Influence of Some Ions on the Membrane Potential of Ascaris Muscle

The influence of several ions on the membrane potential of the somatic muscle of Ascaris has been investigated by changing their concentration in the surrounding solution. When [K]_o is increased at the expense of [Na]_o leaving [Cl]_o constant, the membrane potential is first seen to increase. [K]_o higher than 45 mM reduces the membrane potential with a slope of 23 mv for a tenfold change in [K]_o. However, when [K]_o is increased keeping [Na]_o and [Cl]_o low and constant, the line relating the membrane potential with log [K]_o has a slope of almost 50 mv. If [Cl]_o is reduced in the absence of external Na, after the [K]_o is increased to 45 mM, the membrane potential decreases with a slope of 59 mv per tenfold change in [Cl]_o in close agreement with the Nernst equation. If Cl^- is replaced by SO₄ ^2-, a depolarization is produced, while chloride replacement by NO₃ ^-, Br^-, and I^- results in a hyperpolarization of the membrane. Removal of the external Na⁺ ions increases the average membrane potential by 17 mv.     

Distribution of potassium and chloride permeability over the surface and T-tubule membranes of mammalian skeletal muscle

Summary The distribution of K and Cl permeability,P _K andP _Cl, over the surface and T-tubule membranes of red rat sternomastoid fibers has been determined. Membrane potential,V _m, was recorded with 3-m KCl-filled glass microelectrodes. Changes inV _m with changes in [K]_o or [Cl]_o were used to estimateP _Cl/P _K in normal and detubulated preparations. The results show that the T-tubule membrane has a highP _Cl and is therefore different from the T-tubule membrane of amphibian fibers. Analysis of the time course of depolarization when [K]_o was raised (in SO₄ solutions) showed thatP _K was distributed over the surface and T-tubule membranes. Two observations suggested that T-tubuleP _Cl was higher than the surfaceP _Cl. Firstly, in normal fibers, the depolarization caused by an increase in [K]_o was 3.5 times greater in SO₄ solutions than in Cl solutions. In marked contrast, the depolarization in glycerol-treated fibers was independent of [Cl]_o. Secondly, the rapid change inV _m when [Cl]_o was changed was reduced by 80% after glycerol treatment. Both observations suggest thatP _Cl was low in glycerol-treated fibers.P _Cl/P _K was calculated from theV _m data using Goldman, Hodgkin and Katz equations for Na and K or for Na, K, and Cl. In normal fibersP _Cl/P _K=4.5 and in glycerol-treated fibersP _Cl/P _K=0.28. Since it is unlikely that glycerol treatment would increaseP _K, the reduction in the ratio must follow the loss of Cl permeability channels in the T-tubule membrane.     

Kinetic comparison of ouabain-resistant K:CI fluxes (K:Cl [Co]-transport) stimulated in sheep erythrocytes by membrane thiol oxidation and alkylation

Summary The stimulatory effects of two thiol (SH) group oxidants, methylmethane thiosulfonate (MMTS) and diazene dicarboxylic acid bis [N,N-dimethylamide] (diamide), on the kinetics of ouabain-resistant (OR) K:Cl [co]-transport in low K (LK) sheep red blood cells were compared with the effects of alkylating agents, notably N-ethylmaleimide (NEM). At low concentrations, both MMTS and diamide stimulated K:CI [co]-transportv and with a latency period, as measured by OR zero-trans K efflux and OR uptake of external Rb, Rb_o, as K congener in Cl and NO₃ media. At high concentrations the effect of diamide saturated, and that of MMTS disappeared. The stimulatory effect of MMTS was partially reversed by the reducing agent dithiothreitol (DTT) known to fully restore the diamide-activated K flux (Lauf, J. Memb. Biol. 101:179–188, 1988). In diamide pre-equilibrated LK sheep red cells, the K_m of K:Cl [co]-transport for external Cl, Cl_o, was 84.3 mM, and 18.7 mM for Rb_o, with nearly identical V_max values around 4 mmol Rb/L cells × h for K (Rb) fluxes in Cl and after correction for the small Cl-independent component. Zero net K (Rb) flux existed at K_c (cell K)/Rb_o concentration ratios, [K]_c/[Rb]_c, of 0.8 i.e. when the electrochemical driving forces across the membrane were about equal. The measured K efflux/Rb influx ratios were almost twice those predicted from [K]_c/[Rb]_o and the Cl equilibrium potential suggesting that the diamide-stimulated K (Rb) flux may occur through non-diffusional, carrier-mediated transport. The effects of NEM and of A23187 plus/minus Ca or chelators on K: [co]Cl-transport (Lauf, Am. J. Physiol. 249:C271–278, 1985) consisted primarily of V_max changes. Thus, all chemical interventions resulted in an increase of the number of actively transporting K:Cl [co]-transport units or an augmented turnover number per existing site.     

Non-Linear Current-Potential Relations in an Axon Membrane

The membrane current density, I_m, in the squid giant axon has been calculated from the measured external current applied to the axon, I_o, by the equation See PDF for Equation where V_m is the membrane potential under the current electrode and r₁ and r₂ are the external and internal longitudinal resistances. The original derivation of this equation included in one step an assumption of a linear relation between I_m and V_m. It is shown that the same equation can be obtained without this restricting assumption.     

Nature of the Schwann cell electrical potential. Effects of the external ionic concentrations and a cardiac glycoside

The effects on the Schwann cell electrical potential of external ionic concentrations and of K-strophanthoside were investigated. Increasing (K)_o depolarized the cell. The potential is related to the logarithm of (K)_o in a quasi-linear fashion. The linear portion of the curve has a slope of 45 mv/ten-fold change in (K)_o. Diminutions of (Na)_o and (Cl)_o produced only small variations in the potential. Calcium and magnesium can be replaced by 44 mM calcium without altering the potential. Increase of (Ca)_o to 88 mM produced about 10 mv hyperpolarization. The cell was hyperpolarized by 11 mv and 4 mv within 1 min after applying K-strophanthoside at concentrations of 10^-3 and 10^-5 M, respectively. No variations of cellular potassium, sodium, or chloride were observed 3 min after applying the glycoside. The hyperpolarization caused by 10^-3 M K-strophanthoside was not observed when (K)_o was diminished to 1 or 0.1 mM or was increased to 30 mM. At a (K)_o of 30 mM, 10^-2 M strophanthoside was required to produce the hyperpolarizing effect. In high calcium, the cell was further hyperpolarized by the glycoside. The initial hyperpolarization caused by the glycoside was followed by a gradual depolarization and a decrease of the cellular potassium concentration. The results indicate that the Schwann cell potential of about -40 mv is due to ionic diffusion, mainly of potassium, and to a cardiac glycoside-sensitive ion transport process.     

Simultaneous Measurements of Cytoplasmic K Concentration and the Plasma Membrane Electrical Parameters in Single Membrane Samples of Chara corallina

The electrophysiological properties of cytoplasm-rich fragments (single membrane samples) prepared from internodal cells of Chara corallina were explored in conjunction with K⁺-sensitive microelectrode and current-voltage (I-V) measurements. This system eliminated the problem of the inaccessible cytoplasmic layer, while preserving many of the electrical characteristics of the intact cells. In 0.1 millimolar external K concentration (K_o⁺), the resting conductance (membrane conductance G_m, 0.85 ± 0.25 Siemens per square meter (±standard error)) of the single membrane samples, was dominated by the proton pump, as suggested by the response of the near-linear I-V characteristic to changes in external pH. Initial cytoplasmic K⁺ activities (a_K+), judged most reliable, gave values of 117 ± 67 millimolar; stable a_K+ values were 77 ± 31 millimolar. Equilibrium potentials for K⁺ (Nernst equilibrium potential) (E_K) calculated, using either of these data sets, were near the mean membrane potential (V_m). On a cell-to-cell basis, however, E_K was generally negative of the V_m, despite an electrogenic contribution from the Chara proton pump. When K_o⁺ was increased to 1.0 millimolar or above, G_m rose (by 8- to 10-fold in 10 millimolar K_o⁺), the steady state I-V characteristics showed a region of negative slope conductance, and V_m followed E_K. These results confirm previous studies which implicated a K_o⁺-induced and voltage-dependent permeability to K⁺ at the Chara plasma membrane. They provide an explanation for transitions between apparent K_o⁺-insensitive and K_o⁺-sensitive (`K⁺ electrode') behavior displayed by the membrane potential, as recorded in many algae and higher plant cells.     

Cat Heart Muscle in Vitro : VI. Potassium exchange in papillary muscles

The exchange of cell K with K⁴², J _K, has been measured in cat right ventricular papillary muscle under conditions of a steady state with respect to intracellular K concentration. Within the limits of the measurement, all of cell K exchanged at a single rate. Cells from small cats are smaller and have larger surface/volume ratios than cells from large cats. The larger surface/volume ratio results in larger flux values. J _K increases in an approximately linear manner as the external K concentration is increased twentyfold, from 2.5 to 50 mM, at constant intracellular K concentration. The permeability for K ions, P _K, calculated from the influx and membrane potential, remains very nearly constant over this range of external K concentrations. J _K is not affected by replacement of O₂ by N₂, or by stimulated contractions at 60 per minute, but K influx decreases markedly in 10^-5 M and 10^-8 M ouabain.     

Quinine and quinidine inhibit and reveal heterogeneity of K-Cl cotransport in low K sheep erythrocytes

Low K (LK) sheep red blood cells (SRBCs) serve as a model to study K-Cl cotransport which plays an important role in cellular dehydration in human erythrocytes homozygous for hemoglobin S. Cinchona bark derivatives, such as quinine (Q) and quinidine (QD), are effectively used in the treatment of malaria. In the present study, we investigated in LK SRBCs, the effect of various concentrations of Q and QD on Cl-dependent K efflux and Rb influx (K(Rb)-Cl flux), activated by either swelling in hyposmotic media, thiol alkylation with N-ethylmaleimide (NEM), or by cellular Mg (Mg _i ) removal through A23187 in the presence of external chelators. K efflux or Rb influx were determined in Cl and NO₃ medium and K(Rb)-Cl flux was defined as the Cl-dependent (Cl minus NO₃) component. K(Rb)-Cl flux stimulated by all three interventions was inhibited by both Q and QD in a dose-dependent manner. Maximum inhibition of K(Rb)-Cl flux occurred at Q and QD concentrations ?1 mm. The inhibitory effect of Q was manifested in Cl, but not in NO₃, whereas QD reduced K and Rb fluxes both in Cl and NO₃ media. The mean 50% inhibitory concentration (IC⁵⁰) of Q and QD to inhibit K(Rb)-Cl flux varied between 0.23 and 2.24 mm. From determinations of the percentages of inhibition of the different components of K and Rb fluxes, we found that SRBCs possess a Cl-dependent QD-sensitive and a Cl-dependent QD-insensitive K efflux and Rb influx. These two components vary in magnitude depending on the manipulation and directional flux, but in average they are about 50% of the total Cl-dependent flux. This study raises the possibility that, in SRBCs, the Cl-dependent K(Rb) fluxes are heterogeneous. This work was supported by a grant from the National Institutes of Health (NIH DK5RO1 37,160).     

Ionic processes underlying the decrease in osmotic potential of Chara L-cell fragments in dilute electrolyte solutions

Movements of ions are considered to be governed by the electroneutrality rule. Therefore, a cation moving across the cell membrane into the cell either passively or actively should move together with its counterion, an anion, in equal amounts of charge or in exchange for another cation inside the cell. This means that the net influx of the cation in question should be affected by the permeability of its counterion and/or another cation inside the cell. To examine osmotic and ionic regulation in Chara cells, cell fragments of Chara having a lower osmotic pressure than normal (L-cell fragments) were prepared. The L-cell fragments were individually put into various dilute electrolyte solutions and their osmotic potentials were measured with a turgor balance. Concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^?, NO^?₃. and SO^2?₄. in the external electrolyte solutions in which L-cells had been incubated were also analysed by ion chromatography. The results showed that in 0.5 mM KCL + 0.1 mM CaCl₂ solution, Chara L-cell fragments absorbed K⁺ and Cl^? to maintain electroneutrality and then regained their osmotic potential very rapidly. When the anion was Cl, the cation absorbed at the highest rate was K⁺ On the other hand, when the cation was K, the anion absorbed at the highest rate was Cl, Other ions Ca²⁺, SO^2?₄ and NO^?₃ showed much less permeability than K⁺ and Cl ^?for the Chara plasma membrane. The conclusion from these findings was that due to the constraint of electroneutral transport, the uptake rate of a salt into L-cells is limited by the permeability of the least permeable ion.     

Cat Heart Muscle in Vitro : VIII. Active transport of sodium in papillary muscles

The cells of cat right ventricular papillary muscles were depleted of K and caused to accumulate Na and water by preincubation at 2–3°C. The time courses of changes in cellular ion content and volume and of the resting membrane potential (V_m) were then followed after abrupt rewarming to 27–28°C. At physiological external K concentration ([K]_o = 5.32 mM) recovery of cellular ion and water contents was complete within 30 minutes, the maximal observable rates of K uptake and Na extrusion (Δmmol cell ion/(kg dry weight) (min.)) being 3.4 and 3.6, respectively. The recovery rate was markedly slowed at [K]_o = 1.0 mM. Rewarming caused V_m measured in cells at the muscle surface to recover within from <1 to 9 minutes, but only slight restoration of cellular ion contents (measured in whole muscles) had occurred after 10 minutes. Studies of recovery in NaCl-free sucrose Ringer''s solution made it possible to separate the ouabain-insensitive outward diffusion of Na as a salt from a simultaneous ouabain-sensitive Na extrusion which is associated with a net cellular K uptake. A hypothesis consistent with these observations is that rewarming may activate a ouabain-sensitive "electrogenic" mechanism, most probably the net active transport of Na out of the cell, from which net K uptake may then follow passively.     

Crystal structure and magnetic measurements of [Cr(en)3] [ZnCl4]Cl

The compound [Cr(en)₃][ZnCl₄]Cl has been synthesized by reaction of CrCl₃·6H₂O, Zn and [Cr(en)₃]₂(SO₄)₃ in HCl. Its molecular and crystalline structure was determined by X-ray diffraction methods, being monoclinic, P2₁/c, a=21.215(3), b=12.532(2), c=13.707(2) Å, β=95.21°, V= 3629(2) ų, D_x=1.738g cm⁻³, MW=474.9, Z= 8, F(000)=1928, λ(Mo Kα)=0.71069 Å, μ(Mo Kα)= 27.04 cm⁻¹, 288 K. No significant exchange interactions between Cr(III) cations in the crystalline lattice were found. Curie-Weiss behavior was found in the three directions tested (g₁=2.06±0.02,g₂= 2.08±0.02,g₃=2.09±0.01), T=1.2-1.4 K.     

The chemistry of surface water in prairie ponds

Water analyses from 80 small prairie ponds, 0.17 to 89.8 hectares, in Manitoba and Saskatchewan between 1967 to 1972 exhibited two basic ionic dominance patterns HCO₃ > SO₄ > Cl and SO₄ > HCO₃ > Cl. The order of SO₄ and Cl were reversed in 40 percent of the bicarbonate ponds below 500 µmhos/cm. At salinities above 12,000 µmhos/cm, Cl exceeds SO₄ in several wetlands. Temporary wetlands were characterised by Ca > K > Mg > Na / HCO₃ > Cl > SO₄, semi-permanent ponds Mg > Ca > Na > K / SO₄ > HCO₃ > Cl while the permanent pond structure was Ca > Mg > Na > K / HCO₃ > SO₄ > Cl.Conductivities of the wetlands studied ranged from 47 to 23,000 µmhos / cm.Seasonal changes in salinity varied within and between pond types as well as from year to year. The average salinity increase within season in temporary ponds was 67 percent, 63 percent in semi-permanent ponds and 20 percent for permanent ponds. These changes were affected by evaporation, transpiration, see-page and precipitation patterns. Ionic dominance patterns did not change within a season although Mg, K and HCO₃ increased at higher rates than Ca, Na, CI and SO₄.Temporary wetlands are slightly acidic, averaging pH 6.8 while semi-permanent and permanent ponds were alkaline, pH range 7.1 to 9.2. No stratification of pH with depth and minimal diurnal variation was recorded.At two study areas, Bradwell and St. Denis fhe following ranges (micrograms/litre) were recorded: 3 to 630 PO₄-P, 10 to 650 NO₃-N, 5 to 630 TDFe, 1 to 13 TDCu and 1 to 27 TDZn. Colour was 5 to 190 Hazen units at St. Denis and 10 to 110 for Bradwell.Two herbicides, 2,4-D and 2,4,5-T were detected at levels of 4 to 111 and 1.4 to 13.5 micrograms/litre, respectively, from 15 ponds at St. Denis.     

Nonselective ionic channels inAplysia neurones

Summary Single-channel recordings from outside-out patches ofAplysia neurones in K-free solutions revealed the presence in most membrane patches of ionic channels showing surprising selectivity properties, as deduced from reversal potential measurements. After complete substitution of external NaCl by mannitol (in the presence of internal CsCl), these channels are more permeable to Cl than to Cs, but are also slightly permeable to Cs:P _Cl/P _Cs=4. Furthermore, in the presence of external NaCl, their ability to discriminate cations from anions seems lower than in external mannitol. Substitutions of external Cl by various anions showed that the channels are more permeable to NO₃ than to Cl, and that they are appreciably permeable to isethionate, SO₄ and methanesulfonate. Their elementary conductance is about 100 pS in 600mm symmetrical Cl. However, different conductance states (usually 2 or 3) can often be detected in the same membrane patch. By using voltage ramps, we established theI–V curves corresponding to each of these states and found small but significant differences between the reversal potentials of each state.     

Mechanism of Cl- sensitivity in internal ion receptors of the leech; an inward current gated off by Cl- in the nephridial nerve cells

Summary The nephridial nerve cells of the leech, Hirudo medicinalis, 34 sensory cells, each associated with one nephridium, are sensitive to changes in extracellular Cl^- concentration, an important factor in ion homeostasis. Using single-electrode current- and voltage clamp and ion substitution techniques, the specificity and mechanism of Cl^- sensitivity of the nephridial nerve cell was studied in isolated preparations. Increase of the normally low external Cl^- concentration leads to immediate and sustained hyperpolarization, decrease of the frequency of bursts and decrease of membrane conductance. The response is halogen specific: Cl^- can be replaced by Br^–, but not by organic mono- or divalent anions or inorganic divalent anions.At physiological Cl^- concentrations (36mM extra-cellular Cl^-), the nephridial nerve cell has a high resting conductance for Cl^- and the membrane potential is governed by Cl^-. In high extracellular Cl^- concentrations (110–130 mM), membrane conductance is low, most likely due to the gating off of Cl^- channels. Under these conditions, membrane potential is dominated by the K⁺ distribution and the nephridial nerve cell hyperpolarizes towards E_K.Abbreviations NNC nephridial nerve cell - V _m membrane potential - E _Cl(k) equilibrium potential for Cl (K) - IV-curve current-voltage relationship     

Anion activation of γ-glutamyltransferase from fruiting bodies of Lentinus edodes

Activation by different anions of γ-glutamyltransferase obtained in a. particulate form from fruiting bodies of Lentinus edodes has been studied using either L-γ-glutamyl-p-nitroanlide or lentinic acid as substrate. The mushroom transferase was activated by SCN^?, NO₃^?, Cl^?, Br^?, ClO₃^?, Bro₃^?, N₃^?, I^? and F^?, but not those alkali and earth cations previously believed to activate the animal transferase, nor by citrate, claimed to be effective for the kidney bean transferase. Among anions proved hardly to activate the transferase were ClO₄^?, NO₂?, HCO₃^?, H₂PO₄^?, SO₃^2? and SO₄^2?. A high concentration of these anions more or less impeded the halide activation. Kinetic studies revealed that halides function as activators of increasing V_max while keeping K_m constant. These observations appeared least compatible with the possibility that the anion activation might involve a non-specific effect of high solute concentration, viz. dissociation of the enzyme from the supporting structure in the particulates. The activating effect of halides described here probably extends also to the animal enzymes.     

The Effects of Alkali Metal Cations and Common Anions on the Frog Skin Potential

The effects on the potential difference across isolated frog skin (R. catesbeiana, R. pipiens) of changing the ionic composition of the bathing solutions have been examined. Estimates of mean values and precision are presented for the potential changes produced by substituting other alkali metal cations for Na at the outside border and for K at the inside border. In terms of ability to mimic Na at the outside border of bullfrog skin, the selectivity order is Li > Rb, K, Cs; at the outside border of leopard frog skin, Li > Cs, K, Rb. In terms of ability to mimic K at the inside border of bullfrog and leopard frog skin: Rb > Cs > Li > Na. Orders of anion selectivity in terms of sensitivity of the potential for the outside border of bullfrog skin are Br > Cl > NO₃ > I > SO₄, isethionate and of leopard frog skin are Br, Cl > I, NO₃, SO₄. An effect of the solution composition (ionic strength?) on the apparent Na-K selectivity of the outside border is described. The results of the investigation have been interpreted and discussed in terms of the application of the constant field equation to the Koefoed-Johnsen-Ussing frog skin model. These observations may be useful in constructing and testing models of biological ionic selectivity.     

Active sulfate absorption in rabbit ileum: Dependence on sodium and chloride and effects of agents that alter chloride transport

Summary Unidirectional fluxes of³⁵SO₄ across and into rabbit ileal epithelium were measured under short-circuit conditions, mostly at a medium SO₄ concentration of 2.4mm. Unidirectional mucosa (m)-to-serosa (s) ands-to-m fluxes (J _ms,J _sm) were 0.456 and 0.067 moles hr^–1 cm^–2, respectively.J _ms was 2.7 times higher in distal ileum than in mid-jejunum. Ouabain abolished net SO₄ transport (J _net) by reducingJ _ms. Epinephrine, a stimulus of Cl absorption, had no effect on SO₄ fluxes. Theophylline, a stimulus of Cl secretion, reducedJ _ms without affectingJ _sm, causing a 33% reduction inJ _net. Other secretory stimuli (8-Br-cAMP, heat-stable enterotoxin, Ca-ionophore A23187) had similar effects. Replacement of all Cl with gluconate markedly reducedJ _net through both a decrease inJ _ms and an increase inJ _sm. The anion-exchange inhibitor, 4-acetoamido-4-isothiocyano-2,2-sulfonic acid stilbene (SITS), when added to the serosal side, reducedJ _ms by 94%, nearly abolishingJ _net. SITS also decreasedJ _sm by 75%. Mucosal SITS (50 m) was ineffective. 4,4-diisothiocyano-2,2-sulfonic acid stilbene (DIDS) had effects similar to SITS but was less potent. Measurements of initial rates of epithelial uptake from the luminal side (J _me) revealed the following: (1)J _me is a saturable function of medium concentration with aV _max of 0.94 moles hr^–1 cm^–2 and aK _1/2 of 1.3mm; (2) replacing all Na with choline abolishedJ _me; (3) replacing all Cl with gluconate increasedJ _me by 40%; (4) serosal SITS had no effect onJ _me; and (5) stimuli of Cl secretion had no effect onJ _me or increased it slightly. Determination of cell SO₄ with³⁵SO₄ indicated that, at steady-state, the average mucosal concentration is 1.1 mmoles per liter cell water, less than half the medium concentration. Cell SO₄ was increased to 3.0mm by adding SITS to the serosal side. Despite net transport rates greater than 1.4 Eq hr^–1 cm^–2, neither addition of SO₄ to the SO₄-free medium nor addition of SITS to SO₄-containing medium altered short-circuit current. The results suggest that (1) ileal SO₄ absorption consists of Na-coupled influx (symport) across the brush border and Cl-coupled efflux (antiport) across the basolateral membrane; (2) the overall process is electrically neutral; (3) the medium-to-cell Cl concentration difference may provide part of the driving force for net SO₄ absorption; and (4) since agents affecting Cl fluxes (both absorptive and secretory) have little effect on SO₄ fluxes, the mechanisms for their transcellular transports are under separate regulation.     

Amine transport at the plasmalemma of Riccia fluitans

Green thallus cells of the aquatic liverwort, Riccia fluitans, are rapidly depolarized in the presence of 1–20 μM NH₄Cl and 5–100 μM CH₃NH₃Cl, respectively. Simultaneously, the membrane conductance is increased from 0.41 to 1.2 S · m^?2. Uptake of [¹⁴C]methylamine is stimulated by increasing [K⁺]_o and inhibited by increasing [Na⁺]_o or [H⁺]_o, is highly voltage sensitive, and saturates at low amine concentrations.Double-reciprocal plots of (a) maximal membrane depolarization and (b) methylamine uptake vs. external amine concentration give apparent K_m values of 2 ± 1 μM ammonia and 25–50 μM methylamine; K_m values for changes in conductance and membrane current are greater and voltage dependent. Whereas the amine transport into the cell is strongly inhibited by CN^?, the amine efflux is stimulated.The current-voltage characteristics of the ammonia transport are represented by a sigmoid curve with an equilibrium potential of ?60 mV, and this is understood as a typical carrier curve with a saturation current of about 70 mA · m^?2. It is further concluded that the evidently carrier-mediated transport is competitive for the two amines tested, and that ammonia and methylamine are transported in the protonated form as NH₄⁺ and CH₃NH₃⁺ into the cytoplasm.     

Sulphate transport in the cyanobacterium Synechococcus R-2 (Anacystis nidulans, S. leopoliensis) PCC 7942

Synechococcus R-2 (PCC 1942) actively accumulates sulphate in the light and dark. Intracellular sulphate was 1.35 ± 0.23 mol m^?3 (light) and 0.894 ± 0.152 mol m^?3 (dark) under control conditions (BG-11 media: pH_o, 7.5; [SO₄^2?]_o, 0.304 mol m^?3). The sulphate transporter is different from that found in higher plants: it appears to be an ATP-driven pump transporting one SO₄^2?/ATP [ΔμSO₄^2?_i,o=+ 27.7 ± 0.24 kJ mol^?1 (light) and + 24 ± 0.34 kj mol^?1 (dark)]. The rate of metabolism of SO₄^2?at pH_o, 7.5 was 150 ± 28 pmol m^?2 s^?1 (n = 185) in the light but only 12.8 ± 3.6 pmol m^?2 s^?1 (n = 61) in the dark. Light-driven sulphate uptake is partially inhibited by DCMU and chloramphenicol. Sulphate uptake is not linked to potassium, proton, sodium or chloride transport. The alga has a constitutive over-capacity for sulphate uptake [light (n= 105): K_m= 0.3 ± 0.1 mmol m^?3, V_max, = 1.8 ± 0.6 nmol m^?2 s^?1; dark (n= 56): K_m= 1.4 ± 0.4 mmol m^?3, V_max= 41 ± 22 pmol m^?2 s^?1]. Sulphite (SO₃^2?) was a competitive inhibitor of sulphate uptake. Selenate (SeO₄^2?) was an uncompetitive inhibitor.     

Multiphasic absorption of glucose and 3-o-methyl glucose by aged potato slices

The isotherm for glucose absorption by aged potato (Solanum tuberosum var. Russet Burbank) discs shows four distinct phases in the concentration ranges 1.0 to 75 μm, 75 μm to 1.5 mm, 1.5 to 15 mm, and 15 to 100 mm, respectively. Each segment of the multiphasic isotherm, when plotted reciprocally by the method of Lineweaver and Burk or of Hofstee, without regard for uptake in earlier phases, indicates absorption rate to be a hyperbolic function of concentration. The observations suggest that glucose uptake is carrier-mediated, and that the transport barrier undergoes a series of all-or-none transformations at critical external concentrations, yielding successive new and higher values for the parameters Km and V_max 3-O-Methyl glucose, a nonmetabolizable analogue of glucose, shows the same multiphasic absorption isotherm, with Km values essentially similar to those for glucose uptake, and V_max values somewhat lower than those for glucose absorption. Whereas the first three phases of the absorption isotherm are taken to reflect passage across the plasma membrane, the fourth phase may reflect kinetics of glucose or 3-O-methyl glucose transport to the vacuole.