Flux ratio of valinomycin-mediated K+ fluxes across the human red cell membrane in the presence of the protonophore CCCP

Summary The ratio of valinomycin-mediated unidirectional K⁺ fluxes across the human red cell membrane, has been determined in the presence of the protonophore carbonylcyanidem-chlorophenylhydrazone, CCCP, using the K⁺ net efflux and⁴²K influx. The driving force for the net efflux (V _m –E _K ⁺) has been calculated from the membrane potential, estimated by the CCCP-mediated proton distribution and the Nernst potential for potassium ions across the membrane. An apparent driving potential for the K⁺ net efflux has been calculated from the K⁺ flux ratio, determined in experiments where the valinomycin and CCCP concentrations were varied systematically. This apparent driving force, in conjunction with the actual driving force calculated on basis of the CCCP estimated membrane potential, is used to calculate a flux ratio exponent, which represents an estimate of the deviation of valinomycin-mediated K⁺ transport from unrestricted electrodiffusion, when protonophore is present.In the present work, the flux ratio exponent is found to be 0.90 when the CCCP concentration is 5.0 m and above, while the exponent decreases to about 0.50 when no CCCP is present. The influence of CCCP upon the rate constants in the valinomycin transport cycle is discussed. The significance of this result is that red cell membrane potentials are overestimated, when calculated from valinomycin-mediated potassium isotope fluxes, using a constant field equation.     

Steady-state and transient membrane potentials in human red cells determined by protonophore-mediated pH changes

Summary Protonophores have been used frequently to determine changes in membrane potential in suspensions of red cells, since such changes are reflected by changes in extracellular pH, due to proton and consequently protonophore reequilibration.In a previous paper (Bennekou, P. 1988.J. Membrane Biol. 102:225–234) a kinetic model for the translocation of a protonophore, CCCP, across the human red cell membrane was established. This model accounts for the protonophore reequilibration following abrupt changes in membrane potential.In this paper, the limitations of the method with regard to the estimation of transient membrane potentials are examined, using the transport model to simulate changes in extracellular pH in response to noninstantaneous changes in membrane potential. The temperature and time resolution calculated from the model are reported.Furthermore, it is shown that the transport model established for CCCP is valid for another protonophore, TCS, thus indicating the general validity of the transport scheme for the entire class of protonophores.     

The role of sodium-channel density in the natriferic response of the toad urinary bladder to an antidiuretic hormone

Summary Urinary bladders ofBufo marinus were depolarized, by raising the serosal K concentration, to facilitate voltage-clamping of the apical membrane. Passive Na transport across the apical membrane was then studied with near-instantaneous current-voltage curves obtained before and after eliciting a natriferic response with oxytocin. Fitting with the constant-field equation showed that the natriferic effect is accounted for by an increase in the apical Na permeability. It is accompanied by a small increase in cellular Na activity. Furthermore, fluctuation analysis of the amiloride-induced shot-noise component of the short-circuit current indicated that the permeability increase is not due to increased Na translocation through those Na channels which were already conducting prior to hormonal stimulation. Rather, the natriferic effects is found to be based on an increase in the population of transporting channels. It appears that, in response to the hormone, Na channels are rapidly recruited from a pool of electrically silent channels.     

Anion transport across the red blood cell membrane mediated by dielectric pores

Summary The anion transport across the red blood cell membrane is assumed to occur by ionic diffusion through dielectric pores which are formed by protein molecules spanning the red blood cell membrane. The access of anions to the dielectric pores is regulated by anion adsorption sites positioned at the entrances of the pores. The adsorption of small inorganic anions to the adsorption sites is facilitated by ionizing cationic groups setting up a surface potential at the respective membrane surfaces. Applying the transition state theory of rate processes, flux equations for the unidirectional flux were derived expressing the unidirectional flux as a function of the fractional occupancies of anion adsorption sites at both membrane surfaces.The basic properties of the transport model were investigated. The concentration-dependence and the pH-dependence of the unidirectional fluxes were shown to depend upon the surface charge density and upon the affinity of the transported anion species to the anion binding sites. The concentration-response and the pH-response of the unidirectional fluxes of different anion species may differ substantially even if the anion species are transported by the same anion transport system. The model predicts a characteristic behavior of the Lineweaver-Burk plot and of the Dixon plot.A comparison between computer simulated and experimentally determined flux curves was made. By choosing a suitable set of parameters, the anion transport model is capable of simulating the concentration-dependencies and the pH-dependencies of the unidirectional sulfate and chloride flux. It is sufficient to change one single constant in order to convert the sulfate transport system into a chloride transport system. Furthermore, the model is capable of predicting the inhibitory action of chloride on the sulfate transport system. No attempts were made to fit the experimental data to the model. The behavior of the model was qualitatively in accordance with the experimental results.     

Characterization of the binding of valinomycin to bacteriorhodopsin

Circular dichroism spectroscopy has been used to investigate the binding of valinomycin to bacteriorhodopsin in purple membrane suspensions. Addition of valinomycin to purple membrane suspensions obtained from Halobacterium halobium causes the circular dichroism spectrum to shift from an aggregate spectrum to one resembling a monomer spectrum, indicating a loss of chromophore-chromophore interactions. By observing the spectral change upon titration of valinomycin, an apparent dissociation constant of 30–40 M for valinomycin binding was determined. Kinetics of dark adaptation for valinomycin-treated purple membrane are comparable to those for monomeric bacteriorhodopsin. Centrifugation studies demonstrate that valinomycin-treated purple membrane sediments the same as untreated purple membrane suspensions. These results are consistent with a model in which valinomycin binds specifically to bacteriorhodopsin without disrupting the purple membrane fragments.Abbreviations BR bacteriorhodopsin - CD circular dichroism - Tricine N-[tris-(hydroxymethyl) methyl] glycine     

Chloride conductance of the Amphiuma red cell membrane

Summary Like most other red cells, the giant erythrocytes ofAmphiuma means possess a system for rapid exchange of chloride across the membrane. Also, there are indications that the net transport of chloride in these cells is slow. The size ofAmphiuma erythrocytes allows direct measurements of membrane potential with microelectrodes. The present work exploits the possibility that such measurements can be used to give a quantitative estimate of the chloride conductance (G _Cl) of the Amphiuma red cell membrane. The membrane potential was measured as a function of extracellular chloride concentration (5–120mM), using an impermeant anion (Para-amino-hippurate) as a substitute. Furthermore, the effect of different pH values (6.0–7.2) was studied. For each extracellular chloride concentration the membrane potential was determined at a pH at which hydroxyl, hydrogen, and bicarbonate ions were in electrochemical equilibrium. From these membrane potentials and the corresponding chloride concentrations in the medium (at constant intracellular ion concentrations), theG _Cl of the membrane was calculated to be 3.9×10^–7 {ie27-1} cm^–2. This value is some six orders of magnitude smaller than that calculated from the rate of tracer exchange under equilibrium conditions. The experimental strategy used gives the value for a partial transference number which takes into account only ions which arenot in electrochemical equilibrium. Whereas this approach gives a value forG _Cl, it does not permit calculation of the overall membrane conductance. From the calculated value ofG _Cl it is possible to estimate that the maximal value of the combined conductances of hydroxyl (or proton) and bicarbonate ions is 0.6×10^–7 {ie27-2} cm^–2. The large discrepancy between the rate of exchange of chloride and its conductance is in agreement with measurements on human and sheep red cells employing the ionophore valinomycin to increase the potassium conductance of the membrane. The results in the present study were, however, obtained without valinomycin and an accompanying assumption of a constant field in the membrane. Therefore, the present measurements give independent support to the above mentioned conclusions.     

The kinetic mechanism by which CCCP (carbonyl cyanidem-Chlorophenylhydrazone) transports protons across membranes

Summary We demonstrate that a simple kinetic model describes the transport of protons across lipid bilayer membranes by the weak acid CCCP (carbonyl cyanidem-chlorophenylhydrazone). Four parameters characterize this model: the adsorption coefficients of the anionic and neutral forms of the weak acid onto the interface ( _A and _HA) and the rate constants for the movement of A^– and HA across the membrane (k _A andk _HA). These parameters were determined by equilibrium dialysis, electrophoretic mobility, membrane potential, membrane conductance, and spectrophotometric measurements. From these equilibrium and steady state measurements on diphytanoyl phosphatidylcholine/chlorodecane membranes we found that _A= _HA=1.4 10^-3cm,k _A=175 s^–1 andk _HA=12,000 sec^–1. These parameters and our model describe our kinetic experiments if we assume that the protonation reactions, which occur at the interfaces, remain at equilibrium. The model predicts a single exponential decay of the current in a voltage-clamp experimetn. The model also predicts that the decay in the voltage across the membrane following an intense current pulse of short duration (50 nsec) can be described by the sum of two exponentials. The magnitudes and time constants of the relaxations that we observed in both voltage-clamp and charge-pulse experiments agree well with the predictions of the model for all values of pH, voltage and [CCCP].     

Permeation of membranes by the neutral form of amino acids and peptides: Relevance to the origin of peptide translocation

The flux of amino acids and other nutrient solutes such as phosphate across lipid bilayers (liposomes) is 10⁵ slower than facilitated inward transport across biological membranes. This suggests that primitive cells lacking highly evolved transport systems would have difficulty transporting sufficient nutrients for cell growth to occur. There are two possible ways by which early life may have overcome this difficulty: (1) The membranes of the earliest cellular life-forms may have been intrinsically more permeable to solutes; or (2) some transport mechanism may have been available to facilitate transbilayer movement of solutes essential for cell survival and growth prior to the evolution of membrane transport proteins. Translocation of neutral species represents one such mechanism. The neutral forms of amino acids modified by methylation (creating protonated weak bases) permeate membranes up to 10¹⁰ times faster than charged forms. This increased permeability when coupled to a transmembrane pH gradient can result in significantly increased rates of net unidirectional transport. Such pH gradients can be generated in vesicles used to model protocells that preceded and were presumably ancestral to early forms of life. This transport mechanism may still play a role in some protein translocation processes (e.g., for certain signal sequences, toxins and thylakoid proteins) in vivo.Abbreviations LUV large unilamellar vesicle - pH transmembrane pH gradient - PAH polyaromatic hydrocarbon Correspondence to: A.C. Chakrabarti     

Correlated and uncorrelated fitness landscapes and how to tell the difference

The properties of multi-peaked fitness landscapes have attracted attention in a wide variety of fields, including evolutionary biology. However, relaively little attention has been paid to the properties of the landscapes themselves. Herein, we suggest a framework for the mathematical treatment of such landscapes, including an explicit mathematical model. A central role in this discussion is played by the autocorrelation of fitnesses obtained from a random walk on the landscape. Our ideas about average autocorrelations allow us to formulate a condition (satisfied by a wide class of landscapes we call AR(1) landscapes) under which the average autocorrelation approximates a decaying exponential. We then show how our mathematical model can be used to estimate both the globally optimal fitnesses of AR(1) landscapes and their local structure. We illustrate some aspects of our method with computer experiments based on a single family of landscapes (Kauffman's N-k model), that is shown to be a generic AR(1) landscape. We close by discussing how these ideas might be useful in the tuning of combinatorial optimization algorithms, and in modelling in the experimental sciences.     

Effect of calcium on the membrane potential ofAmphiuma red cells

Summary An increase in extracellular Ca concentration causes the membrane of giant red cells of the salamander,Amphiuma means, to undergo a marked, transient hyperpolarization. This hyperpolarization is caused by an increase in K permeability of the membrane as judged from the K sensitivity of the membrane potential and from the rate of K loss under influence of raised extracellular Ca concentration. At constant external pH, the induction of hyperpolarization by increased extracellular Ca has a relatively well-defined threshold concentration. Furthermore the phenomenon is of an all or none type with most of the cells having membrane potential values either in the normal range (about –15 mV) or in the range –40 to –70 mV. Shortly after suspension in Ringer's with 15mM Ca, most if not all of the individual cells are hyperpolarized. Upon continued exposure (5–20 min) to the higher Ca concentration the membrane potential returns to the normal value in a fashion compatible with an all or none response. The observed Ca effect is sensitive to the pH of the suspending medium. At pH 6.2 the response is absent whereas the hyperpolarization is markedly stronger at pH 8.2 than at pH 7.2. It is argued that a reliable transport number for K under influence of Ca cannot be estimated from the slope of membrane potentialvs. log (extracellular K concentration). This is probably related to the fact that the membrane potentials of the cells in the population do not stay constant in time. The above phenomenon is compared with the Ca-induced K permeability in poisoned human red cells or red cell ghosts. It is important to note that the cells employed in the present study are neither poisoned nor mechanically disrupted. This study emphasizes that the role of Ca in regulating cell membrane permeability to K seems to be a general feature.     

Proton translocation in intact cells of Thiobacillus denitrificans

Proton translocation assessed by the quinacrine fluorescence technique was compared with oxygen uptake during thiosulphate oxidation by cells of Thiobacillus denitrificans. The addition of thiosulphate to cell suspensions resulted in an outwardly directed proton translocation as reflected by an increased quinacrine fluorescence. Compared to the O₂ uptake activity, the proton translocating system was much more sensitive to proton conductors, other ionophores and inhibitors of electron transport. The results indicate that (a) the proton-translocation activity (membrane energization) is enhanced in aged cell suspensions, (b) intactness of the cytoplasmic membrane is essential for establishing a protonmotive force in cells, (c) the fluorescence increase and proton translocation are reversible processes, (d) inhibitors of electron transport may also act as proton conductors by altering the integrity of the cytoplasmic membrane.Abbreviations CCCP carbonyl cyanide m-chlorophenyl-hydrazone - DBP 2,4-dibromophenol - DNP 2,4-dinitrophenol - HOQNO 2-heptyl-4-hydroxyquinoline-N-oxide - PCP pentachlorophenol - TPB tetraphenyl boron - TTFA 1-[thenoyl-(2)]-3,3,3-trifluoracetone     

Modulation of water and urea transport in human red cells: Effects of pH and phloretin

Summary It has previously been shown by Macey and Farmer (Biochim. Biophys. Acta 211:104–106, 1970) that phloretin inhibits urea transport across the human red cell membrane yet has no effect on water transport. Jennings and Solomon (J. Gen. Physiol. 67:381–397, 1976) have shown that there are separate lipid and protein binding sites for phloretin on the red cell membrane. We have now found that urea transport is inhibited by phloretin binding to the lipids with aK ₁ of 25±8 m in reason-able agreement with theK _D of 54±5 m for lipid binding. These experiments show that lipid/protein interactions can alter the conformational state of the urea transport protein. Phloretin binding to the protein site also modulates red cell urea transport, but the modulation is opposed by the specific stilbene anion transport inhibitor, DIDS (4,4-diisothiocyano-2,2-stilbene disulfonate), suggesting a linkage between the urea transport protein and band 3. Neither the lipid nor the protein phloretin binding site has any significant effect on water transport. Water transport is, however, inhibited by up to 30% in a pH-dependent manner by DIDS binding, which suggests that the DIDS/band 3 complex can modulate water transport.     

Transport of branched-chain amino acids in Corynebacterium glutamicum

The transport of branched-chain amino acids was characterized in intact cells of Corynebacterium glutamicum ATCC 13032. Uptake and accumulation of these amino acids occur via a common specific carrier with slightly different affiniteis for each substrate (K _m[Ile]=5.4 M, K _m[Leu]=9.0 M, K _m[Val]=9.5 M). The maximal uptake rates for all three substrates were very similar (0.94–1.30 nmol/mg dw · min). The optimum of amino acid uptake was at pH 8.5 and the activation energy was determined to be 80 kJ/mol. The transport activity showed a marked dependence on the presence of Na⁺ ions and on the membrane potential, but was independent of an existing proton gradient. It is concluded, that uptake of branched-chain amino acid transport proceeds via a secondary active Na⁺-coupled symport mechanism.Abbreviations CCCP Carboxyl cyanide m-chlorophenylhydrazone - dw dry weight - MES 2[N-morpholino]ethanesulfonic acid - mon monensin - nig nigericin - TPP tetraphenylphosphonium bromide - Tris tris[hydroxymethyl]aminomethane - val valinomycin     

The transport of potassium through lipid bilayer membranes by the neutral carriers valinomycin and monactin

Summary Stationary conductance experiments on neutral and negatively charged bilayer membranes in the presence of valinomycin or monactin agree with a recently proposed carrier transport model, which is common to both carrier types. This model assumes an interface reaction between a cation from the aqueous solution and a carrier molecule from the membrane phase to establish charge transport across the interface. The transport across the membrane interior is described by some kind of Eyring model. The discussion of the current-voltage characteristic, the dependence of membrane conductance on the carrier and K⁺ concentrations, and the comparison with appropriate experiments allow correlation of the different rate constants of the transport model. The results show that the rate constants partly depend on the surface charge of the membranes. This dependency can be described by introducing the Gouy-Chapman theory for charged surfaces into the transport model.It was found that the carrier molecules could be added either to the aqueous phase or to the membrane-forming solution. The quantitative treatment of this phenomenon gives an evaluation of the partition coefficient of the carrier molecules between the membrane bulk phase and water.     

In vitro translocation of protein across Escherichia coli membrane vesicles requires both the proton motive force and ATP

The energy requirement for protein translocation across membrane was studied with inverted membrane vesicles from an Escherichia coli strain that lacks all components of F1F0-ATPase. An ompF-lpp chimeric protein was used as a model secretory protein. Translocation of the chimeric protein into membrane vesicles was totally inhibited in the presence of carbonyl cyanide m-chlorophenylhydrazone (CCCP) or valinomycin and nigericin and partially inhibited when either valinomycin or nigericin alone was added. Depletion of ATP with glucose and hexokinase resulted in the complete inhibition of the translocation process, and the inhibition was suppressed by the addition of ATP-generating systems such as phosphoenolpyruvate-pyruvate kinase or creatine phosphate-creatine kinase. These results indicate that both the proton motive force and ATP are required for the translocation process. The results further suggest that both the membrane potential and the chemical gradient of protons (delta pH), of which the proton motive force is composed, participate in the translocation process.     

1-Anilino-8-naphthalenesulfonate: A fluorescent probe of ion and ionophore transport kinetics and trans-membrane asymmetry

Summary The kinetics of the transport of the 1-anilino-8-naphthalenesulfonate (ANS^–, an anionic fluorescent probe of the membrane surface) across phospholipid vesicle membranes have been studied using a stopped-flow rapid kinetic technique. The method has been used to gain detailed information about the mechanism of transport of this probe and to study ionophore-mediated cation transport across the membrane. The technique has also been exploited to study differences between the inside and outside surfaces of vesicles containing phosphatidyl choline (PC).The following is a summary of the major conclusions of this study. (a) Binding of ANS^– on the outside surface occurs within times shorter than 100 sec while permeation occurs in the time range 5–100 sec. (b) Net transport of ANS^– occurs with cotransport of alkali cations. (c) The transport rate is maximal in the region of the crystalline to liquidcrystalline phase transition, and the increase correlates with changes in the degree of aggregation of the vesicles. (d) Incorporation of phosphatidic acid (PA), phosphatidyl ethanolamine (PE) or cholesterol into PC membranes decreases the rate of ANS^– transport. (e) Neutral ionophores (I) of the valinomycin type increase ANS^– permeability in the presence of alkali cations (M ⁺) by a mechanism involving the transport of a ternaryI–M ⁺-ANS^– complex. The equilibrium constants for formation of these complexes and their rate constants for their permeation are presented. The maximal turnover number for ANS^– transport by valinomycin in dimyristoyl PC vesicles at 35°C was 46 per sec. (f) The partitioning of the ionophore between the aqueous and membrane phases and the rate of transfer of an ionophore from one membrane have been determined in kinetic experiments. (g) A method is described for the detection ofI–M ⁺ complexes on the membrane surface by their enhancement effects on ANS^– fluorescence at temperature below the phase transition temperature on monolayer vesicles. The apparent stability constants for severalI–M ⁺ complexes are given. (h) Analysis of the effect of ionic strength on the ANS^– binding to the inside outside surfaces indicates that the electrostatic surface potential (at fixed ionic strength and surface change) is larger for the inside surface than for the outside surface. (i) Analysis of the dependence of the maximal ANS^– binding for the inside and outside surfaces of vesicles made from PC and a variable mole fraction of PA, PE or cholesterol indicate that the latter three are located preferentially on the inside surface.     

Influence of molecular variations of ionophore and lipid on the selective ion permeability of membranes: I. Tetranactin and the methylation of nonactin-type carriers

Summary The manner in which the molecular structure of the carrier and the lipid composition of the membrane modulate the membrane selectivity among monovalent cations has been investigated for nonactin, trinactin, and tetranactin, which differ only in their degrees of methylation, and for membranes made of two lipids, phosphatidyl ethanolamine and glyceryl dioleate, in which equilibrium and kinetic aspects of permeation, respectively, are emphasized. Bilayer permeability ratios for Li, Na, K, Rb, Cs, Tl, and NH₄ have been characterized and resolved into equilibrium and kinetic components using a model for carrier-mediated membrane transport which includes both a trapezoidal energy barrier for translocation of the complex across the membrane interior and a potential-dependence of the loading and unloading of ions at the membrane-solution interfaces. The bilayer permeability properties due to tetranactin have been characterized in each of these lipids and found not only to be regular but to be systematically related to those of the less methylated homologues, trinactin and nonactin. This analysis has led to the following conclusions: (1) The change in lipid composition alters the relative contributions of kineticvs. equilibrium components to the observed carrier-mediated selectivity. (2) Increased methylation of the carrier increases the contribution of the kinetic component to the selectivity relative to that of the equilibrium component and additionally alters the equilibrium component sufficiently that an inversion in Cs–Na selectivity occurs between trinactin and tetranactin. (3) For all ions and carriers examined, the reaction plane for ion-carrier complexation and the width for the diffusion barrier can be represented by the same two parameters, independent of the ion or carrier, so that in all cases the complexation reaction senses 10% of the applied potential and the plateau of the diffusion barrier extends across 70% of the membrane interior.     

Valinomycin pulse-induced phosphorylation of ADP in dark anaerobic cells of the cyanobacteriumAnacystis nidulans

Addition of valinomycin to dark anaerobic suspensions ofAnacystis nidulans resulted in a transient hyperpolarization of the electrical potential across the cell membrane ( _CM) and seen from anilinonaphthalenesulfonate (ANS) fluorescence quenching and from distribution ratios of³H-tetraphenylphosphonium (TPP⁺) and¹⁴C-thiocyanate (SCN^–). At the same time a similar transient increase of intracellular ATP levels was observed, which was paralleled by decreasing ADP levels and eliminated by the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) and the F₀F₁-ATPase inhibitors dicyclohexylcarbodiimide (DCCD) and 7-chloro-4-nitrobenz-2-oxa-1,3-diazole (NBD), and in the presence of K⁺ in the medium. Since the steady-state concentration of K⁺ in dark anaerobic cells was around 150 mM, it is concluded that a valinomycin-induced K⁺ diffusion potential across the cell membrane can serve as an energy source for ATP synthesis by a reversible H⁺-ATPase present in the membrane.     

Cellular chloride depletion inhibits cAMP-activated electrogenic chloride fluxes in HT29-18-C1 cells

Cyclic AMP-activated chloride fluxes have been analyzed in HT29-18-C₁ cells (a clonal cell line derived from a human colon carcinoma) using measurements of cell volume (electronic cell sizing), cell chloride content (chloride titrator) and intracellular chloride activity (6-methoxy-N-(3-sulfopropyl)quinolinium; SPQ). HT29-18-C₁ was shown to mediate polarized chloride transport. In unstimulated cells, the apical membrane was impermeable to chloride and net chloride flux was mediated by basolateral furosemide-sensitive transport. Forskolin (10) (m) increased furosemideinsensitive chloride permeability of the apical membrane, and decreased steady-state intracellular chloride concentration approximately 9%. Cellular chloride depletion (substitution of medium chloride by nitrate or gluconate), caused greater than fourfold reduction in cellular chloride concentration. When chloride-depleted cells were returned to normal medium, cells regained chloride and osmolytes via bumetanide-sensitive transport, but forskolin did not stimulate bumetanideinsensitive chloride uptake. The inhibition of cAMP-activated chloride reuptake was not explained by limiting cation conductance, cell shrinkage, choice of substitute anion, or decreased generation of cAMP in chloridedepleted cells. When cells with normal chloride content were depolarized (135 mm medium potassium + 10 m valinomycin), cAMP activated electrogenic chloride uptake permselective for Cl^–Br^–>NO ₃ ^– >I^–. The electrogenic transport pathway was inhibited in chloridedepleted cells. Results suggest that chloride depletion limits activation of electrogenic chloride flux.The technical assistance of Dwight Derr is gratefully acknowledged. We also thank Dr. Chahrzad Montrose-Rafizadeh for help in performance of the chloride efflux experiments. This work was supported by National Institutes of Health grants RO1-DK42457 and PO1-DK44484.     

Über die Feinstruktur vonLabyrinthula coenocystis Schmoller nach Gefrierätzung

Zusammenfassung Gefrierätz-Untersuchungen an Cysten vonL. coenocystis ergaben, daß jede Zelle von einer gewellt verlaufenden Zellmembran umgeben wird, welche zylindrische Einsenkungen mit einem Durchmesser von 0,3 m aufweist. Diese Einsenkungen stellen die fürLabyrinthula charakteristischen Organellen, die bothrosomes, dar. Sie weisen in ihrem Zentrum 1–3 quergebrochene Erhebungen auf. Diese stellen Poren in der Zellmembran vonL. coenocystis dar. Die Anzahl der bothrosomes beträgt pro Zellhälfte 0–3, so daß die Gesamtzahl pro Zelle auf 5–8 geschätzt wird. Die Verteilung dieser Organellen auf der Zelloberfläche ist nicht regelmäßig, sondern zufällig. Die geringe Anzahl, ihre relativ geringe Größe im Vergleich zur Zelle und ihre unregelmäßige Verteilung auf der Zelloberfläche haben es unwahrscheinlich gemacht, daß die bothrosomes allein für die in ihrem Mechanismus noch völlig ungeklärte kontinuierliche gleitende Bewegung, etwa durch Abscheidung von Substanzen an diesen Organellen nach dem Rückstoßprinzip, verantwortlich gemacht werden können.

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The fine structure ofLabyrinthula coenocystis schmoller as revealed by freeze-etching

Summary Freeze-etched cysts ofL. coenocystis show that each cell is surrounded by a waved cell membrane in which cylindrical invaginations with a diameter of 0.3 m are visible. These invaginations represent the bothrosomes, characteristic organelles inLabyrinthula. Centered in the bothrosomes are 1–3 evaginations, which are cross-fractioned pores in the cell membrane. The number of bothrosomes per half cell is 0–3; so that the total number of these organelles is calculated at 5–8 per cell. The distribution of the bothrosomes on the cell surface is not regular but random. Their low number, their relative small size in comparison to the total cell area, and their random distribution on the cell surface lead us to presume that these organelles alone can not be held responsible for the cell's gliding movement as, for example, by emission of substances from the organelles by the repulsionprincip.

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Wir danken Herrn Prof. Dr. K. G.Grell, Zoologisches Institut der Universität Tübingen, für die Überlassung derLabyrinthula-Kultur und der Deutschen Forschungsgemeinschaft für ihre Unterstützung.