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1.
20% of the acetylcholine receptors in cultured chick skeletal muscle remain unbound following long-term growth of muscle in medium containing a potent, essentially irreversible receptor-blocking agent, α-bungarotoxin. About half the receptors which are unavailable for interaction with extracellular α-bungarotoxin are newly synthesized molecules which presumably are being processed and transported to the plasma membrane. When the muscle cultures are switched to a medium containing 2H, 13C, 15N-amino acids, these receptors are rapidly labeled, the fraction of labeled molecules beginning to plateau at 3 hr. Few labeled receptors appear in the plasma membrane during the first 3 hr of labeling with 2H, 13C, 15N-amino acids. After 3.5 hr of labeling, virtually all the receptors being incorporated into the plasma membrane are labeled receptors. The kinetics of labeling of the “pool” and “surface” receptors with 2H, 13C, 15N-amino acids confirm the “precursor-product” type relationship of pool and surface acetylcholine receptors.In this study, receptors synthesized in medium containing 2H, 13C, 15N-amino acids were resolved from 1H, 12C, 14N-receptors by velocity sedimentation in sucrose-deuterium oxide and sucrose-H2O gradients, and their densities were estimated from sedimentation rates in shallow gradients of various average density. Estimated densities were 1.32 g/cm3 for 1H, 12C, 14N-receptors and 1.41 g/cm3 for 2H, 13C, 15N-receptors. This density difference corresponds to 80% substitution of normal aminoacyl residues by 2H, 13C, 15N-residues in the denser receptor. 相似文献
2.
Identification of Residues in Domain III of Bacillus thuringiensis Cry1Ac Toxin That Affect Binding and Toxicity
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Alanine substitution mutations in the Cry1Ac domain III region, from amino acid residues 503 to 525, were constructed to study the functional role of domain III in the toxicity and receptor binding of the protein to Lymantria dispar, Manduca sexta, and Heliothis virescens. Five sets of alanine block mutants were generated at the residues 503SS504, 506NNI508, 509QNR511, 522ST523, and 524ST525. Single alanine substitutions were made at the residues 509Q, 510N, 511R, and 513Y. All mutant proteins produced stable toxic fragments as judged by trypsin digestion, midgut enzyme digestion, and circular dichroism spectrum analysis. The mutations, 503SS504-AA, 506NNI508-AAA, 522ST523-AA, 524ST525-AA, and 510N-A affected neither the protein’s toxicity nor its binding to brush border membrane vesicles (BBMV) prepared from these insects. Toward L. dispar and M. sexta, the 509QNR511-AAA, 509Q-A, 511R-A, and 513Y-A mutant toxins showed 4- to 10-fold reductions in binding affinities to BBMV, with 2- to 3-fold reductions in toxicity. Toward H. virescens, the 509QNR511-AAA, 509Q-A, 511R-A, and 513Y-mutant toxins showed 8- to 22-fold reductions in binding affinities, but only 509QNR511-AAA and 511R-A mutant toxins reduced toxicity by approximately three to four times. In the present study, greater loss in binding affinity relative to toxicity has been observed. These data suggest that the residues 509Q, 511R, and 513Y in domain III might be only involved in initial binding to the receptor and that the initial binding step becomes rate limiting only when it is reduced more than fivefold. 相似文献
3.
Celia E. Deane-Drummond 《Physiologia plantarum》1986,66(1):115-121
Net nitrate uptake, 36ClO?3/NO?3 influx and 36Cl? influx into Pisum sativum L. cv. Feltham First seedlings have been examined following growth in culture medium containing different combinations of chloride and nitrate. When young (6 days old) seedlings, that had been grown in the absence of N were used, nitrate accumulation stimulated net nitrate uptake and 36ClO?3/NO?3 influx (r2= 0.99) while chloride accumulation inhibited nitrate uptake and 36ClO?3/NO?3 influx (r2= 0.65). When nitrate was provided during growth there was no effect of chloride pretreatment on net nitrate uptake and there was little effect of total [NO?3+ Cl?]i on 36ClO?3/NO?3 influx (r2= 0.26). A direct effect of Cl? on 36ClO?3/NO?3 influx was only found when seedlings had been starved of N for more prolonged periods (14 days). When moderate chloride was supplied during growth, 36Cl? influx was insensitive to nitrate or chloride accumulated, but significantly correlated with loge [NO?3+ Cl?]i (r2= 0.75). When trace amounts of Cl? were supplied during growth 36Cl? influx was inhibited by (a) NO?3 in the external medium and (b) Cl? pretreatment, but was insensitive to NO?3 pretreatment. The sensitivity of 36Cl? influx to external nitrate was not found following Cl? pretreatment in the absence of nitrate. The possibility that there are two populations of chloride carriers which differ in their sensitivity to external nitrate is discussed. Tentative schematic models to account for the regulation of nitrate and chloride uptake are proposed in the context of current hypotheses for regulation of ion transport and control systems theory. 相似文献
4.
The activation of desoxyribonuclease on desoxyribonucleate, known to occur with Mg++ and Mn++, has been shown to occur equally well with Co++, to nearly the same extent with Fe++, and to a lesser extent with Ca++, Ba++, Sr++, Ni++, Cd++, and Zn++. The conditions under which the optimal activation is revealed vary among these ions. Thus, Mg++, Mn++, and Co++ may show marked activation under conditions in which Fe++ is nearly ineffective. Since too high a concentration of an ion may be as ineffective as too little, concentration-activation curves were determined for each ion. Per micromole of nucleic acid phosphorus, the optimal effective amount of each ion in micromoles is as follows: Mg++ 3, Mn++ 3, Co++ 3, Fe++ 0.3, Ni++ 0.3, Ba++ 1.7, Ca++ 3, Sr++ 3, Zn++ 0.3, and Cd++ 0.3.The optimum pH for the activation with Mg++, Co++, and Ca++ is about 6.5, that with Fe++ is at 5.7, while Mn++ shows two optima at pH 6.8 and 8.0.Experiments conducted in Pyrex and in quartz vessels showed the same results, and indicated that there was no activation of desoxy-ribonuclease in the absence of added salts. 相似文献
5.
Permeability of reconstituted sarcoplasmic reticulum vesicles: Reconstitution of the K+, Na+ channel
Permeability properties of reconstituted rabbit skeletal muscle sarcoplasmic reticulum vesicles were characterized by measuring efflux rates of [3H]inulin, [3H]choline+, 86Rb+, and 22Na+, as well as membrane potential changes using the voltage-sensitive probe, 3,3′-dipentyl-2,2′-oxacarbocyanine. Native vesicles were dissociated with deoxycholate and were reconstituted by dialysis. Energized Ca2+ accumulation was partially restored. About of the reconstituted vesicles were found to be ‘leaky’, i.e., permeable to choline+ or Tris+ but not to inulin. The remaining reconstituted vesicles were ‘sealed’, i.e., impermeable to choline+, Tris+ and inulin. Sealed reconstituted vesicles could be further subdivided according to their K+, Na+ permeability. About , previously designated Type I, were readily permeable to K+ and Na+, indicating the presence of the K+, Na+ channel of sarcoplasmic reticulum. The remaining sealed vesicles (Type II) formed a permeability barrier to K+ and Na+, suggesting that they lacked the K+, Na+ channel. These studies show that the K+, Na+ channel of sarcoplasmic reticulum can be solubilized with detergent and reconstituted with retention of activity. Furthermore, our results suggest that part or all of the decreased Ca2+-loading efficiency of reconstituted vesicles may be due to the presence of a significant fraction of leaky vesicles. 相似文献
6.
Cortical thymocytes are devoid of any immune function, as tested by presently available techniques. The ability of this subpopulation to respond to mitogens or antigens in the presence of interleukin 2 (IL-2) produced by activated mature T lymphocytes has been claimed but is still questioned. In an attempt to study the participation of the different thymocyte subsets and especially that of the cortical type, phenotypic modifications were examined during concanavalin A activation in the presence of IL-2. An immunofluorescent double labeling technique with anti-Lyt 1 and anti-Lyt 2 antibodies was used which led to the determination of four different phenotypes: Lyt 1+2+, Lyt 1+2?, Lyt 1?2+, and Lyt 1?2?. Careful analysis of cell viability in culture and expression of the results in absolute numbers of living cells per culture allowed us to follow modifications of small cellular subsets. Cultures of total thymocytes and PNA-agglutinated (enriched in Lyt 1+2+ cells) and non-PNA-agglutinated cells (enriched in Lyt 1+2?, Lyt 1?2+, and Lyt 1?2? cells) were studied. It was shown that thymocyte activation began by early phenotypic modifications which took place within the first 2 hr of culture but only when Con A plus IL-2 were used. These modifications imply the reduction of the Lyt 1+2+ pool and a compensatory enhancement of Lyt 1?2+ and Lyt 1?2? cells, without modification of the total cell number or [3H]thymidine incorporation. These early phenotypic changes are interpreted as the modulation of antigens on the surface of Lyt 1+2+ cells. The second phase of thymocyte activation implies cell death (essentially Lyt 1+2+ cells) and cell proliferation. The cells which specifically proliferate in the presence of Con A and IL-2 are Lyt 1+2? and Lyt 1?2+, the latter always being present in greater number. Cell survival and absolute number of Lyt 1+2? and Lyt 1?2+ cells in the activated PNA?-enriched population are always higher than in total thymocyte and PNA+ cells cultures. Thus, if Lyt 1+2+ cortical thymocytes do not proliferate by themselves, they seem to intervene by providing Lyt 1?2+ cells which proliferate secondarily. 相似文献
7.
Influence of La3+ on the accumulation of trace elements (75Se, 56Co, 83Rb, 48V, 95mTc, and 67Ga) in chloroplasts of cucumber seedling leaves was studied by a radioactive multitracer technique. At the same time, chloroplast
contents of different concentrations of La3+ treatment were calculated. It was observed that chloroplast contents peaked at 0.02 mM La3+ treatment and that the uptake and distribution of these trace elements in chloroplasts of cucumber seedling leaves are different
under different La3+ treatments. With the increase of lanthanum concentrations from 0.002 to 2 mM, the uptake percentages of 75Se, 56Co, and 83Rb presented an obvious increase and then sharply decreased in contrast to the nonlanthanum treatment, whereas there appeared
a sharp decrease and then restored control level in the uptake of 48V. The other two trace elements, namely 95mTc and 67Ga, were accumulated only in the presence of 0.02 mM La3+. The results indicate that lanthanum treatments to growing the cucumber lead to the change of trace element uptake in the
chloroplasts of leaves, which suggest that lanthanum might influence the accumulation of trace elements in chloroplasts of
cucumber seedling leaves by regulation of various ion transport mechanisms, thus affecting the photosystem of leaves. 相似文献
8.
Maize seeds and five-day-old maize seedlings were incubated in media containing Pb2+ at concentrations of 50, 100, and 200 mg 1-1 and Cd2+ at concentrations of 1, 5, 10 and 50 mg 1-1. After five days of incubation, both heavy metals were determined by means of AAS following wet mineralisation of roots and
shoots. The results obtained indicate that Pb2+ were transported to shoots from roots at a lower rate than Cd2+.
Phosphoenolpyruvate carboxylase (PEPC) isolated from germinating maize seeds was inhibited to a comparable degree by solutions
containing 0.001 mmol 1-1 Pb2+, 0.01 mmol 1-1 Cd2+, and 0.005 mmol 1-1 Cu2+. The enzyme was protected against this inhibition by the addition of mercaptoethanol, the substrate (PEP), or the cofactor
(Mg2+). The inhibition increased during a 20 min incubation of the enzyme with salts of the metals. Mn2+, Ni2+, and Co2+ ions could partially substitute for the metal cofactor Mg2+. Km values for these metal ions were as follows: for Mg2+ 0.07 mmol 1-1 in the range from 0 to 0.30 mmol 1-1 Mg2+; 0.71 mmol 1-1 for 0.30 to 2.50 mmol 1-1 Mg2+; for Mn2+ 0.36 mmol 1-1; for Ni2+ 0.34 mmol 1{-1}; and for Co2+ 0.20 mmol 1-1. The activity of the enzyme reached with Mn2+ 85 %, with Ni2+ 65 %, and with Co2+ 55 % of the activity recorded with Mg2+. 相似文献
9.
We report the conformational analysis by 1H‐NMR in DMSO and computer simulations involving distance geometry and molecular dynamics simulations of peptoid analogs of the cyclic hexapeptide c‐[Phe11‐Pro6‐Phe7‐d ‐Trp8‐Lys9‐Thr10] L‐363,301 (the numbering refers to the positions in native somatostatin). The compounds c‐[Phe11‐Nphe6‐Nal7‐d ‐Trp8‐Lys9‐Thr10] ( Nphe 6 ‐ Nal 7 analog 1 ), c‐[Nal11‐Nphe6‐Phe7‐d ‐Trp8‐Lys9‐Thr10] ( Nal 11 ‐ Nphe 6 analog 2 ) and c‐[Phe11‐Nnal6‐Phe7‐d ‐Trp8‐Lys9‐Thr10] ( Nnal 6 analog 3 ), where Nphe denotes N‐benzylglycine and Nnal denotes N‐(1‐naphthylmethyl)glycine, are subjected to SAR studies in order to investigate the influence of the bulky naphthyl aromatic ring on the conformation. The Nal 11 ‐ Nphe 6 and Nphe 6 ‐Nal 7 analogs exhibit potent binding to the hsst2, hsst3 and hsst5 receptors, whereas the Nnal 6 analog has decreased binding affinity to all receptors but is more selective towards the hsst2 than the other two analogs and L‐363,301. The conformational search employing distance geometry, energy minimization and molecular dynamic simulations gives insight into the conformational flexibility of these analogs. The molecules adopt both cis and trans orientations of the peptide bond between residues 11 and 6. The cis isomers of these analogs adopt type II′ β‐turns with d ‐Trp in the i+1 position and type VIa β‐turns with the cis peptide bond between residues 6 and 11. The results of free and distance restrained molecular dynamics simulations at 300 K indicate that the Nphe 6 ‐Nal 7 and Nal 11 ‐Nphe 6 compounds adopt a preferred backbone conformation which can be described as ‘folded’ about residues 7 and 10. The Nnal 6 analog, which binds less effectively to the hsst receptors, has a more flexible backbone structure than the Nal 11 ‐Nphe 6 and Nphe 6 ‐Nal 7 analogs and prefers a ‘flat’ structure with regard to the orientations about Phe7 and Thr10 during molecular dynamics simulations. Copyright © 1999 European Peptide Society and John Wiley & Sons, Ltd. 相似文献
10.
《Journal of receptor and signal transduction research》2013,33(3):323-350
AbstractAssociation of 125I-Bolton-Hunter labelled substance P (125I-BH-SP) to suspended pancreatic acinar cells of the guinea pig was studied. Cellular association at 37°C and 22°C was inhibited by cholecystokinin octapeptide (CCK-8) in concentrations from 10?9 to 10?6M, whereas another pancreatic secretagogue, carbachol, was uneffective. The CCK induced inhibition disappeared at low temperatures. CCK-8 mainly interfered with internalization of 125I-BH-SP into acinar cells. Increased extracellular Ca2+ and the Ca2+ ionophores A23187 and ionomycin reduced association of 125I-BH-SP to cells whereas extracellular Ca2+ chelation with EGTA had the opposite effect. However, extra- and intracellular Ca2+ chelation did not affect the degree of CCK-induced reduction of 125I-BH-SP association to acinar cells but eliminated the effect of the calcium ionophore ionomycin. Three agents known to interfere with receptor recycling, namely monensin, methylamine and ammonium chloride reduced cell-associated 125I-BH-SP. In a series of experiments, the cytoplasmic calcium concentrations ([Ca2+) during exposure to these three agents, to the CCK-8-analogue caerulein and to ionomycin were determined. In all cases, [Ca2+] was raised. The results indicate that endocytosis of receptor-bound 125I-BH-SP is regulated by CCK and that the endocytotic process is influenced by calcium. 相似文献
11.
Influx of na, k, and ca into roots of salt-stressed cotton seedlings : effects of supplemental ca
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High Na+ concentrations may disrupt K+ and Ca2+ transport and interfere with growth of many plant species, cotton (Gossypium hirsutum L.) included. Elevated Ca2+ levels often counteract these consequences of salinity. The effect of supplemental Ca2+ on influx of Ca2+, K+, and Na+ in roots of intact, salt-stressed cotton seedlings was therefore investigated. Eight-day-old seedlings were exposed to treatments ranging from 0 to 250 millimolar NaCl in the presence of nutrient solutions containing 0.4 or 10 millimolar Ca2+. Sodium influx increased proportionally to increasing salinity. At high external Ca2+, Na+ influx was less than at low Ca2+. Calcium influx was complex and exhibited two different responses to salinity. At low salt concentrations, influx decreased curvilinearly with increasing salt concentration. At 150 to 250 millimolar NaCl, 45Ca2+ influx increased in proportion to salt concentrations, especially with high Ca2+. Potassium influx declined significantly with increasing salinity, but was unaffected by external Ca2+. The rate of K+ uptake was dependent upon root weight, although influx was normalized for root weight. We conclude that the protection of root growth from salt stress by supplemental Ca2+ is related to improved Ca-status and maintenance of K+/Na+ selectivity. 相似文献
12.
Microbial diversity, numbers, and metabolic activities in Minnesota peatlands were investigated using a variety of microbial enrichment and enumeration procedures together with radioisotopic measurements of microbial degradative processes. Minnesota peatlands were shown to contain large microbial populations of wide metabolic diversity. Direct counts of bacteria using epifluorescence microscopy indicated bacterial populations of about 108 ml–1 of peatland water, irrespective of depth. Radioisotopic most-probable-number (MPN) counts of heterotrophs able to mineralize14C-labeled substrates to14CO2 showed significant populations of glucose degraders (104–106 ml–1) as well as degraders of benzoate (102–103 ml–1), 2,4-dichlorophenoxyacetate (102–105 ml–1), and sphagnum (103–107 ml–1) in the various peatlands examined. The MPNs of NO3
– reducers varied from 103–106 ml–1, SO4
– reducers from 102–103 ml–1, methanogenic bacteria from 103–106 ml–1, and methane oxidizers from 103–104 ml–1, depending on sampling site and depth. Eighty pure cultures of aerobic bacteria and fungi were isolated from Minnesota peats. Most of those cultures tested were able to grow on at least 20 organic compounds (carbohydrates, aromatic molecules, hydrocarbons, etc.) as sole sources of carbon and energy. One isolate, aBacillus, was able to fix atmospheric N2. Several of the isolates were able to mineralize14C-labeled lignin. 相似文献
13.
Effect of oxygen deficiency on nitrogen assimilation and amino acid metabolism of soybean root segments 总被引:1,自引:0,他引:1
Plants submitted to O2 deficiency present a series of biochemical modifications, affecting overall root metabolism. Here, the effect of hypoxia on the metabolic fate of 15N derived from 15NO3 ?, 15NO2 ? and 15NH4 + in isolated soybean root segments was followed by gas chromatography–mass spectrometry, to provide a detailed analysis of nitrogen assimilation and amino acid biosynthesis under hypoxia. O2 deficiency decreased the uptake of the nitrogen sources from the solution, as ratified by the lower 15NO3 ? and 15NH4 + enrichment in the root segments. Moreover, analysis of endogenous NO2 ? and 15NH4 + levels suggested a slower metabolism of these ions under hypoxia. Accordingly, regardless of the nitrogen source, hypoxia reduced total 15N incorporation into amino acids. Analysis of 15N enrichment patterns and amino acid levels suggest a redirecting of amino acid metabolism to alanine and γ-aminobutyric acid synthesis under hypoxia and a differential sensitivity of individual amino acid pathways to this stress. Moreover, the role of glutamine synthetase in nitrogen assimilation both under normoxia and hypoxia was ratified. In comparison with 15NH4 +, 15NO2 ? assimilation into amino acids was more strongly affected by hypoxia and NO2 ? accumulated in root segments during this stress, indicating that nitrite reductase may be an additional limiting step. NO2 ? accumulation was associated with a higher nitric oxide emission. 15NO3 ? led to much lower 15N incorporation in both O2 conditions, probably due to the limited nitrate reductase activity of the root segments. Overall, the present work shows that profound alterations of root nitrogen metabolism occur during hypoxic stress. 相似文献
14.
Charles Levinson 《Journal of cellular physiology》1976,87(2):235-244
The effect of Cl? on SO4?2 efflux was studied in both Cl?-containing and Cl?-free ascites tumor cells loaded with 35SO4?2 to test the hypothesis that Cl?-SO4?2 exchange is mediated by the same mechanism responsible for SO4?2-self exchange. The addition of Cl?-free, 35SO4?2 loaded cells to a SO4?2-free, Cl? medium results in: (1) SO4?2 efflux that is dependent on the extracellular Cl? concentration (Km = 4.85 mM; ke = 0.048 min?1 at 50 mM Cl?) and (2) net Cl?-uptake that exceeds SO4?2 loss. Both SITS (4-acetamido-4′-isothiocyanostilbene-2,2′-disulfonate) and ANS (1-anilino-8-napthalene sulfonate) inhibit SO4?2 efflux but are without effect on Cl? uptake. The addition of Cl?-containing, 35SO4?2 loaded cells to a SO4?2-free, C1? medium results in: (1) a slight gain in cellular Cl? and (2) k efor SO4?2 efflux identical to that for Cl?-free cells. The results are compatible with the suggestion that: (1) Cl? interacts with a membrane component responsible for transmembrane SO4?2 movement; (2) Cl? interaction stimulates the rate of unidirectional SO4?2 efflux from cells initially free of Cl? as well as the rate of SO4?2 turnover in cells maintained in the steady state with respect to Cl? and SO4?2; and (3) in the case of cells initially free of Cl?, the Cl?-SO4?2 pathway represents only a small fraction of the total unidirectional Cl?-influx the remainder being compatible with the electroneutral accumulation of NaCl and KCl. 相似文献
15.
《Biochimica et Biophysica Acta (BBA)/Molecular Cell Research》1986,889(1):65-71
We investigated the role of extracellular Ca2+ in the Clostridium perfringens enterotoxin-induced alteration of the permeability of the plasma membrane. Enterotoxin released 86Rb and 51Cr from the Vero cells preloaded with the isotope. In the presence of EGTA, however, it released 86Rb but not 51Cr. The binding of enterotoxin to the cells was not influenced by Ca2+ or Mg2+. The effects of various cations on the enterotoxin-induced 51Cr release was also studied. The release depended on extracellular Ca2+ but not on Mg2+; it was inhibited by each of Zn2+, La3+ and Co2+. Zn2+ and Co2+ also inhibited 51Cr release caused by the enterotoxin previously bound to the cell membrane. In contrast, antibody against enterotoxin did not neutralize the toxin once it was bound to the Vero cells. When the cells were treated with enterotoxin, 45Ca influx occurred and reached the plateau in a few minutes, as did 86Rb release. 相似文献
16.
Simultaneous rates of [32P]orthophosphate and 45Ca2+ efflux from prelabeled rat pancreatic islets have been evaluated to assess whether these ions move in concert throughout all phases of “stimulus-secretion coupling”. Perifusion with stimulatory concentrations of glucose elicited immediate but transitory increases in 32P outflow accompanied by initial retardations and subsequent augmentations in net 45Ca2+ outflows. These monophasic 32P and biphasic 45Ca2+ responses to secretory stimulation were abolished completely by membrane stabilization with tetracaine. However, certain manipulations enabled individual components to be modified separately. During stimulation with glucose, inhibition of insulin release by Ni2+ abolished the late increases in 45Ca2+ outflow without affecting the initial retentions of 45Ca2+ or the increased releases of 32P. Under basal conditions, the ionophore A23187 “triggered” increased releases of 45Ca2+ and insulin without prior retentions of 45Ca2+ or enhancements of 32P efflux. Thus, the immediate retardations of 45Ca2+ outflow and heightened efflux of 32P may reflect early events in stimulus-secretion coupling which can be dissociated from the augmented release of 45Ca2+ accompanying activated emiocytosis. 相似文献
17.
Calcium ions serve as a second messenger in signal transduction and metabolic regulation. Effects of Al on calcium homeostasis remain to be elucidated. Short-term net 45Ca2+ uptake by Amaranthus tricolor protoplasts was monitored from uptake media prepared to test the influence of pH, Al, and various inhibitors. Accumulation of 45Ca2+ increased during the first 3 to 6 minutes and then leveled off or declined. Al and Ca2+ channel blockers (verapamil and bepridil) decreased net 45Ca2+ uptake. This decrease was more pronounced when Al and bepridil were both present in uptake media, but Al did not aggravate verapamil-induced reduction of net 45Ca2+ uptake. Erythrosin B and calmidazolium each increased net 45Ca2+ uptake, probably by interfering with Ca2+ efflux. This effect was undetectable in the presence of Al. Mycophenolic acid decreased net 45Ca2+ uptake; guanosine alleviated this effect. Al-induced reduction of net 45Ca2+ uptake was not aggravated by mycophenolic acid. Net 45Ca2+ uptake was generally less at pH 4.5 than at 5.5 for all treatments. It is concluded that Al ions affect net 45Ca2+ uptake by binding to the verapamil-specific channel site that is different from the bepridil-specific one, as well as by interfering with the action of guanosine 5′-triphosphate-binding proteins. 相似文献
18.
Two ionophores, monensin and salinomycin, increased total cell Na+ and ouabain-sensitive 86Rb+ uptake in cultures of smooth muscle cells from rat aorta. Monensin was used to produced graded increases in cell Na+ in order to assess the Na+ dependence of the Na+/K+ pump in the intact cell. The relationship between internal Na+ and ouabain-sensitive 86Rb+ uptake was hyperbolic (K1Na = 3 mM). Monensin did not stimulate 86Rb+ uptake in the absence of external Na+. Loading the cells with Na+ by exposing cultures to a K+-free medium for 3 hr maximally increased cell Na+ and ouabain-sensitive 86Rb+ uptake to the same extent as monensin. Total cell Na+ and pump activity in monensin-treated cells returned to the initial values after removing the ionophore. Monensin was then able to increase total cell Na+ and ouabain-sensitive 86Rb+ uptake to the same extent as the initial treatment with the ionophore. 相似文献
19.
Kinetics of radioactive iron transport were examined in three strains of Bacillus megaterium. In strain ATCC 19213, which secretes the ferric-chelating secondary hydroxamic acid schizokinen, 59Fe3+ uptake from 59FeCl3 or the ferric hydroxamate Desferal-59Fe3+ was rapid and reached saturation within 3 min. In strain SK11, which does not secrete schizokinen, transport from 59FeCl3 was markedly reduced; the two ferric hydroxamates Desferal-59Fe3+ or schizokinen-59Fe3+ increased both total 59Fe3+ uptake and the 59Fe3+ appearing in a cellular trichloroacetic acid-insoluble fraction, although 10 min was required to reach saturation. Certain characteristics of transport from both ferric hydroxamates and FeCl3 suggest that iron uptake was an active process. The growth-inhibitory effect of aluminum on strain SK11 was probably due to the formation of nonutilizable iron-aluminum complexes which blocked uptake from 59FeCl3. Desferal or schizokinen prevented this blockage. A strain (ARD-1) resistant to the ferric hydroxamate antibiotic A22765 was isolated from strain SK11. Strain ARD-1 failed to grow with Desferal-Fe3+ as an iron source, and it was unable to incorporate 59Fe3+ from this source. Growth and iron uptake in strain ARD-1 were similar to strain SK11 with schizokinen-Fe3+ or the iron salt as sources. It is suggested that the ferric hydroxamates, or the iron they chelate, may be transported by a special system which might be selective for certain ferric hydroxamates. Strain ARD-1 may be unable to recognize both the antibiotic A22765 and the structurally similar chelate Desferal-Fe3+, while retaining its capacity to utilize schizokinen-Fe3+. 相似文献
20.
Stereospecific interaction of opiate narcotics in binding of 3H-dihydromorphine to membranes of rat brain 总被引:2,自引:0,他引:2
Membranes from homogenates of corpus striatum bound 3H-dihydromorphine in a saturable fashion with a Km value of 1 × 10?9M. The binding of 3H-dihydromorphine to the membranes was reduced to about 10% by 10?7M levorphanol but not by 10?7M dextrorphan. The binding of 3H-dihydromorphine became less sensitive to 10?7M levorphanol when the concentration of 3H-dihydromorphine was greater than 2 × 10?9M. Other opiate narcotics, e.g. morphine and -methadone, were as effective as levorphanol in competition for the binding 3H-dihydromorphine with ED50 values of 2–4 × 10?9M. -Methadone and dextrorphan were about 1/50 and 1/2000 as effective as their respective levo-isomers. The opiate antagonist, naloxone, also competed effectively for the binding sites with an ED50 value of 3.3 × 10?9M. Substances like acetylcholine, choline, serotonin, norepinephrine and dopamine were ineffective. Only ionophores specific for divalent cations stimulated the binding of 3H-dihydromorphine suggesting that some endogenous divalent cations may be inhibitory to the binding of the opiate narcotic. The receptors of 3H-dihydromorphine probably exist in the membranes of nerve endings and have a density of 6 × 1012 sites per g in corpus striatum. We conclude that the described technique can successfully detect the opiate narcotic receptors in the central nervous system without the usual method of displacement. 相似文献