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1.
Summary The ability of early proximal tubule cells of theNecturus kidney to regulate volume was evaluated using light microscopy, video analysis and conventional microelectrodes.Necturus proximal tubule cells regulate volume in both hyperand hyposmotic solutions. Volume regulation in hyperosmotic fluids is HCO
3
–
dependent and is associated with a decrease in the relative K+ conductance of the basolateral cell membrane and a decrease in the resistance ratio,R
a
/R
bl
. Volume regulation in hyposmotic solutions is also dependent upon the presence of HCO
3
–
but is also inhibited by 2mm Ba2+ in the basolateral solution. Hyposmotic regulation is accompanied by an increase in the relative K+ conductance of the basolateral cell membrane and an increase inR
a
/R
bl
. Neither hypo- nor hyposmotic regulation have any affect on the depolarization of the basolateral cell membrane potential induced by HCO
3
–
removal. We conclude that volume regulation in the early proximal tubule of the kidney involves both HCO
3
–
-dependent transport systems and the basolateral K+ conductance. 相似文献
2.
Cysteamine is widely used in rodents to induce duodenal ulcer. Herein, the pathogenesis of duodenal ulceration in its earliest stages was reviewed using findings from cysteamine-and propionitrile-induced duodenal ulcer in rodent models, especially taking into account changes in the secretion of gastric acid, duodenal and pancreatic bicarbonate as well asgastroduodenal motility. The effect of cysteamine-HCl in inducing ulcers in rats is circadian rhythm-dependent. The effect is greatest from just before the end of diurnal rest to just after the start of nocturnal activity. The chronobiologic effect may be in part due to the circadian rhythm-dependent increased gastric acid production from cysteamine. Titratable acidity was found to be twice as great in the gastric juice of rodents when cysteamine was given by injection at 2000 (just after the start of nocturnal activity) in comparison to when given at 0800 or 1200 (at the beginning or middle span of daily rest). Further studies have shown that adrenalectomy of rats 7 days before cysteamine administration obliterated the observed circadian susceptibility to ulcer formation. Duodenal ulceration, at least in the cysteamine model, appears to be under chronobiologic neuroendocrine control or influence, seemingly mediated by the adrenal glands. 相似文献
3.
It has been known for some time that bicarbonate reverses the inhibition, by formate under HCO3
--depletion conditions, of electron transport in thylakoid membranes. It has been shown that the major effect is on the electron acceptor side of photosystem II, at the site of plastoquinone reduction. After presenting a historical introduction, and a minireview of the bicarbonate effect, we present a hypothesis on how HCO3
- functions in vivo as (a) a proton donor to the plastoquinone reductase site in the D1-D2 protein; and (b) a ligand to Fe2+ in the QA-Fe-QB complex that keeps the D1-D2 proteins in their proper functional conformation. They key points of the hypothesis are: (1) HCO3
- forms a salt bridge between Fe2+ and the D2 protein. The carboxyl group of HCO3
- is a bidentate ligand to Fe2+, while the hydroxyl group H-bonds to a protein residue. (2) A second HCO3
- is involved in protonating a histidine near the QB site to stabilize the negative charge on QB. HCO3
- provides a rapidly available source of H+ for this purpose. (3) After donation of a H+, CO3
2- is replaced by another HCO3
-. The high pKa of CO3
2- ensures rapid reprotonation from the bulk phase. (4) An intramembrane pool of HCO3
- is in equilibrium with a large number of low affinity sites. This pool is a H+ buffering domain functionally connecting the external bulk phase with the quinones. The low affinity sites buffer the intrathylakoid [HCO3
-] against fluctuations in the intracellular CO2. (5) Low pH and high ionic strength are suggested to disrupt the HCO3
- salt bridge between Fe2+ and D2. The resulting conformational change exposes the intramembrane HCO3
- pool and low affinity sites to the bulk phase.Two contrasting hypotheses for the action of formate are: (a) it functions to remove bicarbonate, and the low electron transport left in such samples is due to the left-over (or endogenous) bicarbonate in the system; or (b) bicarbonate is less of an inhibitor and so appears to relieve the inhibition by formate. Hypothesis (a) implies that HCO3
- is an essential requirement for electron transport through the plastoquinones (bound plastoquinones QA and QB and the plastoquinone pool) of photosystem II. Hypothesis (b) implies that HCO3
- does not play any significant role in vivo. Our conclusion is that hypothesis (a) is correct and HCO3
- is an essential requirement for electron transport on the electron acceptor side of PS II. This is based on several observations: (i) since HCO3
-, not CO2, is the active species involved (Blubaugh and Govindjee 1986), the calculated concentration of this species (220 M at pH 8, pH of the stroma) is much higher than the calculated dissociation constant (Kd) of 35–60 M; thus, the likelihood of bound HCO3
- in ambient air is high; (ii) studies on HCO3
- effect in thylakoid samples with different chlorophyll concentrations suggest that the left-over (or endogenous) electron flow in bicarbonate-depleted chloroplasts is due to left-over (or endogenous) HCO3
- remaining bound to the system (Blubaugh 1987).Abbreviations DCMU
3-(3,4-dichlorophenyl)-1, 1-dimethylurea (common name: diuron)
- PSII
photosystem II
- QA
first plastoquinone electron acceptor of PSII
- QB
second plastoquinone acceptor of PS II 相似文献
4.
Chloroplasts with high rates of photosynthetic O2 evolution (up to 120 mol O2· (mg Chl)-1·h-1 compared with 130 mol O2· (mg Chl)-1·h-1 of whole cells) were isolated from Chlamydomonas reinhardtii cells grown in high and low CO2 concentrations using autolysine-digitonin treatment. At 25° C and pH=7.8, no O2 uptake could be observed in the dark by high- and low-CO2 adapted chloroplasts. Light saturation of photosynthetic net oxygen evolution was reached at 800 mol photons·m-2·s-1 for high- and low-CO2 adapted chloroplasts, a value which was almost identical to that observed for whole cells. Dissolved inorganic carbon (DIC) saturation of photosynthesis was reached between 200–300 M for low-CO2 adapted chloroplasts, whereas high-CO2 adapted chloroplasts were not saturated even at 700 M DIC. The concentrations of DIC required to reach half-saturated rates of net O2 evolution (Km(DIC)) was 31.1 and 156 M DIC for low- and high-CO2 adapted chloroplasts, respectively. These results demonstrate that the CO2 concentration provided during growth influenced the photosynthetic characteristics at the whole cell as well as at the chloroplast level.Abbreviations Chl
chlorophyll
- DIC
dissolved inorganic carbon
- Km(DIC)
coneentration of dissolved inorganic carbon required for the rate of half maximal net O2 evolution
- PFR
photon fluence rate
- SPGM
silicasol-PVP-gradient medium 相似文献
5.
Air-grown cells of a marine, small-celled (2 m diameter) strain of Stichococcus bacillaris contained appreciable carbonic-anhydrase activity but this was repressed when cells were grown on air enriched with 5% (v/v) CO2. Assay of carbonic-anhydrase activity using intact cells and cell extracts showed all activity was intracellular in this Stichococcus strain. Measurement of inorganic-carbon-dependent photosynthetic O2 evolution at pH 5.0, where CO2 is the predominant form of inorganic carbon, showed that the concentration of inorganic carbon required for half-maximal rate of photosynthetic O2 evolution [K0.5(CO2)] was 4.0 M for both air- and CO2-grown cells. At pH 8.3 the K0.5(CO2) was 0.3 mM for air-grown and 0.6 mM for CO2-grown cells. Sodium ions did not enhance bicarbonate utilization. Measurement of the internal inorganic-carbon pool (HCO
3
–
+CO2) by the silicone-oil-layer centrifugal filtering technique showed that air- and CO2-grown cells were able to concentrate inorganic carbon up to 20-fold in relation to the external medium at pH 5.0 but not at pH 8.3. In this alga the high affinity for CO2 and inorganic-carbon accumulation in CO2- and air-grown cells results from active CO2 transport that is not dependent on carbonic-anhydrase activity.Abbreviation Hepes
4-(2-hydroxyethyl)-1-piperazine ethanesulfonic acid 相似文献
6.
The response of soil exchangeable sodium percentage levels to nitrate reductase activity, nitrite reductase activity, free proline, DNA, RNA, chlorophyll a and b contents and yield components in lentil (Lens esculenta Moench)cv. PL 406 was studied in a replicated pot experiment. All the biochemical observations were recorded at four growth stages i.e. 30, 60, 90 and 120 days after sowing (DAS). Germination occurred up to exhangeable sodium percentage of 30, but plants survived only up to 25. With increasing exchangeable sodium percentage, there was a continuous decrease in chlorophyll a and b content, nitrate and nitrite reductase enzyme activities and DNA and RNA content. Increasing level of sodicity enhanced the free proline content up to 60 DAS, after which values fell.Number of pods per plant, 1000 grain weight and grain yield were significantly reduced with increasing level of sodicity, but the number of grains per pod was not affected. 相似文献
7.
Bruce A. Biagi 《The Journal of membrane biology》1985,88(1):25-31
Summary Conventional microelectrodes were used to study the effects of SITS (4-acetamido-4-isothiocyanostilbene-2,2-disulfonate) on the basolateral membrane potentialVbl of the superficial proximal straight tubule (PST) of the rabbit kidney perfusedin vitro. Addition of 0.1mm SITS to the bathing solution resulted in a slow and irreversible hyperpolarization ofVbl from –42.5±1.17 (37) mV to –77.3±0.83 (52) mV. The new steady-state potential was reached in 10 to 15 min and was accompanied by visible cell swelling. Associated with thisVbl hyperpolarization was: 1) an increased steady-state depolarization (from 6.2±0.77 (17) mV to 25.7±0.83 (29) mV) in response to increasing bath potassium concentration from 5 to 16.7mm (HK); 2) a decreased transient depolarization (from 19.8±1.88 (8) mV to 0.43±0.37 (8) mV) in response to decreasing bath bicarbonate concentration from 22 to 6.6mm at constant bath pH (L-HCO3); and 3) inhibition of a depolarizing overshoot and a decreased steady-state depolarization (from 35.9±1.84 (12) mV to 4.7±1.37 (13) mV) in response to reducing bath sodium concentration from 144 to zero (0-Na). Sodium, chloride and NMDG (N-methyl-d-glucamine) were used as the substituting ions, respectively. These results are consistent with the presence of a coupled sodium-bicarbonate carrier in the basolateral membrane which is electrogenic and SITS inhibitable. Comparison of the time course of SITS effects on these ion-substitution responses suggests that the inhibition of the bicarbonate exit pathway(s) is the primary event and that the changes inVbl and in the steady-stateVbl responses to HK and 0-Na are secondary events which may be related to changes in intracellular composition and/or basolateral membrane properties. 相似文献
8.
Summary Active HCO
3
t-
secretion in the anterior rectal salt gland of the mosquito larva,Aedes dorsalis, is mediated by a 11 Cl–/HCO
3
–
exchanger. The cellular mechanisms of HCO
3
–
and Cl– transport are examined using ion- and voltage-sensitive microelectrodes in conjunction with a microperfused preparation which allowed rapid saline changes. Addition of DIDS or acetazolamide to, or removal of CO2 and HCO
3
–
from, the serosal bath caused large (20 to 50 mV) hyperpolarizations of apical membrane potential (V
a) and had little effect on basolateral potential (V
bl). Changes in luminal Cl– concentration alteredV
a in a repid, linear manner with a slope of 42.2 mV/decaloga
Cl
l
–. Intracellular Cl– activity was 23.5mm and was approximately 10mm lower than that predicted for a passive distribution across the apical membrane. Changes in serosal Cl– concentration had no effect onV
bl, indicating an electrically silent basolateral Cl– exit step. Intracellular pH in anterior rectal cells was 7.67 and the calculated
was 14.4mm. These results show that under control conditions HCO3 enters the anterior rectal cell by an active mechanism against an electrochemical gradient of 77.1 mV and exits the cell at the apical membrane down a favorable electrochemical gradient of 27.6 mV. A tentative cellular model is proposed in which Cl enters the apical membrane of the anterior rectal cells by passive, electrodiffusive movement through a Cl–-selective channel, and HCO
3
–
exits the cell by an active or passive electrogenic transport mechanism. The electrically silent nature of basolateral Cl– exit and HCO3 entry, and the effects of serosal addition of the Cl–/HCO3 exchange inhibitor, DIDS, on
and transepithelial potential (V
ic) suggest strongly that the basolateral membrane is the site of a direct coupling between Cl– and HCO
3
–
movements. 相似文献
9.
CO2 uptake and transport in leaf mesophyll cells 总被引:4,自引:3,他引:1
Abstract The acquisition of inorganic carbon for photosynthetic assimilation by leaf mesophyll cells and chloroplasts is discussed with particular reference to membrane permeation of CO2 and HCO?3. Experimental evidence indicates that at the apoplast pH normally experienced by leaf mesophyll cells (pH 6–7) CO2 is the principal species of inorganic carbon taken up. Uptake of HCO?3 may also occur under certain circumstances (i.e. pH 8.5), but its contribution to the net flux of inorganic carbon is small and HCO?3 uptake does not function as a CO2-concentrating mechanism. Similarly, CO2 rather than HCO?3 appears to be the species of inorganic carbon which permeates the chloroplast envelope. In contrast to many C3 aquatic plants and C4 plants, C3 terrestrial plants lack specialized mechanisms for the acquisition and transport of inorganic carbon from the intercellular environment to the site of photosynthetic carboxylation, but rely upon the diffusive uptake of CO2. 相似文献
10.
Abdallah Ghalayini Robert E. Anderson 《Biochemical and biophysical research communications》1984,124(2):503-506
Isolated frog () retinas were labeled in the dark with either [32P]PO4-orthophosphate or -[2-3H]inositol for 2.5–4 hrs. After washing the retinas with cold buffer, they were exposed to brief flashes of light (5 secs or 15 secs) and their rod outer segments isolated. Upon separation of labeled phospholipids, a specific decrease in label in phosphatidylinositol 4,5-bisphosphate was observed, whereas there was no significant effect on the labeling of phosphatidylinositol 4-phosphate, phosphatidylinositol, or phosphatidic acid. These results are indicative of a light-activated phosphatidylinositol 4,5-bisphosphate-specific phospholipase C in frog rod outer segments. 相似文献