The influence of external potassium on the inactivation of sodium currents in the giant axon of the squid, Loligo pealei

Isolated giant axons were voltage-clamped in seawater solutions having constant sodium concentrations of 230 mM and variable potassium concentrations of from zero to 210 mM. The inactivation of the initial transient membrane current normally carried by Na⁺ was studied by measuring the Hodgkin-Huxley h parameter as a function of time. It was found that h reaches a steady-state value within 30 msec in all solutions. The values of h _∞, τ_h, α_h,and β_h as functions of membrane potential were determined for various [K _o]. The steady-state values of the h parameter were found to be inversely related, while the time constant, τ_h, was directly related to external K⁺ concentration. While the absolute magnitude as well as the slopes of the h _∞ vs. membrane potential curves were altered by varying external K⁺, only the magnitude and not the shape of the corresponding τ_h curves was altered. Values of the two rate constants, α_h and β_h, were calculated from h_∞ and τ_h values. α_h is inversely related to [K_o] while β_h is directly related to [K_o] for hyperpolarizing membrane potentials and is independent of [K_o] for depolarizing membrane potentials. Hodgkin-Huxley equations relating α_h and β_h to E_m were rewritten so as to account for the observed effects of [K_o]. It is concluded that external potassium ions have an inactivating effect on the initial transient membrane conductance which cannot be explained solely on the basis of potassium membrane depolarization.     

Effect of atoms evaporated from the anode on the structure of the vacuum arc space charge sheath

The structure of the anode space charge sheath of a vacuum arc is studied with allowance for the dependence of the negative anode fall on the ratio of the directed electron velocity v ₀ to the electron thermal velocity v _T for different values of the flux density of atoms evaporated from the anode. Poisson’s equation for the sheath potential is solved taking into account the electron space charge, fast cathode ions, and slow ions produced due to the ionization of atoms evaporated from the anode. The kinetic equation for atoms and slow anode ions is solved with allowance for ionization in the collision integral. Analytic solutions for the velocity distribution functions of atoms and slow ions and the density of slow ions are obtained. It is shown that the flux of slow ions substantially affects the spatial distribution of the electric field E(z) in the sheath. As the flux density increases, the nonmonotonic dependence E(z) transforms into a monotonic one and the sheath narrows. For a given flux of evaporated atoms Πa, the increase in the ratio of the directed electron velocity to the electron thermal velocity leads again to a nonmonotonic dependence E(z). As z increases, the electric field first increases, passes through the maximum, decreases, passes through the minimum E _min, and then again increases toward the anode. There is a limiting value of the ratio (v ₀/v _T )* at which E _min(z) vanishes. At v ₀/v _T > (v ₀/V _T )*, the condition for the existence of a steady-state sheath is violated and the profiles of the field and potential in the sheath become oscillating. The dependence of (v ₀/v _T )* on the flux density of evaporated atoms Π _a is obtained. It is shown that the domain of existence of steady-state solutions in the sheath broadens with increasing Π _a .     

The roles of Rhodobacter sphaeroides copper chaperones PCuAC and Sco (PrrC) in the assembly of the copper centers of the aa3-type and the cbb3-type cytochrome c oxidases

The α proteobacter Rhodobacter sphaeroides accumulates two cytochrome c oxidases (CcO) in its cytoplasmic membrane during aerobic growth: a mitochondrial-like aa₃-type CcO containing a di-copper Cu_A center and mono-copper Cu_B, plus a cbb₃-type CcO that contains Cu_B but lacks Cu_A. Three copper chaperones are located in the periplasm of R. sphaeroides, PCu_AC, PrrC (Sco) and Cox11. Cox11 is required to assemble Cu_B of the aa₃-type but not the cbb₃-type CcO. PrrC is homologous to mitochondrial Sco1; Sco proteins are implicated in Cu_A assembly in mitochondria and bacteria, and with Cu_B assembly of the cbb₃-type CcO. PCu_AC is present in many bacteria, but not mitochondria. PCu_AC of Thermus thermophilus metallates a Cu_A center in vitro, but its in vivo function has not been explored. Here, the extent of copper center assembly in the aa₃- and cbb₃-type CcOs of R. sphaeroides has been examined in strains lacking PCu_AC, PrrC, or both. The absence of either chaperone strongly lowers the accumulation of both CcOs in the cells grown in low concentrations of Cu^2 +. The absence of PrrC has a greater effect than the absence of PCu_AC and PCu_AC appears to function upstream of PrrC. Analysis of purified aa₃-type CcO shows that PrrC has a greater effect on the assembly of its Cu_A than does PCu_AC, and both chaperones have a lesser but significant effect on the assembly of its Cu_B even though Cox11 is present. Scenarios for the cellular roles of PCu_AC and PrrC are considered. The results are most consistent with a role for PrrC in the capture and delivery of copper to Cu_A of the aa₃-type CcO and to Cu_B of the cbb₃-type CcO, while the predominant role of PCu_AC may be to capture and deliver copper to PrrC and Cox11. This article is part of a Special Issue entitled: Biogenesis/Assembly of Respiratory Enzyme Complexes.     

The effects of N-substitution of chitosan and the physical form of the products on the rate of hydrolysis by chitinase from Streptomyces griseus

N-Formyl, N-chloroacetyl, N-glycyl, N-isobutyryl, and N-pentanoyl derivatives of chitosan have been prepared. N-Acetylchitosan was the derivative most susceptible to chitinase from Streptomyces griseus and lysozyme from chicken egg-white, but the susceptibility was not restrictive. The relative rates of hydrolysis by chitinase with respect to R in the RCONH group were CH₃ > CH₃CH₂ > H > CH₃CH₂CH₂ > (CH₃)₂CH > NH₂CH₂ > ClCH₂. Neither enzyme hydrolysed chitosan or its N-methylene, N-benzylidene, N-benzoyl, N-nicotinyl, and N-fatty acyl (C₅C₁₈) derivatives, and lysozyme did not hydrolyse N-butyrylchitosan. N-Acetylhexanoyl-chitosans, which had d.s. ratios of ~0.7: ~0.3 and ~0.3; ~0.7, were hydrolysed at ~0.75 and ~0.04 of the rate of N-acetylchitosan (powder) by chitinase. O-Acylation of N-acylchitosans caused a decrease in the rates of hydrolysis by chitinase. N-Acetylchitosan gels were hydrolysed at 8–13 times the rate for crab-shell chitin. These results indicate that not only N- and O-substituents but also the physical form of the substrates influence the rates of hydrolysis by these enzymes.     

Individual Interactions of the b Subunits within the Stator of the Escherichia coli ATP Synthase

F_OF₁ ATP synthases are rotary nanomotors that couple proton translocation across biological membranes to the synthesis/hydrolysis of ATP. During catalysis, the peripheral stalk, composed of two b subunits and subunit δ in Escherichia coli, counteracts the torque generated by the rotation of the central stalk. Here we characterize individual interactions of the b subunits within the stator by use of monoclonal antibodies and nearest neighbor analyses via intersubunit disulfide bond formation. Antibody binding studies revealed that the C-terminal region of one of the two b subunits is principally involved in the binding of subunit δ, whereas the other one is accessible to antibody binding without impact on the function of F_OF₁. Individually substituted cysteine pairs suitable for disulfide cross-linking between the b subunits and the other stator subunits (b-α, b-β, b-δ, and b-a) were screened and combined with each other to discriminate between the two b subunits (i.e. b_I and b_II). The results show the b dimer to be located at a non-catalytic α/β cleft, with b_I close to subunit α, whereas b_II is proximal to subunit β. Furthermore, b_I can be linked to subunit δ as well as to subunit a. Among the subcomplexes formed were a-b_I-α, b_II-β, α-b_I-b_II-β, and a-b_I-δ. Taken together, the data obtained define the different positions of the two b subunits at a non-catalytic interface and imply that each b subunit has a different role in generating stability within the stator. We suggest that b_I is functionally related to the single b subunit present in mitochondrial ATP synthase.     

Effect of Ethanol on the Sodium and Potassium Conductances of the Squid Axon Membrane

The effects of ethanol on squid giant axons were studied by means of the sucrose-gap technique. The membrane action potential height is moderately reduced and the duration sometimes shortened by ethanol in sea water. Voltage clamp experiments showed that ethanol in sea water reduced the maximum membrane conductances for sodium (g'_Na) and potassium (g'_K). In experiments with multiple application of ethyl alcohol to the same spot of membrane, a reduction of g'_Na to 82 per cent and of g'_K to 80 per cent of their value in sea water was brought about by 3 per cent ethanol (by volume) while 6 per cent caused a decrease of g'_Na to 59 per cent and of g'_K to 69 per cent. Ethanol has no significant effect on the steady-state inactivation of g_Na (as a function of conditioning membrane potential) or on such kinetic parameters as τ_h or the time course of turning on gi g_Na and g_K. It is concluded that ethanol mainly reduces g_Na and g_K in the Hodgkin-Huxley terminology.     

Structure of chain d of the gigantic hemoglobin of the earthworm

The extracellular hemoglobin of the earthworm has four major O₂-binding chains, a, b, c and d, together with additional non-heme structural chains that are required for assembly. Although the abc trimer self-associates extensively at least to (abc)₁₀, addition of chain d results in the formation of a discrete 280 kDa complex corresponding to (abcd)₄. Thus a primary function of chain d is to cap the abc association and convert an abc trimer that binds O₂ with weak cooperativity to a highly cooperative (abcd)₄ complex. Amino-acid sequences of the major globin chains a, b, c have been determined previously by peptide and cDNA analysis. However, the peptide sequence reported for the major chain d (Shishikura, F., Snow, J.W., Gotoh, T., Vinogradov, S.N. and Walz, D.A. (1987) J. Biol. Chem., 262, 3123–3131), has a calculated molecular mass 134–167 Da higher than masses for components of chain d determined by mass spectrometry (Ownby, D.W., Zhu, H., Schneider, K., Beavis, R.C., Chait, B.T. and Riggs, A.F. (1993) J. Biol. Chem. 268, 13539–13547). Reverse-phase HPLC confirms the presence of two distinct polypeptides, d₁ and d₂, together with d₁′, a variant of d₁. cDNA-derived amino-acid sequences have been determined for chains d₁′ and d₂ by application of the polymerase chain reaction with primers based on the NH₂-terminal sequences and oligo-dT. Each of the two cDNA-derived sequences has 140 residues and they differ by 28 substitutions. The data show that the sequence originally reported had been derived from peptides generated from both polypeptides.     

Correlation between the optimal growth pressures of four Shewanella species and the stabilities of their cytochromes c 5

Shewanella species live widely in deep-sea and shallow-water areas, and thus grow piezophilically and piezosensitively. Piezophilic and psychrophilic Shewanella benthica cytochrome c ₅ (SB cytc ₅) was the most stable against guanidine hydrochloride (GdnHCl) and thermal denaturation, followed by less piezophilic but still psychrophilic Shewanella violacea cytochrome c ₅ (SV cytc ₅). These two were followed, as to stability level, by piezosensitive and mesophilic Shewanella amazonensis cytochrome c ₅ (SA cytc ₅), and piezosensitive and psychrophilic Shewanella livingstonensis cytochrome c ₅ (SL cytc ₅). The midpoint GdnHCl concentrations of SB cytc ₅, SV cytc ₅, SL cytc ₅, and SA cytc ₅ correlated with the optimal growth pressures of the species, the correlation coefficient value being 0.93. A similar trend was observed for thermal denaturation. Therefore, the stability of each cytochrome c ₅ is related directly to its host’s optimal growth pressure. Phylogenetic analysis indicated that Lys-37, Ala-41, and Leu-50 conserved in piezosensitive SL cytc ₅ and SA cytc ₅ are ancestors of the corresponding residues in piezophilic SB cytc ₅ and SV cytc ₅, Gln, Thr, and Lys, respectively, which might have been introduced during evolution on adaption to environmental pressure. The monomeric Shewanella cytochromes c ₅ are suitable tools for examining protein stability with regard to the optimal growth pressures of the source species.     

The redox potentials of the b-type cytochromes of higher plant chloroplasts

1. In fresh chloroplasts, three b-type cytochromes exist. These are b-559_HP (λ_max, 559 nm; E_m at pH 7, +370 mV; pH-independent E_m), b-559_LP (λ_max, 559 nm; E_m at pH 7, +20 mV; pH-independent E_m) and b-563 (λ_max, 563 nm; E_m at pH 7, ?110 mV; pH-independent E_m). b-559_HP may be converted to a lower potential form (λ_max, 559 nm; E_m at pH 7, +110 mV; pH-independent E_m).2. In catalytically active b-f particle preparations, three cytochromes exist. These are cytochrome f (λ_max, 554 nm; E_m at pH 7, +375 mV, pK on oxidised cytochrome at pH 9), b-563 (λ_max, 563 nm; E_m at pH 7, ?90 mV, small pH-dependence of E_m) and a b-559 species (λ_max, 559 nm, E_m at pH 7, +85 mV; pH-independent E_m).3. A positive method of demonstration and estimation of b-559_LP in fresh chloroplasts is described which involves the use of menadiol as a selective reductant of b-559_LP.     

The Role of Calcium in the Regulation of the Steady-State Levels of Sodium and Potassium in the HeLa Cell

Studies on HeLa cells in spinner culture at pH 7.0 and 37° have shown that [Na]_i decreased and [K]_i increased with increasing [Ca]_o. In Na-free (choline) medium [K]_i remained high whether or not Ca was present in the medium. [Na]_i and [K]_i approached a new steady state within 1 min after transfer to Ca-free medium and returned to the initial values within 15 min upon readdition of Ca. 40% of the cell Ca exchanged within 1 min followed by a slow exchange of the remaining Ca over several hours. [Ca]_i increased with decreasing [Na]_o but was independent of [K]_o. Equimolar Mg did not substitute for Ca in maintaining low [Na]_i and high [K]_i. Under steady-state conditions about 50% of the cell Na exchanged in accordance with a single rate constant. The initial Na influx was 270, 100, and 2.5 µM/liter of cell water/sec for 0, 0.10, and 1.0 mM [Ca]_o, respectively. When Na transport was inhibited with strophanthidin and [Na]_i and [K]_i allowed to reach a steady state, Na influx was more rapid for cells incubated in Ca-free medium than for cells incubated in medium containing 1.0 mM Ca. These results suggest that Ca competes with Na at the cell membrane and thus controls the passive diffusion of Na into the cell.     

Genetic specifics of the breeding of cotton varieties with cleistogamous flowers

As a prerequisite to studying the genetics and breeding of chasmogamous and cleistogamous flowers, a preliminary experiment was performed to estimate the extent of cross-pollination in cotton varieties and hybrids. Vicinism estimates varied from 0.53 to 15.36%, i.e., the proportion of cross-pollination was relatively high, leading to a biological contamination. As a result of such contamination, genetic collection lines and varieties lose genetic homogeneity and become heterozygous and genetically heterogeneous. The genetic control of the flower type was studied in the Gossipium hirsutum L. × G. barbadense L. interspecific hybrids, and phenotypic segregation of the 3: 1 and 15: 1 types with monogenic (3: 1) and digenic (15: 1) differences of noncumulative polymerization was observed. The corresponding types of genotypic segregation were 1: 2: 1 (1Cg ₁ Cg ₁ cg ₂ cg ₂ : 2Cg ₁ cg ₁ cg ₂ cg ₂ : 1cg ₁ cg ₁ cg ₂ cg ₂ ) and 1: 2: 2: 4: 1: 2: 1: 2: 1 (1) Cg ₁ Cg ₁ Cg ₂ Cg ₂ -1; (2) Cg ₁ Cg ₁ cg ₂ cg ₂ -2; (3) Cg ₁ cg ₁ Cg ₂ Cg ₂ -2; (4) Cg ₁ cg ₁ Cg ₂ cg ₂-4; (5) Cg ₁ Cg ₁ cg ₂ cg ₂ -1; (6) Cg ₁ cg ₁ cg ₂ cg ₂ -2; (7) cg ₁ cg ₁ Cg ₂ Cg ₂-1; (8) cg ₁ cg ₁ Cg ₂ cg ₂ -2; (9) cg ₁ cg ₁ cg ₂ cg ₂ -1. Genotypes (1)–(8) had chasmogamous flowers, while double-recessive genotype (9) had cleistogamous flowers. Based on this, genotypes with individual phenotypic expression were identified in F2, and their correlation with the most important morphological, biological, and agricultural features was studied. Special attention was paid to the productivity of hybrid plants intended for use in breeding to obtain intensive varieties. The study made it possible to isolate forms, families, genetic collection lines, and varieties with isogenic or nonisogenic determination of these characters and chasmogamous and cleistogamous flowers of G. hirsutum L. and G. barbadense L. prototypes by using original methods to examine the two types of flowers; the methods do not have analogs in cotton breeding worldwide.     

Light dependence of the decay of the proton gradient in broken chloroplasts

The initial rates and steady-state values of proton uptake by broken chloroplasts have been measured as functions of light intensity at various concentrations of chlorophyll, pyocyanine, supporting electrolyte, buffer, as well as pH and temperature. Kinetic analysis of the data shows that the rate of decay of proton gradient due to backward leakage depends on light intensity. Under steady illumination, the decay constant k_L is equal to k_D + mR₀, where R₀ is the initial rate of proton uptake which is a function of light intensity, k_D is the decay constant in the dark and m is a parameter which is independent of light intensity. Treatment of chloroplasts with lysolecithin, neutral detergent, 2,4-dinitrophenol, or valinomycin in the presence of K⁺ increases k_D without affecting m. Treatment with N,N′-dicyclohexylcarbodiimide or adenylyl imidodiphosphate under appropriate conditions decreases m without affecting k_D. Treatment with glutaraldehyde makes k_L independent of light intensity and hence m = 0. These results suggest that the light-dependent part (mR₀) of k_L is due to leakage of protons through the coupling factor (CF₁-CF₀) complex which can open or close depending on light intensity and that the light-independent part (k_D) of the decay constant k_L is due to proton leakage elsewhere.     

Mutations That Extend the Specificity of the Endonuclease Activity of λ Terminase

Terminase, an enzyme encoded by the Nu1 and A genes of bacteriophage lambda, is crucial for packaging concatemeric DNA into virions. cosN, a 22-bp segment, is the site on the virus chromosome where terminase introduces staggered nicks to cut the concatemer to generate unit-length virion chromosomes. Although cosN is rotationally symmetric, mutations in cosN have asymmetric effects. The cosN G₂C mutation (a G-to-C change at position 2) in the left half of cosN reduces the phage yield 10-fold, whereas the symmetric mutation cosN C₁₁G, in the right half of cosN, does not affect the burst size. The reduction in phage yield caused by cosN G₂C is correlated with a defect in cos cleavage. Three suppressors of the cosN G₂C mutation, A-E₅₁₅G, A-N₅₀₉K, and A-R₅₀₄C, have been isolated that restore the yield of λ cosN G₂C to the wild-type level. The suppressors are missense mutations that alter amino acids located near an ATPase domain of gpA. λ A-E₅₁₅G, A-N₅₀₉K, and A-R₅₀₄C phages, which are cosN⁺, also had wild-type burst sizes. In vitro cos cleavage experiments on cosN G₂C C₁₁G DNA showed that the rate of cleavage for A-E₅₁₅G terminase is three- to fourfold higher than for wild-type terminase. The A-E₅₁₅G mutation changes residue 515 of gpA from glutamic acid to glycine. Uncharged polar and hydrophobic residues at position 515 suppressed the growth defect of λ cosN G₂C C₁₁G. In contrast, basic (K, R) and acidic (E, D) residues at position 515 failed to suppress the growth defect of λ cosN G₂C C₁₁G. In a λ cosN⁺ background, all amino acids tested at position 515 were functional. These results suggest that A-E₅₁₅G plays an indirect role in extending the specificity of the endonuclease activity of λ terminase.     

四川柏木人工林林下植被生物量与林分结构的关系

森林结构与林下植被生物量的关系是森林持续经营与森林碳计量监测的科学基础,但一直缺乏必要的研究。以四川柏木(Cupressus funebris)人工林为研究对象,揭示林下植被生物量(Wu)、灌木生物量(Ws)和草本生物量(Wh)与林分结构的关系,并试图构建区域性林下植被生物量估测的混合模型。结果表明:(1)乔、灌、草群体共12个结构因子中,灌木群体的平均基径(Ds)、盖度(Cs)、高度(Hs)、体积(Vs)与林下植被生物量关系更紧密,在林下植被生物量模型构建中更有效;(2)多模型拟合与比较表明,柏木林Ws最佳估算模型为Ws=0.0005V1.0411s(R2a=0.762,P0.001,n=40),而Wu的最佳估算模型为ln Wu=0.0158Hs+0.0111Cs-0.5358(R2a=0.695,P0.001,n=40),但对于Wh未能获得较为理想的估算模型(R2a0.410,P0.01,n=40);(3)林分密度(Du)整合进入多元线性模型提高了林下植被生物量的估测精度,ln Wu=a+b Du+c Hs+d Cs(R2a=0.721,P0.001,n=40)。研究为区域性林下生物量估测模型构建提供了新论据。     

Distinct effects of the C-terminal octapeptide of cholecystokinin and of a cholera toxin pretreatment on the kinetics of rat pancreatic adenylate cyclase activity

(1) The kinetic parameters of rat pancreatic adenylate cyclase were evaluated, using GTP, p[NH]ppG or GTPγS as nucleotide activator, cholecystokinin as peptide hormone, and GDPβS and dibutyryl cyclic GMP as inhibitors of guanosine triphosphate and CCK-8, respectively. The time courses of activation and the degree of activation at steady state (E_A/E_TOT) were compatible with a simple two-state model of activation-deactivation based on a pseudo-monomolecular activation process (rate constant k₊₂, and a deactivation process (rate constant k_off) that included, depending on the activating nucleotide, the hydrolysis of GTP (rate constant k₂) and/or the dissociation of the intact nucleotide (rate constant k_?1), so that E_A/E_TOT = k₊₁/(k₊₁ + k₂ + k_?a). (2) The hormone CCK-8 increased the value of k₊₁ with GTP dose-dependently, from 0.2 to 10.9 min^?1. The value of k_?1 increased 0.01 to 0.3 min^?1 but the value of k₂ was unaltered at 7 min^?1, so that E_A/E_TOT increased 15-fold, from 4% to 61%. (3) A cholera toxin pretreatment at 30 μg/ml allowed also a large increase in E_A/E_TOT with GTP (up to 51%) but the underlying mechanism was different. It consisted of a 14-fold decrease in the k_off value of the GTP-activated enzyme (from 7 min^? to 0.5 min^?1) that corresponded to a reduction in GTPase activity. When testing the system with p[NH]ppG, two added effects of the cholera toxin pretreatment were observed: a 4-fold increase in the value of k₊₁ (from 0.2 to 0.8 min^?1) and the occurrence of a significant 0.3 min^?1 value for k_?1.     

Glutamate-induced polymerization of tubulin: characteristics of the reaction and application to the large-scale purification of tubulin

The multiple attack model of the α-amylase mechanism has been analyzed. Theoretical rate coefficients dependent on the degree of polymerization of substrate (n) have been applied. They are: effectiveness of hydrolysis (y_n) which affects V values, and the average number of unitary movements (ζ_n) which affects K_m values. The model explains the dependence of V and K_m on the degree of polymerization of substrates with n values higher than the number of subsites in the enzyme binding site. Distribution of the degree of polymerization of hog pancreas α-amylase products (n_p) and average n_p value has been found experimentally and applied to calculate the maximal number of unitary movements (z) of the enzyme during the single enzyme-substrate meeting. V and K_m values of hog pancreas α-amylase for amylose and different branched substrates have been determined and discussed in terms of the multiple attack theory. Their dependence on theoretic y_n and ζ_n values has been found in support of the model.     

Calculation of the energy confinement time and power threshold for L-H transitions in a tokamak with allowance for the parameters of the transport barrier

The ASTRA-ETL code is used to simulate L-H transition scenarios and calculate the energy confinement time and the threshold power of the L-H transition as functions of the averaged electron density 〈n〉, the averaged magnetic field B, the neutral density n _n , and the neutral temperature T _n , as well as the values of T _Se , T _Si , and n _S at the separatrix. It is shown that the linear dependence of the threshold power of the L-H transition on the averaged electron density, Q _L-H∝〈n〉, is associated with an increase in the viscosity of a poloidally rotating plasma due to charge exchange and is governed exclusively by an increase in the neutral density n _n . When the averaged electron density 〈n〉 is low, the threshold power rises because T _Si and T _Se increase. The accuracy of predictions for the power threshold of the L-H transition can be improved if the scaling of Q _L-H versus 〈n〉 and B is derived by processing experimental data from discharges with close parameter values at the separatrix. The hysteresis effect during an L-H-L transition triggered by varying the input power is modeled. The global energy confinement time τ_E is shown to increase linearly with 〈n〉 in the range 〈n〉<3.6×10¹⁹ m^?3 and to saturate at higher electron densities; this behavior is found to be characteristic of the Ohmic, L-, and H-modes. The saturation is associated with the fact that losses via the ion channel (when the transport coefficients are density-independent) dominate over losses via the electron channel. The dependence of τ _E on the input power is determined from the calculated database and is found to be τ _E =0.12Q _L-H ^?0.46 at a fixed averaged electron density 〈n〉. In the simulations of the L-H transition, the energy confinement time τ _E increases by a factor of 2 only if the thermal diffusivity inside the transport barrier is lower than that in the central plasma by a factor of more than 6.     

Influence of the lamellar phase unbinding energy on the relative stability of lamellar and inverted cubic phases

Based on curvature energy considerations, nonbilayer phase-forming phospholipids in excess water should form stable bicontinuous inverted cubic (Q_II) phases at temperatures between the lamellar (L_α) and inverted hexagonal (H_II) phase regions. However, the phosphatidylethanolamines (PEs), which are a common class of biomembrane phospholipids, typically display direct L_α/H_II phase transitions and may form intermediate Q_II phases only after the temperature is cycled repeatedly across the L_α/H_II phase transition temperature, T_H, or when the H_II phases are cooled from T > T_H. This raises the question of whether models of inverted phase stability, which are based on curvature energy alone, accurately predict the relative free energy of these phases. Here we demonstrate the important role of a noncurvature energy contribution, the unbinding energy of the L_α phase bilayers, g_u, that serves to stabilize the L_α phase relative to the nonlamellar phases. The planar L_α phase bilayers must separate for a Q_II phase to form and it turns out that the work of their unbinding can be larger than the curvature energy reduction on formation of Q_II phase from L_α at temperatures near the L_α/Q_II transition temperature (T_Q). Using g_u and elastic constant values typical of unsaturated PEs, we show that g_u is sufficient to make T_Q > T_H for the latter lipids. Such systems would display direct L_α → H_II transitions, and a Q_II phase might only form as a metastable phase upon cooling of the H_II phase. The g_u values for methylated PEs and PE/phosphatidylcholine mixtures are significantly smaller than those for PEs and increase T_Q by only a few degrees, consistent with observations of these systems. This influence of g_u also rationalizes the effect of some aqueous solutes to increase the rate of Q_II formation during temperature cycling of lipid dispersions. Finally, the results are relevant to protocols for determining the Gaussian curvature modulus, which substantially affects the energy of intermediates in membrane fusion and fission. Recently, two such methods were proposed based on measuring T_Q and on measuring Q_II phase unit cell dimensions, respectively. In view of the effect of g_u on T_Q that we describe here, the latter method, which does not depend on the value of g_u, is preferable.     

Active transport of chloride by the giant neuron of the Aplysia abdominal ganglion

Internal chloride activity, aⁱ _Cl, and membrane potential, E_m, were measured simultaneously in 120 R2 giant neurons of Aplysia californica. aⁱ _Cl was 37.0 ± 0.8 mM, E_m was -49.3 ± 0.4 mv, and E _Cl calculated using the Nernst equation was -56.2 ± 0.5 mv. Such values were maintained for as long as 6 hr of continuous recording in untreated neurons. Cooling to 1°–4°C caused aⁱ _Cl to increase at such a rate that 30–80 min after cooling began, E _Cl equalled E_m. The two then remained equal for as long as 6 hr. Rewarming to 20°C caused aⁱ _Cl to decline, and E _Cl became more negative than E_m once again. Exposure to 100 mM K⁺-artificial seawater caused a rapid increase of aⁱ _Cl. Upon return to control seawater, aⁱ _Cl declined despite an unfavorable electrochemical gradient and returned to its control values. Therefore, we conclude that chloride is actively transported out of this neuron. The effects of ouabain and 2,4-dinitrophenol were consistent with a partial inhibitory effect. Chloride permeability calculated from net chloride flux using the constant field equation ranged from 4.0 to 36 x 10^-8 cm/sec.