Active and Passive Components of Sulfate Uptake in Sunflower Plants

The aim of the investigation was to identify components of active and passive ion uptake and transport in roots of plants and to assess their quantitative relations under different external and internal conditions. The uptake of radiosulfate and water by young sunflower plants from complete nutrient solutions labelled with ³⁵S was studied. The metabolism-linked nature of the sulfate uptake in the root following the passive migration into the apparent free space (AFS) was demonstrated by the addition of sodium. selenate, 2,4-dinitrophenol, potassium cyanide, and sodium azide to the nutrient solutions. The magnitude of the AFS measured on a root volume basis varied between 14 and 57 per cent depending on the pretreatment of the plants and the sulfate concentration of the nutrient solution. The variations were supposed to be due to different capacity to bind sulfate by exchange-adsorption within the AFS. The amounts of sulfate in different fractions of the total AFS-uptake were computed under certain theoretical assumptions. A quantitative connection was proposed between the magnitude of the adsorbed sulfate fraction in the AFS and the rate of active uptake into the symplasm. The exchange-adsorption probably constitutes the initial stage of active ion uptake. The stimulating effect by water on ion uptake would be an increase of the speed of transporting ions to, from, or along the adsorption sites in the AFS. Experiments conducted at temperatures in the nutrient solution between 5 and 35 C elucidated the multistep nature of ion transport within a root.     

Effects of Ionic Strength of Nutrient Solutions on Phosphate Uptake by Sunflower

Active phosphate uptake by the roots of young sunflower plants was stimulated nonspecifically by increasing the total salt concentration of the uptake solution. Inhibition of active uptake by DNP-treatment removed the salt stimulation. Independently of the rate of active uptake the amount of phosphate present in the free space of the roots increased as the salt concentration was raised. It is suggested that at low ionic strength of the nutrient solution the initial passive step of ion transport through the root free space can limit the overall uptake rate.     

Sodium uptake across the apical border of the isolated turtle colon: Confirmation of the two-barrier model

Summary The initial rate of Na uptake by the turtle colon from the mucosal bathing solution consists of two operationally distinct components. One component is a linear function of mucosal Na concentration, is unaffected by amiloride, and appears to represent Na uptake into the paracellular shunt path. The major component of Na uptake is abolished by amiloride and is virtually equal to the short-circuit current over a wide range of. mucosal Na concentrations, suggesting that this portion of Na uptake represents Na movement into Na-transporting cells of the colon. The amiloride-sensitive component of Na uptake, at low mucosal Na concentrations, was unaffected if net Na transport was abolished by ouabain. Similarly, at low mucosal Na concentrations the amiloride-sensitive conductance of the colon was identical in the presence and in the absence of net Na transport.These results show that the isolated turtle colon behaves, as two distinct barriers to transmural Na transport, an apical barrier blocked by amiloride and a more basal-lying barrier where active, transmural Na transport is blocked by ouabain. In addition, these experiments appear to provide the first unambiguous demonstration that the initial-rate isotope uptake technique can provide adirect measure of the properties of the amiloridesensitive barrier to transmural Na movement, presumably the apical membranes of the Na-transporting cells. The results are consistent with the notion that the rate of transmural active Na transport and the conductance of the active Na-transport path are determined by the properties of the apical membrane.     

An Hg-sensitive channel mediates the diffusional component of glucose transport in olive cells

In several organisms solute transport is mediated by the simultaneous operation of saturable and non-saturable (diffusion-like) uptake, but often the nature of the diffusive component remains elusive. The present work investigates the nature of the diffusive glucose transport in Olea europaea cell cultures. In this system, glucose uptake is mediated by a glucose-repressible, H(+) -dependent active saturable transport system that is superimposed on a diffusional component. The latter represents the major mode of uptake when high external glucose concentrations are provided. In glucose-sufficient cells, initial velocities of D- and L-[U-(14)C]glucose uptake were equal and obeyed linear concentration dependence up to 100 mM sugar. In sugar starved cells, where glucose transport is mediated by the saturable system, countertransport of the sugar pairs 3-O-methyl-D-glucose/D-[U-(14)C]glucose and 3-O-methyl-D-glucose/3-O-methyl-D-[U-(14)C]glucose was demonstrated. This countertransport was completely absent in glucose-sufficient cells, indicating that linear glucose uptake is not mediated by a typical sugar permease. The endocytic inhibitors wortmannin-A and NH(4)Cl inhibited neither the linear component of D- and L-glucose uptake nor the absorption of the nonmetabolizable glucose analog 3-O-methyl-D-[U-(14)C]glucose, thus excluding the involvement of endocytic mediated glucose uptake. Furthermore, the formation of endocytic vesicles assessed with the marker FM1-43 proceeded at a very slow rate. Activation energies for glucose transport in glucose sufficient cells and plasma membrane vesicles were 7 and 4 kcal mol(-1), respectively, lower than the value estimated for diffusion of glucose through the lipid bilayer of phosphatidylethanolamine liposomes (12 kcal mol(-1)). Mercury chloride inhibited both the linear component of sugar uptake in sugar sufficient cells and plasma membrane vesicles, and the incorporation of the fluorescent glucose analog 2-NBDG, suggesting protein-mediated transport. Diffusive uptake of glucose was inhibited by a drop in cytosolic pH and stimulated by the protein kinase inhibitor staurosporine. The data demonstrate that the low-affinity, high-capacity, diffusional component of glucose uptake occurs through a channel-like structure whose transport capacity may be regulated by intracellular protonation and phosphorylation/dephosphorylation.     

Chloroquine,a novel inhibitor of amino acid transport by rat renal brush border membrane vesicles

Summary Chloroquine is an antimalarial and antirheumatic lysosomotropic drug which inhibits taurine uptake into and increases efflux from cultured human lymphoblastoid cells. It inhibits taurine uptake by rat lung slices and affects the uptake and release of cystine from cystinotic fibroblasts. Speculations on its mode of action include a proton gradient effect, a non-specific alteration in membrane integrity, and membrane stabilization. In this study, the effect of chloroquine on the uptake of several amino acids by rat renal brush border membrane vesicles (BBMV) was examined. Chloroquine significantly inhibited the secondary active, NaCl-dependent component of 10µM taurine uptake at all concentrations tested, but did not change equilibrium values. Analysis of these data indicated that the inhibition was non-competitive. Taurine uptake was reduced at all osmolarities tested, but inhibition was greatest at the lowest osmolarity. Taurine efflux was not affected by chloroquine, nor was the NaCl-independent diffusional component of taurine transport. Chloroquine (1 mM) inhibited uptake of the imino acids L-proline and glycine, and the dibasic amino acid L-lysine. It inhibited the uptake of D-glucose, but not the neutral-amino acids L-alanine or L-methionine. Uptake of the dicarboxylic amino acids, L-glutamic acid and L-aspartic acid, was slightly enhanced. With regard to amino acid uptake by BBMV, these findings may support some of the currently proposed mechanisms of the action of chloroquine but further studies are indicated to determine why it affects the initial rate of active amino acid transport.     

Active ion transport in the renal proximal tubule. I. Transport and metabolic studies

Various aspects of the interrelationship between ion transport and cellular metabolism were investigated using a suspension of rabbit cortical tubules that were mainly proximal in nature. Using the intact tubules, the compartmentation of K within the renal cell was studied by performing 42K uptake studies. The oxygen consumption (QO2) of the tubules was measured under similar conditions, as well as when the Na pump was stimulated by increasing Na+ entry with nystatin. In addition, the state 3 rate of respiration was measured when the mitochondria of digitonin-permeabilized tubules were stimulated by ADP. At 37 and 25 degrees C, a single-compartmental uptake of 42K was observed, which suggests that extracellular K+ communicates with a single compartment within the renal cell. Between 37 and 15 degrees C, the ouabain- sensitive QO2 and the initial 42K uptake rate were parallel in an Arrhenius-type plot, which indicated that active ion transport and oxidative phosphorylation remain tightly coupled within this temperature range. At all temperatures between 37 and 15 degrees C, nystatin stimulated the QO2, which demonstrates that the entry of Na+ into the renal cells was rate limiting for active Na+ transport throughout this temperature range. Between 37 and 20 degrees C, the nystatin-stimulated QO2 was nearly equal to the state 3 rate of respiration, which suggests that active ion transport may be limited by ATP availability under these conditions. At 15 degrees C, nystatin addition stimulated the QO2 well below the state 3 respiratory rate.     

Evidence for a direct role of tRNA in an amino acid transport system.

The transport of phenylalanine by the general aromatic transport system in spheroplasts of Escherichia coli 9723 has been found to be stimulated by exogenous tRNA. Neither periodate-treated tRNA nor phenylalanine-charged tRNA stimulated, and the latter inhibited, phenylalanine uptake. Among preparations of specific tRNAs, tRNAPhe and tRNATyr were effective in stimulating the uptake of phenylalanine and tyrosine, respectively, and tRNAGlu and tRNAVal gave no detectable stimulation of phenylalanine or tyrosine transport. The preparation of tRNATyr was 10 times as active as unfractionated tRNA and gave as much as 167% stimulation of tyrosine transport. Correspondingly, the preparation of tRNAPhe was at least 3.5 times as active as the unfractionated tRNA and 2.5 times as active as the preparation of tRNATyr in stimulation of phenylalanine transport. Preliminary results in fractionation of the active component of tRNA for stimulating phenylalanine uptake show that the major activity resides in minor isoacceptor(s) tRNAPhe rather than the major component tRNAPhe, and the slight activity of preparations of tRNATyr is probably due to a contamination of the active tRNAPhe. Other preliminary results indicate that this type of stimulation occurs with uptake of other amino acids and their tRNA.     

Calcium uptake into rat small intestinal brush border membrane vesicles: characterization of transmembrane calcium transport at short initial incubation times

Calcium transport into brush border vesicles from rat small intestine was investigated by determining uptake rates at very short incubation periods. At incubation times up to 1 second a linear relationship between calcium uptake and time was observed at free calcium concentrations ranging from 1 microM to 5 mM. At time points above 1 second calcium uptake deviates progressively from linearity. Several lines of evidences (EGTA-wash, dependency on membrane potential, temperature sensitivity and effect of the calcium ionophore A23187) suggest transmembrane transport rather than extravesicular binding of calcium as being responsible for calcium uptake. Saturation experiments performed under initial linear and curvilinear uptake conditions show a saturable transport component in the mu molar and only a tendency to saturate in the molar concentration range. It is concluded that uptake values far from equilibrium are characteristic for transmembrane flux of calcium. Transmembrane flux of calcium is mediated by multiple and potential-sensitive mechanisms.     

Uphill transport of monosaccharides inCandida beverwijkii

The yeastCandida beverwijkii was found to transport several monosaccharides against a concentration gradient. The process is mediated by a mobile carrier and shows a pronounced pH dependence. It is tightly coupled with metabolism and only potassium sorbate uncoupled the equilibrating from the active transport component. Kinetic analysis of uptake and efflux at high monosaccharide concentrations indicates an active transport operating either into or out of cells. T. Deák carried out the work in Prague while supported by the Hungarian Academy of Sciences.     

Uptake and efflux of copper-64 in Menkes''-disease and normal continuous lymphoid cell lines.

The accumulation of copper over 2 h by normal lymphoid cells and those from Menkes'-disease patients (Menkes' cells) was found to be biphasic, with an initial phase of rapid uptake, an approach to steady state at around 40-60 min, followed by a further accumulation phase. The accumulation of copper was not diminished by the addition of a variety of metabolic inhibitors, suggesting that copper uptake is not an active process. The presence of carbonyl cyanide m-chlorophenylhydrazone in the culture medium stimulated the uptake and accumulation of copper in both normal and Menkes' cells to the same absolute level. This effect appeared to be specific for copper, since the accumulation of Zn and Cd was unaffected. Menkes' cells did not differ from normal in their initial rate of copper uptake. Analysis of the uptake curve suggested that the membrane transport of copper involves both passive and facilitated diffusion. Initial rate of efflux from the cells was approximated by two methods. Menkes' cells did not appear to be affected in this function. It seems likely that the basic defect in Menkes' disease involves a step in intracellular copper transport rather than the membrane transport of copper.     

Iron Transport in Escherichia coli: Roles of Energy-dependent Uptake and 2,3-Dihydroxybenzoylserine

Escherichia coli strains B/r and 2276 contain an active transport system for iron. The system is energy-dependent, repressed by excess iron in the growth medium, and capable of accumulating iron inside of the cells at concentrations 2,000-fold higher than those in the medium. Two tonB-trp deletion mutants, strains B/rlt and B/lt7, which are sensitive to chromic ion and require high levels of iron for normal growth, are deficient in this active transport system. A point mutant, strain Chr2, which is also sensitive to chromic ion and requires high levels of iron for growth, has the active uptake system but cannot synthesize a specific chelator for iron, 2,3-dihydroxybenzoylserine (DHBS). Evidence is presented to support the hypothesis that both the active uptake system and chelation of iron by DHBS play a role in iron uptake from iron-deficient medium. The chromium sensitivity of the mutants can be explained by inhibition of uptake of exogenous iron.     

Stimulatory effect of lithium ion on proline transport by whole cells of Escherichia coli.

The effect of monovalent cations on proline transport in whole cells of Escherichia coli K-12 has been examined. Lithium ion added to the uptake medium stimulated proline transport severalfold and K+ and Na+ were slightly effective, whereas Rb+, Cs+, and NH4+ were completely without effect. The stimulatory effect of Li+ on proline transport was not due to an increase in osmolarity of the uptake medium, and d 5 mM p-chloromercuribenzene sulfonic acid completely blocked this effect of Li+ without having any effect on the basal rate of proline transport. The Arrhenius plots for Li+-stimulated transport showed a clear transition point at 35 degrees C in addition to 20 degrees C which was also detectable in the basal transport. Lithium ion stimulated proline transport synergistically in the presence of glucose and succinate as a carbon source. The addition of 2.5 mM KCN or 0.5 mM arsenate did not inhibit this synergistic effect, although the presence of these inhibitors inhibited completely the stimulation of proline transport induced by the addition of carbon source. Carbonylcyanide m-chlorophenylhydrazone and 2,4-dinitrophenol blocked both the basal and Li+-stimulated proline transport. When membrane potential of E. coli cells was measured by the dibenzyldimethylammonium uptake method, the incubation of Li+ with the cells did not affect the preexisting membrane potential. These results suggest that Li+ stimulates proline transport by intact cells of E. coli in a manner somewhat affecting membrane component(s) different from the transport carrier of proline. It is uncertain whether the effect of Li+ is directly involved in the mechanisms of energy coupling of proline transport.     

Active amino acid transport in plasma membrane vesicles from Simian virus 40-transformed mouse fibroblasts. Characteristics of electrochemical Na+ gradient-stimulated uptake.

Selectively permeable membrane vesicles isolated from Simian virus 40-transformed mouse fibroblasts catalyzed Na+ gradient-coupled active transport of several neutral amino acids dissociated from intracellular metabolism. Na+-stimulated alanine transport activity accompanied plasma membrane material during centrifugation in discontinuous dextran 110 gradients. Carrier-mediated transport into the vesicle was demonstrated. When Na+ was equilibrated across the membrane, countertransport stimulation of L-[3H]alanine uptake occurred in the presence of accumulated unlabeled L-alanine, 2-aminoisobutyric acid, or L-methionine. Competitive interactions among neutral amino acids, pH profiles, and apparent Km values for Na+ gradient-stimulated transport into vesicles were similar to those previously described for amino acid uptake in Ehrlich ascites cells, which suggests that the transport activity assayed in vesicles is a component of the corresponding cellular uptake process. Both the initial rate and quasi-steady state of uptake were stimulated as a function of a Na+ gradient (external Na+ greater than internal Na+) applied artificially across the membrane and were independent of endogenous (Na+ + K+)-ATPase activity. Stimulation by Na+ was decreased when the Na+ gradient was dissipated by monensin, gramicidin D or Na+ preincubation. Na+ decreased the apparent Km for alanine, 2-aminoisobutyric acid, and glutamine transport. Na+ gradient-stimulated amino acid transport was electrogenic, stimulated by conditions expected to generate an interior-negative membrane potential, such as the presence of the permeant anions NO3- and SCN-. Na+-stimulated L-alanine transport was also stimulated by an electrogenic potassium diffusion potential (K+ internal greater than K+ external) catalyzed by valinomycin; this stimulation was blocked by nigericin. These observations provide support for a mechanism of active neutral amino acid transport via the "A system" of the plasma membrane in which both a Na+ gradient and membrane potential contribute to the total driving force.     

Comparative transport activity of intact cells, membrane vesicles, and mesosomes of Bacillus licheniformis

Sodium ion was shown to stimulate strongly the transport of l-glutamic acid into cells of Bacillus licheniformis 6346 His(-). Lithium ion had a slight capacity to replace Na(+) in this capacity, but K(+) was without effect. Three of five amino acids tested. l-glutamic acid, l-aspartic acid, and l-alanine, were concentrated against a gradient in the cells. Intracellular pools of these amino acids were extractable with 5% trichloroacetic acid. Pools of l-histidine and l-lysine could not be detected. No evidence of active transport of lysine into cells could be detected, and histidine was taken up in the absence of chloramphenicol but not in its presence. The uptake of glutamic acid by membrane vesicle preparations was strongly stimulated by reduced nicotinamide adenine dinucleotide (NADH) and to a lesser extent by succinate. The presence of phenazine methosulfate increased uptake in the presence of succinate. Either l- or d-lactate and adenosine triphosphate were without effect. None of these compounds stimulated the uptake of glutamic acid by mesosomes, although some mesosome preparations contained separable membrane which was very active. NADH strongly stimulated the uptake of aspartic acid and alanine by membrane vesicles but had only a slight effect on the uptake of histidine and lysine. No evidence of active transport of any of the amino acids into mesosomes could be detected either in the presence or absence of NADH. NADH stimulation of the uptake of glutamic acid by membrane vesicles was destroyed by exposure to light of 360 nm; this inactivation was reversible by vitamin K(2(5)) or K(2(10)). Sodium ion stimulated transport of glutamic acid by membrane vesicles.     

Energetics of osmoregulation: II. Water flux and osmoregulatory work in the euryhaline fish, Fundulus heteroclitus

Teleost fish experience passive osmotic water influx in fresh water (FW) and water outflux in salt water, which is normally compensated by water flow driven by active ion transport mechanisms. Euryhaline fish may also minimize osmotic energy demand by "behavioral osmoregulation", seeking a medium isotonic with their body fluids. Our goal was to evaluate the energy requirement for osmoregulation by the euryhaline fish Fundulus heteroclitus, to determine whether it is of sufficient magnitude to favor behavioral osmoregulation. We have developed a method of weighing small fish repetitively for long periods without apparent damage, which was used to assess changes in water content following changes in external salinity. We found that cold (4 degrees C) inhibits osmoregulatory active transport mechanisms in fish acclimated to warmer temperatures, leading to a net passive water flux which is reversed by rewarming the fish. A sudden change of salinity at room temperature triggers a transient change in water content and the initial slope can be used to measure the minimum passive flux at that temperature. With some reasonable assumptions as to the stoichiometry of the ion transport and ATP-generating processes, we can calculate the amount of respiration required for ion transport and compare it to the oxygen uptake measured previously under the same conditions. We conclude that osmoregulation in sea water requires from 6% to 10% of the total energy budget in sea water, with smaller percentages in FW, and that this fraction is probably sufficient to be a significant selective driving force favoring behavioral osmoregulation under some circumstances.     

Na(+)-dependent D-mannose transport at the apical membrane of rat small intestine and kidney cortex

The presence of a Na(+)/D-mannose cotransport activity in brush-border membrane vesicles (BBMV), isolated from either rat small intestine or rat kidney cortex, is examined. In the presence of an electrochemical Na(+) gradient, but not in its absence, D-mannose was transiently accumulated by the BBMV. D-Mannose uptake into the BBMV was energized by both the electrical membrane potential and the Na(+) chemical gradient. D-Mannose transport vs. external D-mannose concentration can be described by an equation that represents a superposition of a saturable component and another component that cannot be saturated up to 50 microM D-mannose. D-Mannose uptake was inhibited by D-mannose > D-glucose>phlorizin, whereas for alpha-methyl glucopyranoside the order was D-glucose=phlorizin > D-mannose. The initial rate of D-mannose uptake increased as the extravesicular Na(+) concentration increased, with a Hill coefficient of 1, suggesting that the Na(+):D-mannose cotransport stoichiometry is 1:1. It is concluded that both rat intestinal and renal apical membrane have a concentrative, saturable, electrogenic and Na(+)-dependent D-mannose transport mechanism, which is different from SGLT1.     

Studies on avian erythrocyte metabolism. XVII. Kinetics and transport properties of myo-inositol in chicken reticulocytes

The uptake of myo-inositol was determined in a reticulocyte-enriched fraction prepared from chicken blood and compared with uptake in mature erythrocytes. While reticulocytes accumulated inositol at levels more than threefold that of the plasma concentration, erythrocyte levels were only slightly higher than that of the plasma concentration. The rate of uptake in reticulocytes was approximately 66 mumol/ml rbc/h compared to 5 mumol/ml rbc/h in mature erythrocytes when measured at an inositol medium concentration of 250 microM. The kinetic analysis of inositol influx by reticulocytes reveals a two component system: saturable and nonsaturable. The saturable component, which has a Km for inositol of approximately 222 microM, is Na-dependent. This Na-dependent saturable component, which presumably reflects active transport of inositol, accounts for 30-35% of the transport process. The saturable component is completely inhibited by amiloride but to a lesser extent by ouabain and bumetanide. Moreover, in the course of reticulocyte maturation, the saturable component is lost concomitantly with the completion of the synthesis of myo-inositol pentakisphosphate and the drastic decrease in the membrane permeability to inositol. In addition, phloretin and cytochalasin B, which bind to hexose carriers and inhibit hexose sugar transport, also inhibited inositol transport. The uptake of inositol was not affected by excesses of 3-O-methylglucose (100 mM) or by physiological concentrations of D-glucose. Thus, the transport mechanism of myo-inositol appears distinct from that of D-glucose.     

Nitrite uptake and its regulation in the cyanobacterium Anacystis nidulans

Two different components seem to participate in the uptake of nitrite by the cyanobacterium Anacystis nidulans, namely a transport system sensitive to N,N′-dicyclohexylcarbodiimide and a passive influx. The relative contribution of each component depended on the pH of the medium, that of the active system being prevalent at high pH values. The active transport of nitrite appears to be mediated by a high-affinity system, whereas the affinity for nitrite of the passive system is lower, similar to that of nitrite reductase. The utilization of nitrite was inhibited by products of the assimilation of ammonium via glutamine synthetase, apparently acting at the level of the active component involved in nitrite uptake.     

Urea transport-defective strains of Saccharomyces cerevisiae.

Experiments characterizing the urea active transport system in Saccharomyces cerevisiae indicate that (i) formamide and acetamide are strong competitive inhibitors of urea accumulation, (ii) uptake is maximal at pH 3.3 and is 80% inhibited at pH 6.0, and (iii) adenosine 5'-triphosphate generated by glycolysis in conjunction with formation of an ion gradient is likely the driving force behind urea transport. Mutant strains were isolated that are unable to accumulate urea at external concentrations of 0.25 mM. These strains also exhibit a depressed growth rate on 10 mM urea, indicating existence of a relationship between the active transport and facilitated diffusion modes of urea uptake.     

An Hg-sensitive channel mediates the diffusional component of glucose transport in olive cells

In several organisms solute transport is mediated by the simultaneous operation of saturable and non-saturable (diffusion-like) uptake, but often the nature of the diffusive component remains elusive. The present work investigates the nature of the diffusive glucose transport in Olea europaea cell cultures. In this system, glucose uptake is mediated by a glucose-repressible, H⁺-dependent active saturable transport system that is superimposed on a diffusional component. The latter represents the major mode of uptake when high external glucose concentrations are provided. In glucose-sufficient cells, initial velocities of d- and l-[U-¹⁴C]glucose uptake were equal and obeyed linear concentration dependence up to 100 mM sugar. In sugar starved cells, where glucose transport is mediated by the saturable system, countertransport of the sugar pairs 3-O-methyl-d-glucose/d-[U-¹⁴C]glucose and 3-O-methyl-d-glucose/3-O-methyl-d-[U-¹⁴C]glucose was demonstrated. This countertransport was completely absent in glucose-sufficient cells, indicating that linear glucose uptake is not mediated by a typical sugar permease. The endocytic inhibitors wortmannin-A and NH₄Cl inhibited neither the linear component of d- and l-glucose uptake nor the absorption of the nonmetabolizable glucose analog 3-O-methyl-d-[U-¹⁴C]glucose, thus excluding the involvement of endocytic mediated glucose uptake. Furthermore, the formation of endocytic vesicles assessed with the marker FM1-43 proceeded at a very slow rate. Activation energies for glucose transport in glucose sufficient cells and plasma membrane vesicles were 7 and 4 kcal mol^− 1, respectively, lower than the value estimated for diffusion of glucose through the lipid bilayer of phosphatidylethanolamine liposomes (12 kcal mol^− 1). Mercury chloride inhibited both the linear component of sugar uptake in sugar sufficient cells and plasma membrane vesicles, and the incorporation of the fluorescent glucose analog 2-NBDG, suggesting protein-mediated transport. Diffusive uptake of glucose was inhibited by a drop in cytosolic pH and stimulated by the protein kinase inhibitor staurosporine. The data demonstrate that the low-affinity, high-capacity, diffusional component of glucose uptake occurs through a channel-like structure whose transport capacity may be regulated by intracellular protonation and phosphorylation/dephosphorylation.