Zone of apposition in the passive diaphragm of the dog

Boriek, Aladin M., Joseph R. Rodarte, and Susan S. Margulies. Zone of apposition in the passive diaphragm of thedog. J. Appl. Physiol. 81(5): 1929-1940, 1996.Wedetermined the regional area of the diaphragmatic zone of apposition(ZAP) as well as the regional craniocaudal extent of the ZAP(ZAP_ht) of the passive diaphragm in six paralyzedanesthetized beagle dogs (8-12 kg) at residual lung volume (RV),functional residual capacity (FRC), FRC + 0.25 and FRC + 0.5 inspiratory capacity, and total lung capacity (TLC) in prone and supinepostures. To identify the caudal boundary of the ZAP, 17 lead markers(1 mm) were sutured to the abdominal side of the costal and cruraldiaphragms around the diaphragm insertion on the chest wall. Two weekslater, the dogs' caudal thoraces were scanned by the use of thedynamic spatial reconstructor (DSR), a prototype fast volumetric X-raycomputer tomographic scanner, developed at the Mayo Clinic. Thethree-dimensional spatial coordinates of the markers were identified(±1.4 mm), and the cranial boundary of the ZAP was determined from30-40 1.4-mm-thick sagittal and coronal slices in each DSR image.We interpolated the DSR data to find the position of the cranial andcaudal boundaries of the ZAP every 5° around the thorax and computedthe distribution of regional variation of area of the ZAP andZAP_ht as well as the total area of ZAP. TheZAP_ht and area of ZAP increased as lung volume decreasedand were largest near the lateral extremes of the rib cage. We measuredthe surface area of the rib cage cephaled to the ZAP(A_L) in both postures in another six beagle dogs(12-16 kg) of similar stature, scanned previously in the DSR. Weestimated the entire rib cage surface area(A_rc = A_ZAP +A_L). The A_ZAP as a percentageof A_rc increased more than threefold as lung volumedecreased from TLC to RV, from ~9 to 29% of A_rc.

     

Time-dependent stimulation by aldosterone of blocker-sensitive ENaCs in A6 epithelia

To study and define the early time-dependent response (6 h) ofblocker-sensitive epithelial Na⁺channels (ENaCs) to stimulation ofNa⁺ transport by aldosterone, weused a new modified method of blocker-induced noise analysis todetermine the changes of single-channel current (i_Na) channel open probability(P_o), andchannel density(N_T) undertransient conditions of transport as measured by macroscopic short-circuit currents(I_sc). In threegroups of experiments in which spontaneous baseline rates of transportaveraged 1.06, 5.40, and 15.14 µA/cm², stimulation of transportoccurred due to increase of blocker-sensitive channels.N_T variedlinearly over a 70-fold range of transport (0.5-35µA/cm²). Relatively small andslow time-dependent but aldosterone-independent decreases ofP_o occurredduring control (10-20% over 2 h) and aldosterone experimentalperiods (10-30% over 6 h). When theP_o of control andaldosterone-treated tissues was examined over the 70-fold extendedrange of Na⁺ transport,P_o was observedto vary inversely withI_sc, falling from~0.5 to ~0.15 at the highest rates ofNa⁺ transport or ~25% per3-fold increase of transport. Because decreases ofP_o from anysource cannot explain stimulation of transport by aldosterone, it isconcluded that the early time-dependent stimulation ofNa⁺ transport in A6 epithelia isdue exclusively to increase of apical membraneN_T.

     

Regulation of the epithelial Na+ channel by intracellular Na+

The hypothesis that the intracellularNa⁺ concentration([Na⁺]_i)is a regulator of the epithelialNa⁺ channel (ENaC) was tested withthe Xenopus oocyte expression systemby utilizing a dual-electrode voltage clamp.[Na⁺]_iaveraged 48.1 ± 2.2 meq (n = 27)and was estimated from the amiloride-sensitive reversal potential.[Na⁺]_iwas increased by direct injection of 27.6 nl of 0.25 or 0.5 MNa₂SO₄.Within minutes of injection,[Na⁺]_istabilized and remained elevated at 97.8 ± 6.5 meq(n = 9) and 64.9 ± 4.4 (n = 5) meq 30 min after theinitial injection of 0.5 and 0.25 MNa₂SO₄,respectively. This increase of[Na⁺]_icaused a biphasic inhibition of ENaC currents. In oocytes injected with0.5 MNa₂SO₄(n = 9), a rapid decrease of inwardamiloride-sensitive slope conductance(g_Na) to 0.681 ± 0.030 of control within the first 3 min and a secondary, slowerdecrease to 0.304 ± 0.043 of control at 30 min were observed.Similar but smaller inhibitions were also observed with the injectionof 0.25 MNa₂SO₄.Injection of isotonicK₂SO₄(70 mM) or isotonicK₂SO₄made hypertonic with sucrose (70 mMK₂SO₄-1.2M sucrose) was without effect. Injection of a 0.5 M concentration ofeitherK₂SO₄,N-methyl-D-glucamine (NMDG) sulfate, or 0.75 M NMDG gluconate resulted in a much smaller initial inhibition (<14%) and little or no secondary decrease. Thusincreases of[Na⁺]_ihave multiple specific inhibitory effects on ENaC that can betemporally separated into a rapid phase that was complete within 2-3 min and a delayed slow phase that was observed between 5 and 30 min.

     

Endothelial cell dilatory pathways link flow and wall shear stress in an intact arteriolar network

Frame, Mary D. S., and Ingrid H. Sarelius. Endothelialcell dilatory pathways link flow and wall shear stress in an intactarteriolar network. J. Appl. Physiol.81(5): 2105-2114, 1996.Our purpose was to determine whether theendothelial cell-dependent dilatory pathways contribute to theregulation of flow distribution in an intact arteriolar network. Cellflow, wall shear stress (T),diameter, and bifurcation angle were determined for four sequentialbranches of a transverse arteriole in the superfused cremaster muscleof pentobaribtal sodium (Nembutal, 70 mg/kg)-anesthetized hamsters(n = 51). Control cell flow wassignificantly greater into upstream than into downstream branches[1,561 ± 315 vs. 971 ± 200 (SE) cells/s,n = 12]. Tissue exposure to 50 µMN-nitro-L-arginine + 50 µM indomethacin (L-NNA + Indo) produced arteriolar constriction of 14 ± 4% and decreasedflow into the transverse arteriole. More of the available cell flow wasdiverted to downstream branches, yet flow distribution remainedunequal. Control T was higherupstream than downstream (31.3 ± 6.8 vs. 9.8 ± 1.5 dyn/cm²).L-NNA + Indo decreasedT upstream and increasedT downstream to become equal inall branches, in contrast to flow. To determine whether constriction ingeneral induced the same changes, 5%O₂ (8 ± 4% constriction) or10⁹ M norepinephrine (NE;4 ± 3% constriction) was added to the tissue (n = 7). WithO₂, flow was redistributed tobecome equal into each branch. With NE, flow decreased progressivelymore into the first three branches. The changes in flow distributionwere thus predictable and dependent on the agonist. WithO₂ or NE, the spatial changes inflow were mirrored by spatial changes inT. Changes in diameter and incell flux were not related forL-NNA + Indo (r = 0.45),O₂(r = 0.07), or NE(r = 0.36). For all agonists, when thebifurcation angle increased, cell flow to the branch decreasedsignificantly, whereas if the angle decreased, flow was relativelypreserved; thus active changes in bifurcation angle may influence redcell distribution at arteriolar bifurcations. Thus, when theendothelial cell dilatory pathways were blocked, the changes in flowand in T were uncoupled; yet when they were intact, flowand T changed together.

     

Regulation of the epithelial Na(+) channel by extracellular acidification

The effect of extracellular acidification wastested on the native epithelial Na⁺ channel (ENaC) in A6epithelia and on the cloned ENaC expressed in Xenopusoocytes. Channel activity was determined utilizing blocker-inducedfluctuation analysis in A6 epithelia and dual electrode voltage clampin oocytes. In A6 cells, a decrease of extracellular pH(pH_o) from 7.4 to 6.4 caused a slow stimulation of theamiloride-sensitive short-circuit current (I_Na)by 68.4 ± 11% (n = 9) at 60 min. This increaseof I_Na was attributed to an increase of openchannel and total channel (N_T) densities. Similar changes were observed with pH_o 5.4. The effects ofpH_o were blocked by buffering intracellularCa²⁺ with 5 µM1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. Inoocytes, pH_o 6.4 elicited a small transient increase of theslope conductance of the cloned ENaC (11.4 ± 2.2% at 2 min)followed by a decrease to 83.7 ± 11.7% of control at 60 min (n = 6). Thus small decreases of pH_ostimulate the native ENaC by increasing N_T butdo not appreciably affect ENaC expressed in Xenopus oocytes.These effects are distinct from those observed with decreasingintracellular pH with permeant buffers that are known to inhibit ENaC.

     

Aerobic fitness effects on exercise-induced low-frequency diaphragm fatigue

Babcock, Mark A., David F. Pegelow, Bruce D. Johnson, andJerome A. Dempsey. Aerobic fitness effects on exercise-induced low-frequency diaphragm fatigue. J. Appl.Physiol. 81(5): 2156-2164, 1996.We usedbilateral phrenic nerve stimulation (BPNS; at 1, 10, and 20 Hz atfunctional residual capacity) to compare the amount of exercise-induceddiaphragm fatigue between two groups of healthy subjects, a high-fitgroup [maximal O₂consumption (O_{2 max}) = 69.0 ± 1.8 ml ^· kg^{1 ·} min¹,n = 11] and a fit group(O_{2 max} = 50.4 ± 1.7 ml ^· kg^{1 ·} min¹,n = 13). Both groups exercised at88-92% O_{2 max}for about the same duration (15.2 ± 1.7 and 17.9 ± 2.6 min forhigh-fit and fit subjects, respectively,P > 0.05). The supramaximal BPNS test showed a significant reduction (P < 0.01) in the BPNS transdiaphragmatic pressure (Pdi) immediatelyafter exercise of 23.1 ± 3.1% for the high-fit group and23.1 ± 3.8% (P > 0.05)for the fit group. Recovery of the BPNS Pdi took 60 min in both groups.The high-fit group exercised at a higher absolute workload, whichresulted in a higher CO₂production (+26%), a greater ventilatory demand (+16%) throughout theexercise, and an increased diaphragm force output (+28%) over theinitial 60% of the exercise period. Thereafter, diaphragm force outputdeclined, despite a rising minute ventilation, and it was not differentbetween most of the high-fit and fit subjects. In summary, the high-fitsubjects showed diaphragm fatigue as a result of heavy enduranceexercise but were also partially protected from excessive fatigue,despite high ventilatory requirements, because their hyperventilatoryresponse to endurance exercise was reduced, their diaphragm wasutilized less in providing the total ventilatory response, and possiblytheir diaphragm aerobic capacity was greater.

     

LY-294002-inhibitable PI 3-kinase and regulation of baseline rates of Na(+) transport in A6 epithelia

Blocker-inducednoise analysis of epithelial Na⁺ channels (ENaCs) was usedto investigate how inhibition of an LY-294002-sensitive phosphatidylinositol 3-kinase (PI 3-kinase) alters Na⁺transport in unstimulated and aldosterone-prestimulated A6 epithelia. From baseline Na⁺ transport rates(I_Na) of 4.0 ± 0.1 (unstimulated) and9.1 ± 0.9 µA/cm² (aldosterone), 10 µM LY-294002caused, following a relatively small initial increase of transport, acompletely reversible inhibition of transport within 90 min to 33 ± 6% and 38 ± 2% of respective baseline values. Initialincreases of transport could be attributed to increases of channel openprobability (P_o) within 5 min to 143 ± 17% (unstimulated) and 142 ± 10% of control (aldosterone) frombaseline P_o averaging near 0.5. Inhibition oftransport was due to much slower decreases of functional channeldensities (N_T) to 28 ± 4% (unstimulated)and 35 ± 3% (aldosterone) of control at 90 min. LY-294002 (50 µM) caused larger but completely reversible increases ofP_o (215 ± 38% of control at 5 min) andmore rapid but only slightly larger decreases ofN_T. Basolateral exposure to LY-294002 induced nodetectable effect on transport, P_o or N_T. We conclude that an LY-294002-sensitive PI3-kinase plays an important role in regulation of transport bymodulating N_T and P_o ofENaCs, but only when presented to apical surfaces of the cells.

     

Intracellular pH shifts in cultured kidney (A6) cells: effects on apical Na+ transport

We report, for the epithelialNa⁺ channel (ENaC) in A6 cells,the modulation by cell pH (pH_c)of the transepithelial Na⁺ current(I_Na), thecurrent through the individual Na⁺channel (i), the openNa⁺ channel density(N_o), and thekinetic parameters of the relationship betweenI_Na and theapical Na⁺ concentration. Thei andN_o were evaluatedfrom the Lorentzian I_Na noise inducedby the apical Na⁺ channel blocker6-chloro-3,5-diaminopyrazine-2-carboxamide.pH_c shifts were induced, understrict and volume-controlled experimental conditions, byapical/basolateral NH₄Cl pulses orbasolateral arrest of theNa⁺/H⁺exchanger (Na⁺ removal; block byethylisopropylamiloride) and were measured with the pH-sensitive probe2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. Thechanges in pH_c were positivelycorrelated to changes inI_Na and theapically dominated transepithelial conductance. The sole pH_c-sensitive parameter underlyingI_Na wasN_o. Only thesaturation value of theI_Na kinetics wassubject to changes in pH_c.pH_c-dependent changes inN_o may be causedby influencingP_o, the ENaC openprobability, or/and the total channel number,N_T = N_o/P_o.

     

ATP-dependent regulation of inwardly rectifying K+ current in bovine retinal pigment epithelial cells

Inwardlyrectifying K⁺ current(I_Kir) infreshly isolated bovine retinal pigment epithelial (RPE) cells wasstudied in the whole cell recording configuration of the patch-clamptechnique. When cells were dialyzed with pipette solution containing noATP, I_Kir randown completely in <10 min [half time(t_1/2) = 1.9 min]. In contrast, dialysis with 2 mM ATP sustainedI_Kir for 10 min or more. Rundown was also prevented with 4 mM GTP or ADP. When 0.5 mMATP was used,I_Kir ran down by~71%. Mg²⁺ was a criticalcofactor because rundown occurred when the pipette solution contained 4 mM ATP but no Mg²⁺(t_1/2 = 1.8 min).I_Kir also randown when the pipette solution contained 4 mMMg²⁺ + 4 mM5'-adenylylimidodiphosphate(t_1/2 = 2.7 min)or 4 mM adenosine 5'-O-(3-thiotriphosphate)(t_1/2 = 1.9 min),nonhydrolyzable and poorly hydrolyzable ATP analogs, respectively. Weconclude that the sustained activity ofI_Kirin bovine RPE requires intracellular MgATP and that the underlyingmechanism may involve ATP hydrolysis.

     

Interaction between airway edema and lung inflation on responsiveness of individual airways in vivo

Brown, Robert H., Wayne Mitzner, and Elizabeth M. Wagner.Interaction between airway edema and lung inflation onresponsiveness of individual airways in vivo. J. Appl.Physiol. 83(2): 366-370, 1997.Inflammatorychanges and airway wall thickening are suggested to cause increasedairway responsiveness in patients with asthma. In fivesheep, the dose-response relationships of individual airways weremeasured at different lung volumes to methacholine (MCh) before andafter wall thickening caused by the inflammatory mediator bradykininvia the bronchial artery. At 4 cmH₂O transpulmonary pressure(Ptp), 5 µg/ml MCh constricted the airways to a maximum of 18 ± 3%. At 30 cmH₂O Ptp, MCh resultedin less constriction (to 31 ± 5%). Bradykinin increased airwaywall area at 4 and 30 cmH₂O Ptp(159 ± 6 and 152 ± 4%, respectively;P < 0.0001). At 4 cmH₂O Ptp, bradykinin decreasedairway luminal area (13 ± 2%; P < 0.01), and the dose-response curve was significantly lower (P = 0.02). At 30 cmH₂O, postbradykinin, the maximalairway narrowing was not significantly different (26 ± 5%;P = 0.76). Bradykinin produced substantial airway wall thickening and slight potentiation ofthe MCh-induced airway constriction at low lung volume. At high lung volume, bradykinin increased wall thickness but had no effecton the MCh-induced airway constriction. We conclude that inflammatoryfluid leakage in the airways cannot be a primary cause of airwayhyperresponsiveness.

     

Secretory modulation of basolateral membrane inwardly rectified K(+) channel in guinea pig distal colonic crypts

Cell-attached recordings revealedK⁺ channel activity in basolateral membranes ofguinea pig distal colonic crypts. Inwardly rectified currents wereapparent with a pipette solution containing 140 mM K⁺.Single-channel conductance () was 9 pS at the resting membrane potential. Another inward rectifier with  of 19 pS was observed occasionally. At a holding potential of 80 mV,  was 21 and 41 pS,respectively. Identity as K⁺ channels was confirmed afterpatch excision by changing the bath ion composition. From reversalpotentials, relative permeability of Na⁺ overK⁺ (P_Na/P_K)was 0.02 ± 0.02, withP_Rb/P_K = 1.1 andP_Cl/P_K < 0.03. Spontaneous open probability (P_o) of the 9-pSinward rectifier (^gpK_ir) was voltageindependent in cell-attached patches. Both a low(P_o = 0.09 ± 0.01) and a moderate(P_o = 0.41 ± 0.01) activity mode wereobserved. Excision moved ^gpK_ir to the mediumactivity mode; P_o of^gpK_ir was independent of bath Ca²⁺activity and bath acidification. Addition of Cl andK⁺ secretagogues altered P_o of^gpK_ir. Forskolin or carbachol (10 µM)activated the small-conductance ^gpK_ir inquiescent patches and increased P_o inlow-activity patches. K⁺ secretagogues, either epinephrine(5 µM) or prostaglandin E₂ (100 nM), decreasedP_o of ^gpK_ir in activepatches. This ^gpK_ir may be involved inelectrogenic secretion of Cl and K⁺ acrossthe colonic epithelium, which requires a large basolateral membraneK⁺ conductance during maximal Cl secretionand, presumably, a lower K⁺ conductance during primaryelectrogenic K⁺ secretion.

     

Voluntary activation of the human diaphragm in health and disease

Intersubjectcomparison of the crural diaphragm electromyogram, as measured by anesophageal electrode, requires a reliable means for normalizing thesignal. The present study set out 1) to evaluate which voluntary respiratory maneuvers provide high andreproducible diaphragm electromyogram root-mean-square (RMS) values and2) to determine the relativediaphragm activation and mechanical and ventilatory outputs duringbreathing at rest in healthy subjects(n = 5), in patients with severechronic obstructive pulmonary disease (COPD,n = 5), and in restrictive patientswith prior polio infection (PPI, n = 6). In all groups, mean voluntary maximal RMS values were higher duringinspiration to total lung capacity than during sniff inhalation throughthe nose (P = 0.035, ANOVA). The RMS(percentage of voluntary maximal RMS) during quiet breathing was 8% inhealthy subjects, 43% in COPD patients, and 45% in PPI patients.Despite the large difference in relative RMS(P = 0.012), there were no differencesin mean transdiaphragmatic pressure (P = 0.977) and tidal volumes (P = 0.426). We conclude that voluntary maximal RMS is reliably obtainedduring an inspiration to total lung capacity but a sniff inhalationcould be a useful complementary maneuver. Severe COPD and PPI patientsbreathing at rest are characterized by increased diaphragm activationwith no change in diaphragm pressure generation.

     

Ca2+ depolarizes adrenal cortical cells through selective inhibition of an ATP-activated K+ current

Bovine adrenalzona fasciculata cells (AZF) express a noninactivatingK⁺ current(I_AC) whoseinhibition by adrenocorticotropic hormone and ANG II may be coupled tomembrane depolarization andCa²⁺-dependentcortisol secretion. We studiedI_ACinhibition byCa²⁺ and theCa²⁺ionophore ionomycin in whole cell and single-channel patch-clamp recordings of AZF. In whole cell recordings with intracellular (pipette)Ca²⁺concentration([Ca²⁺]_i)buffered to 0.02 µM,I_AC reachedmaximum current density of 25.0 ± 5.1 pA/pF(n = 16); raising[Ca²⁺]_ito 2.0 µM reduced it 76%. In inside-out patches, elevated[Ca²⁺]_idramatically reducedI_AC channelactivity. Ionomycin inhibited I_AC by 88 ± 4% (n = 14) without altering rapidlyinactivating A-type K⁺ current.Inhibition of I_ACby ionomycin was unaltered by adding calmodulin inhibitory peptide tothe pipette or replacing ATP with its nonhydrolyzable analog5'-adenylylimidodiphosphate.I_AC inhibition byionomycin was associated with membrane depolarization. When[Ca²⁺]_iwas buffered to 0.02 µM with 2 and 11 mM1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), ionomycin inhibitedI_AC by 89.6 ± 3.5 and 25.6 ± 14.6% and depolarized the same AZF by 47 ± 8 and 8 ± 3 mV, respectively (n = 4). ANG II inhibitedI_AC significantlymore effectively when pipette BAPTA was reduced from 11 to 2 mM. Raising[Ca²⁺]_iinhibits I_ACthrough a mechanism not requiring calmodulin or protein kinases,suggesting direct interaction withI_AC channels. ANGII may inhibitI_AC anddepolarize AZF by activating parallel signaling pathways, one of whichuses Ca²⁺ asa mediator.

     

Ultrasound evaluation of piglet diaphragm function before and after fatigue

Kocis, Keith C., Peter J. Radell, Wayne I. Sternberger, JaneE. Benson, Richard J. Traystman, and David G. Nichols. Ultrasound evaluation of piglet diaphragm function before and after fatigue. J. Appl. Physiol. 83(5):1654-1659, 1997.Clinically, a noninvasive measure of diaphragmfunction is needed. The purpose of this study is to determine whetherultrasonography can be used to 1)quantify diaphragm function and 2)identify fatigue in a piglet model. Five piglets were anesthetized withpentobarbital sodium and halothane and studied during the followingconditions: 1) baseline (spontaneous breathing); 2) baseline + CO₂ [inhaledCO₂ to increase arterial PCO₂ to 50-60 Torr (6.6-8kPa)]; 3) fatigue + CO₂ (fatigue induced with 30 minof phrenic nerve pacing); and 4)recovery + CO₂ (recovery after 1 hof mechanical ventilation). Ultrasound measurements of the posteriordiaphragm were made (inspiratory mean velocity) in the transverseplane. Images were obtained from the midline, just inferior to thexiphoid process, and perpendicular to the abdomen. M-mode measures weremade of the right posterior hemidiaphragm in the plane just lateral tothe inferior vena cava. Abdominal and esophageal pressures weremeasured and transdiaphragmatic pressure (Pdi) was calculated duringspontaneous (Sp) and paced (Pace) breaths. Arterial blood gases werealso measured. Pdi(Sp) and Pdi(Pace)during baseline + CO₂ were 8 ± 0.7 and 49 ± 11 cmH₂O, respectively, anddecreased to 6 ± 1.0 and 27 ± 7 cmH₂O,respectively, during fatigue + CO₂. Mean inspiratory velocityalso decreased from 13 ± 2 to 8 ± 1 cm/s during theseconditions. All variables returned to baseline during recovery + CO₂. Ultrasonography can beused to quantify diaphragm function and identify piglet diaphragm fatigue.

     

Respiratory tissue properties derived from flow transfer function in healthy humans

Tomalak, W., R. Peslin, and C. Duvivier. Respiratorytissue properties derived from flow transfer function in healthy humans. J. Appl. Physiol. 82(4):1098-1106, 1997.Assuming homogeneity of alveolar pressure, therelationship between airway flow and flow at the chest during forcedoscillation at the airway opening [flow transfer function(FTF)] is related to lung and chest wall tissue impedance (Zti):FTF = 1 + Zti/Zg, where Zg is alveolar gas impedance, which isinversely proportional to thoracic gas volume. By using a flow-typebody plethysmograph to obtain flow rate at body surface, FTF has beenmeasured at oscillation frequencies (f_os) of 10, 20, 30 and 40 Hz in eight healthy subjects during both quiet and deepbreathing. The data were corrected for the flow shunted through upperairway walls and analyzed in terms of tissue resistance (Rti) andeffective elastance (Eti,eff) by using plethysmographically measuredthoracic gas volume values. In most subjects, Rti was seen to decreasewith increasingf_os and Eti,effto vary curvilinearly withf_os²,which is suggestive of mechanical inhomogeneity. Rti presented a weakvolume dependence during breathing, variable in sign according tof_os and amongsubjects. In contrast, Eti,eff usually exhibited a U-shaped patternwith a minimum located a little above or below functional residualcapacity and a steep increase with decreasing or increasing volume(30-80 hPa/l²) on eitherside. These variations are in excess of those expected from the sigmoidshape of the static pressure-volume curve and may reflect the effect ofrespiratory muscle activity. We conclude that FTF measurement is aninteresting tool to study Rti and Eti,eff and that these parametershave probably different physiological determinants.

     

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.

     

Thermal and metabolic responses to cold-water immersion at knee, hip, and shoulder levels

Lee, Dae T., Michael M. Toner, William D. McArdle, IoannisS. Vrabas, and Kent B. Pandolf. Thermal and metabolic responses tocold-water immersion at knee, hip, and shoulder levels.J. Appl. Physiol. 82(5):1523-1530, 1997.To examine the effect of cold-water immersion atdifferent depths on thermal and metabolic responses, eight men (25 yrold, 16% body fat) attempted 12 tests: immersed to the knee (K), hip(H), and shoulder (Sh) in 15 and 25°C water during both rest (R) orleg cycling [35% peak oxygen uptake; (E)] for up to 135 min. At 15°C, rectal (T_re)and esophageal temperatures(T_es) between R and E were notdifferent in Sh and H groups (P > 0.05), whereas both in K group were higher during E than R(P < 0.05). At 25°C,T_re was higher(P < 0.05) during E than R at alldepths, whereas T_es during E washigher than during R in H and K groups.T_re remained at control levels inK-E at 15°C, K-E at 25°C, and in H-E groups at 25°C,whereas T_es remained unchanged inK-E at 15°C, in K-R at 15°C, and in all 25°C conditions (P > 0.05). During R and E, themagnitude of T_re change wasgreater (P < 0.05) than themagnitude of T_es change in Sh andH groups, whereas it was not different in the K group(P > 0.05). Total heat flow wasprogressive with water depth. During R at 15 and 25°C, heatproduction was not increased in K and H groups from control level(P > 0.05) but it did increase in Shgroup (P < 0.05). The increase inheat production during E compared with R was smaller(P < 0.05) in Sh (121 ± 7 W/m² at 15°C and 97 ± 6 W/m² at 25°C) than in H (156 ± 6 and 126 ± 5 W/m²,respectively) and K groups (155 ± 4 and 165 ± 6 W/m², respectively). These datasuggest that T_re andT_es respond differently duringpartial cold-water immersion. In addition, water levels above knee in15°C and above hip in 25°C cause depression of internal temperatures mainly due to insufficient heat production offsetting heatloss even during light exercise.

     

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.

     

Effects of acute hyperoxic exposure on solute fluxes across the blood-gas barrier in rat lungs

Zheng, Lu P., Rui Sheng Du, and Barbara E. Goodman.Effects of acute hyperoxic exposure on solute fluxes across the blood-gas barrier in rat lungs. J. Appl.Physiol. 82(1): 240-247, 1997.We investigatedeffects of acute hyperoxia on solute transport from air space tovascular space in isolated rat lungs. Air spaces were filled withKrebs-Ringer bicarbonate solution containing fluoresceinisothiocyanate-labeled dextran (FD-20; mol wt 20,000) and either²²Na⁺and [¹⁴C]sucrose, orD-[¹⁴C]glucoseandL-[³H]glucose.Apparent permeability-surface area products for tracers over time (upto 120 min) were calculated for isolated perfused lungs from controlrats (room air) and rats exposed to >95%O₂ for 48 or 60 h immediatelypostexposure. After O₂ exposures,mean fluxes for[¹⁴C]sucrose and FD-20were significantly higher than in room-air control lungs. However,amiloride-sensitive Na⁺ and activeD-glucose fluxes were unchangedafter hyperoxic exposure. Therefore, it is unlikely that decreases innet solute transport in this lung-injury model contributed to pulmonaryedema resulting from O₂ toxicity.Increased net solute transport shown to help resolve pulmonary edemaafter acute hyperoxic exposure must therefore begin during the recoveryperiod. In summary, our data show increases in passive solute fluxesbut no changes in active solute fluxes immediately after acutehyperoxic lung injury.

     

Role of aspartate residues in Ca(2+) affinity and permeation of the distal ECaC1

The Ca²⁺ affinity andpermeation of the epithelial Ca²⁺ channel (ECaC1) wereinvestigated after expression in Xenopus oocytes. ECaC1displayed anomalous mole-fraction effects. Extracellular Ca²⁺ and Mg²⁺ reversibly inhibited ECaC1 wholecell Li⁺ currents: IC₅₀ = 2.2 ± 0.4 µM (n = 9) and 235 ± 35 µM (n = 10), respectively. These values compare well with theCa²⁺ affinity of the L-type voltage-gated Ca²⁺(Ca_V1.2) channel measured under the same conditions,suggesting that high-affinity Ca²⁺ binding is awell-conserved feature of epithelial and voltage-gated Ca²⁺channels. Neutralization of D550 and E535 in the pore region had nosignificant effect on Ca²⁺ and Mg²⁺ affinities.In contrast, neutralization of D542 significantly decreasedCa²⁺ affinity (IC₅₀ = 1.1 ± 0.2 mM,n = 6) and Mg²⁺ affinity(IC₅₀ > 25 ± 3 mM, n = 4).Despite a 1,000-fold decrease in Ca²⁺ affinity in D542N,Ca²⁺ permeation properties and theCa²⁺-to-Ba²⁺ conductance ratio remainedcomparable to values for wild-type ECaC1. Together, our observationssuggest that D542 plays a critical role in Ca²⁺ affinitybut not in Ca²⁺ permeation in ECaC1.