Protective effects of intravascular pressure and nitric oxide in ischemic lung injury

Cessation of bloodflow during ischemia will decrease both distending and shearforces exerted on endothelium and may worsen ischemic lung injury bydecreasing production of nitric oxide (NO), which influences vascularbarrier function. We hypothesized that increased intravascular pressure(Piv) during ventilated ischemia might maintain NO productionby increasing endothelial stretch or shear forces, thereby attenuatingischemic lung injury. Injury was assessed by measuring the filtrationcoefficient(K_f) and theosmotic reflection coefficient for albumin(_alb) after 3 h of ventilated(95% O₂-5%CO₂; expiratory pressure 3 mmHg) ischemia. Lungs were flushed with physiological salt solution, and then Piv was adjusted to achieve High Piv (mean 6.7 ± 0.4 mmHg, n = 15) or Low Piv (mean0.83 ± 0.4 mmHg, n = 10).N^G-nitro-L-arginine methyl ester(L-NAME;10⁵ M,n = 10),N^G-nitro-D-argininemethyl ester (D-NAME;10⁵ M,n = 11), orL-NAME(10⁵M)+L-arginine (5 × 10⁴ M,n = 6) was added at the start ofischemia in three additional groups of lungs with High Piv.High Piv attenuated ischemic injury compared with Low Piv(_alb 0.67 ± 0.04 vs. 0.35 ± 0.04, P < 0.05). Theprotective effect of High Piv was abolished byL-NAME(_alb 0.37 ± 0.04, P < 0.05) but not byD-NAME(_alb 0.63 ± 0.07). The effects of L-NAME were overcomeby an excess of L-arginine(_alb 0.56 ± 0.05, P < 0.05).K_f did not differsignificantly among groups. These results suggest that Piv modulatesischemia-induced barrier dysfunction in the lung, and theseeffects may be mediated by NO.

     

Nitric oxide regulates oxygen uptake and hydrogen peroxide release by the isolated beating rat heart

Isolated rat heart perfused with 1.5-7.5µM NO solutions or bradykinin, which activates endothelial NOsynthase, showed a dose-dependent decrease in myocardial O₂uptake from 3.2 ± 0.3 to 1.6 ± 0.1 (7.5 µM NO, n = 18,P < 0.05) and to 1.2 ± 0.1 µM O₂ · min¹ · gtissue¹ (10 µM bradykinin, n = 10,P < 0.05). Perfused NO concentrations correlated with aninduced release of hydrogen peroxide (H₂O₂) inthe effluent (r = 0.99, P < 0.01). NO markedlydecreased the O₂ uptake of isolated rat heart mitochondria(50% inhibition at 0.4 µM NO, r = 0.99,P < 0.001). Cytochrome spectra in NO-treated submitochondrial particles showed a double inhibition of electron transfer at cytochrome oxidase and between cytochrome b andcytochrome c, which accounts for the effects in O₂uptake and H₂O₂ release. Most NO was bound tomyoglobin; this fact is consistent with NO steady-state concentrationsof 0.1-0.3 µM, which affect mitochondria. In the intact heart,finely adjusted NO concentrations regulate mitochondrial O₂uptake and superoxide anion production (reflected byH₂O₂), which in turn contributes to thephysiological clearance of NO through peroxynitrite formation.

     

Isolated mouse pancreatic beta-cells show cell-specific temporal response pattern

The length of the silent lag time beforeelevation of the cytosolic free Ca²⁺ concentration([Ca²⁺]_i) differs between individualpancreatic -cells. One important question is whether thesedifferences reflect a random phenomenon or whether the length of lagtime is inherent in the individual -cell. We compared the lag times,initial dips, and initial peak heights for[Ca²⁺]_i from two consecutive glucosestimulations (with either 10 or 20 mM glucose) in individualob/ob mouse -cells with the fura 2 technique in amicrofluorimetric system. There was a strong correlation between thelengths of the lag times in each -cell (10 mM glucose:r = 0.94, P < 0.001; 20 mM glucose:r = 0.96, P < 0.001) as well as between theinitial dips in [Ca²⁺]_i (10 mM glucose:r = 0.93, P < 0.001; 20 mM glucose:r = 0.79, P < 0.001) and between theinitial peak heights (10 mM glucose: r = 0.51, P < 0.01; 20 mM glucose: r = 0.77, P < 0.001). These data provide evidence that theresponse pattern, including both the length of the lag time and thedynamics of the subsequent [Ca²⁺]_i, isspecific for the individual -cell.

     

Influence of group B streptococci on piglet pulmonary artery response to bradykinin

To study whether a sepsis-induced increase indes-Arg⁹-bradykinin(des-Arg⁹-BK) and bradykinin (BK)B₁-receptor activity participatesin the observed increase in pulmonary vascular resistance in neonatal group B streptococcal sepsis (GBS), isometric force bioassays ofpulmonary artery (PA) rings were studied, after 4-h exposure to eitherKrebs or GBS, by using the following protocols:1) BK dose-response curve,2) vascular response to BK withN^G-nitro-L-arginine methyl ester(L-NAME), and3) response todes-Arg⁹-BK (BK metabolite andB₁ agonist). PA rings exposed toBK resulted in contraction in the GBS group and a decrease in restingtension in the Control group (P = 0.034) at a concentration of10⁵ M. GBS-treated PA ringscontracted more to des-Arg⁹-BKthan did Controls (P < 0.001). BK(10⁶ M) relaxedpreconstricted PA rings incubated in GBS less than BK relaxed Controls(P < 0.001), and preincubation withL-NAME decreased relaxation inboth. These results suggest that GBS decreased endothelium-dependent BKrelaxation and increased contractile response todes-Arg⁹-BK. We speculate thatthis occurs secondary to upregulation of B₁ receptors reflected byB₁-agonist-mediated PA contraction.

     

Hypertonic perfusion inhibits intracellular Na and Ca accumulation in hypoxic myocardium

Muchevidence supports the view that hypoxic/ischemic injury is largely dueto increased intracellular Ca concentration([Ca]_i) resulting from 1) decreasedintracellular pH (pH_i), 2) stimulated Na/H exchangethat increases Na uptake and thus intracellular Na (Na_i),and 3) decreased Na gradient that decreases or reverses net Catransport via Na/Ca exchange. The Na/H exchanger (NHE) is alsostimulated by hypertonic solutions; however, hypertonic media mayinhibit NHE's response to changes in pH_i (Cala PM and Maldonado HM. J Gen Physiol 103: 1035-1054, 1994). Thus wetested the hypothesis that hypertonic perfusion attenuates acid-induced increases in Na_i in myocardium and, thereby, decreasesCa_i accumulation during hypoxia. Rabbit hearts wereLangendorff perfused with HEPES-buffered Krebs-Henseleit solutionequilibrated with 100% O₂ or 100% N₂. Hypertonic perfusion began 5 min before hypoxia or normoxicacidification (NH₄Cl washout). Na_i,[Ca]_i, pH_i, and high-energyphosphates were measured by NMR. Control solutions were 295 mosM, andhypertonic solutions were adjusted to 305, 325, or 345 mosM by additionof NaCl or sucrose. During 60 min of hypoxia (295 mosM),Na_i rose from 22 ± 1 to 100 ± 10 meq/kg dry wt while[Ca]_i rose from 347 ± 11 to 1,306 ± 89 nM.During hypertonic hypoxic perfusion (325 mosM), increases inNa_i and [Ca]_i were reduced by 65 and 60%, respectively (P < 0.05). Hypertonicperfusion also diminished Na uptake after normoxic acidification by87% (P < 0.05). The data are consistent with the hypothesisthat mild hypertonic perfusion diminishes acid-induced Na accumulationand, thereby, decreases Na/Ca exchange-mediated Ca_iaccumulation during hypoxia.

     

Small bowel tonometry is more accurate than gastric tonometry in detecting gut ischemia

Gastric tonometerPCO₂ measurement may help identifygut ischemia in critically ill patients but is frequentlyassociated with large measurement errors. We tested the hypothesis thatsmall bowel tonometer PCO₂measurement yields more accurate information. In 10 anesthetized,mechanically ventilated pigs subject to progressive hemorrhage, wemeasured gut oxygen delivery and consumption. We also measuredtonometer PCO₂ minus arterialPCO₂(PCO₂) and calculated the corresponding intracellular pH from tonometers placed in the stomach and jejunum. We found that the correlation coefficient(r²) forbiphasic gut oxygen delivery-PCO₂relationships was 0.29 ± 0.52 for the gastric tonometer vs. 0.76 ± 0.25 for the small bowel tonometer(P < 0.05). In addition, thecritical gastric tonometer PCO₂was excessively high and variable (62.9 ± 39.6) compared with thecritical small bowel tonometerPCO₂ (17.0 ± 15.0, P < 0.01). Small bowel tonometerPCO₂ was closely correlated withsuperior mesenteric vein PCO₂(r² = 0.81, P < 0.001), whereas gastrictonometer PCO₂ was not(r² = 0.13, P = not significant). Weconclude that measurement of gastric tonometerPCO₂ yields excessively noisy andinaccurate data on the onset of gut anaerobic metabolism in hemorrhagicshock. Small bowel tonometer PCO₂ isless noisy and, as a result, is superior in detecting gut hypoperfusionand the onset of anaerobic metabolism.

     

Effects of N2O narcosis on breathing and effort sensations during exercise and inspiratory resistive loading

Fothergill, D. M., and N. A. Carlson. Effects ofN₂O narcosis on breathing andeffort sensations during exercise and inspiratory resistive loading.J. Appl. Physiol. 81(4):1562-1571, 1996.The influence of nitrous oxide(N₂O) narcosis on the responses toexercise and inspiratory resistive loading was studied in thirteen maleUS Navy divers. Each diver performed an incremental bicycle exercisetest at 1 ATA to volitional exhaustion while breathing a 23%N₂O gas mixture and a nonnarcoticgas of the same PO₂, density, andviscosity. The same gas mixtures were used during four subsequent30-min steady-state submaximal exercise trials in which the subjectsbreathed the mixtures both with and without an inspiratory resistance(5.5 vs. 1.1 cmH₂O ^· s ^· l¹at 1 l/s). Throughout each test, subjective ratings of respiratory effort (RE), leg exertion, and narcosis were obtained with acategory-ratio scale. The level of narcosis was rated between slightand moderate for the N₂O mixturebut showed great individual variation. Perceived leg exertion and thetime to exhaustion were not significantly different with the twobreathing mixtures. Heart rate was unaffected by the gas mixture andinspiratory resistance at rest and during steady-state exercise but wassignificantly lower with the N₂O mixture during incremental exercise (P < 0.05). Despite significant increases in inspiratory occlusionpressure (13%; P < 0.05),esophageal pressure (12%; P < 0.001), expired minute ventilation (4%;P < 0.01), and the work rate ofbreathing (15%; P < 0.001) when the subjects breathed the N₂O mixture,RE during both steady-state and incremental exercise was 25% lowerwith the narcotic gas than with the nonnarcotic mixture(P < 0.05). We conclude that the narcotic-mediated changes in ventilation, heart rate, and RE induced by23% N₂O are not of sufficientmagnitude to influence exercise tolerance at surface pressure.Furthermore, the load-compensating respiratory reflexes responsible formaintaining ventilation during resistive breathing are not depressed byN₂O narcosis.

     

Effect of ventilation on vascular permeability and cyclic nucleotide concentrations in ischemic sheep lungs

Ventilation during ischemia attenuatesischemia-reperfusion lung injury, but the mechanism is unknown.Increasing tissue cyclic nucleotide levels has been shown to attenuatelung ischemia-reperfusion injury. We hypothesized thatventilation prevented increased pulmonary vascular permeability duringischemia by increasing lung cyclic nucleotide concentrations.To test this hypothesis, we measured vascular permeability and cGMP andcAMP concentrations in ischemic (75 min) sheep lungs that wereventilated (12 ml/kg tidal volume) or statically inflated with the samepositive end-expiratory pressure (5 Torr). The reflection coefficientfor albumin (_alb) was 0.54 ± 0.07 and 0.74 ± 0.02 (SE) in nonventilated and ventilatedlungs, respectively (n = 5, P < 0.05). Filtration coefficientsand capillary blood gas tensions were not different. The effect ofventilation was not mediated by cyclic compression of alveolarcapillaries, because negative-pressure ventilation(n = 4) also was protective (_alb = 0.78 ± 0.09). Thefinal cGMP concentration was less in nonventilated than in ventilatedlungs (0.02 ± 0.02 and 0.49 ± 0.18 nmol/g blood-free dry wt,respectively, n = 5, P < 0.05). cAMP concentrations werenot different between groups or over time. Sodium nitroprussideincreased cGMP (1.97 ± 0.35 nmol/g blood-free dry wt) and_alb (0.81 ± 0.09) innonventilated lungs (n = 5, P < 0.05). Isoproterenol increasedcAMP in nonventilated lungs (n = 4, P < 0.05) but had no effect on_alb. The nitric oxide synthaseinhibitor N^G-nitro-L-arginine methylester had no effect on lung cGMP (n = 9) or _alb(n = 16) in ventilated lungs but didincrease pulmonary vascular resistance threefold(P < 0.05) in perfused sheep lungs (n = 3). These results suggest thatventilation during ischemia prevented an increase in pulmonaryvascular protein permeability, possibly through maintenance of lungcGMP by a nitric oxide-independent mechanism.

     

Depressed ventilatory response to hypoxia in hypothermic newborn piglets: role of glutamate

To evaluatewhether changes in extracellular glutamate (Glu) levels in the centralnervous system could explain the depressed hypoxic ventilatory responsein hypothermic neonates, 12 anesthetized, paralyzed, and mechanicallyventilated piglets <7 days old were studied. The Glu levels in thenucleus tractus solitarius obtained by microdialysis, minute phrenicoutput (MPO), O₂ consumption, arterial blood pressure, heart rate, and arterial blood gases weremeasured in room air and during 15 min of isocapnic hypoxia (inspiredO₂ fraction = 0.10) at braintemperatures of 39.0 ± 0.5°C [normothermia (NT)]and 35.0 ± 0.5°C [hypothermia (HT)]. During NT, MPO increased significantly during hypoxia and remained above baseline. However, during HT, there was a marked decrease in MPOduring hypoxia (NT vs. HT, P < 0.03). Glu levels increased significantly in hypoxia during NT;however, this increase was eliminated during HT(P < 0.02). A significant linearcorrelation was observed between the changes in MPO and Glu levelsduring hypoxia (r = 0.61, P < 0.0001). Changes in pH, arterialPO₂, O₂ consumption, arterial bloodpressure, and heart rate during hypoxia were not different between theNT and HT groups. These results suggest that the depressed ventilatoryresponse to hypoxia observed during HT is centrally mediated and inpart related to a decrease in Glu concentration in the nucleus tractussolitarius.

     

Cyclooxygenase inhibition potentiates the renal vascular response to endothelin-1 in humans

Vascular endothelin-receptor stimulation resultsin vasoconstriction and concomitant production of the vasodilatorsprostaglandin I₂ and nitric oxide.The vascular effects of cyclooxygenase (COx) blockade (diclofenacintravenously) and the subsequent vasoconstrictor response toendothelin-1 (ET-1) infusion 30 min after diclofenac were studied inhealthy men. With COx blockade, cardiac output (7%) and splanchnic(14%) and renal (12%) blood flows fell (all P < 0.001). Splanchnic blood flowreturned to basal value within 30 min. Mean arterial blood pressureincreased (4%, P < 0.001). Splanchnic glucose output fell (22%,P < 0.01). Subsequent ET-1 infusioncaused, compared with previous ET-1 infusion without COx blockade (G. Ahlborg, E. Weitzberg, and J. M. Lundberg. J. Appl.Physiol. 77: 121-126, 1994; E. Weitzberg, G. Ahlborg, and J. M. Lundberg. Biochem. Biophys. Res.Commun. 180: 1298-1303, 1991; E. Weitzberg, G. Ahlborg, and J. M. Lundberg. Clin.Physiol. (Colch.) 13: 653-662, 1993),the same increase in mean arterial blood pressure (4%), decreases incardiac output (13%) and splanchnic blood flow (38%), but nosignificant change in splanchnic glucose output. Renal blood flowreduction was potentiated (33 ± 3 vs. 23 ± 2%,P < 0.02), with a total reductioncorresponding to 43 ± 3%(P < 0.01 vs. 23 ± 3%). Weconclude that COx inhibition induces renal and splanchnicvasoconstriction. The selectively increased renal vascularresponsiveness to ET-1 emphasizes the importance of endogenousarachidonic acid metabolites (i.e., prostaglandin I₂) to counteract ET-1-mediatedrenal vasoconstriction.

     

Diaphragm thickening during inspiration

Cohn, David, Joshua O. Benditt, Scott Eveloff, and F. DennisMcCool. Diaphragm thickening during inspiration.J. Appl. Physiol. 83(1): 291-296, 1997.Ultrasound has been used to measure diaphragm thickness(T_di) in thearea where the diaphragm abuts the rib cage (zone of apposition).However, the degree of diaphragm thickening during inspiration reportedas obtained by one-dimensional M-mode ultrasound was greater than thatpredicted by using other radiographic techniques. Becausetwo-dimensional (2-D) ultrasound provides greater anatomic definitionof the diaphragm and neighboring structures, we used this technique toreevaluate the relationship between lung volume andT_di. We firstestablished the accuracy and reproducibility of 2-D ultrasound bymeasuring T_diwith a 7.5-MHz transducer in 26 cadavers. We found thatT_di measured byultrasound correlated significantly with that measured by ruler (R² = 0.89), withthe slope of this relationship approximating a line of identity(y = 0.89x + 0.04 mm). The relationship between lung volume andT_di was thenstudied in nine subjects by obtaining diaphragm images at the fivetarget lung volumes [25% increments from residual volume (RV) tototal lung capacity (TLC)]. Plots ofT_di vs. lungvolume demonstrated that the diaphragm thickened as lung volumeincreased, with a more rapid rate of thickening at the higher lungvolumes[T_di = 1.74 vital capacity (VC)² + 0.26 VC + 2.7 mm] (R² = 0.99; P < 0.001) where lung volumeis expressed as a fraction of VC. The mean increase inT_di between RVand TLC for the group was 54% (range 42-78%). We conclude that2-D ultrasound can accurately measureT_di and that theaverage thickening of the diaphragm when a subject is inhaling from RVto TLC using this technique is in the range of what would be predictedfrom a 35% shortening of the diaphragm.

     

Regional clearance of solute from peripheral airway epithelia: recovery after sublobar exposure to ozone

The influence oflocal exposure to ozone (O₃) onrespiratory epithelial permeability of sublobar lung segments wasstudied by using aerosolized^99mTc-diethylenetriaminepentaacetic acid (DTPA; mol wt, 492). Two bronchoscopes were insertedthrough an endotracheal tube in anesthetized, mechanically ventilated,mixed breed dogs and were wedged into sublobar bronchi located in theright and left lower lobes, respectively. Segments were ventilated viathe bronchoscope with 5% CO₂ inair delivered at 200 ml/min, and an aerosol of^99mTc-DTPA was generated anddelivered through the scope and into the sublobar segment over a 30-speriod. Clearance of ^99mTc-DTPAwas measured simultaneously from right and left lower lung segments atbaseline and 1, 7, and 14 days after a 6-h sublobar exposure tofiltered air or 400 parts per billionO₃.O₃ treatment significantlydecreased the clearance halftime(t₅₀) of^99mTc-DTPA by 50% from thebaseline mean of 32.3 to 16.0 min at 1 day postexposure. After 7 daysof recovery, t₅₀was still reduced by 28.8%; however, by 14 days postexposure,clearance of ^99mTc-DTPA hadrecovered, and thet₅₀ had a meanvalue of 30.0 min. ^99mTc-DTPAclearance was not altered by exposure to filtered air, andt₅₀ values werecomparable to baseline at 1, 7, and 14 days postexposure. These resultsreveal that a single local exposure toO₃ increases transepithelialclearance, but only for epithelia directly exposed toO₃, and that 7-14 days ofrecovery are required before permeability to small-molecular-weightsolutes returns to normal.     

Multiple targets of chemosensitive signaling in locus coeruleus neurons: role of K+ and Ca2+ channels

Westudied chemosensitive signaling in locus coeruleus (LC) neurons usingboth perforated and whole cell patch techniques. Upon inhibition offast Na⁺ spikes by tetrodotoxin (TTX), hypercapnic acidosis[HA; 15% CO₂, extracellular pH (pH_o) 6.8]induced small, slow spikes. These spikes were inhibited byCo²⁺ or nifedipine and were attributed to activation ofL-type Ca²⁺ channels by HA. Upon inhibition of bothNa⁺ and Ca²⁺ spikes, HA resulted in a membranedepolarization of 3.52 ± 0.61 mV (n = 17) thatwas reduced by tetraethylammonium (TEA) (1.49 ± 0.70 mV,n = 7; P < 0.05) and absent(0.97 ± 0.73 mV, n = 7; P < 0.001) upon exposure to isohydric hypercapnia (IH; 15%CO₂, 77 mM HCO, pH_o 7.45).Either HA or IH, but not 50 mM Na-propionate, activatedCa²⁺ channels. Inhibition of L-type Ca²⁺channels by nifedipine reduced HA-induced increased firing rate andeliminated IH-induced increased firing rate. We conclude that chemosensitive signals (e.g., HA or IH) have multiple targets in LCneurons, including TEA-sensitive K⁺ channels andTWIK-related acid-sensitive K⁺ (TASK) channels.Furthermore, HA and IH activate L-type Ca²⁺ channels, andthis activation is part of chemosensitive signaling in LC neurons.

     

Blood flow vs. venous pressure effects on filtration coefficient in oleic acid-injured lung

On the basis ofchanges in capillary filtration coefficient(K_fc) in 24 rabbit lungs, we determined whether elevations in pulmonary venouspressure (Ppv) or blood flow (BF) produced differences infiltration surface area in oleic acid-injured (OA) or control (Con)lungs. Lungs were cyclically ventilated and perfused under zone 3 conditions by using blood and 5% albumin with no pharmacological modulation of vascular tone. Pulmonary arterial, venous, and capillary pressures were measured by using arterial, venous, and double occlusion. Before and during eachK_fc-measurementmaneuver, microvascular/total vascular compliance was measured by usingvenous occlusion.K_fc was measuredbefore and 30 min after injury, by using a Ppv elevation of 7 cmH₂O or a BF elevation from 1 to2 l · min¹ · 100 g¹ to obtain a similardouble occlusion pressure. Pulmonary arterial pressure increased morewith BF than with Ppv in both Con and OA lungs [29 ± 2 vs. 19 ± 0.7 (means ± SE) cmH₂O;P < 0.001]. In OA lungscompared with Con lungs, values ofK_fc (200 ± 40 vs. 83 ± 14%, respectively; P < 0.01) and microvascular/total vascular compliance ratio (86 ± 4 vs. 68 ± 5%, respectively; P < 0.01) increased more with BF than with Ppv. In conclusion, for a given OA-induced increase in hydraulic conductivity, BF elevation increased filtration surface area more than did Ppv elevation. The steep pulmonary pressure profile induced by increased BF could result in therecruitment of injured capillaries and could also shift downstream thecompression point of blind (zone 1) and open injured vessels (zone 2).

     

Furosemide reduces accumulated oxygen deficit in horses during brief intense exertion

Hinchcliff, K. W., K. H. McKeever, W. W. Muir, and R. A. Sams. Furosemide reduces accumulated oxygen deficit inhorses during brief intense exertion. J. Appl.Physiol. 81(4): 1550-1554, 1996.We theorizedthat furosemide-induced weight reduction would reduce the contributionof anaerobic metabolism to energy expenditure of horses during intenseexertion. The effects of furosemide on accumulatedO₂ deficit and plasma lactateconcentration of horses during high-intensity exercise were examined ina three-way balance randomized crossover study. Nine horses completedeach of three trials: 1) a control(C) trial, 2) a furosemide-unloaded(FU) trial in which the horse received furosemide 4 h before running, and 3) a furosemide weight-loaded(FL) trial during which the horse received furosemide and carriedweight equal to the weight lost after furosemide administration. Horsesran for 2 min at ~120% maximalO₂ consumption. Furosemide (FU)increased O₂ consumption (ml ^· 2 min^{1 ·} kg¹)compared with C (268 ± 9 and 257 ± 9, P < 0.05), whereas FL was notdifferent from C (252 ± 8). AccumulatedO₂ deficit (ml O₂ equivalents/kg) wassignificantly (P < 0.05) lowerduring FU (81.2 ± 12.5), but not during FL (96.9 ± 12.4), thanduring C (91.4 ± 11.5). Rate of increase in blood lactateconcentration (mmol ^· 2 min^{1 ·} kg¹)after FU (0.058 ± 0.001), but not after FL (0.061 ± 0.001), was significantly (P < 0.05) lower than after C (0.061 ± 0.001). Furosemide decreased theaccumulated O₂ deficit and rate ofincrease in blood lactate concentration of horses during briefhigh-intensity exertion. The reduction in accumulatedO₂ deficit in FU-treated horseswas attributable to an increase in the mass-specific rate ofO₂ consumption during thehigh-intensity exercise test.

     

Effects of emphysema on diaphragm blood flow during exercise

Chronichyperinflation of the lung in emphysema displaces the diaphragmcaudally, thereby placing it in a mechanically disadvantageous positionand contributing to the increased work of breathing. We tested thehypothesis that total and regional diaphragm blood flows are increasedin emphysema, presumably reflecting an increased diaphragm energeticdemand. Male Syrian Golden hamsters were randomly divided intoemphysema (E; intratracheal elastase 25 units/100 g body wt) andcontrol (C; saline) groups, and experiments were performed 16-20wk later. The regional distribution of blood flow withinthe diaphragm was determined by using radiolabeled microspheres inhamsters at rest and during treadmill exercise (walking at 20 feet/min,20% grade). Consistent with pronounced emphysema, lung volume per unitbody weight was greater in E hamsters (C, 59.3 ± 1.8; E, 84.5 ± 5.0 ml/kg; P < 0.001) and arterialPO₂ was lower both at rest (C, 74 ± 3; E, 59 ± 2 Torr; P < 0.001) and during exercise (C, 93 ± 3; E, 69 ± 4 Torr; P < 0.001). At rest, total diaphragm blood flow was not different between C and Ehamsters (C, 47 ± 4; E, 38 ± 4 ml · min¹ · 100 g¹;P = 0.18). In both C and E hamsters,blood flow at rest was lower in the ventral costal region of thediaphragm than in the dorsal and medial costal regions and the cruraldiaphragm. During exercise in both C and E hamsters, blood flowsincreased more in the dorsal and medial costal regions and in thecrural diaphragm than in the ventral costal region. Total diaphragmblood flow was greater in E hamsters during exercise (C, 58 ± 7; E,90 ± 14 ml · min¹ · 100 g¹;P = 0.03), as a consequence ofsignificantly higher blood flows in the medial and ventral costalregions and crural diaphragm. In addition, exercise-induced increasesin intercostal (P < 0.005) andabdominal (P < 0.05) muscle bloodflows were greater in E hamsters. The finding that diaphragm blood flowwas greater in E hamsters during exercise supports the contention thatemphysema increases the energetic requirements of the diaphragm.

     

H2O2 increases sheep tracheal blood flow, permeability, and vascular response to luminal capsaicin

Wells, U. M., S. Duneclift, and J. G. Widdicombe.H₂O₂increases sheep tracheal blood flow, permeability, and vascular response to luminal capsaicin. J. Appl.Physiol. 82(2): 621-631, 1997.Exogenous hydrogenperoxide(H₂O₂)causes airway epithelial damage in vitro. We have studied the effectsof luminalH₂O₂in the sheep trachea in vivo on tracheal permeability tolow-molecular-weight hydrophilic (technetium-99m-labeleddiethylenetriamine pentaacetic acid;^99mTc-DTPA) and lipophilic([¹⁴C]antipyrine;[¹⁴C]AP) tracers andon the tracheal vascular response to luminal capsaicin, whichstimulates afferent nerve endings. A tracheal artery was perfused, andtracheal venous blood was collected. H₂O₂exposure (10 mM) reduced tracheal potential difference(42.0 ± 6.4 mV) to zero. It increased arterial andvenous flows (56.7 ± 6.1 and 57.3 ± 10.0%,respectively; n = 5, P < 0.01, paired t-test) but not tracheal lymph flow(unstimulated flow 5.0 ± 1.2 µl ^· min^{1 ·} cm¹,n = 4). DuringH₂O₂exposure, permeability to ^99mTc-DTPA increased from2.6 to 89.7 × 10⁷ cm/s(n = 5, P < 0.05), whereas permeability to[¹⁴C]AP (3,312.6 × 10⁷ cm/s,n = 4) was not altered significantly(2,565 × 10⁷cm/s). Luminal capsaicin (10 µM) increased tracheal blood flow (10.1 ± 4.1%, n = 5)and decreased venous ^99mTc-DTPAconcentration (19.7 ± 4.0, P < 0.01), and these effects weresignificantly greater after epithelial damage (28.1 ± 6.0 and45.7 ± 4.3%, respectively,P < 0.05, unpairedt-test). Thus H₂O₂increases the penetration of a hydrophilic tracer into tracheal bloodand lymph but has less effect on a lipophilic tracer. It also enhancesthe effects of luminal capsaicin on blood flow and tracer uptake.

     

Age-related differences in Na+-dependent Ca2+ accumulation in rabbit hearts exposed to hypoxia and acidification

In this study, we test the hypothesisthat in newborn hearts (as in adults) hypoxia and acidificationstimulate increased Na⁺ uptake, in part via pH-regulatoryNa⁺/H⁺ exchange. Resulting increases inintracellular Na⁺ (Na_i) alter the force drivingthe Na⁺/Ca²⁺ exchanger and lead to increasedintracellular Ca²⁺. NMR spectroscopy measuredNa_i and cytosolic Ca²⁺ concentration([Ca²⁺]_i) and pH (pH_i) inisolated, Langendorff-perfused 4- to 7-day-old rabbit hearts. AfterNa⁺/K⁺ ATPase inhibition, hypoxic hearts gainedNa⁺, whereas normoxic controls did not [19 ± 3.4 to139 ± 14.6 vs. 22 ± 1.9 to 22 ± 2.5 (SE) meq/kg drywt, respectively]. In normoxic hearts acidified using theNH₄Cl prepulse, pH_i fell rapidly and recovered,whereas Na_i rose from 31 ± 18.2 to 117.7 ± 20.5 meq/kg dry wt. Both protocols caused increases in [Ca]_i;however, [Ca]_i increased less in newborn hearts than inadults (P < 0.05). Increases in Na_i and[Ca]_i were inhibited by theNa⁺/H⁺ exchange inhibitormethylisobutylamiloride (MIA, 40 µM; P < 0.05), aswell as by increasing perfusate osmolarity (+30 mosM) immediately before and during hypoxia (P < 0.05). The data supportthe hypothesis that in newborn hearts, like adults, increases inNa_i and [Ca]_i during hypoxia and afternormoxic acidification are in large part the result of increased uptakevia Na⁺/H⁺ and Na⁺/Ca²⁺exchange, respectively. However, for similar hypoxia and acidification protocols, this increase in [Ca]_i is less in newborn thanadult hearts.

     

Nociceptive mechanisms modulate ozone-induced human lung function decrements

We have previously suggested that ozone(O₃)-induced pain-relatedsymptoms and inhibition of maximal inspiration are due to stimulationof airway C fibers (M. J. Hazucha, D. V. Bates, and P. A. Bromberg.J. Appl.Physiol. 67: 1535-1541, 1989). If this were so,pain suppression or inhibition by opioid-receptor agonists shouldpartially or fully reverseO₃-induced symptomatic and lung functional responses. The objectives of this study were to determine whether O₃-induced pain limitsmaximal inspiration and whether endogenous opioids contribute tomodulation of the effects of inhaledO₃ on lung function. Theparticipants in this double-blind crossover study were healthyvolunteers (18-59 yr) known to be "weak" (WR;n = 20) and "strong"O₃ responders (SR;n = 42). They underwent either two 2-hexposures to air or two 2-h exposures to 0.42 parts/millionO₃ with moderate intermittentexercise. Immediately afterpost-O₃ spirometry, the WR wererandomly given either naloxone (0.15 mg/kg iv) or saline, whereas SRrandomly received either sufentanil (0.2 µg/kg iv) or saline.O₃ exposure significantly(P < 0.001) impaired lung function.In SR, sufentanil rapidly, although not completely, reversed both thechest pain and spirometric effects (forced expiratory volume in 1 s;P < 0.0001) compared with saline.Immediate postexposure administration of saline or naloxone had nosignificant effect on WR. Plasma -endorphin levels were not relatedto an individual's O₃responsiveness. Cutaneous pain variables showed a nonsignificantweak association with O₃responsiveness. These observations demonstrate that nociceptive mechanisms play a key role in modulatingO₃-induced inhibition ofinspiration but not in causing lack of spirometric response toO₃ exposure in WR.

     

Contribution of Na(+)-K(+)-Cl(-) cotransporter to high-[K(+)](o)- induced swelling and EAA release in astrocytes

We hypothesized that highextracellular K⁺ concentration([K⁺]_o)-mediated stimulation ofNa⁺-K⁺-Cl cotransporter isoform 1 (NKCC1) may result in a net gain of K⁺ and Cland thus lead to high-[K⁺]_o-induced swellingand glutamate release. In the current study, relative cell volumechanges were determined in astrocytes. Under 75 mM[K⁺]_o, astrocytes swelled by 20.2 ± 4.9%. This high-[K⁺]_o-mediated swelling wasabolished by the NKCC1 inhibitor bumetanide (10 µM, 1.0 ± 3.1%; P < 0.05). Intracellular³⁶Cl accumulation was increased from acontrol value of 0.39 ± 0.06 to 0.68 ± 0.05 µmol/mgprotein in response to 75 mM [K⁺]_o. Thisincrease was significantly reduced by bumetanide (P < 0.05). Basal intracellular Na⁺ concentration([Na⁺]_i) was reduced from 19.1 ± 0.8 to16.8 ± 1.9 mM by bumetanide (P < 0.05).[Na⁺]_i decreased to 8.4 ± 1.0 mM under75 mM [K⁺]_o and was further reduced to5.2 ± 1.7 mM by bumetanide. In addition, the recovery rate of[Na⁺]_i on return to 5.8 mM[K⁺]_o was decreased by 40% in the presenceof bumetanide (P < 0.05). Bumetanide inhibitedhigh-[K⁺]_o-induced ¹⁴C-labeledD-aspartate release by ~50% (P < 0.05).These results suggest that NKCC1 contributes tohigh-[K⁺]_o-induced astrocyte swelling andglutamate release.