Endotoxemia increases relative perfusion to dorsal-caudal lung regions.

Changes in the spatial distribution of perfusion during acute lung injury and their impact on gas exchange are poorly understood. We tested whether endotoxemia caused topographical differences in perfusion and whether these differences caused meaningful changes in regional ventilation-to-perfusion ratios and gas exchange. Regional ventilation and perfusion were measured in anesthetized, mechanically ventilated pigs in the prone position before and during endotoxemia with the use of aerosolized and intravenous fluorescent microspheres. On average, relative perfusion halved in ventral and cranial lung regions, doubled in caudal lung regions, and increased 1.5-fold in dorsal lung regions during endotoxemia. In contrast, there were no topographical differences in perfusion before endotoxemia and no topographical differences in ventilation at any time point. Consequently, endotoxemia increased regional ventilation-to-perfusion ratios in the caudal-to-cranial and dorsal-to-ventral directions, resulting in end-capillary PO2 values that were significantly lower in dorsal-caudal than ventral-cranial regions. We conclude that there are topographical differences in the pulmonary vascular response to endotoxin that may have important consequences for gas exchange in acute lung injury.     

Evaluation of estimates of alveolar gas exchange by using a tidally ventilated nonhomogenous lung model

Busso, Thierry, and Peter A. Robbins. Evaluation ofestimates of alveolar gas exchange by using a tidally ventilated nonhomogenous lung model. J. Appl.Physiol. 82(6): 1963-1971, 1997.The purposeof this study was to evaluate algorithms for estimatingO₂ andCO₂ transfer at thepulmonary capillaries by use of a nine-compartment tidallyventilated lung model that incorporated inhomogeneities inventilation-to-volume and ventilation-to-perfusion ratios.Breath-to-breath O₂ andCO₂ exchange at the capillary level and at the mouth were simulated by using realistic cyclical breathing patterns to drive the model, derived from 40-min recordings in six resting subjects. The SD of the breath-by-breath gas exchange atthe mouth around the value at the pulmonary capillaries was 59.7 ± 25.5% for O₂ and 22.3 ± 10.4% for CO₂. Algorithmsincluding corrections for changes in alveolar volume and for changes in alveolar gas composition improved the estimates of pulmonary exchange, reducing the SD to 20.8 ± 10.4% forO₂ and 15.2 ± 5.8% forCO₂. The remaining imprecision ofthe estimates arose almost entirely from using end-tidal measurementsto estimate the breath-to-breath changes in end-expiratory alveolar gasconcentration. The results led us to suggest an alternative method thatdoes not use changes in end-tidal partial pressures as explicitestimates of the changes in alveolar gas concentration. The proposedmethod yielded significant improvements in estimation for the modeldata of this study.

     

Photosynthetic Characteristics of Spinach Leaves Grown with Different Nitrogen Treatments

Spinach plants (Spinacia oleracea L.) were grown hydroponicallywith different concentrations of nitrate nitrogen, ranging from0.5 to 12 mM, in a glasshouse under full sunlight. Using anopen gas exchange system, the rate of CO₂ assimilation, A, wasdetermined as a function of intercellular partial pressure ofCO₂, P_i, with a constant amount of absorbed light per unit Chl.When expressed on a leaf area basis, A measured at high irradianceand at p_i=500 µbar, was proportional to the in vitro rateof uncoupled whole-chain electron transport as well as to Chlcontent. There was a curvilinear relationship between the mesophyllconductance (the slope of the A : P_i curve near the CO₂ compensationpoint) and the in vitro RuBP carboxylase activity. The curvaturedid not appear to be due to enzyme inactivation in vivo in leaveswith high nitrogen contents. The curvature suggested the presenceof a CO₂ transfer resistance between the intercellular spacesand the site of carboxylation of 2.2 m² s bar mol^–1 CO₂,which is similar to that previously observed in wheat. Thisimplied that, while nitrogen deficiency increased the ratioof in vitro activity of electron transport to that of RuBP carboxylase,the two activities remained balanced in vivo. Irradiance response curves were determined by both net CO₂ andO₂ exchange. The two methods gave reasonable agreement at lightsaturation. The quantum yield measured by O₂ evolution was 0.090?0.003mol O₂ mol^–1 absorbed quanta, whereas after correctingfor p_i = 500µbar, the quantum yield for CO₂ assimilationwas only 82% of that measured by oxygen evolution. ²Present address: Plant Environmental Biology Group, ResearchSchool of Biological Sciences, The Australian National University,G.P.O. Box 475, Canberra, A.C.T. 2601, Australia. (Received July 29, 1987; Accepted November 2, 1987)     

Effect of CO2 Concentration During Growth on a CO2 Concentrating Mechanism in White Clover as Predicted from Differential 14CO2/12CO2Uptake

Lehnherr, B. M?chler, F. and N?sberger, J. 1985. Effect of CO₂concentration during growth on a CO₂ concentrating mechanismin white clover as predicted from differential ¹⁴CO₂/¹²CO² uptake.-J. exp. Bot. 36: 1835-1841. White clover was grown at 20 and100 Pa p(CO₂). The CO₂ response of net photosynthesis and differentialuptake of ¹⁴CO₂ and ¹²CO₂ by leaves were measured at varioustemperatures and at various O₂ and CO₂ partial pressures andcompared with predictions from ribulose bisphosphate carboxylase/oxygenasekinetics. Discrepancies between the observed gas exchange characteristicsfor the leaves and those predicted from the enzyme kineticswere interpreted as being due to a CO₂ concentrating mechanism.Plants grown at 20 Pa p(CO₂) showed a higher affinity for CO₂than plants grown at 100 Pa p(CO₂) when measured at 10 ?C. Nodifference in affinity was found at 30 ?C. The postulated CO₂concentrating effect was greater in plants grown at low CO₂than in plants grown at high CO₂ concentration and occurredonly at low temperature and low CO₂ partial pressure. It issuggested that plants grown at the lower CO₂ partial pressurehave a higher affinity for CO₂ due to a more efficient CO₂ concentratingsystem than plants grown at the higher CO₂ partial pressure. Key words: Photosynthesis, CO₂, concentration, RuBP carboxylase/oxygenase     

Refixation of respiratory CO2 in the ears of C3 cereals

The spatial arrangement of tissues within the ears of cereals,and gas exchange measurements on intact ears of barley and durumwheat suggest that respiratory CO₂ associated with grain-fillingprocesses, may be refixed close to its site of evolution. Apparentrefixation of respiratory CO₂ in intact ears was compared withthat in flag leaves, by feeding both organs with ¹⁴C-labelledsucrose and trapping ¹⁴CO₂ released by respiration. Apparentrefixation in the ears was twice that measured in flag leafblades of durum wheat genotype Durelle. In ears, the capacityof refixation of respiratory CO₂ at 210 mmol mol^–1 O₂ranged from 55% in barley genotype Roxana to 75% in barley genotypeHatif, and 60% in duwm wheat genotype Bidi 17. A low O₂ concentrationincreased refixation of respiratory CO₂ by up to 90%, 92% and82%, respectively. The occurrence of CO₂ refixation in the field,in a set of 12 barley genotypes grown under irrigated and rainfedMediterranean field conditions, was consistent with observedcarbon isotope ratios (     

Pulmonary embolization causes hypoxemia by redistributing regional blood flow without changing ventilation

To explore mechanisms of hypoxemia after acutepulmonary embolism, we measured regional pulmonary blood flow andalveolar ventilation before and after embolization with 780-µm beadsin five anesthetized, mechanically ventilated pigs. Regionalventilation and perfusion were determined in~2.0-cm³ lung volumes by using1-µm-diameter aerosolized and 15-µm-diameter injected fluorescentmicrospheres. Hypoxemia after embolization resulted from increasedperfusion to regions with low ventilation-to-perfusion ratios.Embolization caused an increase in perfusion heterogeneity and a fallin the correlation between ventilation and perfusion. Correlationbetween regional ventilation pre- and postembolization was greater thancorrelation between regional perfusion pre- and postembolization. Themajority of regional ventilation-to-perfusion ratio heterogeneity wasattributable to changes in regional perfusion. Regional perfusionredistribution without compensatory changes in regional ventilation isresponsible for hypoxemia after pulmonary vascular embolization in pigs.

     

Interaction of soil moisture and elevated CO2 on the above-ground growth rate, root length density and gas exchange of turves from temperate pasture

Interactions between water availability and elevated atmosphericCO₂ concentrations have the potential to be important factorsin determining future forage supply from temperate pastures.Using large turves from an established pasture, the responseof these communities at 350 or 700 l l^–1 CO₂ to a soilmoisture deficit and to recovery from the deficit in comparisonto turves that were well-watered throughout was measured. Priorto this experiment the turves had been exposed to the CO₂ treatmentsfor 324 d. Net CO₂ exchange continued at elevated CO₂ even when the volumetricsoil moisture content was less than 0.10 m³ m^–3 soil;at the same moisture deficit gas exchange at ambient CO₂ waszero. The additional carbon fixed by the elevated CO₂ turveswas primarily allocated below-ground as shown by the maintenanceof root length density at the same level as in well-wateredturves. When the dry turves were rewatered there was compensatorygrowth at ambient CO₂ so that the above-ground growth rate exceededthat of turves that had not experienced a moisture deficit.At the start of this experiment, the turves that were growingat 700 l I^–1 CO₂ had a greater proportion of legume (principallywhite clover, Trifolium repens L.) in the harvested herbage.There was a trend for the legume content at elevated CO₂ tobe reduced under a soil moisture deficit. The results indicate different strategies in response to soilmoisture deficits depending on the CO₂ concentration. At ambientCO₂, growth stopped, but plants were able to respond stronglyon rewatering; while at elevated CO₂ growth continued (particularlybelow-ground), but no additional growth was evident on rewatering.Ecosystem gas exchange measurements taken at the end of theexperiment (after 429 d of exposure to CO₂) showed 33% moreCO₂ was fixed at elevated CO₂ with only a small (12%) and nonsignificantdownward regulation. Key words: Carbon dioxide, climate change, grassland, gas exchange, soil moisture deficit     

Oxygen Exchange in Relation to Carbon Assimilation in Water-stressed Leaves During Photosynthesis

In a study on metabolic consumption of photosynthetic electronsand dissipation of excess light energy under water stress, O₂and CO₂ gas exchange was measured by mass spectrometry in tomatoplants using ¹⁸O₂ and ¹³CO₂. Under water stress, gross O₂ evolution(E_O), gross O₂ uptake (U_O), net CO₂ uptake (P_N), gross CO₂ uptake(TPS), and gross CO₂ evolution (E_C) declined. The ratio P_N/E_Ofell during stress, while the ratios U_O/E_O and E_C/TPS rose.Mitochondrial respiration in the light, which can be measureddirectly by ¹²CO₂ evolution during ¹³CO₂ uptake at 3000 µll^{–1 13}CO₂, is small in relation to gross CO₂ evolutionand CO₂ release from the glycolate pathway. It is concludedthat PSII, the Calvin cycle and mitochondrial respiration aredown-regulated under water stress. The percentages of photosyntheticelectrons dissipated by CO₂ assimilation, photorespiration andthe Mehler reaction were calculated: in control leaves morethan 50 % of the electrons were consumed in CO₂ assimilation,23 % in photorespiration and 13 % in the Mehler reaction. Undersevere stress the percentages of electrons dissipated by CO₂assimilation and the Mehler reaction declined while the percentageof electrons used in photorespiration doubled. The consumptionof electrons in photorespiration may reduce the likelihood ofdamage during water deficit.     

Gas exchange in dogs in the prone and supine positions.

To determine the cause of the difference in gas exchange between the prone and supine postures in dogs, gas exchange was assessed by the multiple inert gas elimination technique (MIGET) and distribution of pulmonary blood flow was determined using radioactively labeled microspheres in seven anesthetized paralyzed dogs. Each animal was studied in the prone and supine positions in random order while tidal volume and respiratory frequency were kept constant with mechanical ventilation. Mean arterial PO2 was significantly lower (P less than 0.01) in the supine [96 +/- 10 (SD) Torr] than in the prone (107 +/- 6 Torr) position, whereas arterial PCO2 was constant (38 Torr). The distribution of blood flow (Q) vs. ventilation-to-perfusion ratio obtained from MIGET was significantly wider (P less than 0.01) in the supine [ln SD(Q) = 0.75 +/- 0.26] than in the prone position [ln SD (Q) = 0.34 +/- 0.05]. Right-to-left pulmonary shunting was not significantly altered. The distribution of microspheres was more heterogeneous in the supine than in the prone position. The larger heterogeneity was due in part to dorsal-to-ventral gradients in Q in the supine position that were not present in the prone position (P less than 0.01). The decreased efficiency of oxygenation in the supine posture is caused by an increased ventilation-to-perfusion mismatch that accompanies an increase in the heterogeneity of Q distribution.     

Modeling diffusion limitation of gas exchange in lungs containing perfluorocarbon

We reportedchanges in alveolar-arterial PO₂ gradient,ventilation-perfusion heterogeneity, and arterial-alveolarPCO₂ gradient during partial liquid ventilation(PLV) in healthy piglets (E. A. Mates, P. Tarczy-Hornoch, J. Hildebrandt, J. C. Jackson, and M. P. Hlastala. In: OxygenTransport to Tissue XVII, edited by C. Ince. New York: Plenum,1996, vol. 388, p. 585-597). Here we develop two mathematicalmodels to predict transient and steady-state (SS) gas exchangeconditions during PLV and to estimate the contribution of diffusionlimitation to SS arterial-alveolar differences. In the simplest model,perfluorocarbon is represented as a uniform flat stirred layer and, ina more complex model, as an unstirred spherical layer in a ventilatedterminal alveolar sac. Time-dependent solutions of both models showthat SS is established for various inert and respiratory gases within5-150 s. In fluid-filled unventilated terminal units, all times toSS increased sometimes by hours, e.g., SF₆ exceeded 4 h. SSsolutions for the ventilated spherical model predicted minorend-capillary disequilibrium of inert gases and significantdisequilibrium of respiratory gases, which could explain a largeportion of the arterial-alveolar PCO₂ gradient measured during PLV (14). We conclude that, during PLV, diffusion gradients for gases are generally small, except for CO₂.

     

The Influence of NO-3 and NH+4 Nutrition on the Gas Exchange Characteristics of the Roots of Wheat (Triticum aestivum) and Maize (Zea mays) Plants

Respiratory oxygen consumption by roots was 1·4- and1·6-fold larger in NH⁺₄-fed than in NO^-₃-fed wheat (Triticumaestivum L.) and maize (Zea mays L.) plants respectively. Higherroot oxygen consumption in NH⁺₄-fed plants than in NO^-₃-fedplants was associated with higher total nitrogen contents inNH⁺4-fed plants. Root oxygen consumption was, however, not correlatedwith growth rates or shoot:root ratios. Carbon dioxide releasewas 1·4- and 1·2-fold larger in NO⁺3-fed thanin NH⁺₄-fed wheat and maize plants respectively. Differencesin oxygen and carbon dioxide gas exchange rates resulted inthe gas exchange quotients of NH^-₄-fed plants (wheat, 0·5;maize, 0·6) being greatly reduced compared with thoseof NO^-₃-fed plants (wheat, 1·0; maize, 1·1). Measuredrates of HCO^-₃ assimilation by PEPc in roots were considerablylarger in 4 mM NH⁺₄-fed than in 4 NO^-₃ plants (wheat, 2·6-fold;maize, 8·3-fold). These differences were, however, insufficientto account for the observed differences in root carbon dioxideflux and it is probable that HCO^-₃ uptake is also importantin determining carbon dioxide fluxes. Thus reduced root extension in NH⁺₄-fed compared with NO^-₃-fedwheat plants could not be ascribed to differences in carbondioxide losses from roots.Copyright 1993, 1999 Academic Press Triticum aestivum, wheat, Zea mays, maize assimilation, ammonium assimilation, root respiration     

A Fast, Low Cost and Low Power Requiring Device for Improving Closed Loop CO2 Measuring Systems

An improved and fast version of a closed loop system for measuringnet CO₂ exchange rates (NCER) in the field using a portableinfrared gas analyser (IRGA) is described. An electronic timer-differentiatordevice measures and displays very accurately the time periodnecessary to bridge over a constant and preselected CO₂ concentrationdifference. The time required for each measurement from startof the measurement to display of final results is c. 9 s forNCER of 24 µmol m^–2 s^–1 The measurement interval is sufficiently short that no environmentalcontrol in the perspex mini-cuvette is needed. Some satisfactorydata of NCER measurements obtained under field and glasshouseconditions on poplar, sugar bcct and winter wheat are presented. Key words: Infrared gas analysis, mobile gas exchange unit, photosynthesis     

Peroxydisulfate ion, a plant growth inhibitor, and its reaction with IAA

Germination and sprouting tests were used to demonstrate thatperoxydisulfate ion, S₂O₈^–2, reversibly inhibits turnipsprout growth but does not prevent germination. Peroxydisulfateion is kinetically inert to most organic compounds but readilyoxidizes IAA. Activation energy and rate constants for the reactionwere measured and the pH effect studied. Hormone oxidation isproposed as the effective mechanism of growth inhibition. (Received April 28, 1979; )     

The Response of the C3-CAM Tree, Clusia rosea, to Light and Water Stress: I. GAS EXCHANGE CHARACTERISTICS

Gas exchange measurements were undertaken on 2-year-old plantsof Clusia rosea. The plants were shown to have the ability toswitch from C₃-photosynthesis to CAM and vice versa regardlessof leaf age and, under some conditions, CO₂ was taken up continuously,throughout the day and night. The light response was saturatedby 120 µmol m^–2 s^–1 typical of a shade plant. Gas exchange patterns in response to light, water and VPD wereexamined. All combinations of daytime and night-time CO₂ uptakewere observed, with rates of CO₂ uptake ranging from 2 to 11µmol m^–2 s^–1 depending upon water status andlight. Categorization of this plant asC₃, CAM or an intermediateis impossible. Differing VPD affected the magnitude of changesfrom CAM to C₃-photosynthesis (0 to 0.5 and 0 to 6.0 µmolm^–2 s^–1 CO₂, respectively) when plants were watered.Under well-watered conditions, but not under water stress, highPPFD elicited changes from CAM to C₃ gas exchange. This is unusualnot only for a shade plant but also for a plant with CAM. Itis of ecological importance for C. rosea, which may spend theearly years of its life as an epiphyte or in the forest understorey,to be able to maximize photosynthesis with minimal water loss. Key words: Clusia rosea, CAM, C₃, stress     

Acclimation of photosynthesis to low temperature in Spinacia oleracea L. I. Effects of acclimation on CO2 assimilation and carbon partitioning

Acclimation of spinach plants grown at 25C to a temperatureof 10C for 10 d resulted in an increased capacity for leafphotosynthesis in saturating light and CO₂ but not at ambientCO₂ concentrations. Gas exchange and chlorophyll fluorescencemeasurements indicated that acclimation was accompanied by anincreased capacity for the regeneration of ribulose-1,5-bisphosphate.Changes in starch, soluble carbohydrates and activities of sucrose-Psynthase and ADP-glucose pyrophosphorylase were measured duringthe acclimation process. There was an initial increase in starchand sucrose during the first 2 d, but these then declined. Therewas an increase in the capacity for sucrose synthesis duringlow temperature acclimation, evidenced by an increase in themaximum activity of sucrose-P synthase activity and an increasein partitioning of ¹⁴CO₂ into sucrose, but there was no increasein the activity of ADP-glucose pyrophosphorylase or carbon partitioninginto starch. Key words: Acclimation, carbon metabolism, gas exchange, low temperature, spinach, Spinacia oleracea     

Acute effects of 17beta-estradiol on myocardial pH, Na+, and Ca2+ and ischemia-reperfusion injury

Evidence suggests that 1) ischemia-reperfusion injury is due largely to cytosolic Ca²⁺ accumulation resulting from functional coupling of Na⁺/Ca²⁺ exchange (NCE) with stimulated Na⁺/H⁺ exchange (NHE1) and 2) 17-estradiol (E2) stimulates release of NO, which inhibits NHE1. Thus we tested the hypothesis that acute E2 limits myocardial Na⁺ and therefore Ca²⁺ accumulation, thereby limiting ischemia-reperfusion injury. NMR was used to measure cytosolic pH (pH_i), Na⁺ (Na), and calcium concentration ([Ca²⁺]_i) in Krebs-Henseleit (KH)-perfused hearts from ovariectomized rats (OVX). Left ventricular developed pressure (LVDP) and lactate dehydrogenase (LDH) release were also measured. Control ischemia-reperfusion was 20 min of baseline perfusion, 40 min of global ischemia, and 40 min of reperfusion. The E2 protocol was identical, except that 1 nM E2 was included in the perfusate before ischemia and during reperfusion. E2 significantly limited the changes in pH_i, Na and [Ca²⁺]_i during ischemia (P < 0.05). In control OVX vs. OVX+E2, pH_i fell from 6.93 ± 0.03 to 5.98 ± 0.04 vs. 6.96 ± 0.04 to 6.68 ± 0.07; Na rose from 25 ± 6 to 109 ± 14 meq/kg dry wt vs. 25 ± 1 to 76 ± 3; [Ca²⁺]_i changed from 365 ± 69 to 1,248 ± 180 nM vs. 293 ± 66 to 202 ± 64 nM. E2 also improved recovery of LVDP and diminished release of LDH during reperfusion. Effects of E2 were diminished by 1 µM N-nitro-L-arginine methyl ester. Thus the data are consistent with the hypothesis. However, E2 limitation of increases in [Ca²⁺]_i is greater than can be accounted for by the thermodynamic effect of reduced Na accumulation on NCE. myocardial ischemia; Na⁺/H⁺ exchange; Na⁺/Ca²⁺ exchange; nuclear magnetic resonance; ischemic biology; ion channels/membrane transport; transplantation     

Pulmonary diffusing capacities for oxygen-labeled CO2 and nitric oxide in rabbits

Heller, Hartmut, Gabi Fuchs, and Klaus-DieterSchuster. Pulmonary diffusing capacities foroxygen-labeled CO₂ and nitric oxide in rabbits.J. Appl. Physiol. 84(2): 606-611, 1998.We determined the pulmonary diffusing capacity(DL) for¹⁸O-labeledCO₂(C¹⁸O₂)and nitric oxide (NO) to estimate the membrane component of therespective gas conductances. Six anesthetized paralyzed rabbits wereventilated by a computerized ventilatory servo system. Single-breath maneuvers were automatically performed by inflating the lungs with gasmixtures containing 0.9%C¹⁸O₂or 0.05% NO in nitrogen, with breath-holding periods ranging from 0 to1 s forC¹⁸O₂and from 2 to 8 s for NO. The alveolar partial pressures of C¹⁸O₂and NO were determined by using respiratory mass spectrometry. DL was calculated from gasexchange during inflation, breath hold, and deflation. We obtainedvalues of 14.0 ± 1.1 and 2.2 ± 0.1 (mean value ± SD)ml · mmHg¹ · min¹forDL_C¹⁸O2and DL_NO,respectively. The measured DL_C¹⁸O2/DL_NOratio was one-half that of the theoretically predicted value accordingto Graham's law (6.3 ± 0.5 vs. 12, respectively).Analyses of the several mechanisms influencing the determination ofDL_C¹⁸O2and DL_NOand their ratio are discussed. An underestimation of the membranediffusing component for CO₂ isconsidered the likely reason for the lowDL_C¹⁸O2/DL_NOratio obtained.

     

Reactions of birch leaves to changes in light during early ontogeny: comparison between in vivo and in vitro techniques to measure carbon uptake

Trends in several photosynthetic parameters and their responseto changed growth light were followed for 15 d in leaves ofyoung birch saplings using a rapid-response gas exchange measuringequipment. These in vivo measurements were compared to biochemicalassays that were made from the same leaves after the gas exchangestudies. The measurements were made on leaves that were selectedprior to the study and were at that time of similar age. Forthe first 7 d the photosynthetic parameters were followed fromthe growth conditions of moderate light (200 µmol m^–2s^–1; referred to as controls later in the text). On day7 some of the saplings were transferred to grow either underhigh (450 µmol m^–2 s^–1; referred to as highlight plants) or low (75 µmol m^–2 s^–1; referredto as low light plants) light and the capability of the preselectedleaves for acclimation was followed for 6 d. For comparison,at the end of the experiment the measurements were made on bothcontrols and on young leaves that had developed under high andlow light. Generally the in vivo measured rate of CO₂ uptake (gross photosynthesis)both at 310 ppm CO₂ and 2000 ppm CO₂ corresponded very wellto the biochemically determined CO₂ fixation capacity in vitroafter rapid extraction (measured as the initial and total activityof Rubisco, respectively). However, if the flux of CO₂ intothe chloroplasts was limited by the closure of the stomata,as was the case of the high light plants, then the in vitromeasured Rubisco activity was greater than the in vivo measuredCO₂ uptake. V_max, calculated from the mesophyll conductanceat 1% O₂, exceeded the initial activity of Rubisco (assayedat saturating RuBP and CO₂) constantly by 60%. The catalyticactivity of Rubisco in birch leaves was overall very low, evenwhen calculated from the total activity of Rubisco (K_cat 0.63–1.18 s^–1), when compared to herbaceous C₃ species. Signs of light acclimation were not observed in most of thephotosynthetic parameters and in chloroplast structure whenmature birch leaves were subjected to changes in growth lightfor 6 d. However, the change of the growth light either to highor low light caused day-to-day fluctuations in most of the measuredphotosynthetic parameters and in the case of the high lightplants signs of photoinhibition and photodestruction were alsoobserved (decrease in the amount of chlorophyll and increasein chlorophyll a/b ratio). As a result of these fluctuationsthese plants achieved a new and lower steady-state conditionbetween the light and dark reactions, as judged from the molarratio of RuBP to Rubisco binding site. Key words: Acclimation, photosynthesis, light, Rubisco, birch     

Evidence for Na+/Ca2+ exchange in intact single skeletal muscle fibers from the mouse

The myoplasmic free Ca²⁺concentration([Ca²⁺]_i)was measured in intact single fibers from mouse skeletal muscle withthe fluorescent Ca²⁺ indicatorindo 1. Some fibers were perfused in a solution in which theconcentration of Na⁺ was reducedfrom 145.4 to 0.4 mM (low-Na⁺solution) in an attempt to activate reverse-modeNa⁺/Ca²⁺exchange (Ca²⁺ entry in exchangefor Na⁺ leaving the cell). Undernormal resting conditions, application oflow-Na⁺ solution only increased[Ca²⁺]_iby 5.8 ± 1.8 nM from a mean resting[Ca²⁺]_iof 42 nM. In other fibers,[Ca²⁺]_iwas elevated by stimulating sarcoplasmic reticulum (SR)Ca²⁺ release with caffeine (10 mM)and by inhibiting SR Ca²⁺ uptakewith2,5-di(tert-butyl)-1,4-benzohydroquinone(TBQ; 0.5 µM) in an attempt to activate forward-modeNa⁺/Ca²⁺exchange (Ca²⁺ removal from thecell in exchange for Na⁺ influx).These two agents caused a large increase in[Ca²⁺]_i,which then declined to a plateau level approximately twice the baseline[Ca²⁺]_iover 20 min. If the cell was allowed to recover between exposures tocaffeine and TBQ in a solution in whichCa²⁺ had been removed, theincrease in[Ca²⁺]_iduring the second exposure was very low, suggesting thatCa²⁺ had left the cell during theinitial exposure. Application of caffeine and TBQ to a preparation inlow-Na⁺ solution produced a large,sustained increase in[Ca²⁺]_iof ~1 µM. However, when cells were exposed to caffeine and TBQ in alow-Na⁺ solution in whichCa²⁺ had been removed, a sustainedincrease in[Ca²⁺]_iwas not observed, although[Ca²⁺]_iremained higher and declined slower than in normalNa⁺ solution. This suggests thatforward-modeNa⁺/Ca²⁺exchange contributed to the fall of[Ca²⁺]_iin normal Na⁺ solution, but whenextracellular Na⁺ was low, aprolonged elevation of[Ca²⁺]_icould activate reverse-modeNa⁺/Ca²⁺exchange. The results provide evidence that skeletal muscle fibers possess aNa⁺/Ca²⁺exchange mechanism that becomes active in its forward mode when [Ca²⁺]_iis increased to levels similar to that obtained during contraction.

     

E-C coupling failure in mouse EDL muscle after in vivo eccentric contractions

The objectives of this research were to determine thecontribution of excitation-contraction (E-C) coupling failure to the decrement in maximal isometric tetanic force(P_o) in mouse extensor digitorumlongus (EDL) muscles after eccentric contractions and to elucidatepossible mechanisms. The left anterior crural muscles of femaleICR mice (n = 164) wereinjured in vivo with 150 eccentric contractions.P_o, caffeine-,4-chloro-m-cresol-, andK⁺-induced contracture forces,sarcoplasmic reticulum (SR) Ca²⁺release and uptake rates, and intracellularCa²⁺ concentration([Ca²⁺]_i)were then measured in vitro in injured and contralateral control EDLmuscles at various times after injury up to 14 days. On the basis ofthe disproportional reduction inP_o (~51%) compared with caffeine-induced force (~11-21%), we estimate that E-C coupling failure can explain 57-75% of theP_o decrement from 0 to 5 days postinjury. Comparable reductions inP_o andK⁺-induced force (51%), and minorreductions (0-6%) in the maximal SRCa²⁺ release rate, suggest thatthe E-C coupling defect site is located at the t tubule-SR interfaceimmediately after injury. Confocal laser scanning microscopy indicatedthat resting[Ca²⁺]_iwas elevated and peak tetanic[Ca²⁺]_iwas reduced, whereas peak4-chloro-m-cresol-induced[Ca²⁺]_iwas unchanged immediately after injury. By 3 days postinjury, 4-chloro-m-cresol-induced[Ca²⁺]_ibecame depressed, probably because of decreased SRCa²⁺ release and uptake rates(17-31%). These data indicate that the decrease inP_o during the first several daysafter injury primarily stems from a failure in the E-C couplingprocess.