hypoxia did not correlate with changes in circulating aldosterone, renin, atrial natriuretic peptide, vasopressin, or a digoxin-like immunoreactive substance. These findings are compatible with a role of the peripheral arterial chemoreceptors in mediating the renal response to hypoxia in humans. The efferent pathway remains unknown.

Physiological and immunocytochemical evidence for a putative H-K-ATPase in elasmobranch renal acid secretion

The mechanism of renal acid secretion in marine fish is largely unknown. We explored whether H(+)-K(+)-adenosinetriphosphatase (H(+)-K(+)-ATPase) is present and functional in acid secretion in the kidney of the elasmobranch spiny dogfish shark, Squalus acanthias. In whole animal studies, a specific inhibitor of mammalian H(+)-K(+)-ATPase, Sch-28080, abolished greater than 87% of basal (62 mg/kg) and 75% of imidazole-stimulated titratable acid excretion (5 and 62 mg/kg). Antibodies directed against the COOH-terminus hog gastric H(+)-K(+)-ATPase alpha-subunit stained specific subdivisions of the neck, early and late proximal tubule, late intermediate tubule, both segments of the distal tubule, and the early collecting duct of the renal tubule of these fish. These findings are consistent with a major role for a protein similar to the mammalian gastric H(+)-K(+)-ATPase in elasmobranch renal acid secretion.

Comparison of the effect of inhaled selective and non-selective adrenergic agonists on cardiorespiratory parameters in chronic stable asthma

Increased hypoxia has been found after beta 2 adrenoceptor agonists (but not adrenaline) in asthmatics. Combined with hypokalaemia and sympathomimetic stimulation, this may predispose to cardiac arrhythmias. We have compared the effects of nebulized adrenaline and a selective beta 2 agonist (salbutamol) on the arterial oxygen saturation (SaO2), minute ventilation (VE), forced expiratory volume in 1 s (FEV1), plasma potassium and the electrocardiogram (ECG) in patients with chronic stable asthma. Six patients were studied according to a randomized, placebo-controlled, double-blind cross-over protocol. Adrenaline (5 mg), salbutamol (5 mg) and placebo were administered during 4 min tidal breathing using a nebulizer driven by air. There was a fall in SaO2 after both adrenaline (mean % fall (SEM) 3.3 (0.2)) and salbutamol (4.0 (0.7)) associated with an increase in FEV1, with no change in VE. Therefore, the fall in SaO2 must have been caused by increased ventilation-perfusion imbalance. There was an increased heart rate after both adrenaline and salbutamol and ventricular ectopic beats and a short run of parasystole were recorded on the ECG in one patient after adrenaline and in two patients after salbutamol. No change was found in plasma potassium levels. We conclude that both adrenaline and a selective beta 2 agonist salbutamol can cause a fall in SaO2 and ventricular ectopy in some asthmatic patients.

Effects of inspired carbon dioxide on ventilation-perfusion matching in normoxia, hypoxia, and hyperoxia

We studied the effect of low concentrations (2 to 4%) of inspired CO2 on gas exchange and ventilation-perfusion (VA/Q) relationships in healthy normocapnic anesthetized dogs during constant mechanical ventilation by the multiple inert gas elimination technique (MIGET). One group was studied at normal tidal volumes (12 to 14 ml/kg) and rates (13 to 15/min) in normoxia, and the other in mild hyperoxia (FIO2 = 0.50) and hypoxia (FIO2 = 0.15). In normoxic dogs there were progressive increases in arterial PO2 and reductions in the alveolar-to-arterial PO2 and arterial-to-mixed expired PCO2 differences in response to increases in FICO2. This increased gas exchange efficiency was characterized by reductions in both dead space ventilation and VA/Q mismatch. Better VA/Q matching was characterized by reduction in the log standard deviation of ventilation (log SDV) without significant change in the log standard deviation of perfusion (log SDQ). Gas exchange parameters returned to baseline when dogs were returned to CO2-free inspired gas. In the second group, the effects of 3% inspired CO2 were of comparable magnitude in both mild hypoxia and hyperoxia. In this group (taking hyperoxic values as baseline), there were improved gas exchange and less VA/Q heterogeneity with inspired hypoxia, both with and without inspired CO2. In contrast to the effects of added inspired CO2, improved VA/Q matching with hypoxia was characterized by reductions in both log SDV and log SDQ.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and characterization of human neutrophil carbonic anhydrase

Many functions of polymorphonuclear leukocytes (PMNs) appear to alter and be affected by changes in the intracellular and/or extracellular acid-base milieu, suggesting that carbonic anhydrase (CA) may be important. Although small amounts of CA activity in PMNs have been reported, it has not been characterized fully. We therefore studied isolated mature circulating human PMNs and cultured HL-60 cells, an undifferentiated myelopoietic cell line, and compared these to human red cells (RBCs) for CA activity. Activity and sulfonamide inhibition were measured by a modified micromethod assay. Isoenzyme and total CA concentrations were determined by radioimmunoassay for human isozyme CA I, differential inhibition by MK-927, inhibition by 0.2% sodium dodecyl sulfate (SDS), and quantitative sulfonamide binding. Total CA activity (units/10(6) cells) was 0.04 in PMNs, 0.06 in HL-60 cells, and 0.62 in RBCs. Human PMNs have a total CA concentration of 1.3 microM, of which 0.9 microM is CA I and the remainder is CA II. Total loss of CA activity with 100 microM ethoxzolamide and 0.2% SDS ruled out significant CA III or CA IV activity. Subcellular fractionation of PMNs revealed that all CA activity was cytosolic. The absence of CA activity in mitochondrial and microsomal membrane fractions argues against any mitochondrial CA V or membrane-bound CA IV contribution to total CA activity. Neutrophils contain both CA I and II isozymes in roughly the same proportion as RBCs but at much lower concentrations, suggesting that in the course of maturation the CA content of neutrophils is regulated differently from that in erythrocytes.