Circulating Electric Currents Between Acid and Alkaline Zones Associated with Assimilation in Chara

Some charophyte species, like some other freshwater plants,have been shown to take up for photosynthetic fixation. Such plants show localized deposits of CaCO₃ associatedwith localized ‘OH^– excretion’. Chara corallinaproduces acid and alkaline regions in unbuffered external media,in the light, even when CaCO₃ is not being precipitated. Theseregions are associated with uptake and ‘OH^–efflux’ respectively. We have found that large currentscirculate, in the light, between acid and alkaline zones ofChara internodes, by (i) direct measurement of the short-circuitcurrent and (ii) measurement of potential differences in thesolution around the cell. Both methods suggest currents of 0.5–1µA per zone, with local current densities of about 75mA m^–2, or local univalent fluxes of 0.75 µmol m^–2s^–1. These circulating currents produce mean peak-to-peakpotential differences of 7 mV in the external medium. The fluxesare believed to be of and H⁺ or OH^–.While active transport of is strongly indicated, passive uniport of H⁺ or OH^– appears to be sufficient.Control systems for both fluxes would be required.     

Compensatory Changes in Ion Fluxes into Barley (Hordeum vulgare L. cv. Betzes) Seedlings in Response to Differential Root/Shoot Growth Temperature

The effect of growth temperature on the short term influx of⁸⁶Rb⁺/K⁺, ³⁶Cl^–/Cl^–, , ⁴⁵Ca²⁺/Ca²⁺and into barley plants (Hordeum vulgareL. cv. Betzes) has been examined. When barley plants were grownwith a differential root: shoot temperature (15 ?C/25 ?C) therewas a marked stimulation of influx of all ions except Ca²⁺.Q₁₀ measurements were close to 2.0 for all ions except Ca²⁺,where Q₁₀ 1.0 was found. Kinetics of ion influx showed thatthere had been almost complete compensation for the lower growingtemperature over a range of concentrations. The regulation of uptake was affected by growth temperature. On exposure to 15?C efflux/influx fell initially but was restored after 5 d. Sensitivity of net uptake to was increased by growth at a lower temperature. The importance of these observations in relationto application to field experiments and the proposed mechanismfor uptake is discussed. Key words: Growth temperature, Nitrate, Ammonium     

Potassium Transport Across the Membranes of Chara: I. THE RELATIONSHIP BETWEEN RADIOACTIVE TRACER INFLUX AND ELECTRICAL CONDUCTANCE

Smith, J. R., Smith, F. A. and Walker, N. A. 1987. Potassiumtransport across the membrane of Chara. I. The relationshipbetween radioactive tracer influx and electrical conductance.—J.exp. Bot. 38:731–751. The ⁴²K influx () and the electrical conductance (G_m) were measured simultaneously for the ‘membrane’of internodal cells of Chara australis as a function of theexternal [KCl] (K?. In bathing solutions of pH = 5?0, progressively increased from 20?5to 430?60 nmol m^–2 s^–1 and G_m increased from 0?36?0?02to 3?8?0?8 S m^–2 when K? was increased from 0?1 to 10mol m^–3. The resting membrane potential difference (p.d.)was approximately -135 mV for low K? and approached the expectedNernst equilibrium p.d. for K⁺ ions when K? > 1?0 mol m^–3.Measurements of ³⁶Cl influx suggested that the ⁴²K influx waspredominantly electrogenic. The equivalent Goldman permeabilityto K⁺ ions (P_k) was approximately 20–30 nm s^–1 anddid not vary significantly with increasing K?. The equivalentconductance attributable to the electrogenic transport of K⁺ ions was calculated from assuming passive, independent diffusionof K⁺ ions and the ratio was found to be typically close to one. It was also found that themagnitudes of and G_m measuredsimultaneously for each individual cell were also well correlatedfor K? 1?0 mol m^–3, and that the slope of the line ofbest fit was close to one. For each K? it was found that theconductance not attributable to K⁺ translocation and presumablyassociated primarily with the transport of protons or theirequivalents was typically 0?2–0?5 Sm^–2. For K? >1?0 mol m^–3 the results indicated that the transport ofK⁺ ions was essentially independent, i.e. there was no evidencefor flux interactions. The results also indicated that the equivalentconductance derived from the measured ⁴²K influx could usefullyindicate the fraction of the electrical conductance attributableto the translocation of K⁺ ions. Key words: Potassium, conductance, influx     

An electrophysiological study of the membrane in aplanospore-like cell of Boergesenia forbesii

Membrane potential and resistance, each of which was the sumof those of the plasmalemma and tonoplast, measured in the coenocyticthallus of Boergesenia forbesii were 6.7 mv inside positiveand 2.8 k.cm², respectively. Protoplasm squeezed from the thallus into artificial sea water(ASW) formed numerous spherical bodies, which are termed aplanospore-likecells (simply "spores"). The following electrical propertiesof the "spores" 20–40 hr after squeezing were obtained:potential difference (p.d.) across plasmalemma (E_co) was –66mv (– means inside negative), plasmalemma resistance 665cm², p.d. across the tonoplast (E_vc) +73 mv, and tonoplast resistance2.6 k.cm². Tenfold increase in external [K⁺] caused +45 mv changein E_co and +17 mv in E_vc. The plasmalemma was entirely depolarizedin Ca⁺⁺-free ASW or ASW containing Triton X-100. When the "spore" was immersed in potassium-rich (277 mil) ASW,E_co was almost zero and the tonoplast showed two states (I andII, E_ve about +70 mv and +20 mv, respectively). E_vc went backand forth between the two states spontaneously or when a smallcurrent was applied. In most cases oscillatory changes in E^vcoccurred after the lapse of a long time in the K+-rich sea water.Membrane resistances in states I and II were 5 and 9 k.cm²,respectively. (Received July 11, 1977; )     

Calcium Activation of Potassium Channels in the Plasmalemma of Eremosphaera viridis

The role of cytoplasmic calcium activity in activation of K⁺-channelsin the unicellular green alga Eremosphaera viridis has beenstudied. As reported previously, after a ‘light off’signal a voltage independent opening of K⁺-channels in the plasmalemmais observed. This effect is indicated by a transient polarization(TP) with a simultaneous increase of the membrane conductance.TPs can also be triggered by different treatments, which allowinvestigations within a ‘short-circuited’ signalchain. (i) After incubation with EGTA a single extended TP canbe released by a sudden increase of the external calcium concentration.The Ca²⁺-channel inhibitors nifedipine (10 ^–2 mol m^–3)and verapamil (5 ? 10^–2 mol m^–3) suppress the releaseof this TP. (ii) In the presence of external calcium the additionof the ionophore A23187 [GenBank] (10–3 mol m^–3) causes anextremely prolonged TP. (iii) Low external concentrations ofbarium (10^–2 mol m^–3) induce repetitive TPs in thepresence of external calcium. In this case the Ca²⁺-channelinhibitors are less effective. (iv) Strontium (0.1–1.0mol m^–3) is able to trigger repetitive TPs even withoutexternal calcium. Whereas barium may stimulate a calcium influx,strontium can serve as a substitute for calcium to induce anopening of K⁺-channels. These results indicate strongly a Ca²⁺-dependentand voltage-independent activation of K⁺-channels in the plasmalemmaof Eremosphaera. The participation of cytoplasmic calcium inthe signal transduction chain after a ‘light off’signal is discussed. Key words: Ca²⁺-dependent K⁺-channels, Ca²⁺-channel effectors, A23187, transient membrane potential, Eremosphaera     

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     

Potassium Transport Across the Membranes of Chara: IV. INTERACTIONS WITH OTHER CATIONS

Smith, J. R. and Kerr, R. J. 1987. Potassium transport acrossthe membranes of Chara. IV. Interactions with other cations.—J.exp. Bot. 38: 788–799. The ⁴²K influx () and the membrane electrical conductance (G_m were measured simultaneously forintemodal cells of Chara australis bathed in solutions containingdifferent concentrations of various cationic species. It wasfound that the potassium permeability (P_k,) of the membranewas reduced significantly when the bathing [CaSO₄ exceeded 01mol m^–1. Concentrations of tetra-ethylammonium ions (TEA)exceeding 0?3 mol m^–3 were found to reduce significantlyboth and , but even high concentrations (10 mol m^–3)usually did not reduce the fluxes by more than a factor of 3.Na⁺ ions were found to be capable of reducing P_K by a factorof 5?6 to a value of 4 nm s^–1. This appeared to be aminimum value for P_K which was not reduced even if several inhibitorycations were present simultaneously. This suggests that possiblyonly one of two different modes of K⁺ transport can be inhibitedby cations. The possible geometry of the inhibitable K⁺ channelis briefly discussed and the implications of the presence ofNa⁺ and Ca²⁺ ions in many common bathing solutions are considered. Key words: Potassium, calcium, tetraethylammonium, inhibition     

Permeation of Ca2+ and monovalent cations through an outwardly rectifying channel in maize root stelar cells

A depolarization-activated outwardly-rectifying channel (OR),most likely involved in the passive release of K⁺ from the rootsymplasm into the stelar apoplast (for subsequent transportto the shoot via the xylem vessels), has been characterizedin the plasma membrane of maize root stelar cells (Roberts andTester, 1995). In the present study, the selectivity of thischannel was further characterized using single channel current-voltagecurves generated using a voltage ramp protocol. This protocolpermitted the accurate and unambiguous measurement of the reversalpotentials of currents resulting from single channel openings.Using the voltage ramp protocol, it was shown that the OR allowsboth K⁺ efflux and Ca²⁺ influx at potentials positive of E_Kand negative of E_Ca. The OR had a P_Ca/P_K of 1.72–0.21decreasing as extracellular Ca²⁺ was increased. The permeabilityof the OR for monovalent cations other than K⁺ was also investigated.In biionic conditions, a relative permeability sequence of was determined (i.e. Eisenman sequenceIV). The physiological implications of the selectivity of theOR are discussed. Key words: Maize roots, K⁺ channel selectivity, Ca²⁺ permeation     

Comparative effects of a low-carbohydrate diet and exercise plus a low-carbohydrate diet on muscle sarcoplasmic reticulum responses in males

We employed a glycogen-depleting session of exercise followed by a low-carbohydrate (CHO) diet to investigate modifications that occur in muscle sarcoplasmic reticulum (SR) Ca²⁺-cycling properties compared with low-CHO diet alone. SR properties were assessed in nine untrained males [peak aerobic power (O_{2 peak}) = 43.6 ± 2.6 (SE) ml·kg^–1·min^–1] during prolonged cycle exercise to fatigue performed at 58% O_{2 peak} after 4 days of low-CHO diet (Lo CHO) and after glycogen-depleting exercise plus 4 days of low-CHO (Ex+Lo CHO). Compared with Lo CHO, Ex+Lo CHO resulted in 12% lower (P < 0.05) resting maximal Ca²⁺-ATPase activity (V_max = 174 ± 12 vs. 153 ± 10 µmol·g protein^–1·min^–1) and smaller reduction in V_max induced during exercise. A similar effect was observed for Ca²⁺ uptake. The Hill coefficient, defined as slope of the relationship between cytosolic free Ca²⁺ concentration and Ca²⁺-ATPase activity, was higher (P < 0.05) at rest (2.07 ± 0.15 vs. 1.90 ± 0.10) with Ex+Lo CHO, an effect that persisted throughout the exercise. The coupling ratio, defined as the ratio of Ca²⁺ uptake to V_max, was 23–30% elevated (P < 0.05) at rest and during the first 60 min of exercise with Ex+Lo CHO. The 27 and 34% reductions (P < 0.05) in phase 1 and phase 2 Ca²⁺ release, respectively, observed during exercise with Lo CHO were not altered by Ex+Lo CHO. These results indicate that when prolonged exercise precedes a short-term Lo CHO diet, Ca²⁺ sequestration properties and efficiency are improved compared with those during Lo CHO alone. calcium cycling; vastus lateralis; contractile activity; glycogen; phosphorylation potential     

Uptake of Thallium, a Toxic Heavy-Metal, in the Cyanobacterium Synechococcus R-2 (Anacystis nidulans, S. Leopoliensis) PCC 7942

Uptake of the toxic heavy-metal, thallium, was studied in thecyanobacterium Synechococcus R-2 (PCC 7942) using clinicallyavailable ²⁰¹Tl ⁺. Thallium was found to distribute across theplasmalemma passively, and so the accumulation ratio of theion ([Tl⁺]_i/[Tl⁺]_o) could be used to calculate the apparentmembrane potential (­_i,o) of the cells (^ETI⁺_i,o = ­_i,o).The permeability of the plasmalemma to TI⁺ (^PTI⁺ 1 to 5 nms^–1)is higher than that of K⁺. Valinomycin does not increase thepermeability of TI⁺. Transient changes in the ­_i,o of cells,because of electrogenic transport of ions, could be detectedfrom its effects upon the uptake rate of TI⁺. HCO^–₃ hyperpolarizedSynechococcus cells, whereas NH⁺₄, CH₃NH⁺, and K⁺ led to depolarization.The use of TI⁺ as a reporter of ­_i,o has some inherent limitations.Tl⁺ is toxic at very low concentrations (inhibitory effectsare apparent after about 6 h at concentrations as low as 1 mmolm^–3). The rate of equilibration is slow (t_1/25 to 20 min).Equilibration of TI⁺ takes about 2 h, which limits its valueas a membrane potential probe. Large amounts of TI⁺ bind tothe surface of the cells making the method impracticable formeasuring accumulation ratios of less than about 10 (­_i,o)values smaller than about –60 mV). Cultures continuouslyexposed to Tl⁺ (10 mmol m^–3) eventually become TI⁺ resistantby actively extruding TI⁺ (µTI⁺_i,o= –3±0.2kJ mol^–1) and so thallium cannot be used as a ­_i,oprobe in such cells. (Received October 28, 1997; Accepted August 31, 1998)     

Characteristics of Radial Solution Flow in Roots of Cucumber (Cucumis sativus L.)

Low salt roots of Cucumis sativus L. cv. Burpeeana Hybrid weresubjected to iso-ionic treatments in which the external solutionconcentration of K⁺ was maintained at 14 mM. Solution concentrationof varied from 0 to 14 mM, other anions compensating. When Cl^– was the compensating ion, its concentrationin the exudate increased during the first 4 h and thereafterwas nearly the same as that of the external solution in alltreatments containing I mM or more. After 8 h of equilibration the concentration in the exudate increased almost exactly as its concentrationin the external solution. Rates of exudation and K⁺ transportwere almost constant between I and 14 mM KNO₂. More Cl^–was transported from solutions of similar Cl– concentrationwhen was also present. When water transport was inhibited with mannitol in treatments containing both KNO₃and KCI, exudate concentrations of K⁺ and were increased, but exudate concentration of Cl^– was notsignificantly affected except at the highest Cl     

Measurement of Ammonium Ions in Flowing Solution Culture and Diurnal Variation Uptake by Lolium perenne L

An existing system of flowing solution culture, in which pHand the concentration of several nutrient ions are automaticallymonitored and controlled, has been extended to include at 10 mmol N m^–3. A brief account is givenof the use of the equipment with a simulated sward of perennialryegrass (Lolium perenne L.). Uptake of measured over three successive days in June, varied with dailysolar radiation and exceeded 1000 mg N m^–2 d^–1.The uptake of showed a pattern of diurnal variation similar to the variation in solar radiation, but witha lag period for uptake of 5 h. Hourly uptake rates ranged from32 to 67 mg N m^–2h^–1 and solar radiation from 0to 2.8 MJ m^–2 h^–1. During a 24 h period, additionalmeasurements were made of K⁺ uptake and net H⁺ efflux, bothof which showed patterns of diurnal variation with lag periodsof 6 h hand 7 h, respectively. The stoichiometric ratio of thesum of and K⁺ to the net efflux of H⁺ was1.02: 1. Key words: Ion uptake, diurnal variation, Lolium perenne L.     

Effects of Ions and Membrane Potential on the Elongation of the Unicellular Green Alga Closterium

Cells of the unicellular green alga Closterium ehrenbergii elongatedexclusively at septa and for 4–5 hours after cell division.Cell elongation was strongly inhibited by a decrease in eitherthe external concentration of Ca²⁺ or pH, and was also inhibitedby several competitive Ca²⁺ channel blockers. Changes in concentrationsof other external ions had no effect on the elongation. Theaverage concentrations of ions in the intracellular fluid ofthe interphase cell before cell division was as follows (inmM): K⁺=56.5, Na⁺=4.8, Ca²⁺=2.4, Mg²⁺=1.3, Cl^–=59.5; thepH was 7.4. The levels of K⁺, Na⁺ and Cl^– ions decreasedsignificantly with cell elongation, suggesting that this process,which proceeds with water uptake, surpasses ion absorption.The plasma membrane potential (Vm) in both the interphase cellsand in the elongating cells was in the range of –90 to–105 mV (interior negative). The Vm was entirely determinedby the simple diffusion of K⁺. A decrease in the external concentrationof Ca²⁺ caused depolarization, probably by an indirect effectof low Ca²⁺. Changes in the extracellular level of H⁺ and othercations barely affected Vm. Thus, external Ca²⁺ and H⁺ are concludedto affect cell elongation but not via a change in the Vm acrossthe plasma membrane. (Received February 29, 1988; Accepted June 8, 1988)     

Salt-Stimulated Bicarbonate-Dependent Photosynthesis in the Marine Angiosperm Zostera muelleri

Millhouse, J. and Strother, S. 1986. Salt-stimulated bicarbonate-dependentphotosynthesis in the marine angiosperm Zostera muelleri.—J.exp. Bot. 37: 965–976. Photosynthetic oxygen evolution in the seagrass Zostera muelleriIrmisch ex Aschers. was inhibited in iso-osmotic sucrose. Theapparent affinity of the leaves for CO₂ in seawater increasesfrom pH 8?2 to 8?9 indicating that as well as CO₂ may act as a substrate for photosynthesis. Theaffinity for CO₂ was lower in iso-osmotic sucrose and was notaffected by pH. Under these conditions was not a substrate for photosynthesis. The differencebetween the photosynthetic rate in seawater and iso-osmoticsucrose at the same concentration of CO₂ was used to estimate assimilation. The Briggs-Maskell equation, which allows for an unstirred layer around the tissuewas more appropriate than the Michaelis-Menten theory for calculatingthe apparent affinity of the leaf slices for CO₂. The apparentK_m CO₂ was calculated as 116 mmol m^–3 at pH 8?2 by Michaelis-Mentenkinetics but only 8?10 mmol m^–3 by the Briggs-Maskellequation. The stimulation by various ions in Seawater of use was investigated. The cations,in decreasing order of effectiveness were Ca²⁺, Mg²⁺, K⁺ andNa⁺ Anions were ineffective. No single cation at its concentrationin seawater was capable of supporting use at the rate observed in seawater. Acetazolamide,an inhibitor of carbonic anhydrase, inhibited the use of for photosynthesis but had littleeffect on CO₂ photosynthesis. Thus, carbonic anhydrase activityis required for -dependent photosynthesis. Key words: Zostera muelleri, photosynthesis, salinity     

Effects of Cl- on and Malate Fluxes and CO2 Fixation in Carrot and Barley Root Cells

Cl^– and ions interact apparently competitively during influx across the plasmalemma of carrot root cells. Cl^–,however, reduces influx much less than predicted from the effect of on Cl^– influx.Cl^– and plasmalemma influxes both increase with time after excision of carrot tissue. Cl^–and may therefore be transported by a common mechanism. The effect of pH changes on the influx of malate across theplasmalemma in barley roots shows that malate crosses the plasmalemmaas the singly charged anion. Stimulations of influx by bothK₂SO₄ and KCl suggest that the malate anion crosses in associationwith. K⁺. If malate entry is passive, P_mal- is about 2?10^–8cm s^–1, but it is thought that malate entry is partlyan active process. A slight, apparently competitive inhibition by Cl^– ofmalate flux into the vacuole of barley root cells suggests thatthe two anions may be transported by a common process at thetonoplast, but this is not thought to be physiologically significant. The accumulation of ¹⁴C from 1 mM is drastically reduced by 10 mM Cl^–. A quantitative analysis of the kineticsof ¹⁴C exchange shows that Cl^– directly inhibits the formationof malate from . The decreased influx of endogenously produced malate to the vacuole in the presenceof Cl^– is probably a secondary consequence of the fallin the cytoplasmic concentration. The nature of the Cl^–inhibition of malate formation is discussed. In KCl-loaded tissue the influx of external malate and the accumulationof ¹⁴C from external are reduced. The location of these effects is not certain, but the effects suggest thatregulation of malate synthesis and accumulation may be relatedto the negative-feedback regulation of Cl^– and transport.     

Purification by Affinity Chromatography of {alpha}-Amylase--A Main Amylase in Cotyledons of Germinating Vigna mungo Seeds

In Vigna mungo cotyledons, the -amylase activity increased markedlyduring germination at 27°C in the dark, while the activityof other amylases was very low. The -amylase was purified from4-day-old cotyledons by affinity chromatography on epoxyactivatedSepharose 6B substituted with rß-cyclodextrin andby column chromatography on Bio-Gel P-200. Gel filtration andpolyacrylamide gel electrophoresis showed that the enzyme existsmostly as a monomer (43,000 daltons), but partially aggregatesto form dimer, trimer and further multimers. Ca²⁺ protectedthe -amylase against heat inactivation. Incubation of the enzymewith 5 mM EDTA or dialysis against 10 mM EDTA resulted in a50–90% loss of activity. The inactivation was partiallyreversed by the addition of Ca²⁺. Other properties, such asthe amino acid composition, K_m value, pH optimum and activationenergy were similar to those of other plant -amylases. (Received May 6, 1981; Accepted June 22, 1981)     

Role of plasma membrane Ca2+-ATPase in contraction-relaxation processes of the bladder: evidence from PMCA gene-ablated mice

We investigated the roles and relationships of plasma membrane Ca²⁺-ATPase (PMCA), sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA)2, and Na⁺/Ca²⁺ exchanger (NCX) in bladder smooth muscle contractility in Pmca-ablated mice: Pmca4-null mutant (Pmca4^–/–) and heterozygous Pmca1 and homozygous Pmca4 double gene-targeted (Pmca1^+/–Pmca4^–/–) mice. Gene manipulation did not alter the amounts of PMCA1, SERCA2, and NCX. To study the role of each Ca²⁺ transport system, contraction of circular ring preparations was elicited with KCl (80 mM) plus atropine, and then the muscle was relaxed with Ca²⁺-free physiological salt solution containing EGTA. We measured the contributions of Ca²⁺ clearance components by inhibiting SERCA2 (with 10 µM cyclopiazonic acid) and/or NCX (by replacing NaCl with N-methyl-D-glucamine/HCl plus 10 µM KB-R7943). Contraction half-time (time to 50% of maximum tension) was prolonged in the gene-targeted muscles but marginally shortened when SERCA2 or NCX was inhibited. The inhibition of NCX significantly inhibited this prolongation, suggesting that NCX activity might be augmented to compensate for PMCA4 function in the gene-targeted muscles under nonstimulated conditions. Inhibition of SERCA2 and NCX as well as gene targeting all prolonged the relaxation half-time. The contribution of PMCA to relaxation was calculated to be 25–30%, with that of SERCA2 being 20% and that of NCX being 70%. PMCA and SERCA2 appeared to function additively, but the function of NCX might overlap with those of other components. In summary, gene manipulation of PMCA indicates that PMCA, in addition to SERCA2 and NCX, plays a significant role in both excitation-contraction coupling and the Ca²⁺ extrusion-relaxation relationship, i.e., Ca²⁺ homeostasis, of bladder smooth muscle. ATP2B; sarco(endo)plasmic reticulum Ca²⁺-ATPase 2; Na⁺/Ca²⁺ exchanger; homeostasis     

Interaction of Benzoquinones with QA- and QB- in Oxygen-Evolving Photosystem II Particles from the Thermophilic Cyanobacterium Synechococcus elongatus

The interactions of benzoquinones with the reduced forms ofthe bound plastoquinone acceptors, Q_A and Q_B, were studied withoxygen-evolving photosystem II (PS II) particles from the thermophiliccyanobacterium Synechococcus elongatus, which largely lack poolplastoquinone molecules [Takahashi and Katoh (1986) Biochim.Biophys. Acta 845: 183]. Oxygen evolution in the presence ofvarious electron acceptors was determined and flash-inducedchanges in absorbance in the blue region were analyzed in termsof difference spectra, dependence on the concentration of benzoquinoneand on temperature, and sensitivity to 3-(3,4-dichlorophenyl)-1,1-dimethylurea(DCMU). The more hydrophobic the quinone molecule, the higherwas the rate of oxygen evolution, and the maximum rate of 3,000µmoles O₂.(mg chlorophyll)^–1.h^–1 was recordedin the presence of phenyl- and dichloro-p-benzoquinones. DCMUinhibited oxygen evolution by more than 95%. However, spectrophotometricstudies revealed that, even though electrons were transferredto benzoquinones predominantly via the direct oxidation of by added benzoquinones occurred in such a way as to indicate thatabout 40% of PS II reaction centers were not associated withfunctional Q_B sites. was very stable in the presence of ferricyanide. However, benzoquinonesinduced the slow oxidation of . The characteristics of the benzoquinone reductioin in thePS II preparation is discussed. ¹Present address: Department of Life Sciences, Faculty of Science,Himeji Institute of Technology, Shosha 2167, Himejishi, Hyogo-ken,671-22 Japan (Received May 8, 1990; Accepted August 14, 1990)     

Phosphate Uptake in the Cyanobacterium Synechococcus R-2 PCC 7942

Phosphate uptake rates in Synechococcus R-2 in BG-11 media (anitrate-based medium, not phosphate limited) were measured usingcells grown semi-continuously and in continuous culture. Netuptake of phosphate is proportional to external concentration.Growing cells at pH_o 10 have a net uptake rate of about 600pmol m^–2 s^–1 phosphate, but the isotopic flux for³²P phosphate was about 4 nmol m^–2 s^–1. There appearsto be a constitutive over-capacity for phosphate uptake. TheK_m and V_max, of the saturable component were not significantlydifferent at pH_o 7.5 and 10, hence the transport system probablyrecognizes both H₂PO^–₄and HPO^2–₄. The intracellularinorganic phosphate concentration is about 3 to 10 mol m^–3,but there is an intracellular polyphosphate store of about 400mol m^–3. Intracellular inorganic phosphate is 25 to 50kJ mol^–1 from electrochemical equilibrium in both thelight and dark and at pH_o 7.5 and 10. Phosphate uptake is veryslow in the dark ( 100 pmol m^–2 s^–1) and is light-activated(pH_o 7.51.3 nmol m^–2 s^–1, pH_o 10600 pmol m^–2s^–1). Uptake has an irreversible requirement for Mg²⁺in the medium. Uptake in the light is strongly Na⁺-dependent.Phosphate uptake was negatively electrogenic (net negative chargetaken up when transporting phosphate) at pH_o 7.5, but positivelyelectrogenic at pH_o 10. This seems to exclude a sodium motiveforce driven mechanism. An ATP-driven phosphate uptake mechanismneeds to have a stoichiometry of one phosphate taken up perATP (1 PO_{4 in}/ATP) to be thermodynamically possible under allthe conditions tested in the present study. (Received June 16, 1997; Accepted September 4, 1997)     

Effects of blue light deficiency on acclimation of light energy partitioning in PSII and CO2 assimilation capacity to high irradiance in spinach leaves

Blue light effects on the acclimation of energy partitioningcharacteristics in PSII and CO₂ assimilation capacity in spinachto high growth irradiance were investigated. Plants were grownhydroponically in different light treatments that were a combinationof two light qualities and two irradiances, i.e. white lightand blue-deficient light at photosynthetic photon flux densities(PPFDs) of 100 and 500 µmol m^–2 s^–1. The CO₂assimilation rate, the quantum efficiency of PSII (PSII) andthermal dissipation activity / in young, fully expanded leaves were measured under 1,600 µmol m^–2 s^–1white light. The CO₂ assimilation rate and PSII were higher,while / was lower in plants grown under high irradiancethan in plants grown under low irradiance. These responses wereobserved irrespective of the presence or absence of blue lightduring growth. The extent of the increase in the CO₂ assimilationrate and PSII and the decrease in / by high growth irradiance was smaller under blue light-deficient conditions. These resultsindicate that blue light helps to boost the acclimation responsesof energy partitioning in PSII and CO₂ assimilation to highirradiance. Similarly, leaf N, Cyt f and Chl contents per unitleaf area increased by high growth irradiance, and the extentof the increment in leaf N, Cyt f and Chl was smaller underblue light-deficient conditions. Regression analysis showedthat the differences in energy partitioning in PSII and CO₂assimilation between plants grown under high white light andhigh blue-deficient light were closely related to the differencein leaf N.