The K+ channel in the plasma membrane of rye roots has a multiple ion residency pore

The permeation of K⁺ and Na⁺ through the pore of a K⁺ channel from the plasma membrane of rye roots was studied in planar 1-palmitoyl-2-oleoyl phosphatidylethanolamine bilayers. The pore contains at least two ion-binding sites which can be occupied simultaneously. This was indicated by: (i) biphasic relationships with increasing cation concentration of both channel conductance at the zero-current (reversal) potential of the channel (E _rev) and unitary-current at a specified voltage and (ii) a decline in E _rev in the presence of equimolar Na⁺ (cis):K⁺ (trans) as the cation concentration was increased. To determine the spatial characteristics and energy profiles for K⁺ and Na⁺ permeation, unitary-current/ voltage data for the channel were fitted to a three energy-barrier, two ion-binding site (3B2S) model. The model allowed for simultaneous occupancy of binding sites and ionic repulsion within the pore, as well as surface potential effects. Results suggested that energy peaks and energy wells (ion binding sites) were situated asymmetrically within the electrical distance of the pore, the trans energy-well being closer to the center of the pore than its cis counterpart; that the energy profile for K⁺ permeation differed significantly from that of Na⁺ in having a higher cis energy peak and a deeper cis energy well; that cations repelled each other within the pore and that vestibule surface charge was negligible. The model successfully simulated various aspects of K⁺ and Na⁺ permeation including: (i) the complexities in current rectification over a wide range of contrasting ionic conditions; (ii) the biphasic relationships with increasing cation concentration of both channel conductance at E _rev and unitary-current at a specified voltage; (iii) the decline in E _rev in equimolar Na⁺ (cis):K⁺ (trans) as cation concentrations were increased and (iv) the complex relationships between mole fraction and E _rev at total cation concentrations of 100 and 300 mm.We thank Prof. O. Alvarez (Universidad de Chile, Santiago, Chile) for supplying the computer program AJUSTE and Prof. D. Sanders (University of York, UK), Prof. D. Gradmann and Dr. G. Thiel (University of Göttingen, Germany) for stimulating ideas. This work was supported by the Agricultural and Food Research Council.     

Effect of phospholipid surface charge on the conductance and gating of a Ca2+-activated K+ channel in planar lipid bilayers

Summary A Ca-activated, K-selective channel from plasma membrane of rat skeletal muscle was studied in artificial lipid bilayers formed from either phosphatidylethanolamine (PE) or phosphatidylserine (PS). In PE, the single-channel conductance exhibited a complex dependence on symmetrical K⁺ concentration that could not be described by simple Michaelis-Menten saturation. At low K⁺ concentrations the channel conductance was higher in PS membranes, but approached the same conductance observed in PE above 0.4m KCl. At the same Ca²⁺ concentration and voltage, the probability of channel opening was significantly greater in PS than PE. The differences in the conduction and gating, observed in the two lipids, can be explained by the negative surface charge of PS compared to the neutral PE membrane. Model calculations of the expected concentrations of K⁺ and Ca²⁺ at various distances from a PS membrane surface, using Gouy-Chapman-Stern theory, suggest that the K⁺-conduction and Ca²⁺-activation sites sense a similar fraction of the surface potential, equivalent to the local electrostatic potential at a distance of 9 Å from the surface.     

Phosphate transport in human red blood cells: Concentration dependence and pH dependence of the unidirectional phosphate flux at equilibrium conditions

Summary The concentration dependence and the pH dependence of the phosphate transport across the red cell membrane were investigated. The unidirectional phosphate fluxes were determined by measuring the³²P-phosphate self-exchange in amphotericin B (5 mol/liter) treated erythrocytes at 25°C.The flux/concentration curves display anS-shaped increase at low phosphate concentrations, a concentration optimum in the range of 150 to 200mm phosphate and a self-inhibition at high phosphate concentrations. The apparent half-saturation concentrations,P _(0.5), range from 50 to 70mm and are little affected by pH. The self-inhibition constants, as far as they can be estimated, range from 400 to 600mm. The observed maximal phosphate fluxes exhibit a strong pH dependence. At pH 7.2, the actual maximal flux is 2.1×10^–6 moles·min^–1·g cells^–1. The ascending branches of the flux/concentration curves were fitted to the Hill equation. The apparent Hill coefficients were always in the range of 1.5–2.0. The descending branches of the flux/concentration curves appear to follow the same pattern of concentration response.The flux/pH curves were bell-shaped and symmetric with regard to their pH dependence. The pH optimum is at approximately pH 6.5–6.7. The apparent pK of the activator site is in the range of 7.0 to 7.2, while the apparent pK for the inactivating site is in the range of 6.2 to 6.5. The pK-values were not appreciably affected by the phosphate concentration.According to our studies, the transport system possesses two transport sites and probably two modifier sites as indicated by the apparent Hill coefficients. In addition, the transport system has two proton binding sites, one with a higher pK that activates and one with a lower pK that inactivates the transport system. Since our experiments were executed under self-exchange conditions, they do not provide any information concerning the location of these sites at the membrane surfaces.     

Periodic responses in squid axon membrane exposed intracellularly and extracellularly to solutions containing a single species of salt

Summary A periodic membrane potential change was found to occur in squid giant axons which were internally and externally perfused with solutions of an identical composition and were hyperpolarized by passing a sustained inward current. The solution contained Co²⁺ or Mn²⁺ as the sole cation species at a concentration of 1–10mm. The amplitude of the response was roughly 100 mV. The current intensity and the ion concentration had large effects on the response. The voltage-clamp technique revealed an N-shapedI-V characteristic of the membrane system. The membrane emf of the resting and excited states was almost the same but the membrane conductance was increased in the excited states. The response was suppressed with 4-aminopyridine reversibly but unchanged with tetrodotoxin or D-600. Those unusual ionic conditions did not deprive axons of their ability to produce ordinary action potentials in physiological solutions. The experimental conditions employed and the results obtained were very close to those for some of the artificial membrane models. Applicability of the physico-chemical theories developed for these models is discussed.     

A light and electron microscopic study of the quadriflagellate green algaSpermatozopsis exsultans

The green flagellateSpermatozopsis exsultans Korshikov has been studied in culture by light and electron microscopy. The organism is naked, bears four flagella and is conspicuously spirally twisted. The ultrastructure and location of cell organelles (except the flagellar apparatus) has been investigated in detail using an absolute configuration analysis. With the exception of a doubling of the flagella and of the secondary cytoskeletal microtubule system,S. exsultans has the exact same complement of organelles occupying the same relative positions as has been described forS. similis. The two species are therefore correctly placed in the same genus. The usefulness of absolute orientations of cell organelles for green algal taxonomy and phylogeny is stressed.Dedicated to Prof.M. Mix on the occasion of her 60th birthday.     

Vergleichende untersuchung des peptonabbaus und der damit verknüpften ciliatenbesiedlung in strömenden und stagnierenden modellgewässern

Summary The decomposition of peptone and the associated species numbers and individual counts of ciliates has been investigated in flowing (current speed 40 cm/sec) and stagnant model ecosystems. In order to imitate the ecological conditions in natural waters receiving sewage small quantities of peptone were added at regular intervals.The most important environmental factors such as pH, dissolved oxygen, NH₄ ⁺-, NO₃ ^--levels, and the organisms occurring in the free water as well as in the periphyton community on microscopic slides, were investigated for three weeks. Both the population dynamics of organisms and the environmental conditions within the ecosystems are figured (Figs. 1–6).The ecosystem with flowing water, and the aerated stagnant model showed high contents of O₂ and fast mineralisation of peptone up to the NO₃ ^--level. The unaerated model showed a retarded decomposition of peptone and accumulation of ammonia.The unaerated stagnant ecosystem showed the highest individual counts of bacteria and ciliates, while in the flowing model a limited number of organisms only was found.Most of the differences between flowing and stagnant waters are due to the varying O₂-level. Only two species were directly inhibited by the current speed tested in these experiments.

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Zoologisches Institut der Universität Bonn Hydrobiologische Arbeitsgruppe (Leiter: Prof. Dr. H. Bick)

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Mit Untersützung der Weltgesundheitsorganisation.

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Herrn Prof. Dr. Rolf Danneel zum 70. Geburtstag gewidmet.

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Anschrift der Verfasser: PProf. Dr. H. Bick und Diol. Biol. W. Schmerenbeck, Hydrobiologische Arbeitsgruppe am Zoologischen Institut der Universität, D05300 Bonn 1, Poppelsdorfer Schlo\.     

The effects of adenohypophysectomy on the hypothalamic-hypophysial neurosecretory system and the adrenal gland of the toad Bufo arenarum Hensel

Summary The effects of adenohypophysectomy were studied on the hypothalamichypophysial neurosecretory system and the adrenal glands of Bufo arenarum Hensel. An increase in vascularization of the pars intermedia was found and the neurosecretory material (NSM) in the glandular region of the median eminence disappeared. Its reappearance later was accompanied by hypertrophy of the pars intermedia; differentiated chromophil cells appeared in the pars intermedia around the vessels and, at the same time, the adrenal reverted to normal. These findings are interpreted as hypertrophy and differentiation of the pars intermedia with replacement, at least partly, of the functions of the pars distalis. The probable mechanism of differentiation and the functions of chromophil cells of the pars intermedia are discussed.This paper was presented at the VI Reunión de la Asociación Latinoamericana de Ciencias Fisiológicas (A.L.A.C.F.), Viña del Mar, Chile, 1964. It was carried out under the auspices of the Consejo Nacional de Investigaciones Científicas y Técnicas (Argentina) and the Rockefeller Foundation (School grant RF-58028).Fellow of the Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina.The authors wish to thank Prof. M. H. Burgos for his constant interest, Prof. B. A. Houssay, Prof. H. Heller and Dr. J. H. Tramezzani for their criticisms, Miss B. Rodriguez and Mr. L. Castro for their technical assistance.     

Modulation of apical Na permeability of the toad urinary bladder by intracellular Na,Ca, and H

Summary The Na conductance of the apical membrane of the toad urinary bladder was measured at different concentrations of Na both in the external medium and in the cell. Bladders were bathed in high K-sucrose medium to reduce basal-lateral resistance and voltage, and the transepithelial currents measured under voltage-clamp conditions. Amiloride was used as a specific blocker of the apical Na channel. At constant external Na, the internal Na concentration was increased by blocking the basallateral Na pump with ouabain. With high Na activity in the mucosal medium (86mm), increases in intracellular Na activity from 10 to over 40mm increased the amiloride-sensitive slope conductance at zero voltage while apical Na permeability, estimated from current-voltage plots using the constant field equation, decreased by less than 20%. Lowering the serosal Ca concentration from 1 to 0.1mm had no effect on the change inP _Na with increasing Na_c, but increasing serosal Ca to 5mm enhanced the reduction inP _Na with increasing Na _c , presumably by increasing Ca influx into the cell.P _Na was also reduced by serosal vanadate (0.5mm), a putative blocker of ATP-dependent Ca extrusion from the cell, and by acute exposure to CO₂, which presumably acidifies the cytoplasm. Current-voltage relationships of the amiloridesensitive transport pathway were also measured in the absence of a Na gradient across the apical membrane. These plots show that outward current passes through the channels somewhat less easily than does inward current. The shape of theI-V relationships was not significantly altered by changes in cellular Na, Ca or H, indicating that the effects of these ions onP _Na are voltage independent.     

Characterization of the High-Affinity Verapamil Binding Site in a Plant Plasma Membrane Ca2+-selective Channel

Despite biochemical evidence for the existence of high-affinity phenylalkylamine receptors in higher plants, their effects on channel activity have only been demonstrated at relatively high concentrations. We have performed a quantitative single-channel analysis of the changes induced by extracellular verapamil in the rca channel [a wheat root plasma membrane Ca²⁺-selective channel (Pi?eros & Tester, 1995. Planta 195:478–488)]. Concentrations as low as 0.5 μm verapamil induced a blockade of the inward current, with no evident reduction of the single-channel current amplitude. Blockade by verapamil was concentration and voltage dependent. Preliminary analysis suggested the blockade was due to a reduction in the maximum open state probability rather than a change in V_0.5. Further analysis of the association and dissociation rate constants revealed a binding site located 56 to 59% down the voltage drop from the extracellular face of the channel, with a K _d (0) of 24 to 26 μm. This results in a K _d at −100 mV of 2 μm. Methoxyverapamil had qualitatively the same effects. This intra-pore binding site can be accessed directly from the extracellular side of the rca channel, but apparently not from the cytosolic side. Received: 15 August 1996/Revised: 23 December 1996     

Ionic Channels of the Sugar Beet Tonoplast are Regulated by a Multi-ion Single-file Permeation Mechanism

Ionic channels of the sugar beet tonoplast were studied using the patch-clamp technique. At micromolar concentrations of cytosolic calcium, several (at least four) distinct single-channel current levels were routinely identified. On the basis of channel voltage dependence, kinetic properties and conductance of single openings, the largest channel (103 ± 2 pS in symmetric 150 mm KCl) corresponds to the slow vacuolar (SV) channel already identified by Hedrich and Neher (1987). The majority of the whole-vacuole current was ascribed to this time-dependent slow-activating channel elicited by positive vacuolar potentials. The channel of intermediate amplitude (41 ± 1 pS in 150 mm KCl) did not show any voltage dependence and delay in the activation upon the application of voltage steps to both positive and negative transmembrane potentials. Owing to its voltage independence this channel was denominated FV1. The opening probability of the SV-type channel increased by increasing the cytoplasmic calcium concentration, while the activity of the FV1 channel did not increase appreciably by changing the calcium concentration in the range from 6 μm to 1 mm. All the channels identified showed a linear current-voltage characteristic in the range ±100 mV and at least the three most conductive ones displayed potassium selectivity properties. Substitution of potassium with tetramethylammonium (TMA) on the cytosolic side demonstrated that both the SV and FV1 channels are impermeable to TMA influx into the vacuole and support the potassium selectivity properties of these two channels. Moreover, the single channel conductances of all the channels identified increased as a function of the potassium concentration and reached a maximum conductivity at [K⁺] ∼0.5 m. This behavior can be explained by a multi-ion occupancy single-file permeation mechanism. Received: 26 December 1995/Revised: 10 July 1996     

Block of the sheep cardiac sarcoplasmic reticulum Ca2+-release channel by tetra-alkyl ammonium cations

Summary The purified ryanodine receptor channel of the sheep cardiac muscle sarcoplasmic reticulum (SR) membrane functions as a calcium-activated cation-selective channel under voltage-clamp conditions following reconstitution into planar phospholipid bilayers. We have investigated the effects of the tetra-alkyl ammonium (TAA) cations, (C _n H_2n+1)₄N⁺ and the trimethyl ammonium cations, ethyltrimethyl ammonium and propyltrimethyl ammonium, on potassium conductance through the receptor channel. Small TAA cations (n = 1–3) and the trimethyl ammonium derivatives act as asymmetric, voltage-dependent blockers of potassium current. Quantitative analysis of the voltage dependence of block indicates that the conduction pathway of the sheep cardiac SR ryanodine receptor channel contains two distinct sites for the interaction of these small organic cations. Sites are located at approximately 50% for tetramethyl ammonium (TMA ⁺) and 90% for tetraethyl ammonium (TEA⁺) and tetrapropyl ammonium (TPrA⁺) of the voltage drop across the channel from the cytosolic face of the protein. The chemical substitution of an ethyl or propyl group for one of the methyl groups in TMA⁺ increases the voltage dependence of block to a level similar to that of TEA ⁺ and TPrA⁺. The zero-voltage dissociation constant (K _b(0)) falls with the increasing number of methyl and methylene groups for those blockers acting 90% of the way across the voltage drop. This is interpreted as suggesting a hydrophobic binding site at this point in the conduction pathway. The degree of block increases as the concentration of small TAA cations is raised. The concentration dependence of tetraethyl ammonium block indicates that the cation interacts with a single site within the conduction pathway with a K _m of 9.8±1.7 mm (mean±sd) at 40 mV. Larger TAA cations (n = 4–5) do not induce voltage-dependent block of potassium current of the form seen with the smaller TAA cations. These data support the contention that the sheep cardiac SR ryanodine receptor channel may be occupied by at most one ion at a time and suggest that a large proportion of the voltage drop falls over a relatively wide region of the conduction pathway.This work was supported by funds from the Medical Research Council and the British Heart Foundation. We would like to thank Richard Montgomery for his considerable help with the chemical synthesis. We are grateful to Drs. John Chambers, Nick Price and staff for showing us the intricacies of NMR spectroscopy.     

An irreversible thermodynamics treatment of electrodiffusion in uniform membranes

Summary In a previous paper (Ciani andGliozzi, 1966) we presented the integration of the phenomenological equations of flow of irreversible thermodynamics, describing the movement of water and two monovalent counterions across a uniform permselective membrane. The present paper generalizes those results, by introducing less restrictive assumptions and by allowing for the case that two counterions of different valences or one counterion and one coion are present in the membrane.The analytical expressions for the voltage-current relationship, the fluxes of the mobile species and the profiles of the concentrations are derived. The membrane resistance and the rectification properties of the system are analyzed in detail.The expression of the flux of water is found to allow for negative electroosmosis.Some comparative remarks between the present treatment and the earlier theory bySchlögl andSchödel (Schlögl andSchödel, 1955) are presented in the last section of the paper.     

Voltage-dependent K channels in protoplasts of trap-lobe cells ofDionaea muscipula

Summary The outward rectification of the K⁺ current in mesophyll cell protoplasts from trap-lobes ofDionaea muscipula was studied with the patch-clamp technique. The rectification had instantaneous and time-dependent components. Changes in [K⁺] _i strongly affected the conductance voltage relation of the plasma membrane while changes in [K⁺] _o had little effect on the relation. Thus, the outward rectification depends on the membrane voltage and the concentration of intracellular K⁺. Corresponding single-channel activities were observed both in the intact membrane (cell-attached recording) and in excised patches. The single-channel conductance was about 3.3 pS with symmetrical solutions containing 30mm K⁺.     

The Action of Blocking Agents Applied to the Inner Face of Ca2+-activated K+ Channels from Human Erythrocytes

The actions of clotrimazole and cetiedil, two drugs known to inhibit the Gardos channel, have been studied on single intermediate conductance calcium-activated potassium (IK_Ca) channels in inside out patches from human red blood cells, and compared with those of TEA and Ba²⁺ applied to the cytoplasmic face of the membrane. TEA produced a fast block which was observed as a reduction in the amplitude of the single channel current. This effect was weakly voltage dependent with the fraction of the membrane potential sensed by TEA at its binding site (δ) of 0.18 and a K _d at 0 mV of 20.5 mm. Ba²⁺ was a very potent blocker of the channel, breaking the single channel activity up into bursts, interspersed with silent periods lasting several seconds. The effect of Ba²⁺ was very voltage sensitive, δ= 0.44, and a K _d at 0 mV of 0.15 μm. Clotrimazole applied to the inner face of the membrane at a concentration ≤1 μm produced a slow block resulting in bursts of channel activity separated by quiescent periods lasting many seconds. The effect of clotrimazole was mimicked by a quaternary derivative UCL 1559, in keeping with an action at the cytoplasmic face of the channel. A high concentration of cetiedil (100 μm) produced only a weak block of the channel. The kinetics of this action were very slow, with burst and inter-burst intervals lasting several minutes. While inhibition of the Gardos channel by cetiedil is unlikely to involve an intracellular site of action, if clotrimazole is able to penetrate the membrane, part of its effect may result from binding to an intracellular site on the channel. Received; 18 February 1998/Received: 5 June 1998     

Slow inactivation of tetrodotoxin-insensitive Na+ channels in neurons of rat dorsal root ganglia

Summary Whole-cell patch-clamp experiments were performed with neurons cultured from rat dorsal root ganglia (DRG). Two types of Na⁺ currents were identified on the basis of sensitivity to tetrodotoxin. One type was blocked by 0.1 nm tetrodotoxin, while the other type was insensitive to 10 m tetrodotoxin. The peak amplitude of the tetrodotoxin-insensitive Na⁺ current gradually decreased after depolarization of the membrane. The steady-state value of the peak amplitude was attained several minutes after the change of holding potential. Such a slow inactivation was not observed in tetrodotoxin-sensitive Na⁺ current. The slow inactivation of the tetrodotoxin-insensitive Na⁺ current was kinetically distinct from the ordinary short-time steady-state inactivation. The voltage dependence of the slow inactivation could be described by a sigmoidal function, and its time course had a double-exponential process. A decrease of external pH partially antagonized the slow inactivation, probably through an increased diffusion potential across the membrane. However, the slow inactivation was not due to change in surface negative charges, since a shift of the kinetic parameters along the voltage axis was not observed during the slow inactivation. Due to the slow inactivation, the inactivation curves for the tetrodotoxininsensitive Na⁺ current were shifted in the negative direction as the prepulse duration was increased. Consequently, the window current activated at potentials close to the resting membrane potential was markedly reduced. Thus, the slow inactivation may be involved in the long-term regulation of the excitability of sensory neurons.We thank Prof. Hirosi Kuriyama for his support and advice and Dr. M. Yoshii for helpful discussions. This study was supported by the Japanese Ministry of Education (Scientific Research 02670090).     

Long-lasting inward current in snail neurons in barium solutions in voltage-clamp conditions

Summary The inward membrane current was recorded under voltage clamp from nonbursting neurons of the snailHelix pomatia in Na-free solutions containing Ba ions but no other divalent cations. The inward current was separated into two components: (i) an early fast inactivating component and (ii) a smaller long-lasting component. Both components were dependent on the external Ba concentration. It is concluded that both components of the inward current are carried by Ba ions. The activation of the early fast inactivating component of the inward current occurred at more positive membrane potential than that of the long-lasting component. The shape of the inactivation curve for the peak value of the inward current was similar to that for the long-lasting component. The potentials of half-inactivation for the peak value of the inward current and for its long-lasting component were –28 and –22 mV, respectively. The blocking effect of Co⁺⁺ on the early fast inactivating component was substantially greater. In some neurons after treatment with 15mm Co⁺⁺ only the long-lasting component was recorded. The activation kinetics of the long-lasting component of the inward current were analyzed using the Hodgkin-Huxley equations. The results could be explained by assuming that two components of the inward current in Na–Ca-free solution with Ba ions flowed through the two different channels. The significance of the long-lasting inward current for the normal spike generation is discussed.     

Dimerization constant and single-channel conductance of Gramicidin in thylakoid membranes

Summary The effect of the pore-forming antibiotic gramicidin on pure lipid membranes is well characterized. We studied its action in protein-rich thylakoid membranes that contain less than 25% (wt/wt) acyl lipids. A transmembrane voltage was induced by flashing light, and its decay was measured and interpreted to yield the distribution of gramicidin over thylakoids, its dimerization constant and its single-channel conductance in this membrane. The distribution of gramicidin over the ensemble of thylakoids was immediately homogeneous when the antibiotic was added under stirring, while it became homogeneous only after 20 min in a stirred suspension that was initially heterogeneous. The dimerization constant, 5×10¹⁴ cm²/mol, was about 10 times larger than in pure lipid membranes. This was attributed to the upconcentration of gramicidin in the small fractional area of protein free lipid bilayer and further by a preference of gramicidin for stacked portions of the membrane. The latter bears important consequences with regard to bioenergetic studies with this ionophore. As gramicidin was largely dimerized from a concentration of 1 nm (in the suspension) on, the membrane's conductance then increased linearly as a function of added gramicidin. When the negative surface potential at the thylakoid membrane was screened, the conductance of a single gramicidin dimer agreed well with figures reported for bilayers from neutral lipid (about 0.5 pS at 10 mm NaCl). The modulation of the conductance by the surface potential in spinach versus pea thylakoids and between different preparations is discussed in detail.We would like to thank Ms. H. Kenneweg for photographs. financial support by the DFG (SFB 171/B3) is gratefully acknowledged.This paper is dedicated to the Late Prof. Peter Läger.     

Basolateral K+ Conductance Establishes Driving Force for Cation Absorption by Outer Sulcus Epithelial Cells

Outer sulcus epithelial cells were recently found to actively reabsorb cations from the cochlear luminal fluid, endolymph, via nonselective cation channels in the apical membrane. Here we determined the transport properties of the basolateral membrane with the whole-cell patch clamp technique; the apical membrane contributed insignificantly to the recordings. Outer sulcus epithelial cells exhibited both outward and inward currents and had a resting membrane potential of −90.4 ± 0.7 mV (n= 78), close to the Nernst potential for K⁺ (−95 mV). The reversal potential depolarized by 54 mV for a tenfold increase in extracellular K⁺ concentration with a K⁺/Na⁺ permeability ratio of 36. The most frequently observed K⁺ current was voltage independent over a broad range of membrane potentials. The current was reduced by extracellular barium (10⁻⁵ to 10⁻³ m), amiloride (0.5 mm), quinine (1 mm), lidocaine (5 mm) and ouabain (1 mm). On the other hand, TEA (20 mm), charybdotoxin (100 nm), apamin (100 nm), glibenclamide (10 μm), 4-aminopyridine (1 mm) and gadolinium (1 mm) had no significant effect. These data suggest that the large K⁺ conductance, in concert with the Na⁺,K⁺-ATPase, of the basolateral membrane of outer sulcus cells provides the driving force for cation entry across the apical membrane, thereby energizing vectorial cation absorption by this epithelium and contributing to the homeostasis of endolymph.     

Action potential-like responses due to the inward rectifying potassium channel

Summary This paper describes experiments carried out in the absence of sodium and calcium in the external solution. Frog atrial trabeculae were stimulated in current clamp with the double sucrose gap technique. The voltage responses looked like slow action potentials with a clear threshold. These responses were not suppressed in the presence of EGTA, in the presence of sodium or calcium channel blockers, or when sulfate ions replaced chloride. Guinea pig isolated ventricular myocytes were studied in whole cell clamp mode with a pathch pipette. Under current clamp, they displayed also voltage responses with a threshold. These responses were resistant to cadmium (5mm), and were suppressed by barium (0.5mm). A negative slope conductance is required to take into account these results. The membrane current in current clamp can be estimated by plotting the response in the phase plane. This analysis shows that on both types of preparations, the current responsible for the negative slope is not time dependent. This current is suppressed by barium. It can be concluded that it is the outward current flowing through the inward rectifying potassium channels. To confirm this hypothesis, data obtained in voltage clamp on the same preparations were introduced into a computer model to predict the response in current clamp. The results were in agreement with the experiments. Similar responses could be recorded and analyzed on skeletal muscle in isotonic potassium solution. These results show that the inward rectifier can induce by itself properties looking like excitability on different preparations. The physiological significance of this effect in normal conditions is discussed. The voltage responses described in this paper look similar to the slow action potentials on heart, which are sensitive to modifications of the calcium channels, but also of the potassium channels. Some implications in cardiac pharmacology are discussed.     

Beobachtungen anLempholemma-Arten aus dem Ostalpenraum (Lichenes,Lichinaceae)

The phycobiont of all species ofLempholemma analyzed is attacked by haustoria. The haustoria of these five species ofLempholemma and of two species with unknown apothecium but with a thallus quite similar toLempholemma are described. They show distinct morphological and functional characters which allow species delimitation.

Frau Prof. Dr.Elisabeth Tschermak-Woess zu ihrem 70. Geburtstag gewidmet.