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1.
Evidence is presented for the proton-coupled transport of sucrose and glutamine in purified plasma membrane vesicles isolated from cotyledons ofRicinus communis. Imposition of a pH gradient (internal alkaline) across the plasma membrane resulted in a rapid uptake of sucrose and glutamine which was inhibited in the presence of carbonyl cyanide-m-chlorophenyl hydrazone. Imposition of a pH gradient plus an internal negative membrane potential stimulated uptake further. Glucose and fructose uptakes were negligible under these conditions. Sucrose uptake into the vesicles demonstrated saturation kinetics with a Km of 0.87 mol·m-3, indicating carrier-mediated transport. In support of this, uptake was very sensitive to the protein-modifying reagentp-chloromercuribenzenesulphonic acid. N-Ethylmaleimide, another sulphydryl reagent, was only slightly inhibitory. However, both reagents strongly inhibited sucrose uptake into intact cotyledons; the possible reasons for the difference between the intact and isolated systems are assessed. The value of this system for the study of sucrose and amino acid carriers is discussed.  相似文献   

2.
Summary To investigate directly whether a sodium-potassium-chloride cotransport system is operating in the mammalian thick ascending limb of Henle's loop (TALH) and in the elasmobranch rectal gland, plasma membrane vesicles were prepared from TALH cells isolated from rabbit kidney outer medulla and from rectal glands ofSqualus acanthias, and chloride uptake was measured by a rapid filtration technique. Chloride uptake into TALH vesicles in the presence of a 25 mM Na2SO4, 25 mM K2SO4 gradient reached 70% of equilibrium at 2.5 min. In the presence of both sodium and potassium, the 15 s chloride uptake was inhibited 35% by 1 mM bumetanide. When either sodium or potassium was removed from the incubation medium, chloride uptake decreased to the level observed in the presence of 1 mM bumetanide. 0.5 mM SITS had no effect on chloride uptake by the plasma membrane vesicles. This sodium and potassium dependent, bumetanide sensitive chloride uptake was also observed under tracer exchange conditions. Chloride uptake into rectal gland plasma membrane vesicles in the presence of a 50 mM Na2SO4, 50 mM K2SO4 gradient reached 80% of equilibrium at 2.5 min. 1 mM bumetanide inhibited the 15 s uptake of chloride by 34% and removal of either sodium or potassium from the incubation medium reduced chloride uptake to the level observed in the presence of bumetanide under both gradient and tracer exchange conditions. These studies provide additional support for the hypothesis that a sodium-potassium-chloride cotransport system is operating in these epithelia.Abbreviations SITS 4-acetamido-4-isothiocyanato-stilbene-2,2-disulfonic acid - TALH thick ascending limb of Henle's loop  相似文献   

3.
Vesicles have been prepared from Ehrlich cells by using a method based on the early experiments of Forte et al. (Forte, J.G., Forte, T.M. and Heinz, E. (1973) Biochim. Biophys. Acta 298, 827–841) with some minor modifications, using the filter technique. From electron micrographs and from the sensitivity of these vesicles towards osmotic pressure changes in the medium induced by (nonpermeant) sucrose, it is concluded that the vesicles are closed. The counterflow phenomenon with glycine and Na+-linked cotransport of glycine appears to indicate that these vesicles are still functioning. The observation of the overshoot phenomenon interpreted in terms of theoretical predictions confirms that the active accumulation of glycine is energized by the Na+ electrochemical potential gradient. In particular, the contribution of the electrical components of these gradients is evidenced by the effects of the anion of the added sodium salt or of the addition of valinomycin. In contrast to observations by others, we found that ouabain does not directly affect Na+-linked cotransport of glycine whereas HgCl2 does so. Nor could any significant overshoot be demonstrated in the absence of an Na gradient. Since these vesicles were not metabolically active, these experiments do not exclude the possibility that in intact cells glycine is in addition transported primarily or partially actively.  相似文献   

4.
We investigated the contribution of the Na+/l-carnitine cotransporter in the transport of tetraethylammonium (TEA) by rat renal brush-border membrane vesicles. The transient uphill transport of l-carnitine was observed in the presence of a Na+ gradient. The uptake of l-carnitine was of high affinity (Km=21 μM) and pH dependent. Various compounds such as TEA, cephaloridine, and p-chloromercuribenzene sulfonate (PCMBS) had potent inhibitory effects for l-carnitine uptake. Therefore, we confirmed the Na+/l-carnitine cotransport activity in rat renal brush-border membranes. Levofloxacin and PCMBS showed different inhibitory effects for TEA and l-carnitine uptake. The presence of an outward H+ gradient induced a marked stimulation of TEA uptake, whereas it induced no stimulation of l-carnitine uptake. Furthermore, unlabeled TEA preloaded in the vesicles markedly enhanced [14C]TEA uptake, but unlabeled l-carnitine did not stimulate [14C]TEA uptake. These results suggest that transport of TEA across brush-border membranes is independent of the Na+/l-carnitine cotransport activity, and organic cation secretion across brush-border membranes is predominantly mediated by the H+/organic cation antiporter.  相似文献   

5.
The effect of a variety of ions and other solutes on the accumulation of the β-amino acid, taurine, was examined in rat renal brush-border membrane vesicles. Initial taurine uptake (15 and 30 s) is sodium-dependent with a typical overshoot. This Na+ effect was confirmed by exchange diffusion and gramicidin inhibition of taurine uptake. External K+ or Li+ do not increase taurine accumulation more than Na+-free mannitol, except that the combination of external K+ and Na1 in the presence of nigericin enhances uptake. Of all anions tested, including more permeant (SCN and NO3) or less permeant (SO42−), chloride supported taurine accumulation to a significantly greater degree. Preloading vesicles with choline chloride reduced taurine uptake, suggesting that external Cl stimulates uptake. Since this choline effect could be related to volume change, due to the slow diffusion of choline into vesicles, brush-border membrane vesicles were pre-incubated with LiCl, LiNO3 and LiSO4. Internal LiCl, regardless of the final Na+ anion mixture, reduced initial rate (15 and 60 s) and peak (360 s) taurine uptake. Internal LiNO3 or LiSO4 with external NaCl resulted in similar or higher values of uptake at 15, 60 and 360 s, indicating a role for external Cl in taurine uptake in addition to Na+ effect. Although uptake by vesicles is greatest at pH 8.0 and inhibited at acidic pH values (pH less than 7.0), an externally directed H+ gradient does not influence uptake. Similarly, amiloride, an inhibitor of the Na+/H+ antiporter, had no influence on taurine accumulation over a wide variety of concentrations or at low Na+ concentrations. Taurine uptake is blocked only by other β-amino acids and in a competitive fashion. d-glucose and p-aminohippurate at high concentrations (> 10−3 M) reduce taurine uptake, possibly by competing for sodium ions, although gramicidin added in the presence of d-glucose inhibits taurine uptake even further. These studies more clearly define the nature of the renal β-amino acid transport system in brush-border vesicles and indicate a role for external Cl in this uptake system.  相似文献   

6.
The uptake of l-phenylalanine into brush border microvilli vesicles and basolateral plasma membrane vesicles isolated from rat kidney cortex by differential centrifugation and free flow electrophoresis was investigated using filtration techniques.Brush border microvilli but not basolateral plasma membrane vesicles take up l-phenylalanine by an Na+-dependent, saturable transport system. The apparent affinity of the transport system for l-phenylalanine is 6.1 mM at 100 mM Na+ and for Na+ 13 mM at 1 mM l-phenylalanine. Reduction of the Na+ concentration reduces the apparent affinity of the transport system for l-phenylalanine but does not alter the maximum velocity.In the presence of an electrochemical potential difference for Na+ across the membrane (ηNa0 >ηNa1) the brush border microvilli accumulate transiently l-phenylalanine over the concentration in the incubation medium (overshoot phenomenon). This overshoot and the initial rate of uptake are markedly increased when the intravesicular space is rendered electrically more negative by membrane diffusion potentials induced by the use of highly permeant anions, of valinomycin in the presence of an outwardly directed K+ gradient and of carbonyl cyanide p-trifluoromethoxyphenylhydrazone in the presence of an outward-directed proton gradient.These results indicate that the entry of l-phenylalanine across the brush border membrane into the proximal tubular epithelial cells involves cotransport with Na+ and is dependent on the concentration difference of the amino acid, on the concentration difference of Na+ and on the electrical potential difference. The exit of l-phenylalanine across the basolateral plasma membranes is Na+-independent and probably involves facilitated diffusion.  相似文献   

7.
A Na+-specific and Na+-stimulated active α-aminoisobutyric acid transport system was reconstituted from plasma membranes isolated from mouse fibroblast BALB/c 3T3 cells transformed by simian virus 40. The plasma membranes were treated with dimethylmaleic anhydride and then extracted with 2% cholate. The cholate-solubilized supernatant proteins were combined with exogenous phospholipids and eluted through a Sephadex G-50 column. This yielded reconstituted vesicles which in the presence of Na+ could actively transport α-aminoisobutyric acid as shown by the transient accumulation above the equilibrium level (overshoot). The overshoot was not obtained with other monovalent cations such as K+, Li+, and choline+. The electrochemical effect of the lipophilic anion, SCN?, led to greater α-aminoisobutyric acid uptake as compared to that observed with Cl? or SO42?. The Na+-stimulated transport of a-aminoisobutyric acid was a saturable process with an apparent Km of 2 mm. Studies of the inhibition of α-aminoisobutyric acid transport by other amino acids showed that methylaminoisobutyric acid [specifically transported by A system (alanine preferring)]had a pronounced inhibitory effect on a-aminoisobutyric acid uptake in contrast to the slight inhibitory effect produced by phenylalanine [primarily transported by L system (leucine preferring)]. The results show that the reconstituted vesicles, prepared from partially purified membrane proteins and exogenous phospholipids, regained the same important transport properties of native membrane vesicles, i.e., Na+-specific and Na+-stimulated concentrative α-aminoisobutyric acid uptake.  相似文献   

8.
 One- and 2-year-old Pinus sylvestris saplings were exposed to chronic doses of ozone (O3) and sulphur dioxide (SO2) in short-term (3 months) and long-term (18 months) experiments. Microsomal and plasma membrane fractions were purified by phase partitioning from current-year needles. The following membrane enzyme activities were determined in the microsomal and/or purified plasma membrane fractions: K+, Mg2+-ATPase (EC 3.6.1.3), NADH ferricyanide oxidoreductase (EC 1.6.99.3), NADH-duroquinone reductase (EC 1.6.5.1), NADH oxidase type I (EC 1.6.99.2), NADH oxidase type II or peroxidase-like enzyme (EC 1.11.1.7) and pyrophosphatase (EC 3.6.1.1). NADH oxidase type I was slightly stimulated in the microsomal fraction after a short-term exposure to O3 whereas NADH-dependent duroquinone reductase was not affected by this pollutant. However, in the long term experiment, NADH oxidase type II measured in the plasma membrane fraction was more than 2-fold stimulated in the SO2 treated pines and more than 4-fold when O3 was added to SO2. However, pyrophosphatase was decreased by 50% in trees treated with SO2+O3 and remained unchanged in the SO2 treatment, indicating that this enzyme is probably sensitive to oxidation. K+, Mg2+-ATPase showed a trend towards an enhancement of activity when exposed to chronic concentrations of air pollutants, this enhancement was more important in the long-term experiment after the combined effect of SO2 and O3. However, the K+-stimulated component was inhibited by the combination of both pollutants. Finally, NADH ferricyanide reductase was significantly enhanced after O3 and SO2+O3 exposures appearing as the most sensitive oxidoreductase to these air pollutants. The stimulation of ATPase and membrane oxidoreductases could facilitate the adaptation and defense of trees by maintaining an adequate redox potential in the plasma membrane region and perhaps stimulating the reduction of extracellular electron acceptors generated by the exposure to air pollutants. Received: 15 September 1997 / Accepted: 4 May 1998  相似文献   

9.
The absorbance change of the weak base dye probe, Acridine orange, was used to monitor alterations of pH gradients across renal brush border membrane vesicles. The presence of Na+/H+ or Li+/H+ exchange was demonstrated by diluting Na2SO4 or Li2SO4 loaded vesicles into Na+- or Li+-free solutions, which caused dye uptake. About 20% of the uptake was abolished by lipid permeable cations such as valinomycin-K+ or tetraphenylphosphonium, indicating perhaps the presence of a finite Na+ conductance smaller than electroneutral Na+/H+ exchange. The protonophore tetrachlorosalicylanilide raised the rate of dye uptake under these conditions, hence the presence of an Na+ conductance greater than the H+ conductance was suggested. K+ gradients also induced changes of pH, at about 10% of the Na+ or Li+ rate. Partial inhibition (21%) was seen with 0.1 mM amiloride indicating that K+ was a low affinity substrate for the Na+/H+ exchange. Acceleration both by tetrachlorosalicylanilide (2-fold) and valinomycin (4-fold) suggested the presence of 2 classes of vesicles, those with high and those with low K+ conductance. The larger magnitude of the valinomycin dependent signal suggested that 75% of the vesicles had a low K+ conductance. Inward Cl? gradients also induced acidification, partially inhibited by the presence of tetraphenylphosphonium, and accelerated by tetrachlorosalicylanilide. Thus both a Cl? conductance greater than the H+ conductance and a Cl?/OH? exchange were present. The rate of Na+/H+ exchange was amiloride sensitive with a pH optimum of 6.5 and an apparent Km for Na+ or Li+ of about 10 mM and an EA of 14.3 kcal per mol. A 61-fold Na2SO4 gradient resulted in a pH gradient of 1.64 units which increased to 1.8 with gramicidin. An equivalent NaCl gradient gave a much lower ΔpH even in the presence of gramicidin showing that the H+ and Cl? pathways could alter the effects of the Na+/H+ exchange.  相似文献   

10.
Fumigation of leaves with SO2 can reduce the capacity for photosynthetic CO2 uptake even in the absence of visible symptoms of damage. In vitro studies suggest that this invisible injury to intact leaves could be affected by damage to each of the main stages in the photosynthetic process. Reduced stomatal apertures may also reduce photosynthesis following SO2 fumigation. The responses of CO2 uptake by leaves to intercellular CO2 concentration and to absorbed light provide information for quantitative separation of the in vivo contribution of the different stages of photosynthesis to reduction in overall rate. This study uses these techniques to examine the basis of reduction in CO2 uptake in Zea mays cv. LG11 leaves following short-term fumigation with SO2. Fumigation with 33 μmol m–3 SO2 for 30 min reduced light saturated CO2 uptake by about one-third. An even greater reduction in light limited CO2 uptake was observed and with no significant change in light absorptance this was attributed to a reduced quantum yield of photosynthesis. The light saturated CO2 uptake rate and the stomatal conductance decreased in parallel. However, the relationship of CO2 uptake to the intercellular CO2 concentration suggested that the reduced stomatal conductance did not account for the reduced rate of CO2 uptake following fumigation. Both the initial slope and plateau of this relationship were significantly reduced, suggesting that both carboxylation efficiency and capacity for regeneration of CO2 acceptor were diminished by SO2 fumigation. The operating intercellular CO2 concentration indicated that both processes were co-limiting, before and after fumigation. The time required for induction of photosynthetic CO2 uptake on illumination was approximately doubled following SO2 fumigation, showing that fumigation impairs the ability of the photosynthetic apparatus to adapt to fluctuations in light level.  相似文献   

11.
The kinetic characteristics of Na+ -Ca2+ exchange in isolated sarcolemma vesicles from new-borne chick heart, which contain about 70% of right-side-out vesicles, were compared with those of cultured embryonic chick heart cells. Na+ -Ca2+ exchange was monitored as Nai-dependent Ca2+ uptake. Increase in the internal concentration of Na+ ([Na+]i) in these two preparations caused increase in both the initial rate and the saturation-level of Ca2+ uptake. Plots of the rate of Ca2+ uptake against [Na+]i showed similar saturation-kinetics in these two preparations. The apparent Michaelis constant (Km) (0.35 mM) for Ca2+ uptake by the intact cells was much higher than that (0.031 mM) for Ca2+ uptake by the vesicles. The degree of inhibition by Mg2+ was also higher in the cells than in the vesicles. Some possible reasons (age of the chicks used, membrane potential, etc.), for these differences were examined and are discussed.  相似文献   

12.
The characteristics of carnosine (β-alanyl-l-histidine) transport have been studied using purified brush-border membrane vesicles from mouse small intestine. Uptake curves did not exhibit any overshoot phenomena, and were similar under Na+, K+ or choline+ gradient conditions (extravesicular > intravesicular). However, uptake of histidine showed an overshoot phenomenon in the presence of a Na+-gradient. There was no detectable hydrolysis of carnosine during 15 min of incubation with membrane vesicles under conditions used for transport experiments. Analysis of intravesicular contents further showed the complete absence of the constituent free amino acids of carnosine, and indicates that intact carnosine is transported. Studies on the effect of concentration on peptide uptake revealed that transport occurred by a saturable process conforming to Michaelis-Menten kinetics with a Km of 9.6 ± 1.4 mM and a Vmax of 2.9 ± 0.2 nmol / mg protein per 0.4 min. Uptake of carnosine was inhibited by both di- and tripeptides with a maximum inhibition of 68% by glycyl-l-leucyltyrosine. These results clearly demonstrate that carnosine is transported intact by a carrier-mediated, Na+-independent process.  相似文献   

13.
Summary In order to investigate whether the loop diuretic sensitive, sodium-chloride cotransport system described previously in shark rectal gland is in fact a sodium-potassium chloride cotransport system, plasma membrane vesicles were isolated from rectal glands ofSqualus acanthias and sodium and rubidium uptake were measured by a rapid filtration technique. In addition, the binding of N-methylfurosemide to the membranes was investigated. Sodium uptake into the vesicles in the presence of a 170mm KCl gradient was initially about five-fold higher than in the presence of a 170mm KNO3 gradient. In the presence of chloride, sodium uptake was inhibited 56% by 0.4mm bumetanide and 40% by 0.8mm N-methylfurosemide. When potassium chloride was replaced by choline chloride or lithium chloride, sodium uptake decreased to the values observed in the presence of potassium nitrate. Replacement of potassium chloride by rubidium chloride, however, did not change sodium uptake. Initial rubidium uptake into the membrane vesicles was about 2.5-fold higher in the presence of a 170mm NaCl gradient than in the presence of a 170mm NaNO3 gradient. The effect of chloride was completely abolished by 0.4mm bumetanide. Replacement of the sodium chloride gradient by a lithium chloride gradient decreased rubidium uptake by about 40%; replacement by a choline chloride gradient reduced the uptake even further. Rubidium uptake was also strongly inhibited by potassium. Sodium chloride dependence and bumetanide inhibition of rubidium flux were also found in tracer exchange experiments in the absence of salt gradients. The isolated plasma membranes bound3[H]-N-methylfurosemide in a dose-dependent manner. In Scatchard plots, one saturable component could be detected with an apparentK D of 3.5×10–6 m and a number of sitesn of 104 pmol/mg protein. At 0.8 m, N-methylfurosemide binding decreased 51% when sodium-free or low-potassium media were used. The same decrease was observed when the chloride concentration was increased from 200 to 600mm or when 1mm bumetanide or furosemide were added to the incubation medium. These studies indicate that the sodium-chloride cotransport system described previously in the rectal gland is in fact a sodium-potassium chloride cotransport system. It is postulated that this transport system plays an essential role in the secondary active chloride secretion of the rectal gland.  相似文献   

14.
D J Rouse  L Lack 《Life sciences》1979,25(1):45-52
The ion requirements for intestinal taurocholate transport were studied using vesicles prepared from the brush borders of guinea pig small intestines. For each experimental electrolyte, parallel uptake experiments were performed with vesicles from jejunal and ileal brush border membranes to differentiate between uptake by passive fluxes and non-specific binding and uptake by the ileal bile salt active transport system. Uptake of taurocholate prior to the addition of electrolyte was the same for vesicles prepared from jejunal and ileal tissue. During the presence of a sodium gradient (extravesicular concentration greater than intravesicular), only ileal vesicles displayed the enhanced uptake which is characteristic of the overshoot phenomenon. When NaCl was replaced by KCl or LiCl, the overshoot was not observed. Replacement of NaCl with NaCNS, Na2SO4, or NaSO3C2H4OH, however, resulted in no significant difference in the initial uptake values observed in either the jejunal or ileal vesicles. This pattern of taurocholate transport independence of relative anion permeability differs from the pattern observed by others for the Na+ dependent transport of D-glucose by intestinal brush border membrane vesicles. This difference may be attributed in part to the fact that, unlike the situation with glucose, the binding of a taurocholate anion and a sodium cation by the hypothetical carrier would result in an electroneutral addition.  相似文献   

15.
The carbohydrate metabolism of the needles of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) has been examined in trees that were exposed to SO2, and O3, in an open-air fumigation experiment located in the Liphook forest in southern England. Two-year-old seedlings were planted in 1985 in seven experimental plots. Five plots received fumigation treatments of SO2, O3 or a combination of these gases to give a 2 × 3 factorial design with one additional ambient plot Fumigation with SO2, occurred from May 1987 to December 1990 and O3, fumigation occurred from March to December 1988, May to December 1989 and February to December 1990. Five samples of needles for investigation of carbohydrate metabolism were taken between February and July 1989. The concentrations of soluble carbohydrates (including sucrose and hexoses) were greatly reduced in the needles taken from Scots pine growing in the treated plots, and were also reduced, but to a lesser extent, in the needles taken from Norway spruce. Little variation in the concentration of starch in the needles of either species was detected. The activities of the two final enzymes of sucrose synthesis, sucrose phosphate synthase and sucrose 6-phos-phate phosphatase, were greatly reduced in the needles of Scots pine and were also reduced, but to a lesser extent, in the needles of Norway spruce in the fumigated plots. These reductions could be correlated with decreases in rates of photosynthetic CO2 assimilation determined by independent groups of researchers working on the Liphook site.  相似文献   

16.
Summary HeLa cells, labeled with Na2 35SO4, release into the culture medium35SO4 bound to plasma membrane vesicles next to35SO4-glycoproteins and free35SO4. Plasma membrane vesicles, experimentally produced by treatment with formaldehyde, contain35SO4 and their surface can be stained with high iron diamine. Scanning of chromatograms of the trypsinate from labeled cells demonstrates radioactivity on the spot of heparan sulfate. It is concluded that HeLa cells synthesize heparan sulfate, which is incorporated at the plasma membrane and released by shedding of small vesicles.Supported by a grant from the Algemene Spaar- en Lijfrentekas Cancer Fund, Brussels, Belgium.  相似文献   

17.
A previous study of energy-independent in vitro Ca2+ uptake by rat intestinal epithelial membrane vesicles demonstrated that uptake by Golgi membrane vesicles was greater than that by microvillus or lateral-basal membrane vesicles, was markedly decreased in vitamin D-deficient rats, and responded specifically to 1,25-(OH)2D3 repletion (R. A. Freedman, M. M. Weiser, and K. J. Isselbacher, 1977, Proc. Nat. Acad. Sci. USA74, 3612–3616; J. A. MacLaughlin, M. M. Weiser, and R. A. Freedman, 1980, Gastroenterology78, 325–332). In the present study, properties of Ca2+ uptake and release by intestinal Golgi membrane vesicles have been investigated. The initial rate of uptake was found to be saturable, suggesting carrier-mediated uptake. Uptake was markedly inhibited by Mg2+ and Sr2+, but not by Na+ or K+. Lowering the external [H+] or raising the internal [H+] resulted in enhancement of the initial rate of uptake; the intial rate was found to correlate with the internal-to-external [H+] gradient. The initial rate of uptake could be enhanced by preloading the vesicles with MgCl2 or SrCl2 but not CaCl2, NaCl, or KCl. Vesicles preloaded with K2SO4 failed to show enhanced uptake in the presence of valinomycin, suggesting that enhancement in uptake by vesicles preloaded with MgCl2 was not due to transmembrane potentials. The internal volume of the Golgi membrane vesicles was determined and found to be 9 μl/mg protein; this volume could accomodate less than 1% of the Ca2+ uptake maintained at equilibrium. Therefore, the remainder of the Ca2+ taken up was presumably bound to the Golgi membranes. A dissociation constant of 3.8 × 10?6m was found for this binding. The bound Ca2+ could be rapidly released by external Mg2+ or Sr2+, but not Ca2+, Na+, or K+. Release of bound Ca2+ could also be induced by raising the [H+] of the external medium. Failure of external Ca2+ to release bound Ca2+ suggested that the release induced by external Mg2+, Sr2+, or H+ was not due to competitive displacement of Ca2+ from its binding sites. These results indicated that Ca2+ uptake by intestinal Golgi membrane vesicles consists of carrier-mediated transport followed by binding of Ca2+ to the vesicle. The effects of H+, Mg2+, and Sr2+ on Ca2+ uptake and release suggest the existence of cation countertransport in the Golgi membrane vesicles.  相似文献   

18.
Na+-dependent leucine uptake was greater in potassium loaded brush-border membrane vesicles compared with controls. This effect was not mediated by an electrical potential difference, since it was still present in voltage-clamped conditions. Inhibition experiments indicate the same Na+-dependent leucine transport activity in the presence or in the absence of potassium. The affinity of sodium for the cotransporter was identical at 10 or 100 mM potassium. Leucine kinetics at different potassium concentrations showed a maximum 2.4-fold increase in Vmax, while Km was unaffected. The secondary plots of the kinetic results were not linear. This kinetic behaviour suggests that K+ acts as a non-essential activator of Na+-dependent leucine cotransport. A charge compensation of sodium-leucine influx is most probably a component of the potassium effect in the presence of valinomycin.  相似文献   

19.
20.
Experiments allowing Na+-dependent short-term uptake measurements by ileal brush border vesicles were described. Glucose uptake was compared with taurocholate uptake in the presence of NaCl, NaSCN and Na2SO4. In contrast to the observation made with glucose, taurocholate transport was the same for the three electrolytes, indicating electroneutral taurocholate transport.  相似文献   

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