Calcification in the Green Alga Halimeda: II. THE EXCHANGE OF CA2+ AND THE OCCURRENCE OF AGE GRADIENTS IN CALCIFICATION AND PHOTOSYNTHESIS

In Halimeda cylindracea and H. tuna segments, the concentrationof CaCO₃, MgCO₃, protein, and chlorophyll, as well as segmentvolume and wet and dry weight, increase with ‘age’i.e. from the apex of a branch downwards. Photosynthetic andcalcification rates decrease with age as does the degree oflight stimulation of calcification. Studies of the exchange of ⁴⁵Ca between the Halimeda thallusand the sea water under various conditions showed that mostof the Ca exchange is between the cell walls, the aragonitecrystals, and the intercellular space. The cell wall has twodistinguishable phases with half-times (t_0?5) of 200 and 35min while the CaCO₃ has a rapidly exchanging phase with a t_0?5of approximately 6 min. The t_0?5 of the exchange of Ca betweenthe intercellular space and the external medium is estimatedat about 6 min, on the basis of uptake studies. If the integrityof the barrier between the intercellular space and the externalsea water, created by the adpressed peripheral utricles is destroyedthe t_0?5 is smaller (<<3 min). These kinetic studies as well as comparative measurements ofcalcification rates by both isotopic and chemical methods showthat the ⁴⁵Ca method for measuring calcification rates overestimatesthe calcification rate, due to binding of ⁴⁵Ca in the cell wallsand retention of ⁴⁵Ca in the intercellular space. The ¹⁴C methodgives more accurate results and has the further advantage ofallowing simultaneous measurement of the photosynthetic andcalcification rate on the same segment.     

An Experimental Study of Calcium Acquisition and its Effects on the Calcifuge Moss Pleurozium schreberi

In a field experiment to investigate the sources and effectson growth of Ca in the calcifuge moss Pleurozium schreberi,significant quantities of Ca reached the growing shoot apicesfrom a CaCO₃ layer placed on the mineral soil surface Top applicationsof 0.5 and 5 mol m^–3 CaCl₂ raised the exchangeable andintracellular Ca concentrations and displaced natural exchangeableK and Mg The 5 mol m^–3 CaCl₂ treatment also caused a significantreduction in intracellular Mg indicating that Mg uptake is dependenton an initial exchange step No growth differences were notedbetween treatments, possibly because ionic changes had not reacheda detrimental level within the 28 weeks of the experiment ina second experiment, shoot apices of Pleurozium schreberi, Pseudoscleropodiumpurum and Calliergon cuspidatum were grown on nylon gauze underintermittent distilled-water mist At weekly intervals the shootswere saturated with CaCl₂ solutions providing factorial combinationsof Ca and pH Growth of C cuspidatum and P purum from chalk soilwas reduced at high (0.01) Ca concentration whereas Pleuroziumschreberi and Pseudoscleropodium purum from acidic clay wereunaffected The pH treatments did not significantly affect mossgrowth Initial tissue levels of K and Mg were lower in the mossesfrom chalk and it is suggested that the CaCl₂ treatments causednutrient deficiencies in these plants Mosses from acidic soilcontained less exchangeable Ca than the chalk plants and grewpoorly in the absence of CaCl₂, perhaps due to the developmentof Ca deficiency Bryophyte growth, calcium uptake, pH, mineral nutrition, Pleurozium schreberi, Pseudoscleropodium purum, Calliergon cuspidatum     

Ca2+-activated Cl- current in cultured myenteric neurons from murine proximal colon

Whole cell patch-clamprecordings were made from cultured myenteric neurons taken from murineproximal colon. The micropipette contained Cs⁺ to removeK⁺ currents. Depolarization elicited a slowly activatingtime-dependent outward current (I_tdo), whereasrepolarization was followed by a slowly deactivating tail current(I_tail). I_tdo andI_tail were present in ~70% of neurons. Weidentified these currents as Cl currents(I_Cl), because changing the transmembraneCl gradient altered the measured reversal potential(E_rev) of both I_tdo andI_tail with that for I_tailshifted close to the calculated Cl equilibrium potential(E_Cl). I_Cl areCa²⁺-activated Cl current[I_Cl(Ca)] because they were Ca²⁺dependent. E_Cl, which was measured from theE_rev of I_Cl(Ca) using agramicidin perforated patch, was 33 mV. This value is more positivethan the resting membrane potential (56.3 ± 2.7 mV), suggestingmyenteric neurons accumulate intracellular Cl.-Conotoxin GIVA [0.3 µM; N-type Ca²⁺ channelblocker] and niflumic acid [10 µM; knownI_Cl(Ca) blocker], decreased theI_Cl(Ca). In conclusion, these neurons haveI_Cl(Ca) that are activated by Ca²⁺entry through N-type Ca²⁺ channels. These currents likelyregulate postspike frequency adaptation.

     

Changes in intracellular Ca2+ concentration induced by L-type Ca2+ channel current in guinea pig gastric myocytes

We investigatedthe relationship between voltage-operatedCa²⁺ channel current and thecorresponding intracellular Ca²⁺concentration([Ca²⁺]_i)change (Ca²⁺ transient) in guineapig gastric myocytes. Fluorescence microspectroscopy was combined withconventional whole cell patch-clamp technique, and fura 2 (80 µM) wasadded to CsCl-rich pipette solution. Step depolarization to 0 mVinduced inward Ca²⁺ current(I_Ca) andconcomitantly raised[Ca²⁺]_i.Both responses were suppressed by nicardipine, an L-typeCa²⁺ channel blocker, and thevoltage dependence of Ca²⁺transient was similar to the current-voltage relation ofI_Ca. When pulseduration was increased by up to 900 ms, peakCa²⁺ transient increased andreached a steady state when stimulation was for longer. The calculatedfast Ca²⁺ buffering capacity(B value), determined as the ratio ofthe time integral ofI_Ca divided bythe amplitude of Ca²⁺ transient,was not significantly increased after depletion of Ca²⁺ stores by the cyclicapplication of caffeine (10 mM) in the presence of ryanodine (4 µM).The addition of cyclopiazonic acid (CPA, 10 µM), a sarco(endo)plasmicreticulum Ca²⁺-ATPase inhibitor,decreased B value by ~20% in areversible manner. When KCl pipette solution was used,Ca²⁺-activatedK⁺ current[I_K(Ca)]was also recorded during step depolarization. CPA sensitivelysuppressed the initial peak and oscillations of I_K(Ca) withirregular effects on Ca²⁺transients. The above results suggest that, in guinea pig gastric myocyte, Ca²⁺ transient is tightlycoupled to I_Caduring depolarization, and global[Ca²⁺]_iis not significantly affected byCa²⁺-inducedCa²⁺ release from sarcoplasmicreticulum during depolarization.

     

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

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

     

The Biological Properties of Cyclopenin and Cyclopenol

Cyclopenin (C₁₇H₁₄O₃N₂) and cyclopenol (C₁₇H₁₄O₄N₂), isolatedfrom an abberent strain of Penicillium cyclopium (NRRL 6233),significantly inhibited the growth of etiolated wheat (Triticumaestivum) coleoptile segments. The former inhibited at 10^–3and 10^–4 M, the latter at 10^–3 M. Cyclopenin producedmalformation of the first set of trifoliate leaves in bean (Phaseolusvulgaris) at 10^–2 M and necrosis and stunting in corn(Zea mays) at 10^–2 M. Cyclopenol induced no apparent effectsin bean or corn plants. Neither compound changed the growthor morphology of tobacco (Nicotiana tabacum) plants. Cyclopenininduced intoxication, prostration and ataxia in day-old chicksat 500 mg/kg, but they recovered within 18 hours. Cyclopenolwas inactive against chicks when dosed at levels up to 500 mg/kg. (Received October 11, 1983; Accepted December 15, 1983)     

Control of ascorbic acid efflux in rat luteal cells: role of intracellular calcium and oxygen radicals

In luteal cells, prostaglandin (PG)F_2a mobilizes intracellular calcium concentration ([Ca]_i), generates reactive oxygen species (ROS), depletes ascorbic acid (AA) levels, inhibits steroidogenesis, and ultimately induces cell death. We investigated the hypothesis that [Ca]_i mobilization stimulates ROS, which results in depletion of cellular AA in rat luteal cells. We used a self-referencing AA-selective electrode that noninvasively measures AA flux at the extended boundary layer of single cells and fluorescence microscopy with fura 2 and dichlorofluorescein diacetate (DCF-DA) to measure [Ca]_i and ROS, respectively. Menadione, a generator of intracellular superoxide radical (), PGF_2a, and calcium ionophore were shown to increase [Ca]_i and stimulate intracellular ROS. With calcium ionophore and PGF_2a, but not menadione, the generation of ROS was dependent on extracellular calcium influx. In unstimulated cells there was a net efflux of AA of 121.5 ± 20.3 fmol · cm^–1 · s^–1 (mean ± SE, n = 8), but in the absence of extracellular calcium the efflux was significantly reduced (10.3 ± 4.9 fmol · cm^–1 · s^–1; n = 5, P < 0.05). PGF_2a and menadione stimulated AA efflux, but calcium ionophore had no significant effect. These data suggest two AA regulatory mechanisms: Under basal conditions, AA efflux is calcium dependent and may represent recycling and maintenance of an antioxidant AA gradient at the plasma membrane. Under luteolytic hormone and/or oxidative stress, AA efflux is stimulated that is independent of extracellular calcium influx or generation of ROS. Although site-specific mobilization of calcium pools and ROS cannot be ruled out, the release of AA by PGF_2a-stimulated luteal cells may occur through other signaling pathways. luteolysis; apoptosis; self-referencing microelectrode     

Further Characteristics of Salt-Dependent Bicarbonate Use by the Seagrass Zostera muelleri

Millhouse, J. and Strother, S. 1987. Further characteristicsof salt-dependent bicarbonate use by the seagrass Zostera muelleri.—J.exp. Bot. 38: 1055–1068. The contribution of HCO^–₃to photosynthetic O₂ evolutionin the seagrass Zostera muelleri Irmisch ex Aschers. increasedwith increasing salinity of the bathing seawater when the inorganiccarbon concentration was kept constant. K_1/2 (seawater salts)for HCO^–₃ -dependent photosynthesis was 66% of seawatersalinity. Both short- and long-term pretreatment at low salinitiesstimulated photosynthesis in full strength seawater. Twentyfour hours pre-incubation of seagrass plants in 3·0 molm^–3 NaHCO₃ resulted in increased photosynthesis at allsalinities, apparently due to stimulation of HCO^–₃ use(K_1/2 (seawater salts) = 26%). V_max (HCO^–₃) was not affectedby low salinity pretreatment. The kinetics of HCO^–₃ stimulationby the major seawater cations was investigated. Ca²⁺ was themost effective cation with the highest V_max (HCO^–₃) andwith K_1/2(Ca²⁺) = 14 mol m^–3. Mg²⁺ was also very effectiveat less than 50 mol m^–3 but higher concentrations wereinhibitory. This inhibition cannot be accounted for solely byprecipitation of MgCO₃. Na⁺ and K⁺ were both capable of stimulatingHCO^–₃ use. Stimulation was in two distinct parts. Up to500 mol m^–3, both citrate and chloride salts gave similarresults (K_1/2(Na⁺) 81 mol m^–3, V_max(HCO^–₃) 0·26µmol O₂ mg^–1 chl min^–1), but use of citratesalts above 500 mol m^–2 caused a second stimulation ofHCO^–₃ use (K_1/2(Na⁺) 830 mol m^–3, V_max(HCO^–₃)0·68 µmol O₂ mg^–1 chl min^–1). V_max(HCO^–₃)for the second-phase Na⁺ or K⁺ stimulation was of the same orderas for Ca²⁺-stimulated HCO^–₃ use. To further characterizesalt-dependent HCO^–₃ use, the sensitivity of photosynthesisto Tris and TES buffers was investigated. The effects of Trisappear to be due to the action of Tris⁺ causing stimulationof HCO^–₃ -dependent photosynthesis in the absence of salt,but inhibition of HCO^–₃ use in saline media. TES has noeffect on photosynthesis. External carbonic anhydrase, althoughimplicated in salt-dependent HCO^–₃ use in Z. muelleri,could not be detected in whole leaves. Key words: Zostera muelleri, HCO^–₃ use, salinity     

Cyclic nucleotide-gated cation channels mediate sodium and calcium influx in rat colon

We found mRNA for the three isoforms ofthe cyclic nucleotide-gated nonselective cation channel expressed inthe mucosal layer of the rat intestine from the duodenum to the colonand in intestinal epithelial cell lines in culture. Because thesechannels are permeable to sodium and calcium and are stimulated by cGMPor cAMP, we measured 8-bromo-cGMP-stimulated sodium-mediatedshort-circuit current (I_sc) inproximal and distal colon and unidirectional⁴⁵Ca²⁺fluxes in proximal colon to determine whether these channels couldmediate transepithelial sodium and calcium absorption across the colon.Sodium-mediatedI_sc, stimulatedby 8-bromo-cGMP, were inhibited by dichlorobenzamil andl-cis-diltiazem, blockers of cyclicnucleotide-gated cation channels, suggesting that these ion channelscan mediate transepithelial sodium absorption. Sodium-mediated I_sc and nettransepithelial⁴⁵Ca²⁺absorption were stimulated by heat-stable toxin fromEscherichia coli that increases cGMP.Addition of l-cis-diltiazem inhibited the enhanced transepithelial absorption of both ions. These results suggest that cyclic nucleotide-gated cation channels simultaneously increase net sodium and calcium absorption in the colon of the rat.

     

Operation of the reductive pentose phosphate cycle daring the induction period of photosynthesis in Chlorella

When Chlorella oulgaris ll h cells grown in air containing 4%CO₂ (high-CO₂ cells) were given low concentrations of¹⁴CO₂ (<150ppm), the initial rate of photosynthetic ¹⁴CO₂ fixation wasvery low and linear ¹⁴CO₂ fixation was observed after an inductionperiod which lasted for ca. 45 min. No such induction period was observed when high-CO₂ cells weregiven high concentrations of ¹⁴CO₂ (10,000 ppm) or when IOW-CO₂cells were given either low or high concentrations of ¹⁴CO₂,supporting the observations by Briggs and Whittingham (l). However,irrespective of CO₂ concentrations during growth and of ¹⁴CO₂concentrations during the experiments, most of the ¹⁴C was incorporatedinto phosphate esters during the initial periods of photosynthetic¹⁴CO₂ fixation. These results are in sharp contrast to the reportby Graham and Whittingham (4). ¹ Requests for reprints should be addressed to S. Miyachi, RadioisotopeCentre, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan. (Received June 30, 1979; )     

Metabolic inhibition with cyanide induces calcium release in pulmonary artery myocytes and Xenopus oocytes

We examined the effectsof metabolic inhibition on intracellular Ca²⁺ release insingle pulmonary arterial smooth muscle cells (PASMCs). Severemetabolic inhibition with cyanide (CN, 10 mM) increased intracellularcalcium concentration ([Ca²⁺]_i) and activatedCa²⁺-activated Cl currents[I_Cl(Ca)] in PASMCs, responses that were greatlyinhibited by BAPTA-AM or caffeine. Mild metabolic inhibition with CN (1 mM) increased spontaneous transient inward currents andCa²⁺ sparks in PASMCs. In Xenopus oocytes, CNalso induced Ca²⁺ release and activatedI_Cl(Ca), and these responses were inhibited by thapsigarginand cyclopiazonic acid to deplete sarcoplasmic reticulum (SR)Ca²⁺, whereas neither heparin nor anti-inositol1,4,5-trisphosphate receptor (IP₃R) antibodies affected CNresponses. In both PASMCs and oocytes, CN-evoked Ca²⁺release was inhibited by carbonyl cyanidem-chlorophenylhydrazone (CCCP) and oligomycin or CCCP andthapsigargin. Whereas hypoxic stimuli resulted in Ca²⁺release in pulmonary but not mesenteric artery myocytes, CN induced release in both cell types. We conclude that metabolic inhibition withCN increases [Ca²⁺]_i in both pulmonary andsystemic artery myocytes by stimulating Ca²⁺ release fromthe SR and mitochondria.

     

Mineral nutrition and auxin-induced growth of Triticum coleoptile sections

A revised method has been described for assaying auxin by thegrowth of Triticum coleoptile sections. With additions of Ca(NO₃)₂10^–4 and MgSO₄ 10^–5 mol liter^–1 the sensitivityand accuracy have been increased. This is mainly due to Ca.The coleoptiles obviously suffer from Ca-deficiency. The importanceof a strict time schedule for manipulations is emphasiced. Theduration of the tests is limited to 6 hr.Indole-3-acetic acidcan be determined in concentrations down to 10^–9mol liter^–1;from 10^–10 to 10^–9mol liter^–1 the log-logrelation between concentration and growth is linear. Above 10^–7mol liter^–1 elongation takes place under an abnormal increasein elastic extension, indicating that growth is limited by someunknown wallstabilizing factor. The interrelation between auxin,an anti-auxin and Ca are discussed. (Received May 8, 1973; )     

Growth Stage Modulates Salinity Tolerance of New Zealand Spinach (Tetragonia tetragonioides, Pall.) and Red Orach (Atriplex hortensis L.)

The response of two speciality vegetable crops, New Zealandspinach (Tetragonia tetragonioides Pall.) and red orach (Atriplexhortensis L.), to salt application at three growth stages wasinvestigated. Plants were grown with a base nutrient solutionin outdoor sand cultures and salinized at 13 (early), 26 (mid),and 42 (late) d after planting (DAP). For the treatment saltconcentrations, we used a salinity composition that would occurin a typical soil in the San Joaquin Valley of California usingdrainage waters for irrigation. Salinity treatments measuringelectrical conductivities (EC_i) of 3, 7, 11, 15, 19 and 23 dSm^-1were achieved by adding MgSO₄, Na₂SO₄, NaCl and CaCl₂to thebase nutrient solution. These salts were added to the base nutrientsolution incrementally over a 5-d period to avoid osmotic shockto the seedlings. The base nutrient solution without added saltsserved as the non-saline control (3 dS m^-1). Solution pH wasuncontrolled and ranged from 7.7 to 8.0. Both species were saltsensitive at the early seedling stage and became more salt tolerantas time to salinization increased. For New Zealand spinach,the salinity levels that gave maximal yields (C_max) were 0,0 and 3.1 dS m^-1and those resulting in a 50% reduction of biomassproduction (C₅₀) were 9.1, 11.1 and 17.4 dS m^-1for early, midand late salinization dates, respectively. Maximal yield ofred orach increased from 4.2 to 10.9 to 13.7 dS m^-1as the timeof salinization increased from 13, to 26, to 42 DAP, respectively.The C₅₀value for red orach was unaffected by time of salt imposition(25 dS m^-1). Both species exhibited high Na⁺accumulation evenat low salinity levels. Examination of K-Na selectivity dataindicated that K⁺selectivity increased in both species withincreasing salinity. However, increased K-Na selectivity didnot explain the increased salt tolerance observed by later salinization.Higher Na-Ca selectivity was determined at 3 dS m^-1in New Zealandspinach plants treated with early- and mid-salinization plantsrelative to those exposed to late salinization. This correspondedwith lower C_maxand C₅₀values for those plants. Lower Ca uptakeselectivity or lower Ca levels may have inhibited growth inyoung seedlings. This conclusion is supported by similar resultswith red orach. High Na-Ca selectivity found only in the early-salinizationplants of red orach corresponded to the lower C_maxvalues measuredfor those plants. Copyright 2000 Annals of Botany Company New Zealand spinach, Tetragonia tetragonioides Pall., red orach, Atriplex hortensis L., salinity, stage of growth, ion accumulation, selectivity, plant nutrition     

Utilization Modes of Inorganic Carbon for Photosynthesis in Various Species of Chlorella

Rates of CO₂ and HCC₃^– fixation in cells of various Chlorellaspecies in suspension were compared from the amounts of ¹⁴Cfixed during the 5 s after the injection of a solution containingonly ¹⁴CO₂ or H¹⁴CO₃^–. Results indicated that irrespectiveof the CO₂ concentration during growth, Chlorella vulgaris 11h and C. miniata mainly utilized CO₂, whereas C. vulgaris C-3,C. sp. K. and C. ellipsoidea took up HCO₃^– in additionto CO₂. Cells of C. pyrenoidosa that had been grown with 1.5%CO₂ (high-CO₂ cells) mainly utilized CO₂, whereas those grownwith air (low-CO₂ cells) utilized HCO₃^– in addition toCO₂. Cells that utilized HCO₃^– had carbonic anhydrase(CA) on their surfaces. The effects of Diamox and CA on the rates of CO₂ and HCO₃^–fixation are in accord with the inference that HCO₃^– wasutilized after conversion to CO₂ via the CA located on the cellsurface. CA was found in both the soluble and insoluble fractions;the CA on the cell surface was insoluble. Independent of the modes of utilization, the apparent K_m (NaHCO₃)for photosynthesis was much lower in low-CO₂ cells than in high-CO₂ones. The fact that the CA in the soluble fraction in C. vulgarisC-3 was closely correlated with the K_m(NaHCO₃) indicates thatsoluble CA lowers the K_m. ¹ Dedicated to the late Professor Joji Ashida, one of the foundersand first president of the Japanese Society of Plant Physiologists. ⁴ On leave from Research and Production Laboratory of Algology,Bulgarian Academy of Sciences, Sofia. (Received September 14, 1982; Accepted March 1, 1983)     

Alterations in Ca2+ cycling by lysoplasmenylcholine in adult rabbit ventricular myocytes

We previously reported thatlysoplasmenylcholine (LPlasC) altered the action potential (AP) andinduced afterdepolarizations in rabbit ventricular myocytes. In thisstudy, we investigated how LPlasC alters excitation-contractioncoupling using edge-motion detection, fura-PE3 fluorescent indicator,and perforated and whole cell patch-clamp techniques. LPlasC increasedcontraction, myofilament Ca²⁺ sensitivity, systolic anddiastolic free Ca²⁺ levels, and the magnitude ofCa²⁺ transients concomitant with increases in the maximumrates of shortening and relaxation of contraction and the rising anddeclining phases of Ca²⁺ transients. In some cells, LPlasCinduced arrhythmias in a pattern consistent with early and delayedaftercontractions. LPlasC also augmented the caffeine-inducedCa²⁺ transient with a reduction in the decay rate.Furthermore, LPlasC enhanced L-type Ca²⁺ channel current(I_Ca,L) and outward currents. LPlasC-induced alterations in contraction and I_Ca,L wereparalleled by its effect on the AP. Thus these results suggest thatLPlasC elicits distinct, potent positive inotropic, lusitropic, andarrhythmogenic effects, resulting from increases in Ca²⁺influx, Ca²⁺ sensitivity, sarcoplasmic reticular (SR)Ca²⁺ release and uptake, SR Ca²⁺ content, andprobably reduction in sarcolemmal Na⁺/Ca²⁺ exchange.

     

Hypertonic perfusion inhibits intracellular Na and Ca accumulation in hypoxic myocardium

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

     

Photosynthesis of Stands of Tomato, Cucumber and Sweet Pepper Measured in Greenhouses under Various CO2-concentrations

The rate of net photosynthesis (P) of whole plant stands oftomato (Lycopersicon esculentum Mill.), cucumber (Cucumis sativusL.) and sweet pepper (Capsicum annuum L.) was measured in sixlong-term experiments in large greenhouses under normal operatingconditions and CO₂-concentrations between 200 and 1200 µmolmol^-1. The objective was to quantify the responses to lightand carbon dioxide and to obtain data sets for testing simulationmodels. The method of measuring canopy photosynthesis involvedan accurate estimation of the greenhouse CO₂ balance, usingnitrous oxide (N₂O) as tracer gas to determine, on-line, theexchange rate between greenhouse and outside air. The estimatedrelative error in the observed P was about ± 10%, exceptthat higher relative errors could occur under particular conditions. A regression equation relating P to the photosynthetically activeradiation, the CO₂ concentration and the leaf area index explained83-91% of the variance. The main canopy photosynthesis characteristicscalculated with the fitted regression equations were: canopyP_max 5-9 g m^-2 h^-1 CO₂ uptake; ratio P_max/LAI 1·5-3 gm^-2 h^-1; light compensation point 32-86 µmol s^-1 m^-2;light use efficiency (quantum yield) at low light 0·06-0·10µmol µmol^-1 and CO₂ compensation point 18-54 µmolmol^-1. The results were related to the prevailing conditions.Copyright1994, 1999 Academic Press Canopy photosynthesis, Capsicum annuum L., carbon dioxide, CO₂, CO₂ balance, CO₂ use efficiency, cucumber, _{Cucumis sativus} L., glasshouse, greenhouse, light use efficiency, Lycopersicon esculentum Mill., sweet pepper, tomato, tracer gas     

Membrane Potentials in Excitable Cells of Aldrovanda vesiculosa Trap-Lobes

The resting membrane potential in excitable cells of Aldrovandatrap-lobes is composed of diffusion and electrogenic potentials.The diffusion potential, about –100 mV in artificial pondwater, was determined from the external K⁺ and Na⁺ concentrations.The permeability ratio, P_Na/P_K of the membrane was estimatedto be about 0.3. The electrogenic potential hyperpolarized themembrane to about –140 mV. The peak value of the actionpotential increased by +26 mV with a tenfold increase in theexternal Ca²⁺ concentration. The action potential was blockedby an application of the Ca²⁺ chelater or the Ca channel blocker,LaCl₃. Cells showed additional Ca²⁺ influx (7.8 pmole/cm² impulse)during membrane excitation. These facts suggest that the transientincrease in Ca²⁺ influx causes the action potential presentin cells of Aldrovanda trap-lobes. ¹ Present address: Jerry Lewis Neuromuscular Research Center,School of Medicine, University of California Los Angeles, LosAngeles, CA90024, U.S.A. ² Present address: Biological Laboratory, Kyoritsu Women's University,Hachioji 193, Japan. (Received September 21, 1983; Accepted September 7, 1984)     

Mechanism of shortened action potential duration in Na+-Ca2+ exchanger knockout mice

In cardiac-specific Na⁺-Ca²⁺ exchanger (NCX) knockout (KO) mice, the ventricular action potential (AP) is shortened. The shortening of the AP, as well as a decrease of the L-type Ca²⁺ current (I_Ca), provides a critical mechanism for the maintenance of Ca²⁺ homeostasis and contractility in the absence of NCX (Pott C, Philipson KD, Goldhaber JI. Excitation-contraction coupling in Na⁺-Ca²⁺ exchanger knockout mice: reduced transsarcolemmal Ca²⁺ flux. Circ Res 97: 1288–1295, 2005). To investigate the mechanism that underlies the accelerated AP repolarization, we recorded the transient outward current (I_to) in patch-clamped myocytes isolated from wild-type (WT) and NCX KO mice. Peak I_to was increased by 78% and decay kinetics were slowed in KO vs. WT. Consistent with increased I_to, ECGs from KO mice exhibited shortened QT intervals. Expression of the I_to-generating K⁺ channel subunit Kv4.2 and the K⁺ channel interacting protein was increased in KO. We used a computer model of the murine AP (Bondarenko VE, Szigeti GP, Bett GC, Kim SJ, and Rasmusson RL. Computer model of action potential of mouse ventricular myocytes. Am J Physiol Heart Circ Physiol 287: 1378–1403, 2004) to determine the relative contributions of increased I_to, reduced I_Ca, and reduced NCX current (I_NCX) on the shape and kinetics of the AP. Reduction of I_Ca and elimination of I_NCX had relatively small effects on the duration of the AP in the computer model. In contrast, AP repolarization was substantially accelerated when I_to was increased in the computer model. Thus, the increase in I_to, and not the reduction of I_Ca or I_NCX, is likely to be the major mechanism of AP shortening in KO myocytes. The upregulation of I_to may comprise an important regulatory mechanism to limit Ca²⁺ influx via a reduction of AP duration, thus preventing Ca²⁺ overload in situations of reduced myocyte Ca²⁺ extrusion capacity. genetically altered mice; cardiac myocytes; short QT interval; transient outward current     

影响叶螨磷酸酯酶活性的四因子数学模型

应用二次回归通用旋转组合设计,组建了影响叶螨磷酸酯酶（酸性和碱性）活性的四因子（缓冲液Ph值X₁、温浴时间X₂、反应温度X₃、底物浓度X₄）数学模型: Y_酸性=0.456380+0.107889X₂+0.069027X₃-0.026836X₁²-0.030794X₃², F=24.98,P<0.01;Y_碱性=0.267286-0.200736X₁+0.049541X₂+0.030930X₃-.049063X₁X₂+0.053585X₁²-0.049665X₂^2, F=57.68,P<0.01。结果表明,温浴时间是影响叶螨酸性磷酸酯酶活性的关键因子,在缓冲液pH 4.4、底物浓度8.5×10^-3 mol/L、42℃温浴40 min测得该酶活性最强。影响碱性磷酸酯酶活性的关键因子则是缓冲液pH值,pH 9.0、37℃恒温30 min、底物7.5×10^-3 mol/L的条件下,光密度值最大。两种酶的最大吸收峰波长为405 nm。