Accelerative exchange diffusion kinetics of glucose between blood and brain and its relation to transport during anoxia.

An earlier study showed that unidirectional glucose transport from blood to brain decreases during perfusion with anoxic blood (Betz, A. L., Gilboe, D. D. and Drewes, L. R. (1974) Brain Res. 67, 307-316). Brain glucose levels also decrease during anoxia. Therefore, the present study was designed to investigate whether the decreased transport might be the result of decreased accelerative exchange diffusion when brain glucose levels are low. The rate of undirectional transport into brain (v) of D-[6-3H]glucose was studied in 22 isolated, perfused dog brains by means of an indicator dilution technique using 22Na as the intravascular reference. The kinetics of transport were determined over a range of blood glucose concentrations (S1) at each of five different brain glucose levels (S2). The existence of accelerative exchange diffusion for glucose was indicated by a decrease in the intercept (increase of apparent V) of a double reciprocal plot (1/v versus 1/S1) as S2 increased. This phenomenon is consistent with a model for facilitated diffusion in which the mobility of the loaded carrier is greater than that of the unloaded carrier. Although the data predict a decrease in glucose transport during anoxia, the predicted decrease (5%) is less than the observed decrease (35%). It is concluded that the simple mobile-carrier model for facilitated diffusion cannot, by itself, describe all properties of blood-brain glucose transport.     

Kinetics of active and passive components of water exchange between the air and a mite, Dermatophagoides farinae

Female North American house dust mites were found to exchange water with the ambient air from two compartments. At humidities above the critical equilibrium activity (CEA), transpiration out of a single large compartment was observed using HTO as a tracer for water. Total sorption into this compartment was also observed by following changes in the specific radioactivity. The sorption data required that an active process or pump be present. The water in this pump is the second compartment above the CEA. Below the CEA the large compartment could be identified as a compartment characterized by a small transpiration rate constant. The pump below the CEA becomes a rapidly transpiring fast compartment. By separating the water pool into two compartments, it was possible to relate a_v to k and $\text{m}\text{?}{}_{\mathrm{<mtext>S</mtext>}}$. The major effect of a_v on k was related to its effect on the permeability of the cuticle. The influence of a_v on $\text{m}\text{?}{}_{\mathrm{<mtext>S</mtext>}}$ was different for active and passive sorption. Above the CEA the pump operated at full capacity and active $\text{m}\text{?}{}_{\mathrm{<mtext>S</mtext>}}$ was directly proportional to a_v. Passive sorption was influenced by a_v in two ways. The driving force for $\text{m}\text{?}{}_{\mathrm{<mtext>S</mtext>}}$ was further reduced below saturation by the effect of a_v on the permeability of the exchange surface.     

Analysis of chlorophyll fluorescence induction curves in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea as a function of magnesium concentration and NADPH-activated light-harvesting chlorophyll ab-protein phosphorylation

The effect of Mg²⁺ concentration and phosphorylation of light-harvesting chlorophyll $\text{a}\text{b-}\text{protein}$ on various chlorophyll fluorescence induction parameters of isolated pea thylakoids has been studied. (1) Lowering the Mg²⁺ concentration from 3 to 0.4 mM decreases only the variable fluorescence (F_v) and the area above the induction curve while at the same time increasing the slow exponential component of the rise (β_max). (2) A further decrease in Mg²⁺ concentration from 0.4 to 0 mM decreases the initial (F₀) fluorescence level such that the ratio $\text{F}{}_{\mathrm{<mtext>v</mtext>}}\text{F}{}_{\mathrm{<mtext>m</mtext>}}$ increases slightly as does the area above the induction curve and β_max. (3) Thylakoid membranes, phosphorylated at 5 mM Mg²⁺, show an equal decrease in F_v and F₀, no change in the area above the induction curve and an increase in β_max. At 2 mM Mg²⁺, however, phosphorylation induced a more extensive quenching of F_v so that the $\text{F}{}_{\mathrm{<mtext>v</mtext>}}\text{F}{}_{\mathrm{<mtext>m</mtext>}}$ ratio was lowered and the area above the induction curve decreased while β_max increased. (4) When phosphorylated membranes were subsequently suspended in an Mg²⁺-free medium the effect on F₀ due to phosphorylation was found to be additive to that due to the absence of Mg²⁺. The effect of membrane phosphorylation on fluorescence is discussed in relation to the control of excitation energy distribution and shows that different mechanisms operate depending on the background Mg²⁺ levels. At high Mg²⁺ the phosphorylation seems to affect the absorption cross-section of Photosystem II while at lower Mg²⁺ levels there is an additional effect of increased spillover from Photosystem II to I.     

Derivation of the equations that describe the effects of unstirred water layers on the kinetic parameters of active transport processes in the intestine

Unidirectional flux of solutes into the intestinal mucosal cells is determined by the rate of movement of these molecules across both an unstirred water layer and the microvillus membrane of the epithelial cell. Therefore, an equation is derived in this paper that describes the velocity of active transport as a function of the characteristics of both the transport carrier in the membrane and the resistance of the overlying unstirred water layer. Using this equation a series of curves are presented that depict the effect on the kinetics of active transport of varying the thickness (d) or surface area (S_w) of the unstirred water layer, the free diffusion coefficient (D) of the solute, the distribution of active transport sites along the villus ($\text{?}{}_{\mathrm{<mtext>n</mtext>}}$), the maximal transport velocity (J^m_d) and the true Michaelis constant (K_m). These theoretical curves illustrate the serious limitations inherent in interpretation of previously published data dealing with active transport processes in the intestine.     

A comparison of the effects of ouabain and 2-deoxy-d-glucose on the thermodynamic variables of the frog skin

Previous studies support the validity of a linear thermodynamic formalism relating the rates of active Na⁺ transport and oxygen consumption J_r to the electrical potential difference ΔΨ an the affinity α (negative free energy) of the metabolic driving reaction. The formulation was further tested in paired control and experimental hemiskins by the use of two inhibitors of Na⁺ transport. Ouabain, a specific inhibitor of the Na⁺ pump, might be expected to diminish the dependence of J_r on ΔΨ without affecting α, whereas 2-deoxy-d-glucose, a competitive inhibitor of glucose metabolism, should be expected to diminish α. Both inhibitors were used at concentrations adequate to depress Na⁺ transport (i.e. short-circuit current J_o) to some 50°_o of control level. Measurements were made of I_o and $\text{d}\text{J}{}_{\mathrm{<mtext>r</mtext>}}\text{d}\text{(ΔΨ)}$, and the apparent value of the affinity α_app was calculated according to the thermodynamic formulation. Ouabain depressed $\text{d}\text{J}{}_{\mathrm{<mtext>r</mtext>}}\text{d}\text{(ΔΨ)}$ without affecting α_app whereas 2-deoxy-d-glucose depressed α_app without affecting $\text{d}\text{J}{}_{\mathrm{<mtext>r</mtext>}}\text{d}\text{(αΨ)}$. The demonstration of these effects indicates the utility of the formalism.     

Lactose transport in Escherichia coli cells Dependence of kinetic parameters on the transmembrane electrical potential difference

We determine the kinetic parameters $\text{V}$ and $\text{K}{}_{\mathrm{<mtext>T</mtext>}}$ of lactose transport in Escherichia coli cells as a function of the electrical potential difference (Δψ) at pH 7.3 and $\text{Δ}\text{pH}\text{= 0}$. We report that transport occurs simultaneously via two components: a component which exhibits a high $\text{K}{}_{\mathrm{<mtext>T</mtext>}}$ (larger than 10 mM) and whose contribution is independent of Δψ, a component which exhibits a low $\text{K}{}_{\mathrm{<mtext>T</mtext>}}$ independent of Δψ (0.5 mM) but whose $\text{V}$ increases drastically with increasing Δψ. We associate these components of lactose transport with facilitated diffusion and active transport, respectively. We analyze the dependence upon Δψ of $\text{K}{}_{\mathrm{<mtext>T</mtext>}}$ and $\text{V}$ of the active transport component in terms of a mathematical kinetic model developed by Geck and Heinz (Geck, P. and Heinz, E. (1976) Biochim. Biophys. Acta 443, 49–63). We show that within the framework of this model, the analysis of our data indicates that active transport of lactose takes place with a H⁺/lactose stoichiometry greater than 1, and that the lac carrier in the absence of bound solutes (lactose and proton(s)) is electrically neutral. On the other hand, our data relative to facilitated diffusion tend to indicate that lactose transport via this mechanism is accompanied by a H⁺/lactose stoichiometry smaller than that of active transport. We discuss various implications which result from the existence of H⁺/lactose stoichiometry different for active transport and facilitated diffusion.     

The reduction of the oxygen-evolving system in chloroplasts by thylakoid components

Using thoroughly dark-adapted thylakoids and an unmodulated Joliot-type oxygen electrode, the following results were obtained. (i) At high flash frequency (4 Hz), the oxygen yield at the fourth flash (Y₄) is lower compared to Y₃ than at lower flash frequency. At 4 Hz, the calculated S₀ concentration after thorough dark adaptation is found to approach zero, whereas at 0.5 Hz the apparent $\text{S}{}_0\text{(}\text{S}{}_0\text{+}\text{S}{}_1\text{)}$ ratio increases to about 0.2. This is explained by a relatively fast donation ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 1.0–1.5}\text{s}$) of one electron by an electron donor to S₂ and S₃ in 15–25% of the Photosystem II reaction chains. The one-electron donor to S₂ and S₃ appears to be rereduced very slowly, and may be identical to the component that, after oxidation, gives rise to ESR signal II_s. (ii) The probability for the fast one-electron donation to S₂ and S₃ has nearly been the same in triazine-resistant and triazine-susceptible thylakoids. However, most of the slow phase of the S₂ decay becomes 10-fold faster ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 5–6}\text{s}$) in the triazine-resistant ones. In a small part of the Photosystem II reaction chains, the S₂ decay was extremely slow. The S₃ decay in the triazine-resistant thylakoids was not significantly different from that in triazine-susceptible thylakoids. This supports the hypothesis that S₂ is reduced mainly by Q^?_A, whereas S₃ is not. (iii) In the absence of CO₂/HCO^?_A and in the presence of formate, the fast one-electron donation to S₂ and S₃ does not occur. Addition of HCO^?₃ restores the fast decay of part of S₂ and S₃ to almost the same extent as in control thylakoids. The slow phase of S₂ and S₃ decay is not influenced significantly by CO₂/HCO^?₃. The chlorophyll a fluorescence decay kinetics in the presence of DCMU, however, monitoring the Q^?_A oxidation without interference of Q_B, were 2.3-fold slower in the absence of CO₂/HCO^?₃ than in its presence. (iv) An almost 3-fold decrease in decay rate of S₂ is observed upon lowering the pH from 7.6 to 6.0. The kinetics of chlorophyll a fluorescence decay in the presence of DCMU are slightly accelerated by a pH change from 7.6 to 6.0. This indicates that the equilibrium Q^?_A concentration after one flash is decreased (by about a factor of 4) upon changing the pH from 7.6 to 6.0. When direct or indirect protonation of Q^?_B is responsible for this shift of equilibrium Q^?_A concentration, these data would suggest that the pK_a value for Q^?_B protonation is somewhat higher than 7.6, assuming that the protonated form of Q^?_B cannot reduce Q_A.     

Cryopreservation of Plasmodium chabaudi I: Protection by glycerol and dimethyl sulfoxide during cooling and by glucose following thawing

Two additives, glycerol and dimethyl sulfoxide (Me₂SO), were investigated for toxic and protective effects for the intraerythrocytic stages of Plasmodium chabaudi. After incubation for 15 min, at 0 °C in Me₂SO and at 37 °C in glycerol, with various concentrations of these additives, half the blood from each treatment was cryopreserved in glass capillary tubes cooled at approximately 3600 °C min^?1 by plunging into liquid nitrogen. Warming was rapid, approximately 12000 °C min^?1, produced by agitation in a water bath at 40 °C for 1 min. The effect of dilution in phosphate-buffered saline (PBS) supplemented with various concentrations (5 to 25% $\text{v}\text{v}$) of glucose was also investigated in conjunction with the two cryoprotectants. Survival of both the frozen and the unfrozen control parasites was assayed by the mean time taken for the parasitemia in groups of five mice to reach a level of 2% following intraperitoneal injection of 10⁶ parasitized erythrocytes into each mouse. Glycerol was toxic at concentrations above 10% $\text{v}\text{v}$ and Me₂SO above approximately 15%. The use of glucose in the recovery medium resulted in a substantial improvement in the survival of frozen and unfrozen parasites previously incubated in either cryoprotectant. The amount of glucose required varied with the concentration of additive used, and optimum survival of cryopreserved parasites was obtaind with 10% $\text{v}\text{v}$ glycerol or 15% $\text{v}\text{v}$ Me₂SO and with 15% $\text{w}\text{v}$ glucose in the diluent medium.     

Sulphate transport by rat ileum. Effect of molybdate and other anions

Kinetic constants for SO₄^2? transport by upper and lower rat ileum in vitro have been determined by computer fitting of rate vs concentration data obtained using the everted sac technique. MoO₄^2? inhibition of this transport is competitive, and kinetic constants for the inhibition were similarly determined. Transport is also inhibited by the anions WO₄^2?, S₂O₃^2? and SeO₄^2?, in the order $\text{S}{}_2\text{O}{}_3{}^{\mathrm{2?}}\text{> SeO}{}_4{}^{\mathrm{2?}}\text{≧ MoO}{}_4{}^{\mathrm{2?}}\text{> WO}{}_4{}^{\mathrm{2?}}$. These anions have no effect on the transport of l-valine. Low SO₄^2? transport rates were observed in sacs from animals fed a high-molybdenum diet. The significance of the results with respect to the problem of molybdate toxicity in animals is discussed, and related to the known protective effect of SO₄^2?.     

Lateral and transversal diffusion and phase transitions in erythrocyte membranes. An excimer fluorescence study

The lateral diffusion of the excimer-forming probe pyrene decanoic acid has been determined in erythrocyte membranes and in vesicles of the lipid extracts. The random walk of the probe molecules is characterized by their jump frequency, v_j, within the lipid matrix. At T = 35°C a value of v_j = 1.6 · 10³ s^?1 is found in erythrocyte membranes. A somewhat slower mobility is determined in vesicles prepared from lipid extracts of the erythrocyte membrane. Depending on structure and charge of the lipids we obtain jump frequencies between 0.8 · 10⁸ s^?1 and 1.5 · 10⁸ s^?1 at T = 35°C. The results are compared with jump frequencies yielded in model membranes.The mobility of molecules perpendicular to the membrane surface (transversal diffusion) is investigated. Erythrocyte ghosts doped with pyrene phosphatidylcholine were mixed with undoped ghosts in order to study the exchange kinetics of the probe molecule. A fast transfer between the outer layers of the ghost cells ($\text{τ}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 1.6}\text{min at}\text{T = 37°}\text{C}$) is found. The exchange process between the inner and the outer layer of one erythrocyte ghost (flip-flop process) following this fast transfer occurs with a half-life time value of $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 100}\text{min at}\text{T = 37°}\text{C}$.The application of excimer-forming probes presumes a fluid state of the membrane. Therefore we investigated the phase transition behaviour using the excimer technique. Beside a thermotropic phase transition at T = 23°C and T = 33°C we observed an additional fluidity change at T = 38°C in erythrocyte ghosts. This transition is attached to a separation of the boundary lipid layer from the intrinsic proteins. No lipid phase transition is observed in liposomes from isolated extracts of the erythrocyte membrane with our methods.     

Variations on stoichiometry of ribosomal proteins in Escherichia coli

Experiments are described in which the Stoichiometry of the ribosomal proteins before and after ribosome release from mRNA is compared. Polysomes labeled with ³H (or ¹⁴C) and run-off 70 S particles (Subramanian el al., 1969) labeled with ¹⁴C (or ³H) were separately isolated, mixed, and the ribosomal proteins extracted and fractionated by two-dimensional gel electrophoresis. The measurement of the isotopic ratios shows that 47 proteins out of the 53 investigated are present in the same amount in polysomes as in run-off ribosomes indicating that they remain with the ribosome during the release step. Proteins S₁, S₂, S₆, S₂₁, $\text{L}{}_{\mathrm{<mtext>7</mtext>}}\text{L}{}_{\mathrm{<mtext>12</mtext>}}$ (Wittmann et al., 1971), however, show higher amounts in polysomes than in run-off ribosomes. The significance of these results is discussed.     

Developmental changes in membrane transport and permeability in the early mouse embryo

Electrophysiological and isotope tracer flux experiments were performed to measure the membrane ion permeability and transport properties of the two-cell mouse embryo. The results show that the internal exchangeable Na and K are 151 and 130 mM, respectively, and their membrane permeabilities are (P_Na) 16 × 10^?8 cm sec^?1 and (P_K) 21 × 10^?8 cm sec^?1. These values predict a membrane potential of ?24 mV (inside negative) which agrees well with ?19 mV measured with microelectrodes. Ouabain-sensitive isotope fluxes demonstrate a Na/K pump mechanism with a stoichiometry of 1.7:1 (Na:K). An external-Na-dependent Na efflux is demonstrated by the reduction of unidirectional Na efflux in Na-free medium, but there is no evidence for a similar mechanism of K efflux at this stage of development. These results are compared with the values reported for the mouse oocyte [Powers, R. D., and Tupper, J. T. (1974). Develop. Biol.38, 320–331; (1975). Exp. Cell Res.91, 413–421]. The hyperpolarization of the membrane potential as compared with the oocyte (?13 mV) results primarily from the increased $\text{P}{}_{\mathrm{<mtext>K</mtext>}}\text{P}{}_{\mathrm{<mtext>Na</mtext>}}$ ratio. A similar phenomenon has been noted in other developing embryos. The increase in pump-mediated K influx at the two-cell stage is accompanied by a decrease of similar magnitude in the external-K-dependent K efflux which is found in the oocyte. This suggests that the KK exchange mechanism may be converted to an active pump. Because of the changes in ion concentrations and movements and the unusual metabolic requirements of the mouse embryo, the effect of external Na on the uptake of glucose and pyruvate in the oocyte and two- and eight-cell stage was examined. No Na-dependent carbohydrate transport could be found at these stages.     

Energization of sulfate transport in yeast

Sulfate uptake by Saccharomyces cerevisiae is stimulated about 12-fold by preincubation of cells with 1% d-glucose or 1% ethanol. The $\text{K}{}_{\mathrm{<mtext>T</mtext>}}$ remains unchanged (0.34–0.38 mM), the $\text{J}{}_{\mathrm{<mtext>mar</mtext>}}$ increase from 18–20 to 195–230 and 170–185 nmol/min per g dry wt., respectively, after glucose and ethanol preincubation. The stimulation involves protein synthesis (it is suppressed by cycloheximide), has a half-time of 18 min and requires mitochondrial respiration (no or low effect in respiration-deficient mutants and those lacking ADP-ATP transport in mitochondria, as well as after anaerobic preincubation of the wild-type strain, and in low-phosphate cells). The presence of NH₄⁺ and some amino acids (e.g., leucine, aspartate, cysteine and methionine) depressed the stimulation while that of cationic amino acids (typically arginine and lysine) and of K⁺ increased it by 50–80%. The stimulated (i.e., newly synthesized) transport system was degraded with a half-life of about 10 min.     

Procedure for measurement of amino acid transport from blood to brain in small animals

The exponential plasma specific activity curve 2.5 to 12.5 min after injection (sc) of [¹⁴C]tyrosine was integrated and divided by time to obtain the mathematical relationship between the average equivalent specific activity $\text{S}$ and the measured specific activity S in any individual animal. $\text{S}$ is the constant, average value of S that is equivalent to the curvllinearly varying quantity that the body tissues are actually exposed to. Dividing the total brain radioactivity by $\text{S}$ gave the tissue Tyr uptake U. The function $\text{dU}\text{dt}$ is linear from 2.5 to 12.5 min and represents the rate of uptake of the amino acid. Incorporation into protein was similarly measured. Brain uptake of Tyr averaged 7.06, and the apparent protein incorporation was 1.99 nmol/g of brain per min. The γ-glutamyl cycle inhibitor l-methionine-RS-sulfoximine reduced total brain uptake of tyrosine by 42.8% and the apparent rate of protein incorporation by 39.0%.     

Amino acid uptake by yeasts: IV. Effect of thiol reagents on l-leucine transport in Saccharomyces cerevisiae

(1) $\text{N-}\text{Ethylmaleimide}$ (a penetrating SH- reagent) inactivated l-[¹⁴C]leucine entrance (binding and translocation) into Saccharomyces cerevisiae, the extent of inhibition depending on the time of preincubation with $\text{N-}\text{ethylmaleimide}$, $\text{N-}\text{ethylmaleimide}$ concentration, the amino acid external and internal concentration, and the energization state of the yeast cells. With d-glucose-energized yeast, $\text{N-}\text{ethylmaleimide}$ inhibited l-[¹⁴C]leucine entrance in all the assayed experimental conditions, but with starved yeast and low (0.1 mM) amino acid concentration, it did not inhibit l-[¹⁴C]leucine binding, except when the cells were preincubated with l-leucine. With the rho^? respiratory-deficient mutant (energized cells), $\text{N-}\text{ethylmaleimide}$ inhibited l[¹⁴C]leucine entrance as with the energized wild-type, though to a lesser extent. (2) Analysis of the $\text{N-}\text{ethylmaleimide}$ effect as a function of l-[¹⁴C]leucine concentration showed a significant decrease of $\text{J}{}_{\mathrm{<mtext>max</mtext>}}$ values of the high- (S₁) and low- (S₂) affinity amino acid transport systems, but $\text{K}{}_{\mathrm{<mtext>T</mtext>}}$ values were not significantly modified. (3) When assayed in the presence of d-glucose, $\text{N-}\text{ethylmaleimide}$ inhibition of d-glucose uptake and respiration contributed significantly to inactivation of l-[¹⁴C]leucine entrance. Pretreatment of yeast cells with 2,4-dinitrophenol enhanced the effect of l-[¹⁴C]leucine binding and translocation. (4) Bromoacetylsulfanilic acid and bromoacetylaminoisophthalic acid, two non-penetrating SH- reagents, did not inactivate l-[¹⁴C]leucine entrance, while $\text{p-}\text{chloromercuribenzoate}$, a slowly penetrating SH- reagent, inactivated it to a limited extent. When compared with the effect of $\text{N-}\text{ethylmaleimide}$, these negative results indicate that thiol groups of the l-[¹⁴C]leucine carrier were not exposed on the outer surface of the yeast cell permeability barrier.     

Influence of sterols on ion transport through lipid bilayer membranes

Charge-pulse relaxation experiments with the negatively charged lipophilic ions, dipicrylamine and tetraphenylborate, (as well as with the positively charged carrier system Rb⁺-valinomycin) have been carried out in order to study the influence of sterols on the ion transport through the lipid bilayer membrane. The mol fraction of the sterols (cholesterol, epicholesterol, ergosterol, stigmasterol, dihydrocholsterol, epicoprostanol and cholesterololeate) as referred to total lipid was varied in a wide range (mol fractions 0–0.8).The monoolein/sterol or dioleoylphosphatidylcholine/sterol mixtures were dissolved in $\text{n}$-hexadecane in order to minimize effects of the sterol on the membrane thickness.Cholesterol had a strong influence on the transport of the lipophilic ions. Its incorporation into monoolein membranes increased the rate constant ${}_{\mathrm{<mtext>i</mtext>}}$ of translocation up to 8-fold, but incorporation into phosphatidylcholine membranes had virtually no influence on $\text{k}{}_{\mathrm{<mtext>i</mtext>}}$. The other sterols with one hydroxy group and cholesterololeate had no influence on the rate constant or the partition coefficient β. The results are discussed on the basis of a possible change of dipole potential of the membrane caused by cholesterol and its derivatives.In the case of valinomycin-mediated Rb⁺ transport only cholesterol had a strong influence on transport properties. The rate constants of association ($\text{k}{}_{\mathrm{<mtext>R</mtext>}}$) as well as the rate constants of translocation of the complex ($\text{k}{}_{\mathrm{<mtext>MS</mtext>}}$) and of the free carrier ($\text{k}{}_{\mathrm{<mtext>S</mtext>}}$) were reduced by incorporation of cholesterol up to eight-fold. The decrease of $\text{k}{}_{\mathrm{<mtext>S</mtext>}}$ and $\text{k}{}_{\mathrm{<mtext>MS</mtext>}}$ are possibly caused by a decrease of membrane fluidity, whereas the decrease of $\text{k}{}_{\mathrm{<mtext>R</mtext>}}$ may be due to an increase of surface potential. The different action of cholesterol on the two transport systems is discussed under the assumption that the adsorption plane of the lipophilic ion is located more towards the aqueous side and that of the ion-carrier complexes more towards the hydrocarbon side of the dipole layer.     

A chloride requirement for Na+-dependent amino-acid transport by brush border membrane vesicles isolated from the intestine of a mediterranean teleost (Boops salpa)

The uptake of d-glucose, 2-aminoisobutyric acid and glycine was studied with intestinal brush border membrane vesicles of a marine herbivorous fish: Boops salpa. The uptake of these three substances is stimulated by an Na⁺ electrochemical gradient ($\text{C}{}_{\mathrm{<mtext>out</mtext>}}\text{C}{}_{\mathrm{<mtext>in</mtext>}}$). For glucose, an increase of the electrical membrane potential generated by a concentration gradient of the liposoluble anion, SCN^?, increases the Na⁺-dependent transport. This responsiveness to the membrane potential was confirmed by valinomycin. Differently from glucose, uptake of glycine and 2-aminoisobutyric acid requires, besides the Na⁺ gradient, the presence of Cl^? on the external side of the vesicles. In the absence of Cl^?, amino acid uptake is not stimulated by the Na⁺ gradient and is not influenced by an electrical membrane potential generated by SCN^? gradient ($\text{C}{}_{\mathrm{<mtext>out</mtext>}}\text{>C}{}_{\mathrm{<mtext>in</mtext>}}$) or by a K⁺ diffusion potential ($\text{C}{}_{\mathrm{<mtext>in</mtext>}}\text{>C}{}_{\mathrm{<mtext>out</mtext>}}$). This Cl^? requirement differs from the Na⁺ requirement, since a Cl^? gradient ($\text{C}{}_{\mathrm{<mtext>out</mtext>}}\text{>C}{}_{\mathrm{<mtext>in</mtext>}}$) does not result in an accumulation of glycine or 2-aminoisobutyric acid similar to that produced by an Na⁺ gradient.