Role for anions in pulmonary endothelial permeability

Griffin, M. Pamela. Role for anions in pulmonaryendothelial permeability. J. Appl.Physiol. 83(2): 615-622, 1997.-Adrenergic stimulation reduces albumin permeation across pulmonary artery endothelial monolayers and induces changes in cell morphology that aremediated by Cl flux. Wetested the hypothesis that anion-mediated changes in endothelial cellsresult in changes in endothelial permeability. We measured permeationof radiolabeled albumin across bovine pulmonary arterial endothelialmonolayers when the extracellular anion was Cl,Br,I,F, acetate(Ac), gluconate(G), and propionate(Pr). Permeability toalbumin (P_albumin)was calculated before and after addition of 0.2 mM of thephosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX), whichreduces permeability. InCl, theP_albumin was 3.05 ± 0.86 × 10⁶ cm/s andfell by 70% with the addition of IBMX. The initialP_albumin was lowest forPr andAc. InitialP_albumin was higher inBr,I,G, andF than inCl. A permeability ratiowas calculated to examine the IBMX effect. The greatest IBMX effect wasseen when Cl was theextracellular anion, and the order among halide anions wasCl > Br > I > F. Although the level ofextracellular Ca²⁺ concentration([Ca²⁺]_o)varied over a wide range in the anion solutions,[Ca²⁺]_odid not systematically affect endothelial permeability in this system.When Cl was theextracellular anion, varying[Ca²⁺]_ofrom 0.2 to 2.8 mM caused a change in initialP_albumin but no changein the IBMX effect. The anion channel blockers4-acetamido-4-isothiocyanotostilbene-2,2-disulfonic acid(0.25 mM) and anthracene-9-carboxylic acid (0.5 mM) significantly altered initialP_albumin and the IBMXeffect. The anion transport blockers bumetanide (0.2 mM) and furosemide(1 mM) had no such effects. We conclude that extracellular anionsinfluence bovine pulmonary arterial endothelial permeability and thatthe pharmacological profile fits better with the activity of anionchannels than with other anion transport processes.

     

Combined heart rate and activity improve estimates of oxygen consumption and carbon dioxide production rates

Moon, Jon K., and Nancy F. Butte. Combined heart rateand activity improve estimates of oxygen consumption and carbon dioxideproduction rates. J. Appl. Physiol.81(4): 1754-1761, 1996.Oxygen consumption(O₂) andcarbon dioxide production (CO₂) rates were measuredby electronically recording heart rate (HR) and physical activity (PA).Mean daily O₂ andCO₂ measurements by HR andPA were validated in adults (n = 10 women and 10 men) with room calorimeters. Thirteen linear and nonlinear functions of HR alone and HR combined with PA were tested as models of24-h O₂ andCO₂. Mean sleepO₂ andCO₂ were similar to basalmetabolic rates and were accurately estimated from HR alone[respective mean errors were 0.2 ± 0.8 (SD) and0.4 ± 0.6%]. The range of prediction errorsfor 24-h O₂ andCO₂ was smallestfor a model that used PA to assign HR for each minute to separateactive and inactive curves(O₂, 3.3 ± 3.5%; CO₂, 4.6 ± 3%). There were no significant correlations betweenO₂ orCO₂ errors and subject age,weight, fat mass, ratio of daily to basal energy expenditure rate, orfitness. O₂,CO₂, and energy expenditurerecorded for 3 free-living days were 5.6 ± 0.9 ml ^· min^{1 ·} kg¹,4.7 ± 0.8 ml ^· min^{1 ·} kg¹,and 7.8 ± 1.6 kJ/min, respectively. Combined HR and PA measured 24-h O₂ andCO₂ with a precisionsimilar to alternative methods.

     

Changes in maximum oxygen uptake during prolonged training, overtraining, and detraining in horses

Tyler, Catherine M., Lorraine C. Golland, David L. Evans,David R. Hodgson, and Reuben J. Rose. Changes in maximum oxygenuptake during prolonged training, overtraining, and detraining inhorses. J. Appl. Physiol. 81(5):2244-2249, 1996.Thirteen standardbred horses were trained asfollows: phase 1 (endurance training, 7 wk),phase 2 (high-intensity training, 9 wk),phase 3 (overload training, 18 wk), andphase 4 (detraining, 12 wk). Inphase 3, the horses were divided intotwo groups: overload training (OLT) and control (C). The OLT groupexercised at greater intensities, frequencies, and durations than groupC. Overtraining occurred after 31 wk of training and was defined as asignificant decrease in treadmill run time in response to astandardized exercise test. In the OLT group, there was a significantdecrease in body weight (P < 0.05).From pretraining values of 117 ± 2 (SE)ml ^· kg^{1 ·} min¹,maximal O₂ uptake(O_{2 max}) increased by15% at the end of phase 1, and when signs of overtraining werefirst seen in the OLT group,O_{2 max} was 29%higher (151 ± 2 ml ^· kg^{1 ·} min¹in both C and OLT groups) than pretraining values. There was nosignificant reduction inO_{2 max} until after 6 wk detraining whenO_{2 max} was 137 ± 2 ml ^· kg^{1 ·} min¹.By 12 wk detraining, meanO_{2 max} was134 ± 2 ml ^· kg^{1 ·} min¹,still 15% above pretraining values. When overtraining developed, O_{2 max} was notdifferent between C and OLT groups, but maximal values forCO₂ production (147 vs. 159 ml ^· kg^{1 ·} min¹)and respiratory exchange ratio (1.04 vs. 1.11) were lower in the OLTgroup. Overtraining was not associated with a decrease inO_{2 max} and, afterprolonged training, decreases inO_{2 max} occurredslowly during detraining.

     

Mechanical and metabolic determination of VO2 and fatigue during repetitive isometric contractions in situ

Repetitiveisometric tetanic contractions (1/s) of the caninegastrocnemius-plantaris muscle were studied either at optimal length(L_o) or shortlength (L_s;~0.9 · L_o),to determine the effects of initial length on mechanical and metabolicperformance in situ. Respective averages of mechanical and metabolicvariables were(L_o vs.L_s, allP < 0.05) passive tension (preload) = 55 vs. 6 g/g, maximal active tetanic tension(P_o) = 544 vs. 174 (0.38 · P_o)g/g, maximal blood flow () = 2.0 vs. 1.4 ml · min¹ · g¹,and maximal oxygen uptake(O₂) = 12 vs. 9 µmol · min¹ · g¹.Tension at L_odecreased to0.64 · P_o over20 min of repetitive contractions, demonstrating fatigue; there were nosignificant changes in tension atL_s. In separatemuscles contracting atL_o, was set to that measured atL_s (1.1 ml · min¹ · g¹),resulting in decreased O₂(7 µmol · min¹ · g¹),and rapid fatigue, to0.44 · P_o. Thesedata demonstrate that 1)muscles at L_ohave higher andO₂ values than those at L_s;2) fatigue occurs atL_o with highO₂, adjusting metabolic demand (tension output) to match supply; and3) the lack of fatigue atL_s with lowertension, , andO₂ suggestsadequate matching of metabolic demand, set low by shortmuscle length, with supply optimized by low preload. Thesedifferences in tension andO₂ betweenL_o andL_s groupsindicate that muscles contracting isometrically at initial lengthsshorter than L_oare working under submaximal conditions.

     

Greater rate of decline in maximal aerobic capacity with age in physically active vs. sedentary healthy women

Tanaka, Hirofumi, Christopher A. DeSouza, Pamela P. Jones,Edith T. Stevenson, Kevin P. Davy, and Douglas R. Seals. Greater rate of decline in maximal aerobic capacity with age in physically active vs. sedentary healthy women. J. Appl.Physiol. 83(6): 1947-1953, 1997.Using ameta-analytic approach, we recently reported that the rate of declinein maximal oxygen uptake(O_{2 max}) with age inhealthy women is greatest in the most physically active and smallest inthe least active when expressed in milliliters per kilogram per minuteper decade. We tested this hypothesis prospectively underwell-controlled laboratory conditions by studying 156 healthy, nonobesewomen (age 20-75 yr): 84 endurance-trained runners (ET) and 72 sedentary subjects (S). ET were matched across the age range forage-adjusted 10-km running performance. Body mass was positivelyrelated with age in S but not in ET. Fat-free mass was not differentwith age in ET or S. Maximal respiratory exchange ratio and rating ofperceived exertion were similar across age in ET and S, suggestingequivalent voluntary maximal efforts. There was a significant butmodest decline in running mileage, frequency, and speed with advancingage in ET.O_{2 max}(ml · kg¹ · min¹)was inversely related to age (P < 0.001) in ET (r = 0.82) and S(r = 0.71) and was higher atany age in ET. Consistent with our meta-analysic findings,the absolute rate of decline inO_{2 max} was greater inET (5.7ml · kg¹ · min¹ · decade¹)compared with S (3.2 ml · kg¹ · min¹ · decade¹;P < 0.01), but the relative (%)rate of decline was similar (9.7 vs 9.1%/decade; notsignificant). The greater absolute rate of decline inO_{2 max} in ET comparedwith S was not associated with a greater rate of decline in maximalheart rate (5.6 vs. 6.2beats · min¹ · decade¹),nor was it related to training factors. The present cross-sectional findings provide additional evidence that the absolute, but not therelative, rate of decline in maximal aerobic capacity with age may begreater in highly physically active women compared with theirsedentary healthy peers. This difference does not appear to be relatedto age-associated changes in maximal heart rate, bodycomposition, or training factors.

     

Age-related declines in maximal aerobic capacity in regularly exercising vs. sedentary women: a meta-analysis

Fitzgerald, Margaret D., Hirofumi Tanaka, Zung V. Tran, andDouglas R. Seals. Age-related declines in maximal aerobic capacityin regularly exercising vs. sedentary women: a meta-analysis. J. Appl. Physiol. 83(1): 160-165, 1997.Our purpose was to determine the relationship between habitualaerobic exercise status and the rate of decline in maximal aerobiccapacity across the adult age range in women. A meta-analytic approachwas used in which mean maximal oxygen consumption(O_{2 max}) values fromfemale subject groups (ages 18-89 yr) were obtained from thepublished literature. A total of 239 subject groups from 109 studiesinvolving 4,884 subjects met the inclusion criteria and werearbitrarily separated into sedentary (groups = 107; subjects = 2,256),active (groups = 69; subjects = 1,717), and endurance-trained (groups = 63; subjects = 911) populations.O_{2 max} averaged 29.7 ± 7.8, 38.7 ± 9.2, and 52.0 ± 10.5 ml · kg¹ · min¹,respectively, and was inversely related to age within each population (r = 0.82 to 0.87, allP < 0.0001). The rate of decline inO_{2 max} withincreasing subject group age was lowest in sedentary women (3.5ml · kg¹ · min¹· decade¹), greater inactive women (4.4ml · kg¹ · min¹· decade¹), andgreatest in endurance-trained women (6.2ml · kg¹ · min¹ · decade¹)(all P < 0.001 vs. each other). Whenexpressed as percent decrease from mean levels at age ~25 yr, therates of decline inO_{2 max} were similarin the three populations (10.0 to 10.9%/decade). Therewas no obvious relationship between aerobic exercise status and therate of decline in maximal heart rate with age. The results of thiscross-sectional study support the hypothesis that, in contrast to theprevailing view, the rate of decline in maximal aerobic capacity withage is greater, not smaller, in endurance-trained vs. sedentary women.The greater rate of decline inO_{2 max} in endurance-trained populations may be related to their higher values asyoung adults (baseline effect) and/or to greater age-related reductions in exercise volume; however, it does not appear to berelated to a greater rate of decline in maximal heart rate with age.

     

Reductions in visceral fat during weight loss and walking are associated with improvements in {V}O2 max

The accumulation ofvisceral fat is independently associated with an increased risk forcardiovascular disease. The aim of this study was to determine whetherthe loss of visceral adipose tissue area (VAT; computed tomography) isrelated to improvements in maximal O₂ uptake(O_2 max) during a weight loss(250-350 kcal/day deficit) and walking (3 days/wk, 30-40 min)intervention. Forty obese [body fat 47 ± 1 (SE) %], sedentary(O_2 max 19 ± 1 ml · kg¹ · min¹)postmenopausal women (age 62 ± 1 yr) participated in the study. The intervention resulted in significant declines in body weight (8%), total fat mass (dual-energy X-ray absorptiometry; 17%), VAT(17%), and subcutaneous adipose tissue area (17%) with no changein lean body mass (all P < 0.001). Women with anaverage 10% increase in O_2 max reducedVAT by an average of 20%, whereas those who did not increaseO_2 max decreased VAT by only 10%,despite comparable reductions in body fat, fat mass, and subcutaneousadipose tissue area. The decrease in VAT was independently related tothe change in O_2 max(r² = 0.22; P < 0.01) andfat mass (r² = 0.08; P = 0.05). These data indicate that greater improvements inO_2 max with weight loss and walking areassociated with greater reductions in visceral adiposity in obesepostmenopausal women.

     

VO2 max is associated with ACE genotype in postmenopausal women

Relationships have frequently been found betweenangiotensin-converting enzyme (ACE) genotype and various pathologicaland physiological cardiovascular outcomes and functions. Thuswe sought to determine whether ACE genotype affected maximalO₂ consumption (O_{2 max}) and maximalexercise hemodynamics in postmenopausal women with different habitualphysical activity levels. Age, body composition, and habitual physicalactivity levels did not differ among ACE genotype groups. However, ACEinsertion/insertion (II) genotype carriers had a 6.3 ml · kg¹ · min¹higher O_{2 max}(P < 0.05) than the ACEdeletion/deletion (DD) genotype group after accounting for the effectof physical activity levels. The ACE II genotype group also had a 3.3 ml · kg¹ · min¹higher O_{2 max}(P < 0.05) than the ACEinsertion/deletion (ID) genotype group. The ACE ID group tended to havea higher O_{2 max} thanthe DD genotype group, but the difference was not significant. ACEgenotype accounted for 12% of the variation inO_{2 max} among womenafter accounting for the effect of habitual physical activity levels.The entire difference inO_{2 max} among ACEgenotype groups was the result of differences in maximal arteriovenousO₂ difference (a-vDO₂).ACE genotype accounted for 17% of the variation in maximal a-vDO₂ inthese women. Maximal cardiac output index did not differ whatsoeveramong ACE genotype groups. Thus it appears that ACE genotype accountsfor a significant portion of the interindividual differences inO_{2 max} among thesewomen. However, this difference is the result of genotype-dependentdifferences in maximala-vDO₂ andnot of maximal stroke volume and maximal cardiac output.

     

Association of chest wall motion and tidal volume responses during CO2 rebreathing

Yan, Sheng, Pawel Sliwinski, and Peter T. Macklem.Association of chest wall motion and tidal volume responses during CO₂ rebreathing.J. Appl. Physiol. 81(4):1528-1534, 1996.The purpose of this study is to investigate theeffect of chest wall configuration at end expiration on tidal volume(VT) response duringCO₂ rebreathing. In a group of 11 healthy male subjects, the changes in end-expiratory andend-inspiratory volume of the rib cage (Vrc,E andVrc,I, respectively) and abdomen (Vab,E and Vab,I, respectively) measured by linearizedmagnetometers were expressed as a function of end-tidalPCO₂(PET_CO2). The changes inend-expiratory and end-inspiratory volumes of the chest wall(Vcw,E and Vcw,I,respectively) were calculated as the sum of the respectiverib cage and abdominal volumes. The magnetometer coils were placed atthe level of the nipples and 1-2 cm above the umbilicus andcalibrated during quiet breathing against theVT measured from apneumotachograph. TheVrc,E/PET_CO2 slope was quite variable among subjects. It was significantly positive (P < 0.05) in fivesubjects, significantly negative in four subjects(P < 0.05), and not different fromzero in the remaining two subjects. TheVab,E/PET_CO2slope was significantly negative in all subjects(P < 0.05) with a much smallerintersubject variation, probably suggesting a relatively more uniformrecruitment of abdominal expiratory muscles and a variable recruitmentof rib cage muscles during CO₂rebreathing in different subjects. As a group, the meanVrc,E/PET_CO2,Vab,E/PET_CO2, andVcw,E/PET_CO2slopes were 0.010 ± 0.034, 0.030 ± 0.007, and0.020 ± 0.032 l / Torr, respectively;only theVab,E/PET_CO2 slope was significantly different from zero. More interestingly, theindividualVT/PET_CO2slope was negatively associated with theVrc,E/PET_CO2(r = 0.68,P = 0.021) and Vcw,E/PET_CO2slopes (r = 0.63,P = 0.037) but was not associated withtheVab,E/PET_CO2slope (r = 0.40, P = 0.223). There was no correlation oftheVrc,E/PET_CO2 andVcw,E/PET_CO2slopes with age, body size, forced expiratory volume in 1 s, orexpiratory time. The groupVab,I/PET_CO2 slope (0.004 ± 0.014 l / Torr) was not significantlydifferent from zero despite theVT nearly being tripled at theend of CO₂ rebreathing. Inconclusion, the individual VTresponse to CO₂, althoughindependent of Vab,E, is a function ofVrc,E to the extent that as theVrc,E/PET_CO2slope increases (more positive) among subjects, theVT response toCO₂ decreases. These results maybe explained on the basis of the respiratory muscle actions andinteractions on the rib cage.

     

Muscle capillarization, O2 diffusion distance, and VO2 kinetics in old and young individuals

Chilibeck, P. D., D. H. Paterson, D. A. Cunningham, A. W. Taylor, and E. G. Noble. Muscle capillarization,O₂ diffusion distance, andO₂ kinetics in old andyoung individuals. J. Appl. Physiol.82(1): 63-69, 1997.The relationships between muscle capillarization, estimated O₂diffusion distance from capillary to mitochondria, andO₂ uptake(O₂) kineticswere studied in 11 young (mean age, 25.9 yr) and 9 old (mean age, 66.0 yr) adults. O₂kinetics were determined by calculating the time constants () forthe phase 2 O₂ adjustment to andrecovery from the average of 12 repeats of a 6-min, moderate-intensityplantar flexion exercise. Muscle capillarization was determined fromcross sections of biopsy material taken from lateral gastrocnemius.Young and old groups had similarO₂ kinetics(O₂-on = 44 vs. 48 s;O₂-off = 33 vs. 44 s, for young and old, respectively), muscle capillarization, andestimated O₂ diffusion distances.Muscle capillarization, expressed as capillary density or averagenumber of capillary contacts per fiber/average fiber area, and theestimates of diffusion distance were significantly correlated toO₂-off kinetics in theyoung (r = 0.68 to 0.83;P < 0.05). We conclude that1) capillarization andO₂ kinetics during exerciseof a muscle group accustomed to everyday activity (e.g., walking) arewell maintained in old individuals, and2) in the young, recovery of O₂ after exercise isfaster, with a greater capillary supply over a given muscle fiber areaor shorter O₂ diffusion distances.

     

Mitochondrial redox state as a potential detector of liver dysoxia in vivo

Dysoxia canbe defined as ATP flux decreasing in proportion toO₂ availability with preserved ATPdemand. Hepatic venous -hydroxybutyrate-to-acetoacetate ratio(-OHB/AcAc) estimates liver mitochondrial NADH/NAD and may detectthe onset of dysoxia. During partial dysoxia (as opposed to anoxia),however, flow may be adequate in some liver regions, diluting effluentfrom dysoxic regions, thereby rendering venous -OHB/AcAc unreliable.To address this concern, we estimated tissue ATP whilegradually reducing liver blood flow of swine to zero in a nuclearmagnetic resonance spectrometer. ATP flux decreasing withO₂ availability was taken asO₂ uptake(O₂) decreasing inproportion to O₂ delivery(O₂);and preserved ATP demand was taken as increasingP_i/ATP.O₂, tissueP_i/ATP, and venous -OHB/AcAcwere plotted againstO₂to identify critical inflection points. Tissue dysoxia required meanO₂for the group to be critical for bothO₂ and forP_i/ATP. CriticalO₂values for O₂ andP_i/ATP of 4.07 ± 1.07 and 2.39 ± 1.18 (SE) ml · 100 g¹ · min¹,respectively, were not statistically significantly different but notclearly the same, suggesting the possibility that dysoxia might havecommenced after O₂ begandecreasing, i.e., that there could have been"O₂ conformity." CriticalO₂for venous -OHB/AcAc was 2.44 ± 0.46 ml · 100 g¹ · min¹(P = NS), nearly the same as that forP_i/ATP, supporting venous -OHB/AcAc as a detector of dysoxia. All issues considered, tissue mitochondrial redox state seems to be an appropriate detector ofdysoxia in liver.

     

Effects of hyperthermia on contraction and dilatation of rabbit femoral arteries

To analyze the effect of hyperthermia on thevascular response, the isometric response of isolated rabbit femoralartery segments was recorded at 37°C and hyperthermia (41 and44°C). Contraction to potassium (5 × 10³-5 × 10² M) was significantlygreater at 41 and 44 than at 37°C and increased by inhibition ofnitric oxide (NO) synthesis withN-nitro-L-arginine(L-NNA;10⁴ M) or endotheliumremoval at 37°C but not at 41 or 44°C. Norepinephrine (10⁹-10⁴M) produced a concentration-dependent contraction greater at 41 or 44 than at 37°C and not modified by endothelium removal orL-NNA at either temperature.Phenylephrine(10⁹-10⁴M) produced a contraction increased by warming to 44°C but not to41°C. The specific₂-adrenoceptor agonist BHT-920produced a weak contraction, reduced by the₁-adrenoceptor antagonist prazosin (10⁶ M) andincreased at 44°C but not at 41°C. The concentration-dependent contraction to endothelin-1 (ET-1;10¹¹-10⁷M) was increased by warming to 41 and 44°C and by endothelium removal or L-NNA at 37°C butnot at 41 or 44°C. Response to ET-1 was reduced by endothelinET_A-receptor antagonist BQ-123(10⁵ M) andET_B-receptor antagonist BQ-788(10⁵ M). In arteriesprecontracted with ET-1(10⁸-3 × 10⁸ M), relaxation tosodium nitroprusside(10⁸-10⁴M) was increased at 41 and 44°C vs. at 37°C, but that of ACh (10⁸-10⁴M) or adenosine(10⁸-10⁴M) was not different at all temperatures studied. Relaxation to ACh,but not adenosine, was reduced similarly byL-NNA at all temperaturesstudied. These results suggest hyperthermia in muscular arteries mayinhibit production of, and increase dilatation to, NO, resulting inunchanged relaxation to ACh and increased constriction to KCl and ET-1,and may increase constriction to stimulation of₁-adrenoceptors byNO-independent mechanisms.

     

Training-induced alterations of carbohydrate metabolism in women: women respond differently from men

We examined the hypothesis that glucose flux wasdirectly related to relative exercise intensity both beforeand after a 12-wk cycle ergometer training program [5days/wk, 1-h duration, 75% peakO₂ consumption(O_{2 peak})] inhealthy female subjects (n = 17; age23.8 ± 2.0 yr). Two pretraining trials (45 and 65% of O_{2 peak})and two posttraining trials [same absolute workload (65% of oldO_{2 peak})and same relative workload (65% of new O_{2 peak})] wereperformed on nine subjects by using a primed-continuous infusion of[1-¹³C]- and[6,6-²H]glucose.Eight additional subjects were studied by using[6,6-²H]glucose.Subjects were studied postabsorption for 90 min of rest and 1 h ofcycling exercise. After training, subjects increased O_{2 peak} by 25.2 ± 2.4%. Pretraining, the intensity effect on glucose kinetics wasevident between 45 and 65% ofO_{2 peak} with rates ofappearance (R_a: 4.52 ± 0.25 vs. 5.53 ± 0.33 mg · kg¹ · min¹),disappearance (R_d: 4.46 ± 0.25 vs. 5.54 ± 0.33 mg · kg¹ · min¹),and oxidation (R_ox: 2.45 ± 0.16 vs. 4.35 ± 0.26 mg · kg¹ · min¹)of glucose being significantly greater(P  0.05) in the 65% thanin the 45% trial. Training reducedR_a (4.7 ± 0.30 mg · kg¹ · min¹),R_d (4.69 ± 0.20 mg · kg¹ · min¹),and R_ox (3.54 ± 0.50 mg · kg¹ · min¹)at the same absolute workload (P  0.05). When subjects were tested at the same relative workload,R_a,R_d, andR_ox were not significantlydifferent after training. However, at both workloads after training,there was a significant decrease in total carbohydrate oxidation asdetermined by the respiratory exchange ratio. These results show thefollowing in young women: 1)glucose use is directly related to exercise intensity;2) training decreasesglucose flux for a given power output;3) when expressed asrelative exercise intensity, training does not affect the magnitude ofblood glucose flux during exercise; but4) training does reduce totalcarbohydrate oxidation.

     

PKA and arachidonic acid activation of human recombinant ClC-2 chloride channels

An HEK-293 cell line stably expressing the humanrecombinant ClC-2 Cl channel was used in patch-clampstudies to study its regulation. The relative permeabilityP_x/P_Cl calculated fromreversal potentials was I > Cl = NO₃ = SCNBr. Theabsolute permeability calculated from conductance ratios wasCl = Br = NO₃  SCN > I. The channel was activatedby cAMP-dependent protein kinase (PKA), reduced extracellular pH, oleicacid (C:18 cis9), elaidic acid (C:18trans9), arachidonic acid (AA; C:20cis5,8,11,14), and by inhibitors of AA metabolism,5,8,11,14-eicosatetraynoic acid (ETYA; C:20trans5,8,11,14),-methyl-4-(2-methylpropyl)benzeneacetic acid (ibuprofen), and2-phenyl-1,2-benzisoselenazol-3-[2H]-one (PZ51, ebselen). ClC-2Cl channels were activated by a combination of forskolinplus IBMX and were inhibited by the cell-permeant myristoylated PKAinhibitor (mPKI). Channel activation by reduction of bath pH wasincreased by PKA and prevented by mPKI. AA activation of the ClC-2Cl channel was not inhibited by mPKI or staurosporine andwas therefore independent of PKA or protein kinase C activation.

     

Influence of muscle fiber type and pedal frequency on oxygen uptake kinetics of heavy exercise

Barstow, Thomas J., Andrew M. Jones, Paul H. Nguyen, andRichard Casaburi. Influence of muscle fiber type and pedal frequency on oxygen uptake kinetics of heavy exercise.J. Appl. Physiol. 81(4):1642-1650, 1996.We tested the hypothesis that the amplitude ofthe additional slow component ofO₂ uptake(O₂) during heavy exerciseis correlated with the percentage of type II (fast-twitch) fibers inthe contracting muscles. Ten subjects performed transitions to a workrate calculated to require aO₂ equal to 50% betweenthe estimated lactate (Lac) threshold and maximalO₂ (50%).Nine subjects consented to a muscle biopsy of the vastus lateralis. Toenhance the influence of differences in fiber type among subjects,transitions were made while subjects were pedaling at 45, 60, 75, and90 rpm in different trials. Baseline O₂ was designed to besimilar at the different pedal rates by adjusting baseline work ratewhile the absolute increase in work rate above the baseline was thesame. The O₂ response after the onset of exercise was described by a three-exponential model. Therelative magnitude of the slow component at the end of 8-min exercisewas significantly negatively correlated with %type I fibers at everypedal rate (r = 0.64 to 0.83, P < 0.05-0.01). Furthermore,the gain of the fast component forO₂ (asml ^· min^{1 ·} W¹)was positively correlated with the %type I fibers across pedal rates(r = 0.69-0.83). Increase inpedal rate was associated with decreased relative stress of theexercise but did not affect the relationships between%fiber type and O₂parameters. The relative contribution of the slow component was alsosignificantly negatively correlated with maximalO₂(r = 0.65), whereas the gainfor the fast component was positively associated(r = 0.68-0.71 across rpm). Theamplitude of the slow component was significantly correlated with netend-exercise Lac at all four pedal rates(r = 0.64-0.84), but Lac was notcorrelated with %type I (P > 0.05).We conclude that fiber type distribution significantly affects both thefast and slow components ofO₂ during heavy exerciseand that fiber type and fitness may have both codependent andindependent influences on the metabolic and gas-exchange responses toheavy exercise.

     

Importance of the lactate anion in control of breathing

Hardarson, Thorir, Jon O. Skarphedinsson, and TorarinnSveinsson. Importance of the lactate anion in control ofbreathing. J. Appl. Physiol. 84(2):411-416, 1998.The purpose of this study was to examine theeffects of raising the arterialLa andK⁺ levels on minute ventilation(E) in rats. EitherLa or KCl solutions wereinfused in anesthetized spontaneously breathing Wistar rats to raisethe respective ion arterial concentration ([La] and[K⁺]) gradually tolevels similar to those observed during strenuous exercise.E, blood pressure, and heart rate wererecorded continuously, and arterial[La],[K⁺], pH, and bloodgases were repeatedly measured from blood samples. To prevent changesin pH during the Lainfusions, a solution of sodium lactate and lactic acid was used. Raising [La] to13.2 ± 0.6 (SE) mM induced a 47.0 ± 4.0% increase inE without any concomitant changes ineither pH or PCO₂. Raising[K⁺] to 7.8 ± 0.11 mM resulted in a 20.3 ± 5.28% increase inE without changes in pH. Thus ourresults show that Laitself, apart from lactic acidosis, may be important in increasing E during strenuous exercise, and weconfirm earlier results regarding the role of arterial[K⁺] in the control ofE during exercise.

     

Susceptibility to periodic breathing with assisted ventilation during sleep in normal subjects

Assisted ventilation with pressure support (PSV)or proportional assist (PAV) ventilation has the potential to produceperiodic breathing (PB) during sleep. We hypothesized that PB willdevelop when PSV level exceeds the product of spontaneous tidal volume (VT) and elastance(VT_sp · E)but that the actual level at which PB will develop[PSV(PB)] will be influenced by thePCO₂ (difference between eupneicPCO₂ andCO₂ apneic threshold) and by RR[response of respiratory rate (RR) to PSV]. We also wishedto determine the PAV level at which PB develops to assess inherentventilatory stability in normal subjects. Twelve normal subjectsunderwent polysomnography while connected to a PSV/PAV ventilatorprototype. Level of assist with either mode was increased in smallsteps (2-5 min each) until PB developed or the subject awakened.End-tidal PCO₂,VT, RR, and airway pressure (Paw) were continuously monitored, and the pressure generated byrespiratory muscle (Pmus) was calculated. The pressure amplification factor (PAF) at the highest PAV level was calculated from[(Paw + Pmus)/Pmus], where Paw is peak Paw  continuous positive airway pressure. PB with central apneas developedin 11 of 12 subjects on PSV. PCO₂ranged from 1.5 to 5.8 Torr. Changes in RR with PSV were small andbidirectional (+1.1 to 3.5min¹). With use ofstepwise regression, PSV(PB) was significantly correlated withVT_sp(P = 0.001), E(P = 0.00009),PCO₂ (P = 0.007), and RR(P = 0.006). The final regressionmodel was as follows: PSV(PB) = 11.1 VT_sp + 0.3E  0.4 PCO₂  0.34 RR  3.4 (r = 0.98). PBdeveloped in five subjects on PAV at amplification factors of1.5-3.4. It failed to occur in seven subjects, despite PAF of upto 7.6. We conclude that 1) aPCO₂ apneic threshold exists duringsleep at 1.5-5.8 Torr below eupneicPCO₂,2) the development of PB during PSVis entirely predictable during sleep, and3) the inherent susceptibility to PBvaries considerably among normal subjects.

     

Smaller lungs in women affect exercise hyperpnea

We subjected 29 healthy young women (age: 27 ± 1 yr) with a wide range of fitness levels [maximal oxygenuptake (O_{2 max}): 57 ± 6 ml · kg¹ · min¹;35-70ml · kg¹ · min¹]to a progressive treadmill running test. Our subjects had significantly smaller lung volumes and lower maximal expiratory flow rates, irrespective of fitness level, compared with predicted values for age-and height-matched men. The higher maximal workload in highly fit(O_{2 max} > 57 ml · kg¹ · min¹,n = 14) vs. less-fit(O_{2 max} < 56 ml · kg¹ · min¹,n = 15) women caused a higher maximalventilation (E) with increased tidal volume (VT)and breathing frequency (f_b) atcomparable maximal VT/vitalcapacity (VC). More expiratory flow limitation (EFL; 22 ± 4% ofVT) was also observed duringheavy exercise in highly fit vs. less-fit women, causing higherend-expiratory and end-inspiratory lung volumes and greater usage oftheir maximum available ventilatory reserves.HeO₂ (79% He-21%O₂) vs. room air exercise trialswere compared (with screens added to equalize external apparatusresistance). HeO₂ increasedmaximal expiratory flow rates (20-38%) throughout the range ofVC, which significantly reduced EFL during heavy exercise. When EFL wasreduced with HeO₂, VT,f_b, andE (+16 ± 2 l/min) weresignificantly increased during maximal exercise. However, in theabsence of EFL (during room air exercise),HeO₂ had no effect onE. We conclude that smaller lungvolumes and maximal flow rates for women in general, and especiallyhighly fit women, caused increased prevalence of EFL during heavyexercise, a relative hyperinflation, an increased reliance onf_b, and a greater encroachment onthe ventilatory "reserve." Consequently,VT andE are mechanically constrained duringmaximal exercise in many fit women because the demand for highexpiratory flow rates encroaches on the airways' maximum flow-volumeenvelope.

     

Contributions of pulmonary perfusion and ventilation to heterogeneity in VA/Q measured by PET

Treppo, Steven, Srboljub M. Mijailovich, and José G. Venegas. Contributions of pulmonary perfusion and ventilation toheterogeneity in A/measured by PET. J. Appl. Physiol. 82(4): 1163-1176, 1997. To estimate the contributions of the heterogeneity in regionalperfusion () and alveolar ventilation(A) to that of ventilation-perfusionratio (A/), we haverefined positron emission tomography (PET) techniques to image localdistributions of andA per unit of gas volume content(s and sA,respectively) and VA/ indogs. sA was assessed in two ways:1) the washout of ¹³NN tracer after equilibrationby rebreathing (sA_i), and2) the ratio of an apneic image after a bolus intravenousinfusion of ¹³NN-saline solution to an image collectedduring a steady-state intravenous infusion of the same solution(sA_p).sA_p was systematically higher than sA_i in allanimals, and there was a high spatial correlation betweens andsA_p in both body positions(mean correlation was 0.69 prone and 0.81 supine) suggesting thatventilation to well-perfused units was higher than to those poorlyperfused. In the prone position, the spatial distributions ofs, sA_p, and A/ were fairlyuniform with no significant gravitational gradients; however, in thesupine position, these variables were significantly more heterogeneous,mostly because of significant gravitational gradients (15, 5.5, and10%/cm, respectively) accounting for 73, 33, and 66% of thecorresponding coefficient of variation (CV)² values. Weconclude that, in the prone position, gravitational forces in blood andlung tissues are largely balanced out by dorsoventral differences inlung structure. In the supine position, effects of gravity andstructure become additive, resulting in substantial gravitationalgradients in s andsA_p, with the higherheterogeneity inA/ caused by agravitational gradient in s, only partially compensated by that in sA.

     

Adrenergic beta 1- and beta 1+2-receptor blockade suppress the natural killer cell response to head-up tilt in humans

Klokker, M., N. H. Secher, P. Madsen, M. Pedersen, and B. K. Pedersen. Adrenergic ₁-and ₁₊₂-receptor blockade suppress the natural killer cell response to head-up tilt in humans. J. Appl. Physiol. 83(5):1492-1498, 1997.To evaluate stress-induced changes in bloodleukocytes with emphasis on the natural killer (NK) cells, eight malevolunteers were followed during three trials of head-up tilt withadrenergic ₁- (metoprolol) and₁₊₂- (propranolol) blockade andwith saline (control) infusions. The ₁- and₁₊₂-receptor blockade did notaffect the appearance of presyncopal symptoms, but the head-up tiltinduced a transient lymphocytosis that was abolished by₁₊₂-receptor blockade but notby ₁-receptor blockade. Head-uptilt also resulted in delayed neutrophilia, which was insensitive to-receptor blockade. Lymphocyte subset analysis revealed that thehead-up tilt resulted in a twofold increase in the percentage andabsolute number of CD3/CD16⁺andCD3/CD56⁺NK cells in peripheral blood and that this increase was partially blocked by metoprolol and abolished by propranolol. The NKcell activity on a per NK cell basis did not change during head-up tilt, indicating that the cytotoxic capability of NK cells recruited tocirculation is unchanged. The data suggest that the head-up tilt-induced lymphocytosis was due mainly toCD16⁺ andCD56⁺ NK cells and that theirrecruitment to the blood was inhibited by₁- and especially₁₊₂-receptor blockade. Thusstress-induced recruitment of lymphocytes, and of NK cells inparticular, is mediated by epinephrine through activation of-receptors on the lymphocytes.