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1.
《Comparative biochemistry and physiology. A, Comparative physiology》1994,107(4):949-956
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
2.
Sustained steady contractural or catchlike tension (CT) occurs in the metathoracic extensor tibiae muscle of the primitive insect the weta (Orthoptera: Stenopelmatidae) during its characteristic leg-extension defense behavior or following leg-position conditioning. Similar action occurs occasionally in semi-intact preparations and is abruptly turned off by a single peripheral inhibitory impulse. These phenomena were reproduced routinely by first infusing saline containing 10?8M (or stronger) octopamine into the muscle for 12 min, and then stimulating the slow excitatory motor neuron SETi with a brief burst. Direct stimulation of the dorsal unpaired median neuron, innervating the extensor tibiae (DUMETi) prior to SETi stimulation, also led to CT. Both octopamine and DUMETi markedly enhanced the tension developed in response to a burst of impulses in SETi. 相似文献
3.
Evidence for Na-Independent HCO(3) Uptake by the Cyanobacterium Synechococcus leopoliensis 总被引:10,自引:10,他引:0
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At low levels of dissolved inorganic carbon (DIC) and alkaline pH the rate of photosynthesis by air-grown cells of Synechococcus leopoliensis (UTEX 625) was enhanced 7- to 10-fold by 20 millimolar Na+. The rate of photosynthesis greatly exceeded the CO2 supply rate and indicated that HCO3− was taken up by a Na+-dependent mechanism. In contrast, photosynthesis by Synechococcus grown in standing culture proceeded rapidly in the absence of Na+ and exceeded the CO2 supply rate by 8 to 45 times. The apparent photosynthetic affinity (K½) for DIC was high (6-40 micromolar) and was not markedly affected by Na+ concentration, whereas with air-grown cells K½ (DIC) decreased by more than an order of magnitude in the presence of Na+. Lithium, which inhibited Na+-dependent HCO3− uptake in air-grown cells, had little effect on Na+-independent HCO3− uptake by standing culture cells. A component of total HCO3− uptake in standing culture cells was also Na+-dependent with a K½ (Na+) of 4.8 millimolar and was inhibited by lithium. Analysis of 14C-fixation during isotopic disequilibrium indicated that standing culture cells also possessed a Na+-independent CO2 transport system. The conversion from Na+-independent to Na+-dependent HCO3− uptake was readily accomplished by transferring cells grown in standing to growth in cultures bubbled with air. These results demonstrated that the conditions experienced during growth influenced the mode by which Ssynechococcus acquired HCO3− for subsequent photosynthetic fixation. 相似文献
4.
We characterized the uptake of carnitine in brush-border membrane (BBM) and basolateral membrane (BLM) vesicles, isolated from mouse kidney and intestine. In kidney, carnitine uptake was Na+-dependent, showed a definite overshoot and was saturable for both membranes, but for intestine, it was Na+-dependent only in BLM. The uptake was temperature-dependent in BLM of both kidney and intestine. The BBM transporter in kidney had a high affinity for carnitine: apparent Km=18.7 μM; Vmax=7.85 pmol/mg protein/s. In kidney BLM, similar characteristics were obtained: apparent Km=11.5 μM and Vmax=3.76 pmol/mg protein/s. The carnitine uptake by both membranes was not affected within the physiological pH 6.5-8.5. Tetraethylammonium, verapamil, valproate and pyrilamine significantly inhibited the carnitine uptake by BBM but not by BLM. By Western blot analysis, the OCTN2 (a Na+-dependent high-affinity carnitine transporter) was localized in the kidney BBM, and not in BLM. Strong OCTN2 expression was observed in kidney and skeletal muscle, with no expression in intestine in accordance with our functional study. We conclude that different polarized carnitine transporters exist in kidney BBM and BLM. L-Carnitine uptake by mouse renal BBM vesicles involves a carrier-mediated system that is Na+-dependent and is inhibited significantly by specific drugs. The BBM transporter is likely to be OCTN2 as indicated by a strong reactivity with the anti-OCTN2 polyclonal antibody. 相似文献
5.
Na/H Antiport in Isolated Tonoplast Vesicles from Storage Tissue of Beta vulgaris 总被引:33,自引:13,他引:20
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The pH-dependent fluorescence quenching of acridine orange was used to study the Na+- and K+-dependent H+ fluxes in tonoplast vesicles isolated from storage tissue of red beet and sugar beet (Beta vulgaris L.). The Na+-dependent H+ flux across the tonoplast membrane could be resolved into two components: (a) a membrane potential-mediated flux through conductive pathways; and (b) an electroneutral flux which showed Michaelis-Menten kinetics relationship to Na+ concentration and was competitively inhibited by amiloride (Ki = 0.1 millimolar). The potential-dependent component of H+ flux showed an approximately linear dependence on Na+ concentration. In contrast, the K+-dependent H+ flux apparently consisted of a single component which showed an approximately linear dependence on K+ concentration, and was insensitive to amiloride. Based on the Na+- and K+-dependent H+ fluxes, the passive permeability of the vesicle preparation to Na+ was about half of that to K+.
The apparent Km for Na+ of the electroneutral Na+/H+ exchange varied by more than 3-fold (7.5-26.5 millimolar) when the internal and external pH values were changed in parallel. The results suggest a simple kinetic model for the operation of the Na+/H+ antiport which can account for the estimated in vivo accumulation ratio for Na+ into the vacuole.
相似文献6.
Ciliary ganglion neurons in cell culture: high affinity choline uptake and autoradiographic choline labeling 总被引:1,自引:0,他引:1
Chick ciliary ganglion neurons grown in dissociated cell culture have a high affinity uptake mechanism for choline that has the properties expected for cholinergic neurons. The uptake has an apparent Km of ca. 0.3 μM and is blocked by addition of 10 μM hemicholinium-3 or replacement of Na+ by Li+ in the uptake medium. When the choline uptake mechanism is used to label ciliary ganglion neuron-myotube cultures autoradiographically, over 99% of the neurons are labeled. A few cells with neuronal morphologies in such cultures (<1%) are labeled by γ-[3H]aminobutyric acid uptake. The number of [3H]choline-labeled neurons and the amount of Na+-dependent choline uptake is the same for ciliary ganglion neurons grown with and without skeletal myotubes. Rat superior cervical ganglion neurons, grown in cell culture under conditions that induce them to synthesize acetylcholine and form cholinergic synapses, are labeled by [3H]choline uptake, though not as heavily as ciliary ganglion neurons. In contrast, chick dorsal root ganglion neurons, a presumed population of noncholinergic neurons, are not labeled by [3H]choline uptake. Thus high affinity choline uptake can be used to label autoradiographically the cholinergic neurons tested, while at least one population of noncholinergic neurons remains unlabeled. 相似文献
7.
Joseph D. Robinson 《Archives of biochemistry and biophysics》1973,156(1):232-243
Inactivation of the (Na+ + K+)-dependent ATPase by 50 μm BeCl2 occurred during brief incubations in the presence of both Mg2+ and K+. Inactivation followed, initially, a first-order time course, with rate constants sensitive to the concentration of K+ (other components held constant). From these data dissociation constants can be calculated for K+ binding to sites controlling inactivation. Comparisons of relative affinities for K+ analogs (T1+ and NH4+), and of sensitivity to reagents altering K+ activation (phlorizin and dimethylsulfoxide) indicate that the same K+ sites operate for both Be2+ inactivation and enzyme activation. With 3 mm MgCl2 the dissociation constant, KD, for K+ was 1.4 mm, but decreased 20-fold on addition of both Na+ and CTP. Alone, Na+ increased the apparent KD for K+, either by direct competition or indirectly from its own site, with a KD of 7 mm. The data suggest a model for K+ transport with K+ sites on the outer membrane surface that increase in affinity after formation of the phosphorylated enzyme intermediate, sufficiently to bind K+ in a high Na+ environment. Translocation may occur by an “oscillating pore” mechanism discharging K+ at the inner surface, while leaving demonstrable sites of moderate affinity at the outer end of the pore (which preclude attempts to document low-affinity discharge sites). 相似文献
8.
D. Parker 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1996,178(2):243-252
The effect of octopamine on the fast extensor and the flexor tibiae motor neurones in the locust (Schistocerca gregaria) metathoracic ganglion, and also on synaptic transmission from the fast extensor to the flexor motor neurones, was examined. Bath application or ionophoresis of octopamine depolarized and increased the excitability of the flexor tibiae motor neurones. 1 mM octopamine reduced the amplitude of the fast extensor-evoked EPSP in the slow but not the fast flexor motor neurones, whereas 10 mM octopamine could reduce the EPSP amplitude in both. Octopamine broadened the fast extensor action potential and reduced the amplitude of the afterhyperpolarization, the modulation requiring feedback resulting from movement of the tibia. Octopamine also increased the frequency of synaptic inputs onto the tibial motor neurones, and could cause rhythmic activity in the flexor motor neurones, and reciprocal activity in flexor and extensor motor neurones. Octopamine also increased the frequency of spontaneous spiking in the octopaminergic dorsal unpaired median neurones. Repetitive stimulation of unidentified dorsal unpaired median neurones could mimic some of the effects of octopamine. However, no synaptic connections were found between dorsal unpaired median neurones and the tibial motor neurones. The diverse effects of octopamine support its role in mediating arousal. 相似文献
9.
《International journal for parasitology》1987,17(3):847-850
Metacercariae of Clinostomum marginatum excysted from yellow perch, Perca flavescens, appear to have two systems for transporting glucose across the tegument, facilitated diffusion and active transport. These systems were distinguished by their differential sensitivities to Na+, phlorizin and phloretin. In Ringer's saline for cold-blooded vertebrates, 0.1 mm phlorizin and phloretin were incomplete, but similarly effective inhibitors of glucose uptake in 3 min incubations; worms accumulated in 1 h nonmetabolized 3-O-methylglucose against an apparent concentration difference demonstrating the active transport component. In Na+-free saline, phlorizin sensitivity and active transport capacity disappeared, but a phloretin sensitive, Na+-independent component remained. The Vmax and K1 of the Na+-independent system were 3.0 ± 0.54 μmol/g ethanol-extracted dry wt/h, and 0.8 ± 0.36 mm, respectively. Vmax and K1 of the Na+-dependent system, estimated by subtracting the Na+-independent values from those obtained in Ringer's saline, were 1.3 ± 0.27 μ mol/g ethanol-extracted dry wt/h, and0.7 ± 0.36mm, respectively. 相似文献
10.
《生物化学与生物物理学报:生物膜》1987,896(2):263-274
Using vesicles from the plasma membrane of hog thyroid, we have characterized its Na+-dependent I− transport system. We have found it to be totally Na+ dependent; K+ cannot substitute and Li+ can partially substitute for Na+; the Na+:I− flux ratio is larger than one; the system is electrogenic, being stimulated by a Δψ negative inside the vesicles. A number of large, lipophilic anions are fully-competitive inhibitors of Na+-dependent I− uptake; the closer their atomic radii are to that of iodine, the smaller their Ki values. 相似文献
11.
The metathoracic extensor tibiae muscle of the cricket Teleogryllus oceanicus is innervated by two excitatory axons; one of which leaves the metathoracic ganglion through nerve 5, the other through nerve 3. Axons in nerve 5 frequently regenerate to reinnervate the extensor tibiae if the nerve is sectioned in a late nymphal stage; functional reinnervation is rare if the nerve is sectioned in young adults. The muscle may become reinnervated by several axons regenerating through nerve 5, and individual muscle fibres may receive inputs from two regenerated axons. Axons regrowing through nerve 5 to a partially-denervated extensor tibiae preferentially innervate fibres in the central portion of the muscle, which is the normal innervation field of nerve 5. If the muscle is totally denervated by transection of both nerve 5 and nerve 3b, reinnervation is less specific and fibres throughout the muscle may be reinnervated by axons in either nerve. Reinnervation by regenerating axons is progressive. The proportion of muscles which are functionally reinnervated by regenerated axons increases with survival time as does the proportion of fibres within a muscle with reinnervation. The amplitude of excitatory junctional potentials and of muscle contraction evoked by regenerated axons both increase with survival time. 相似文献
12.
The uptake of glycine in rabbit renal brush border membrane vesicles was shown to consist of glycine transport into an intravesicular space. An Na+ electrochemical gradient (extravesicular>intravesicular) stimulated the initial rate of glycine uptake and effected a transient accumulation of intravesicular glycine above the steady-state value. This stimulation could not be induced by the imposition of a K+, Li+ or choline+ gradient and was enhanced as extravesicular Na+ was increased from 10 mM to 100 mM. Dissipation of the Na+ gradient by the ionophore gramicidin D resulted in diminished Na+-stimulated glycine uptake. Na+-stimulated uptake of glycine was electrogenic. Substrate-velocity analysis of Na+-dependent glycine uptake over the range of amino acid concentrations from 25 μM to 10 mM demonstrated a single saturable transport system with apparent Km = 996 μM and Vmax = 348 pmol glycine/mg protein per min. Inhibition observed when the Na+-dependent uptake of 25 μM glycine was inhibited by 5 mM extravesicular test amino acid segregated dibasic amino acids, which did not inhibit glycine uptake, from all other amino acid groups. The amino acids d-alanine, d-glutamic acid, and d-proline inhibited similarly to their l counterparts. Accelerative exchange of extravesicular [3H]glycine was demonstrated when brush border vesicles were preloaded with glycine, but not when they were preloaded with l-alanine, l-glutamic acid, or with l-proline. It is concluded that a single transport system exists at the level of the rabbit renal brush border membrane that functions to reabsorb glycine independently from other groups of amino acids. 相似文献
13.
Ionic and Metabolic Requirements for High-Affinity Choline Uptake and Acetylcholine Synthesis in Nerve Terminals at a Neuromuscular Junction 总被引:2,自引:1,他引:1
Abstract: We have shown previously that in the chick ciliary nerve-iris muscle preparation Na+-dependent high-affinity choline uptake was confined to the nerve terminals. In this paper the sodium-dependent high-affinity choline uptake (SDHACU), which is coupled to acetylcholine (ACh) synthesis, was further characterized by measuring uptake of [3H]choline and its conversion to [3hjach under a variety of ionic and metabolic perturbations. Mannitol equilibration with the extracellular space was found to occur in less than 1 min in this preparation. Na+-dependent choline (Ch+) uptake was shown to be linear for 16 min and to reach an equilibrium before Na+-independent Ch+ uptake, which continued to increase for 60 min. Elevated [K+]0 concentrations inhibited Ch+ uptake and ACh synthesis. Glycolytic and respiratory inhibitors also reduced both processes, as did ouabain and omission of [K+]0. Incubation conditions that reduce transmitter release had no effect on inhibition by high [K+]0. Reduction of SDHACU and sodium-dependent ACh synthesis by depolarization with high [K+]0 or by inhibition of Na, K-ATPase implies that the electrochemical gradients for Ch+ and Na+ are important in providing a driving force for high-affinity Ch+ uptake. The inhibition by metabolic blockers suggests active transport, but the effects may be indirect, caused by reduced Na, K-ATPase activity and alterations in membrane potential. While most metabolic inhibitors exerted parallel effects on both Ch+ uptake and ACh synthesis, in some cases Ch+ uptake was more strongly inhibited than ACh synthesis. This occurred in preparations incubated with high [K+]0 and ouabain. Na+-dependent Ch+ uptake and ACh synthesis were found to be temperature-dependent with a Q10 (20–30°) of 3.6 and 6.6, respectively and a Q10 (30–40°) of 1.3 and 1.0, respectively. Inhibition of acetylcholinesterase by paraoxon increases to 92% the proportion of the Ch+ taken up which is converted to ACh. ACh did not reduce Ch+ transport when present at 100 μM. 相似文献
14.
Our previous studies showed that when ethylmercurithiosalicylate (thimerosal) interacts with the transport ATPase of the guinea pig kidney under specified conditions, the Na+ + K+-dependent ATPase activity is inhibited, while the Na+-dependent ATPase, the Na+ + ATP-dependent phosphorylation of the enzyme, and the K+-dependent discharge of the phosphoenzyme seem to be unaffected. Here we describe other properties of the thimerosal-treated enzyme: Na+-dependent ADP-ATP exchange, Na+-dependent UTPase, and K+-dependent p-nitrophenylphosphatase activities of the modified enzyme are not inhibited. Kinetics of the Na+ effect on the UTPase activities of the native and the modified enzyme are the same. However, K+ has a greater inhibitory effect on the Na+-UTPase of the modified enzyme than on the Na+-UTPase of the native enzyme. The increase in the apparent affinity of the thimerosal-treated enzyme for K+ is also evident from the kinetics of the K+ effect on p-nitrophenylphosphatase. Neither the native enzyme nor the modified enzyme catalyzes a P1-ATP exchange. The uninhibited activities of the thimerosal-treated enzyme are sensitive to ouabain. These data provide further support for those reaction mechanisms in which the existence of two ATP sites within the enzyme is assumed. 相似文献
15.
《FEBS letters》1997,400(2-3):191-195
The different murine D2-type dopamine receptors (D2L, D2S, D3L, D3S, and D4) were expressed in Xenopus laevis oocytes. The D2-type receptors were all similarly and efficiently expressed in Xenopus oocytes and were shown to bind the D2 antagonist [125I]sulpride. They were all shown to activate Cl− influx upon agonist stimulation. Using the diagnostic inhibitor bumetanide, we were able to separate the Na+/K+/2Cl− cotransporter component of the Cl− influx from the total unidirectional Cl− influx. The D3L subtype was found to operate exclusively through the bumetanide-insensitive Cl− influx whereas the other D2-type receptors acted on the Na+/K+/2Cl− cotransporter as well. The pertussis toxin sensitivity of the receptor-activated chloride influx via the Na+/K+/2Cl− cotransporter varied between the various D2-type receptors showing that they may couple to different G proteins, and activate different second messenger systems. 相似文献
16.
Elisa Herrero M. Carmen Aragon Javier Diez-Guerra Fernando Valdivieso Cecilio Gimenez 《Neurochemical research》1985,10(5):579-589
The uptake of tryptophan at various stages of development was examined in plasma membrane vesicles derived from rat brain. The total uptake has two components Na+-dependent and Na+-independent respectively. The Na+-dependent component of the transport system appears around the 5th postnatal day and increases with the age. TheK
m value of the system does not vary during development. The Vmax increases five-fold between 14 and 35 day of postnatal life. Plasma membrane vesicles derived from T3-treated rats are able to accumulate nearly three-fold more tryptophan than nontreated rats. The results support the idea that thyroid hormones at the earlier stages of life, promote the establishment of neurotransmission in the developing nervous system. 相似文献
17.
The effect of external and internal K+ on Nao+-dependent Ca2+ efflux was studied in dialyzed squid axons under constant membrane potential. With axons clamped at their resting potentials, external K+ (up to 70 mM) has no effect on Na+?Ca2+ exchange. Removal of Ki+ causes a marked inhibition in the Nao+-dependent Ca2+ efflux component. Internal K+ activates the Na+?Ca2+ exchange with low affinity . Activation by Ki+ is similar in the presence or in the absence of Nai+, thus ruling out a displacement of Nai+ from its inhibitory site. Axons dialyzed with ATP also show a dependency of Ca2+ efflux on Ki+. The present results demonstrate that Ki+ is an important cofactor (partially required) for the proper functioning of the forward Na+?Ca2+ exchange. 相似文献
18.
C.F. Dick A.L.A. Dos-Santos D. Majerowicz L.S. Paes N.L. Giarola K.C. Gondim A. Vieyra J.R. Meyer-Fernandes 《Biochimica et Biophysica Acta (BBA)/General Subjects》2013
Background
Orthophosphate (Pi) is a central compound in the metabolism of all organisms, including parasites. There are no reports regarding the mechanisms of Pi acquisition by Trypanosoma cruzi.Methods
32Pi influx was measured in T. cruzi epimastigotes. The expression of Pi transporter genes and the coupling of the uptake to Na+, H+ and K+ fluxes were also investigated. The transport capacities of different evolutive forms were compared.Results
Epimastigotes grew significantly more slowly in 2 mM than in 50 mM Pi. Influx of Pi into parasites grown under low Pi conditions took place in the absence and presence of Na+. We found that the parasites express TcPho84, a H+:Pi-symporter, and TcPho89, a Na+:Pi-symporter. Both Pi influx mechanisms showed Michaelis–Menten kinetics, with a one-order of magnitude higher affinity for the Na+-dependent system. Collapsing the membrane potential with carbonylcyanide-p-trifluoromethoxyphenylhydrazone strongly impaired the influx of Pi. Valinomycin (K+ ionophore) or SCH28028 (inhibitor of (H+ + K+)ATPase) significantly inhibited Pi uptake, indicating that an inwardly-directed H+ gradient energizes uphill Pi entry and that K+ recycling plays a key role in Pi influx. Furosemide, an inhibitor of the ouabain-insensitive Na+-ATPase, decreased only the Na+-dependent Pi uptake, indicating that this Na+ pump generates the Na+ gradient utilized by the symporter. Trypomastigote forms take up Pi inefficiently.Conclusions
Pi starvation stimulates membrane potential-sensitive Pi uptake through different pathways coupled to Na+ or H+/K+ fluxes.General significance
This study unravels the mechanisms of Pi acquisition by T. cruzi, a key process in epimastigote development and differentiation to trypomastigote forms. 相似文献19.
Brian E. Peerce 《The Journal of membrane biology》1989,110(2):189-197
Summary The substrate stoichiometry of the intestinal Na+/phosphate cotransporter was examined using two measures of Na+-dependent phosphate uptake: initial rates of uptake with [32P] phosphate and phosphate-induced membrane depolarization using the potential-sensitive dye diSC3(5). Isotopic phosphate measures electrogenic and electroneutral Na+-dependent phosphate uptake, while phosphate-induced membrane depolarization measures electrogenic phosphate uptake. Using these measures of Na-dependent phosphate uptake, three parameters were compared: substrate affinity; phenylglyoxal sensitivity and labeling; and inhibiton by mono- and di-fluorophosphates. Na+/phosphate cotransport was found to have similar Na+ activations (apparentK
0.5's of 28 and 25mm), apparentK
m
's for phosphate (100 and 410 m), andK
0.5's for inhibition by phenylglyoxal (70 and 90 m) using isotopic phosphate, uptake and membrane depolarization, respectively. Only difluorophosphate inhibited Na+-dependent phosphate uptake below 1mm at pH 7.4.Difluorophosphate also protected a 130-kDa polypeptide from FITC-PG labeling in the presence of Na+ with apparentK
0.5 for phosphate of 200 m; similar to the apparentK
m
for phosphate uptake, andK
0.5 for phosphate protection against FITC-PG inhibition of Na+-dependent phosphate uptake and FITC-PG labeling of the 130-kDa polypeptide. These results indicate that the intestinal Na+/phosphate cotransporter is electrogenic at pH 7.4, that H2PO
4
–
is the transport-competent species, and that the 130-kDa polypeptide is an excellent candidate for the intestinal Na+/phosphate cotransporter. 相似文献
20.
The uptake of l-glutamic acid into brush-border membrane vesicles isolated from rat renal proximal tubules is Na+-dependent. In contrast to Na+-dependent uptake of d-glucose, pre-equilibration of the vesicles with K+ stimulates l-glutamic acid uptake. Imposition of a K+ gradient () further enhances Na+-dependent l-glutamic acid uptake, but leaves K+-dependent glucose transport unchanged. If K+ is present only at the outside of the vesicles, transport is inhibited. Intravesicular Rb+ and, to a lesser extent, Cs+ can replace intravesicular K+ to stimulate l-glutamic acid uptake. Changes in membrane potential incurred by the imposition of an H+-diffusion potential or anion replacement markedly affect Na+-dependent glutamic acid uptake only in the presence of K+. Experiments with a potential-sensitive cyanine dye also indicate that, in the presence of intravesicular K+ a charge movement is involved in Na+-dependent transport of l-glutamic acid.The data indicate that Na+-dependent l-glutamic acid transport can be additionally energized by a K+ gradient. Furthermore, intravesicular K+ renders Na+-dependent l-glutamic acid transport sensitive to changes in the transmembrane electrical potential difference. 相似文献