The ability of acidic pH,growth inhibitors,and glucose to increase the proton motive force and energy spilling of amino acid-fermenting <Emphasis Type="Italic">Clostridium sporogenes</Emphasis> MD1 cultures

Clostridium sporogenes MD1 grew rapidly with peptides and amino acids as an energy source at pH 6.7. However, the proton motive force (p) was only –25 mV, and protonophores did not inhibit growth. When extracellular pH was decreased with HCl, the chemical gradient of protons (ZpH) and the electrical membrane potential () increased. The p was –125 mV at pH 4.7, even though growth was not observed. At pH 6.7, glucose addition did not cause an increase in growth rate, but increased to –70 mV. Protein synthesis inhibitors also significantly increased . Non-growing, arginine-energized cells had a of –80 mV at pH 6.7 or pH 4.7, but was not detected if the F₁F₀ ATPase was inhibited. Arginine-energized cells initiated growth if other amino acids were added at pH 6.7, and and ATP declined. At pH 4.7, ATP production remained high. However, growth could not be initiated, and neither nor the intracellular ATP concentration declined. Based on these results, it appears that C. sporogenes MD1 does not need a large p to grow, and p appears to serve as a mechanism of ATP dissipation or energy spilling.Mandatory disclaimer: Proprietary or brand names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product, and exclusion of others that may be suitable.     

Determination of the overall volumetric mass transfer coefficient kLa,by a temperature dependent change of gas solubility

Summary The solubility of oxygen in the liquid phase of a bioreactor was changed by a ramp change of temperature, and k_La was determined from the resulting return to equilibrium of dissolved oxygen activity. The maximum k_La that can be measured by this method in a standard laboratory scale bioreactor is 145 h^–1 corresponding to a temperature change rate of 320°C h^–1.Nomenclature p Difference between p_G and p_L (% saturation) - T Ramp change of temperature (°C) - E Temperature-compensated output from the oxygen electrode (A) - E_u Uncompensated output from the oxygen electrode (A) - k_La Overall volumetric mass transfer coefficient (h^–1) - k_La_Tm Overall volumetric mass transfer coefficient at temperature T_m (h^–1) - P_G Dissolved oxygen activity in equilibrium with the gas phase (% saturation) - p_L Dissolved oxygen activity (% saturation) - p_Lm Dissolved oxygen activity at time t_m (% saturation) - t Time (h) - t_m Time of maximum p (h) - T Temperature (°C) - T_cal Calibration temperature of the oxygen electrode (°C) - T_m Final temperature after a temperature shift (°C) - T_n Temperature at time t_n     

Sterols in seeds and leaves of oats (Avena sativa L.)

In seeds and leaves of oats (Avena sativa L.) 12 different sterols (cholesterol, cholstanol, ⁷-cholestenol, campesterol, campestanol, stigmasterol, lophenol, sitosterol, stigmastanol, ⁵-avenasterol, ⁷-avenasterol and ⁷-stigmastenol) have been identified. The sterol pattern is qualitatively the same, but the relative composition is different in leaves and in seeds. Leaves contain mainly sitosterol, stigmasterol, cholesterol and campesterol, but only minor portions of avenasterols. Seeds contain sitosterol, ⁵- and ⁷-avenasterol, campesterol, but only minor amounts of stigmasterol and cholesterol. In leaf lipids 1-hexacosanol (2.35 wt % of total lipid) has also been identified.     

Foam behavior of biological media

Summary The influence of the alcohol concentration on the foaminess, , of-BSA-solutions is considered. This effect is calculated by means of the function (C_BSA . f), where f=1 for pure protein solutions and f>1 for alcohol solutions. f is calculated by f = 2^TT_eff. Here, where T_T is the turbidity temperature change due to solvent structure effects and T_D, the temperature correction due to alcohol-protein interaction. The constants necessary to calculate T_T and T_D are tabulated. The agreement between the calculated and measured foaminess , as a function of the n-propanol concentration is satisfactory and for methanol or ethanol excellent.     

Lactate conversion to acetate,CO2 and H2 in cell suspensions of Desulfovibrio vulgaris (Marburg): indications for the involvement of an energy driven reaction

Cell suspensions of Desulfovibrio vulgaris were found to catalyze, in the absence of sulfate, the complete conversion of 1 lactate to 1 acetate, 1 CO₂, and 2 H₂ (G₀=-8.8 kJ/mol) and of 1 pyruvate to 1 acetate, 1 CO₂, and 1 H₂ (G₀=-52 kJ/mol). Protonophores, the proton translocating ATPase inhibitor N,N-dicyclohexylcarbodiimide, and arsenate specifically inhibited H₂ formation from lactate but not from pyruvate. The results suggest that lactate oxidation to pyruvate and H₂ (G ₀=+43.2 kJ/mol) is energy driven.     

Uptake of methylamine via an inducible,energy-dependent transport system in the facultative methylotroph Arthrobacter P1

Cytoplasmic membrane vesicles were prepared by a lysozyme-salt treatment from Arthrobacter P1 grown on methylamine as the carbon and energy source. In the presence of an ascorbate-phenazine methosulphate electron donor system, these vesicles accumulated methylamine in unmodified form by an inducible transport system. This system has a high affinity for methylamine (K_app=20–25 M). The effect of the ionophores valinomycin and nigericin combined with membrane potential () and pH-gradient (pH) measurements demonstrated that methylamine uptake is electrogenic and driven by the . Optimal activity is observed at pH 6.5 and 30°C. Methylamine uptake was not affected by the presence of ammonium ions but was inhibited by the primary amines ethylamine (competitively), propylamine, butylamine and benzylamine. In addition, formaldehyde and acetate, at a concentration of 1 mM, inhibited methylamine uptake almost completely. These compounds were shown to be non-competitive inhibitors. A strong inhibition observed in the presence of plumbagin could be relieved by addition of dithiothreitol. This indicates that the oxidation-reduction state of, probably, carrier dithiol-disulfide-groups is an important factor in methylamine translocation in Arthrobacter P1.     

Theoretical stability diagram of solvent-containing black lipid films

Recently, we have developed an analytical, semi-microscopic theory for the macroscopic behavior of a solvent-containing black lipid film subjected to an electric cross film voltage, . Here we employ the theoretical expressions derived for the disjoining pressure, _D, the film elasticity, _F, and the film tension, _F, to construct the stability diagram of the film, in the _D-. Depending on its state (_D, ), the film is stable or is prone to squeezing or bending deformations. For a monooleate film we show how the destruction of the plane film due to a periodic thickness fluctuation (squeezing) is facilitated by two mechanisms: i) lowering of _D at fixed ; ii) lowering of at fixed _D, provided that the film is in a stable state characterized by _D<–7.03×10³ dyne/cm² and >0 mV. Bending of a low tension film (single interface tension _s 0.025 dyne/cm¹) can be achieved only for >170 mV and _D > –8.7 × 10⁴ dyne/cm². Finally, we demonstrate the existence of a marginal state ( _D ⁰ , ⁰) where the film is predicted to exhibit strong fluctuations both in the squeezing and in the bending mode.     

Preparation of a series of sulfated tetrasaccharides from shark cartilage chondroitin sulfate D using testicular hyaluronidase and structure determination by 500 MHz1H NMR spectroscopy

Six tetrasaccharide fractions were isolated from shark cartilage chondroitin sulfate D by gel filtration chromatography followed by HPLC on an amine-bound silica column after exhaustive digestion with testicular hyaluronidase. Their structures were determined unambiguously by one- and two-dimensional 500 MHz¹H NMR spectroscopy in conjunction with HPLC analysis of chondroitinase AC-II digests of the tetrasaccharides. One fraction was found to contain two tetrasaccharide components. All the seven tetrasaccharides shared the common core structure GlcA1-3GalNAc1-4GlcA1-3GalNAc with various sulfation profiles. Four were disulfated comprising of two monosulfated disaccharide units GlcA1-3GalNAc(4-sulfate) and/or GlcA1-3GalNAc(6-sulfate), whereas the other three were hitherto unreported trisulfated tetrasaccharides containing a disulfated disaccharide unit GlcA(2-sulfate)1-3GalNAc(6-sulfate) and a monosulfated disaccharide unit GlcA1-3GalNAc(4-or 6-sulfate). These sulfated tetrasaccharides were demonstrated to serve as appropriate acceptor substrates for serum -N-acetylgalactosaminyltransferase, indicating their usefulness as authentic oligosaccharide substrates or probes for the glycobiology of sulfated glycosaminoglycans.Abbreviations NFU National formulary unit - COSY correlation spectroscopy - HOHAHA homonuclear Hartmann-Hahn - 1D or 2D one- or two-dimensional - IdoA l-iduronic acid - GlcA d-gluco-4-enepyranosyluronic acid - Di-0S GlcA1-3GalNAc - Di-4S GlcA1-3GalNAc(4-sulfate) - Di-4S GlcA1-3GalNAc(4-sulfate) - Di-6S GlcA1-3GalNAc(6-sulfate) - Di-6S GlcA1-3GalNAc(6-sulfate) - Di-diS _d GlcA(2-sulfate)1-3GalNAc(6-sulfate) - Di-diS_E GlcA1-3GalNAc(4, 6-disulfate) - U G, U, 2S, 4S, and 6S represent GlcA, GalNAc, GlcA, 2-O-sulfate, 4-O-sulfate, and 6-O-sulfate, respectively     

Effect of L-triiodothyronine on liver microsomal Δ6 and Δ5 desaturase activity of male rats

The effect of different doses of L-triiodothyronine (T₃) on the activity of 6 and 5 desaturases and lipid fatty acid composition was studied in liver microsomes of male rats. The activity of 6 and 5 desaturases was decreased 24 and 28%, respectively, in animals administered a daily intraperitoneal dose of 1000g T₃/100g body wt. for 5 days, whereas with 500g T₃/100g body wt. only 6 desaturase activity was decreased. On the other hand, no enzyme activity changed at a shorter period of hormone treatment. Changes in microsomal fatty acid composition did not seem to be a direct consequence of desaturation activity, since after 1 and 5 days of T₃ treatment, the concentrations of 18:2 (n-6) and 20:3 (n-6) decreased and only after 1 day that of 20:4 (n-6) increased in spite of unchanged or decreased 6 and 5 desaturase activities. Other factors than desaturation activity must be involved in fatty acid composition of thyroid hormonetreated rats, such as diet, membrane lipid synthesis and degradation, fatty acid turn-over and oxidation. (Mol Cell Biochem121: 149–153, 1993)     

Optomotorische Untersuchung des visuellen systems einiger Augenmutanten der Fruchtfliege Drosophila

Summary The optical properties of the compound eye of Drosophila have been analysed using the optomotor reactions of flies with normal and mutant eye pigmentation. The stimulus was provided by cylindrical patterns with different periodic intensity distributions rotating at different speeds. The response consists of a torque about the vertical axis and was recorded under conditions of fixed flight. (Maximum reaction is about 0.04 dyn · cm). The transfer characteristics of the optical system are determined by the inter-ommatidial angle , influencing the resolving power and by the width of the visual field of single ommatidia , influencing the response at high spatial frequencies. The values = 4.6° and = 3.5° are obtained from stimulus-response experiments with Drosophila. They are independent of the presence of screening pigments. Differences in the response of flies with strong (+, se), weak (w ^a), and missing (w) pigmentation can be explained by the increased amount of scattered light in the pigment-deficient eyes. The overall intensities in the equally illuminated receptors are expected to be in the ratio 11825, respectively. The perception of motion depends only on the temporal, not on the spatial phase relations between periodic intensity variations in neighbouring ommatidia. Therefore the inhomogeneous distribution of the inter-ommatidial angle changes the resolving power in different parts of the eye without changing the response to motion. Different simultaneous stimuli of equal strength in different parts of the eye are averaged in the perceptive system of Drosophila according to the number of ommatidia in these parts.     

Effects of cannabinoids on the activities of mouse brain lipases

Cannabinoids were found to augment phospholipase activities and modify lipid levels of mouse brain synaptosomes, myelin and mitochondria. Delta-1-tetrahydrocannabinol (¹-THC) and several of its metabolites induced a dose-dependent (0.32–16 M) stimulation of phospholipase A₂ (PLA₂) activity resulting in the increased release of free arachidonic acid from exogenous [1-¹⁴C]phosphatidylcholine (PC). The potencies of the cannabinoids in modulating PLA₂ activity were approximately of the order: 7-OH-¹-THC > ¹-THC > 7-oxo-¹-THC > ¹-THC-7oic acid = 6 OH-¹-THC 6-OH-¹-THC. The hydrolysis of phosphatidylinositol (PI) by synaptosomal phospholipase C (PLC) was enhanced significantly by ¹-THC and promoted diacylglyceride levels by greater than 100 percent compared to control values. In contrast, arachidonate was the major product resulting from phospholipase activities of a 20,000g pellet. Synaptosomal diacylglyceride lipase activity was inhibited by ¹-THC. [1-¹⁴C]Arachidonic acid was readily incorporated into subcellular membrane phospholipids and after exposure to cannabinoids led to diminished phosphoglyceride levels and concomitant increases in released neutral lipid products. These data suggest that cannabinoids control phospholipid turnover and metabolism in mouse brain preparations by the activation of phospholipases and, through this mechanism, may exert some of their effects.     

Effect of abscisic acid on membrane potential and transport of glucose and glycine in Lemna gibba G1

The membrane potential of Lemna gibba G1 was measured with a microelectrode; glucose and glycine uptake were measured with ¹⁴C-labeled substances. The membrane potential was increased by 85 mV on the average, after the plants had been pretreated with 10 M abscisic acid (ABA) for more than 30 min. This effect is not linked to the endogenous level of soluble sugars. The concentration of these soluble sugars was increased to more than 200% by pretreatment of the plants with ABA, however, the respiration of the plants was not affected. ABA stimulated uptake of glucose and glycine. Glucose- and glycine-dependent depolarization and repolarization of the membrane was altered: depolarization was less and repolarization was slower; during uptake of glycine, the first typical phase of repolarization was suppressed. The data suggest that ABA interferes with the primary steps of substrate uptake.Abbreviations ABA abscisic acid - FW fresh weight - IAA indole acetic acid - pd membrane potential difference - 1× perfusing solution (see methods) - H⁺ electrochemical proton gradient - pd solute-induced maximum depolarization of the membrane     

Protonmotive force in Brevibacterium flavum: the lack of intracellular pH homeostasis

Brevibacterium flavum 22LD-P cells were shown to maintain a transmembrane pH gradient (pH) from 0.6 to 1.8–2 units and a transmembrane electric potential difference () from 0 to 200 mV depending on the pH and ionic composition of the incubation medium, grwoth substrate and concentration of cells. decreased from 120–140 mV to 0 when medium pH was lowered from neutral to 5.0–5.5 and increased to 180–200 mV when medium pH was raised to 8–9 in cells utilizing acetate or endogenous substrate. Cells growing on sucrose, kept around 100–120 mV at neutral as well as acidic medium pH. Intracellular pH in the acetate utilizing or endogenously respiring cells was maintained with the range of 8.9 to 5.5 at medium pH ranging from 9.1 to 4.0, respectively. Sucrose grown cells were able to maintain a more stable intracellular pH. Endogenously respiring cells in potassium phosphate buffer at high biomass concentrations maintained larger pH and relatively smaller , than the same cells in diluted suspensions. Cells in sodium phosphate buffer possessed larger and almost no pH, but was still dependent on biomass concentration.The lack of intracellular pH homeostasis and the collapse of at acid medium pH are discussed in the context of cell membrane proton permeability.     

Deletion ΔF508 and haplotype analysis of CFTR gene region in Slovak CF patients

Summary Analysis of a sample of 50 unrelated cystic fibrosis (CF) patients and 46 nuclear families from Slovakia (Czechoslovakia) by the polymerase chain reaction and Southern hybridization revealed that the proportion of the F508 mutation was 58% in this population, and that the frequency of the B (i.e., KM19/XV2c [1–2]) haplotype was increased in both F508 and nonF508 CF chromosomes (98% and 46%, respectively). These results support the view that the trans-European gradient of the F508 frequency is of a geographical rather than of an ethnic origin, and that in Slavonic populations, there exists an as yet unidentified but frequent CF mutation other than F508, associated with the B haplotype.     

Subcellular heterogeneity of mitochondrial membrane potential in anther tapetum

Studies on animal material have revealed that changes in the mitochondrial permeability transition pore (PTP), which cause a reduction in the mitochondrial transmembrane potential (_m) followed by release of cytochrome c, belong to the earliest manifestations of some types of apoptosis. We have attempted to monitor the _m of mitochondria during programmed cell death (PCD) of the secretory tapetum using JC-1, a fluorochrome dye that detects mitochondrial membrane potential and to relate changes in this potential to mitochondrial ultrastructure. Analysis of tapetal cells isolated from Ornithogalum virens anthers revealed that the _m of mitochondria in the tapetal cells alters during development; the change, however, is not uniform in the mitochondrial population within a single tapetal cell. In young tapetal cells, at the tetrad stage, we detected only the red fluorescence of JC-1 aggregates in all tapetal mitochondria, which indicates highly negative _m. In an advanced stage of PCD at the late microspore stage, in each tapetal cell we detected both mitochondria with red (as formerly) and mitochondria with green fluorescence. The green fluorescence of JC-1 monomers indicates mitochondria with depolarised membranes. These changes in _m are related to observed changes in mitochondria ultrastructure. This is the first documentation of intracellular heterogeneity of _m during anther tapetum development. Alteration in _m suggests a relationship between mitochondrial function and PCD processes in tapetal cells.     

Transmembrane signal defect and absence of cancer extract induced leukocyte adherence inhibition (LAI) for leukocytes from patients with advanced cancer

Summary Leukocytes from patients with early cancer exhibit leukocyte adherence inhibition (LAI) when incubated with extracts of cancer of the same organ and histogenesis, whereas leukocytes from patients with advanced cancer seldom do. To understand the reason for this refractory state, tumor antigen-induced LAI and transmembrane signalling were measured in the same leukocytes. Transmembrane signalling was measured by changes in membrane potential () by the [³H]tetraphenylphosphonium equilibration technique. When leukocytes from patients with early breast cancer were incubated with extracts of breast cancer and malignant melanoma they showed changes consisting of depolarization and hyperpolarization beginning within 0.5 min after addition of the breast cancer extract and finishing 15 min later. Moreover, they showed no changes when incubated with extracts of normal breast tissue. Leukocytes from subjects without cancer seldom showed changes. In criss-cross experiments, leukocytes from patients with melanoma only exhibited changes when incubated with the melanoma extract. There was a strong correlation between cancer extract-induced change and LAI. The change was triggered by leukotriene-like mediators from antibody-dependent monocytes. Authentic leukotrienes triggered changes in all subpopulation of leukocytes. Leukocytes from patients with advanced breast cancer when incubated with breast cancer extract did not transmit a signal or show LAI. Brief elevation of intracellular cyclic AMP restored both change and LAI induced by breast cancer extracts, indicating that reactive leukocytes are present but in a refractory state. We conclude that leukocytes from patients with advanced cancer do not react in LAI because tumor antigen does not trigger a transmembrane signal to initiate the cascade of biochemical reactions and physiological changes for LAI.Abbreviations CCCP carbonyl cyanide m-chlorophenylhydrazone - cyclic AMP cyclic adenosine monophosphate - ETYA eicosatetraynoic acid - HEPES 4-(2-hydroxyethyl)-1-piperazine ethansulfonate - LAI leukocyte adherence inhibition - NAI nonadherence index - OSN organ-specific cancer neoantigen - PBL peripheral blood leukocytes - PGE₂ prostaglandin E₂ - [³H]TPP⁺ [phenyl³H]tetraphenylphosphonium bromide - transmembrane potential     

Cytoplasmic gel and water relations of axon

Summary A previous method of measuring the swelling pressure ( _g ) of the cytoplasmic gel of the giant axon ofLoligo vulgaris was refined. The estimates of _g made with the improved method were consistent with those made with the earlier method. In these methods the activity of the solvent in the gel is measured by increasing the activity of the solvent in the internal phase of the gel by application of hydrostatic pressure to the gel directly. Comparable values for the activity of the solvent in the gel were obtained also by an alternate method, in which the deswelling of the gel is measured upon decreasing the activity of the solvent in the external phase by addition of a nonpenetrating high mol wt polymer (i.e., Ficoll).Additional support was obtained for the earlier suggestion that _g contributes to the swelling and shrinkage pattern of the whole axon. In part, the new evidence involved two consecutivedirect measurements of intraxonal pressure. The first measurement was that of a mixed pressure composed of _g and _m ( _m being the effective osmotic pressure due to the intra-extraxonal gradient in the activity of mobile solutes). The subsequent measurement was that of _g alone. The latter measurement was made feasible by destroying the axolemma, thereby eliminating the contribution of _m . An estimate of _m was obtained by subtracting _g from the total pressure measured initially. The _m determined by the above method was two orders of magnitude smaller than the theoretical osmotic pressure. This is consistent with the _m determined previously, where osmotic intra-extraxonal filtration coefficients were compared to the hydrostatic. The mixed pressure experiments lend credence to the idea that the substantial contribution of _g to the water relations of the whole axon is due to _g being of the same order of magnitude as _m .The degree of free swelling of axoplasmic gels was studied as a function of pH, salt concentration, and hydration radius of the anion of the salt used. The swelling increased with an increase in the reciprocal of the hydration radius, a decrease in salt concentration, and at pH below or above 4.5.The nature of the constraints to the free swelling of axoplasm in axons immersed in seawater was studied. With the seawater employed, these constraints appeared to be due more to the retractive forces of the sheath than to _m .     

Phytochrome action in Oryza sativa L.

Summary In-vivo phytochrome determinations in totally etiolated rice seedlings with a dual-wavelength spectrophotometer showed that on a fresh weight basis phytochrome concentration was highest in the coleoptile apex (0.175 of mean) ( O.D.) g^-1 (fresh weight). The age of the seedlings had little effect on the pattern of phytochrome distribution in the coleoptiles.The extent of growth inhibition observed 2 days after the irradiations was proportional to the logarithm of P _fr amount in the coleoptiles at the time of initial exposure to either red or blue light. Ultraviolet irradiation, however, did not induce either reversible growth inhibition or optically detectable phytochrome changes in vivo.After the conversion of P _r to P _fr bya brief red irradiation, non-photochemical transformation of phytochrome was observed in intact coleoptile tissues. Most of the optically measurable P _fr disappeared within 6 hours at 27°, when the total ( O.D.) decreased to about one fifth of the original level. The optical data did not agree with the fact that 50% of the initial physiological reversibility was still observed 9 hours later. No significant difference in dark transformation rate was seen between intact and excised coleoptile tissues.Abbreviations P _r red light absorbing form of phytochrome - P _fr far-red light absorbing form of phytochrome - ( O.D.) the change in the optical density difference reading at two wavelengths, following irradiation of the sample with actinic sources of red and far-red light - UV ultraviolet light     

Stereospecificity and electrogenicity of amino acid transport in Riccia fluitans

In the aquatic liverwort Riccia fluitans, membrane depolarization (_m), change in membrane conductance (g_m), and current-voltage (I-V) characteristics in the presence of different amino acids as well as the uptake of ¹⁴C-labeled amino acids were measured. L-isomers of the tested amino acids generate larger electrical effects (_m, g_m) than D-isomers, and the I-V characteristics show that the positive electrical inward-current of 20 mA m^-2 generated by 0.5 mM D-serine is only about 50% of the current generated by adding 0.5 mM L-serine. Whereas - and -amino acids rapidly depolarize the membrane to the same extend, with -aminobutyric acid (-AB) and dipeptides no significant electrical effects have been measured. The uptake kinetics of ¹⁴C-labeled amino acids display three components: (I) A saturable high-affinity component with K_s-values of 48 M D-alanine, 12 M -aminoisobutyric acid (AIB), 9 M L-alanine, 8 M L-proline, and 6 M L-serine, respectively; (2) an apparently linear low-affinity component, and (3) an also linear but unspecific component at concentrations >20 times the given K_s-value. Uptake of ¹⁴C-labeled AIB can be inhibited competitively by all tested neutral amino acids, the L-isomers being more effective than the D-isomers, as well as by ammonium or methylamine. Vice versa, AIB competitively inhibits uptake of L-serine and L-alanine. It is concluded that an uncharged stereospecific carrier for the investigated amino acids exists in the plasmalemma of Riccia fluitans. Accumulation ratios of about 50 suggest secondary active transport driven by a transmembrane electro-chemical gradient (mainly _m) which is generated by the electrogenic proton pump. It is suggested that this carrier binds to the amino group forming either a charged binary complex with positively charged amines (Felle 1980), or an uncharged complex with -AB or dipeptides, whereas electrogenic transport of - and -amino acids is mediated by a ternary carrier complex, probably charged by a proton.Symbols and Abbreviations _m membrane potential (mV) - E_co equilibrium potential (mV) of the transport system - g_m membrane (slope) conductance (Sm^-2) - g_m change in g_m - I-V curve current-voltage curve - AIB -aminoisobutytric acid - -AB -aminobutyric acid